Piping Design and Operations Guideobook...
Piping Design and Operations Guidebook Volume 1
Piping Design and Operations Guidebook Volume 1
Table of Contents Facts at Your Fingertips: Fluid Flow______________________________________________________ 3 Facts at Your Fingertips: Tubing for Peristaltic Dosing Pumps_______________________________ 4 Piping for Process Plants Part 1: The Basics______________________________________________ 5 Piping Design Part 2: Flanges__________________________________________________________ 11 Piping Design Part 3: Design Elements__________________________________________________ 17 Piping for Process Plants Part 4: Codes and Fabrication___________________________________ 25 Piping Design Part 5: Installation and Cleaning___________________________________________ 33 Piping for Process Plants Part 6: Testing and Verification__________________________________ 42 Stress Analysis for Piping Systems Resting on Supports___________________________________ 49 A Method for Quantifying Pipe Vibrations________________________________________________ 53 New Piping Code for High-Purity Processes_____________________________________________ 57 Piping Design for Hazardous Fluid Service_______________________________________________ 62 Active Management of Pipespool Fabrication____________________________________________ 69 Reduce Gas Entrainment in Liquid Lines_________________________________________________ 75 Designing Safer Process Plants________________________________________________________ 78 Designing for a Safe Process___________________________________________________________ 83
Fluid Flow Department Editor: Kate Torzewski ¨ N N4 · f 16 ©1 1 He 1 3 He7 ¸ 3 f NRe ¸ NRe © 6 N Re d¹ d ª d DVR NRed Md
DEFINITIONS Newtonian fluid. A fluid is known to be Newtonian when shear stresses associated with flow are directly proportional to the shear rate of the fluid Power law fluid. A structural fluid has a structure that forms in the undeformed state, but then breaks down as shear rate increases. Such a fluid exhibits “power law” behavior at intermediate shear rates Bingham plastic fluid. A plastic is a material that exhibits a yield stress, meaning that it behaves as a solid below the stress level and as a fluid above the stress level
Laminar Pipe flow For steady flow in a pipe (whether laminar or turbulent), a momentum balance on the fluid gives the shear stress at any distance from the pipe centerline.
T rx $&r T w r 2L R
N He
R
Q P ∫¾ r 2G˙ rx dr
(2) Newtonian fluid. For a Newtonian fluid, τrx = µγ•rx, which gives the following volumetric flowrate, known as the Hagen-Poiseuille equation. 4
(3)
It can be written in dimensionless form in Equation (4) with the two terms defined in Equations (5) and (6).
f = 16 / NRe f
P 2 D5 $& 32RLQ2
NRe
τ Q=π ⎞ w⎞ ⎠mR⎠
n
⎞ n ⎞R ⎠3n + 1⎠
3n+ 1 n
(7)
Equation (7) can be rearranged into the following dimensionless form. (8) f = 16 / NRe, pl
NRe, pl
7 3 n
2
RQ
12 · ¨¥ ´ 1 ¸ f 2 ©¦ 8 µ 1 . 5 ©§ NRe ¶ A B ¸¹ ª
n
¥ 37, 530 ´ B ¦ µ § NRe ¶
4 PR3T w ¨© 4 ¥ T 0 ´ 1 ¥ T 0 ´ ·¸ 1 ¦ µ ¦ µ Q 4 M d © 3 § T w ¶ 3 §T w ¶ ¸ ¹ ª
The equivalent dimensionless form is given by Equations (11), (12) and (13).
(10)
12
(16)
16
16
(17) (18)
Power law. For a power-law fluid, the friction factor depends only upon Equation (9) and the flow index, as represented by Equations (19)–(25) [3].
f = (1 − α ) f L + fL
(9)
Bingham plastic. In this case, there is a solid-like “plug flow” region from the pipe centerline (where τrx = 0) to the point where –τrx = τ0 (that is, at r = r0 = R x τ0/ τw). The result is a flow integral modified from Equation (2). For a Bingham plastic, –τrx = τ0 + µ∞(–γ•rx). Using this expression and the modified flow integral, the Buckingham-Reiner Equation (10) is found.
1
¨ ´· ¥ © µ¸ ¦ 1 µ¸ A ©2.457 ln¦ 0 . 9 © ¦¥ 7 ´ 0.27E µ ¸¸ © µ ¦§ ¶ § Re D ¶ ¹¾ ª¾
2 n
¨3n 1 · mP 2 n D 4 3n © n¸ ª ¹
(15)
The Churchill Equation [2] represents the entire Moody diagram, from laminar, through transition flow, to fully turbulent flow. It is presented here as Equations (16), (17), and (18).
(6)
1
(14)
1 = – 4 log ⎡ ε D ⎤ ⎢⎣ 3.7 ⎥⎦ fT
Power law. A fluid that follows the power law model obeys the relationship τrx = –µ(–γ•rx)n. This gives the following equation.
· ¸ ¹¾
When NRe is very large, the friction factor depends only on ε/D. This condition is noted with fT as the “fully turbulent” friction factor in Equation (15).
(5)
4QR PDM
(13)
1 4 log ¨ E D 1.255 © f ª¾ 3.7 NRe f
(4)
(12)
Since most turbulent flows cannot be analyzed from a purely theoretical perspective, data and generalized dimensionless correlations are used. Newtonian fluid. The friction factor for a Newtonian fluid in turbulent flow is a function of both NRe and the pipe relative roughness, ε/D, which can be read off the Moody diagram [5]. The turbulent part of the Moody diagram (for NRe > 4,000) is accurately represented by the Colebrook equation (14).
0
Q P$&D 128 ML
Turbulent Pipe flow
(1)
In Equation (1), Φ = P + ρgz. The volumetric flowrate Q can be related to the local shear rate by doing an integration by parts of Equation (2).
D2 RT 0 M d2
(11)
fT fTr
16 NRe, pl
[f
α
−8 T
−8 + fTr
]
1
(19)
8
nomenclature a Dimensionless parameter A Dimensionless parameter B Dimensionless parameter D Diameter, m f Fanning friction factor, dimensionless fL Laminar friction factor, dimensionless fT Fully turbulent friction factor, dimensionless fTr Transition friction factor, dimensionless g Gravitational acceleration, m/s2 L Length of cylinder or pipe, m m Consistency coefficient, (N)(s)/m2 n Power law fluid flow index, dimensionless NHe Hedstrom number, dimensionless NRe Reynolds Number, dimensionless NRe,pl Power law Reynolds Number, dimensionless NRe,plc Power law Reynolds Number at transition from laminar to turbulent flow, dimensionless NRe∞ Bingham-plastic Reynolds Number, dimensionless P Pressure, Pa Q Volumetric flowrate, m3/s r Radial position in a pipe or a cylinder, m R Pipe or cylinder radius, m V Velocity, m/s z Vertical elevation above a horizontal reference plane, m α Dimensionless parameter γ•rx Shear rate in tube flow, s–1 ε Wall roughness, m µ Newtonian viscosity, Pa–s µ∞ Bingham Plastic limiting viscosity, Pa–s ρ Density, kg/m3 τ0 Yield stress, N/m2 τrx Stress due to force in x direction acting on r surface, N/m2 τw Stress exerted by fluid on tube wall, N/m2 Φ Flow potential, P + ρgz, Pa ∆Φ Ιncrease in flow potential, Pa Bingham plastic. For the Bingham plastic, fT is solely a function of NRe∞ and NHe, as represented by Equations (26)–(29).
f f Lm fTm a fT 10 0.193 NRe
d
;
0.0682n [ NRe, pl ]
1
m 1.7
0.5
(1.87 2.39 n)
(21)
(26)
m
(27)
40, 000 NRe
5
N He
=
(28) (29)
References
0.414 0.757 n [ 5.24 n] e 1.79 s 10 4 NRe, pl (22)
1 A 1 4 $ $ NRe, pl NRe, plc
(23)
(24) The value of NRe where transition from laminar to turbulent flow occurs (NRe,plc) is given by Equation (25).
NRe, plc 2, 100 875(1 n)
1
a 14.7 1 0.146 e 2.9 s 10
(20)
(25)
1.Darby, R., Take the Mystery Out of Non-Newtonian Fluids, Chem. Eng., March 2001, pp. 66–73. 2.Churchil, S. W., Friction Factor Equation Spans all FluidFlow Regimes, Chem. Eng., November 1997, p. 91. 3.Darby, R., and Chang, H. D., A Generalized Correlation for Friction Loss in Drag-reducing Polymer Solutions, AIChE J., 30, p. 274, 1984. 4.Darby, R., and Chang, H. D., A Friction Factor Equation for Bingham Plastics, Slurries and Suspensions for all Fluid Flow Regimes, Chem. Eng., December 28, 1981, pp. 59–61. 5.Darby, R., “Fluid Mechanics for Chemical Engineers,” Vol. 2, Marcel Dekker, New York, N.Y., 2001.
Tubing for Peristaltic Dosing Pumps
Department Editor: Scott Jenkins
P
eristaltic pumps work by compressing a tube against a circular pump housing with rollers on a rotating arm. The fluid that is ahead of the roller gets pushed forward, while new fluid is drawn into the tube by the vacuum generated as the tube returns to its relaxed state. Peristaltic pumps are a type of positive displacement pump that can be used in industrial chemical dosing applications and others, including medical applications. The tubing used to convey the material into and out of the pump mechanism is a critical aspect of pump performance. The following are considerations for selecting tubing materials for use with a peristaltic pump.
Advantages and disadvantages
As dosing pumps, peristaltic-based systems have a number of advantages, along with some limitations (Table). Reducing the risk of contamination by pump components is a distinct advantage of peristaltic pumps, but the flow is non-uniform, which can present problems in certain applications requiring continuous flow.
Tubing materials
Peristaltic pump tubing is a key component, and needs to be selected thoughtfully. Major considerations for tubing are chemical compatibility, elastomeric performance and tube life. Tubing for peristaltic pumps needs to be constructed of an elastomeric material in order to maintain the circular crosssectional shape, even after millions of squeeze-cycles inside the pump. Because of this requirement, many non-elastomeric polymer materials that are effective at resisting chemical attack must be eliminated from consideration in these applications. Materials such as PTFE (polytetrafluoroethylene), polyolefins, PVDF (polyvinylidene fluoride) and so on should not be considered as material for pump tubing unless they are used as a lining of another tubing material. Popular elastomers for pump tubing are silicone, PVC (polyvinyl chloride), EPDM (ethylene propylene diene monomer)+polypropylene (as in Santoprene), polyurethane and Neoprene. Of these materials, the EPDM+polypropylene (“-prenes”) have the best fatigue resistance and a wide range of chemical compatibility. Silicone is popular with water-based fluids, such as in the biopharma industry, but have limited range of chemical compatibility in other industries. To help select tubing materials, many tubing suppliers provide chemical compatibility charts, but it is important for engineers to use a chart designed specifi-
Advantages and disadvantages of peristaltic pumps Advantages
Disadvantages
•
•
F lexible tubing tends to degrade with time and requires periodic replacement
•
T he flow is pulsed, particularly at low rotational speeds, so peristaltic pumps are less suitable where a smooth, consistent flow is required
•
N ot as effective for continuous process duties, as opposed to intermittent duties, because hose and coolant replacements are needed
•
L argest sizes are limited to 10–15 gal/min
•
•
• •
Dosing accuracy is high, and is not affected by line pressure and fluid viscosity Maintenance can be minimal due to the absence of valves, seals, pipework, strainers and so on Contamination is virtually eliminated because the only part of the pump in contact with the fluid being pumped is the interior of the tube Handling slurries, highly viscous, shear-sensitive and aggressive fluids is possible Pump design prevents backflow and syphoning without valves
cally for pump tubing rather than for general use. Tubing that gets an acceptable rating for general contact with a given chemical might not withstand exposure to the same chemical when subjected to the physical stresses of peristaltic pumping. When using compatibility charts, endusers should check the compatibility of each component of the solution, rather than just the main ingredient. Even trace levels of some acids or solvents can be enough to destroy pumps with exposure over longer periods of time. Chemical resistance decreases as temperature increases. Chemicals that have no effect on the tubing material at room temperature could attack the tubing at elevated temperatures.
Immersion test
If information on chemical compatibility cannot be found, or if a plant’s operating conditions are significantly different from those used to determine the chemical-resistance ratings, an immersion test can be performed. In an immersion test, a small length of tubing is weighed accurately, and its diameter and length measured. The tubing is then immersed in a closed vessel containing the chemical in question for 48 h. The test piece is then rinsed, dried, weighed and measured again, and changes are recorded. The tubing should also be examined for signs of softening or embrittlement, which would indicate chemical attack on the tubing.
Tube squeezing
The amount of squeeze applied to the tubing affects pumping performance and the tube life — more squeezing decreases the tubing life dramatically, while less squeezing decreases the pumping efficiency, especially in high-pressure pumping.
Thicker-walled tubes generate greater suction when they return to their original shape after being squeezed, so they are generally better for pumping more viscous fluids. For longer tube life, larger-bore tubes at lower pumping speeds should be used.
Pressure capabilities
Peristaltic pump applications are typically limited by the pressure capabilities of the tubing. Typical pump tubing materials have working pressure ratings from 10 to 40 psi, with softer materials such as silicone at the low end and firmer materials at the higher end. Recent material advances are expanding the pressure ranges for peristaltic pump applications. Pressure sources in a fluid-handling system can vary. Backpressure can be generated by the fluid passing through a filter or by the fluid pushing through the flowmeters or the valves. Backpressure can also come from the fluid pumping into a pressurized reaction vessel. Peristaltic pumps deliver fixed amounts of fluid with each pass of a roller over the tube, so the size of the tube has a direct effect on the amount of fluid delivered. Variations in tubing dimensions can mean compromised consistency and repeatability, so a tighter tubing-dimension tolerance is better.
References 1. Hall, J. Process Pump Control. Chem. Eng., November 2010, p. 30–33. 2. Ebelhack, A. Peristaltic Pumps: Matching the Tubing to the Fluid. Cole-Parmer Technical Resource Library, article 576. September 2009. Accessed from www.coleparmer.com, March 2012. 3. Cole-Parmer Metering Pump Selection Guide. Cole-Parmer Technical Resource Library, article 681. April 2008. Accessed from www.coleparmer.com, March 2012.
Feature Report
Piping for Process Plants Part 1: The Basics Pipe, fittings and related equipment are fundamental to the operation of chemical process plants. The series of articles beginning with this one spells out the details W. M. Huitt W. M. Huitt Co.
T
his is the first in a series of articles that will cover a wide range of piping topics. The topics will cross process-industry lines, pertaining to, for example, the chemical, petroleum-refining, pulp-and-paper and pharmaceutical and other industries.The main intent of these articles to address questions and misunderstandings as they relate to use of piping on a general basis. Typical of the topics that will be covered in this series are the following: • With respect to ASME flange ratings — Is the correct terminology 150- and 300-pound flange, or is it Class 150 and Class 300 flange? And do the 150 and 300 actually mean anything, or are they simply identifiers? Similarly, with respect to forged fittings, is the terminology 2,000-pound and 3,000-pound, or is it Class 2000 and Class 3000? • How do you determine which Class of forged fitting to select for your specification? 42
• How do you determine and then assign corrosion allowance for piping? • How do you select the proper bolts and gaskets for a service? • How is pipe wall thickness established? • What is MAWP? • What is operating and design pressure, and how do they differ? Similarly, what are operating and design temperature? How do design pressure and temperature relate to a PSV set point and leak testing? • For a given process application, under what Code should the design be carried out? • What kind of problems might be expected with sanitary clamp fittings, and how can they be avoided or alleviated? • What is ASME-BPE? And how do ASME B31.3 and ASME-BPE work in concert with one another? What is ASME BPE doing to bring accreditation to the pharmaceutical industry? The catch-all terminology for pipe and tubing is “tubular products.” This term
includes pipe, tube and their respective fittings. The term, “piping,” itself refers to a system of pipe, fittings, flanges, valves, bolts, gaskets and other inline components that make up an entire system used to convey a fluid. As for the simple distinction between pipe and tubing, it is that tubing is thin-walled pipe with a diameter different from that of nominally comparable pipe.
Piping and tubing
Piping and tubing can basically be grouped into three broad classifications: pipe, pressure tube and mechanical tube. Based on user requirements, these classifications come in various types, such as standard pipe, pressure pipe, line pipe, water well pipe, oilcountry tubular goods, conduit, piles, nipple pipe and sprinkler pipe. The two types of main relevance to the chemical process industries are standard and pressure pipe. Distinguishable only from the standpoint of use, standard pipe is intended for lowpressure, non-volatile use, whereas pressure pipe is intended for use in higher-integrity services, namely, services in which the pipe is required to convey high-pressure, volatile or nonvolatile liquids and gases, particularly at sub-zero or elevated temperatures. Pipe (standard or pressure) is manufactured to a nominal pipe size (NPS) in which the outside diameter (OD) of
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“P
Industries and Standards
ipe is pipe”. This is a euphemism quite often used among piping designers and engineers. Taken at face value, this is a true statement — pipe is certainly pipe. However, taken in context, the statement means that no matter which procerss industry you work in when designing piping systems, the issues are all the same. And in that context, it could not be further from the truth. Consider in particular the pharmaceutical industry. Although not new per se, it is a relative newcomer to the idea of dedicated design, engineering and construction principles, when compared to other process industries, such as petroleum refining, bulk chemicals, and pulp and paper industries; indeed, even in comparison with nuclear power, and with semiconductor manufacture. Here is a frame of reference, in terms of relevant standard-setting organizations: the American Society of Mechanical Engineers (ASME) was established in 1880; the American Petroleum Institute (API) was established in 1919; 3-A Standards (for the food and dairy industry) were first developed in the 1920’s; the ASME committee for BPVC (Boiler Pressure Vessel Code) Section III for nuclear power was proposed in 1963; the Semiconductor Equipment and Materials Institute (SEMI) was established in 1973; the International Society of Pharmaceutical Engineers (ISPE) was established in 1980; and ASME Biopharmaceutical Equipment (BPE) issued its first standard in 1997. Prior to ASME-BPE, the aforementioned 3A piping standards were the common recourse for facilitating the design of pharmaceutical facilities. While some of the above standards organizations, and their resulting codes and standards, are specific to a particular industry, others are more generalized in their use and are utilized across the various industries. For example, the design and construction of a large pharmaceutical facility depends upon not only pharmaceutical-based standards, codes, guidelines and industry practices such as those generated by ISPE and ASME-BPE; it also avails itself of standards created for other industries. In other words, when designing and constructing a bulk pharmaceutical finishing facility, or a bulk Active Pharmaceutical Ingredient (API) facility, the engineers and constructors will be working under some of the same standards and guidelines as they would when designing and building in other industries such as a petroleum refinery or bulk chemical facility.
a given nominal size remains constant while any change in wall thickness is reflected in the inside diameter (ID). Pipe wall thicknesses are specified by Schedule (Sch.) Numbers 5, 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. Add the suffix ‘s’ when specifying stainless steel or other alloys. Wall thickness is also specified by the symbols Std. (Standard), XS (Extra Strong) and XX (Double Extra Strong). Pipe of NPS 12 in. and smaller has an OD that is nominally larger than that specified, whereas pipe with a NPS 14 in. and larger has an OD equal to the size specified. Steel and alloy tubing is manufactured to an OD equal to that specified; this means, for example, that ¼-in. tubing will in fact have a ¼-in. OD, and that 2-in. tubing will have a 2-in. OD. This practice also pertains to copper tubing for air conditioning and refrigeration. Copper tubing for other purposes has an OD that is always 1/8
The point is not that the pharmaceutical industry itself is young; as already stated, it is not. The point is that the standards and accepted practices appropriate for state-of-the-art design, engineering and manufacture are. As recently as the past fifteen or so years, industry practice, including dimensional standards for high purity fittings, were left to the resources of the pharmaceutical company owner or their engineering firm (engineer of record). The same point applied to construction methods and procedures, including materials of construction. These requirements were basically established for each project and were very dependent upon what the owner’s personnel and the engineering firm brought to the table. Industry standards did not exist. With regard to materials of construction, the ongoing evolution of technology (science and engineering alike) has raised expectations throughout industry. For instance, out of the research and development that went into the Hubble Space Telescope came new methodology and technology to better measure and define the limits of surface roughness required in material used in hygienic-fluid-service contact piping. This capability is of particular interest to the pharmaceutical and biopharmaceutical industries (as well as the semiconductor industry), where cross-contamination at the molecular level cannot be tolerated in many cases. This requires surfaces to be very cleanable. Surface roughness used to be expressed as polish numbers (i.e., #4 or #7) then grit numbers such as 150, 180 or 240). The problem with either of these two methods lay in their subjectivity and their generality. These indicators were not specific enough and the accept/reject result relied too much on a subjective visual verification. There will be more on surface finish requirements in a subsequent installment. With acute awareness of the ongoing problems currently faced in the pharmaceutical industry and, for altogether different reasons, the semiconductor industry, various standards organizations have taken steps to alleviate the consistent problems that have plagued the industry in the past with, for instasnce, high purity welding issues, standardization of fittings, and guidelines for industry practice. This series of articles will discuss some of the finer points of these issues, and, in some cases, what the standards organizations, are doing to promote and consolidate some of the better thinking in this industry and in this field. ❏
in. larger than the diameter specified. As an example, ½-in. copper tubing will have a 5/8-in. OD, and 1-in. tubing will have a 1 1/8-in. OD.Wall thickness for tubing is specified in the actual decimal equivalent of its thickness.
Manufacturing methods
Pipe is manufactured in three basic forms: cast, welded and seamless. Tubing is manufactured in two basic forms: welded and seamless. Cast Pipe: Cast pipe is available in four basic types: white iron, malleable iron, gray iron and ductile iron. White iron has a high content of carbon in the carbide form. Carbides give it a high compressive strength and a hardness that provides added resistance to wear, but leaves it very brittle. The absence of graphite bestows a light colored appearance. Malleable iron is white cast iron that has been heat treated for added ductility. If white cast iron is reheated
in the presence of oxygen-containing materials such as an iron oxide, and allowed it to cool very slowly, the free carbon forms small graphite particles. This gives malleable iron excellent machinability and ductility properties, along with good shock resistant properties. Gray iron is the oldest form of cast iron pipe and is synonymous with the name, “cast iron.” It contains carbon in the form of flake graphite, which gives it its characteristic gray color. Gray cast iron has virtually no elastic or plastic properties, but has excellent machining and self-lubricating properties due to the graphite content Ductile iron is arguably the most versatile of the cast irons. It has excellent ductile and machinable properties while also having high strength characteristics. Welded Steel Pipe (and Tubing): Statements made about pipe in the this section also pertain to tubing.
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Feature Report $Y
Welded steel pipe is manufactured by furnace welding or by fusion welding. Furnace welding is achieved by heating strip steel, also referred as skelp, to welding temperature then forming it into pipe. The continuous weld, or buttweld, is forged at the time the strip is formed into pipe. This is a process generally used to manufacture low-cost pipe 3 ½ in. OD and below. Fusion Welded pipe is formed from skelp that is cold rolled into pipe and the edges welded together by resistance welding, induction welding or arc welding. Electric resistance welding (ERW) can be accomplished by flash welding, high-frequency or low-frequency resistance welding. A scarfing tool is used to remove upset material along the seam of flash-welded pipe. Flash welding produces a highstrength steel pipe in NPS 4 in. through 36 in. Low-frequency resistance welding can be used to manufacture pipe through NPS 22 in. Highfrequency resistance welding can be used to manufacture pipe through NPS 42 in. High-frequency induction welding can be used for high-rate production of small-NPS (6 in. and less) pipe. This is a cleaner form of welding in which scarfing, or the cleaning of upset material along the seam, is normally not required. Arc welding the longitudinal seam of production pipe is accomplished with submerged arc welding (SAW), inert gas tungsten arc welding (GTAW) also called tungsten inert gas welding (TIG), or gas shielded consumable metal arc welding (MIG). As will be discussed later in this series, the type of weld seam used in the manufacture of pipe is a factor when calculating the Pressure Design Thickness (t) of the pipe wall. Some types of longitudinal pipe seam welding are not as strong as others, reducing the overall integrity of the pipe wall by a percentage factyor given in ASME B31.3 based on the type of longitudinal seam weld. Seamless Steel Pipe and Tubing: Statements in the following also pertain to tubing. Seamless steel pipe, made using various extrusion and mandrel mill methods, is manufactured by first cre44
U/PNJOBMQJQF XBMMUIJDLOFTT $Y .JO UCVU OPUMFTT UIBOJO NN
ating a tube hollow from a steel billet, which is a "QQSPYJO solid steel round. The billet NN
4FF is heated to its hot metal CFGPSFXFMEJOH .JOJNVN forming temperature, then GMBU$Y pierced by a rotary piercer or by a press piercer to create the tube hollow, which will have a larger diam- Figure 1. Socketweld fittings are available in a wide range of sizes eter and thicker wall than its final pipe form. The tube hollow is cannot, for instance, automatically then hot-worked by the mandrel mill transfer the pressure/temperature limprocess, the Mannesmann plug-mill its of a flange joint in ASME B16.5 to process, or the Ugine Sejournet extru- that of a fitting in B16.3. sion process. Cast Steel: Cast steel, stainless steel Upon completion of these processes, and alloy steel flanged fittings, under the pipe is referred to as hot-finished. ASME B16.5, are available in Class If further work is required to achieve 150, 300, 400, 600, 900, 1500 & 2500 more accuracy in the diameter or in sizes ½ in. though 24 in. wall thickness or improve its finish, Cast Brass: Cast brass, as well as the pipe can be cold-finished, or cold- bronze, threaded fittings, under ASME worked. If the pipe is cold-finished, it B16.15, are available in Class 125 and will then require heat treating to re- 250, in sizes NPS 1/8 in. through 4 in. move pipe-wall stress created during for Class 125, and 1/4” through 4 in. the working in its cold state. for Class 250. There are also two forging processes Cast Copper: Cast copper solder used in the manufacture of large di- joints, under ASME B16.18, are availameter (10 to 30 inch) pipe with heavy able in sizes ¼ in. through 6 in. wall thickness (1.5 to 4 inch). The two forging methods are called forged and Forged fittings bored, and hollow forged. Before discussion of forged fittings, it is illuminating to consider the difPipe Fittings ference between forged and wrought Pipe fittings are manufactured by the fittings. The term, forging, actually following processes: cast, forged and dates from the times when metal was wrought. worked by hand. A bar of steel would be placed into a forge and heated until Cast fittings it reached its plastic state, at which Cast fittings are available in cast iron, time the metal would be pulled out malleable iron, ordinary steel, stain- of the forge and hammered into some less steel, brass, bronze, and other desired shape. Today, forging metal alloy material as follows: basically means working the metal Cast Iron: Cast iron threaded fittings, by means of hydraulic hammers to covered by ASME B16.4, are available achieve the desired shape. in Class 125 and Class 250 for sizes Wrought iron is corrosion resistant, NPS ¼ in. through 12 in. Cast iron has excellent tensile strength and flanged fittings, under ASME B16.1, welds easily, and in its plastic range are available in Class 25, 125 and 250 is said to be like working taffy candy. in sizes NPS 1 in. through 48 in. What gives wrought iron these attriMalleable Iron: Malleable iron fit- butes is the iron silicate fibers, or slag tings, under ASME B16.3, are avail- added to the molten iron with a small able in Class 150 and Class 300 in sizes percentage of carbon, whereas cast NPS 1/8 in. though 6 in. for Class 150, iron, having a high carbon content, is and ¼ in. through 3 in. for Class 300. more brittle and not as easily worked. Be aware that Classifications such as The smelters, where the iron ore 150 and 300 are not universal through- was melted to produce wrought iron, out the ASME Standards. They are were called bloomeries. In a bloomery, instead specific to the Standard with the process did not completely melt which they are associated. One thus the iron ore; rather the semi-finished
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Plastic-lined pipe
I
n the main body of this article, we have touched on just some of the key points related to metal pipe and fittings, while not considering plastic lined pipe systems and nonmetallic piping. Nonmetallic piping merits a discussion on its own, and should not be relegated to a paragraph or two here. On the other hand, since plastic lined pipe is steel pipe with a liner, and is so widely used in the process industries, it is worthwhile to present the relevant basics here. When first introduced, plastic lined pipe filled a large fluid-handling gap in industry, but brought with it some technical issues. In particular, when various manufacturers began producing lined pipe and fittings, industry standards for them did not exist. Consequently, there were no standard fitting dimensions, and the availability of size and type of fittings would vary from one company to another (as they still do, to a much lesser degree). Due to the autonomous nature of lined pipe manufacturing during its initial stages, the piping designer for a process plant would have to know early in the design process which manufacturer he or she were going to use. Particularly in fitting-makeup situations, in which a 90-deg elbow might be bolted to a tee, which in turn might br bolted to another 90-deg elbow it was important to know in advance what those makeup dimensions were going to be, and thus the identity of the fitting manufacturer. While the lack of industry standard dimensions was a design problem, other operational type problems existed as well. Some of the fluid services for which these lined pipe systems were specified for (and still are) would normally be expected to operate under a positive pressure, but at times would phase into a negative pressure. The liners in the early systems were not necessarily vacuumrated, and consequently would collapse at times under the negative internal pressure, plugging the pipeline. There was an added problem when gaskets were thrown into the mix. Gaskets were not normally required unless frequent dismantling was planned; even so, many firms, both engineering and manufacturers, felt more secure in specifying gaskets at every joint. When required, the gasket of choice, in many cases, was an envelope type gasket made of PTFE (polytetrafluoroethylene) with an inner core of various filler material, such as EPDM. These gaskets had a tendency to creep under required bolt-torque pressure at ambient conditions. From the time at which a system was installed to the time it was ready to hydrotest, the gaskets would, on many occasion, creep, or relax to the point of reducing the compressive bolt load of the joint enough to where it would not stand up to the hydrotest pressure. Quite often, leaks would become apparent during the fill cycle prior to testing.
product was a spongy molten mass called a bloom, a term derived from the red glow of the molten metal, which is likewise how the process gets its name. The slag and impurities were then mechanically removed from the molten mass by twisting and hammering, which is where the term wrought originates. Today forged and wrought are almost synonymous. ASTM A234, “Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service” states in Para 4.1 and in Para 5.1 that wrought fittings made under A234 are actually manufactured or fabricated from material pre-formed by one of the methods listed previously, which includes forging. In ASTM A961, “Standard Specification for Common Requirements for
Other problems that still exist are those of permeation with regard to PTFE liner material, as well as that of internal and external triboelectric charge generation and accumulation (static electricity). But, due to the diligent efforts of the lined pipe and gasket industries, these types of problems have either been largely eliminated or controlled. Even so, the designer employing lined pipe should keep the potential for static-electricity problems in mind. If electrical charge generation is allowed to continually dissipate to ground, then there is no charge buildup and no problem. That is what occurs with steel pipe in contact with a flowing fluid: charge generation has a path to ground, and does not have an opportunity to build up. With regard to thermoplastic lined pipe, there are two issues to be considered: external charge accumulation and internal charge accumulation. Experience and expertise are needed in order to analyze a particular situation. A subsequent installment of this series will provide basic information that will at least allow you to be familiar with the subject, and help you to understand the issues. Fitting dimensions for lined pipe have been standardized through ASTM F1545 in referencing ASME B16.1 (cast iron fittings), B16.5 (steel fittings) and B16.42 (ductile iron fittings). Note 3 under SubPara. 4.2.4 of ASTM F1545 states, “Center-to-face dimensions include the plastic lining,” which means that the dimensions given in the referenced ASME standards are to the bare metal face of the fittings. However, when lined fittings are manufactured, the metal casting is modified to accommodate the liner thickness being included in that same specified center-to-face dimension. With regard to vacuum rating, liner specifications have been greatly improved, but it is prudent to check the vacuum ratings of available pipe and fittings with each manufacturer under consideration. This rating is likely to vary from manufacturer to manufacturer depending on diameter, fitting, liner type, pressure and temperature. Gasket materials such as PTFE/Silicate composite or 100% expanded PTFE, have been developed to reduce the gasket creep rate in a gasket material. Permeation issues with PTFE liners (these issues also arise, to a lesser extent, with other liner material) have been accommodated more than resolved with the use of vents in the steel pipe casing, the application of vent components at the flange joint, and increased liner thickness. Standard sizes of plastic lined pipe and fittings range from NPS 1 in. through 12 in. And at least one lined-pipe manufacturer, also manufactures larger-diameter pipe and fittings: from NPS 14 in. through 24 in., and when requested can manufacture spools to 144 in. diameter. ❏
Steel Flanges, Forged Fittings, Valves and Parts for Piping Applications,” the definition for the term Forged is, “the product of a substantially compressive hot or cold plastic working operation that consolidates the material and produces the required shape. The plastic working must be performed by a forging machine, such as a hammer, press, or ring rolling machine, and must deform the material to produce a wrought structure throughout the material cross section.” The difference, therefore, between forged and wrought fittings is that forged fittings, simply put, are manufactured from bar, which while in its plastic state is formed into a fitting with the use of a hammer, press or rolling machine. Wrought fittings, on the other hand, are manufactured from killed steel, forgings, bars, plates
and seamless or fusion welded tubular products that are shaped by hammering, pressing, piercing, extruding, upsetting, rolling, bending, fusion welding, machining, or by a combination of two or more of these operations. In simpler terms wrought signifies “worked”. There are exceptions in the manufacture of both, but that is the general difference.* *A point concerning the ASTM specifications is worth noting. In referring to ASTM A961 above, I am quoting from what ASTM refers to as a General Requirement Specification. Such a specification is one that covers requirements typical for multiple individual Product Specifications. In this case, the individual Product Specifications covered by A961 are A105, A181, A182, A360, A694, A707, A727 and A836. The reason I point this out is that many designers and engineers are not aware that when reviewing an A105 or any of the other ASTM individual Product Specifications you may need to include the associated General Requirement Specification in that review. Reference to a General Requirement Specification can be found in the respective Product Specification.
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Feature Report
Hygienic Piping
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ajor characteristics of piping for the pharmaceutical and semiconductor industries are the requirements for highpurity, or hygienic, fluid services. These requirements, as dictated by current Good Manufacturing Practices (cGMP) and defined and quantified by the International Soc. of Pharmaceutical Engineers (ISPE) and by ASME Bio Processing Equipment (ASMEBPE), are stringent with regard to the manufacture, documentation, fabrication, installation, qualification, validation and quality control of hygienic piping systems and components. The hours that the engineer or designer requires in generating, maintaining and controlling the added documentation required for hygienic fabrication and installation addds up to 30% to 40% of the overall cost of fabrication and installation. A subsequent installment in this series will cover in more detail the specific requirements of hygienic fabrication, and, accordingly, where that added cost comes from. Hygienic is a term defined in ASME-BPE as: “of or pertaining to equipment and piping systems that by design, materials of construction, and operation provide for the maintenance of cleanliness so that products produced by these systems will not adversely affect animal or human health.” While system components such as tube, fittings, valves, as well as the hygienic aspects of the design itself, can apply to the semiconductor industry, the term “hygienic” itself does not; it instead pertains strictly to the health aspects of a clean and cleanable system for pharmaceuticals manufacture. The semiconductor industry requires a high, or in some cases higher, degree of cleanliness and cleanability than do the hygienic systems in the pharmaceutical industry, for altogether different reasons. A term that can more ap-
Forged steel and alloy steel socketweld (Figure 1) and threaded fittings, under ASME B16.11, are available in sizes NPS 1/8 in. through 4 in. Forged socketweld fittings are available in pressure rating Classes 3000, 6000 and 9000. Forged threaded fittings are available in pressure rating Classes 2000, 3000 and 6000. Misapplication of the pressure rating in these forged socketweld and threaded fittings is not infrequent; the person specifying components on many cases does not fully understand the relationship between the pressure Class of these fittings and the pipe they are to be used with. In ASME B16.11 is a table that associates, as a recommendation, fitting pressure Class with pipe wall thickness, as follows: Table 1. Correlation of
Pipe Wall Thickness & Pressure Rating
Pipe wall thickness. 80 or XS 160 XXS 46
Threaded 2000 3000 6000
Socketweld 3000 6000 9000
propriately be interchanged between these two industries is “highpurity;” this implies a high degree of cleanliness and cleanability without being implicitly connected with one industry or the other. For what is referred to as product contact material, the absence of surface roughness, minimal dead-legs and an easily cleanable system are all imperative. Therefore, the pharmaceutical industry had to make a departure from the 3-A standards (created for the food and dairy industries) of which it had availed itself early on, in order to develop a set of guidelines and standards that better suit its industry. Enter ASME-BPE, which has taken on the task of providing a forum for engineers, pharmaceutical manufacturers, component and equipment manufacturers, and inspectors in an effort to develop consensus standards for the industry where none existed before. Hygienic piping was, up until just recently, referred to as sanitary piping. Because this term has been so closely associated with the plumbing industry and with sanitary drain piping, it is felt by the pharmaceutical industry that the change in terminology to hygienic is more appropriate. In both the pharmaceutical and semiconductor industries, the need for crevicefree, drainable systems is a necessity. This translates into weld joint quality, mechanical joint design requirements, interior pipe surface roughness limits, system drainability and dead-leg limitations. There are two basic types of fitting joints in hygienic piping: welded and clamp. The welded fittings, unlike standard buttweld pipe fittings, have an added tangent length to accommodate the orbital welding machine. The orbital welding machine allows the welding operator to make consistent high-quality autogenous welds (welds made without filler metal). Fusion is made between
The ASME recommendation is based on matching the I.D. of the barrel of the fitting with the I.D. of the pipe. The shoulder of the fitting (the area of the fitting against which the end of the pipe butts), whether socketweld, as shown in Fig. 1, or threaded, is approximately the same width as the specified mating pipe wall thickness, with allowance for fabrication tolerances. As an example, referring to Table 1, if you had a specified pipe wall thickness of Sch. 160 the matching threaded forged fitting would be a Class 3000, for socketweld it would be a Class 6000. The fitting pressure class is selected based on the pipe wall thickness. Referring to Fig. 1, one can readily see that by not matching the fitting class to the pipe wall thickness it will create either a recessed area or a protruding area the length of the barrel of the fitting, depending on which side you error on. For forged reinforced branch fittings refer to MSS Standard SP-97 – “Integrally Reinforced Forged Branch Outlet Fittings - Socket Welding, Threaded and Buttwelding Ends.”
Wrought fittings
Wrought steel butt-weld fittings under ASME B16.9 (standard-radius 1.5D elbows and other fittings) are available in sizes ½ in. through 48 in. Wrought steel butt-weld fittings under B16.28 (short-radius 1D elbows), are available in sizes ½ in. through 24 in. There is no pressure/temperature rating classification for these fittings. In lieu of fitting pressure classifications, both B16.9 and B16.28 require that the fitting material be the same as or comparable to the pipe material specification and wall thickness. Under ASME B16.9, given the same material composition, the fittings will have the same allowable pressure/temperature as the pipe. ASME requires that the fittings under B16.28, short radius elbows, be strength-rated at 80% of the value calculated for straight seamless pipe of the same material and wall thickness. These fittings can be manufactured from seamless or welded pipe or tubing, plate or forgings. Laterals, because of the elongated opening cut from the run pipe section are strength-rated at 40% of the strength calculated for
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the parent metals of the two components being welded by means of tungsten inert gas welding. Pipe welding will be covered in more detail in an upcoming installment. The photograph shows an example of an orbital, or automatic, welding machine mounted on its workpiece. In this example, the piece happens to be a 90-deg elbow being welded to a cross. One can see in this example why the additional straight tangent section of automatic weld fittings is needed — that extra length provides a mounting surface for attaching the automatic welding machine. As for the clamp connection, it is a mechanical connection whose design originated in the food and dairy industry, but whose standardization has been under development by ASMEBPE. Due to a lack of definitive standardization, most companies that use this type connection require in their specifications that both the ferrule (the component upon which the clamp fits) and the clamp itself come from the same manufacturer. This precaution is to ensure a competent fit. There are no specific dimensions and tolerances for the clamp assembly, except for those being developed by ASME-BPE. Currently, it is possible to take a set of ferrules from one manufacturer, mate them together with a gasket, attach a clamp from a different manufacturer and tighten up on the clamp nut. In some cases, one can literally rotate the clamp by hand about the ferrules, with no significant force being applied on the joint seal. The clamp joint is the clamp that applies the force that holds the ferrules together. The fact that this can occur begs the need for
straight seamless pipe of the same material and wall thickness. If a full strength lateral is required, either the wall thickness of the lateral itself can be increased or a reinforcement pad can be added at the branch to compensate for the loss of material at the branch opening. Wrought copper solder joint fittings, under ASTM B88 and ASME B16.22, are available in sizes ¼ in. through 6 in. These fittings can be brazed as well as soldered. The pressure/temperature rating for copper fittings are based on the type of solder or brazing material and the tubing size. The rating will vary too, depending on whether the fitting is a standard fitting or a DWV (Drain, Waste, Vent) fitting, which has a reduced pressure rating. As an example, using alloy Sn50, 50-50 Tin-Lead Solder, at 100ºF, fittings ½ in. through 1 in. have a pressure rating of 200 psig, and fittings 1½ in. through 2 in. have a pressure rating of 175 psig. DWV fittings 1½ in. through 2 in. have a pressure rating of 95 psig. Using alloy HB, which is a Tin-Anti-
standardization to a greater degree than what currently exists. Another issue that currently exists with the clamp joint is gasket intrusion into the pipe inside wall, due to inadequate compression control of the gasket. Gasket intrusion is a problem in pharmaceutical service for two reasons: • Depending on the hygienic fluid service and the gasket material, the gasket protruding into the fluid stream can break down and slough off into the fluid flow, contaminating the hygienic fluid • The intrusion of the gasket into pipe on a horizon tal line can also cause fluid holdup. This can result in the loss of residual product, cause potential cross-contamina tion of product, and promote microbial growth. Some manufacturers are attempting to overcome these issues by improving on the concept of the clamp joint. One company has developed ferrules whose design provides compression control of the gasket while also controlling the creep tendency that is inherent in, arguably, the most prevalent gasket material used in high purity piping, namely,Teflon. Another firm manufactures a clamp joint (also provided as a bolted connection) that does not require a gasket.This type of joint is currently in use in Europe. While this connection alleviates the issues that are present with a gasketed joint, added care would need to be applied in its handling. Any scratch or ding to the faced part of the sealing surface could compromise its sealing integrity. Nevertheless this is a connection design worth consideration. ❏
mony-Silver-Copper-Nickel (Sn-Sb-AgCu-Ni) solder, having 0.10% maximum lead (Pb) content, at 100ºF, fittings ½ in. through 1 in. have a pressure rating of 1,035 psig and fittings 1½ in. through 2 in. have a pressure rating of 805 psig. DWV fittings 1½ in. through 2 in. would have a pressure rating of 370 psig. It can be seen that, within a given type of fitting, there is a significant difference in the pressure ratings of soldered joints, depending on the type of filler metal composition. Much of the difference is in the temperature at which the solder or brazing filler metal fully melts. This is referred to as its liquidus state. The temperature at which the filler starts to melt is referred to as its solidus temperature. The higher the liquidus temperature, the higher the pressure rating of the joint.
