Process Plant Piping Design

A piping system conveys fluid from one location to another. Within a process plant, the locations are typically one or more equipment items (e.g., pumps, pressure vessels, heat exchangers, process heaters, etc.), or individual process plants that are within the boundary of a process facility.

A piping system consists of:  Pipe sections  Fittings (e.g., elbows, reducers, branch connections, etc.)  Flanges, gaskets, and bolting  Valves  Pipe supports and restraints Each individual component plus the overall system must be designed for the specified design conditions.

Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 2 indicates a pipe whose outside diameter is 2.375 in. The NPS 12 and smaller pipe has outside diameter greater than the size designator (say, 2, 4, 6, . . .). However, the outside diameter of NPS 14 and larger pipe is the same as the size designator in inches. For example, NPS 14 pipe has an outside diameter equal to 14 in. The inside diameter will depend upon the pipe wall thickness specified by the schedule number Diameter nominal (DN) is also a dimensionless designator of pipe size in the metric unit system, developed by the International Standards Organization (ISO).

PIPE SCHEDULE :

1000 P SN = S

SN = Schedule Number P = Service Pressure (psi) S = Allowable Fiber Stress (psi)

Std (Standard) ; S (STRONG ; XS (extra strong)

A106 Grade B Carbon Steel Pipes - Pressure and Temperature Ratings Pressure (psig) and temperature (deg F) ratings of A106 Grade B carbon steel pipes temperatures ranging 100 oF - 750 oF

 The accepted formula is Schedule Number = 1,000 x (P/S) where, P = internal pressure, pounds-per-square-inch-gauge (psig), S = allowable fiber stress (ultimate tensile strength of the steel in psi).  Rearrange terms to solve for P, assuming schedule number and S are known. Therefore P = Schedule number x S/1,000  Calculate internal pressure (P) based on Schedule 40 steel pipe, and an allowable fiber stress (S) value of 60,000-psi (ultimate tensile strength for A106 Grade B Carbon Steel Pipe). Therefore, P = 40 x 60,000/1,000 = 2,400-psi.  This is reasonable, based on a current-day published value of 2,847-psi for 1-inch Schedule 40 steel pipe (with the range temperature 1000F – 7500F).

B31.1 - Power Piping B31.2 - Fuel Gas Piping

B31.3 - Process Piping B31.4 - Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids B31.5 - Refrigeration Piping and Heat Transfer Components B31.8 - Gas Transmission and Distribution Piping Systems B31.8S - Managing System Integrity of Gas Pipelines B31.9 - Building Services Piping B31.11 - Slurry Transportation Piping Systems B31G - Manual for Determining Remaining Strength of Corroded Pipelines

Terminologi yang digunakan pada material yang digunakan pada pipa maupun sistem perpipaan.

PIPING MATERIALS (GRADE)

UTS : Ultimate Tensile Strength

YS : Yields Strength

A106 Grade B Carbon Steel Pipes (Pressure-Temperature Rating)

1)

STD (standard) = schedule 40

2)

XS (extra strong) = schedule 80

PIPING STANDARDS (CLASS)

Provides requirements for: • Erection • Design • Inspection • Materials • Fabrication • Testing

Piping and piping components, all fluid services:  Raw, intermediate, and finished chemicals  Petroleum products  Gas, steam, air, and water  Fluidized solids  Refrigerants  Cryogenic fluids  Interconnections within packaged equipment

 Piping systems for internal gauge pressures at or above zero but less than 15 psi, provided that the fluid is nonflammable, nontoxic, and not damaging to human tissue, and its design temperature is from -20°F through 366°F.  Power boilers that are designed in accordance with the ASME Boiler and Pressure Vessel Code Section I and external boiler piping that must conform to ASME B31.1.  Tubes, tube headers, crossovers, and manifolds that are located inside a fired heater enclosure.  Pressure vessels, heat exchangers, pumps, compressors, and other fluid-handling or processing equipment. This includes both internal piping and connections for external piping.

General  Normal operating conditions  Design conditions Design pressure and temperature  Identify connected equipment and associated design conditions  Consider contingent conditions  Consider flow direction  Verify conditions with process engineer

σl σc σr t P

= Axial or Longitudinal Stress = Circumferential (Hoop) Stress = Radial Stress = Wall Thickness = Internal Pressure

The stress in circumferential direction at a point in the tube or cylinder wall can be expressed as: σc = [(pi ri2 - po ro2) / (ro2 - ri2)] - [ri2 ro2 (po - pi) / r2 (ro2 - ri2)] Where, σc = stress in circumferential direction (MPa, psi) pi = internal pressure in the tube or cylinder (MPa, psi) po = external pressure in the tube or cylinder (MPa, psi) ri = internal radius of tube or cylinder (mm, in) ro = external radius of tube or cylinder (mm, in) r = radius to point in tube or cylinder wall (mm, in) maximum stress when r = ri (inside pipe or cylinder) t = wall tube thickness = r0 - ri

σc < Maximum Allowable Stress of Pipe Materials

Function of :  Material properties  Temperature  Safety factors

Established to avoid:  General collapse or excessive distortion from sustained loads  Localized fatigue failure from thermal expansion load  Collapse or distortion from occasional loads

A piping system must be modified to add a new, spare heat exchanger. You have been assigned the responsibility to determine the required wall thickness for the pipe from the heat exchanger to several pumps.  The piping system will have a design temperature of 650°F.  The design pressure is 1,380 psig.  The pipe outside diameter is 14 in.  The material is ASTM A335, Gr. P11 (1