Acknowledgement
I wish to thank Earl Lamson, senior Project Manager with Eli Lilly and Co., for taking time out of a busy schedule to read through the draft of this article. He obliged me by reviewing this article with the same skill, in-
telligence and insight he brings to everything he does. His comments kept me concise and on target. ■ Edited by Nicholas P. Chopey
Recommended Reading 1. Cox, John, Avoid Leakage in Pipe Systems, Chem. Eng., January 2006, pp. 40–43. 2. Sahoo, Trinath, Gaskets: The Weakest Link, Chem. Eng., June 2005, pp. 38–40.
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp & paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, Owner expectations and good design practices. Bill is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sets on two corporate specification review boards. He can be reached at: W. M. Huitt Co., P O Box 31154, St. Louis, MO 63131-0154, (314)966-8919
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Engineering Practice
Piping Design, Part 2 — Flanges W. M. Huitt W. M. Huitt Co.
P
ipe flanges are used to mechanically connect pipe sections to other pipe sections, inline components, and equipment. Flanges also allow pipe to be assembled and disassembled without cutting or welding, which eliminates the need for those two operations when dismantling is required. In providing a breakable joint, however, flanges unfortunately provide a potential leak path for the process fluid contained in the pipe. Because of this, the usage of flanges needs to be minimized where possible, as with all other joints. The most prevalent flange standards to be used in the process industries are based on those of the American Soc. of Mechanical Engineers (ASME). These include: B16.1 – Cast Iron Pipe Flanges and Flanged Fittings B16.5 - Pipe Flanges and Flanged Fit tings (NPS 1/2 through NPS 24, where NPS is nominal pipe size; see Part 1 of this series, CE, February, pp. 42–47) B16.24 – Cast Copper Alloy Pipe Flanges and Flanged Fittings B16.36 – Orifice Flanges B16.42 – Ductile Iron Pipe Flanges and Flanged Fittings Large Diameter Steel Flanges (NPS* 26 through NPS 60) B16.47 – Large Diameter steel flanges (NPS 26 through NPS 60) *NPS, indicated above, is an acronym for Nominal Pipe Size.
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The engineer or designer must choose among several flange options. Additional decisions involve facing and surface finishes, and the appropriate gaskets, bolts and nuts Flanges are available with various contact facings (the flange-to-flange contact surface) and methods of connecting to the pipe itself. The flanges under B16.5, a standard widely relevant to the process industries, are available in a variety of styles and pressure classifications. The different styles, or types, are denoted by the way each connects to the pipe itself and/or by the type of face. The types of pipe-to-flange connections include the following: • Threaded • Socket welding (or socket weld) • Slip-on welding (or slip on) • Lapped (or lap joint) • Welding neck (or weld neck) • Blind
Flange types
Threaded: The threaded flange (Figure 1), through Class 400, is connected to threaded pipe in which the pipe thread conforms to ASME B1.20.1. For threaded flanges in Class 600 and higher, the length through the hub of the flange exceeds the limitations of ASME B1.20.1. ASME B16.5 requires that when using threaded flanges in Class 600 or higher, Schedule 80 or heavier pipe wall thickness be used,
and that the end of the pipe be reasonably close to the mating surface of the flange. Note that the term “reasonably close” is taken, in context, from Annex A of ASME B16.5; it is not quantified. In order to achieve this “reasonably close” requirement, the flange thread has to be longer and the diameters of the smaller threads must be smaller than that indicated in ASME B1.20.1. When installing threaded flanges Class 600 and higher, ASME B16.5 recommends using power equipment to obtain the proper engagement. Simply using arm strength with a hand wrench is not recommended. The primary benefit of threaded flanges is in eliminating the need for welding. In this regard, these flanges are sometimes used in high-pressure service in which the operating temperature is ambient. They are not suitable where high temperatures, cyclic conditions or bending stresses can be potential problems. Socketweld: The socketweld flange is made so that the pipe is inserted into the socket of the flange until it hits the shoulder of the socket. The pipe is then backed away from the shoulder approximately 1/16 in. before being welded to the flange hub.
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Figure 2. Socketweld flanges have been commonly used for small pipe sizes
If the pipe were resting against the shoulder (this is the flat shelf area depicted in Figure 2 as the difference between diameters B and B2) of the socket joint during welding, heat from the weld would expand the pipe longitudinally into the shoulder of the socket, forcing the pipe-to-flange weld area to move. This could cause the weld to crack. The socketweld flange was initially developed for use on small size, highpressure piping in which both a backside hub weld and an internal shoulder weld was made. This provided a static strength equal to the slip-on flange (discussed below), with a fatigue strength 1.5 times that of the slip-on flange. Because having two welds was labor intensive, it became the practice to weld only at the hub of the flange. This practice relegated the socketweld flange to be more frequently used for small pipe sizes (NPS 2 in. and below) in non-highpressure, utility type service piping. The socketweld flange is not approved above Class 1500. Slip on: Unlike the socketweld flange, the slip-on flange (Figure 3) allows the pipe to be inserted completely through its hub opening. Two welds are made to secure the flange to the pipe. One fillet weld is made at the hub of the flange, and the second weld is made at the inside diameter of the flange near the flange face. The end of the pipe is offset from the face of the flange by a distance equal to the lesser of the pipe wall thickness or ¼ in. plus approximately 1/16 in. This is to allow for enough
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Figure 4. A lap-joint flange can yield savings in material costs
room to make the internal -FOHUI -FOHUI -FOHUI fillet weld without damaging the flange face. UO UO UO The slip-on flange is a preferred flange for many appli0% 0% 0% cations because of its initial lower cost, the reduced need for cut length accuracy and S S S U U U the reduction in end prep time. However, the final in5ZQF" 5ZQF# 5ZQF$ stalled cost is probably not much less than that of a weld-neck flange. "/4* "/4* The strength of a slip4MJQPO MBQKPJOU on flange under internal GMBOHF GMBOHF pressure is about 40% less than that of a weld-neck flange, and the fatigue rate 5ZQFT#BOE$ 5ZQFT" is about 66% less. The slip UZQF$TIPXO
on flange is not approved above Class 1500. Figure 5. Stub-ends serve to complete lap joints Lap joint: The lap-joint flange (Figure 4) requires a compan- cases, the designer can elect to use a ion lap joint, or Type A stub end (stub stub end specified with the same maends are described below) to complete terial as the pipe, but use a less costly, the joint. The installer is then able to perhaps carbon-steel, lap-joint flange. rotate the flange. This capability al- This strategy prevents the need of lows for quick bolthole alignment of having to weld a more costly compatthe mating flange during installation ible alloy flange to the end of the pipe. without taking the extra precautions Stub ends are prefabricated or cast required during prefabrication of a pipe flares that are welded directly to welded flange. the pipe. They are available in three Their pressure holding ability is different types (Figure 5): Type A, about the same as that of a slip-on (which is the lap-joint stub end), Type flange. The fatigue life of a lap-joint/ B and Type C. stub-end combination is about 10% Type A is forged or cast with an that of a weld-neck flange, with an outside radius where the flare beinitial cost that is a little higher than gins. This radius conforms to the that of a weld-neck flange. radius on the inside of the lap-joint The real cost benefit in using a lap- flange. The mating side of the flare joint flange assembly is realized when has a serrated surface. installing a stainless-steel or other Type B is forged or cast without costly alloy piping system. In many the radius where the flare begins. It Chemical Engineering www.che.com March 2007
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Engineering Practice 9 " #
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is used to accommodate the slip-on flange or plate flange as a back-up flange. Type C is fabricated from pipe using five suggested methods indicated in ASME B31.3. The most prevalent of these is the machine flare. This method consists of placing a section of pipe into a flaring machine, flaring the end of the pipe and then cutting it to length. As you can see in the assembly detail of Figure 5, stub-end Types B & C have no radius at the flare, while Type A does. This allows Type A to conform to the lap-joint flange. Due to the radius of the Type A stub end, a slip-on flange would have a poor fit, creating non-uniform loading of the flare face as well as an undesirable point load at the radius of the flare. Weld neck: The reinforcement area of the weld-neck flange (Figure 6) distinguishes it from other flanges. This reinforcement area is formed by the added metal thickness, which tapers from the hub of the flange to the weld end. The bore of the flange needs to be specified in order to obtain the same wall thickness at the weld end as the pipe it will be welded to. This will give it the same ID bore as the pipe. The weld-neck flange is the most versatile flange in the ASME stable of flanges. Much of its use is for fitting-to-fitting fabrication, in which the flange can be welded directly to a fitting, such as an elbow, without the need for a short piece of pipe, as would be required with a slip-on flange. It can be used in low-pressure, non-hazardous fluid services as well as highpressure, high-cyclic and hazardous fluid services. While the initial cost of the weldneck flange may be higher than that of a slip-on flange, the installed cost reduces that differential. And for conditions of possible high thermal 58
0
#MJOE Figure 7. Blind flanges are commonly used to cap off pipeline ends
loading, either cryogenic or elevated temperatures, the weld-neck flange is essential. Blind: While the blind flange (Figure 7) is used to cap off the end of a pipeline or a future branch connection, it is also used for other purposes. It can be drilled and tapped for a threaded reducing flange or machined out for a slip-on reducing flange. The reduced opening can be either on-center or eccentric.
Flange pressure ratings
ASME B16.5 flange pressure ratings have been categorized into material groupings. These groupings are formulated based on both the material composition and the process by which the flange is manufactured. The available pressure Classifications under ASME B16.5 are: 150, 300, 400, 600, 900, 1500 and 2500. The correct terminology for this designation is Class 150, Class 300, and so on. The term 150 pound, 300 pound and so on is a carryover from the old ASA (American Standards Association) Classification. ASA is the precursor to the American National Standards Institute (ANSI).* Development of ASME B16.5 began in 1920. In 1927 the American Tentative Standard B16e was approved. This eventually became what we know today as ASME B16.5. Until the 1960s, the pressure classifications, as addressed earlier, were referred to as 150 pound, 300 pound, etc. It was at this point the pressure clas-
sification was changed to the class designation. These designations have no direct correlation with pounds of pressure. Rather, they are a factor in the pressure rating calculation found in B16.5. In a subsequent part of this series, we will discuss how these designations are factored into the design of the flange. Flanges, whether manufactured to ASME, API (American Petroleum Institute), MSS (Manufacturer’s Standardization Soc.), AWWA (American Water Works Assn.) or any other standard, are grouped into pressure ratings. In ASME, these pressure ratings are a sub-group of the various material groups designated in B16.5. Tables 1 and 2 in this article break out information from the Table 2 series in ASME B16.5. The Table 2 series is a series of tables that list the working pressures of flanges based on material groupings, temperature and classification. There are 34 such tables, segregated into three material categories: carbon and low alloy steels, austenitic stainless steels, and nickel alloys. These are further segregated into more defined material sub-groups. Tables 1 and 2 of this article show Table 2-1.1 from B16.5, which indicates, in reverse sequence, Subcategory 1 of Material group 1 (carbon and low alloy steels). If you had an ASME B16.5, Class 150, ASTM A105 flange, this is the table you would use to determine the working pressure limit of the flange. To find the working pressure of the
*ANSI was founded as a committee whose responsibility was to coordinate the development of standards and to act as a standards traffic cop for the various organizations that develop standards. Its basic function is not to develop standards, but rather to provide accreditation of those standards Originating as the American Engineering Standards Committee (AESC) in 1918, ANSI had, over its first ten years, outgrown its Committee status and in 1928 was reorganized and renamed as the American Standards Association (ASA). In 1966 the ASA was reorganized again under the name of the United States of America Standards Institute (USASI). In 1969 ANSI adopted its present name. While the B16 and B31 Standards have previously carried the ASA and ANSI prefix with its various standards titles, ASME has always been the administrative sponsor in the development of those standards. In the 1970s the prefix designation changed to ANSI/ASME and finally to ASME. Referring to ANSI B16. or ANSI B31. is no longer correct. Instead, it is correct to refer to a standard as ANSI/ASME B16. in that it indicates an ANSI-accredited ASME standard. Or one can simply refer to the standard as ASME B16. or ASME B31.
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Figure 8. Flange facings are available in several varieties
abovementioned flange, enter the column of this table designated as 150 then move down the column to the operating temperature. For intermediate temperatures, linear interpolation is permitted. The previous paragraph refers to “operating temperature” when one is looking to determine the working pressure of a flange. “Operating” and “working” are synonymous. The indication of a working pressure and temperature of a fluid service is the same as indicating the operating pressure and temperature. There exists some confusion in this area. That confusion becomes apparent when the engineer is determining design pressure and temperature and applying them to the flange rating. On the surface, there appears to be a conflict in rating a flange for design conditions when Table 2 only indicates working pressures. Operating and design pressures and temperatures will be explained in more detail in a subsequent article in this series. For now, be aware that every service should have an operating pressure/temperature as well as a design pressure/temperature. A design condition is the maximum coincidental pressure and temperature con-
dition that the system is expected or allowed to see. This then becomes the condition to which you should design for, and to which the leak test is based on, not the operating condition. Table 2, as it indicates, represents the working or operating pressures of the flange at an indicated temperature for a specific class. The maximum hydrostatic leak-test pressure for a Class 150 flange in Table 2-1.1 is 1.5 times the rated working pressure at 100°F, or 285 x 1.5 = 427.5 rounded off to the next higher 25 psi, or 450 psig. We can extrapolate that piece of information to say that since hydrostatic leak-test pressure is based on 1.5 times design pressure, the working pressure limit given in the Table 2 matrix ostensibly becomes the design pressure limit. When one is working with ASME B31.3 Category D fluid services, and initial service leak testing is performed, the working pressure limit then remains the working pressure limit because testing is performed at operating or working pressures. However, there are caveats that address the fact that not all Category D fluid services (see next paragraph) should waive the hydrostatic leak test for an initial service leak test.
These conditions, such as steam service, will also be discussed in a subsequent article. Category D fluid services are those fluid services that are nonflammable, nontoxic and not damaging to human tissue. Additionally, Category D fluids do not exceed 150 psig and 366º F. In initial service leak testing, the test fluid is the service fluid. Leak testing occurs during or prior to initial operation of the system. As the service fluid is introduced to the piping system and brought to operating pressure, in pressure increments, all joints are observed for possible leaks. If no leaks are detected, the pipeline simply remains in service. Other ASME B31.3 fluid services may be expected to operate at one set of conditions, but are designed for another set. For those systems, which might include periodic steamout (cleaning, sterilization, sanitization) or passivation, you therefore want to base your flange-rating selection on those more-extreme, periodic design conditions. To clarify “periodic” in this context, the sanitization process may be done as frequently as once per week and last for up to one-and-ahalf shifts in duration.
Facings and surface finishes
Standard flange-facing designations (Figure 8) are as follows: flat face, raised face, ring joint, tongue and groove, large and small male and female, small male and female on end of pipe, and large and small tongue and groove. The height of the raised face for Class 150 and 300 flanges is 0.06 in. The height of the raised face for Class 400 and above is 0.25 in. Industry wide, not discounting the lap-joint flange and stub-end combination, the two most widely used flange facings are the flat face and the raised face. The surface finish of standard raised-face and flat-face flanges has a serrated concentric or serrated spiral
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Table 1. Pressure Temperature ratings for Groups 1.1 through 3.16 Materials
Engineering Practice surface finish with an average roughness of 125 × 10–6 in. to 250 × 10–6 in. The cutting tool used for the serrations will have a 0.06 in. or larger radius, and there should be from 45 to 55 grooves per inch.
Bolts, nuts and gaskets
Sealing of the flange joint and the hygienic-clamp joint (as discussed last month in Part 1) is paramount in providing integrity to the overall piping system. This is achieved with the use of bolts, nuts and gaskets. Making the right selection for the application can mean the difference between a joint with integrity and one without. ASME B16.5 provides a list of appropriate bolting material for ASME flanges. The bolting material is grouped into three strength categories — high, intermediate and low — that are based on the minimum yield strength of the specified bolt material. The high-strength category includes bolt material with a minimum yield strength of not less than 105 kilopounds per square inch (ksi). The intermediate-strength category includes bolt material with a minimum yield strength of between 30 ksi and 105 ksi. The low-strength category includes bolt material with a minimum yield strength no greater than 30 ksi. As defined in ASME B16.5, the high-strength bolting materials “. . . . may be used with all listed materials and all gaskets.” The intermediatestrength bolting materials “. . . . may be used with all listed materials and all gaskets, provided it has been verified that a sealed joint can be maintained under rated working pressure and temperature”. The low-strength bolting materials “. . . . may be used with all listed materials but are limited to Class 150 and Class 300 joints,” and can only be used with selected gaskets as defined in ASME B16.5. ASME B31.3 further clarifies in Paragraph 309.2.1, “Bolting having not more than 30 ksi specified minimum yield strength shall not be used for flanged joints rated ASME B16.5 Class 400 and higher, nor for flanged joints using metallic gaskets, unless calculations have been made showing 60
Ratings for group 1.1 materials Nominal designation
Forgings
Castings
Plates
C-Si
A 105 (1)
A 216 Gr. WCB (1)
A 515 Gr. 70 (1)
C-Mn-Si
A 350 Gr. LF2 (1)
A 516 Gr. 70 (1)(2) A 537 Cl. 1 (3)
Notes: (1) Upon prolonged exposure to temperature above 800°F, the carbide phase of steel may be converted to graphite. Permissible, but not recommended for prolonged use above 800°F. (2) Not to be used over 850°F (3) Not to be used over 700°F
Table 2. Working Pressures by classes, psig Temp., °F -20 to 100 200 300 400 500 600 650 700 750 800 850 900 950 1,000
150 285 260 230 200 170 140 125 110 95 80 65 50 35 20
300 740 675 655 635 600 550 535 535 505 410 270 170 105 50
400 990 900 875 845 800 730 715 710 670 550 355 230 140 70
adequate strength to maintain joint tightness.” B31.3 additionally states in Paragraph 309.2.3, “…If either flange is to the ASME B16.1 (cast iron), ASME B16.24 (cast copper alloy), MSS SP42 (valves with flanged and buttweld ends), or MSS SP-51 (cast flanges and fittings) specifications, the bolting material shall be no stronger than low yield strength bolting unless: (a) both flanges have flat faces and a full face gasket is used: or, (b) sequence and torque limits for bolt-up are specified, with consideration of sustained loads, displacement strains, and occasional loads (see Paragraphs. 302.3.5 and 302.3.6), and strength of the flanges.” In specifying flange bolts, as well as the gasket, it is necessary to consider not only design pressure and temperature but also fluid service compatibility, the critical nature (if any) of the fluid service, and environmental conditions, all in conjunction with one another. To aid in understanding the relationships among these criteria, some clarification follows: • The design pressure and temperature jointly determine the pressure class of a flange set. That in turn, along with flange size, will determine the number and size of the flange bolts. The flange class will also determine the compressibility range of the gasket material
Class 600 1,480 1,350 1,315 1,270 1,200 1,095 1,075 1,065 1,010 825 535 345 205 105
900 2,220 2,025 1,970 1,900 1,795 1,640 1,610 1,600 1,510 1,235 805 515 310 155
1,500 3,705 3,375 3,280 3,170 2,995 2,735 2,685 2,665 2,520 2,060 1,340 860 515 260
2,500 6,170 5,625 5,470 5,280 4,990 4,560 4,475 4,440 4,200 3,430 2,230 1,430 860 430
• Fluid service compatibility will help determine the most suitable gasket material. The critical nature of the fluid will determine the degree of integrity required in the joint. This requirement will help determine bolt strength and material as well as gasket type • Environmental conditions (corrosive atmosphere, wash-down chemicals, other) will also help determine the best bolt material In short, all of the variables that come together in making up a flange-joint specification have to do so in a complementary fashion. Simply selecting a gasket based on material selection and not taking into account the pressure rating requirement could provide a gasket that would get crushed under necessary torque requirements rather than withstand the bolt load and create a seal. Selecting a low-strength bolt to be used with a Class 600 flange joint with proper gasketing will require the bolts to be torqued beyond their yield point, or, at the very least, beyond their elastic range. To explain this briefly, bolts act as springs when they are installed and loaded properly. In order for the flange joint to maintain a gasket seal, it requires dynamic loading. Dynamic loading of flange bolts allows expansion and contraction movement in and
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around the joint while maintaining a seal. This is achieved by applying sufficient stress to the bolt to take it into the material’s elastic range. If the bolts are not stressed sufficiently into their elastic range, any relaxation in the gasket could reduce the sealing ability of the joint. To the other extreme, if the bolts were stressed beyond their elastic range and into the plastic range of their material of construction the same issue would apply; they would lose their dynamic load on the gasket. In that case, if they did not shear, they would take a set. Any relaxation in the gasket will then result in the reduction or elimination of the joints sealing ability. The nut should be selected to complement the bolt. The bolt material specification will steer you, either partially or completely, into the proper nut selection. ASTM A307, a material standard for bolts in the low-strength category,
states that the proper grade for bolts to be used for pipe flange applications is Grade B. The standard goes further to state that when used for pipe flanges, Grade B bolts require a Heavy Hex Grade A nut under ASTM A563. In writing a pipe specification that included the A307 bolt, you would not need to specify the nut, since it is already defined in A307. However, ASTM A193, alloy and stainless-steel bolts, goes only so far when it states that nuts shall conform to ASTM A194 — there are several grades of A194 nuts to select among. This is an example of where the matching nut is not always explicitly called out in the ASTM standard. Because the ASTM standards are inconsistent in that regard, the specification writer must make sure that the nut is covered in a specification. In summary, all four components — flanges, bolts, nuts and gaskets — have to be selected in conjunction
with one another in order for the joint assembly to perform in a way that it is expected to for a given application. ■ Edited by Nicholas P. Chopey
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp & paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, Owner expectations and good design practices. Bill is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sets on two corporate specification review boards. He can be reached at: W. M. Huitt Co., P O Box 31154, St. Louis, MO 63131-0154, (314)966-8919. His email address is
[email protected]
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Feature Report Engineeering Practice
Piping Design, Part 3 — Design Elements W. M. Huitt W. M. Huitt Co.
P
iping design is the job of configuring the physical aspects of pipe and components in an effort to conform with piping and instrumentation diagrams (P&IDs), fluid-service requirements, associated material specifications, equipment-data sheets, and current good manufacturing practices (GMP) while meeting owner expectations. All of this must be accomplished within a pre-determined, three-dimensional assigned space, while coordinating the activity with that of the architecture, structural steel, HVAC (heating, ventilation air conditioning), electrical, video, data-and-security conduit and trays, and operational requirements. Pulling together and coordinating these activities to achieve such a compilation of design requires a systematic methodology, planning, technical ability, interdisciplinary coordination, foresight, and above all, experience. This third part in a series on piping design* discusses a number of key elements, including how to prepare specifications and guidelines, and some insights on flanges, surface finish, design temperature and pressure, and charge accumulation. Although computer-aided design (CAD) has become an integral part of piping design, it will not be discussed in this article.
Specs and guidelines
One of the first activities the piping engineer will be involved with is development of piping specifications (specs) and guidelines on design and construc*Part 1: The Basics, CE February, pp. 42–47; Part 2: Flanges, CE March, pp. 56–61
50
Design requires a systematic methodology, planning, technical ability, interdisciplinary coordination, foresight and, above all, experience tion. Piping specifications, as an overview, should provide essential material detail for design, procurement and fabrication. Guidelines, both design and construction, should provide sufficient definition in a well organized manner to allow the designer and constructor the insight and direction they need in order to provide a facility that will meet the expectation of the owner with minimal in-process direction from the owner or construction manager.
Piping specifications
A piping specification is the document that will describe the physical characteristics and specific material attributes of pipe, fittings and manual valves necessary to the needs of both design and procurement personnel. These documents also become contractual to the project and the contractors that work under them. Designers will require a sufficient degree of information in a specification that will allow for determining the service limitations of the specification and what fluid services the specification’s material is compatible with. For example, a project may have, among other fluid services, sulfuric acid and chilled water. The economic and technical feasibility of the material selection for chilled water service would not be technically feasible for sulfuric acid. Inversely, the economic and technical material selection for sulfuric acid service would not be economically feasible for chilled water service.
Procurement personnel, too, will need detailed specifications to limit the assumptions they will have to make or the questions they will have to ask in preparing purchase orders. The piping specification should make clear exactly what the material of construction is for each component, and to what standard that component is manufactured. Also included in the component description should be pressure rating, end-connection type and surface finish where required. There are a few rather common mistakes that companies make in developing or maintaining specifications: 1. The specification itself is either not definitive enough or too definitive; 2. The specifications are not updated in a timely manner; and 3. The specifications are too broad in their content. Let’s consider each of these points in more detail. Point 1. When defining pipe and components in a specification, you should provide enough information to identify each component without “hamstringing” yourself or procurement personnel in the process. In other words, do not get so specific or proprietary with the specification that only one manufacturer is qualified to provide the component (unless that is the actual intent). With standard pipe and fittings, it’s difficult to provide too much information. However, with valves and other inline equipment, overspecification can happen quite easily. A common practice is to write a
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Figure 1. Shown here is a magnified image (2,000x) of a biofilm [1]
specification for a generic type valve, one that can be bid on by multiple potential suppliers, by using the description of one particular valve as a template. What happens is that proprietary manufacturer trade names, such as some of the trim materials, are carried over to the generic valve spec. When the procurement person for the mechanical contractor, or whoever is buying the valves for the project, gets ready to purchase this valve, the only manufacturer that can supply it with the specified proprietary trim is the one from which the spec was copied. You would think that doing this would eliminate multiple bids for the valve based on the unintentional proprietary requirements in the spec. Instead, it creates confusion and propagates questions. The valve bidders, other than the one the spec was based on, will bid the valve with an exception to the proprietary material, or they will contact the purchasing agent for clarification. Since the purchasing agent won’t have the answer, the question or clarification goes back to the engineer and/or the owner. The time necessary for responding to these types of issues is better spent on more pressing matters. When developing a spec, be specific, but try not to include proprietary data unless you intend to. For example, when specifying Viton you are specifying a generic DuPont product — generic in that there are several different types of Viton, such as Viton A, Viton B, Viton GF, Viton GFLT and so on. Each of these has a specific formulation, which gives it different fluidservice compatibility and pressure and temperature ranges. Viton is a type of fluorocarbon. Fluorocarbons are designated FKM under ASTM D-1418, so when specifying “Viton” you are identifying a specific
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Figure 2. The proper surface roughness can maximize the cleaning of biofilm from a pipe [1]
product from a specific manufacturer — almost. By almost, what is meant is that, if you write the spec as Viton you would most likely get the original formulation, which is Viton A. The fluid service may be more suited for an FKM with polytetrafluoroethylene in it (Viton GF) or an FKM suitable for colder temperatures may be a better choice (Viton GFLT). Be specific for those who have to use the specs for design and purchase of the material. If, in developing a specification, you wish to establish minimum requirements for a component or a material, it is certainly acceptable to identify a specific proprietary item as a benchmark. In doing this — and we’ll stay with the fluorocarbon gasket or seal example — you could identify Viton GF or equal, which would indicate that a comparable material from one of the other fluorocarbon manufacturers would be acceptable so long as the fluid service compatibility and pressure/temperature ranges were equal to or greater than the Viton GF material. Point 2. All too often after a specification is developed it will reside in the company’s database without being periodically reviewed and updated. However, industry standards change, part numbers change, manufacturers are bought and sold, manufacturers improve their products, and so on. All of these things constitute the need and necessity to review and revise specifications on a timely basis. A company that houses its own set of specifications should review them at least every two years. This timing works out for a couple reasons. Firstly, industry standards, on average, publish every two years, and secondly, capital projects, from design through close-out, will arguably have an average duration of two years. Lessonslearned from projects can then be
considered for adoption into company specs, prompting a new revision. Point 3. Specs that are too broad in their content refers to an attempt at making the specs all-inclusive. A piping specification should contain only those components and information that would typically be used from job to job. That would include the following (as an example): 1. Pressure and temperature limit of the specification 2. Limiting factor for pressure and temperature 3. Pipe material 4. Fitting type, rating and material 5. Flange type, rating and material 6. Gasket type, rating and material 7. Bolt and nut type and material 8. Manual valves, grouped by type 9. Notes 10. Branch chart matrix with corro sion allowance These ten line items provide the primary component information and notations required for a typical piping system. Some specifications are written to include components, such as steam traps, sight glasses, three- or four-way valves, strainers, and other miscellaneous items. These miscellaneous items are better referred to as specialty items (or some other similarly descriptive name) and are sized and specified for each particular application. This does not make them good candidates for inclusion into a basic pipe specification. To explain the above we can use, as an example, a carbon-steel piping system that is specified to be used in a 150-psig steam service. The pipe, flanges, fittings, bolts, gaskets and valves can all be used at any point in the system as specified. The specification for a steam trap, however, will vary depending on its intended application. And depending on its applica-
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Engineeering Practice tion, the load requirements for each trap may vary. For example, a steamtrap application at a drip leg will have a light steady load, whereas a steamtrap application at a shell-and-tube heat exchanger may have a heavier modulating load. And that doesn’t take into account the need for the different types of traps, including F&T (floatand-thermostatic), inverted bucket, and thermodynamic. You could, depending on the size of the project, have multiple variations of the four basic types of steam traps with anywhere from 30 to 300 or more traps in multiple sizes and various load requirements. I think you can see why this type of requirement needs to be its own specification and not a part of the piping specification. A piping specification should be concise, definitive and repeatable. Adding specialty type items to the specification makes it convoluted and difficult to control and interpret. Users of these specifications are designers, bidders, procurement personnel, fabricators, receipt verification clerks, validation and maintenance personnel. With this in mind, you can better understand, or at least value the fact, that these documents have to be interpreted and used by a wide range of personnel. These personnel are looking for particular information, written in a concise manner that will allow them to design and order or verify components within that specification. Inclusion of the specialty type items will, at the very least, complicate and exacerbate the process.
Design/construction guidelines
In conjunction with the piping specifications, the design and construction guidelines should convey to the designer and constructor point-by-point requirements as to how a facility is to be designed and constructed. The guidelines should not be a rhetorical essay, but instead should follow an industry standard format, preferably a CSI (Construction Specifications Institute) format. Look at it this way: the material specifications tell the designer and constructor what material to use; the guidelines should tell them how to assimilate and use the material 52
specifications in applying them to JOUZQJDBM good design practice. Without these (SPVOEJOHMVHMPDBUJPO guidelines as part of any bid package or request-for-proposal package, Figure 3. Incorporating a grounding the owner is essentially leaving it up lug into the pipe will ensure proper grounding, even if the pipe has been painted to the engineer and/or constructor to bring their own set of guidelines to the table. And this may or may not fications and guidelines. In the older, be a good thing. Leaving the full facil- more established petroleum-refining ity’s delivery to the engineer and con- and chemical companies you will see structor depends a great deal on the entire departments whose mission is qualifications of the engineer and the to create, maintain and refine all of constructor, and whether or not consis- the specifications and guidelines nectency from plant to plant and project essary to execute a project. When a to project is an issue. project is approved to go out for bid If the owner approaches a proj- to an engineer, the necessary specifiect with expectations as to how they cations and guidelines along with the would like their plant or facility de- requisite drawings are assembled, signed and built, then some prepara- packaged and provided to the engineer tion, on the owner’s part, is in order. as bid documents, and beyond that as Preparation should include, not only working documents in the design, enmaterial specifications as described gineering and construction efforts. earlier, but also the guidelines and In Scenario 2, the owner, or cusnarratives (yes, narratives) necessary tomer, has some specifications and to define the design and construction guidelines that have possibly not been requirements. updated for several years. These are I mention the use of narratives here provided to the engineer with the unbecause a narrative helps facilitate derstanding and stipulation that any the understanding and conveys the errors or omissions in the documents magnitude of the, in most cases, reams should be addressed and corrected of specifications and guidelines neces- by the engineer. These, too, would be sary to build an industrial facility of used in the bid process as well as on any appreciable size. the project itself. In general, a narrative should exIn Scenario 3, the owner, or cusplain in simple, straight-forward lan- tomer, brings no specifications or guage, for each discipline: the number- guidelines to the project table. Speciing scheme used for the specifications fication development becomes part of and guidelines; association between the overall project engineering effort. the material specifications and the Scenarios 1 and 3 are at opposite guidelines; an explanation as to why ends of the spectrum, but afford the the project is governed by a particular best situation for both the owner and code or codes; and a brief description engineer/constructor. By providing of expectation. the engineer and constructor, as in The narrative allows you to be more Scenario 1, with a full set of current explanatory and descriptive than a specifications and well articulated formal point-by-point specification. It guidelines, the assumption is made gives the bidder/engineer a “Readers that both the engineer and construcDigest” version of the stacks of speci- tor are qualified for the level of work fications and guidelines they are ex- required, and can very effectively expected to read through and assimilate ecute the design, engineering and conwithin a matter of a few weeks. struction for the project. How piping specifications are delivScenario 3 allows the engineer and ered to a project can have a significant constructor to bring their own gameimpact on the project itself. There are, plan to the project. This too is effective, generally speaking, three scenarios in due only to the fact that the learning which project specifications and guide- curve is minimal. Most engineering lines are delivered to a project. In Sce- firms will be prepared to execute a nario 1, the owner, or customer, has project with their own set of specifideveloped a complete arsenal of speci- cations and guidelines. This applies
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to qualified constructors as well. The down side of this is in the project-toproject inconsistency in specifications and methodology when using different engineers and constructors. Scenario 2 is a worse case situation. Ineffective and outdated owner specifications create confusion and inefficient iterations in both the bid process and the execution of a project. Scenario 2 additionally creates the greatest opportunity for conflicts between owner documents and the engineer’s documents. For project management, this translates into change orders at some point in a project. A guideline should explain to the engineering firm or constructor, in a concise, definitive manner, just what the owner expects in executing the design and construction of a facility. By actively and methodically developing a set of guidelines, an owner or customer does not have to rely on an outside resource, such as an engineering firm or constructor, to provide the facility required and hoped for. Developing guidelines to convey your company’s requirements and expectations can be accomplished using one or both of the following two basic methods: 1. A formal point-by-point format that covers all necessary criteria that you, as the owner, require on a proprietary basis, plus a listing and description of the necessary code and GMP requirements
2. A narrative for each discipline that allows the writer to expand and define, in a much more descriptive manner, the points that aren’t made clear enough, or readily apparent in the more formal format The guideline can be structured on one of the CSI formats. The format examples provided by CSI give a company sufficient flexibility in writing guidelines, or specifications for that matter, to allow the document to conform to its own particular brand of requirements and nuances. The format also lends a degree of intra-industry conformity to the guidelines and specifications, providing a degree of familiarity to the engineers and constructors who will have to adhere to them.
Design elements
In the first paragraph of this article, I described the act of designing piping systems for a facility as bringing a number of technical components together to make the pipe conform to a specific set of requirements, within a prescribed area. That’s pretty simplistic, and does not really convey the magnitude of the experience, technical background or the imagination required to execute such a task. Experience is the essential component here. And that is simply because, aside from whatever innate ability a good designer might possess, the required knowledge is not taught through formal education, but
is instead learned by experience. Ongoing learning can be in the form of organized classes, a mentor or any other means available to help learn and understand the physical requirements and restraints of various systems and industries. Since we do not have enough space here to cover all of the design elements, I will key in on a few topics for clarification. (And this doesn’t even scratch the surface.) We will discuss flanges, pipe internal-surface finish, weld seam factor, pipe wall thickness, MAWP and MADP, design pressure and temperature, and charge accumulation.
Flanges
In Parts 1 and 2 of this series of articles (see footnote on first page), we discussed ASME flanges and their classifications. Most designers are familiar with ASME flange classifications such as 150, 300, 400, and so on. And even though verbally stating 150 pound flange (the origin of this term is discussed in Part 2) rolls off the tongue much easier and is still an industry accepted term, Class 150 is the proper terminology and designation. What may be less familiar is that the class designation is a factor in the calculation for determining the rated working pressure of a flange. That calculation is: PT Pr S1 / 8, 750 b Pc
(1) where Pc = Ceiling pressure, psig, as specified in ASME B16.5, paragraph D3, at temperature T PT = Rated working pressure, psig, for the specified material at temperature T Pr = Pressure rating class index, psi (for instance, Pr = 300 psi for Class 300). Note: This definition of Pr does not apply to Class 150. See ASME B16.5, paragraphs D2.2, D2.3 and D2.4 S1 = Selected stress, psi, for the specified material at temperature T. See ASME B16.5, paragraphs D2.2, D2.3 and D2.4
Pipe internal-surface finish
Internal surface roughness is a topic that is specific to the pharmaceutical, bio-pharmaceutical and semiconduc-
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Engineeering Practice tor sectors, but can also be an issue throughout the CPI. Quantifying and specifying a maximum surface roughness for internal pipe wall for use in what is referred to as direct impact fluid services, is a necessity in the above-mentioned sectors. Direct impact piping systems are those systems that carry product or carry a fluid service that ultimately comes in contact with product. The need for a relatively smooth internal pipe wall is predicated on three primary issues: 1. Cleanability and drainability; 2. The ability to hinder the growth of biofilm and to enhance the ability to remove it once it does appear; and 3. To reduce, to a microscopic level, crevices in which microscopic particles can reside and at some point dislodge and get carried along in the fluid stream to damage product. Regarding the first point, cleanability and drainability are associative; in order for a system to be fully cleanable it has to be designed and laid out in a manner that will eliminate any pockets and provide enough slope to eliminate any residual liquid (drainable). Not only is this residual liquid (or holdup) a contaminant — from both a bacterial standpoint and as a cross batch contaminant — but it can also be expensive due to the high cost of some drug products. Along those lines, the ASME-BPE Standard provides criteria for minimum slope, maximum deadleg, gasket intrusion, gasket concavity, and many other criteria for design of cleanable and drainable hygienic piping systems. Regarding the second point, biofilm is defined as a bacterial population composed of cells that are firmly attached as microcolonies to a solid surface (see Figure 1). At a recent ASME-BPE symposium [1], Frank Riedewald, a senior process engineer with Lockwood-Greene IDC Ltd., explained the results of testing that was performed to determine the relationship between the formation of biofilm, pipe wall-surface finish and pipe wall-surface cleanability. One of the many interesting factors that came from these studies is the fact that the internal surface of the pipe wall can actually be too smooth. Referring to the graph in Figure 2, re54
sults indicate that the surface finish range best suited to reduce biofilm adherence to the internal pipe wall surface is from 0.4Ra µm to 1.0Ra µm (15.7Ra µin. to 58.8Ra µin.). What this implies is that, while we currently do not have the means to prevent the onset of biofilm on the internal walls of hygienic or semiconductor piping systems, we can facilitate its removal in the cleaning process by specifying the proper surface finish of the internal pipe walls. The accepted maximum surface finish in the pharmaceutical and biopharmaceutical industries is 25Ra µin. (0.6 µm). In the semiconductor industry you might typically see surface finishes in the range of 7Ra µin. to 15Ra µin., particularly in gas delivery systems. While the pharmaceutical industry is concerned with bacterial growth and cross contamination, the semiconductor industry is concerned more with particulate damage to product on the microscopic level. This pertains to point three above.
Pipe weld seam factor
Part 2 of this series of articles mentioned the fact that the weld seam in longitudinally welded pipe is a factor in the pipe-wall-pressure-design thickness calculation. In ASME B31.3, there are two pipewall thicknesses for calculations. One is pressure design thickness (t) and the other is minimum required thickness (tm). There are two equations for finding pressure-design thickness for straight pipe under internal pressure. Equation 2 is where t < D/6, where D is the actual pipe outer diameter (OD); this calculation is based on internal pressure, the actual (not nominal) OD of the pipe, stress value of the material at design temperature, joint efficiency factor, and the coefficient Y [a factor used to adjust internal pressure (P) for a nominal material at temperature]. Equation 3 is used when t ≥ D/6; this calculation is based on the abovelisted criteria except that ID is used instead of OD, and the sum of all mechanical allowances is included. t=
PD 2( SE + PY )
(2)
for when t < D/6 t=
P ( d + 2c) 2[ SE − P (1 − Y )]
for when t ≥ D/6
(3)
tm = t + c
(4) where t = Pressure design thickness tm = Minimum required thickness, including mechanical, corrosion and erosion allowances c = Sum of the mechanical allowances (thread or groove depth) plus corrosion and erosion allowances. For threaded components, the nominal thread depth (dimension h of ASME B1.20.1, or equivalent) shall apply. For machined surfaces or grooves where the tolerance is not specified, the tolerance shall be assumed to be 0.02 in. (0.5 mm) in addition to the specified depth of the cut D = Actual pipe OD d = Pipe ID P = Internal design gage pressure S = Stress value for material from ASME B31.3 Table A-1, at design temperature E = Quality factor, or joint efficiency factor Y = Coefficient from ASME B31.3 Table 304.1.1 To determine wall thickness for pipe under external pressure conditions, refer to the Boiler and Pressure Vessel Code (BPVC) Section VIII, Division 1, UG-28 through UG-30 and ASME B31.3, paragraph 304.1.3. Keep in mind that for seamless pipe, E will be removed from Equations 2 and 3.
Determining MAWP
Taking a page from the BPVC, we will go through a few brief steps to determine maximum-allowable working pressure (MAWP) for straight pipe. But let me begin by saying that MAWP is not a B31.3 expression, it comes from the BPVC. We will instead transpose this term to MADP (maximum-allowable design pressure), which is also not a B31.3 term, but more closely relates to piping. When a vessel goes into design it is assigned a coincidental design pressure and temperature. These are the
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(minimum or maximum) expected during service. B31.3 goes on to state: 'MBOHF The most severe condition 5IJTQPJOU BMXBZTBU is that which results in QJQFJOWFSU the greatest required component thickness and the highest component rating. How do you determine these values and where Figure 5. Internal linings of nonconducting plastic in pipe can lead to undesirable and dangerous charge accumulation. This can be prevented by introducing a conductivity orifice-plate do you apply them? We’ll assembly, such as the one shown here (left, crosssectional view; right, side view) cover the where first. The discussion on determin'JHVSF iEJBTIPVMECFJOEJB maximum conditions the vessel is ex- The Barlow formula: ing pipe wall thickness was based on SFNPWF 1MBUFBOEKVTULFFQJOXJEF pected DIBOHFiUPJOFWFSZXIFSF -HUPMBSHF IEUPIFBE to experience while in service, design conditions, in which P is the 2 × TF × ST 3FNPWF4FDUJPO""BUCPUUPN and what the engineers will design internal design gage pressure and S PBA = D 'JHVSF it’s the vessel to handle. The material, (5) is the stress value at the design temDIBOHFiUPJO thickness, welds, nozzles, flanges, and The Boardman formula: perature. Design conditions are also UIFDBMMPVUTBUUIFGBSSJHIUBSFNJTTJOHBSSPXIFBET so on are all designed predicated on used to determine component ratings 2 × TF × ST PBO = this predetermined design criteria. and as a basis for determining leak D − (0.8 × T ) Throughout design, the vessel’s in(6) test pressure. tended maximum pressure is referred The Lamè formula: There is no published standard, or 2 2 to as its design pressure. All calculagenuine industry consensus, on how S × (D − d ) PL = T 2 tions are based on specified material to determine design conditions. It ba( D + d2 ) and component tolerances along with (7) sically comes down to an owner’s or fabrication specifics, meaning types where engineer’s experience. What I will proand sizes of welds, reinforcement and PBA = Burst pressure, psig (Barlow) vide here is a resultant philosophy deso on. Not until after the vessel is fab- PBO = Burst pressure, psig (Board- veloped from many sources along with ricated can the engineer know what man) my own experiences. the actual material thickness is, the PL = Burst pressure, psig (Lamè) To understand what constitutes detype and size of each weld, thickness D = Actual pipe OD, in. sign conditions, we first need to define of each nozzle neck, and so on. Only d = Pipe ID, in. them. The following are some accepted when all of the factual data of con- TF = Wall thickness (minus factory terms and their definitions: struction is accumulated and entered tolerance), in. System operating pressure: The into vessel engineering programs can ST = Minimum tensile strength, psi, pressure at which a fluid service is exthe MAWP be determined. This value, from B31.3 Table A-1 pected to normally operate. once determined, then replaces the Sf = Safety factor, a factor of 3 or 4 System design pressure: Unless exdesign pressure, and is calculated is applied to burst pressure to tenuating process conditions dictate based on the installed configuration of determine MADP otherwise, the design pressure is the the vessel (that is, mounted vertically Using any of the three results from pressure at the most severe coincior horizontally; mounted on legs; or any one of the above equations we dent of internal or external pressure mounted on lugs). can then determine MADP (M) as fol- and temperature (minimum or maxiThe difference between the design lows: mum) expected during service, plus pressure and the MAWP is that the the greater of 30 psi or 10%. Pi (8) System operating temperature: The engineer will design to the design M = S f pressure, but the final MAWP is the temperature at which a fluid service is limiting pressure of the vessel. The where the subscript i is BA, BO, or L, expected to normally operate. MAWP may exceed the design pres- depending on which formula is used. System design temperature: Unless sure, but it can never be less than the extenuating process conditions dictate design pressure. Design pressure & temperature otherwise, the design temperature, for In applying this to piping we will The ASME B31.3 definition for design operating temperatures between 32°F first calculate the burst pressure pressure and design temperature is and 750°F, this value shall be equal to of the pipe and then determine the stated as two separate definitions. I the maximum anticipated operating MAWP, or, as was mentioned earlier, will integrate them into one by stat- temperature, plus 25°F rounded off to a term more closely related to piping, ing: The design pressure and tempera- the next higher 5°. the MADP. ture of each component in a piping Applying a sort of philosophy creThere are three equations generally system shall be not less than the most ated by the above definitions is used in calculating burst pressure for severe condition of coincident internal somewhat straightforward for utility pipe. They are: or external pressure and temperature services, such as steam, water, and $POEVDUJWF PSJGJDF QMBUF
0GGDFOUFS PSJGJDF
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Engineeering Practice non-reactive chemicals. However, that part of the above definitions for design conditions that provide the caveat, “…extenuating process conditions…” implies a slightly different set of rules for process systems. Extenuating process conditions can mean increased pressure and temperature, beyond that defined above, due to chemical reaction, loss of temperature control in heat transfer, and so on.
Charge buildup in lined pipe
Internal and external charge accumulation, known as static electricity, or more technically known as triboelectric charge accumulation, is the result of charge that is unable to dissipate. If a charge generated in a flowing fluid is allowed to dissipate to ground, as it does in grounded metallic pipe, then there is no problem. However, if a charge cannot dissipate and is allowed to accumulate, as it may in nonconductive pipe liners, it now becomes a problem by potentially becoming strong enough to create an electrostatic discharge (ESD). With regard to thermoplastic lined pipe there are two forms of this to be considered: external charge accumulation (ECA) and internal charge accumulation (ICA). ECA. This is a concern with lined pipe due to the possibility of not achieving spool-to-spool continuity during installation due, in large part, to improved paint primer on flanges. When pipe spools (lined or unlined) are joined by flanges using non-metallic gaskets, the only thing that completes the spool-to-spool continuity is the bolting. The improved paint primer on lined pipe flanges makes this more difficult to achieve because normal bolt tightening doesn’t guarantee metalto-metal contact between the nut and the flange. Pipe generally does not come with a prime coat of paint; however, lined pipe does. Since flange bolts are used to complete continuity from spool to spool, the installer has to make certain, when installing lined pipe, that the bolts, at least one of the bolts, has penetrated the primer and made contact with bare metal. This was achieved in the past by using star washers on at least one flange bolt while assuming pos56
sible bare metal contact with the other bolts, allowing the washers, as they were tightened, to scrape away the prime coat so that contact was made with the bare metal of the flange. With improved prime coat material this is no longer a guarantee. If continuity from spool to spool is not achieved, any charge generation resulting from an internal or an external source cannot readily dissipate to ground. The voltage in triboelectric charge generation will build until it is strong enough to jump to the closest grounded object creating an undesired spark of electricity (ESD). ICA. With regard to pipe, ICA is unique to thermoplastic lined pipe and solid thermoplastic pipe. Without being impregnated with a conductive material, thermoplastics are not good conductors of electricity. PTFE (polytetrafluoroethylene), as an example, has a high (>1016 Ohms/unit area), resistivity factor. This is a relatively high resistance to conductivity, which means that any charge created inside the pipe cannot readily be conducted away to ground by way of the PTFE liner. Instead, the charge will be allowed to build until it exceeds its total dielectric strength and burns a pinhole in the liner to the internal metal wall of the casement pipe. It isn’t charge generation itself that is the problem, it’s the charge accumulation. When the rate of charge generation is greater than the rate of charge relaxation (the ability of material to conduct away the generated charge), charge accumulation occurs. The dielectric strength of PTFE is 450 to 500 volts/mil. This indicates that for every 0.001 in. of PTFE liner 450 V of triboelectric charge will be required to penetrate the liner. For a 2-in. pipeline with a 0.130-in. thick liner, this translates into 58,500 V of triboelectric charge to burn through the liner thickness. When the liner is penetrated by an accumulated charge, two additional problems are created: 1. Corrosive fluid (a major use of lined pipe) is now in contact with and corroding the metal pipe wall and at some point, depending on rate of corrosion, will fail locally and cause fluid to leak to
TABLE 1.
RECOMMENDED VELOCITIES Liquid BS 5958 recomconductivity mended flow velocity >1,000 pS/m No restriction 50 – 1,000 pS/m
Less than 7 m/s
Less than 50 pS/m
Less than 1 m/s
Note: pS/m (picosiemens/meter)
the environment, and 2. The initial charge that burned through the liner is now charging the outer metal pipe. If continuity has not been achieved for the outer pipe, a spark of triboelectric charge is, at some point, going to jump to ground and cause a spark.
Corrective action
ECG. The simplest method to ensure continuity is to sand away any primer on the back side of each flange to ensure good metal-to-metal contact between nut and flange. Aside from that or the use of a conductive prime paint, the current ready-made solution to the external continuity problem is the addition of stud bolts located in close proximity to flanges on both pipe spools and fittings (see Figure 3). These studs can be applied at the factory or in the field. At each flange joint a grounding strap (jumper) is then affixed to a stud on one spool with a nut, extended over the flange joint and attached to a stud on the connecting spool completing continuity throughout the chain of connecting spools and fittings. Another method of creating continuity at flange joints, while being less obtrusive and more integral, is described as follows. Referring to Figure 4, flanges would be purchased pre-drilled and tapped in the center of the outer edge of the flange between the backside of the flange and the face side of the flange. The drilled and tapped hole in each flange will need to be centered between bolt holes so that they line up after the flange bolts are installed. The tapped hole is 1/4-in. dia. x 1/2-in. deep. After a flange set is installed and fully bolted, the continuity plate (Figure 4) can be installed using two 1/4-in. x 1/2-in. long hex-head screws and two lock washers. The Continuity Plate has two 0.312-in. slotted
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boltholes allowing for misalignment and movement. The entire continuity plate assembly is relatively simple to install, unobtrusive and establishes integral contact with the pipeline. ICG. One of the first options in preventing internal charge accumulation is by minimizing charge generation. This can be done by adjusting the flow velocity relative to the liquid’s conductivity. To minimize design impact, cost and even schedule impact on a project, ICG needs to be evaluated early in the project due to the possibility of a change in line size. To retard charge generation by reducing flow velocities, British Standard (BS) suggests the values presented Table 1 (per BS 5958). If velocity reduction is not an option, or if further safeguards against charge accumulation are warranted, then a mechanical solution to provide a path to ground for ICG might be nrcessary. One method for conducting charge accumulation from the interior of the pipe to ground is indicated in Figure 5. What is shown is an orifice plate made of conductive (static dissipative) material that is compatible with the fluid service. The orifice itself is off center to the OD of the plate and the pipeline itself. With the shallow portion of the ID at the invert of the pipe, the orifice allows the piping to drain in horizontal runs. The tab portion of the plate extends beyond the flange OD. On the tab is a bolthole for attaching the modified continuity flange plate. The plate is designed to come in contact with the interior surface of the liner wall as well as protrude into the flowing fluid to provide a conduit for internally generated charge. Continuity is achieved by attaching the plate to the flange OD that is in contact with the piping, which is, in turn, grounded through equipment.
Recommendations
It is difficult to pre-determine what fluid services and systems will be candidates for charge accumulation prevention and electrostatic discharge protection. The simplest and most conservative answer is to as-
sume that all fluid services in lined pipe systems are susceptible. In saying that, we then have to declare that a company’s pipe specifications need to reflect a global resolution that will affect all installations. With regard to ECA, the recommendation for future installations with the least impact would be to specify pipe with no prime coat or at least no primer on the flanges, or a prime coat using a conductive paint. The unprimed pipe would be primed prior to installation with care given to primer touchup on flanges after installation. This would better ensure spool-tospool external continuity. For existing installations, either the studs or the continuity plate installation would work. It can also be suggested that the continuity plates can be tacked on to one flange rather than drilling and tapping both flanges. For dissipating ICG, the orifice plate, as shown in Figure 5, is the only recommendation. ■ Edited by Gerald Ondrey
References 1. Riedewald, Frank, “Microbial Biofilms — Are they a problem in the pharmaceutical industry?”ASME-BPE Symposium, Cork, Ireland, June 2004.
Acknowledgement I wish to thank Earl Lamson, senior project manager at Eli Lilly and Co., for taking time out of a busy schedule to read this article with the same skill, intelligence and insight he brings to everything he does. His comments kept me concise and on track.
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. (P.O. Box 31154, St. Louis, MO 63131-0154. Phone: 314-966-8919; Email:
[email protected]) a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp & paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, owner expectations and good design practices. Bill is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sits on two corporate specification review boards.
Bioengineering Inversina – the gentle way of mixing. The Inversina mixes solids or liquids thoroughly and efficiently. The process is clean, because mixing takes place in closed containers that can be quickly interchanged. The Inversina mixes a diverse range of components rapidly and in an extremely gentle way. Segregation does not occur, even after extended mixing times, by virtue of the eversion phenomenon ( Paul Schatz principle ) . Applications for the Inversina: analytical labs, metal finishing shops, powder metallurgy and nuclear industry, manufacture of batteries, cement, ceramics, cosmetics, dental products, diamond tools, dyes and pigments, electrical and electronic devices, explosives and pyrotechnics, foods, homeopathic products, household products, medicines and pharmaceuticals, plastics, printing inks and many other products. The Bioengineering Inversina is available with capacities of 2, 20, 50, 100 and 300 L .
Bioengineering, Inc. Waltham, MA 02451, USA Bioengineering AG 8636 Wald, Switzerland
[email protected] www.bioengineering.ch Circle 44 on p. 82 or go to adlinks.che.com/6897-44
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Feature Report Engineering Practice
Piping for Process Plants, Part 4:
Codes and Fabrication William M. Huitt W.M. Huitt Co.
T
his fourth in a series of articles* on piping for process plants examines two topics that may, at first, seem to fall outside the scope of chemical engineering — piping codes and the pipe fabrication. Obviously chemical engineers will not be welding pipes together, but understanding the benefits and limitations of different types of welding processes, for example, can help the engineer when designing the system that needs to be welded. But before we get into fabrication, a general overview of piping codes is presented in order to answer the following questions: Why is it necessary to comply with piping codes? What is the difference between a code and a concensus standard? Which code should I follow?
PIPING CODE Codes and standards
The querry, “Why do we, as a company, need to comply with a piping code?” is actually a trick question. Code, by definition is law with statutory force. Therefore the reason for complying with a code is because you literally have to, or else be penalized for non-compliance. A better question would be, “Why comply with or adopt a piping con* Part 1: The Basics, CE February, pp. 42–47; Part 2: Flanges, CE March, pp. 56–61; Part 3: Design Elements, CE July, pp. 50–57)
68
Besides flanges, there are also several different types of joints and welding processes to choose from. Additional decisions involve piping codes sensus standard?” When phrased this way, the question supports the author’s contention that many engineers and designers do not fully understand the difference between a code and a standard. And it doesn’t help matters when some standards are published as a code, and some codes are published as a standard. This is certainly nothing to get excited about, but it is something worth pointing out. My take on the reason for the misunderstanding of these two closely related terms, standard and code, is that they get bounced around so often in the same context that designers and engineers simply begin interchanging the two terms without much consideration for their different meanings. The difference between a standard and a code will be explained shortly, but first lets respond to the first question.
Why comply?
Consensus standards such as those published by ASME (American Soc. of Mechanical Engineering), ANSI (American National Standards Inst.), API (Americal Petroleum Inst.), NFPA (National Fire Protection Assn.), ASTM
(American Soc. for Testing and Materials), International Plumbing Code and others are not mandatory in and of themselves. However, federal, state, city and other local codes are mandatory. In these municipal codes you will find regulations that establish various requirements taken in whole, or in part from the standards published by the above listed organizations, and others, as legally binding requirements. These standards, as adopted, then become code, which is enforceable by law. When not addressed on a municipal level, but included in corporate specifications, the standard becomes a legal code on a contractual basis. Compliance with these codes, irrespective of government regulations or corporate requirements, doesn’t cost the builder any more than if it didn’t comply. It does, however, cost more to fabricate and install piping systems that have a high degree of integrity as opposed to systems that don’t. Hiring non-certified welders and plumbers, bypassing inspections, examinations and testing, using material that may potentially not withstand service pressures and temperatures,
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4:
n
and supporting this type of system with potentially inadequate supports is less costly initially, but there’s too much at risk. I don’t think anyone in good conscience would intentionally attempt to do something like that in order to save money. If anyone intends on fabricating and installing a piping system plans to perform any of the following points, then they are essentially complying with code: • Use listed material • Specify material that meets the requirements for fluid service, pressure and temperature • Inspect the material for MOC (material of construction), size and rating • Use certified welders and plumbers • Inspect welds and brazing • Adequately support the pipe • Test the pipe for tightness The code simply explains how to do each of these activities in a formal, well thought-out manner. There is not a reason sufficiently good enough to not comply with appropriate industry standards and codes. If there was a fee involved for compliance, this might be a stimulus for debate. But there is no fee, and there is usually just too much at stake to ignore them. Even with utility systems in an administration building or an institutional facility, the potential damage from a ruptured pipeline, or a slow leak at an untested joint could easily overshadow any savings gained in non-compliance. That’s without considering the safety risk to personnel. The first thing that someone should do, if they are considering to do otherwise, is check local and state codes. They may find regulations that require adherence to ASME, the International Plumbing Code or some of the other consensus standards. If not already included, this should be a requirement within any company’s specifications. Finally, it is worth taking a historical aside to make a point. ASME published the first edition of the Boiler and Pressure Vessel Code in 1914– 1915. Prior to creation of the code, and what played a large part in instigating its creation, was that between 1870 and 1910 approximately 14,000 boilers had exploded. Some were devastating to both people and property.
Those numbers fell off drastically as the code was adopted. Uniformity and regulation does have its place.
Which code to follow?
Like the seatbelt law, code compliance is not just the law, it makes good sense. A professional consensus standard is, very simply put, a code waiting to be adopted. Take the ASME Boiler and Pressure Vessel Code (BPVC): since its first publication in 1915 it has been adopted by 49 states, all the provinces of Canada, and accepted by regulatory authorities in over 80 countries. On May 18, 2005, it was finally adopted by the 50th state, South Carolina. And this doesn’t mean the BPVC is adopted in its entirety. A state, or corporation for that matter, can adopt a single section or multiple sections of the BPVC, or it can adopt the code in its entirety. Until South Carolina adopted the BPVC, it was actually no more than a standard in that state and only required compliance when stipulated in a specification. However, in all honesty you would not get a U.S. boiler or pressure vessel manufacturer to bypass code compliance. That is, unless you wanted to pay their potential attorneys’ fees. With regard to code compliance, the question often asked is, “How do I determine which piping code, or standard, I should comply with for my particular project?” Determining proper code application is relatively straightforward and at the same time comes with a certain degree of latitude to the owner in making the final determination. In some cases that determination is made for the engineer or contractor at the state level, the local level or by an owner company itself. Providing guidelines for code adoption on a project basis is direction that should be included in any company’s set of specifications, but quite often is not. This can cause a number of disconnects through design and construction. In order to answer the question about code assignment some history has to be told. In keeping this brief I will just touch on the high points. In 1942, ASA B31.1 — American Standard Code for Pressure Piping was published by the American Standards
Bioengineering Inversina – the gentle way of mixing. The Inversina mixes solids or liquids thoroughly and efficiently. The process is clean, because mixing takes place in closed containers that can be quickly interchanged. The Inversina mixes a diverse range of components rapidly and in an extremely gentle way. Segregation does not occur, even after extended mixing times, by virtue of the eversion phenomenon ( Paul Schatz principle ) . Applications for the Inversina: analytical labs, metal finishing shops, powder metallurgy and nuclear industry, manufacture of batteries, cement, ceramics, cosmetics, dental products, diamond tools, dyes and pigments, electrical and electronic devices, explosives and pyrotechnics, foods, homeopathic products, household products, medicines and pharmaceuticals, plastics, printing inks and many other products. The Bioengineering Inversina is available with capacities of 2, 20, 50, 100 and 300 L .
Bioengineering, Inc. Waltham, MA 02451, USA Bioengineering AG 8636 Wald, Switzerland
[email protected] www.bioengineering.ch Circle 51 on p. 122 or go to adlinks.che.com/6900-51
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Engineering Practice Association (ASA). This would later change to B31.1 — Power Piping. In the early 1950’s the decision was made to create additional B31 Codes in order to better define the requirements for more specific needs. The first of those Standards was ASA B31.8 — Gas Transmission and Distribution Piping Systems, which was published in 1955. In 1959 the first ASA B31.3 — Petroleum Refinery Piping Standard was published. After some reorganization and organizational name changes the ASA became ANSI. Subsequent code revisions were designated as ANSI Codes. In 1978, ASME was granted accreditation by ANSI to organize the B31 Committee as the ASME Code for Pressure Piping. This changed the code designation to ANSI/ASME B31. Since 1955 the B31 Committee has continued to categorize, create and better define code requirements for specific segments of the industry.
Through the years since then they have created, not necessarily in this order: B31.4 — Liquid Transportation Piping; B31.5 — Refrigeration Piping; B31.9 — Building Services Piping; and B31.11 — Slurry Transportation Piping. Each of these standards is considered a stand-alone section of the ASME Code for Pressure Piping, B31. What the B31 committee has accomplished, and is continuing to improve upon, are standards that are better focused on specific segments of industry. This alleviates the need for a designer or constructor building an institutional type facility from having to familiarize themselves with the more voluminous B31.3 or even a B31.1. They can work within the much less stringent and extensive requirements of B31.9, a standard created for and much more suitable to that type of design and construction. As mentioned above, ASME B31.1 — Power Piping, was first published in
S
ince 1956 the employees of Mueller Steam Specialty have been dedicated to the manufacture of high quality products delivered on time and with superior customer service. Our core line of rugged strainers is available in a wide range of types and materials. Whether you require basket strainers, Y strainers, “Tee” type strainers, duplex strainers, or even temporary strainers, Mueller will deliver your order from stock or custom engineer and manufacture it to your requirements. In addition to its strainer line, Mueller offers a full line of check valves, butterfly valves, pump protection and specialty products for a variety of industries and applications. Choose Mueller Steam Specialty for your next project.
Circle 57 on p. 122 or go to adlinks.che.com/6900-57 70
1942. Its general scope reads: “Rules for this Code Section have been developed considering the needs for applications which include piping typically found in electric power generating stations, in industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems.” The general scope of ASME B31.3 — Process Piping, reads: “Rules for the Process Piping Code have been developed considering piping typically found in petroleum refineries, chemical, pharmaceutical, textile, paper, semiconductor and cryogenic plants; and related processing plants and terminals.” ASME B31.5 — Refrigeration Piping, applies to refrigerant and secondary coolant piping systems. Closely related to B31.1, but not having the size, pressure or temperature range, B31.9 was first published in 1982. It was created to fill the need for piping in limited service requirements. Its scope is narrowly focused on only those service conditions that may be required to service the utility needs of operating a commercial, institutional or residential building. From its shear scope of responsibility, B31.3 encompasses virtually all piping, including those also covered by B31.1 (except for boiler external piping), B31.5 and B31.9. The difference, and distinction, as to which code should apply to a particular project, lies with the definition and scope of the project itself. If a project includes only the installation of perhaps a refrigeration system, B31.5 would apply. If a project’s scope of work consists of an office, laboratory, research facility, institutional facility or any combination thereof, B31.1 or B31.9 and possibly B31.5 would apply. A laboratory or research facility could possibly require fluid services beyond the fluid service limits of B31.9. In that case, B31.3 would be adopted for those services. In the case of a process manufacturing facility, B31.3 would be the governing code. Since B31.3 covers all piping, B31.5 or B31.9 would not need to be included, not even necessarily with associated laboratory, office and research facilities. The only time B31.5 or B31.9 would become governing codes, in as-
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should be well defined. The final determination as to what constitutes a governing code, within " # $ the purview of the above Figure 1. Problems can arise with a clamped joint mentioned codes, is left if not properly installed. Overtightening the clamp can to the owner and/or to the cause the gasket to intrude into the tubing (A), whereas local governing jurisdicundertightening results in pockets where residue can tion. Engineering specifiaccumulate (B). The ideal situation is joint C cations should clarify and sociation with a manufacturing facil- reflect the intent of the owner and the ity, is if a refrigeration unit, or an of- respective codes in an attempt to profice, laboratory and/or research facility vide consistency and direction across were under a separate design/construct all projects within a company. contract from the process manufacturing facility. Or if it was a substantial PIPE FABRICATION part of the overall project. Entering this section on fabrication As an example, project XYZ consists does not mean that we leave engineerof a process manufacturing facility, ing behind. Indeed, the majority, if not related office building and lab facili- all, fabricators (referring to the fabrities. If the utility service piping for the cators that are qualified for heavy inoffice and lab facilities is a small per- dustrial work) will have an engineercentage of the overall project, and/or ing staff. the design and construction contracts As a project moves from the design for those facilities are a part of the phase into the construction phase, overall process manufacturing facility, anyone with a modicum of project exall piping, with code exclusions, could perience can acknowledge the fact that be governed by B31.3. there will most certainly be conflicts, If, however, the office and labora- errors and omissions, no matter how tory facilities were a substantial part diligent one thinks he or she is during of the overall project, or they were to design. This is inherent in the methgo to a separate constructor, it may odology of today’s design/engineering be more beneficial to determine bat- process. Although there are methods tery limits for those facilities and and approaches to design in which designate anything inside those bat- this expected result can be minimized, tery limits as B31.1 or B31.9 and/or it is always prudent to be prepared for B31.5. In such a case, separate pipe such errors and omissions. specifications may have to be issued If, on the other hand, the assumpfor those portions of the project des- tion is made that the Issued for Conignated as being governed by B31.9. struction design drawings will faciliThis is due to the range of fluid ser- tate fabrication and installation with vices and the corresponding pressure minimal problems, then you can exand temperature limits of B31.9 com- pect to compound whatever problems pared to those of B31.3. These differ- do occur because you weren’t prepared ences in code assignment and battery to handle them. The greatest asset a limits may be a driver for the project’s project manager can have is the abilcontracting strategy. ity to learn from past experience and Many piping service requirements, the talent to put into practice what he such as steam, air, chilled water and or she has learned. so on, can come under the auspices of Pipe fabrication, in the context of multiple codes. These fluid services, this article, is defined as the construcwhich fall within the definition of tion of piping systems by forming and B31.3 Category D fluid services, can assembling pipe and components with just as easily fall within the require- the use of flanged, threaded, clamped, ments of B31.1 or B31.9 as well. In an grooved, crimped and welded joints. effort at maintaining a high degree of In Part 2 of this series, we discontinuity in the process of making cussed the flange joint; the others the determination of which code to will be discussed here. There are varapply to a project, company guidelines ious factors, or considerations, that
prompt the decision as to which type of connection to use in the assembly of a piping system. To start with, any mechanical joint is considered a potential leak point and should be minimized. Also, the decision as to which type of joint should be specified comes down to accessibility requirements, installation requirements and joint integrity. Using that as our premise, we can continue to discuss the various joining methods.
Threaded joint
Pipe thread, designated as NPT (National Pipe Taper) under ASME B1.20.1, is the type of thread used in joining pipe. This is a tapered thread that, with sealant, allows the threads to form a leak-tight seal by jamming them together as the joint is tightened. The same criteria described (in Part 2) for the threaded flange joint apply also to threaded fittings, in which the benefits of the threaded joint is both in cost savings and in eliminating the need for welding. In this regard, threaded components are sometimes used in high-pressure service in which the operating temperature is ambient. They are not suitable where high temperatures, cyclic conditions or bending stresses can be potential concerns.
Hygienic clamp joint
The clamped joint refers to the sanitary or hygienic clamp (Figure 1). Three installed conditions of the hygienic joint, minus the clamp are presented in Figure 1. Joint A represents a clamp connection that has been over tightened causing the gasket to intrude into the inner diameter (ID) of the tubing. This creates a damming effect, preventing the system from completely draining. In joint B, the clamp wasn’t tightened enough and left a recess at the gasket area. This creates a pocket where residue can accumulate, so cleanability becomes an issue. Joint C represents a joint in which the proper torque was applied to the clamp leaving the ID of the gasket flush with the ID of the tubing. The clamp C representation is the result that we want to achieve with the hygienic clamp. The problem is that this is very difficult to control on a repeatable basis. Even when the gas-
Chemical Engineering www.che.com October 2007
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Engineering Practice
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ket and ferrules are initially lined up with proper assembly and torque on the joint, some gasket materials have a tendency to creep (creep relaxation), or cold flow. Creep relaxation is defined as: A transient stress-strain condition in which strain increases concurrently with the decay of stress. More simply put, it is the loss of tightness in a gasket, measurable by torque loss. Cold flow is defined as: Permanent and continual deformation of a material that occurs as a result of prolonged compression or extension at or near room temperature. There have been a number of both gasket and fitting manufacturers that have been investing a great deal of research in attempting to resolve this issue with the clamp joint. Some of the solutions regarding fittings were addressed in Part 2 of this series. Additionally, gasket manufacturers and others have been work-
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ing on acceptable gasket materials that have reduced creep relaxation factors, as well as compression controlled gasket designs. What is meant by acceptable gasket material is a gasket that is not only compatible with the hygienic fluid service, but also meets certain U.S. FDA (or comparable) requirements. Those requirements include gasket material that complies with USP Biological Reactivity Test #87 & 88 Class VI for Plastics and FDA CFR Title 21 Part 177.
Grooved joint
The grooved joint (Figure 2), from a static internal-pressure-containment standpoint, is as good as or, in some cases, superior to the ASME Class 150 flange joint. In the smaller sizes (1 to 4 in.), the working pressure limit will be equal to that of a Class 300, carbon-steel, ASTM A105, ASME B16.5 flange. The main weakness of the grooved joint is the bending and torsional stress
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allowable at the coupling. This stress can be alleviated with proper support. Because of this design characteristic, the manufacturers of grooved joint systems have focused their efforts and created a niche in the fire-protection and utilityfluid service requirements, with the exception of steam and steam condensate. The grooved joint is comparatively easy to install, which is particularly important in areas that would require a fire card for welding. Since no welding is required, modifications can be made while operation continues. Some contractors choose to couple at every joint and fitting, while others choose to selectively locate couplings, much as you would selectively locate a flange joint in a system. It’s a decision that should be made based on the particular requirements or preference of a project or facility.
Pressed joint
The pressed joint (Figure 3) is actually a system that uses thin wall pipe, up through 2-in. NPT, to enable the joining of pipe and fittings with the use of a compression tool. Welding is not required, and threading is only necessary when required for instrument or equipment connection. These types of systems are available from various manufacturers in carbon steel, 316 and 304 stainless steel and copper. Because of the thin wall pipe, corrosion allowance becomes a big consideration with carbon steel. While the static internal pressure rating of these systems is comparable to an ASME Class 150 flange joint, there are additional fluid-service and installation characteristics that need to be considered. With axial and torsional loading being the weak spots in these systems, they are not practical where water hammer is a potential,
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(tungsten inert gas); SMAW GMAW process is better used indoors (shielded metal arc welding) or or in an area protected from the wind. MMA (manual metal arc) or stick If the shielding gas is disturbed, the welding; and FCAW (flux cored weld area can be affected. automatic welding). GTAW: Most often referred to as TIG, *OTFSUJPO GMAW: Often referred to as MIG, welding, GTAW can be automatic or 1BUFOUFE NBSL #FBE )PVTJOH 1SFTTGJUUPPM GMAW can be an automatic or manual. It uses a nonconsumable JOEFOU semi-automatic welding process. tungsten electrode to make the weld &YBHHFSBUFEGPSDMBSJUZ It is a process by which a shield- (Figure 4b), which can be done with Figure 3. Welding is not required for the ing gas and a continuous, consum- filler metal or without filler metal pressed joint, but corrosion can be an issue able wire electrode is fed through (autogenous). The TIG process is more due to the thin walls the same gun (Figure 4a). The exacting, but also more complex and such as in steam-condensate service. shielding gas is an inert or semi-inert slower than MIG welding. The axial load consideration carries gas such as argon or CO2 that protects In Part 2 of this series, the use of over to supporting the pipe as well. the weld area from atmospheric gases, orbital welding was mentioned for Ensure that vertical runs of this pipe which can detrimentally affect the hygienic tube welding. Orbital weldare supported properly from beneath. weld area. ing uses the GTAW method. Once the Do not allow joints in vertical runs to There are four commonly used orbital welder is programmed for the be under tension. They must be sup- methods of metal transfer used in material it is welding, it will provide ported properly from the base of the GMAW. They are: excellent welds on a consistent basis • globular vertical run. — provided, that is, that the chemistry • short-circuiting of the base material is within allow• spray Welded joint able ranges. The welded joint is by far the most in- • pulsed-spray A wide differential in sulfur content tegrated and secure joint you can have. With the use of a shielding gas, the between the two components being When done properly, a welded joint is as strong as the pipe itself. The key to a weld’s integrity lies in the craftsmanship of the welder or welding operator, the performance qualification of the welder or welding operator, and LEADING WORLDWIDE IN MIXING TECHNOLOGIES the weld procedure specification. Before going further, I want to explain the difference between the terms welder and welding operator. A welder is someone who welds by hand, or manually. A welding operator is someone who operates an automatic The EKATO GROUP provides their customers with the technical excellence and welding machine. The ends of the pipe experience of a global market leader. The companies within the EKATO GROUP still have to be prepared and aligned operate across the spectrum of mixing technologies. manually, and the automatic welding machine has to be programmed. The advantage of machine welding is apparent in doing production welds. This is shop welding in which there is a quantity of welds to be made on the same material type, wall thickFrom simple laboratory mixers to turnkey production plants, the EKATO GROUP provides a range of ness and nominal pipe size. Once the engineering services and custom-made solutions for the most challenging customer applications. The machine is set up for a run of typical synergies within the EKATO GROUP ensure that reliable and cost-effective solutions can be provided pipe like this, it is very efficient and to the highest quality standards for every application. This is supported by a global service network. consistent in its weld quality. This is another topic that could easily stand alone as an article, but instead, here we will focus on some Your contact in the USA Your contact in Europe of the primary types of welding used Tel.: +1 201 825 4684 Tel.: +49 7622 29-0 with pipe. Those types include the folwww.ekato.com e-mail:
[email protected] e-mail:
[email protected] lowing: GMAW (gas metal arc welding) or MIG (metal inert gas); GTAW (gas tungsten arc welding) or TIG
A d v a n c e d P r o c e s s S o l u t i o ns
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Circle 58 on p. 122 or go to adlinks.che.com/6900-58
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Engineering Practice
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inspection summary 7. Mechanical and electropolishing procedures 8. Examiner qualification 9. Inspector qualification 10. Welder qualification summary 11. Gage calibration certifications 12. Weld continuity report 13. WPSs (weld procedure specifica- tions) 14. PQRs (procedure qualification record) 15. Weld coupon log 16. Weld maps 17. Slope maps 18. Weld logs 19. Leak test reports 20. Inspection reports 21. Passivation records 22. Detail mechanical layouts 23. Technical specifications for com- ponents 24. As-built isometrics 25. Original IFC isometrics 26. Documentation recording any changes from IFC to as-built isometrics The above listed documentation, which closely parallels the list in ASME-BPE, is that which is generally required to move an installed hygienic system through validation, commissioning and qualification (C&Q). And this isn’t all that’s required. There is additional supporting documentation such as P&ID’s, procedural documents, and so on, which are also required. Depending on the size and type of a project it can be a massive undertaking. If not properly set up and orchestrated, it can become a logistical nightmare. What you do not want to do is discover during C&Q that you are missing a portion of the required documentation. Resurrecting this information is labor intensive and can delay a project’s turnover significantly. I cannot stress strongly enough just how
imperative it is that all necessary documentation be identified up front. It needs to be procured throughout the process and assimilated in a turnover (TO) package in a manner that makes it relatively easy to locate needed information while also allowing the information to be cross indexed and traceable within the TO package. The term validation is a broad, generalized, self-defining term that includes the act of commissioning and qualification. Commissioning and qualification, while they go hand in hand, are two activities that are essentially distinct within themselves. n Edited by Gerald Ondrey
Acknowledgement: The author wishes to thank Earl Lamson, senior project manager with Eli Lilly and Company, for being kind enough in taking time out of a busy schedule to read through the draft of this article. Earl has a remarkable set of project and engineering skills that set him apart from many I have worked with. That and the fact that I value his opinion are the reasons I asked him to review this article.
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. (P.O. Box 31154, St. Louis, MO 63131-0154. Phone: 314-966-8919; Email:
[email protected]) a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp & paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, owner expectations and good design practices. Huitt is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sits on two corporate specification review boards.
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Feature Report Engineering Practice
Piping Design Part 5:
Installation and Cleaning These practical guidelines for deciding which installation procedure to follow, and for cleaning a new pipeline system can prevent problems from happening during startup W. M. Huitt W. M. Huitt Co.
T
his fifth in a series of articles [1–4] on piping design discusses the practical issues of installation and cleaning.
Pipe installation
The installation of pipe follows its fabrication and is very frequently a part of it. The installation of pipe can be accomplished in the following four primary ways, or combinations thereof: 1. Field fabricate and install 2. Shop fabricate and field erected 3. Skid fabrication, assembly and installation 4. Modular construction
Field fabricate and install
In the first method, the pipe is fabricated onsite, either in place or in segments, at an onsite field-fabrication area and then erected. A number of factors will dictate whether or not it is feasible to field fabricate, including the following: the size and type of the project; pipe size and material; the facility itself; weather conditions; availability of qualified personnel; existing building operations; cleanliness requirements; and time available to do the work. Efficiency, quality and safety are the imperatives that are factored in when considering field fabrication. And cost is the fallout of those factors. Logistically speaking, if all pipe could be fabricated onsite in a safe and efficient manner — maintaining qual48
ity while doing so — it would make sense to do it in that manner. However, before making that final decision, let’s look at some of the pros and cons of field fabrication: Pros: • Only raw material (pipe, fittings, valves and so on) need to be shipped to the site location. Such materials are much easier to handle and store than multi-plane configurations of pre-fabricated pipe • No time-consuming need to carefully crib, tie-down and chock pre-fabricated spool* pieces for transport to the job site • Reduced risk of damage to spool pieces • More efficient opportunity to fabricate around unexpected obstacles (structural steel, duct, cable tray, and so on) • Fabricate-as-you-install reduces the rework risk assumed when pre-fabricating spools, or the cost related to field verification prior to shop fabrication • The field-routing installation of pipe through an array of insufficiently documented locations of existing pipe and equipment, on a retrofit project, is quite frequently more effective than attempting to pre-fabricate pipe based on dimensional assumptions *Spool pieces are the pre-fabricated sections of pipe that are fabricated and numbered in the shop, then shipped to the job site for installation.
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Cons: • Weather is arguably the biggest deterrent. If the facility under construction is not enclosed, then protection from the elements will have to be provided • When welding has to be done in conditions that are not environmentally controlled, then pre-heating will be required if the ambient temperature (not the metal surface temperature) is 0°F or below • In a new facility, as opposed to having to route piping through an array of poorly located existing pipe and equipment, field fabrication of buttwelded pipe is not as efficient and cost effective as shop fabrication • There may be concerns about safety and efficiency when working in a facility while it is in operation in advance of a turnaround or to begin advance work on a plant expansion Generally speaking, threaded, socketweld, grooved, and other proprietary-type joints that do not require buttwelding are field fabricated and installed. Buttwelding of small, 1 1/2-in. NPS and less, are very often field fabricated and installed because
of the added risk of damage during transport, in pre-fabricated form, from the shop to the site.
Shop fabricate and install
Shop fabrication refers to, generally speaking, any pipe, fittings and components that are assembled by welding into spool assemblies at the fabricator’s facility. The spools are then labeled with an identifier and transported to the job site for installation. Each spool piece needs its own identifier marked on the piece itself in some fashion that will make it easy to know where its destination is in the facility and where it belongs in a multi-spool system of pipe. This will allow the installer to efficiently stage the piece and ready it for installation. As part of the process of developing spool sections, field-welded joints need to be designated. These are welded joints that connect the pre-fabricated spools. In doing this the designer or fabricator will identify two different types of field-welded joints: field weld (FW) and field closure weld (FCW). FW indicates a joint in which the end of a pipe segment is prepared for the installer to set in place and weld to its connecting joint without additional modification in the field. This means that the length of pipe that is joined to another in the field is cut precisely to length and the end prepared in the shop for welding. FCW provides the installer with an additional length of pipe, usually 4 to 6 in. longer than what is indicated on the design drawings, to allow for field adjustment. What has to be considered, and what prompts the need for a FCW, is the actual, as-installed, location of both the fixed equipment that the pipe assemblies may connect to and the actual
installed location of the pipe assembly itself. Odds are that all equipment and piping will not be installed exactly where indicated on design drawings. The dimensional location of the equipment items given on design drawings is not a finite location, it is merely an intended location, as are dinensional locations on drawings for building steel, pipe supports and others. What factors into the installation of shop-fabricated pipe is the actual location of the equipment nozzle it will be connecting to in relation to the pipe’s installed location. In connecting to equipment there is a build-up, or stack-up, of tolerances that will effectively place the actual, or final, location of the nozzle at some point in three-dimensional space, other than where the design drawing indicates. The tolerance stack-up results from the following circumstances: • Manufacturing tolerances in material forming, nozzle location, and vessel support location • The actual set-in-place location of the vessel • Load cell installation (when applicable) • The actual set-in-place pipe runup location In order to allow for these inevitable deviations between the drawing dimensions used to fabricate the vessel, set the vessel and install the pipe assembly and the actual installed location of the connecting points, a fieldclosure piece, or two, will be required for that final adjustment. The field-closure piece is a designated section of the pipe assembly in which a field-closure weld has been indicated.
Skid (super skid) fabrication
A skid is a pre-packaged assembly that may contain all or some of the following that make up an operating system: vessels, rotating equipment, piping, automation components, operator interfaces, instrumentation, gages, electrical panels, wiring and connectors, framework, supports, inline piping components, and insulation. A single process or utility system may fit onto one skid or, depending on size restraints, may comprise multiple skids. After fabrication of a skid is complete, it will typically go through fac-
tory-acceptance testing (FAT) at the fabricator’s facility. The skid is then shipped to the job site where it is installed in its final location. After installation it would typically go through a follow-up site-acceptance test (SAT), including additional hydrotesting. This is basically a system shake-down to determine that everything is intact, and that those things that did not remain intact during transport are discovered and repaired. Logistics and the necessary skill set required for the installation, connection and startup of a particular skid package will dictate to what extent the skid fabricator will be involved after it is shipped to the job site.
Modular construction
The term module or modular construction is quite often, in this context, interchanged with the term skid fabrication. A module can refer to pre-fabricated units that actually form the structure of a facility as each is installed. Or, the units may be smaller sub-assemblies that, when combined, make up a complete process or utility system. Modules also consist of all or some of the following: vessels, rotating equipment, piping, automation components, HVAC, instrumentation, electrical wiring and connectors, framework, walls, architectural components, lighting, supports, inline piping components, and insulation. This, as an example, allows a complete locker-room module to be placed and connected to a complete water-treatment module. The smaller sub-assembly modules, in many cases, are interchanged with the term skid. Misconception can be avoided when a company defines these terms, both for internal discussion and for the purpose of making it clear to outside contractors, as to what is meant when using the term module.
Installation approach
Now that we have a general idea of the four primary approaches to piping installations how do we decide which is the best method, or combination of methods, to use for a particular project? Each project is unique with its own particular set of decision drivers with regard to a selected execution approach. There are no hard and fast
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49
Engineering Practice rules for determining a best approach. It requires experienced personnel to assign values to the various aspects of project execution, overlay a timeline, and then assess logistics. It sounds simple, but in actuality can be a very complex process. Therefore, the following is a guideline and not a hard and fast set of rules. There are simply too many project variables and complexities otherwise. When considering an approach, keep in mind that the method of installation needs to be weighed against a contractor’s preferred methodology. This does not imply that the contractor’s preferred methodology should drive your decision on how to execute a job. On the contrary, once you determine how the job needs to be executed, then look to only those contractors whose preferred methodology agrees with your project execution plans. Some contractors prefer to do most, if not all, fabrication in the shop, others prefer to set up at the job site, while others are flexible enough to utilize the best of both methods. The three main criteria discussed above — efficiency, quality and safety — would apply here as well. Using these three elements as a basis for making a determination, let us look at some common variables. Environment: The environment is only a factor when work has to be done in an open-air structure or other outdoor installation (such as tank farm, pipeline, pipe rack or yard piping). Working in an open-air structure will require protection from the elements (such as rain, snow, wind and cold). In addition, there may also be a requirement to work in elevated areas with the use of scaffolding. All of this can have a potential impact on safety and efficiency. Pipe-rack installation consists mainly of straight runs of pipe, and will not necessarily have a requirement or need for pre-fabrication. That is, unless it is pre-fabricated as modular-skid units. Depending on the project, it could be cost effective on an overall strategic basis to modularize the pipe rack, steel and all. The big advantage to shop fabrication is the controlled environment in which it’s done. This includes the qual50
ity control aspect, better equipment (generally speaking), a routine methodology of how a piece of work progresses through the shop, and better control, through a developed routine of required documentation. Industry: The various sectors of the chemical process industries (CPI) can be grouped into two categories: clean/ indoor build and non-clean/outdoor build. Realizing that there will be exceptions to this generalization, we can include in the clean/indoor built category: pharmaceutical, biopharmaceutical, semiconductor and food and dairy. Under non-clean/outdoor build we can include: petroleum refining; bulk chemicals; pulp and paper; off-shore; pipeline (oil and gas); and power generation. The clean-build philosophy comes from the need to construct certain facilities with a more stringent control on construction debris. Those industries included in this category often require a facility — at least a portion of a facility — to be microbial and particulate free, as stipulated by the design. There can be no debris, organic or inorganic, remaining after construction in accessible or inaccessible spaces of the facility. Of particular concern with pharmaceutical, biopharm and food-and-dairy facilities are food waste and hidden moisture. Food waste can entice and support rodents and insects, and hidden moisture can propagate mold, which can eventually become airborne. If these intruders are not discovered until the facility is in operation, the impact, upon discovery, can potentially be devastating to production. Such contamination can be found in one of two ways. Discovery at the source, possibly behind a wall or some other out-of-the-way place, means that not only does current production have to cease, but product will have to be analyzed for possible contamination. Once found, it then has to be remediated. The other method of discovery comes from the continuous testing and validation of the product stream. If a contaminant is discovered in the product, the production line is stopped, and the problem becomes an investigation into finding the source of the contamination.
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The clean-build philosophy, therefore, dictates more stringent and strict requirements for controlling and inspecting for debris on an ongoing basis throughout construction and startup. It will be necessary, on a clean-build site, to adhere to the following rather simple rules: • Smoking or smokeless tobacco products of any kind are not allowed on the site property • Provide for offsite break and lunch areas; no food or drink, other than water, are allowed on the site premises • Do not begin installing pipe, duct or equipment until, at the very least, a roof is installed • After roof and walls are installed, ensure that there is no standing water remaining in the facility • Prior to and during the construction of hollow walls, such as those framed and dry-walled, ensure on a daily basis that there is no moisture or debris in the wall cavity • Duct work delivered to the job site shall have the ends covered with a plastic sheet material, which shall remain on the ends until connected in place • Fabricated pipe delivered to the job site shall have the ends covered in a suitable fashion with suitable material, and the cover shall remain on the ends until pipe is ready to be connected in place • During and after flushing and testing of pipelines, all water spills shall be controlled to the extent possible and shall be cleaned after each flushing and testing or at the end of the work day
Type of project
While the type of project is not the main influence in determining how you approach the execution of a project, it does play a key role. It will help drive the decision as to how the piping should be fabricated and installed. For example, if the project is a retrofit, it will require much of the pipe, regardless of size and joint connection, to be field fabricated and installed. This is due simply to the fact that the effort and cost necessary to verify the location of all existing pipe, equipment, walls, columns, duct and
Table 1. Shop versus field fabrication Size (in.) ≤1½ ≤1½ ≥2 ≥2 ≤1 ≤1 ≥1½
Material Pipe Pipe Pipe Pipe Tubing Tubing Tubing
Joint 1, 2, 3, 6 4&5 3&6 4&5 5 5 5
Shop or field Field Shop Field Shop Field Shop (a, b) Shop
so on, in a somewhat precise manner, would not be very practical. You would be better served by field verifying Joint Type: the approximate location of 1 = Socketweld the above items with existing 23 == Threaded Grooved – Fully (Grooved fittings and pipe ends.) drawings, for planning and 4 = Grooved – Partially (Shop-welded spools with grooved logistic purposes, then shop 5 = ends.) Buttweld or field fabricate, verify and 6 = Flanged – Lined or unlined Pipe Notes: install as you go. a. Hygienic tubing A fast track project, one that b. Special cribbing and support for transport has a compressed schedule, will require parallel activities where to a minimum at the job site is not possible. Shop and skid fabrication an absolute, but is one of the key conwould be utilized as much as possible siderations to the efficiency of pipe simply to expend more man-hours installation. Following that logic, over a shorter time period while at- most of the buttwelded pipe should tempting to maintain efficiency, even be shop fabricated. A couple of things though there may be added cost to to consider, when determining which this approach. This approach is time buttwelded pipe to shop fabricate, are driven and not budgetary driven. size and material. A new grassroots facility still requires routing verification as you go, Pipe material and size range but certainly not the much-more in- Shop-fabricated spools need to be volved need to locate previously in- transported to the job site, which restalled obstructions that is necessary quires handling. Handling and transwhen working with an existing facility. porting small diameter pipe and thinIf the project is a clean-build project wall tubing spools create the potential inside an environmentally controlled for damage to those spools. area, it will be more practical to shop If you are shop fabricating everyfabricate or utilize skid or modular fab- thing and the distance from shop to rication for most, if not all of the piping. site is across town, the risk to damThis will reduce the number of person- aging small-diameter pipe spools is a nel and the amount of fabrication de- great deal less than if they have to be bris in the facility, and provide better shipped halfway across the U.S., Eucontrol for keeping it out of the pipe rope or Asia, or even across an ocean. itself. With personnel, you could have In transporting spools over long food wrappers, drink cans and bottles, distances, unless there is a great deal food waste, and clothing items. Fabrica- of thought and care given to cribbing tion debris could include metal filings, the load of spools, it may not be bencutting oil, pieces of pipe, weld-rod and eficial to transport buttwelded pipe weld-wire remnants, and so on. spools NPS 1 1/2 in. and less. It may be If the project is not a clean-build, but more practical to fabricate these sizes is still inside an environmentally con- on site, unless you are fabricating hytrolled facility, the same logic does not gienic or semiconductor piping; these necessarily apply. The decision to shop types of systems require a great deal fabricate and install or to field fabri- more control and a cleaner fabrication, cate and install becomes one based meaning that pipe fabrication will reon efficiency rather than how best to quire a clean shop area onsite, or the maintain a clean area. But that’s not to pipe will need to be fabricated at an say that if it doesn’t qualify as a clean- offsite, better controlled shop facility. build project then the construction deA practical rule of thumb in deterbris can just be allowed to pile up. mining what to fabricate in the shop There is still safety and efficiency or in the field is provided in Table 1. to consider on any project, and a clean Dictates of the project and a contracjob site is a major part of that. Main- tors’ standard operating proceedures taining a clean job site is an integral will determine how best to define component of good project execution. what is shop fabricated and what is Keeping personnel and equipment field fabricated. Circle 30 on p. 76 or go to adlinks.che.com/7371-30
Table 2. General cleaning scenarios
Engineering Practice Petroleum-refining and bulk-chemical projects are generally open-air projects in which field fabrication and installation of pipe are exposed to the elements. While a clean build is not a requirement on these types of projects efficiency and, above all, safety are. Because of this, it would make sense to utilize shop fabrication as much as possible. Fabricating pipe spools under bettercontrolled shop conditions will provide improved efficiency and safer-per-hour working conditions over what you will generally find in the field. This translates into fewer accidents. Referring back to Table 1, with respect to the potential for damage during transport, pipe sizes NPS 2–3 in. and larger ship much better than smaller pipe sizes, particularly when working with thin-wall tubing.
Location
Job-site location is one of the key markers in determining shop or field fabrication. In many cases, building a facility in a remote location will be a driver for utilizing a disproportionate amount of skid or module fabrication — disproportionate in the sense that project management may look at modularizing the entire job, rather than mobilize the staffing and facilities needed to fabricate and install on or near the job site. This would constitute a larger amount of modularization over what might normally be expected for the same type project in a more metropolitan region, or an area with reasonable access to needed resources. To expand on that thought; it was pointed out to me by Earl Lamson, senior project manager with Eli Lilly and Co., that project resources, even in metropolitan areas, are quite frequently siloed around a specific industry segment. In certain regions of the U.S. for example, you may discover that there is an abundance of craftsman available when building a refinery, but that same region may have difficulty, from a trained and experienced personnel perspective, in supporting the construction of a semiconductor facility. Consequently when building a pharmaceutical facility in another region you may find a sufficient population 52
Category C-1 C-2 C-3 C-4
Description Flush only (water, air or inert gas) Flush, clean with cleaning solution, flush Clean with cleaning solution, flush Flush, clean, passivate, flush
of trained and experienced craftsman for Table 3. General Leak Testing Scenarios that industry, but may Category Description not find resources adInitial service leak test equate when building T-1 T-2 Hydrostatic leak test a chemical plant. T-3 Pneumatic leak test Building a project in T-4 Sensitive leak test a remote location reT-5 Alternative leak test quires the project team to rethink the job-asusual methodology. From a logistics Flushing. This is a process by which standpoint, mobilization of personnel water, air or an inert gas is forced and material become a major factor through a piping system either in in determining the overall execution preparation for chemical cleaning or of such a project. Project planning is as the only cleaning process. Flushing a big component in project execution, can be accomplished by using dynamic but is more so when attempting to pressure head or released static presbuild in remote areas. And this doesn’t sure head, as in a fill-and-dump proceeven touch on the security aspect. dure. Blow-down can be considered as Nowadays, when constructing in flushing with a gas. any number of remote areas, security Passivation. In this process, a chemiis a real concern that requires real cal solution, usually with a base of consideration and real resolution. Re- nitric, phosphoric, citric acid or other duced onsite staffing is a good counter mild oxidant, is used to promote or acmeasure in reducing risk to personnel celerate the formation of a thin (25–50 when building in remote or even non- Å), protective oxide layer (a passive remote third-world areas. layer) on the internal surfaces of pipe, fittings and equipment. In stainless PIPE SYSTEM CLEANING steels — the most commonly used alloy While there are requirements in at present — passivation removes any ASME for leak testing, cleaning re- free iron from the pipe surface to form quirements do not exist. ASTM A 380 a chromium-rich oxide layer to protect and 967 has standards on cleaning, the metal surface from aggressive liqdescaling and passivation, but there uids such as high-purity waters. is nothing in ASTM on simply flushNote that the terms cleaning and ing and general cleaning. Defining the flushing can be interchanged when requirements for the internal cleaning the process only requires water, air or of piping systems falls within the re- an inert gas to meet the required level sponsibilities of the owner. of cleanliness. When the term “cleanThe term “cleaning”, in this context, ing” is used in this context it may infer is a catch-all term that also includes what is defined as flushing. flushing, chemical cleaning, and passivation. So before we go further, let Cleaning and testing me provide some definition for these With regard to cleaning and leak testterms as they apply in this context, be- ing, and which to do first, there are cause these terms are somewhat flex- drivers for both and different schools ible in their meaning, depending on of thought on the overall process. Each source and context, and could be used contractor will have its preference. It to describe activities other than what is in the owner’s best interest to deteris intended here. mine its preference or be at risk in just leaving it to the contractor. In either Definitions case you should have a line of thought Cleaning: This is a process by which on the process, if for no other reason water, solvents, acids or proprietary than to be able to understand what cleaning solutions are flushed through the contractor is proposing to do. a piping system to remove contamiAt the very least, in advance of leak nants such as cutting oils, metal fil- testing, perform either a basic flush of ings, weld spatter, dirt and other un- a test circuit, or perform an internal wanted debris. visual examination as the pipe is in-
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Engineering Practice stalled. A walk-down of the test circuit should be done just prior to filling the system with any liquid. The last thing you want to happen is to discover too late that a joint wasn’t fully connected or an inline component was taken out of the pipeline. In a facility that is not a clean-build, it can simply be a mess that has to be cleaned. In a clean-build facility, an incident such as this can potentially be costly and time consuming to remediate. Tables 2 and 3 list general cleaning and testing procedures along with easy-to-use indicators. Since this article is concerned with new pipe installations, we will not include steam-out cleaning or pipeline pigging in our discussion. These are cleaning procedures that are used on in-service piping to clean the fluid service residue buildup from interior pipe walls after a period of use. Before subjecting the system to an internal test pressure, you should first perform a walk down of the piping to make certain, as mentioned earlier, that there are no missing or loose components. The system is then flushed with water or air to make sure that there are no obstacles in the piping. Over the years, we have discovered everything from soda cans to shop towels, work gloves, nuts and bolts, weld rod, Styrofoam cups, candy wrappers, and other miscellaneous debris, including dirt and rocks in installed piping systems. After an initial flush, which could also be the only flush and cleaning required, the system is ready for chemical cleaning or leak testing. In large systems, it may be beneficial to leak test smaller test circuits and then perform a final cleaning once the entire system is installed and tested. This would include a final completed system leak test that would test all of the joints that connect the test circuits. That is, unless these joints were tested as the assembly progressed. On large systems, if it is decided to leak test smaller segments, or test circuits as they are installed (prior to flushing the entire system), the piping needs to be examined internally as it is installed. This is to prevent any large-debris items from remaining in the piping during the test. 54
Now that we have Table 4. Rate of flushing liquid (gal/ touched on generalimin) needed to maintain a velocity of approximately 10 ft/s ties, let’s take a look at each of the clean- Pipe Pipe size (in.) ing categories listed in Sch. ½ ¾ 1 1½ 2 3 4 Table 2 and see how to 5S 12 20 34 77 123 272 460 apply them. 40 10 16 27 64 105 230 397 Cleaning Category 80 7 13 22 55 92 — — C-1: This is simply a flush with water, air or inert gas. The one non-manual locity of 10 ft/s. This gets a little cumassist that water requires in order bersome and costly unless you have for it to clean the inside of a piping pumps or compressors in place that system is velocity. But what velocity can achieve the necessary flowrate. is necessary? The alternative for liquid flushing The main concept behind flushing the larger pipe sizes other than using a pipeline is to dislodge and remove source line pressure or a pump is to suspected debris. In order to dislodge, perform a fill-and-dump. In this prosuspend and remove this unwanted cess, the pipe system is completely material in the piping system, it is filled with liquid and then drained necessary that water or air be forced through a full-line-size, quick-openthrough the piping system at a veloc- ing valve. In doing this, there has to ity sufficient to suspend the heaviest be enough static head to generate sufsuspected particles and move them ficient force and velocity to achieve along the pipeline. essentially the same result as the The velocity required to suspend pumped or line pressure liquid. the particles and move them along Cleaning Category C-2: This is a the pipeline for removal is dependent three-step process by which the piping upon their size and weight, and the system is initially flushed out with a flush medium. Metal filings, arguably liquid to remove most of the loose dethe heaviest particles normally found bris. This is followed by the circulation in newly fabricated pipe, will have a of a cleaning solution, which is then terminal mid-range settling veloc- followed by a final flush of water. ity, in water, of approximately 10 ft/s. Cleaning solutions are, in many Therefore, a flushing velocity of ap- cases, proprietary detergent or acidproximately 10 ft/s should be achieved based solutions each blended for speduring the flush. (This does not apply cific uses. Detergent-based solutions to acid cleaning.) Table 4 indicates the are generally used for removing dirt, rate of flow required to achieve ap- cutting oils and grease. Acid-based soproximately 10 ft/s of velocity through lutions are used to remove the same various sizes and schedules of pipe. contaminants as the detergent-base Purging a piping system clear of de- plus weld discoloration and residue. bris with air requires a velocity of ap- The acid-based solution also passivproximately 25 ft/s. Table 5 indicates ates the pipe wall. the air flowrate required to achieve apAs defined earlier, passivation proximately 25 ft/s of velocity through provides a protective oxide barrier various sizes and schedules of pipe. against corrosion. The acids used in One thing you might notice is that some cleaning solutions for ferrous the size range only extends to 4-in. and copper materials leave behind a NPS for both the liquid flush and for passivated interior pipe surface as a the air or gas blow-down. The reason result of the cleaning process. In utilfor that is the volume of liquid or gas ity water services, such as tower and required to achieve the necessary ve- chilled water, this barrier against corlocity through the larger pipe sizes is rosion is maintained with corrosion quite significant. inhibitors that are injected into the For example, a 6-in. NPS pipeline fluid stream on an ongoing basis. would require approximately 900 Keep in mind that the formation of to 1,000 gal/min, depending on wall passivated surfaces is a natural occurthickness of the pipe, to achieve a ve- rence with metals in an oxygen envi-
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ronment; the acid merely initiates and speeds up the process. When using stainless alloys — usually 316L, in hygienic-water services such as water for injection (WFI), purified water, deionized (DI) water and in some cases soft water — passivation is a final step in the preparation for service of these pipelines. Passivation is also a periodic ongoing preventative-maintenance procedure. High-purity water is very corrosive and attacks any free iron found on the surface of stainless-steel pipe. Free iron has a tendency to come out of solution when material is cold worked, as in bending or forming pipe without the benefit of heat. It also occurs with the threading of alloy bolts, which are solution annealed (heat treated) after threading. Passivation removes this free iron while also accelerating, in the presence of O2, the oxidation rate of the stainless steel, providing a chromium-rich, oxide corrosion barrier as defined above. Over time (and this is one hypothetical thought on the subject), this very thin corrosion barrier tends to get depleted or worn off, particularly at high impingement areas of the piping system, such as elbows, tees and pump casings. Once the passive layer wears through, any free iron exposed to the high purity water will oxidize, or rust. This will show up as surface rouge. Rouging is an unwanted surface discoloration that is periodically removed by means of a derouging process. This is an operational, as-needed chemicalcleaning process that will remove all or most of the rouge and also re-passivate the internal pipe surface. Discussions and research on the topic of rouging continue. This is a subject that has more questions than answers at the present time. Currently, the ASME-BPE is looking into this issue. One of the questions to be answered is whether or not rouge is actually detrimental to product streams. Cleaning Category C-3: This is a two-step cleaning process that uses a detergent- or acid-based solution to clean the pipe interior of any unwanted residue or debris. This is then followed by a final flush of water. Cleaning Category C-4: This is a three- or four-step process generally
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Engineering Practice used in hygienic service piping. In most cases, simply due to the clean fabrication approach used in hygienic pipe fabrication, only a water flush with deionized- (DI) quality water, or better, would be necessary for cleaning ,followed by passivation of the piping system, then a final flush of water. There are variations to each of these primary cleaning functions and it would be in an owner’s best interest to define these requirements, by fluid service, in advance of the work to be done.
Cleaning procedures
This section describes some fundamental cleaning procedures as they might appear in a specification or guideline and includes the leak-test procedures that will follow in Part 6. This will give you some idea as to what you might consider developing for your own set of specifications. Assuming that if your company repeatedly executes projects you will have cleaning and testing guidelines, in some form, prepared for your contractor. If not, you may not get what you expect. It’s better to give some forethought to these activities rather than be surprised at the results. Once a menu of these cleaning and testing procedures are developed, using pre-assigned symbols, similar to those given in the following, they can then be specified in the line list with the respective fluid services as you require. In this manner, there is no second guessing during construction. Each piping circuit is assigned a specific clean and test protocol in advance. Many pre-developed procedures I have seen over the years, those developed by owners in particular, have been very simplistic, and typically out of date. This is an indicator to most contractors that the owner’s representative will most likely not attempt to enforce them. The contractor, in making that assumption, may simply ignore them and perform their own procedures. Your procedural guidelines should be explicit and current to ensure that the contractors know that someone has given some thought to how he or she wants that work accomplished, making it far more likely that the contractors will execute your procedure instead of their own. It is certainly acceptable to accom56
Table 5. Rate of air flow (ft3/s) to maintain a velocity of approximately 25 ft/s Pipe Sch. Press. 15 psig Press. 50 psig
5S 40 80 5S 40 80
Pipe Sizes (in.) ½ ¾ 0.14 0.23 0.11 0.19 0.08 0.15 0.30 0.51 0.23 0.41 0.18 0.33
1 0.39 0.30 0.25 0.84 0.66 0.55
modate suggestions to a procedure from a contractor when they don’t compromise the intent of the owner’s requirements and are likely to improve the efficiency of the contractor. If a submitted alternate procedure does not compromise the intent of the owner, it is recommended that it be accepted. This will allow the owner to see if that efficiency is really there. With that in mind, let’s create a couple of general cleaning procedures. A general practice in the flushing and cleaning process (also indicated in leak testing), is the evacuation of air when using liquids. Always provide high-point vents for evacuating air during the fill cycle and low point drains for clearing out all of the liquid when the process is complete. Using the same terminology in Table 2 these cleaning procedures will be categorized as follows: Category C-1: Flush or blowdown only (water, air or inert gas) C-1.1 — These systems shall be flushed with the fluid that the system is intended for. There shall be no hydrostatic or pneumatic leak test. An initial-service leak test will be performed. a. Connect system to its permanent supply line. Include a permanent block valve at the supply line connection. All outlets shall have temporary hoses run to drain. Do not flush through coils, plates, strainers or filter elements. b. Using supply line pressure, flush system through all outlets until water is clear and free of any debris at all outlet points. Flush a quantity of fluid through each branch not less than three times that contained in the system. Use Table 6 to estimate volume of liquid in the system. c. These systems are required only to undergo an initial-service leak test.
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1½ 0.86 0.71 0.62 1.88 1.56 1.35
2 1.39 1.18 1.04 3.02 2.56 2.26
3 3.06 2.59 2.32 6.67 5.65 5.05
4 5.17 4.47 4.03 11.3 9.73 8.79
During the flushing procedure, and as the system is placed into service, all joints shall be checked for leaks. d. Any leaks discovered during the flushing process, or during the process of placing the system into service, will require the system to be drained and repaired. After which the process will start over with Step 2. C-1.2 — These systems shall be flushed clean with potable water. a. Connect a flush/test manifold at a designated inlet to the system, and a temporary hose or pipe on the designated outlet(s) of the system. b. Route temporary hose or pipe from potable water supply, approved by owner, and connect to flush/test manifold. Route outlet hose or pipe to sewer, or as directed by owner represenative. Secure end of outlet. c. Using a once through procedure (not a re-circulation), and the rate of flow in Table 4, perform an initial flush through the system with a quantity of potable water not less than three times that contained in the system. Use Table 6 to estimate volume of liquid in the system. Discharge to sewer, or as directed by owner representative. d. After the initial flush, insert a conical strainer into a spool piece located between the discharge of the piping system and the outlet hose. Perform a second flush with a volume of potable water not less than that contained in the system. e. After the second flush (Step d), pull the strainer and check for debris; if debris is found repeat Step c. If no debris is found the system is ready for leak testing. Category C2: Flush then clean with cleaning solution, followed by a neutralization rinse. Because of the thoroughness of the flush, clean and rinse process there should be no need to
Engineering Practice Table 6.Volume of water (gal) per lineal foot of pipe Sch. 5S 20 40 80
Pipe Sizes (in.) 1/2 3/4 1 .021 .035 .058 — — — .016 .028 .045 .012 .023 .037
11/2 .129 — .106 .093
2 .207 — .176 .154
check for transient debris, only for neutralization. However, if circumstances dictate otherwise, then a final check for debris may be warranted. C-2.1 — These systems shall be preflushed with potable water, cleaned with (indicate cleaning agent) then a rinse/neutralization followed by leak testing with potable water. If it is determined that the system will be installed and tested progressively in segments, the sequence of cleaning and testing can be altered to follow the segmented installation, thereby leak testing segments of a piping system as they are installed without cleaning. The entire system would then be cleaned once installed and tested. a. Hook up flush/test manifold at a designated temporary inlet to the system between the circulating pump discharge and the system inlet. Install a temporary hose or pipe on the designated outlet(s) of the system. b. Route temporary hose or pipe from potable water supply, approved by owner, and connect to flush/test manifold. Route outlet hose or pipe to sewer, or as directed by owner’s representative. c. Close valve between the circulating pump (if no valve is included in the system design, insert a line-blind or install a blind flange with a drain valve) discharge and flush/test rig. Open valve between flush/test manifold and piping system. d. Using the once-through procedure (meaning the cleaning fluid is not re-circulated), and the rate of flow in Table 4, perform an initial flush through the system, bypassing the circulation pump, with a quantity of potable water equal to not less than three times that contained in the system. Use Table 6 to estimate volume of liquid in the system. (Note: During the water flush, check the system for leaks. Verify no leaks prior to introducing chemical cleaning solution to the piping system.) 58
3 .455 — .386 .345
4 .771 — .664 .60
6 1.68 — 1.51 1.36
8 — 2.71 2.61 —
10 — 4.31 4.11 —
12 — 6.16 5.84 —
e. Discharge to sewer, or as directed by owner’s representative. f. After completing the initial flush, drain remaining water in the system. Or, retain water if cleaning chemicals will be added to the circulating water. g. Configure valves and hoses to circulate through pump. Connect head tank, or other source containing cleaning agent, to connection provided on circulation loop. h. Fill the system with the pre-measured (indicate preferred cleaning agent and mixing ratio or percentage by volume) and circulate through the system for 48 h. To minimize corrosion, if anticipated, circulate cleaning agent at a lowvelocity rate prescribed by the cleaning-agent manufacturer. i. Drain cleaning agent to sewer or containment, as directed by owner. j. Reconnect, as in Step a, for the once through flush/neutralization, and flush system with potable water using a quantity not less than three times that of the system volume. Since the (name cleaning agent) solution has a neutral pH, the rinse water will have to be visually examined for clarity. Rinse until clear. The rinse must be started as quickly after the cleaning cycle as possible. If cleaning residue is allowed to dry on the interior pipe wall, it will be more difficult to remove by simply flushing. The final rinse and neutralization must be accomplished before any possible residue has time to dry. k. Test pH for neutralization. Once neutralization is achieved proceed to Step l. l. Remove pump and temporary circulation loop, then configure the system for leak testing. This may include removal of some components, insertion of line-blinds, installation of temporary spools pieces and so on. These three examples should pro-
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14 — 7.34 9.22 —
16 — 9.70 9.22 —
18 — 12.4 14.5 —
20 — 15.2 14.5 —
24 — 22.2 — —
vide an idea as to the kind of dialog that needs to be created in providing guidance and direction to the contractor responsible for the work. And, as stated earlier, these procedures, for the most part, are flexible enough to accommodate suggested modifications from the contractor. ■ Edited by Gerald Ondrey
Acknowledgement
The author’s deep appreciation again goes to Earl Lamson, senior project manager with Eli Lilly and Co., for taking the time to review these articles. His comments help make this article, and the others, better documents than they otherwise would have been. He obliged me by applying the same skill, intelligence and insight he brings to everything he does. His comments kept me concise and on target.
References 1. Huitt, W.H., Piping for Process Plants: The Basics, Chem. Eng. February 2007, pp. 42–47. 2. Huitt, W.H., Piping for Process Plants: Flanges, Chem. Eng. March 2007, pp. 56–61. 3. Huitt, W.H., Piping for Process Plants: Design Elements, Chem. Eng. July 2007, pp. 50–57. 4. Huitt, W.H., Piping for Process Plants: Codes and Fabrication, Chem. Eng. February 2007, pp. 68–76.
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. (P.O. Box 31154, St. Louis, MO 63131-0154. Phone: 314-966-8919; Email: wmhuitt@aol. com) a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp & paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, owner expectations and good design practices. Bill is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sits on two corporate specification review boards.
Feature Report Engineering Practice
Piping for Process Plants Part 6:
Testing & Verification Proper documentation, determination of the fluid service category and operating conditions are among the factors necessary to perform the correct leak test on a piping system W. M. Huitt W. M. Huitt Co.
T
his sixth and final part of a series of articles [1–5] on piping for process plants discusses practical issues of leak testing and verification of piping systems.
Leak testing
Leak testing and pressure testing are often used synonymously. However, pressure testing is a misnomer when referring to leak testing of piping systems. By definition, a pressure test is the procedure performed on a relief valve to test its set-point pressure. The intent, when pressure testing a relief valve, is not to check for leaks, but to test the pressure set point of the valve by gradually adding pressure to the relief valve until it lifts the valve off of the seat. A leak test, on the other hand, is performed to check the sealing integrity of a piping system by applying internal pressure to a pre-determined limit, based on design conditions, then checking joints and component seals for leaks. It is not intended that the MAWP (maximum allowable working pressure) of a piping system be verified or validated. Before discussing the various types of leak tests and leak-test procedures I would like to briefly talk about controlling and tracking this activity. Testing, like many aspects of a project, should be a controlled process. There should be a formal method of documenting and tracking this activity as the contractor proceeds through the leak testing process. 48
Documentation
In documenting the leak testing activity there are certain forms that will be needed. They consist of the following: 1. A dedicated set of piping and instrumentation diagrams (P&IDs) to identify the limits and number the test circuits 2. A form to record components that were either installed or removed prior to testing 3. A checklist form for field supervision to ensure that each step of the test process is accomplished 4. Leak-test data forms The two sets of documents, from those listed above, that need to be retained are the P&ID’s and the leaktest data forms. The other two sets of forms are procedural checklists. The leak-test data forms should contain key data such as the following: 1. Test circuit number 2. P&ID number(s) 3. Date of test 4. Project name or number, or both 5. Location within facility 6. Line number(s) 7. Design pressure 8. Test pressure 9. Test fluid 10. Test fluid temperature 11. Time (military) recorded test begins 12. Pressure at start of test 13. Time (military) recorded test ends 14. Pressure at end of test 15. Total elapsed time of test 16. Total pressure differential (plus or minus) from the beginning to the end of test period 17. Comment section (indicate if leaks were found and system was repaired and retested or if system passed)
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18. Signatures and dates Also make certain that the testing contractor has current calibration logs of his or her test instruments, such as pressure gages.
Primary leak tests
ASME B31.3 defines five primary leak tests as follows: Initial service leak test. This applies only to those fluid services meeting the criteria as defined under ASME B31.3 Category D fluid service. This includes fluids in which the following apply: • The fluid handled is nonflammable, nontoxic, and not damaging to human tissue • The design gage pressure does not exceed 1,035 kPa (150 psi) • The design temperature is from –29°C (–20°F) through 186°C (366°F) The initial service leak test is a process by which the test fluid is the fluid that is to be used in the intended piping system at operating pressure and temperature. It is accomplished by connecting to the fluid source with a valved connection and then gradually opening the source valve and filling the system. In liquid systems, air is purged during the fill cycle through high point vents. A rolling examination of all joints is continually performed during the fill cycle and for a period of time after the system is completely filled and is under line pressure. In a situation in which the pipeline that is being tested has distribution on multiple floors of a facility, there will be pressure differentials between the floors due to static head differences. This will occur in operation
PT = 1.5 ⋅ P
and is acceptable under initial service test conditions. The test pressure achieved for initial service testing is what it will be in operation. The only difference is that the flowing fluid during operation will incur an amount of pressure drop that will not be present during the static test. Hydrostatic leak test. This is the most commonly used leak test and is performed by using a liquid, normally water, and in some cases with additives to prevent freezing, under a pressure calculated by Equation (1): 1.5 ⋅ P ⋅ ST (1) S where PT = Test pressure, psi P = Internal design gage pressure, psig ST = Stress value at test temperature, psi (see ASME B31.3, Table A-1) S = Stress value at design temperature, psi (see B31.1, Table A-1) However, as long as the metal temperature of ST remains below the temperature at which the allowable stress value for ST begins to diminish and the allowable stress value of S and ST are equal, then ST and S cancel each other leaving the simpler Equation (2): PT =
(2)
Unlike initial service testing, pressure variations due to static head differences in elevation have to be accommodated in hydrostatic testing. That means the calculated test pressure is the minimum pressure required for the system. When hydrostatically testing a multi-floor system, the minimum calculated test pressure shall be realized at the highest point. This is not stated, but is inferred in B31.3. Pneumatic leak test. This test is performed using air or a preferred inert gas. This is a relatively easy test to perform simply from a preparation and cleanup standpoint. However, this test has a hazardous potential because of the stored energy in the pressurized gas. And for that reason alone it should be used very selectively. When pneumatic testing is performed, it must be done under a strictly controlled procedure with onsite supervision in addition to coordination with all other crafts and personnel in the test area. The test pressure for pneumatic leak testing under B31.3 is calculated using Equation (3), for B31.9 it is calculated using Equation (4), and for B31.1 it is calculated using Equation (5). PT = 1.1 ⋅ P
(3)
PT = 1.4 ⋅ P
(4)
PT = 1.2 ⋅ P to 1.5 ⋅ P
(5) One misconception with pneumatic leak testing is in its procedure, as described in B31.3. There is a misconception that the test pressure should be maintained while the joints are examined. This is not correct. As B31.3 explains, pressure is increased gradually until the test pressure is reached. At that point, the test pressure is held until piping strains equalize throughout the system. After a sufficient amount of time is allowed for piping strains to equalize, the pressure is then reduced to the design pressure (see Reference [3] for
a discussion of the design pressure). While design pressure is held, all joints are examined for leaks. It is not required that the examination take place while holding test pressure. There is more to the entire procedure that is not included here. Please refer to B31.3 or B31.1 for full details on pneumatic leak testing. Sensitive leak test. This leak test is performed when there is a higherthan-normal potential for fluid leakage, such as for hydrogen. I also recommend its use when a fluid is classified as a Category M fluid service. B31.1 refers to this test as Mass-Spectrometer and Halide Testing. In B31.3, the process for sensitive leak testing is as follows: The test shall be in accordance with the gas and bubble test method specified in the BPV Code, Section V, Article 10, or by another method demonstrated to have equal sensitivity. Sensitivity of the test shall be not less than 10–3 atm. mL/s under test conditions. a. The test pressure shall be at least the lesser of 105 kPa (15 psi) gage, or 25% [of] the design pressure. b. The pressure shall be gradually increased until a gage pressure the lesser of one-half the test pressure or 170 kPa (25 psi) gage is attained, at which time a preliminary check shall be made. Then the pressure shall be gradually increased in steps until the test pressure is reached, the pressure being held long enough at each step to equalize piping strains. In testing fluid services that are extremely difficult to seal against, or fluid services classified as a Category M fluid service, I would suggest the following in preparation for the process described under B31.3: • Prior to performing the sensitive leak test, perform a low-pressure test (15 psig) with air or an inert gas using the bubble test method. Check every mechanical joint for leakage • After completing the preliminary low-pressure pneumatic test, purge all of the gas from the system using helium. Once the system is thoroughly purged, and contains no less than 98% He, continue using He to perform the sensitive leak test with a mass spectrometer tuned to He. Helium is the trace gas used in this
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49
Engineering Practice process and has a size that is close to that of the hydrogen molecule; this makes it nearly as difficult to seal against as H2 without the volatility. Test each mechanical joint using the mass spectrometer to determine leak rate, if any. Alternative leak test. In lieu of performing an actual leak test, in which internal pressure is used, the alternative leak test takes the examination and flexibility analysis approach. This test is conducted only when it is determined that either hydrostatic or pneumatic testing would be detrimental to the piping system or the fluid intended for the piping system, an inherent risk to personnel, or impractical to achieve. As an alternative to testing with internal pressure, it is acceptable to qualify a system through examination and flexibility analysis. The process calls for the examination of all groove welds, and includes longitudinal welds used in the manufacture of pipe and fittings that have not been previously tested hydrostatically or pneumatically. It requires a 100% radiograph or ultrasonic examination of those welds. Where applicable, the sensitive leak test shall be used on any untested mechanical joints. This alternative leak test also requires a flexibility analysis as applicable. Very briefly, a flexibility analysis verifies, on a theoretical basis, that an installed piping system is within the allowable stress range of the material and components under design conditions if a system: (a) duplicates or replaces without significant change, a system operating with a successful service record; (b) can be judged adequate by comparison with previously analyzed systems; and (c) is of uniform size, has no more than two points of fixation, no intermediate restraints, and falls within the limitations of empirical Equation (6). D⋅ y ≤ K1 L −U 2 ( )
(6)
where D = Outer dia. of pipe, in. (or mm) y = Resultant of total displacement strains to be absorbed by piping system, in. (or mm) 50
L = Developed length of piping between anchors, in. (or mm) U = Anchor distance, straight line between anchors, ft (or m) K1 = 208,000 SA/Ea, (mm/m)2 = 30 SA/Ea, (in./ft)2 SA = Allowable displacement stress range per Equation (1a) of ASME B31.3, ksi (MPa) Ea = Reference modulus of elasticity at 70°F (21°C), ksi (MPa) One example in which an alternative leak test might be used is in making a branch tie-in to an existing, in-service line using a saddle with an o-let branch fitting with a weld-neck flange welded to that, and a valve mounted to the flange. Within temperature limitations, the fillet weld used to weld the saddle to the existing pipe can be examined using the dye penetrant or magnetic particle method. The circumferential butt or groove weld used in welding the weld neck and the o-let fitting together should be radiographically or ultrasonically examined. And the flange joint connecting the valve should have the torque of each bolt checked after visually ensuring correct type and placement of the gasket. There are circumstances, regarding the tie-in scenario we just discussed for alternative leak testing, in which a hydrostatic or pneumatic test can be used. It depends on what the fluid service is in the existing pipeline. If it is a fluid service that can be considered a Category D, then it is quite possible that a hydrostatic or pneumatic leak test can be performed on the described tie-in. By capping the valve with a blind flange modified to include a test rig of valves, nipples and hose connectors, you can perform a leak test rather than an alternative leak test. As mentioned, this does depend on the existing service fluid. If the existing fluid service is steam or a cryogenic fluid, then you might want to consider the alternative leak test.
More on documentation
As seen in Equations (1–5), the leak test pressure, except for initial service testing, is based on design pressure and design temperature, both of which are described in Reference [3]. A few
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general procedures for cleaning and testing are presented below. As in all other project functions, control and documentation is a key element in the cleaning and testing of piping systems. It does, however, need to be handled in a manner that is dictated by the type of project. That means you don’t want to bury yourself in unwarranted paperwork and place an unnecessary burden on the contractor. Building a commercial or institutional type facility will not require the same level of documentation and stringent controls that an industrial type facility would require. But even within the industrial sector there are varying degrees of required testing and documentation. To begin with, documentation requirements in industry standards are simplistic and somewhat generalized, as is apparent in ASME B31.3, which states in Para. 345.2.7: Records shall be made of each piping system during the testing, including: (a) Date of test (b) Identification of piping system tested (c) Test fluid (d) Test pressure (e) Certification of results by examiner These records need not be retained after completion of the test if a certification by the inspector that the piping has satisfactorily passed pressure testing as required by this Code is retained. ASME B31.3 goes on to state, in Para. 346.3: Unless otherwise specified by the engineering design, the following records shall be retained for at least 5 years after the record is generated for the project: (a) Examination procedures; and (b) Examination personnel qualifications Standards that cover such a broad array of industrial manufacturing, do not, as a rule, attempt to get too specific in some of their requirements. Beyond the essential requirements, such as those indicated above, the owner, engineer or contractor has to assume responsibility and know-how for providing more specific and proprietary requirements for a particular project specific to the particular needs of the
Table 1. Areas under Consideration for Category D
owner. The following will help, to some extent, fill that gap.
Which fluid service category?
While Category-D fluid services qualify for initial service leak testing, there are caveats that should be considered. This is a situation in which ASME provides some flexibility in testing by lowering the bar on requirements where there is reduced risk in failure, provided that if failure should occur, the results would not cause catastrophic damage to property or irreparable harm to personnel. The owner’s responsibility for any fluid service selected for initial service leak testing lies in determining what fluid services to place into each of the fluid service categories: Normal, Category D, Category M, and High Pressure. Acids, caustics, volatile chemicals and petroleum products are usually easy to identify as those not qualifying as a Category-D fluid service. Cooling tower water, chilled water, air and nitrogen are all easy to identify as qualifyiers for Category-D fluid services. The fluid services that fall within the acceptable Category D guidelines, but still have the potential for being hazardous to personnel are not so straight forward. Consider water as an example. At ambient conditions, water will simply make you wet if you get dripped or sprayed on. By OSHA standards, once the temperature of water exceeds 140°F (60°C), it starts to become detrimental to personnel upon contact. At this point, the range of human tolerance becomes a factor. However, as the temperature continues to elevate, it eventually moves into a range that becomes scalding upon human contact. Human tolerance is no longer a factor because the water has become hazardous and the decision is made for you. Before continuing, a point of clarification. The 140ºF temperature mentioned above is with respect to simply coming in contact with an object at that temperature. Brief contact at that temperature would not be detrimental. In various litigation related to scalding it has been determined that an approximate one-second exposure to 160°F water will result in
Group
Description
1
Personnel occupied space
Yes
√
2
Corridor frequented by personnel
√
3
Sensitive equipment (MCC, control room, and so on)
4
Corridor infrequently used by personnel
√
5
Maintenance & operations personnel only access
√
third degree burns. An approximate half-minute exposure to 130°F water will result in third degree burns. And an approximate ten minute exposure to 120°F water can result in thirddegree burns. With the maximum temperature limit of 366°F (185.5°C) for CategoryD fluid services, what the owner needs to consider are three factors: (1) within that range of 140°F (60°C), the temperature at which discomfort begins to set in, to 366°F (185.5°C), the upper limit of Category-D fluids, what do we consider hazardous; (2) what is the level of opportunity for risk to personnel; and (3) what is the level of assured integrity of the installation Assured integrity means that, if there are procedures and protocols in place that require, validate and document third-party inspection of all pipe fabrication, installation and testing, then there is a high degree of assured integrity in the system. If some or all of these requirements are not in place then there is no assured integrity. All three of these factors — temperature, risk of contact and assured integrity — have to be considered together to arrive at a reasonable determination for borderline Category-D fluid services. If, for instance, a fluid service is hot enough to be considered hazardous, but is in an area of a facility that sees very little personnel activity, then the fluid service could still be considered as a Category-D fluid service. One factor I have not included here is the degree of relative importance of a fluid service. If a system failed, how big of a disruption would it cause in plant operation, and how does that factor into this process? For example, if a safety shower water system has to be shut down for leak repair, the downtime to make the repairs has little impact on plant operations. This system would therefore be of relatively low importance and not a factor in this evaluation process. If, on the other hand, a chilled water system has to be shut down for leak repair to a main header, this could have
No
√
a significant impact to operations and production. This could translate into lost production and could be considered a high degree of importance. You could also extend this logic a bit further by assigning normal fluid-service status to the primary headers of a chilled water system and assigning Category D status to the secondary distribution branches, then leak test accordingly. You need to be cautious in considering this. By applying different category significance to the same piping system it could cause more confusion than it is worth. In other words it may be more value added to simply default to the more conservative category of normal. Once it has been established that there is a high assured integrity value for these piping systems, there are two remaining factors to be considered. First, within the temperature range indicated above, at what temperature should a fluid be considered hazardous? Second, how probable is it that personnel could be in the vicinity of a leak, should one occur? For this discussion, let us determine that any fluid at 160°F (71°C) and above is hazardous upon contact with human skin. If the fluid you are considering is within this temperature range, then it has the potential of being considered a normal fluid, as defined in B31.3, pending its location as listed in Table 1. For example, if you have a fluid that is operating at 195°F (90.6°C), it would be considered hazardous in this evaluation. But, if the system is located in a Group 5 area (Table 1) it could still qualify as a Category D fluid service.
Leak test examples
After the above exercise in evaluating a fluid service, we can now continue with a few examples of leak test procedures. Using the designations given in Table 2, these leak test procedures will be categorized as follows: Testing Category T-1. T-1.1 — This category covers liquid piping systems categorized by ASME B31.3 as Category-D fluid service and
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51
Table 2. General Leak Testing Scenarios
Engineering Practice will require initial service leak testing only. 1. If the system is not placed into service or tested immediately after flushing and cleaning, and has set idle for an unspecified period of time, it shall require a preliminary pneumatic test at the discretion of the owner. In doing so, air shall be supplied to the system to a pressure of 10 psig and held there for 15 min to ensure that joints and components have not been tampered with, and that the system is still intact. After this preliminary pressure check, proceed. 2. After completion of the flushing and cleaning process, connect the system, if not already connected, to its permanent supply source and to all of its terminal points. Open the block valve at the supply line and gradually feed the liquid into the system. 3. Start and stop the fill process to allow proper high-point venting to be accomplished. Hold pressure to its minimum until the system is completely filled and vented. 4. Once it is determined that the system has been filled and vented properly, gradually increase pressure until 50% of operating pressure is reached. Hold that pressure for approximately two minutes to allow piping strains to equalize. Continue to supply the system gradually until full operating pressure is achieved. 5. During the process of filling the system, check all joints for leaks. Should leaks be found at any time during this process, drain the system, repair leak(s) and begin again with Step 1. (Caveat: Should the leak be no more than a drip every minute or two on average at a flange joint, it could require simply checking the torque on the bolts without draining the entire system. If someone forgot to fully tighten the bolts, then do so now. If it happens to be a threaded joint you may still need to drain the system, disassemble the joint, clean the threads, add new sealant and reconnect the joint before continuing.) 6. Record test results and fill in all required fields on the leak test form. T-1.2. — This category covers pneumatic piping systems categorized by ASME B31.3 as Category-D fluid ser52
vice and will require initial service leak testing. 1. After completion of the blow-down process, the system shall be connected to its permanent supply source, if not already done so, and to all of its terminal points. Open the block valve at the supply line and gradually feed the gas into the system. 2. Increase the pressure to a point equal to the lesser of one-half the operating pressure or 25 psig. Make a preliminary check of all joints by sound or bubble test. If leaks are found, release pressure, repair leak(s) and begin again with Step 1. If no leaks are identified, continue to Step 3. 3. Continue to increase pressure in 25 psi increments, holding that pressure momentarily (approximately 2 min) after each increase to allow piping strains to equalize, until the operating pressure is reached. 4. Check for leaks by sound or bubble test, or both. If leaks are found, release pressure, repair leak(s) and begin again with Step 2. If no leaks are found, the system is ready for service. 5. Record test results and fill in all required fields on the leak test form. Category T-3.1 — Hydrostatic Leak Test. T-3.1. — This category covers liquid piping systems categorized by ASME B31.3 as normal fluid service. 1. If the system is not placed into service or tested immediately after flushing and cleaning, and has set idle for an unspecified period of time, it shall require a preliminary pneumatic test at the discretion of the owner. In doing so, air shall be supplied to the system to a pressure of 10 psig and held there for 15 minutes to ensure that joints and components have not been tampered with, and that the system is still intact. After this preliminary pressure check, proceed. 2. After completion of the flushing and cleaning process, with the flush/test manifold still in place and the temporary potable water supply still connected (reconnect if necessary), open the block valve at the supply line and complete filling the system with potable water. 3. Start and stop the fill process to
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Category
Description
T-1
Initial service leak test
T-2
Hydrostatic leak test
T-3
Pneumatic leak test
T-4
Sensitive leak test
T-5
Alternative leak test
allow proper high-point venting to be accomplished. Hold pressure to its minimum until the system is completely filled and vented. 4. Once it is determined that the system has been filled and vented properly, gradually increase pressure until 50% of the test pressure is reached. Hold that pressure for approximately two minutes to allow piping strains to equalize. Continue to supply the system gradually until test pressure is achieved. 5. During the process of filling the system and increasing pressure to 50% of the test pressure, check all joints for leaks. Should any leaks be found, drain system, repair leak(s) and begin again with Step 1. 6. Once the test pressure has been achieved, hold it for a minimum of 30 min or until all joints have been checked for leaks. This includes valve and equipment seals and packing. 7. If leaks are found, evacuate system as required, repair and repeat from Step 2. If no leaks are found, evacuate system and replace all items temporarily removed. 8. Record all data and activities on leak test forms. The three examples above should provide an idea as to the kind of guideline that needs to be created in providing direction to the contractor responsible for the work.
Preparation
For leak testing to be successful on your project, careful preparation is key. This preparation starts with gathering information on test pressures, test fluids, and the types of tests that will be required. The most convenient place for this information to reside is the piping line list or piping system list. A piping line list and piping system list achieve the same purpose, only to different degrees of detail. On some projects, it may be more practical to compile the information by entire service fluid systems. Other projects may require a more detailed approach
by listing each to and from line along with the particular data for each line. The line list itself is an excellent control document that might include the following for each line item: 1. Line size 2. Fluid 3. Nominal material of construction 4. Pipe specification 5. Insulation specification 6. P&ID 7. Line sequence number 8. From and to information 9. Pipe code 10. Fluid service category 11. Heat tracing 12. Operating pressure 13. Design pressure 14. Operating temperature 15. Design temperature 16. Type of cleaning 17. Test pressure 18. Test fluid 19. Type of test Developing this type of information on a single form provides everyone involved with the basic information needed for each line. Having access to this line-byline information in such a concise, wellorganized manner reduces guess-work and errors during testing. Test results, documented on the test data forms, will be maintained under separate cover. Together, the line list provides the required information on each line or system, and the test-data forms provide signed verification of the actual test data of the test circuits that make up each line or system.
VALIDATION
The process of validation has been around for longer than the 40 plus years the author has been in this business. You may know it by its less formal namesakes walk-down and checkout. Compared to validation, walk-down and checkout procedures are not nearly as complex, stringent, or all inclusive. Validation is actually a subset activity under the umbrella of commissioning and qualification (C&Q). It is derived from the need to authenticate and document specifically defined requirements for a project and stems indirectly from, and in response to, the Code of Federal Regulation 29CFR Titles 210 and 211 current Good Man-
ufacturing Practice (cGMP) and U.S. Food and Drug Administration (FDA) requirements. These CFR Titles and FDA requirements drove the need to demonstrate or prove compliance. These requirements can cover everything from verification of examination and inspection, documentation of materials used, software functionality and repeatability to welder qualification, welding machine qualification, and so on. The cGMP requirements under 29CFR Titles 210 and 211 are a vague predecessor of what validation has become, and continues to become. From these basic governmental outlines, companies, and the pharmaceutical industry as a whole, have increasingly provided improved interpretation of these guidelines to meet many industry-imposed, as well as self-imposed requirements. To a lesser extent, industrial projects outside the pharmaceutical, food and beverage, and semi-conductor industries, industries not prone to require such in-depth scrutiny, could benefit from adopting some of the essential elements of validation, such as: material verification, leak-test records, welder and welding operatorqualification records, and so on. At face value this exercise would provide an assurance that the fabricating and installing contractor is fulfilling its contractual obligation. The added benefit is that, in knowing that this degree of scrutiny will take place, the contractor will take extra measures to minimize the possibility of any rejects. This is not to imply that all contractors are out to get by with as little as they can. Just the opposite is actually true. Most contractors qualified to perform at this level of work are in it to perform well and to meet their obligations. Most will already have their own verification procedure in place. The bottom line is that the owner is still responsible for the end result. No one wants to head for the litigation table at the end of a project. And the best way to avoid that is for the owner to be proactive in developing its requirements prior to initiating a project. This allows the specification writers and reviewers the benefit
of having time to consider just what those requirements are and how they should be defined without the time pressures imposed when this activity is project driven. Performing this kind of activity while in the heat of a project schedule tends to force quick agreement to specifications and requirements written by parties other than those with the owner’s best interest at heart. Validating a piping system to ensure compliance and acceptability is always beneficial and money well spent.
Final remarks
Before concluding this series of articles, there are just a couple of final points to be made.
Evolving standards
We have previously discussed industry standards and how they are selected and applied on a project [4]. What was not covered is the fact that most projects will actually have a need to comply with multiple industry standards. In a large grassroots pharmaceutical project you may need to include industry compliance standards for much of the underground utility piping, ASME B31.1 for boiler external piping (if not included with packaged boilers), ASME B31.3 for chemical and utility piping throughout the facility, and ASME-BPE for any hygienic piping requirements. These and other standards, thanks in large part to the cooperation of the standards developers and ANSI, work hand-in-hand with one another by referencing each other where necessary. These standards committees have enough work to do within their defined scope of work without inadvertently duplicating work done by other standards organizations. An integrated set of American National Standards is the reason that, when used appropriately, these standards can be used as needed on a project without fear of conflict between those standards. One thing that should be understood with industry standards is the fact that they will always be in a state of flux; always changing. And this is a good thing. These are changes that reflect updating to a new understanding, expanded clar-
Chemical Engineering www.che.com June 2008
53
Engineering Practice ification on the various sections that make up a standard, staying abreast of technology, and simply building the knowledge base of the standard. For example, two new parts are being added to the seven parts currently existing in ASME-BPE. There will be a Metallic Materials of Construction Part (MMOC), and a Certi-
fication Part (CR). This is all part of the ever-evolving understanding of the needs of the industrial community and improved clarification, through discussion and debate on content.
Conclusion
This series of articles attempted to cover a wide range of topics on in-
dustrial piping in order to provide a basic broad understanding of some key points, without going into great detail on any specific topic. It is hoped that the readers of this series will dig deeper into this subject matter to discover and learn some of the more finite points of what was discussed in this and previous articles. It is hoped that this series provides enough basic knowledge of piping for you to recognize when there is more to a piping issue than what you are being told. n Edited by Gerald Ondrey
Acknowledgement
My deep appreciation again goes to Earl Lamson, senior project manager with Eli Lilly and Co., for taking the time to review each of these articles. His comments help make the articles better documents than they otherwise would have been. He obliged me by applying the same skill, intelligence and insight he brings to everything he does. His comments kept me concise and on target.
References 1. Huitt, W.H., Piping for Process Plants: The Basics, Chem. Eng. February 2007, pp. 42–47. 2. Huitt, W.H., Piping for Process Plants: Flanges, Chem. Eng. March 2007, pp. 56–61. 3. Huitt, W.H., Piping for Process Plants: Design Elements, Chem. Eng. July 2007, pp. 50–57. 4. Huitt, W.H., Piping for Process Plants: Codes and Fabrication, Chem. Eng. October 2007, pp. 68–76. 5. Huitt, W.H., Piping for Process Plants: Installation and Cleaning, Chem. Eng. April 2008, pp. 48–58.
Author
54
Circle 27 on p. 86 or go to adlinks.che.com/7373-27 Chemical Engineering www.che.com June 2008
W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. (P.O. Box 31154, St. Louis, MO 63131-0154. Phone: 314966-8919; Email:
[email protected]) a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp and paper, nuclear power, and coal gasification. He has written numerous specifications including engineering and construction guidelines to ensure that design and construction comply with code requirements, owner expectations and good design practices. Bill is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a contributor to ASME-BPE and sits on two corporate specification review boards.
Feature Report Engineering Practice
Stress Analysis for Piping Systems Resting on Supports Avoid erroneous analysis that may result from using computer programs Liang-Chuan Peng Peng Engineering
P
iping-flexibility and stress analysis are required in the design of most piping systems before the piping is installed in a chemical-process or other plant. It is intended to ensure the safety of the plant and thus protect the interests of the owner and the general public. Owing to the availability of powerful computer software packages, the analysis has become simple and routine. However, due to misconceptions of some software approaches, some analyses performed do not reflect the actual situation of the piping. The engineers have unwittingly performed many erroneous analyses that put the safety of the plants in jeopardy. One of the most common misconceptions adopted by some computer programs is the method of analyzing the piping with regards to resting supports. The most common and economical approach in dealing with the numerous lines of piping in a process plant is to 48
rest the piping on pipe racks or other support structures. The piping is either supported directly on the pipe wall or through pipe shoes, which are attachments placed under the pipe to distribute the pipe weight and other loads to the support. These types of supports are generally called resting supports. These supports are single-acting, because they only stop the pipe from moving downward but allow the pipe to move up freely. Due to the nature of this non-linearity, exact solutions cannot be expected for piping that goes through various temperature cycles. Therefore, three major schools of thought have been conceived in the pipe-stress-software community with regards to resting supports and temperature cycles. Unfortunately, two of these are in apparent violation of the code requirements. As a basis for explaining how these computer methods violate the code, the requirements of the code are summarized in the following.
CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2006
ASME code
ASME B31 code for pressure piping is an American National Standard. It also becomes a safety code when adopted by federal, state, or local governments. Nowadays, most nonnuclear piping systems in the U.S. and in many other countries are designed according to ASME B31 code. Among many other things, the code requires that the piping shall be designed to meet the limitation of the following categories of stresses. a. Internal pressure stress, Shp: Stresses due to internal pressure shall be less than the basic code allowable stress, including longitudinal joint efficiency, of the pipe at design temperature, Sh(E). b. Longitudinal (sustained) stress, SL: The sum of longitudinal stresses due to pressure, weight, and other sustained loadings shall not exceed the allowable stress, excluding longitudinal joint efficiency, at design tempera-
NOMENCLATURE E Longitudinal-joint efficiency of the pipe e Strain f Stress-range-reduction factor, f =1.0 for 7,000 or fewer operation cycles
S Stress SA Allowable stress range for thermal expansion and displacement stresses Sc Basic allowable stress of the pipe material at ambient (cold) temperature
Sh Basic allowable stress of the pipe material at operating (hot) temperature
The stress range due to thermal expansion of pipe, movements of restraints, earthquake or wind sway, tidal movement, and temperature change in connected equipment shall not exceed the allowable displacement stress range SA defined as follow
�� ���������
SE Expansion stress range
SEC Expansion stress at cool-down (cold) state
��
SA = f.(1.25Sc + 0.25Sh)
�
�
��������� �
���
where Sc is the basic allowable stress at minimum metal temperature expected during the displacement cycle under analysis. When Sh is greater than SL, the difference between them may be added to the above. That is SA = f.[ 1.25.Sc + 0.25.Sh + (Sh – SL)] = f.[1.25.(Sc + Sh) – SL ]
FIGURE 1. Any kind of significant stress will cause some yielding or relaxation in the pipe. Shown here is an example of reverse expansion stress due to yielding (see text)
ture, Sh. The sustained longitudinal stress normally consists of only the longitudinal pressure stress, Slp, and the weight bending stress, Slw. In this case, the code requires that Slp + Slw < Sh
(1)
To get an idea of the weight stress limitation, Slp can be taken as 0.5.Sh (one half of the hoop stress). Thus, Equation (1) becomes Slw < 0.5.Sh
(3)
(2)
c. Stress due to occasional loads, Soc : The effects of pressure, weight, other sustained loads, and occasional loads including earthquake and wind, shall not exceed k.Sh. The k value varies from 1.15 to 1.33 depending on the duration of loading and the type of plant. Wind and earthquake forces need not be considered as acting concurrently. d. Displacement stress range, SE:
(4)
Longitudinal joint efficiency need not be included. Sh is the basic allowable stress at maximum metal temperature expected during the displacement cycle under analysis. Longitudinal joint efficiency need not be included. f is a stress range reduction factor, which is equal to 1.0 for 7,000 or less cycles. The displacement stress range is traditionally called the thermal-expansion stress range. For most applications, the number of cycles expected is much less than 7,000. Therefore, for discussion purposes f can be taken as unity. That is SA + SL = 1.25.(Sc + Sh)
(5)
Since SE is limited to SA, the code requires that the total stress (SE + SL) shall satisfy the following (SE + SL) < 1.25.(Sc + Sh)
(6)
The total stress includes mostly cyclic stresses such as expansion stress from ambient to operating temperature, pressure stress from zero to operating pressure, and weight stress from empty to full and also from cold to hot. At low to moderate tempera-
SL Sustained longitudinal stress Slp Sustained longitudinal stress due to pressure Slw Sustained longitudinal stress due to weight Sy Yield strength of the pipe material
tures where the yield strength governs the allowable stress, 1.25.(Sc + Sh) is roughly equivalent to 1.56. Sy for ASME B31.1 code, and 1.67. Sy for ASME B31.3 code, where Sy is the yield strength of the pipe material. This shows that the allowable value for the calculated total stress is over 1.5.Sy for 7.000 cycles of operation. Furthermore, since the stress intensification factor for ASME B31 code is only one half of the theoretical value, the actual total local stress limit is greater than 3.0.Sy for 7,000 cycles of operation. From this brief deduction, it is clear that with any kind of significant stress, some yielding or relaxation in the pipe will occur. Figure 1 shows the situation when the pipe is stressed beyond the yield point and to Point A. In this situation, the actual stress remains the same as the yield stress Sy. However, the calculated stress is the elastic equivalent stress SE which is used in the coderequirement evaluation. The pipe will stay at Point A throughout the operation or it might relax to a lower stress point. Assuming the relaxation does not occur, the pipe will cool down to Point B at ambient condition. This produces a reverse-expansion stress at the cold condition. This stress reversal is very important in the evaluation of the analysis method. In order to make the matter less confusing, only the sustained stress and displacement stress range will be discussed in this paper. From the above summary of ASME B31 code requirements, it can be concluded that a piping system should be designed so the following stress conditions are satisfied: • The sustained stress due to weight, pressure, and other sustained loads, shall be smaller than the yield strength of the pipe, to avoid gross
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�������������� ���������
Engineering Practice deformation. It should be noted that the sustained stress is often limited by the creep strength of the pipe at higher temperatures • The sum of the sustained stress and displacement stress range, among other things, can reach as high as 1.5 times the yield strength. If the pipe involves a stress intensification factor, this sum can go as high as 3.0 times the yield strength The reason that the allowable displacement stress can go higher than the yield strength is because the displacement stress is self-limiting in nature. Although it is allowed to exceed the yield strength, once the amount of applied displacement is reached the movement is stopped and the expansion will go no further. This kind of self-limiting stress will not cause failure in one application. Therefore, the failure mode of displacement stress is fatigue through many cycles of repeated operations. Also, because the displacement stress normally relaxes due to yielding, creeping, or temperature normalization, the initial stress value will be greatly reduced after a certain period of operation. The sign of the stress will then reverse when the piping is cooled down to ambient temperature. Therefore it is important to note that the initial stress has very little meaning for the displacement load. The important measure is the potential maximum strain range expected.
An example
A simple example demonstrates the merits and pitfalls of some analysis approaches designed to satisfy the code requirements and philosophy. Figure 2 shows a typical piping system resting on a support structure. The piping has one end connected to a process tower and the other end connected to another piece of process equipment. The piping is supported at three locations. As the temperature of the process fluid increases, the tower expands upward and the pipe expands. With the tower connection gradually moving upwards, the piping system also goes through the following sequence of changes: • With a small movement, the piping is held down on all supports by the weight of the piping including fluid 50
������������ ���������� ���������
�� ��
��
������������� ���������
FIGURE 2. Shown here is a typical piping system on resting supports. The picture is a 2D isometric plot of a pipe with a horizontal bend. Care must be taken to account for for the piping lifting off the supports (see text example)
and attachments. Some thermal expansion (displacement) stress is generated, but the weight stress remains the same as in the cold condition • As the movement increases somewhat, the piping will lift from the first support, support 20. A further increase of the tower movement will lift the pipe off support 30, thus making a large portion of the piping unsupported. This increases greatly the sustained weight stress • As the system reaches the maximum operating temperature, the tower connection moves up some more, but the pipe is still being supported at support 40. The expansion stress increases, corresponding to the larger movement. The sustained weight stress remains the same as no additional piping is lifting off its support • If the process system is held at this maximum operating temperature for a period of time, the thermal expansion stress (displacement stress) will be relaxed somewhat. The amount of relaxation depends on the stress level and the operating temperature. However, the sustained weight plus pressure stress remains unchanged • When the plant cools down, the piping moves back on to its supports. This reduces the weight stress to its initial cold-condition weight stress. The system, however, generates some reverse thermal expansion stress due to relaxation at operating temperature. • If considerable yielding or creep occurs at hot condition, the pipe may return to the support point while the temperature is still considerably higher than the ambient temperature. A continued cooling down to ambient temperature will cause high thermal stresses and loads due
CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2006
to stoppage of the support that prevents the pipe from moving further down • In the next operating cycle, the weight stress goes back to the hot condition stress sustained, but the expansion stress will be reduced to a level corresponding to the relaxed state Three main approaches have been adopted by computer software packages in dealing with systems like the one shown in Figure 2. General, straightforward approach: This is an approach commonly adopted by general-purpose, finite-element programs. In this approach, the sustained stress and the expansion stress will be calculated separately without checking the influence of the one on the other. The sustained stress is calculated considering only the weight and pressure loads at the ambient state. All supports are considered active, as no temperature and support displacement is involved. The expansion stress range is calculated only with the temperature change. No weight influence is considered. If the pipe lifts off from the support due to temperature, it is considered inactive for the expansion analysis. This approach may mishandle both the sustained and expansion stresses. First, the sustained stress calculated is the stress at ambient condition. The most important sustained stress at the hot condition is not calculated. Secondly, the expansion stress may be under estimated, because the restraining effects of the supports, over which the piping is held down by weight, are ignored. Algebraic-subtraction approach: In this approach, the sustained stress is calculated considering only the weight and pressure loads at ambi-
ent state. All supports are active, as no temperature and support displacement is involved. The expansion stress range is calculated by subtracting algebraically the temperature-plusweight condition (hot operating condition) minus the weight condition at ambient state (initial cold condition). Three major issues are at stake in this approach. First, the sustained stress calculated is the stress at ambient condition; the most important sustained stress at the hot condition is not calculated. Secondly, this approach tries to include the cyclic weight stress range, changing from ambient to hot conditions, in the expansion stress range. This is not consistent with the code philosophy of separating sustained stress from self-limiting expansion stress. Furthermore, the cyclic sustained stress involves not only the weight stress change from ambient to hot, but also the pressure stress change from zero to operating pressure, and the initial weight stress change from empty to full. Thirdly, the stress for the temperature-plus-weight condition depends greatly on the signs of the moments of the two loads included. If the moment of the weight change is in the opposite direction of the moment of the temperature change, the calculated expansion stress will be smaller than that calculated by the temperature change alone. This is not correct, as relaxation can change the sign of the expansion stress during the course of operation. It is important to note that the stresses involved in Equations (5) and (6) are to be added absolutely. Operating-condition approach: In this approach, all supports are checked at the operating condition, which normally involves temperature plus weight and pressure. If the pipe lifts off from a support at operating condition, that particular support is then treated as inactive for both the sustained weight plus pressure stress and the expansion stress calculations. By the same token, if the pipe is weighted down on a support at operating condition, that support is treated as active for both sustained and expansion stress calculations. With this method the sustained stress and the expansion stress are calculated inde-
pendently once the activity of the supports is determined. The sustained weight plus pressure stress calculated with this approach is the true sustained stress at hot operating condition, when the stress is high and the pipe is weak. The one thing that may appear to be improper to some inexperienced analysts is the weight displacement that may show a downward movement at support locations. This downward displacement represents only the movement of pipe from a thermally lifted condition. At support locations, the operational displacements combining weight and temperature will be either zero or in the upward direction. The expansion stress calculated is the potential stress range, recognizing that the sign and the magnitude may change throughout the operating cycles. This expansion stress is combined absolutely with the sustained stress in the evaluation of the total stress given by Equation (6). From the above discussions, it is obvious that the third (operating-condition) approach is the only method that meets the code philosophy and requirements. The other two approaches all have flaws in calculating the sustained stress and the expansion stress range.
Final remarks
Analysis of the piping resting on supports is nothing new. Engineers have analyzed this kind of piping routinely for more than two decades. The erroneous concepts of some computer software packages and the blind acceptance of computer results by engineers, however, are new. Attracted by the glamorous nature of thermalflexibility analysis, many engineers have forgotten that sustained stress is much more important than expansion stress. Sustained stress is the primary stress, whereas the expansion stress is a secondary stress. From a comparison of Equations (2) and (3), it is clear that sustained weight stress is much more critical than the expansion stress. At low temperatures, when the hot allowable stress has the same value as the cold allowable stress, the weight allowable stress limit is only about one-third of the expansion-stress allowable limit. At higher operating
temperatures in the creep range, the weight allowable stress limit can be as low as only one tenth of the expansion allowable limit. Therefore, it is important to note that the first priority of the analysis is to accurately determine the sustained weight stress at hot operating condition. This is not to say that expansion stress is unimportant. A good analysis shall calculate as accurately as possible both sustained and expansion stresses. It should be noted that by calculating weight stress at cold condition, the analysis result is not expected to indicate where a spring support is needed. It is only when the weight stress at hot operating condition is calculated that the engineer will be able to detect when a spring support is needed. A spring support is made from precompressed coil springs; as the pipe moves up and down, the spring is stretched or compressed, causing the load to change. With a properly selected spring support, the pipe is always well supported, regardless of any up-ordown movement. Spring supports are thus used to reduce the weight stress at hot operating condition. The operating-condition approach may be somewhat conservative for pipes that only lift up a very small amount from the support. In this case, the rule of thumb is to consider the support double acting to check both sustained and expansion stresses. If both stresses are within the code allowable, then the system should be ■ considered as acceptable. Edited by Gerald Ondrey
Author Liang-Chuan Peng, P.E., is the president of Peng Engineering (3010 Manila Lane, Houston, Tex. 77043; Phone: 713-462-7390; Fax: 713-4626930; Email: Lcpeng@aol. com). Previously, he has been employed by M.W. Kellogg, Foster Wheeler, Brown & Root, Bechtel, Taiwan Power and others. Peng has over 35 years of experience in piping-stress analysis and engineering. He is the original co-author of NUPIPE software, and has developed the PENGS and SIMFLEX series of pipe-stress-analysis computer programs. Peng has performed troubleshooting on piping systems and taught piping-engineering seminars in more than a dozen countries. He has published 18 technical papers on piping engineering. He earned a M.S. in mechanical engineering from Kansas State University. Peng is a member of ASME and a registered professional engineer in Texas and California.
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Feature Report Engineering Practice
A Method for Quantifying Pipe Vibrations Simply supported pipe
A technique to quantify vibration forces can help prevent pipe failures due to vibration-induced fatigue S. Saha Reliance Refinery
F
ailures of piping due to vibrationinduced fatigue are a serious problem in the chemical process industries (CPI) and a matter of concern for the safety and reliability of plant operations. Due to the complexity of flow-induced vibrations in pipes, no closed-form design solutions — those that can be expressed in terms of wellknown functions — are available. In this article, we present a method for quantifying vibration forcing functions for the optimal design of metal piping systems in the CPI, as well as an example of its use. The method is an analytical technique based on the theory of vibrations in the frequency domain (Inverse Theory of vibrations). The method can be easily adopted by practicing engineers.
vibration measurement
Piping systems experience various vibratory loads throughout their lifecycles. If not controlled, these pipe vibrations will lead to fatigue failures at points of high stress intensity and can even damage pipe supports. These failure scenarios could result in plant outages or in more severe consequences, such as fire or loss of human life. Thus, it is imperative that piping systems be safeguarded against such failures. To avoid fatigue failures in piping systems, engineers carry out dynamic analyses of vibrations during a design adequacy check for a piping system. The major difficulty in dealing with the vibration problems lies in estimating the forcing function. If the exciting forces 46
ML
MO
U X
L
Figure 1. The span of pipe between two supported points can be measured for vibrations
acting on the pipe can be Vessel quantified precisely, the nozzle system response can be determined with great Compressor accuracy by the existing discharge piping analytical methods. But unfortunately, this is not Compressor nozzle B readily possible in most cases, since the vibrations A in an operating pipeline are flow-induced. Region of failure The complexity of flow patterns and the mechanism of force-coupling Figure 2. The mid-point between two supported render the determination points on a pipe is often where vibration failure occurs of the forcing function extremely difficult. In such a scenario, petroleum refining and petrochemical data — in the form of field vibration industries, these charts are used exmeasurements in conjunction with tensively. However, they are typically analytical methods — can provide a found to yield conservative estimates. basis for estimating the dynamic force Another widely used tool is the and stress [1–3]. ASME OM Code [5] — a standard folIn our method, we analyze the prob- lowed for piping in the nuclear power lem in terms of the theory of vibrations industry. Here the vibration velocity in the frequency domain. We present a for a piping span between two nodes simple numerical technique that can is the criterion. The limiting value for be easily built into any of the common pipe-vibration velocity is determined spreadsheet computer programs with by an empirical relationship, which the help of macros. involves coefficients that depend on several parameters, such as weld arrangements, mass lumping, and othCurrent vibration approaches The current practice for exploring ers. When the peak value for the vepipe vibrations is the vibration screen- locity is less that 12.7 mm/s, it may be ing criteria method. In this method, assumed that the piping has sufficient vibration response parameters, such dynamic capacity. If the vibration exas velocity or displacement, are mea- ceeds this level, however, the ASME sured in situ and compared against guide recommends reviewing the visome established acceptance criteria, brations with more information on the usually in the form of graphs known potential causes and taking steps to as vibration severity charts [4]. In the reduce vibration levels.
Chemical Engineering www.che.com May 2012
FFT Plot for Velocity Modulus coefficient, mm
Pk velocity, mm/s
60 50 40 30 20 10 0
8
Stress distribution
L
0.02
With error Base curve
5,000
4 2
Fig. 5: 5. Stress Distribution. Figure The vibration stresses exceed the endurance limit of the piping
We have observed the above methods to be conservative and to provide a “cookbook” or a “go/no-go” approach. They tell us only whether or not the vibrations are within acceptable levels. It is not possible to generate a quantitative estimate of the forcing function and of the actual stress levels on the pipes, both of which are essential for a design adequacy check. We studied the problem within the framework of Inverse Theory. We will focus on steady-state vibrations, because they have been found to cause maximum damage.
Proposed method
Theoretically, for a simply supported pipe, the response at any location along the span may be determined by the vibration measurements at two distinct points in the span. The span is a straight portion between two fixed points or supports (Figure 1). A single point measurement near the mid-span is also sufficient. Further mathematical details are included in the second part of this article. The measurements could be realtime displacement, velocity or acceleration with the post-processed fast Fourier transform (FFT) plots. The calculations are straightforward and amenable to simple spreadsheet programming with macros.
4,000
End reactions
Differential operator
ω
Circular frequency
σ
Stress
m
Mass per unit length
U
Displacement of pipe
Û Fourier transform of U
Reaction @x=L Reaction @x=0
k
Wave number
3,000
FFT Fast Fourier transform
2,000
R
Reaction force
1,000
EI
Bending modulus
j
Square root of –1
0
180 184 188 192 196 200 204 208 Frequency, Hz
Length of pipe span
x Distance along the span
6,000
6
0
Nomenclature
0.04
Figure 4. The peak at 200 Hz indicates the excitation frequency due to compressor pulsations
Force, N
Modulus stress, MPa
10
0.06
180 184 188 192 196 200 204 208 Frequency, Hz
Figure 3. A fast Fourier tranform plot for a mid-span point shows high vibration speed
12
A B C D
0.08
0 180 184 188 192 196 200 204 208 Frequency, Hz
14
Coefficients
0.1
70
180 184 188 192 196 200 204 208 Frequency, Hz
FIGURE 6. After error is introduced, the variation in the solution is similar to maximum error
Steps for implementation
The following are the steps needed to implement the method. The notations and equations mentioned in the steps are shown on p. 49. 1. Identify the pipe span in which the vibration is severe. 2. Take velocity readings at two points in the span. One of the points should preferably (but not necessarily) be the mid-span. The measurements can be made using any portable handheld accelerometers or realtime velocity-measuring devices. Finally, the time history readings are to be converted into FFT plots as output (a part of the post-processing features of the measurement devices). 3. As explained on p. 49, construct matrix G of size 4 × 4, as in Equation (10). The elements of the matrix are based on the material and the damping properties of the material. 4. Note that the matrix elements are complex quantities having real and imaginary parts. 5. Construct the vector V using Equation (12). The first two elements are 0; the remaining two are the measured FFT responses at the two points obtained from Step 2. The elements of V are also complex quantities. 6. The coefficients (A,B,C,D) are obtained as a solution vector X from Equation (13). As the quantities are
(
)T
Vector norm
η
Loss factor
Z
Section modulus
complex, a suitable complex matrix solution routine is used. 7. The displacements at any location in the span can be calculated with the help of the coefficients using Equation (5). 8. The stresses and end reactions are calculated from Equations (14)–(16). 9. Repeat the procedure for a range of frequencies. The frequencies chosen should cover the peaks of maximum response. 10. From the above, the frequency variation of the output parameters, such as stress, and the reactions, are obtained. These responses are combined to obtain the results for stress and end reactions (for example, Equations (15)–(16)). Resultant values may be compared with those allowable, as an adequacy check. For example, the endurance limit may be considered as the allowable for the stresses for fatigue evaluations. The support member may be checked for the dynamic reactions. 11. If the response parameters are within allowable limits, terminate the procedure. Otherwise, make a modification based on engineering judgment, and repeat the procedure. The numerical tool required is a simple matrix-solution routine for complex quantities. Such modules are readily available or may be easily
Chemical Engineering www.che.com May 2012
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Example problem
0.035 0.03 0.025 0.02
Displacement plot Benchmark Present method
0.015 0.01 0.005 0
180 184 188 192 196 200 204 208 Frequency, Hz
6,000
Reaction at x = 0 Present method
4,000 Reaction, N
programmed using macros available in a standard spreadsheet.
Displacement, mm
Engineering
0.04
Benchmark
2,000
0
End Reaction)
180 184 188 192 196 200 204 208
Frequency, Hz The method has been applied to vibrations in the dis- Figure 7. Results of the proposed method FIGURE 8. The proposed method requires no information on natural boundary conditions charge piping (8 in. nominal match those using finite element analysis bore) leading from a refinery fuel gas (RFG) screw compressor up to As a check, a direct solution the oil separator. Figure 2 shows the (benchmark) based on calcumodel for numerical simulation. The lated end moments was obrotor frequency is around 3,000 rpm. tained through finite element Heavy vibrations, along with failures, analysis (FEA) by standard in the small-bore connections have commercial software. The rebeen reported. The goal was to study sults show a close match with the problem and provide a solution for those of the proposed method reducing vibration levels and prevent- (Figures 7 and 8). ing such failures in the future. Figures 5 and 6 show the Vibration measurements were results after the introduction taken at the points of failure. An FFT of the error. The variation in displacement plot of a point in the the solution is about the same mid-span is shown in Figure 3. There order of magnitude of the FIGURE 9. A view of the final configuration of the is a peak at 200 Hz (that is, four times maximum error, which is also piping shows additional pipe supports the running speed), which is typical in agreement with the theory. of screw compressor pulsations. The A distinguishing feature vibration velocity is around 62 mm/s, of this method is that no inwhich is much higher than the ASME formation is required on the limit of 12.7 mm/s [5]. Hence for a com- natural boundary conditions prehensive design check, the actual (BCs). This is remarkable stresses and the support reactions are since in the direct theory, the New support required. Also, there is no excitation solution depends on the BCs, source of forces in the span. The exci- whereas in this inverse probtations are by the end moments. lem, the BCs do not play a role. This is also significant in the sense that practically, it Numerical simulation FFT plots of displacements at points is almost impossible to assess 0.25 and 0.5 of the pipe span have the true support conditions. In order to reduce the FIGURE 10. New supports can be added to rebeen considered as inputs. As the quantities are complex, both modulus stresses, the modes around duce vibration stresses and phase were required. From Equa- the observed frequency of tion (14), the coefficients are solved. 200 Hz were identified. The modes Final assessment The plots of coefficients A to D are were then iteratively shifted by means Vibration failure in operational pipshown in Figure 4. On their basis, of additional restraints. The end mo- ing is a serious problem that requires the response (the stress and end reac- ments were applied to determine the comprehensive study and analysis tions) were calculated (Figures 5 and stresses and the reactions. The final to solve. In this sense, the proposed 6). As a part of the error analysis, a configuration was achieved by further method has tremendous practical random error with a peak magnitude fine-tuning considering practical con- value. A quantitative method with of 1% was introduced into the mea- straints. Figures 9 and 10 show the proper mathematical basis has been surements. The exercise was repeated final configuration of the piping. provided as an alternative to the Vibration readings were again cookbook approach. and the resultant plots are also shown taken after the implementation of the in Figures 5 and 6 for comparison. The method provides a basis for a recommendations (Figure 11). The proper engineering design, and can be maximum reported vibration velocity easily adopted by engineers involved Reduction of vibration stress The plots (Figures 4–6) show peaks is around 5 mm/s. The results show in troubleshooting. It should be acat 200 Hz, which is the excitation fre- a drastic reduction in the vibration knowledged, however that troublequency due to pulsations generated levels, which proves the success of shooting vibrations in plant piping is by the compressor. The stresses are the resolution and vindicates the pro- the job of a specialist with experience in this field. high and exceed the endurance limit. posed method.
48
Chemical Engineering www.che.com May 2012
FFT Plot for Velocity Peak velocity, mm/s
70 60 50
Initial Final
40 30
pressed as:
20 10
0 180 184 188 192 196 200 204 208 Frequency, Hz
FIGURE 11. After modification, the maximum vibration velocity was reduced drastically
Mathematical background
The basic pipe configuration is shown in Figure 1. Considering the Bernoulli-Euler formulation and structural damping, the dynamic equation of motion in the frequency domain [6] is as follows:
(9)
GX = V
where G is the matrix
(10)
(11) (12)
(2)
Here, X is the solution vector and V is the vector comprising the displacement measurements (from the FFT) at points x1 and x2 in the span. It can be observed that the determinant of G is nonzero. Hence, G is invertible and X can be solved uniquely as:
(5) The complex coefficients A,B,C and D are independent of x, but dependent on ω. The first two terms of Equation (5) represent travelling waves from the left and right ends respectively. The last two terms represent evanescent waves that rapidly decay away from the boundaries. The complex wave number k may be expressed as follows [7]: (6)
The complex coefficients can be obtained by the following matrix system:
(1)
(3) Equation (1) pertains to steady-state vibrations with the frequency dependence on ω. Here, the variables Û, M0, ML are complex, arising out of the Fourier Transform. BCs (Equation (3)) imply that the excitation at the ends is by moments, which is the source of vibration of the pipe in this span. The damping component has been expressed in terms of the loss factor η [7], which is a function of ω. The solution of Equation (1) (which is also termed a wave solution [6,7]) can be written as:
(8)
(7)
Here, kre is the wave number for the undamped case and kim may be ex-
X = G–1V
(13)
After the coefficients are obtained, other response quantities like velocity and stress can be computed. For stress, we have the expression as follows:
(14)
The stress function is a complex quantity and has a continuous dependence on frequency, which varies theoretically from −∞ to ∞. However, for practical purposes, the response is dominated by some finite number N modes or frequencies. We can define the total stress as the square root sum of squares (SRSS) combination of the individual components.
(15)
Here, σi= σ(x,ωi). Because the quantity is complex, the modulus has been used for the combination. In the same vein, the end reactions may be obtained as follows:
(16)
The SRSS method has been used for the computation of the resultants for stress and the reaction forces. This method is simple, reasonably accurate and also widely used. Alternatively, for a more rigorous analysis, other combinations for cumulative fatigue evaluation, such as the rain-flow counting method or the more recent Dirlik’s method [8] may be used. The number of measurement points may be reduced to one. This is because of the exponential terms in the matrix G. One of the coefficients, C or D, becomes negligible and we are left with three coefficients. ■ Edited by Scott Jenkins
References 1. Saha, S. Estimation of Point Vibration Loads for Industrial Piping. Journal of Pressure Vessel Technology, Vol. 131, 2009, ASME, New York. 2. Moussa, W.A., Abdel Hamid. A.N. On the Evaluation of Dynamic Stresses in Pipelines Using Limited Vibration Measurements and FEA in the Frequency Domain. Journal of Pressure Vessel Technology, Vol. 121, 1999, ASME, New York. 3. Dobson, B.J. and Rider, E., A Review of the Indirect Calculation of Excitation Forces from Measured Structural Response Data. Jour. Mech. Eng. Sci. 204, 1990. 4. Wachel, J.C. Piping Vibration and Stress, Proc. Machinery Vibration Monitoring & Analysis, Vibration Institute, USA, 1981. 5. ASME –OM. Code for Operation and Maintenance of Nuclear Power Plants, ASME, New York, 2004. 6. MacDaniel, J. and others, A Wave Approach to Estimating Frequency-Dependent Damping Under Transient Loading, Journal of Sound and Vibration, Vol. 231, 2000. 7. Goyder, H. Method and Applications of Structural Modeling from Measured Structural Frequency Response Data. Journal of Sound & Vibration. Vol.68(2),1980. 8. Dirlik, T., Ph.D. Thesis. Application of Computers to Fatigue Analysis, Warwick University, 1985.
Author S. Saha is is currently the head of the piping engineering dept. at Reliance Refinery (Jamnagar, India; Email:
[email protected]). His area of specialization is finite element analysis (FEA), as well as stress and dynamic analysis of mechanical and structural systems. He has wide consultancy experience in piping design for the refinery, petrochemical and power (both nuclear and conventional) industries. Dr. Saha holds a B.Tech. (Hons.) degree in mechanical engineering from the Indian Institute of Technology (Kharagpur, India) and a Ph.D. from the Indian Institute of Technology (Kanpur, India). He has several publications in international journals and conferences.
Chemical Engineering www.che.com May 2012
49
Solids PristineProcessing Processing
New Piping Code for High-Purity Processes Simplified bioethanol process diagram
It is essential that industry codes, standards and regulations keep up with evolving technology and changing demands of the chemical process industries
Biomass feedstock
Enzyme production Distribution
Pretreatment
Cellulose hydrolysis (saccharification}
William M. Huitt W.M. Huitt Co. Barbara K. Henon representing Arc Machines, Inc. Vicencio B. Molina III Air Products and Chemicals, Inc.
I
n the emerging and ever-expanding areas of bioprocessing, where maintaining hygienic designs and practices is of paramount importance, and semiconductor manufacturing, which has its own stringent purity requirements, there is a need to standardize the essential codes and standards that are available. The goal is to consistently achieve process systems that meet the highly refined cleanliness and cleanability requirements that these industries demand. In addition to cleanliness and cleanability requirements, process operators must integrate safety into all high-puritydesign philosophies and standardization efforts. This article discusses the importance of, and need for, engineering codes and standards that govern the design of high-purity process piping systems. The focus of this article is the new Chapter X (High Purity Piping) that is found in the 2010 issue of the American Society of Mechanical Engineers (ASME) B31.3 Process Piping code. This chapter deals mainly with the bioprocessing and semiconductor industries, but also includes a sub-
Glucose fermentation
Pentose fermentation
Ethanol product
Lignin utilization
Figure 1. CPI manufacturing involving biological or biochemical processes requires high-purity system design that provides an environment that is conducive to desired bacteria while preventing, through its integrated cleanability, any unwanted bacterial contamination
set of bioprocessing-related industries, such as pharmaceuticals manufacturing, biofuels production, food-anddairy production and others.
Evolving purity requirements
Early on (in the 1920s), the food-anddairy industry, through the cooperative effort of the International Association of Food Industry Suppliers (IAFIS; now the Food Processing Suppliers Assn.), the International Assn. for Food Protection (IAFP), and the Milk Industry Foundation (MIF) — formed the 3-A Sanitary Standards organization, or simply “3-A SSI.” 3-A SSI was instrumental in establishing the first set of standards, protocols and methodologies to ensure that this industry could produce food products on a repeatable basis that were free from pathogenic bacteria. Such bacteria are potentially derived from contaminated piping systems as a result of an inadequate cleanability design, an insufficient cleaning regimen, or cross contamination of dissimilar products. Until the late 1990s, the food-industry standards that were initiated by
3-A SSI were widely utilized by two other industry sectors — pharmaceuticals and semiconductors — that both require a particularly high degree of purity throughout their processes and utility systems, but for very different reasons. The pharmaceutical industry, like the food-and-dairy industry, expends great effort to design, install and maintain its process systems to ensure a high degree of hygienic purity. In general, process systems used by pharmaceutical manufacturers require added care and documentation during both the manufacture of individual components that make up these systems, and the fabrication and installation of the complete systems. While the pharmaceutical and food-and- dairy industries both require high degrees of cleanliness, they each have their own differing set of guidelines on how to achieve and maintain the desired cleanliness. Piping used throughout the semiconductor industry, on the other hand, requires a degree of purity that is even higher than that required of the pharmaceutical, food-and-dairy
Chemical Engineering www.che.com July 2011
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Pristine Processing industries, but for altogether different reasons. During semiconductor manufacturing, bacterial contamination is not the driving consideration that it is in the food-and-dairy, and pharmaceutical industries. Rather, semiconductor operations have a critical need to mitigate the potential for particulate contamination, which can be devastating to today’s highly miniaturized electronic components. Microscopic particles in semiconductor facilities, whether coming from equipment, tubing, or the various fluids used during the manufacture of silicon chips, can render the chip useless, or at the very least, out of specification. In the face of such exacting purity requirements, the widely used ASME B31.3 Process Piping code proves its adaptability, in terms of keeping pace with changing technology demands across these varied industry segments. Three primary segments — food-anddairy, semiconductor and pharmaceutical — have served as initiators and proponents during the development of standards to meet the needs of their respective industries. In particular, the industry-specific standards developed by 3-A SSI, Semiconductor Equipments and Materials International (SEMI), and ASME’s Bioprocessing Equipment (BPE) Committee led the way in establishing criteria for the high-purity component design, system design, fabrication and installation requirements to meet the needs of these specific industry sectors. While individual, industry-specific codes are in place, the ASME B31.3 piping code is also relevant to all of these industries, as it establishes engineering practices to ensure piping system integrity and safety. While the industry-specific standards define the particular degree of purity and cleanability required in those sectors, and establish the component and system designs needed to meet those requirements, ASME B31.3 has recently expanded its content to incorporate requirements that establish structural integrity and safety parameters for high-purity applications. In order to meet these integrity and safety requirements, the reader or user of an industry-specific standard can now be referred to the ASME 50
B31.3 Process Piping Code, and more specifically to its latest Chapter X. In an effort to harmonize its efforts and dovetail seamlessly with the prevailing codes and standards mentioned above, ASME recognized the fact that while many of the B31.3 sections and paragraphs referenced by 3-A, SEMI, and BPE could be applied appropriately as written, there was concern that B31.3 did not meet all of the needs of the bioprocessing and semiconductor industries, especially when it comes to high-purity fabrication, examination, testing and inspection. This was the impetus for the development of the new Chapter X addition to the ASME B31.3 Process Piping code.
Chapter X: High Purity Piping
The ASME B31.3 Process Piping code has developed over time to become the preeminent piping code for the chemical process industries (CPI). The 2008 issue of B31.3 consisted of nine chapters. Chapters I through VI are considered to be the base code. These chapters are essentially written for metallic piping that is intended for fluid services that can be categorized according to what B31.3 defines as normal and Category D fluid services. [Author’s note: Shortly after the writing of this article, the 2010 issue of the ASME B31.3 Process Piping code was published in March 2011.] The requirements for nonmetallic piping and piping lined with nonmetallic materials can be found in Chapter VII, and are supplemental to the base code. Nonmetals were initially introduced to the code in its 1976 publication, but not given their own chapter until the 1980 publication. The paragraphs in Chapter VII are numbered with respect to the paragraphs in the base code with the added prefix A. Requirements associated with handling toxic fluids, defined by ASME B31.3 as Category M fluid services in Chapter VIII, were first added in the 1976 publication. This chapter establishes more-stringent requirements for toxic fluid services, and was also developed to supplement the base code. The paragraphs in Chapter VIII are numbered with respect to the paragraphs in the base code with the added prefix M.
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Chapter IX, added in the 1984 publication, provides supplemental requirements for operations involving high-pressure fluids. The paragraphs in Chapter IX are numbered with respect to the paragraphs in the base code with the added prefix K. Adding to those supplemental chapters is the latest Chapter X High Purity Piping, which is included in the 2010 issue of the ASME B31.3 code (as noted, the latest issue was published in March 2011). As in Chapters VII, VIII, and IX, Chapter X is supplemental to the base code, so that the respective base code paragraphs included in Chapter X carry the added prefix U, to establish their connection with the high-purity piping requirements detailed in Chapter X.
Application of Chapter X
As noted, Chapter X is a supplement to the base code of B31.3. It provides supplemental recommendations to augment those paragraphs in the base code where additional requirements are needed for high-purity applications. However, readers should note that while ASME B31.3 is considered by many to be the preeminent piping code, it is not a design guide. Specifically, as stated in its introduction: “The designer is cautioned that the code is not a design handbook; it does not do away with the need for the designer or for competent engineering judgment.” High-purity fluid service is defined in B31.3 as “A fluid service that requires alternative methods of fabrication, inspection, examination, and testing not covered elsewhere in the code with the intent to produce a controlled level of cleanliness. The term thus applies to piping systems defined for other purposes as high purity, ultra high purity, hygienic, or aseptic.” This definition touches on the relevant points in which the requirements that are spelled out in the supplemental B31.3 Chapter X are needed — specifically during the fabrication, inspection, examination and testing of high-purity piping systems. However, depending on the industry- or case-specific requirements related to material attributes and specific installation requirements, the designer or engineer may need to go beyond
Organizations and standards related to high-purity piping 3-A Sanitary Standards, Inc. (3-A SSI; 6888 Elm St., Suite 2D, McLean, VA 22101; 3-a.org) • P3-A 002:2008 Pharmaceutical 3-A Sanitary/Hygienic Standards for Materials for Use in Process Equipment and Systems • P3-A 003:2008 P3-A End Suction Centrifugal Pumps for Active Pharmaceutical Ingredients American Society of Mechanical Engineers (ASME; Three Park Ave., New York, NY 10016-5990; asme.org) • ASME B31.3 2008 and 2010 editions • ASME-Bioprocessing Equipment (BPE) Standard 2009 American Welding Society (AWS; 550 N.W. LeJeune Rd., Miami, FL 33126; aws.org) • AWS D18.1 Specification for Welding of Austenitic Stainless Steel Tube and Pipe Systems in Sanitary (Hygienic) Applications International Society of Pharmaceutical Engineers (ISPE; 3109 W. Dr. Martin Luther King, Jr. Blvd., Suite 250, Tampa, FL 33607-6240; ispe.org) • ISPE Baseline Pharmaceutical & Engineering Baseline Guide
B31.3 Chapter X and refer to the other industry-specific design requirements, as mentioned earlier.
Safety considerations
Chapter X in B31.3 also integrates safety into high-purity piping systems, by adapting the B31.3 code to incorporate some preferential, safety-related
Semiconductor Equipment and Materials International (SEMI; 805 East Middlefield Road, Mountain View, CA 94043; semi.org) • SEMI E49.8-2003 Guide for High-Purity and Ultrahigh Purity Gas Distribution Systems in Semiconductor Manufacturing Equipment • SEMI F1-96 Specification for Leak Integrity of High-Purity Gas Piping Systems and Components • SEMI F19-0310 Specification for the Surface Condition of the Wetted Surfaces of Stainless Steel Components • SEMI F20—0706E Specification for 316L Stainless Steel Bar, Forgings, Extruded Shapes, Plate, and Tubing for Components Used in General Purpose, High Purity and UltraHigh Purity Semiconductor Manufacturing Applications • SEMI F22—1102 Guide for Gas Distribution Systems • SEMI F78—0703 Practice for Gas Tungsten Arc (GTA) Welding of Fluid Distribution Systems in Semiconductor Manufacturing Applications • SEMI F81—1103 Specification for Visual Inspection and Acceptance of Gas Tungsten Arc (GTA) Welds in Fluid Distribution Systems in Semiconductor Manufacturing Applications ❏
nuances that are associated with those industries that utilize high-purity piping systems. It does so by adapting its basic philosophy for safety to that of the industry-specific components, material joining methods, and purity requirements. For example, achieving acceptable, repeatable welds is a key element dur-
ing the fabrication of high-purity piping systems. These high-purity welds are accomplished most efficiently by means of a certified welding operator using an orbital welder. In addition to the requirements for acceptable gas tungsten arc (GTA) welds listed in B31.3, the user will need to refer to the criteria for acceptability of these types
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51
Pristine Processing of welds as defined in ASME-BPE, or the respective SEMI standards. Biochemical processes. The addition of Chapter X could not be more timely. Over the past few decades, the breadth and depth of bioprocessing operations has continued to grow. For instance, the growth of the biofuels industry, coupled with the introduction of many new and evolving biotechnology-based processes throughout the CPI, has increased demand for process systems that are able to reliably handle biologics, both refined and industrial, while controlling the risk of contamination. This has pushed the demand for high-puritysystem design beyond the boundaries of the pharmaceutical industry, and has cascaded into industries that are typically unfamiliar with the need for system cleanability. Biochemical processes utilizing hybrid cellulase enzymes and bacteria as catalysts demand a very different set of design guidelines compared to chemical processes that do not use living organisms. During biochemical processing, operators must maintain an environment that ensures that the specialized enzymes and bacteria can thrive and perform their consumption and processing of the pretreated feedstock. However, efforts to maintain an environment that is conducive to the survival of the intended bacteria and enzymes also creates a suitable environment for unwanted bacteria to thrive. If the proper steps are not taken throughout the process, a proliferation of unwanted bacteria can devastate colonies of the desired bacteria, ruining the process. Specifically, efforts to prevent the contamination of a biochemical process, such as the one shown in Figure 1, requires a system design that is conducive to clean-inplace (CIP) or steam-in-place (SIP) capabilities (Note: SIP systems are often also defined as sterilize-in-place or sanitize-in-place systems; the terms are considered to be synonymous). Figure 1 shows the key stages in the biochemical manufacture of ethanol. The only segments of this process that would require high-purity-piping design concepts are those that handle the enzymes (the primary catalyst enzymes for the process), namely in the 52
enzyme production (if the enzymes are produced onsite rather than outsourced), saccharification and fermentation steps of the process. Readers should note that the term high-purity, in the case of Figure 1, should not be misconstrued as an application for hygienic piping. During the production of ethanol, for example, the process system does not need to achieve a hygienic-level of cleanliness. But, it does have to be cleanable from an engineering standpoint. This is where ASME B31.3 Chapter X and BPE work well together, by establishing acceptable design and fabrication requirements that are needed to achieve a cleanable system. These include criteria that define acceptable welds, surface finishes, mechanical joint connections, required slope, examination requirements and more. Semiconductor manufacturing. Unlike bioprocessing operations, the semiconductor industry has a relatively narrow bandwidth of technological requirements that are used by other industries. Specifically, the high degree of purity, testing and the extremely sensitive instrumentation required by semiconductor manufacturers do not readily translate into practical use by many other industries. Thus, the semiconductor industry is relatively autonomous in that respect. With some semiconductor manufacturers producing chips with dimensions at the 32-nanometer (nm) level, and research going on at the 15-nm level, it is easy to see why the design, fabrication, and maintenance practices required to ensure exacting purity requirements of their process fluid distribution systems are of paramount importance. During semiconductor device fabrication, a variety of ultrahigh purity gases and chemicals are used during many of the processing steps, such as dry etching, wet etching, plasma etching, chemical vapor deposition, physical vapor deposition, and chemical-mechanical planarization. Engineering steps must be taken to ensure that these fluids be of ultrahigh purity, and must ensure that all associated tubing and components that distribute these fluids be maintained in an ultra-highpurity mode, as well.
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To meet these demands, semiconductor manufacturers can now use B31.3 Chapter X in conjunction with the prevailing SEMI standards, as these two documents bring together the necessary criteria to establish acceptable design attributes, acceptable materials of construction, fabrication quality, testing protocols, validation, examination and inspection requirements.
The impact of Chapter X
As mentioned earlier, the addition of Chapter X to the content of B31.3 could not be timelier. Chapter X augments not only the B31.3 base code, but the ASME-BPE, ISPE baseline guide, and SEMI standards, as well, at a time when all of these high-purity industries are undergoing significant changes and facing more-rigorous purity requirements than ever before. This preliminary movement of the ASME B31.3 piping code into the realm of high-purity process requirements is just an initial step. Once a segment of industry is adopted by ASME in such a manner, it adds a whole new level of thinking and evaluation to the standardization of that high-purity industry. The American National Standards Institute (ANSI) accreditation program. to which ASME adheres, legitimizes the standardization process and institutes an ongoing review process, which brings fresh new insights and technological advances to the continuing evolution of the industries it touches. In general, many of the standards that have been developed specifically for high-purity industries have been driven and guided by the participation of active standards-development committee members, who are directly associated with the pharmaceutical and semiconductor industries. The addition of Chapter X invites the involvement of a more-diverse array of experts from a broader group of industries (for instance, the biofuels industry and other CPI sectors) that also have demanding purity and cleanability requirements. This promises to bring new vision and cross-industry collaboration when it comes to the ongoing evolution of high-purity pipingsystem standardization. ■ Edited by Suzanne Shelley
Acknowledgements
The following individuals provided invaluable input during the development of this article: Philip Guerrieri, Sr., president of Integrated Mechanical Services, Inc., Phillip E. Robinson, Consultant to Parker Hannifin, LLC, Gerald A. Babuder, Swagelok Co., and Kenneth A. NislyNagele, Archer Daniels Midland Co.
Authors
S
ince 1956 the employees of Mueller Steam Specialty have been dedicated to the manufacture of high quality products delivered on time and with superior customer service. Our core line of rugged strainers is available in a wide range of types and materials. Whether you require basket strainers, Y strainers, “Tee” type strainers, duplex strainers, or even temporary strainers, Mueller will deliver your order from stock or custom engineer and manufacture it to your requirements. In addition to its strainer line, Mueller offers a full line of check valves, butterfly valves, pump protection and specialty products for a variety of industries and applications. Choose Mueller Steam Specialty for your next project.
W. M. (Bill) Huitt is president of W.M. Huitt Co., a piping consulting firm founded in 1987 (P.O. Box 31154, St. Louis, MO 63131-0154; Phone: 314-966-8919; Cell: 314-330-4068; Email:
[email protected]). He has been involved in industrial piping design, engineering and construction since 1965. Prior positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager. His experience covers both the engineering and construction fields and crosses industry lines to include petroleum refining, the production of chemicals, petrochemicals, pharmaceuticals, pulp & paper, nuclear power, biofuels, and coal gasification. He has written numerous specifications, guidelines, papers, and magazine articles on the topic of pipe design and engineering. A Watts Water Technologies Company Huitt is a member of ISPE (International Society WSMC-Comi-Condor-Tomoe 1-3 page Black & Blue - Chem Eng (r... https://nymail.accessintel.com/ex of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a member Circle 27 on p. 62 or go to adlinks.che.com/35067-27 of three ASME-BPE subcommittees, several Task Groups, ASME B31.3 Subgroup H on High Purity Piping, API Task Group on RP-2611, and sits on two corporate specification review boards. He also serves on the advisory board for the ChemInnovations Conference. Barbara K. Henon is a contract employee for Arc Machines, Inc., a manufacturer of orbital GTAW tube and pipe welding equipment (Arc Machines, Inc., 10500 Orbital Way, Pacoima, CA 91331). She holds a Ph.D. in biological sciences from the University of Southern California. She has more than 15 years of experience training orbital welding operators and engineers for high-purity applications in the pharmaceutical and semiconductor industries. Henon is a member of ISPE and has been an instructor at the annual ASME Bioprocess Technology Seminars since 1989. Henon is the former vice chair of the ASME Bioprocessing Equipment (BPE) Main Committee, and is a current member of the BPE Materials Joining, Surface Finishes, and General Requirements Subcommittees, as well as the BPE Main Committee and Executive Committee. She is a member of the ASME B31.3 Process Piping Section Committee, and a member of Subgroup H that developed Chapter X High Purity Piping for the ASME B31.3 Process Piping Code. Henon is also active on the AWS D18 and D10 Standards writing committees and was on the committee for writing the SEMI F81 and SEMI F78 standards for orbital welding of semiconductor-fluid-distribution systems. Vicencio B. (Vince) Molina III is an engineering manager for the HYCO PST of Air Products & Chemicals, Inc. (555 West Arrow Highway, Claremont CA 91711; Phone: (909)447-3976, Email:
[email protected]). He has been a member of the ASME Section Committee since 1997, and is currently the chairman of ASME B31.3 Subgroup on High Purity Piping.
Circle 28 on p. 62 or go to adlinks.che.com/35067-28 Chemical Engineering www.che.com July 2011
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Feature Report
Piping Design for Hazardous Fluid Service William M. Huitt W.M. Huitt Co.
I
ncorporating fire safety into plant design takes on two fundamental goals: to prevent the occurrence of fire and to protect the initially uninvolved piping and equipment long enough for operations personnel to perform their duties and for emergency responders to get the fire under control. While it is impractical to completely eliminate the potential risk of an accidental fire in a complex process-plant facility that is expected to handle and process hazardous chemicals, it is reasonable to assume that certain aspects of design can be incorporated to reduce that risk. Designing facilities that use and store hazardous chemicals requires a demanding set of requirements, at times beyond what can practically be written into industry codes and standards. It is ultimately the responsibility of the engineer of record (EOR) and the owner to fill in those blanks and to read between the lines of the adopted codes and standards to create a safe operating environment, one that minimizes the opportunity for fire and its uncontrolled spread and damage. This article will not delve into the various trigger mechanisms of how a fire might get started in a process facility, but will instead discuss containment and control of the fuel component of a fire that resides in piping systems that contain combustible, explosive or flammable fluids. In the design of piping systems containing such fluids, there are critical aspects that need additional considerations beyond those involved in the design of piping systems containing non-hazardous fluids. There are two key safety aspects that need to be incorporated into the design, namely system integrity and fire safety.
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Extra considerations and precautions are needed beyond the requirements of codes and standards System integrity
System integrity describes an expectation of engineering that is integrated into the design of a piping system in which the selected material of construction (MOC), system joint design, valve selection, examination requirements, design, and installation have all been engineered and performed in a manner that instills the proper degree of integrity into a piping system. While this approach is certainly needed for the piping design of so-called normal fluid service it is absolutely critical for hazardous fluid systems. The design of any piping system, hazardous or non-hazardous, is based, in large part, on regulations and industry accepted standards published by such organizations as the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API). The standards published by these organizations include tables that establish joint-pressure ratings based on MOC and temperature. Where the joint-design consideration for hazardous fluid services departs from that of non-hazardous fluid services is in gasket and seal material specifications. This is due to the need for sealing material to contain hazardous chemicals for as long as possible while surrounded by a fire or in close proximity to a fire. The effect of heat from a fire on an otherwise uninvolved piping system can only be delayed for a relatively short period of time. And the first thing to fail will be the mechanical type joints. Depending on the type of fire and whether the piping is directly in the fire or in close proximity, the window of opportunity, prior to joint seal failure, for an emergency response team to get
Chemical Engineering www.che.com June 2010
the fire under control is anywhere from a few hours to less than 30 minutes. As you will see, a number of factors dictate the extent of that duration in time. A system in which the gasket material is selected on the basis of material compatibility, design pressure, and design temperature may only require a solid fluoropolymer. In a fire, this non-metallic material would readily melt, allowing the contents of the pipe to discharge from the joint once sealed by the gasket. Specifying a gasket that is better suited to hold up in a fire for a longer period of time gives the emergency responders time to bring the initial fire under control, making it quite possible to avoid a major catastrophe.
Fire-safe system
Preventing the potential for a fire requires operational due diligence as well as a proper piping-material specification. However, controlling and restricting the spread of fire goes beyond that. Results of the assessment reports of catastrophic events coming from the U. S. Chemical Safety and Hazard Investigation Board (CSB; Washington, D.C.) have shown that many of the occurrences of catastrophic incidents have actually played out through a complex set of circumstances resulting from design flaws, instrumentation problems, pipe modifications, inadequate fire-proofing and human error. Events, such as a fire, are not necessarily then the result of a hazardous fluid simply escaping through a leaky joint and then coming into contact with an ignition source. There are usually a complex set of events leading up to a fire incident. Its subsequent spread,
Incident No. 1 Valero-McKee Refinery, Sunray, Tex., Feb. 16, 2007
W
ithout going into great detail as to the circumstances that led up to this incident, piping handling liquid propane in a propane deasphalting (PDA) unit ruptured. The location of the rupture was in a section of isolated piping that had been abandoned in place several years prior. A valve, intended to isolate the active flow of liquid propane from the abandoned-in-place piping, had been unknowing left partially open due to an obstruction inside the valve. Water had gradually seeped in past the valve seat over the years and being heavier than the liquid Figure 1. propane, settled at a low-point control station where it eventually froze during a cold period. The expanding ice inside the pipeline subsequently cracked the pipe. When the temperature outside began to warm, the ice thawed allowing liquid propane to escape from the active pipeline, through the partially closed valve, and out the now substantial crack. The resultant cloud of propane gas drifted toward a boiler house where it found an ignition source. The flame of the ignited gas cloud tracked back toward its source where the impending shockwave from the explosion ripped apart piping attached to the PDA extractor columns causing ignited propane to erupt from one of the now opened nozzles on the column at such a velocity as to create a jet fire. The ensuing jet fire, which is a blow-torch like Figure 2. flame, discharged toward a main pipe rack approximately 77 ft away, engulfing the pipe rack in the jet fire. As the temperature of the non-fire-proofed structural steel of the pipe rack reached its plastic range and began to collapse in on itself, the piping in the rack, which contained additional flammable liquids, collapsed along with it (Figure 1). Due to the loss of support and the effect of the heat, the pipes in the pipe rack, unable to support its own weight, began to sag. The allowable bending load eventually being exceeded from the force of its unsupported weight, the rack piping ruptured spilling its flammable contents into the already catastrophic fire. The contents of the ruptured piping, adding more fuel to the fire, caused the flames to erupt into giant fireballs and thick black smoke. The non-fire-proofed support steel (seen on the left in Figure 1 and on the right in Figure 2) was actually in compliance with API recommendations. Those recommendations can be found in Publication 2218 — Fireproofing Practices in Petroleum and Petrochemical Processing Plants; API Publications 2510 — Design and Construction of LPG Installations; and 2510A — Fire-Protection
into a possible catastrophic event, can then be the result of inadequate design requirements that extend beyond the piping itself. While this discussion touches only on piping issues, know that this is only a part of the overall integration of safety into the design of a facility that handles hazardous fluids. What follows are recommended piping design considerations that are intended to substantially reduce the risk of the onset of fire and its uncontrollable spread throughout a facility. In discussing the spread of fire, it will be necessary to include discussion regarding the needs for disciplines other
A collapsed pipe rack as a result of heat from a jet flame
The same collapsed pipe rack as Figure 1 seen from above
Considerations for the Design and Operation of Liquefied Petroleum Gas (LPG) Storage Facilities. In these issues of the publications it was recommended that pipe-rack support steel within 50 ft of an LPG vessel be fire proofed. The collapsed support steel was approximately 77 ft from the extractor columns, which is beyond the 50-ft recommended distance. While the EOR was in compliance with the governing code, with regard to fire proofing, there may have been a degree of complacency in defaulting to that minimum requirement. This goes back to a point made earlier in which it was said that industry standards are not intended to be design manuals. They instead provide, “… the minimum requirements necessary to integrate safety into the design, fabrication, inspection, installation, and testing of piping systems…” Proprietary circumstances make it the imperative responsibility of the EOR or the owner to make risk assessments based on specific design conditions and go beyond the minimum requirements of an industry code or standard when the assessment results and good engineering practices dictate. ❏
than piping, namely fire proofing of structural steel.
General codes and standards
From a fire-safety standpoint, some requirements and industry regulations are stipulated in the International Fire Code (IFC), published by the International Code Conference (ICC) under IFC 3403.2.6.6. There are also requirements by the National Fire Protection Assn. (NFPA) under NFPA 1 and NFPA 30. Test requirements for fire-rated valves can be found under API 607 — “Fire Test for Soft Seated Quarter Turn Valves.” Starting with the 4th edition of this API standard,
it was added that, among other things, the tested valve has to be operated from fully closed to fully open after the fire test. Prior to the 4th edition a soft-seated fire-rated valve had to only remain sealed when exposed to fire without having to be operated, or rotated. Additional fire test requirements can be found as published by the BSI Group (formerly known as British Standards Institution) as BS-6755-2 “Testing of Valves. Specification for Fire Type-Testing Requirements,” and FM Global FM-7440 “Approval Standard for Firesafe Valves.” With exception to the specific requirements covered in the valve test-
Chemical Engineering www.che.com June 2010
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Feature Report ing standards, the codes and standards mentioned above provide generalized requirements that touch on such key aspects of safety as relative equipment location, mass volume versus risk, electrical classifications, valving, and so on. They cannot, and they are not intended to provide criteria and safeguards for every conceivable situation. Designing safety into a particular piping system containing a hazardous liquid goes beyond what should be expected from an industry-wide code or standard and falls to the responsibility of the owner or EOR. As ASME B31.3 states in its introduction, “The designer is cautioned that the code is not a design handbook; it does not do away with the need for the designer or for competent engineering judgment.” When designing piping systems to carry hazardous liquids, the design basis of a project or an established protocol for maintenance needs to incorporate a mitigation strategy against two worse-case scenarios: (a) A leak at a pipe joint containing a hazardous liquid, and (b) The rupture or loss of containment, during a fire, of surrounding hazardous piping systems, not otherwise compromised that would add fuel to the fire. The occurrence of those two failures, one initiating the incident and the other perpetuating and sustaining the incident, can be minimized or eliminated by creating a design basis that provides the following: • Added assurance against the potential for joint failure • Added assurance of containment and control of a hazardous liquid during a fire • Safe evacuation of a hazardous liquid from the operating unit under distress
Fire prevention through design
Piping joints. When designing piping systems to contain hazardous liquids, one of the key objectives for the design engineer should be taking the necessary steps to minimize the threat of a leak, steps beyond those typically necessary in complying with the minimum requirements of a code. There are certainly other design issues that warrant consideration, and they will be touched on much later. However, while 38
PTFE envelope
Primary sealing element
Profiled inner ring
Monel* windings
Secondary sealing element
Flexible graphite filler
* Monel is a registered trademark of international Nickel
the pipe, valves, and instrumentation all have to meet the usual criteria of material compatibility, pressure, and temperature requirements there are added concerns and cautions that need to be addressed. Those concerns and cautions are related to the added assurance that hazardous liquids will stay contained within their piping system during normal operation and for a period of time during a fire as expressed in such standards as API-607, FM-7440, and BS-6755-2. Designing a system, start to finish, with the intent to minimize or eliminate altogether the potential for a hazardous chemical leak will greatly help in reducing the risk of fire. If there is no fuel source there is no fire. In the design of a piping system, leak prevention begins with an assessment of the piping and valve joints. There are specified minimum requirements for component ratings, examination, inspection, and testing that are required for all fluid services. Beyond that, there is no guidance given for fire safety with regard to the piping code other than a statement in B31.3 Para. F323.1 in which it states, in part: “The following are some general considerations that should be evaluated when selecting and applying materials in piping: (a) the possibility of exposure of the piping to fire and the melting point, degradation temperature, loss of strength at elevated temperature, and combustibility of the piping material under such exposure, (b) the susceptibility to brittle failure or failure from thermal shock of the piping material when exposed to fire or to fire-fighting measures, and possible hazards from fragmentation of the material in the event of failure, (c) the ability of thermal insulation to protect piping against failure under fire exposure (for example, its stability, fire resistance, and ability to remain in place during a fire).” The code does not go into specifics on this matter. It is the engineer’s respon-
Chemical Engineering www.che.com June 2010
Carbon steel outer ring
Figure 3. If flanged joints are necessary, it is suggested that fire-safe spiral-wound type gaskets with graphite filler be specified
sibility to raise the compliance-level requirements to a higher degree where added safety is warranted and to define the compliance criteria in doing so. Joints in a piping system are its weak points. All joints, except for the full penetration buttweld, will de-rate a piping system to a pre-determined or calculated value based on the type of joint. This applies to pipe longitudinal weld seams, circumferential welds, flange joints and valve joints such as the body seal, stem packing, and bonnet seal, as well as the valve seat. For manufactured longitudinal weld seams, refer to ASME B31.3 Table A-1B for quality factors (E) of the various types of welds used to manufacture welded pipe. The quality factor is a rating value, as a percentage, of the strength value of the longitudinal weld in welded pipe. It is used in wall thickness calculations as in the following equations for straight pipe under internal pressure:
(1)
(2) Where: c = sum of mechanical allowances D = outside dia. of pipe d = inside dia. of pipe E = quality factor from Table A-1A and A-1B P = internal design gage pressure S = stress value for material from Table A-1 t = pressure design thickness W = weld-joint strength-reduction factor y = coefficient from Table 304.1.1 Also found in Para. 304 of B31.3 are wall thickness equations for curved and mitered pipe. With regard to circumferential welds, the designer is responsible for assigning a weld-joint reduction factor (W) for welds other than longitudinal welds. What we can do, at
Incident No. 2: Formosa Plastics Corp., trailer being towed by a forklift operator down a pipe rack alley in the Olefins II operating unit of Formosa’s Point Comfort facility attempted to back the trailer up into an open area between pipe rack support columns in an effort to turn the rig around. When the operator, in the process of pulling back into the pathway, began to pull forward the trailer struck a protruding 2-in. blow-down valve on a vertically mounted Y-strainer that was connected to a 4-in. NPS liquid propylene line subsequently ripping the valve and nipple from the strainer (Figure 4). Liquid propylene under 216 psig pressure immediately began discharging into a liquid pool from the 2-in. opening and partially vaporizing into a flammable cloud. The flammable cloud eventually found an ignition source, ignited and exploded, in-turn igniting the pool of liquid propylene. The fire burned directly under the pipe rack and an attached elevated structure containing process equipment and piping. About 30 min into the event, non-fire-proofed steel sections of the pipe rack and the elevated structure containing process equipment collapsed (Figure 5). The collapse caused the rupture of equipment and additional piping containing flammable liquids, adding more fuel to an already catastrophic fire. The flare header was also crimped in the collapse and ruptured, causing flow that should have gone to the flare stack to be discharged into the heart of the fire. The fire burned for five days. Again, as in Incident No. 1, you can see in Figure 5 the result of insufficient fire proofing of steel beams and columns in close proximity to process units. And fire protection does not apply only to vertical columns. As you can see, it is not sufficiently effective to have the vertical columns protected while the horizontal support steel is left unprotected and susceptible to the heat from a fire. Another key factor in the Formosa fire was the ambiguous decision by the designer to orient the Y-strainer blow-down in such a position of vulnerability. While there is absolutely nothing wrong with installing the Y-strainer in the vertical position, as this one was, they are normally installed in a horizontal position with the blow-down at the bottom, inadvertently making it almost impossible to accidentally strike it with enough force to dislodge the valve and nipple. However, orienting the blow-down in such a manner, about the vertical axis, should have initiated the need to evaluate the risk and make the determination to rotate the blow-down about its vertical axis to a less vulnerable location, or to provide vehicle protection
least for this discussion, is to provide, as a frame of reference, some quality rankings for the various circumferential welds based on the stress intensification factor (SIF) assigned to them by B31.3. In doing so, the full penetration buttweld is considered to be as strong as the pipe with an SIF = 1.0. The double fillet weld at a slip-on flange has an SIF = 1.2. The socketweld joint has a SIF = 2.1. Any value in excess of 1.0 will de-rate the strength of the joint below that of the pipe. With that said, and assuming an acceptable weld, the weld joint, and particularly the full penetration buttweld, is still the joint with the highest degree of integrity. In a fire,
4-in. Propylene product line
Column
A
Point Comfort, Tex., Oct. 6, 2005
Strainer Pipe nipple – 2 ft
Figure 4. The impact point (left) showing the damaged Y-strainer
Figure 5. Collapse of non-fire-proofed structural steel
around the blow-down in the form of concrete and steel stanchions. Both of these precautionary adjustments were overlooked. The plant did perform a hazard and operability study (HAZOP) and a pre-startup safety review (PSSR) of the Olefins II operating unit. In the CSB report, with regard to process piping and equipment, it was stated that, “During the facility siting analysis, the hazard analysis team [Formosa] discussed what might occur if a vehicle (for instance, fork truck, crane, man lift) impacted process piping. While the consequences of a truck impact were judged as “severe,” the frequency of occurrence was judged very low (that is, not occurring within 20 years), resulting in a low overall risk rank [The ranking considered both the potential consequences and likely frequency of an event]. Because of the low risk ranking, the team considered existing administrative safeguards adequate and did not recommend additional traffic protection.” ❏
the last joint type to fail will be the welded joint. The threaded joint has an SIF = 2.3 and requires a thread sealant applied to the threads, upon assembly, to maintain seal integrity. With flame temperatures in a fire of around 2,700–3,000ºF the thread sealant will become completely useless if not vaporized, leaving bare threads with no sealant to maintain a seal at the joint. The flange-joint-sealing integrity, like the threaded joint, is dependent upon a sealant, which, unlike the threaded joint, is a gasket. Flange bolts act as springs, providing a constant live load so long as all things remain constant. Should the gasket
melt or flow due to the heat of a fire, the initial tension that was given the bolts when the joint was assembled will be lost. Once the gasket has been compromised the sealing integrity of the joint is gone. Knowing that the mechanical type threaded and flange joints are the weak points in a piping system, and the primary source for leaks, it is suggested that their use be minimized to the greatest extent possible. Consider the following design points: • Do not specify flange joints solely for installation purposes • Specify flange joints only where required for equipment connections and for break-out spools
Chemical Engineering www.che.com June 2010
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Feature Report
Discharge to safe area
Line D
Slope RD-1
• If a lined pipe system is required, use the type requiring the liner to be fused, a coupling installed and one that is suitable for multi-axis bending Threaded joints should be limited to instrument connections and then only if the instrument is not available with a flange or welded connection. If a threaded connection is used, it should be assembled without thread compound then seal-welded. This may require partial dismantling of the instrument to protect it from the heat of the welding process. It is recommended that piping systems be welded as much as possible and flanged joints be minimized as much as possible. That includes using welded end valves and inline components where possible. If flanged joints are necessary for connecting to equipment nozzles, flanged valves, inline components, or needed for break-out joints, it is suggested that a spiral-wound type gasket with graphite filler be specified. This material can withstand temperatures upwards of 3,000ºF. There are also gasket designs that are suitable for when a fluoropolymer material is needed for contact with the chemical, while also holding up well in a fire. These are gaskets similar in design to that shown in Figure 3. Valves. A fire-rated valve meeting the requirements of API 607 (Fire Test for Soft Seated Quarter Turn Valves) is designed and tested to assure the prevention of fluid leakage both internally along the valve’s flow path, and externally through the stem packing, bonnet seal, and body seal (where a multi-piece body is specified). Testing under API 607 subjects a valve to well defined and controlled fire conditions. It requires that after exposure to the fire test the valve shall be in a condition that will allow it to be rotated from its closed position to its fully open position using only the manual operator fitted to the test valve. Quarter turn describes a type of valve that goes from fully closed to fully open within the 90 deg rotation of its operator. It includes such valve types as ball, plug, and butterfly with a valve seat material of fluoropolymer, elastomer, or some other soft, non-metallic material. Standards such as FM-7440 and 40
XV-1 Line A
Flammable liquid in
VA-1 Operating unit battery limits
LT-1 SG-1
VA-6
VA-3
Flammable liquid to recovery
VA-4 PG-1
XV-2
VA-5
Flammable liquid out
Line B VA-2
Pump
Line C XV-3
XV-4
Figure 6. A simplified P&ID used in the discussion about process systems
BS-6755-2, touched on earlier, apply to virtually any valve type that complies with their requirements. Under the FM and BS standards, valve types such as gates, globes, and piston valves with metal seats can also make excellent fire-rated valves when using a body and bonnet gasket and stem packing material similar in temperature range to that of a graphite or graphite composite. Process systems. At the onset of a fire within an operating unit, initially unaffected process piping systems should not be a contributor to sustaining and spreading what is already a potentially volatile situation. There are basic design concepts that can be incorporated into the physical aspects of a process system that will, at the very least, provide precious time for operators and emergency responders to get the situation under control. In referring to the simplified piping and instrumentation diagram (P&ID) in Figure 6, there are seven main points to consider: 1. Flow supply (Line A), coming from the fluid’s source outside the operating unit, needs to be remotely shut off to the area that is experiencing a fire 2. The flow path at the systems use point valves (VA-1) needs to remain open 3. The flow path at drain and vent valves (VA-2) needs to remain sealed 4. The external path through stem packing and body seals needs to remain intact during a fire 5. The bottom outlet valve (XV-2) on a vessel containing a flammable liquid should have an integral fusible link for automatic shut-off, with its valve seat, stem packing and body seals remaining intact during a fire 6. Pipeline A should be sloped to allow all liquid to drain into the vessel
Chemical Engineering www.che.com June 2010
7. The liquid in the vessel should be pumped out to a safe location until the fusible link activates, closing the valve. There should be an interlock notifying the control room and shutting down the pump Those seven points, with the help of the P&ID in Figure 6, are explained as follows: Point 1. The supply source, or any pipeline supplying the operating unit with a flammable liquid, should have an automated, fire-rated isolation valve (XV-1) located outside the building or operating unit area and linked to the unit’s alarm system with remote on/off operation (from a safe location) at a minimum. Point 2. Any point-of-use valve (VA-1) at a vessel should remain open during a fire. The area or unit isolation valve (XV-1) will stop further flow to the system, but any retained or residual fluid downstream of the automatic shut-off valve needs to drain to the vessel where the increasing overpressure, due to heat from the fire, will be relieved to a safe location, such as a flare stack, through RD-1. If the Valves, XV-1 and VA-1, are closed in a fire situation the blocked-in fluid in a heated pipeline will expand and potentially rupture the pipeline; first at the mechanical joints such as seals and packing glands on valves and equipment, as well as flange joints, and then ultimately the pipe itself will rupture (catastrophic failure). During a fire, expanding liquids and gases should have an unobstructed path through the piping to a vessel that is safely vented. Point 3. Valves at vents and drains (VA-2 & VA-6) need to be fire-rated and remain closed with seals and seat intact for as long as possible during a fire.
Incident No. 3: BP Refinery, Texas City, Tex., July 8, 2005
I
High-temperature n the design layout of a duplex heathydrogen to furnance exchanger arrangement (Figure 7) in the resid-hydrotreater unit of the BP Refinery in Preheat gas Texas City, Tex., the designer duplicated the fabrication dimensions of the 90-deg fabricated elbow-spool assemblies shown in FigFigure 8. Severed 8-in. Heat ure 7 as Elbows 1, 2, and 3. While the pipe NPS hydrogen piping exchanger A sizes and equipment nozzle sizes were the Elbow 3 same, permitting an interchangeability of the (failure location) fabricated elbow spool assemblies, the service Elbow 2 conditions prohibited such an interchange. Bolted flange The shell side conditions on the upstream (typical) side (at Elbow 1) were 3,000 psig at 400ºF. The shell side conditions on the downstream 11/4 chrome side (at Elbow 3) were 3,000 psig at 600ºF. alloy piping The intermediate temperature at Elbow 2 was not documented. In the initial design, Elbow 1 the material for Elbow 1 was specified as carbon steel carbon steel, Elbow 3 was specified as a Heat exchanger B 1 - 1/4 chrome/moly alloy. The reason for the difference in material of construction (MOC) is that carbon steel is susceptible to Preheat gas high temperature hydrogen attack (HTHA) to separator above ~450°F at 3,000 psig, therefore the Low-temperature chrome/moly alloy was selected for the 3,000 psig higher temperature Elbow 3. hydrogen feed 11/4 Chrome alloy pipe At 3,000 psig and temperatures above Carbon steel pipe 450°F hydrogen permeates the carbon steel and reacts with dissolved carbon to form Figure 7. Heat exchanger flow diagram methane gas. The degradation of the steel’s The one thing you can take tensile strength and ductility due to decarburization, coupled with the formation of methane gas creating localized stresses, away from this incident is: Do not dimensionally replicate weakens the steel until it ultimately fatigues and ruptures. In January 2005, scheduled maintenance was performed on the piping spools or assemblies of heat exchanger assembly. The piping connected to the heat ex- different materials. The other changers was dismantled and stored for the next 39 days. After underlying, but significant maintenance was completed, the piping was retrieved from stor- component you can also take away is this: In the initial de- Figure 9. Fragments of age and reinstalled. the failed 8-in. NPS carbonThe elbows of different material were not marked as such and sign of a plant facility the ensteel spool gineer of record will routinely the maintenance contractor was not warned of the different MOC for the elbows. Elbows 1 and 3 were unknowingly in- hold formal design reviews stalled in the wrong locations. On July 8, 2005, approximately that will include all key personnel with vested interest in the projfive months after re-installing the piping around the heat ex- ect. In doing so, include, among the attendees, key operations changers, the elbow in the #3 position catastrophically failed as and management plant personnel from one of the owner’s operating facilities, if available. These individuals typically bring a shown in Figure 8. As you can see in Figure 9 the carbon steel, after becoming lot of insight and knowledge to a review. Whereas the designers progressively weakened by HTHA, fractured on the inside of may not have the wherewithal to think along the lines of issues the pipe and catastrophically failed. The incident injured one that might pertain to a facility turnaround, the plant personnel person in operations responding to the emergency and cost the will. These are issues that they normally think long and hard company $30MM. about. Make use of this resource. ❏
Point 4. During a fire, another source for valve leakage is by way of stem packing and body seal, as mentioned earlier. Leakage, at these seal points, can be prevented with valves that are not necessarily fire-rated, but contain stem packing and body seal gasket material specified as an acceptable form of graphite (flexible graphite, graphoil and so on). This is a fire-safe material which is readily available in non-fire-rated valves. Point 5. The valve on the bottom of the vessel should be fire-rated with a fusible link or a fail closed position.
Relying on an air or electric operated valve actuator may not be practical. A fusible link is most certainly needed on a manually operated valve. The contents of a vessel containing a hazardous liquid needs to get pumped to a safe location during a fire until such time as the fusible link is activated, closing the tank bottom valve, or the pump fails. All valved gage and instrument connections (SG-1) mounted on a vessel should have a graphite-type stem packing and body-seal-gasket material at a minimum. Flange gaskets at these gage and instrument con-
nections should be of a spiral-wound fire-safe gasket type similar to those mentioned earlier. Specialty tankbottom valves (XV-2) should be given special consideration in their design by considering a metal-to-metal seat, or a piston valve design along with fire-rated seal material. Point 6. As mentioned in Point 2, the residual fluid in Line A, after flow has been stopped, should be drained to the vessel. To help the liquid drain, the pipeline should be sloped toward the vessel. The intent, as mentioned above, is to prevent sections of any
Chemical Engineering www.che.com June 2010
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Feature Report pipeline that do not contain a relief device from being blocked and isolated during a fire. If the piping system for flammable fluid service is designed properly, the contents will be able to drain or expand into a vessel where over-pressurization can be relieved and safely vented. Point 7. It will be necessary to evacuate as much of the hazardous fluid as possible from tanks and vessels in the fire area to a safe location. The pumpout should continue until there is inadequate pump suction head, or until the fusible link on XV-2 is activated. At that time the pump interlocks would shut down the pump. With regard to tank farms, the following is a suggested minimum consideration for a safe design: Drain valves should be of a fire-rated type. Tank outlet valves should be of a firesafe type with a fusible link. Tank nozzles used for gages or instrument connections should have, at a minimum, valves containing stem packing and seal gasket material specified as an acceptable form of graphite, as mentioned above, or some other firesafe material. Gaskets used at nozzle flange joints should be a fire-safe gasket similar to the spiral wound gaskets mentioned earlier or the gasket shown in Figure 3. Inline valves in piping downstream of the tank outlet valve, such as pump transfer lines and recirculation lines, do not necessarily need to be firerated, but should have stem packing and seal gasket material that is firesafe as mentioned earlier. Situations will arise that do not fall neatly into what has been described above. If there is any doubt with regard to valving then default to a fire-rated valve. Each piping system identified as needing to be fire-safe should be designated as such. Where individual fire-safe valves are to be strategically located in a system, they should be designated on their respective P&IDs either by notation or through the assigned pipe material specification. The pipe-material specification should be indicated on each pipeline of the P&ID. The specification itself should therefore be descriptive enough for the designer to know which valve to apply at each location.
Chemical Engineering www.che.com June 2010
Lessons learned from incidents
While this particular discussion is specific to piping leaks and joint integrity it bares touching on a few subjects that are integrally associated with piping safety: pipe rack protection, protecting piping from vehicle traffic, and designing for disaster (HAZOP). In Incident Number 1 (box, p. 37), the onset of a fire that might otherwise have been quickly controlled becomes a catastrophic event because piping mounted on the unprotected structural steel of a pipe rack, outside the extent of the initial occurrence, becomes collateral damage adding more fuel to the fire causing it to sustain itself, increase in intensity and continue to spread. In Incident Number 2 (box, p. 39), an unprotected and protruding pipeline component (Y-strainer) is damaged, causing a major leak that operating personnel were unable to stop. The ensuing fire lasted for five days. In Incident Number 3 (box, p. 41), two dimensionally identical spool pieces were designed for a system in which the two were fabricated from different materials because their service conditions were very different. It can only be assumed that this was an erroneous attempt at trying to achieve duplication of pipe spools in an effort to assist the fabricator in their productivity of pipe fabrication. Instead it ultimately caused injury to one person and cost the plant owner $30MM. ■ Edited by Gerald Ondrey
Author W. M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. (P.O. Box 31154, St. Louis, MO 63131-0154; Phone: 314-966-8919; Email:
[email protected]; URL: www.wmhuitt.com), a piping consulting firm founded in 1987. His experience covers both the engineering and construction fields and crosses industrial lines to include petroleum refining, chemical, petrochemical, pharmaceutical, pulp and paper, nuclear power, biofuel, and coal gasification. He has written numerous specifications, guidelines, papers, and magazine articles on the topic of pipe design and engineering. Huitt is a member of ISPE (International Society of Pharmaceutical Engineers), CSI (Construction Specifications Institute) and ASME (American Society of Mechanical Engineers). He is a member of three ASME-BPE subcommittees, several task groups, an API task group, and sits on two corporate specification review boards.
Feature Report Engineering Practice
Active Management of
Pipespool Fabricators Stephen Wyss Bechtel Oil, Gas, & Chemical
I
n today’s fast track schedules for constructing new capital facilities, the process of designing, delivering and erecting piping often falls in the project’s critical path. This is particularly true for facilities constructed in emerging economies, where the facility generally resides in a remote location, posing significant logistical challenges. Squarely positioned in the center of the piping design, deliver and erect (DDE) process sits the subprocess of fabricating piping components into erectable sections of piping, or pipespools. The engineering, procurement and construction (EPC) contractor’s management approach to this fabrication is key because it impacts the project team’s ability to manage the overall schedule. In fact, if handled properly, management of pipespool fabricators can get the piping DDE process off the project’s critical path altogether. Before we address how the EPC contractor’s active spool management of the pipespool fabricator may positively affect schedule and, ultimately, the construction cost of the facility, we need to understand the challenging nature of the piping DDE process in today’s fast-track-project environment; and we need to briefly address the level and nature of pipespool-fabricator services on a given project.
Evolving piping design process
In general, as a project unfolds, the overall facility piping design evolves as high-level process requirements translate into a physical design of equipment linked by piping systems. These piping systems are complex. A grassroots petroleum refinery, for instance, can require as many as 10,000 piping inventory codes identifying 40
Contractors need to integrate and engage to improve deliveries and shorten project schedules unique piping components, given metallurgical, mechanical, and configuration related factors. Ideally, the acquisition of piping materials would occur as the piping design becomes firm enough to confidently ascertain requirements. Practically, however, fast track schedules dictate that the material acquisition process be executed in parallel with the evolving piping design. This places the EPC contractor in the difficult position of attempting to balance the timing of piping materials acquisition between two scenarios: 1) waiting until the design is firm, risking schedule delays due to late arriving materials, or 2) purchasing early on poorly defined requirements, and risking purchase of the wrong materials. This latter scenario — purchasing the wrong materials — is a double edged sword. Not only does it require subsequent purchasing activity to replace the wrong materials, which often results in late deliveries, but it also incurs surplus of materials left over at the end of the project, in this case, the incorrectly purchased items. Nevertheless, given that schedule delays and negative project economics are virtual certainties if the project follows the first scenario (waiting until materials requirements are firm), most EPC contractors choose to manage the parallel process of acquiring piping materials as the design evolves (commonly referred to as the “piping prebuy” process) to guide the project to a successful on-time completion. As we will see below, active management of a pipespool fabricator presents opportunities to recapture schedule time often lost in the piping prebuy effort, furthering successful project execution. The piping prebuy process and the
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significant negative impact on the project’s bottom line that surplus often incurs, are both subjects in and of themselves. They are addressed here only briefly to establish the schedule pressure they place into the overall piping DDE process and to emphasize the need for the EPC contractor to exercise every means possible to reduce the cycle time of the overall pipingDDE process.
Pipespool fabricators
Pipespool fabricators offer varying levels of services, most often influenced by the project setting and complexity, but for all practical purposes they fall into two broad categories: 1) those that supply the piping components and fabricate the pipespools, and 2) those that fabricate pipespools from spool components supplied to them by the EPC contractor. Full-service pipespool fabricators (those that both supply the materials and fabricate the pipespools) are generally found in industrialized settings. These fabricators maintain on-hand inventory — at least for piping components of common metallurgy, wall schedules, and pressure ratings — and use their inventory to jumpstart fabrication. Full-service fabricators also are attractive to an EPC contractor because excess material can be carried over to future projects, at least for commonly used piping components. For an EPC contractor, who approaches each project as a unique cost center, and often ends up shedding surplus at a fraction of value, this approach offers a means to minimize surplus. Fullservice pipespool fabricators become less attractive when the project entails a significant amount of piping compo-
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4DFOBSJP$BTDBEF"MMPDBUJPOmNPOUIGBCEFMJWFSDZDMF Spool 1 BOM
Spool 2 BOM
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#FHJOOJOH .BUFSJBM4UBUVT Inventory Allocations Pipe – 1 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 0 Tee – 1 Tee – 0
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Spool 1 allocation: Pipe & Flange allocated 3FDFJQUT 'FCSVBSZ
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Spool 1 allocation: Pipe & Flange allocated
Result: not constructable
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Spool 2 allocation: Pipe, Flange, & Tee allocated
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*O1SPDFTT .BUFSJBM4UBUVT Allocations Inventory Pipe – 1 LM Pipe – 1 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 1 Tee – 1 Tee – 1 Result: constructable
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Action: hold allocation
*O1SPDFTT .BUFSJBM4UBUVT Inventory Allocations Pipe – 0 LM Pipe – 0 LM Elbow – 1 Elbow – 0 Flange – 1 Flange – 0 Tee – 0 Tee – 0
3FMFBTFTUP GBCSJDBUJPO none
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Action: clear allocation
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Result: not constructable
Releases 4QPPM/FFEFEJO.BZ 4QPPM/FFEFEJO%FD
#FHJOOJOH .BUFSJBM4UBUVT Inventory Allocations Pipe – 1 LM Pipe – 0 LM Elbow – 1 Elbow – 0 Flange – 1 Flange – 0 Tee – 0 Tee – 0
Spool 1 allocation: Pipe, Elbow, & Flange allocated
Result: constructable
*O1SPDFTT .BUFSJBM4UBUVT Inventory Allocations Pipe – 1 LM Pipe – 1 LM Elbow – 1 Elbow – 1 Flange – 1 Flange – 1 Tee – 0 Tee – 0
3FMFBTFTUP GBCSJDBUJPO Spool 1
Action: hold allocation
.POUIFOEJOH .BUFSJBM4UBUVT Inventory Allocations Pipe – 0 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 0 Flange – 0 Tee – 0 Tee – 0 LM: Lineal meters BOM: Bill of materials ROS: Required onsite
Figure 1. In January, this scenario returns Spool 1, which is required onsite in May, back to the pool of allocatable materials, resulting in Spool 2 being delivered nine months early, while Spool 1 (of higher-priority) is delivered seven months late
nents not stocked by the fabricator, and where the project is not geographically close to the fabricator, particularly where there are logistical challenges. Pipespool fabricators that fabricate from materials supplied by the EPC contractor (a process referred to as free-issue) are commonly found in emerging economies. Most have limited procurement capacity, or tend to fabricate for projects where it is not economical to establish and maintain on-hand inventory, in particular for projects where there are unique requirements, or where a significant portion of the piping components are not easily usable on another project. Many operate in settings with limited infrastructure, sometimes setting up a project-specific facility adjacent to the project site. Many times these fabricators, due to their location or project set-up, are able to satisfy the all-toocommon, emerging-economy project requirement for local content.
As such, for the projects that seem to have most challenges relative to the piping DDE process — such as those in emerging economies and those with remote locations and logistical challenges — the prevailing approach is to utilize a fabricator whose scope is limited to fabrication of free-issue materials supplied by the EPC contractor. Such projects, while offering the greatest challenge, also offer the greatest opportunity for an EPC contractor to actively manage the fabricator to deliver pipespools to favorably support the project schedule. The factors discussed below, while primarily directed toward positive, active management of a limited-scope pipespool fabricator, none the less apply to a lesser extent to a full-service pipespool fabricator.
Key management issues
With this background of the piping DDE process and a perspective of pipespool fabricators, we’ll now take a
look at issues that present opportunities for an EPC contractor to actively manage the pipespool fabricator to facilitate delivery of spools, thereby optimizing piping erection, and overall construction of the facility. After reviewing the key issues, we’ll look at the impact of these issues and the potential for positively affecting the project through proactive management.
Integration
First and foremost is the issue of integration. Both EPC contractors and pipespool fabricators operate these days in a highly automated mode. Virtually all EPC contractors design using a 3D model that is integrated with other design-related automation systems, particularly full spectrum (specify/ design/purchase/receive/control/issue) materials management (MM) systems. In parallel, most pipespool fabricators engineer their spools using software that produces their fabrication draw-
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41
Engineering Practice ings. Most fabricators will also have an MM system tailored to their needs and linked to the software that designs and details the spools. Fabricators that are not full service, but rely on free-issue materials, will generally have a limited spectrum (receive/control/issue) MM system. Quite often, both the EPC contractor and the fabricator will be using software suites that are compatible, enabling the fabricator to upload data from the EPC contractor’s 3D model to initiate the spool design and detailing processes. What is not as common is for the EPC contractor and the fabricator to link their MM systems such that each can see what the other sees, as it relates to spool component delivery. For the EPC contractor to effectively and actively manage the process, the first step is to be able to see what the fabricator sees relative to on-hand materials, issued materials, wastage, and constructability at the same increment as the fabricator. Conversely, as we will see below, the key to implementing a just-in-time fabrication program resides in the ability of the fabricator to see the EPC contractor’s delivery data.
Speaking similar languages
EPC contractors generally erect piping using piping isometric (Iso) drawings extracted from the 3D model. An Iso will generally contain several pipespools and the related installing materials (valves, bolts, gaskets, and so on). The Iso will also possess a bill of materials (BOM), which generally identifies the materials required (commonly referred to as takeoff) and splits the BOM between field materials and shop materials, with the shop materials comprising the free-issue materials for the pipespools. The Iso has also historically been the increment by which the EPC contractor managed materials. For instance, the Iso BOM for shop materials generally shows the total requirements for all pipespools on that Iso without distinguishing what is needed for each individual pipespool. As such, Iso BOMs are the increment by which EPC contractors’ MM systems generally operate. On the other hand, the fabricator has no interest in the Iso other than as a reference and always manages ma42
terials at the increment of the spool. Fabricator’s spool cut sheets (detailed spool drawings) possess a BOM just for the materials required for that spool, and the fabricator’s MM systems will manage materials required for the spool at this level or increment. So historically, there has been a disconnect between the EPC contractor MM system and the fabricator MM system, an “apples and oranges” comparison. In today’s computing environment, where the EPC contractor’s MM system is often not designed to extract BOM data at the spools level, the fabricator can easily pass spool BOM-level data to the EPC contractor, and a sufficiently robust EPC contractor MM system can then be configured to manage the free-issue materials at the same increment, for instance, the spool.
Priorities
For the EPC contractor to manage the fabricator such that spools are fabricated and delivered in the sequence at which the EPC contractor intends to erect them, the EPC contractor needs to communicate priorities, and must do so at a granularity that facilitates the allocation process, which is discussed below. Any good MM system will possess an allocation system, and generally, the finer the level of priority granularity, the better the system is able to allocate materials to support desired fabrication and erection sequencing. However, there needs to be a balance here, as specifying too many priorities can have its own downside.
Conflicting goals and processes
The goals of an EPC contractor on a given project and that of the pipespool fabricator are rarely in concert. The pipespool fabricator desires to operate his facility efficiently and at a constant level. This is best accomplished by scheduling groups of spools in a common metallurgy to be released for fabrication together, by scheduling together a group of spools in the same pipe diameter to simplify handling and optimize use of pipe “drops”, and by releasing spools at a constant production rate by 1) building up and maintaining a backlog of constructable spools and 2) by releasing spools at a rate that does not deplete the backlog.
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An EPC contractor, by contrast, generally wants spools delivered in sequence according to planned erection starts of piping in areas at the construction site, per the project schedule. In general, each area will contain a mix of metallurgies and a wide array of pipe diameters. The EPC contractor wants delivery of spools as soon as they are constructable in accordance with the priorities released into fabrication. The EPC contractor has no concern if this might cause a spike in production, and thus resources, at the fabricator, or if it depletes the fabricator’s backlog. Given that the goals of the two parties are not aligned, it should not be surprising that the allocation process each prefers to utilize also conflicts. Allocation processes are routines that form the core of the “control” aspect of an MM system. An allocation process takes the tens of thousands of pipe fittings and hundreds of thousands of feet of pipe — all spread over thousands of inventory codes as they are required on thousands of spools — and, according to the project priorities and control logic, determines which spools should be fabricated in what order.
Typical fabricator allocation
Most fabricators utilize what is commonly referred to as a cascade allocation. This process is designed to maximize current fabrication; for instance, identifying as many spools as possible that are constructable with the current on-hand inventory. Most will have the ability to interject EPC contractor priorities, so that spools are processed sequentially according to the priorities provided by the EPC contractor. But a cascade system is very different from a strict construction-priority allocation, which is commonly utilized by EPC contractors as noted below. Here is how the cascade allocation works. Sequentially by priority, the MM system will look at the first spool BOM, and on an inventory code basis, ascertain if there is unallocated on-hand inventory for that inventory code. If there is, available stock will be allocated to this BOM and deducted from the available pool for following spools. The process then moves to the next inventory code for that spool and executes the
January
Scenario 2 – Construction Priority Allocation – 3 month fab /deliver cycle Spool 1 BOM
Spool 2 BOM
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ROS date – May
ROS date – Dec.
Receipts February
Elbow – 1
Result: not constructable
Beginning Material Status Inventory Allocations Pipe – 1 LM Pipe – 0 LM Elbow – 1 Elbow – 0 Flange – 2 Flange – 0 Tee – 1 Tee – 0
Spool 1 allocation: Pipe, Elbow, & Flange allocated March — Aug.
Release schedule Jan. Spool 2 February – August Sept. Spool 1
Beginning Material Status Inventory Allocations Pipe – 1 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 0 Tee – 1 Tee – 0
Receipts Pipe – 1 LM Flange – 2 Tee – 1
Spool 1 allocation: Pipe & Flange allocated
Result: constructable
In Process Material Status Inventory Allocations Pipe – 1 LM Pipe – 1 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 1 Tee – 1 Tee – 0 Action: hold allocation
Spool 2 allocation: Flg & Tee allocated
Receipts Spool 1: Needed in May Spool 2: Needed in Dec.
0 spools
Impact on Construction Spool 2 received nine months early Spool 1 received seven months late
In Process Material Status Allocations Inventory Pipe – 1 LM Pipe – 1 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 2 Tee – 1 Tee – 1 Result: not constructable
In Process Material Status Inventory Allocations Pipe – 1 LM Pipe – 1 LM Elbow – 1 Elbow – 1 Flange – 2 Flange – 1 Tee – 1 Tee – 0 Action: hold allocation
released in Sept. released in Jan.
Spool 2 allocation: Flange & Tee allocated
Month ending Material Status Inventory Allocations Pipe – 1 LM Pipe – 1 LM Elbow – 0 Elbow – 0 Flange – 2 Flange – 2 Tee – 1 Tee – 1
Action: hold allocation
In Process Material Status Allocations Inventory Pipe – 1 LM Pipe – 1 LM Elbow – 1 Elbow – 1 Flange – 2 Flange – 2 Tee – 1 Tee – 1 Result: not constructable
Releases to fabrication none
received in Dec. received in April
Releases to fabrication Spool 1
Month ending Material Status Inventory Allocations Pipe – 0 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 1 Flange – 1 Tee – 1 Tee – 1
Action: hold allocation
Receipts none
Receipts Pipe – 1 LM September
Releases Spool 1: Needed in May Spool 2: Needed in Dec.
Beginning Material Status Inventory Allocations Pipe – 1 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 1 Flange – 0 Tee – 1 Tee – 0 Spool 1 allocation: Pipe, Flange, & Tee allocated
Result: constructable
In Process Material Status Inventory Allocations Pipe – 1 LM Pipe – 1 LM Elbow – 0 Elbow – 0 Flange – 1 Flange – 1 Tee – 1 Tee – 1
Releases to fabrication Spool 2
Action: hold allocation
Month ending Material Status Inventory Allocations Pipe – 0 LM Pipe – 0 LM Elbow – 0 Elbow – 0 Flange – 0 Flange – 0 Tee – 0 Tee – 0 LM: Lineal meters BOM: Bill of materials ROS: Required onsite
Figure 2. In the construction-priority allocation scenario, the fabrication of Spool 1 is held until materials are available, thereby keeping its materials from being allocated to a lesser-priority spool. As a result, both spools arrive on time
same evaluation. Once all inventory codes for that given spool have been evaluated, the system will look to see if all inventory codes for that spool have been satisfied, in other words if the spool is “constructable”. If so, the allocations are retained. If not, the spool will be considered non-constructable if as little as a single inventory code has not been allocated to on-hand inventory. As a result, the allocations for each inventory code on that spool will be returned to the pool of allocatable material for the remaining spools. The process will then move to the next spool BOM and perform the same analysis. This will continue until all spool BOMs have been assessed. Typical EPC allocation A strict, construction-priority-allocation process allocates on-hand inventory, as the name says, strictly by priority. As with the fabricator cascade system, the strict construction-
priority allocation looks at the spool BOMs on an inventory code basis — doing so sequentially by priority — and allocates on-hand inventory if available, subtracting from the available pool accordingly. Contrary to the cascade process, however, the strict construction-priority process does not look to see if the spool is constructable before moving on to the next spool, nor does it return allocations to the allocatable pool if the spool is not constructable. Allocations once made, are retained, at least until the next run of the process. This process is not intended to ascertain the maximum amount of constructable spools in the current timeframe, but instead is designed to see that priority spools are truly given priority. While this process might appear to be counter-productive when compared to the “cascade” process, as we will see below, the opposite is actually true.
Just-in-time fabrication
“Just-in-time” fabrication implies just what it says, fabrication just as the spool components arrive. This is similar to the just-in-time delivery concept used in manufacturing processes, but here the just-in-time concept applies to the end product, not the component. As noted above, fabricators generally try to maintain a good backlog of constructable spools, usually four to eight weeks worth, so that they don’t find their workers standing by idly with no spools to fabricate. For projects where fabrication proceeds from free-issue materials, the fabricator’s MM system rarely has any knowledge of future deliveries and is limited to planning work according to on-hand inventory. This is another issue where integration with the EPC contractor, either by linking MM systems and downloading delivery data into a capable fabricator MM system, or by providing the fabricator access to the EPC contractor’s MM sys-
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Engineering Practice tem, or some melding of processes in between, allows the fabricator to use future deliveries as backlog. We will discuss this more in the sections below.
Benefits of active spool management
Now that we have highlighted key issues, let’s look at how they facilitate active spool management and how active spool management increases the likelihood of spool deliveries to support the project’s planned erection schedule.
Integration
Integration is the thread that runs through all of the key management issues. Without integration, the EPC contractor must rely on the pipespool fabricator to provide spool status and on-hand inventory of spool components at the fabricator’s facility. Conversely, the fabricator only knows what he has, not what is coming. By allowing each to see what the other sees, communication is much more open, and where either one desires information from the other, that information is often available by looking, instead of asking for it and waiting for a response.
Management at the spool level
Where an EPC contractor leaves his or her MM system incremented at the Iso BOM level, efforts to address why specific spools have not been released — for which the EPC contractor’s MM system appears to show constructability — are often futile. Basically, unless the EPC contractor is managing at the same increment as the fabricator, and is actively reviewing constructability data at the spool level, the EPC contractor simply must rely on the fabricator to assess and ascertain constructability. This can often lead to significant frustration on the part of the EPC contractor, and unnecessary efforts expended by the fabricator to justify what has been released to fabrication. This is particularly true given the conflicting goals the two parties tend to work toward, which, without open communication, can cause unnecessary friction between the parties. On some projects, an EPC contractor who has not integrated and does not have the means to manage at the 44
F
Case Study: Cascade vs. Strict Construction Priority
igures 1 and 2 take us through a very simple set of examples of the two differing allocation processes. Here we have two spools with slightly different, but overlapping requirements. Scenario 1 shows the individual steps in the process for a cascade allocation; Scenario 2 does the same for the strict construction-priority allocation. The only substantive difference in the two scenarios occurs in the month of January where the cascade process returns the pipe for the higher-priority Spool 1 (with a required-onsite or ROS date in May) back to the pool of allocatable materials, because this spool is not constructable, and then allocates the pipe to lower priority Spool 2 (required-onsite or ROS date in December), because Spool 2 is constructable. The strict construction-priority process as shown holds this allocation for priority Spool 1. Looking at the net result of these two processes from an aggregate delivery perspective, independent of priorities, the cascade process appears superior; it gets a spool into fabrication one month earlier also getting the spool onsite one month earlier. Looking at the same net result from a priority focused aggregate perspective, however, the cascade allocation process has a devastating effect on planned erection. Scenario 2 gets one spool into fabrication, and thus onsite, one month later but gets both spools onsite when needed. Scenario 1 gets one spool onsite nine months early and one spool onsite seven months late. This case study also demonstrates another negative effect of a lack of integration. Where the fabricator MM system is ignorant of future deliveries — generally the case in a free-issue scenario — it has no way of knowing that the elbow, which is restraining Spool 1 in January, is scheduled to arrive in February making priority Spool 1 not constructable until then. The EPC contractor MM system, which has this data, however, is not thusly impaired. r
spool level will attempt to direct the fabricator to release spools based on Iso constructability. By this, we mean the EPC contractor waits until all shop materials on an Iso are allocated. This, however, can significantly delay release of spools that would otherwise be constructable. An Iso will contain anywhere from one to five or six spools, averaging about three spools. When managing at the Iso level, it only takes one item — something as insignificant as a minor fitting — to make the Iso nonconstructable. Where a single item is holding up the Iso, it is unnecessarily holding up spools that are otherwise constructable. The solution here is, of course, to manage at the spool level.
Manageable priority granularity
Too few priorities tend to clog the allocation process, yielding a slug of pipespools being delivered all at once. Where EPC contractor construction resources for piping erection are limited and need to be spread out, this scenario delays erection commencement and causes unacceptable construction resource peaks. Conversely, a very high level of priority often leads to priority reshuffling, which tends to have a detrimental impact on planning. The best scenario is where the EPC contractor has thoughtfully planned the work, breaking the project scope into manageable areas coinciding with schedule events (such as area access, equipment erection, system handover, and so on), and where this sequencing
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is translated into a set of priorities at a granularity level where the sequencing can be expected to hold.
Supporting project goals
In the case study in the box, we demonstrated the potential for detrimental impact of the cascade process on deliveries of spools to the construction site to support planned erection. A small project will have 5,000 or so spools; a large project may have 50,000 to 75,000. If we multiply the disconnect of deliveries relative to planned erection of the two spools in our example by 10,000 (to represent a medium size project with 20,000 spools) the impact becomes clear. Thousands of spools will arrive early, requiring unnecessary storage. Thousands will arrive late, causing construction delays. If the fabricator’s system is limited to a cascade allocation, the only way to implement a strict construction-priority allocation is via the EPC contractor’s MM system. And without both integration and management at the spool level, this will be very difficult.
Accelerated releases
As noted above, a fabricator that cannot see what his backlog includes, or who does not have the information to predict workload into the short-term (at least two to three months) can expect some unpleasant surprises, both due to pressure from production peaks, and from idle staff in an unforeseen production valley. Most fabricators try to avoid such surprises by scheduling
production based on what they can see from on-hand inventory. In a situation where the EPC contractor and the fabricator have integrated, and where the shop load can be predicted from both on-hand inventory and future deliveries, the fabricator can schedule production to release all constructable spools, up to his capacity level, in the current timeframe. Instead of establishing a backlog of spools from on-hand inventory alone, the fabricator can include spools that show to be constructable in the short-term based on both on-hand inventory and deliveries scheduled in the short-term. In a situation where the fabricator would otherwise build up a four to eight week backlog of on-hand inventory, the production schedule could ideally be brought forward four to eight weeks. The net result would be to move the entire production schedule forward (sooner) four to eight weeks, yielding a net result to the project of all spools being deliv-
ered four to eight weeks earlier. By itself, independent of allocation process issues, just-in-time fabrication can gain the project one to two months of schedule, that is, if the piping DDE process is on the critical path. Or it might get the piping DDE process off the critical path, allowing the EPC contractor to refocus resources elsewhere to improve schedule. In any case, just-in-time fabrication cannot be achieved outside of an integrated relationship, and only if the EPC contractor is proactively engaged in management of the fabrication process, working to see that critical spool component deliveries are on track to support just-in-time fabrication.
Conclusion
EPC contractors, and pipespool fabricators who work with them, will continue to be challenged to meet the demanding fast track schedules presented by projects in emerging econo-
mies, particularly those with logistical challenges. By closing communication gaps and actively engaging pipespool fabricators, EPC contractors can be much better positioned to succeed. ■ Edited by Rebekkah Marshall
Author Stephen Wyss is a Materials Manager at Bechtel Oil, Gas, and Chemicals, Inc, (3000 Post Oak Blvd, Houston, TX 77056-6503; Phone: 713-235-4625; Email:
[email protected]) and has 33 years experience working with EPC contractors, including previous tenures at Black & Veatch Pritchard, CF Braun, and Intergraph. His current duties entail coordinating materials related aspects of engineering, procurement, suppliers, and construction for large capital process plant projects, in general for bulk materials such as piping, electrical, and structural, but particularly for complex fabricated systems such as pipespools and structural steel. His project experience has generally been in emerging economy environments with logistical challenges including the Middle East, India, and Africa. A registered mechanical engineer in Texas and California, he holds a J.D. degree in law from Loyola Law School (Los Angeles) and an A.B. degree in architecture from the University of California at Berkeley.
Circle 22 on p. 58 or go to adlinks.che.com/23010-22
Feature Report Engineering Practice
Reduce Gas Entrainment In Liquid Lines P0 T0
V-1
h
Follow these tips to properly size self-venting lines and vortex breakers
P1, T1
H0
P2, T2
H1
V-2
H2
Tamagna Ukil and Thomas Mathew Reliance Industries Ltd.
Theoretical basis
A typical GLS arrangement with gravity flow is shown in Figure 1. The operating pressure of the first vessel 42
(V-1) is P0 (psig) and its operating temperature is T0 (°F). The operating pressure and temperature of the second vessel (V-2) are P2 and T2, respectively. The pressure and temperature of the liquid at the exit nozzle of V-1 are P1 and T1, respectively. In Figure 1, the region from the exit of V-1 to the inlet of V-2 is highlighted with a dashed outline. It shows that the associated piping of the system consists of pipes and elbows. The following assumptions are considered for this system: • Liquid flowing through the line is incompressible • The system is in steady state • There is no flashing of liquid • Pressures P0, P1 and P2 are constant • The pipe size is uniform
System equations
Step 1. The pipeline is sized for liquid flow using a conventional line-sizing approach for typical velocity considerations and least annual cost. Table 1 shows typical liquid velocities in steel pipelines. Table 1 shows typical velocities in steel pipelines with liquid flow [3]. It provides a good estimate for the preliminary selection of the pipeline size with respect to its nominal bore (N.B.) dimensions. As Table 1 provides generalized data, readers can use the values provided for any type of pipes, irrespective of metallurgy or material of construction. By applying the lowest-annual-cost
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FIGURE 1. Shown here is a typical gas-liquid separator, with gravity flow from V-1 to V-2 [1] Dollar per year per foot of pipe
A
common practice in the chemical process industries (CPI) is to implement so-called valuemaximization projects (VMP) to increase production or reduce production costs in order to increase profit margins. With such projects, one main objective of the design team is to incur minimum capital expenditures. Because most VMPs aim to increase throughput or production yield, many such projects involve changes to the process that result in an increase in the volume of feed flowing into a gasliquid separator (GLS). The system modifications that are required often call for: • The design of a new GLS to accommodate the increased flow, or • The modification of the vessel internals and associated piping to handle the increased feed flow Increased feed flow into any GLS can lead to the entrainment of gases into the liquid lines. Such gas entrainment can lead to pulsating flows in the line, which can result in vibration and potentially destabilize the downstream processes. In many cases where GLS are provided with “gravity-flow pipelines” — a common approach, as it provides an inexpensive way to transport liquids — the use of self-venting pipelines coupled with properly sized vortex breakers can mitigate the problem of entrainment of gases into liquid lines.
Ground
20
15
10
5
0
1
2 3 4 6 Nominal pipe size, ft
8
FIGURE 2. The relationship between amortized capital cost per foot of pipe and nominal pipe size is shown here [3]
approach as stated by Moharir [3], the cost of the pipe material per unit length for a run of pipe with diameter D is calculated using Equation (1):
(1)
Along with the pipe, the cost of accessories and fittings must also be factored in, hence their number must also be computed on a per-unit-length basis. For instance, if a pipeline of 100 ft has 5 gate valves, 4 long-radius elbows of 90 deg, 2 tees and 7 weld joints, then its per-unit fitting cost can be taken collectively as a factor F. If the amortization rate is AM and the annual maintenance cost is a fraction G of the capital cost, then the annualized capital plus maintenance cost of the pipeline, CP, is calculated using Equation (2):
Table 1. Typical velocities in steel pipelines with liquid flow [3]
Start
Nominal pipe size, in.
2 or less
3 to 10
10 to 20
Liquid and line
Velocity, ft/s
Velocity, ft/s
Velocity, ft/s
Pump suction
1 to 2
2 to 4
3 to 6
Pump discharge (long)
2 to 3
3 to 5
4 to 7
Discharge heads (short) 4 to 9
5 to 12
8 to 14
Boiler feed
5 to 9
5 to 12
8 to 14
Drains
3 to 4
3 to 5
Water Obtain initial pipe diameter using Table 1 Optimize the diameter using annual cost approach to get D'
NO
Check Fr < 0.31
Select the N.B. of pipe such that Fr < 0.31
YES Select the D' and size vortex breakers
FIGURE 3. This flowsheet illustrates the types of decisions that must be made to properly size gravity flow lines and vortex breakers, to reduce gas entrainment
(2)
Rearranging Equations (1) and (2) produces Equation (3):
(3)
In most cases, another component, CF, is needed to calculate is the operating cost. However, in this case, the operating cost component CF is not considered due to the absence of any rotary equipment. Differentiating CP with respect to D, to obtain optimum diameter of the pipeline (D′) and setting it to zero, Equation (3) can then be simplified as follows:
3 to 5
4 to 7
(Normal viscosities)
1.5 to 2.2
2 to 4
3 to 6
Pump suction
2.5 to 3.5
3 to 5
4 to 7
Discharge heads (long)
4 to 9
5 to 12
8 to 15
Boiler feed
3 to 4
3 to 5
Hydrocarbon liquids
——
——
——
——
Pump suction
——
——
——
Viscous oils
Stop
——
Drains
Vortex breakers to be of 2D X 2D dimension
Sloped sewer
——
(4)
Figure 2 shows the relationship between the amortized annual cost per unit length of pipe (ft) and nominal pipe size (nominal bore). From the two methods described above, D′ is obtained as an initial line size in terms of nominal pipe size (nominal bore) of the pipe. Step 2. The next step is to carry out the Froude number analysis for the line using the diameter obtained from Step 1. As per Simpson’s article [2], if
Medium viscosity
——
1.5 to 3
2.5 to 5
Tar and fuel oils
——
0.4 to 0.75
0.5 to 1
Discharge (short)
——
3 to 5
4 to 6
Drains
1
the fluid inside a vessel does not rotate and if the liquid level in the vessel is below a certain height, then gas will get sucked into the liquid line. A conservative estimate of this level was derived by Harleman et al. [1], Harleman’s equation is:
(5) Equation (5) can be used to estimate the height of the liquid inside V-1 below which the gas would be sucked into the liquid line. Experiments on 13/16-in. pipeline and on 1-in. to 4-in. pipelines by Simpson and Webb [2], respectively, show that if the Froude number in the pipeline is less than 0.31, then gas will not be entrained. If the Froude number of the liquid flowing in the pipeline is greater than 0.31, then gas starts getting swept up by the liquid. High, two-phase pulsating flow is observed when the Froude number is between 0.31 and 1. This is the basis of design for selfventing lines: Any provision for selfventing lines should ensure that the Froude number remains between 0
1.5 to 3
——
and 0.31. The typical velocity of liquid in self-venting pipelines is in the range of 1 ft/s. Step 3. When the flow inside a vessel is rotational, vortex breakers should be provided to prevent gas entrainment into liquid lines. If V-1 has a feed entry point that is tangential to the vessel, it will induce a swirling motion in the liquid, like a whirlpool. If this swirling motion is strong enough to reach the liquid exit nozzle of V-1, then it would lead to entrainment of gas into the liquid pipeline. Borghei’s experiments [4] in pipelines of 2-in. to 4-in. show that vortex breakers with dimensions double the nominal bore of the pipe are highly efficient in reducing the vortex effect inside the vessel. Thus in V-1, with a self-venting liquid exit line, the vortex breaker arrangement should be in the form of a cross (+). When the vertical and horizontal dimension of the plates that are used to fabricate the vortex breaker have a dimension of 2D’, each can substantially reduce the entrainment of gas into the liquid exit. The steps described above can be summarized in the flowsheet shown in Figure 3.
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Engineering Practice Authors
Nomenclature AM Amortized cost per unit length of pipe, $/ft CD Cost per unit length of pipe, $/ft CP Total capital cost per unit length of pipe, $/ft F Pipe fitting cost per unit length of pipe, $/ft Fr Froude number G Maintenance cost per unit length of pipe, $/ft g Acceleration due to gravity, ft/s2
The following conclusions can be made from the discussion above: 1. The line size full of liquid will always be smaller than the self-venting line. 2. The work described in Refs. 2 and 4 are based on small lines (up to 4-in. nominal bore). 3. If liquid flow varies during operation, the pipe should be sized to accommodate the maximum possible flow.
h Height inside V-1, ft D Initial pipe dia., in. D’ Optimum pipe dia., in. D” Dia. of self-venting line, in. P0, P1, P2 Pressure shown in Fig. 1, psig T0, T1, T2 Temperature shown in Fig. 1, °F V Velocity of the liquid through the pipeline, ft/s X Cost per unit length of 2-in. nominal bore pipe of the same material and schedule, $/ft
4. D′′ obtained from Equation 5 should be rounded off to the higher nominal bore of pipe of standard available size. n Edited by Suzanne Shelley
References 1. Yu, F.C., Hydrocarbon Proc., Nov. 1997. 2. Simpson, L.L., Chem. Eng., June 17, 1960, p. 191. 3. Moharir, A.S., Pipe hydraulics and sizing, IIT Bombay, May 7, 2008. 4. Borghei, S.M. Partial reduction of vortex in vertical intake pipe, Scientiairanica, Vol 17, Issue 2.
Tamagna Ukil is the Manager of PTA-Process at Reliance Industries Ltd. (Reliance Corporate Park, Ghansoli, 7-B Ground Floor, Navi Mumbai Maharashtra, India; Phone: +912-244-783-452; Email:
[email protected]). He holds a B.S.Ch.E. from Utkal University. He is a Certified Piping Engineer from IIT Bombay, and has been working with Reliance Technology Group, PTA Division, to provide advanced technical services in the field of design, simulation and process optimization for the manufacture of purified terephthalic acid (PTA). Thomas Mathew is president of Reliance Industries Ltd. He graduated as a Chemical Engineer from Kerala University (Trichur Engineering College), and spent the first 16 years of his career involved in the production of ammonia from numerous raw materials, including natural gas, naphtha, fuel oil and coal. Mathew participated in the startup of two coal gasification plants and served as plant manager for five years in the coal gasification plant at Ramagundam, India. He joined Reliance in 1985 and took charge of the commissioning and startup of several petrochemical plants, before heading the manufacturing operations of the Reliance’s Patalganga Complex. He leads the Centre of Excellence in PTA and Gasification within Reliance.
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Chemical Engineering www.che.com June 2011
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Feature Report Engineering Practice
6
Designing
5 4 3
Safer
2 1
Process Plants Several often-overlooked strategies to increase inherent safety are discussed here Victor H. Edwards, P.E., Aker Solutions
M
any individuals and organizations have made important contributions to the creation of inherently safer (IS) products, processes and process plants [1–3]. A brief survey of successful case histories shows that most reported applications relied on only a few of the core IS principles. This paper emphasizes the opportunities presented by three particular — and often-overlooked — possibilities for inherently safer processes. The methods proposed here ensure integration of IS methods beginning with process conception and continuing through process plant engineering design. Particular emphasis is given to matching the IS principles with the state of the project. For example, substitution is best applied during product and process research, while limitation of effects is most effective during plot plan layout and equipment arrangement. The chemical process industries (CPI) face the challenge of working with processes and products that present many hazards, such as the following: • The manufacture of fuels uses and produces products that burn with significant energy release • Certain basic chemicals, such as mineral acids and halogens are toxic and/or corrosive • Many manufacturing processes either release or require significant 44
energy transfer to achieve chemical transformation • Some manufacturing processes produce benign products but require hazardous chemical intermediates in their manufacture For these reasons, rigorous process and product safety practices must be used throughout the lifecycle of process plants and must be applied to their associated raw materials and products. In recent years, this has led to major efforts in green chemistry and engineering to develop products, manufacturing processes, and plants that are safer for both people and the environment. Before green chemistry and engineering achieved prominence, there were pioneering insights in the design of safer process plants. Early approaches to safer processes often employed additional instrumentation and procedures. These measures were often helpful and necessary, but instrumentation and operators can fail, especially when faced with complexity. Trevor Kletz [1] recognized that “What you don’t have can’t leak”, when he first proposed the concept of the inherently safer chemical processes in 1977. His approach placed an emphasis on the inherent nature of the process. Since then, important related concepts such as product design for safety and safer products, process and plant lifecycles have also advanced. Creation of IS processes has been the
Chemical Engineering www.che.com April 2011
1. Process design 2. Basic controls, process alarms and operator supervision 3. Critical alarms, operator supervision and manual intervention 4. Automatic action safety-instrumented systems (SIS) or ESD 5. Physical protection (relief devices) 6. Physical protection (dikes) 7. Plant emergency response 8. Community emergency response
FIGURE 1. Shown here are some typical layers of protection that can be employed in a modern process plant [4]. At the core is an inherently safe process design. Moving outward from the core, the proposed options move through the spectrum from inherent to passive to active to procedural or administrative controls, which are considered to be progressively less reliable
objectives of a number of creative individuals and organizations since Kletz’s path finding proposal, with many notable successes. Complete coverage of the entire product/process/plant lifecycle is needed to assure optimum health, safety and environmental performance of a chemical enterprise. This article focuses on how to ensure maximum incorporation of IS processes into the creation of a process plant by beginning at the product and process research stages and concluding with the detailed design. No effort is made to address the application of inherently safer principles beyond plant design, although these are also important.
Layers of protection
The classical onion diagram (Figure 1) illustrates the safety layers that technical professionals throughout
Often-overlooked IS concepts
FIGURE 2. The Flixborough tragedy ushered in a new era in process safety [6]
the CPI use to prevent process plant incidents. This diagram helps to explain the following four basic process risk-management strategies: Inherent, passive, active, and procedural or administrative Inherent safety is at the core of the onion — the process design. A process that cannot have a major fire, explosion or toxic release is inherently safer than one that could if one or more layers of protection were to fail. Passive safety layers represent the addition of such safety features as a dike or a blast wall. Because passive layers of protection require no active intervention by a human or by a machine, they are deemed more reliable than active layers of protection or procedural layers of protection. Nonetheless, the ability to make an explosion impossible — when possible — is clearly better than trying to mitigate the effects of a potential explosion by adding a blast wall. Active layers of protection represent such features as the basic process control system, a safety-instrumented system, and mechanical interlocks. Procedural or administrative safety layers are generally considered to be the least reliable and include operating procedures and operator intervention. Depending on the site-specific hazard, procedural or administrative controls may be entirely appropriate. In general, the preferred ranking of methods to control process risks is shown below: Inherent > passive > active > procedural or administrative
Basic concepts
Inherently safer process concepts are summarized below [1]: • Substitution • Minimization or intensification • Moderation or attenuation • Simplification • Limitation of (hazardous) effects • Avoiding knock-on effects • Making incorrect assembly impossible • Make status clear • Tolerance of error • Ease of control • Administrative controls or procedures In 2007, the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers (AIChE) concluded that these eleven basic concepts could be reduced to the following four principles [2]: • Minimize • Substitute • Moderate • Moderate and simplify This more concise set of principles makes IS practices simpler to understand and easier to apply. The excellent new CCPS book (2009) goes on to distinguish between first-order and second-order IS: • First-order IS efforts change the chemistry of a process • Second-order IS efforts change the process variables As can be seen by a survey of the process safety literature, most published work has applied one or more of the first four concepts of the eleven cited by Kletz and Amyotte [1] For this reason, this article emphasizes three other promising concepts.
Three underutilized IS concepts are presented here and illustrated with examples: 1. Hybridization or transformation. One relatively new IS concept is based on the recent innovative work by Chen [5] who reports an inherently safer process for the partial oxidation of cyclohexane. Partial oxidation processes often involve hazardous conditions, as illustrated by the Flixborough, England, tragedy in 1974 — which killed 28 people, destroyed a plant, led to new process safety regulations, and inspired Trevor Kletz to propose his inherently safer design concept. The Flixborough plant carried out liquidphase oxidation of large inventories of hot cyclohexane in large pressurized vessels. When containment was lost, a large flammable vapor cloud formed, ignited, and exploded with devastating effect (Figure 2, from Mannan [6]). The traditional cyclohexane-oxidation process to produce a mixture of cyclohexanone and cyclohexanol (K/A oil or ketone/alcohol oil) was operated at low conversion rates (typically 3–5%) to avoid formation of unwanted byproducts. The K/A oil was subsequently converted into adipic acid and caprolactam for the production of nylon. Oxidation of cyclohexane with air instead of oxygen is common practice to reduce risks of transition from a partial oxidation reaction to an uncontrolled deflagration in bubbles or in the vapor space in the reactor. Low conversions and reaction rates led to large inventories of liquid cyclohexane. During systematic research on the flammability and deflagration hazards of cyclohexane, air and oxygen mixtures, Chen [5] discovered that the addition of a small amount of water — which is inert and does not participate in the reaction — helped to inert the otherwise flammable vapors. Cyclohexane and water are known to form minimum-boiling azeotropes. The increase in the vapor pressure of the cyclohexane/water liquid results from the increased vapor pressure of the water. The water vapor inerts the vapor mixture by lowering the upper flammable limit of the vapor [5]. Chen’s work suggests that it will be
Chemical Engineering www.che.com April 2011
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E
Engineering Practice
46
Btu/h
safe and practical to use pure oxygen for cyclohexane oxidation. Benefits include both IS operation and improved productivity. They also suggest that this approach could be extended to safer processes for partial oxidation of other liquid hydrocarbons using pure oxygen. Chen’s approach is a first-order IS process innovation because it changes the chemistry of the gas phase in a gas-liquid reaction and prevents the unwanted side reaction of combustion from occurring in the gas phase. Although reference [5] did not claim to have demonstrated a new IS concept, Chen’s work is different from the classical definition of the Substitute principle because the same reactants, chemical reactions, and products are involved. If the name Substitute were broadened to names such as Change in Chemistry or Hybridize, then it could be lumped in with the many successful applications that are possible when using the Substitute concept. Chen’s innovation permits rapid cyclohexane oxidation at lower temperatures and pressures, and could thus be said to be an example of the inherently safer principle Moderate. However, Chen’s approach enables more moderate conditions by narrowing the flammability limits through the addition of a new component, water. It is thus an example of supplementation or hybridization. Although not proposed by Chen [5] himself, his work suggests that there may be many other opportunities for transformation or hybridization of other potentially hazardous reactions to make them inherently safer. Although water would be high on anyone’s list as a potentially transforming additive, it probably will not help many potentially hazardous reactions. However, there are many other chemicals that may be inert to the reaction and thus also be capable of inerting the vapor phase involved in an otherwise reactive liquid-vapor reaction. For instance, there are many examples of azeotropic mixtures in the literature and there are many compounds that could prove inert to oxidation reactions (such as, certain halocarbons). Applications are not limited to partial oxidation with air or oxygen;
Qout
3 D
Qout
B
Qheat generated
Heat removed Qout
other oxidations include 1 2 chlorination and bromination reactions, for exC ample. And there may be A other examples of vaporliquid reactions, such as Reactor temperature hydrogenation reactions, where addition of a new FIGURE 3. Heat-generation (Q heat generated) and heatchemical could improve removal (QOut) rates as a function of reactor temperature the safety of the process. for three different heat-removal designs [9]. Heat genAddition of an ad- eration is equal to heat removal at points A, C, D, E, and so steady state operation is possible. However, the ditional compound to a B, reactor is not stable at point D without the addition of reaction mixture to min- controls or a modification of the design imize hazardous reactions may add complexity to the puri- in the process variables and is a form fication process, but it may be justified of Moderate, a second-order inherently safer design. by the increased safety. CCPS [2] also cites the work of Chen’s [5] paper on cyclohexane oxidation illustrates transformation Luyben and Hendershot [8] that highor hybridization, in which the basic lights how minimization or intensifichemistry is maintained, but the ad- cation in a reaction system that is indition of another chemical component tended to improve process safety may transforms a potentially hazardous re- lead to less robust processes with the opposite effect. action process into a much safer one. I propose here that Stabilize or En2. Create a robust process to stabilize or ensure dynamic stability. sure Dynamic Stability be added to Not all process designs are inherently the list of IS concepts to be sure that stable, and if the process design is to be it is not overlooked in the quest for insafe, the process engineer must ensure herently safer processes. Application of some of the other IS dynamic stability as well as ensuring that the steady-state mass and energy principles can adversely affect the dybalances are achieved. A number of namic stability of a process. For examprocesses exist that have narrow safe- ple, reduced liquid inventories (Minioperating limits but have been made mize) in a distillation train make the stable by the addition of control sys- process inherently safer from one pertems. Dynamic stability and control spective because the smaller process of chemical processes has been exten- inventory decreases the consequences of loss of containment. However, the sively studied [7]. Designing the process to be more smaller inventory also shortens the inherently stable to process upsets response time of the distillation syswith and without control systems is tem to process upsets, increasing the clearly inherently safer, although this risk that the basic control system will principle is not addressed in most dis- not be able to restore the distillation cussions of IS. The IS principle Ease of system to the desired operating condiControl has usually been interpreted tions and avoid a potentially unsafe to mean a process with a control sys- operating condition and/or an untem that the operator can understand scheduled process shutdown [2]. Chemical reactors carrying out clearly and manage effectively. CCPS briefly mentions the advan- exothermic chemical reactions are tages of designing processes that are perhaps the best known examples of processes that can be dynamically inherently more stable or robust [2]: “It is inherently safer to develop unstable. Harriott [9] provides the ilprocesses with wide operating limits lustration of an irreversible first-order that are less sensitive to variations in chemical reaction being conducted in the operating parameters...Sometimes a continuous-flow, stirred-tank reactor this type of process is referred to as a (CSTR). Figure 3 shows the heat-generation rate by the chemical reaction forgiving or robust process.” Designing a robust process increases as a function of reactor temperature. inherent safety by imposing a change Heat-generation rates are low at low
Chemical Engineering www.che.com April 2011
Tools for Inherently Safer Process Plant Design • P rocess hazards reviews • Chemical interaction matrices • Dow Fire and Explosion Index and Chemical Exposure Index • Fire, explosion and toxic-release consequence modeling and risk assessments • Layer of protection analysis • Spacing tables for units and for process equipment
temperatures, but as temperature increases, the reaction rate increases rapidly because of the exponential dependence of the reaction rate coefficient on temperature. At higher reactor temperatures, the shrinking concentration of reactant (due to conversion to product) reduces the reaction rate and partially overcomes the still-increasing reaction-rate coefficient. The heat-generation rate eventually reaches a constant maximum value when the reaction has reached complete conversion. Figure 3 also shows three different straight lines for the heat-removal rate from the reactor for three different reactor-cooling-system designs. To achieve a steady-state energy balance, the rate of heat generation (Qheat generated) by the chemical reaction must equal the rate of heat removal (Qout) by the reactor cooling system. That energy balance occurs when the heat generation curve intersects the heat removal curve (where Qheat generated = Qout). In Figure 3, the three different heat-removal-rate lines intersect the reactor heat generation rate curve at five points. At four of these points (A, B, C, E), the steady-state energy balance solution is stable. At each of these points, if there is an increase in temperature, the rate of heat removal increases more rapidly than the rate of heat generation by the reaction and the reactor temperature tends to return to the desired operating point. Similarly, if the temperature drops slightly at one of these four operating conditions, the rate of heat removal decreases more than the rate of heat generation by the reactor and the temperature trends back up to the desired operating condition. In contrast, point D in Figure 3 is an inherently unstable operating condition even though the steady state rate of heat generation by the reactor equals the rate of heat removal by the reactor cooling system. At point D, an increase in reactor temperature increases the rate of heat generation by the reactor
• Dynamic process simulation • Inherent safety analysis • P eriodic design reviews during product and process research, development and design • R eviews of plant siting, plot plan, equipment arrangement and 3-D computer models
more than it increases the rate of heat removal by the reactor cooling system, so the reactor temperature increases more instead of cooling back to the desired operating point. This further increase in reactor temperature then leads to an even larger rate of heat generation rate by the reactor and additional heating of the reactor. Without any effective control actions, the reactor temperature will tend to increase to point E in Figure 3 before it stabilizes. Similarly, in Figure 3 a decrease in reactor temperature at point D could eventually lead to the reactor temperature and conversion dropping back to point C. Clearly, of the three reactor coolingsystem designs represented by the three straight lines in Figure 3, the reactor cooling system represented by line CDE is the least desirable from a dynamic-stability perspective. Addition of an effective control system might be able to provide dynamic stability — but at the cost of installation and maintenance of the control system and at the cost of residual risk if the control system fails. Another example of potential sources of process instability results from efforts to improve energy efficiencies in distillation trains through heat integration. In these cases, the feed to a column may be preheated by the bottoms product of a second downstream column. This may increase the risk of process upsets due to increased interactions between the two columns. While avoidance of add-on controls has always been a goal of inherently safer design, achievement of that goal has seldom mentioned the concepts of Ensure dynamic stability or Stabilize as tools of the process engineer. It should be considered when considering other means to assure inherently safer processes during process design. The process engineer should work closely with the control systems engineer to address the dynamic stability
• O ccupied building evaluation and design • Area electrical classification • Safety integrity level assessments and safety instrumented systems • Human factors reviews • Ergonomics reviews • Safety case development • The design process itself
of both the uncontrolled process and the controlled process to ensure a robust process. 3. Limit hazardous effects during conceptual and detailed engineering. David Clark published a seminal paper [10] on the limitation of effects when siting and designing process plants. He reminds us that there is a strong, non-linear decrease of fire, explosion, and toxic effects with separation distance. Comparatively small decreases in separation distance have a major effect, while larger increases in separation offer diminishing returns. Methods, such as the Dow Fire and Explosion Index [11] and the Dow Chemical Exposure Index [12, 13], provide quantitative screening estimates of the hazards from various parts of a chemical process. Other indices have been developed and evaluated to perform a similar objective to the Dow indices [1, 2, 14]. These screening tools can identify those parts of a process where increased separation distances are needed to limit potential escalation of an incident. In one typical plant design, a 10% increase in separation distances for all units increases total plant investment cost by only 3%. Similarly, doubling the separation distance for a hazardous unit representing 10% of the investment cost of the plant would cost only 3% more. Because of the nonlinear effect of separation distance, doubling the separation distance for a hazardous unit could reduce explosion overpressures on the adjacent units by a factor of four or more. The strong decrease in hazardous effects with modest increases in separation distances will often more than justify increased capital cost. Spacing also offers important benefits in crane and other maintenance access, ergonomic advantages and decreased risk of incident escalation. Future plant expansions or process improvements are also facilitated, although expansions that decrease spacing may increase hazardous effects.
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47
Engineering Practice Applying different IS principles
As discussed, the different IS principles are best applied at different stages of the process plant timeline. Although IS checklists are often used at the screening process hazards analysis (PHA) level, much more is needed throughout the development and design of a process plant. For example, Substitute is best done during the product and process research phases before significant investments of time and resources in a particular product and process are made. Hybridize or Transform is best done during process research and development, as is Moderate. Minimize, Simplify, and Error tolerance have the best result when applied during the process development, conceptual design and detail design phases. Stabilize or Ensure Dynamic Stability is also best done during design development. Limitation of effects, which is closely related to passive protection, has its greatest impact during development of the plot plan and equipment arrangement.
IS processes and plants
As mentioned previously, the CCPS [2] defines two levels of inherent safety: • First-order inherent safety results
References 1. Kletz, Trevor A., and Amyotte, Paul, “Process Plants – a Handbook of Inherently Safer Design,” 2nd Ed., Taylor and Francis, Philadelphia, PA, 2010. 2. Center for Chemical Process Safety (CCPS), “Inherently Safer Chemical Processes – A Life Cycle Approach,” 2nd Ed., AIChE, New York, NY, 2009. 3. Hendershot, Dennis C., An overview of inherently safer design, Process Safety Progress, Vol. 25, No. 2, 98–107, June 2006. 4. Dowell, III, Arthur M., Layer of protection analysis and inherently safer processes, Process Safety Progress, Vol. 18, No. 4, 214–220, Winter 1999. 5. Chen, Jenq-Renn, An inherently safer process of cyclohexane oxidation using pure oxygen – An example of how better process safety leads to better productivity, Process Safety Progress, Vol. 23, No. 1, 72–81, March 2004. 6. Mannan, Sam, Ed., “Lee’s Loss Prevention in the Process Industries,” 3rd Ed., Elsevier Butterworth Heinemann, Oxford, U.K., 2005. 7. Edgar, Thomas F., and others, Process Control, Section 8 in “Perry’s Chemical Engineers Handbook,” 8th Edition, Don W. Green, Editor-in-Chief, McGraw-Hill Book, New York, NY, 2008. 8. Luyben, W.L., and Hendershot, D.C., “Dynamic disadvantages of intensification in inherently safer process design,” Industrial Engineering Chemistry Research, Vol. 43, No. 2 (2004) cited in CCPS, 2009. 9. Harriott, Peter, “Process Control,” McGrawHill, New York, NY, 1964.
48
from changes in the chemistry of a process that reduces the hazards of the chemicals used or produced. Substitute or Hybridize efforts lead to first-order inherent safety • Second-order inherent safety results from changes in the process variables. Examples include Minimize, Simplify and Stabilize the operations. It is also helpful to distinguish between IS processes and IS plants. Even when hazards cannot be eliminated from the chemistry of the process, the plant using the potentially hazardous process can be made inherently safer through judicious design. Note also that even with IS process chemistry, it is essential to employ IS principles during the process and plant design to ensure an IS plant.
Tools for IS plant design
There are a number of tools available to aid in designing process plants that are inherently safer (Box, p. 18). Although inherently safer reviews are a valuable tool for identifying opportunities for improvement, it is important to keep the principles of inherently safer in mind throughout the design process. n Edited by Suzanne Shelley 10. Clark, David G., Applying the ‘limitation of effects’ inherently safer processing strategy when siting and designing facilities, Process Safety Progress, Vol. 27, No. 2, 121–130, June 2008. 11. “Dow’s Fire and Explosion Index Hazard Classification Guide”, 7th Ed., American Institute of Chemical Engineers, New York, NY, 1994. 12. “Dow’s Chemical Exposure Index Guide”, American Institute of Chemical Engineers, New York, NY, 1994. 13. Suardin, Jaffee, Mannan, M. Sam, and ElHalwagi, Mahmoud, The integration of Dow’s Fire and Explosion Index (F&EI) into process design and optimization to achieve inherently safer design, Journal of Loss Prevention in the Process Industries, Vol. 20, pp. 79–90, 2007. 14. Khan, Faisal I., and Amyotte, Paul R., How to make inherent safety practice a reality, Canadian Journal of Chemical Engineering, Vol. 81, No. 2, 2–16, February 2003.
Additional suggested reading 1. Edwards, David, Editorial – Special Topic Issue – Inherent safety – Are we too safe for inherent safety?, “Process Safety and Environmental Protection – Transactions of the Institution of Chemical Engineers Part B,” Vol. 81, No. B6, 399–400, November 2003. 2. Englund, Stanley M., Inherently safer plants: Practical applications, Process Safety Progress, Vol. 14, No. 1, 63–70, January 1995. 3. French, Raymond W., Williams, Donald D., and Wixom, Everett D., Inherent safety, health, and environmental (SHE) reviews, Process Safety Progress, Vol. 15, No. 1, 48–51, Spring 1996.
Chemical Engineering www.che.com April 2011
Acknowledgments I gratefully acknowledge the process safety insights from my colleagues at Aker Solutions and at the leading operating companies whose facilities we have helped to design, from Professors Sam Mannan, Trevor Kletz, Ron Darby, Harry West and the Mary Kay O’Connor Process Safety Center at Texas A & M University, and from many others in the community of process safety professionals. The financial support of Aker Solutions is also appreciated.
Author Victor H. Edwards, P.E., is director of process safety for Aker Solutions Americas Inc., (3010 Briarpark Drive, Houston, TX 77042; Phone: 713-270-2817; Fax: 713-270-3195; Émail: vic.edwards@akersolutions. com). In his 28 years with Aker, Edwards’ experience includes process engineering, safety management and process, biochemical and environmental technologies. He has received numerous accolades in the areas of safety and environmental engineering, including five DuPont awards, and has contributed extensively to the engineering literature. His earlier experience includes assistant professor of chemical engineering at Cornell University, an assignment at the National Science Foundation, pharmaceutical research at Merck, alternate energy research at United Energy Resources, visiting professor at Rice University and process engineering at Fluor Corp. Edwards earned his B.A.Ch.E from Rice University and his Ph.D. in chemical engineering from the University of California at Berkeley. A registered professional engineer in Texas, he is an AIChE Fellow, and a member of ACS, AAAS, NFPA, NSPE, and the N.Y. Academy of Sciences. Note: This article is based on a paper presented at the Mary Kay O’Connor International Symposium, Texas A & M University, October 27-28, 2009. 4. Gupta, J.R., and Edwards, D.W., Inherently safer design — Present and future, “Process Safety and Environmental Protection — Transactions of the Institution of Chemical Engineers Part B,” Vol. 80, 115–125, May 2002. 5. Gupta, J.R., Hendershot, D.C., and Mannan, M.S., The real cost of process safety — A clear case for inherent safety, “Process Safety and Environmental Protection – Transactions of the Institution of Chemical Engineers Part B,” Vol. 81, No. B6, 406–413, November 2003. 6. Hendershot, Dennis C., et al., Implementing inherently safer design in an existing plant, Process Safety Progress, Vol. 25, No. 1, 52–57, March 2006. 7. Kletz, Trevor A., Inherently safer design: The growth of an idea, Process Safety Progress, Vol. 15, No. 1, 5–8, Spring 1996. 8. Lutz, William K., Take chemistry and physics into consideration in all phases of chemical plant design, Process Safety Progress, Vol. 14, No. 3, 153–160, July 1995. 9. Lutz, William K., Advancing inherent safety into methodology, Process Safety Progress, Vol. 16, No. 2, 86–88, Summer 1997. 10. Maxwell, Gary R. Edwards, Victor H., Robertson, Mark, and Shah, Kamal, Assuring process safety in the transfer of hydrogen cyanide manufacturing technology, Journal of Hazardous Materials, Vol. 142, pp. 677–684, June 2007. 11. Overton, Tim and King, George M., Inherently safer technology: An evolutionary approach, Process Safety Progress, Vol. 25, No. 2, 116–119, June 2006. 12. Study, Karen, A real-llife example of choosing an inherently safer process option, Process Safety Progress, Vol. 25, No. 4, 274–279, December 2006.
Feature Cover Story Report
Part 1
Designing for A Safe Process Phil Leckner CH2M Hill Lockwood Greene
A
ccidents do happen. While not everything can be predicted, addressing safety concerns throughout the design of a process can help to prevent accidents from occurring. Designing with safety in mind can also help to minimize potentially serious consequences that would result if an accident did occur. On April 12, 2004, toxic allyl alcohol and allyl chloride were released from a reactor at a facility in Dalton, Ga. The consequences included injuries and chemical contamination to people and property in the surrounding area. According to their report [1], the U.S. Chemical Safety and Hazard Investigation Board (CSB) concluded that “better process design, engineering, and hazard analysis would likely have prevented the 2004 runaway chemical reaction and toxic vapor cloud release...” On March 23, 2005, an explosion at a refinery in Texas City, Tex. killed 15 workers and injured 180 others when flammable liquid and vapor overfilled a blowdown drum during the startup of the refinery’s isomerization unit [1]. All of the fatalities and many of the injuries occurred in and around trailers that had been positioned near the isomerization unit to support maintenance activities on adjacent refinery units. The CSB report on this incident [1] recommended that new guidelines be developed for the placement of these and similar temporary structures around hazardous areas. Having a procedure in place to promote safe process design can help ensure that safety concerns are considered at appropriate phases in the design. The methodology presented here, as outlined in Table 1, may help prevent accidents such as those in 30
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Dalton, Ga. and Texas City, Tex. This methodology applies to process designs throughout the chemical process industries (CPI), which include not just chemical production, but also, for example, wastewater-treatment facilities, pharmaceutical and food-andbeverage plants. While following these guidelines will cost time and money, the practice can be a very inexpensive way to help prevent the much more costly consequences of not providing the safest design possible.
Basic Engineering Process flow diagrams
As a process engineer, there are two types of flow diagrams that interest me: the block flow diagram (BFD) and the process flow diagram (PFD). The BFD (Figure 1) presents an overall picture of the process, showing only major process steps. These steps are shown as “black boxes” with simple descriptions. Equipment can be depicted singly or grouped together as a system. The PFD (Figure 2), meanwhile, depicts major and minor equipment with specific symbols that are typi-
cally used in the CPI. Equipment is usually identified and shown with an alphanumeric designation. The PFD includes major and some minor process streams as well as utility streams, such as steam, condensate and cooling media. This diagram can also be used to show process safety requirements, such as proposed locations for relief valves. Often, a heat-and-material balance and major control loops are included. The PFD should be used as the basis for generating the more detailed piping-and-instrument diagram (P&ID). Prior to P&ID preparation, the PFD is reviewed by the design team and issued for design (IFD), with a revision number of zero. At a minimum, the basic design package should include the BFD and the PFD. However, I believe that another document, which takes the PFD to a new level, better promotes a safe process design. This document, which I call the process definition drawing (PDD), is not ordinarily a part of the basic design package, but is a great tool for the process design engineer. The PDD includes operating-and-
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design conditions for each equipment item, each control-valve station and all relief devices. The benefit of this document is that the process engineer can quickly recognize inconsistent or conflicting operating-and-design conditions. It helps the engineer think about the process in terms of how it will operate and what needs to be done to make it safe. Detailed engineering work is also aided by the PDD, since much of the information included on this document will be used in the generation of process-equipment, instrumentation and safety-device duty specifications. The PDD is a living document and will change as the process design progresses. As such, it is often sketched using rudimentary drawing software rather than CAD, thereby making it simple to construct and maintain.
Preliminary safety review
After the PFDs are created and approved for design, a preliminary safety review (PSR) is undertaken. The PSR is the gathering of documentation with an emphasis on process safety. Included are items such as the project scope definition, the process design basis, the process description and a material safety data sheet (MSDS) for each substance used. An extensive list of documentation that may be included in the PSR can be found in Ref. [2]. The MSDS is an important docu-
ment that provides a wealth of information including the proper handling of a substance, special storage requirements (such as keep out of the sun) and required personal-protection equipment (PPE), such as breathing apparatuses. The MSDS also provides basic physical-property and toxicity data, exposure limits and flammability ranges. It may describe what to do in case of spills. The manufacturer or supplier of the raw materials and various websites on the Internet are sources for MSDSs. If the facility is producing a finished product, then the plant owner will have to develop an MSDS for that product and make the document available to potential users. Table 2 lists information typically found in an MSDS. Once the documentation is gathered, the PSR is assembled into a formal report and issued to all pertinent members of the design team for comment. The report is given to project management for distribution as a “revision 0” issue. Note that the PSR is a living document and is subject to change as the design progresses. The PSR document is issued as soon as feasibly possible because it will form the foundation for the balance of the safety review of the project.
Design safety review
Once the PSR is issued, the process can be reviewed for major safety con-
Figure 2. Process flow diagrams give more detail than BFDs, and include at least major process streams and equipment items. Symbols that are typical for the CPI are used
cerns in the design safety review. Using the most up-to-date version of the PSR, all features associated with safety, environmental and layout issues are reviewed, including the following: • Defining the hazardous location classification (HLC), or the electrical area classification as it is sometimes called — The HLC is used to determine electrical-design criteria, such as equipment that may require explosion-proof motors. The HLC boundaries should be shown on appropriate documents such as the PFD or equipment-layout drawings (if available). One source of definitions for HLCs can be obtained from Ref. [3]. • Locating major pressure-relief devices, such as relief valves and rupture disks, explosion panels and flame arrestors — These devices should be indicated on the PFD and, in more detail on the PDD. The destination of the vent from these particular safety devices needs to be considered. The applicability and use of alternate safety systems, such as safety-instrumented systems (SIS) is also evaluated at this time. (For more, see Part 2 of this report, p.34) • Evaluating the layout with respect to minimizing hazards — As noted earlier in the Texas City, Tex. accident, personnel placement is a very important consideration. For
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Cover Story equipment placement, the American Petroleum Institute (API; www.api. org) recommends that all equipment with a ground area of 2,500 to 5,000 ft2 should be considered part of the same fire-relief scenario [4]. Determine if larger-sized equipment can be moved outside the common fire zone to reduce overall relieving capacity. Also be cognizant of where chemicals are stored in proximity to each other. You may not want acids stored near bases, for example. • Locating flares in safe areas, taking into account their radiation effects at ground level. • Evaluating the need for and placement of, fire and gas detectors (for more, see p.18). With the major pressure-relief devices located and the operating and design conditions fully defined, it is appropriate to evaluate modifications that might make the system safer. For example, determine if there are safety advantages to changing storage requirements from concentrated solutions to more dilute solutions, or vise-versa. Evaluate the advantages and disadvantages of breaking unit operations into smaller, more discrete pieces to make process equipment, such as heat exchangers and reactors, smaller. Preliminary hazard analysis. Finally, some type of preliminary hazard analysis (PrHA) should be undertaken before moving into the detailed process engineering phase of the project. During a PrHA, team members visualize ways in which a process design can malfunction or be operated incorrectly. The PrHA can take one of many forms, such as a pre-HAZOP, a what if, failure mode-and-effects analysis (FMEA) or FMEA check list. The various types of hazard analyses that are acceptable to the U.S. Occupational Safety and Health Administration (OSHA) are outlined in Refs. [2] and [5]. The PrHA is performed on the detailed PFD that is issued for design. The documents collected during the PSR and the PDD provide reference materials. Note that a preliminary or IFD issue of the P&ID may have been developed by this time and if so, would be part of the PrHA. At the conclusion of the design safety 32
review, all documents and results are collated and issued. This compilation is included as part of the front-end design package and the design proceeds to the next phase of the project, the detailed process engineering.
Detailed Engineering Piping and instrument diagram
At the start of the detailed processengineering phase of design, all outstanding issues and especially those brought up during the design safety review are examined and addressed. The P&IDs, PFDs and the PDDs are updated as required. The P&IDs are then reviewed in a formal setting as a team, which should include the process, mechanical, and piping and instrumentation engineers. The review will expose any last minute safety and design issues that must be addressed before the more detailed, and required, process hazard analysis (PHA) is undertaken. Changes are documented by again updating the P&IDs and PFDs. I suggest that these documents be issued with a separate revision number established exclusively for the PHA.
The process hazard analysis
The PHA evaluates the design in terms of both safety and operability. The analysis should be performed on the process, as well as instrumentation and control systems, such as the digital control system (DCS). PHAs are mandatory for all plants that fall within the scope of OSHA 29 CFR 1910.119 [5]. This scope applies to plants that meet the following two criteria: 1. Those whose processes involve one or more of certain chemicals (listed in appendix A of the regulation), and in quantities at or above the threshold given 2. Those whose processes involve flammable liquid or gas onsite in one location in a quantity of 10,000 lb or more with the exception of: a. Hydrocarbon fuels used solely for workplace consumption as a fuel b. Flammable liquids stored in atmospheric tanks and kept below their normal boiling point without the aid of chillers or refrigeration
Table 1.
A methodology to promote a safe process design I. Basic process engineering Create the process flow diagram Perform a formal review of the process flow diagram Conduct a preliminary safety review Perform a design safety review
II. Detailed process engineering Create the piping and instrumentation diagram Conduct a formal P&ID review Perform the process hazard analysis on the process Perform the process hazard analysis on the control system
III. Implement a management of change procedure
Facilities that do not fall within 29 CFR 1910.119 would still benefit greatly from the PHA if not for the safety aspects of the process, then for process operability. I can’t stress this enough: not being required to do a PHA by law should not exclude you from doing one to ensure that your process is indeed safe in design and operation. The team make-up. The PHA team should include a facilitator and a scribe (who serves the function of recording secretary), the design firm’s area process engineer, the plant process engineer and representatives from the plant’s safety-and-environmental and operations-and-maintenance departments. If vendor packages are involved, a vendor representative is also advised as a team member. An extensive list of possible participants can be found in Ref. [2]. The PHA team makeup and the extent of their efforts will vary based on the nature and complexity of the process design. The facilitator should be chosen with the following criteria in mind: • The person should be knowledgeable in the type of PHA to be performed • The person should not be intimately involved in the process design (that’s what the process engineers are for) • The person does not necessarily need to be very knowledgeable about the particular process to be reviewed The facilitator’s function is to guide
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Table 2. INFORMATION typically found in an msds Item
Remarks
Chemical name and any common names
The common name will be the same name on the label
Date of preparation or revision The date the MSDS was prepared or revised List of contacts that can provide more information Physical characteristics
Includes smell, color, appearance, flash point and vapor pressure
Physical hazards
For example, if the substance is subject to violent reactions such as explosions or fires
Health hazards
Describes if and how a substance can cause harm to human health and also provides symptoms of exposure
Route of entry information
Describes how the substance can enter the body, for example by ingestion or inhalation
Exposure limits
The maximum amount of exposure a person should have to the substance
Carcinogenic status
Whether the substance causes cancer
Safe handling and use
Explains precautions and protective measures needed when using and handling, including spill control
Control measures
Suggested engineering controls, work practices and personal protective equipment
Emergency and first aid procedures
How to deal with releases and exposure
the team, keep the PHA on track and motivate participation. The facilitator is not the person who brings up all of the issues. A facilitator who is too involved with the process is analogous to someone proofreading his or her own material — things are going to be missed that would otherwise not. Preparation. Having the right documentation available is key to a smooth PHA. The team should at a minimum have all the documents from the PSR and the DSR, as well as the P&IDs and PFDs, plot plans, equipment layouts, hazardous classification drawings, operating-and-maintenance procedures, batch sheets (if applicable), a summary of relief-device calculations and specifications for equipment, instrumentation and piping. A PHA can be very time consuming and expensive to implement. Poor documentation and the failure to perform a comprehensive P&ID review prior to the PHA can contribute significantly to the cost. More time spent
on the earlier steps mentioned in this methodology can reduce the cost and duration of the PHA. Documenting the PHA. One common problem is that many PHAs almost become P&ID reviews and “fix it” sessions. Any problems identified with the P&ID that need “fixing” should be recorded for further action and not discussed in detail during this analysis. Note that once the P&ID “fix” is implemented, the change needs to be re-evaluated. After the PHA is completed, a report is issued documenting what was checked and any actions that need to be addressed, such as additional relief devices, changing instrumentation and adding information to what will eventually become standard operating procedures. These action items are to be addressed in a timely manner and the plant design should be revised as required. Once all issues are addressed and design changes implemented, the design becomes “fixed” in terms of
References 1. The U.S. Chemical Safety and Hazard Investigation Board website (www.csb.gov) 2. The Center for Chemical Process Safety, “Guidelines for Hazard Evaluation Procedures – with Worked Examples,” 2nd ed., AIChE, 1992. 3. National Fire Protection Association, “NFPA 70, National Electric Code,” Chapter 5, 2005. 4. American Petroleum Institute, “Recommended
Practice 521, Guide for Pressure-Relieving and Depressuring Systems”, 4th ed., March, 1997. 5. The Occupational Safety and Health Administration, Process Safety Management of Highly Hazardous Chemicals, in “29 CFR 1910.119”, OSHA, Washington, D.C., 1992. 6. The Center for Chemical Process Safety, “Plant Guidelines for Technical Management of Chemical Process Safety (Revised Edition)”, AIChE, 1992, 1995.
safety. The design documents should be issued as “process safety management” approved. More specifics on PHAs can be found in Refs. [2] and [5] and in an abundant number of books and government publications (OSHA), that the reader is encouraged to research and review.
Management of change
As in all designs, things change even after the PHA is completed. These changes must be captured as they can affect the safety of the design and create new problems. Management of change (MOC) procedures capture these changes and should be strictly followed. A MOC procedure is simply a written way of documenting and informing people about changes made to the design after the PHA. The MOC may even outline when a change necessitates a new PHA. For example, if a valve is added to a line that was not there when the original PHA was performed, a PHA must be performed on this area of the design to ensure that no additional safety hazards were introduced, or if they were, are addressed accordingly. There is no single way to design and implement a MOC procedure. It is up to the discretion of the project team as to how it should be done. The MOC complexity will depend on the complexity of the process. The key is to ensure that the MOC procedure is easy to follow and that it allows easy documentation of all required activities and includes a system for approvals by the appropriate project team members [6]. n Edited by Dorothy Lozowski
Author Phil Leckner is a senior process engineer with CH2MHILL Lockwood Greene (Phone: 732-868-2277; Email:
[email protected]). Phil has over 30 years experience in process design and project engineering, and commissioning and startup for the chemical, petrochemical, foodand-beverage and bio-pharmaceutical industries. Over the past 10 years, he has been deeply involved with process-safety issues with emphasis on reliefsystem design. He has been involved in a number of PHAs, including serving as HAZOP and “What if?” facilitator. Phil received his B.S.Ch.E. from Lowell Technological Institute, which is now part of the University of Massachusetts.
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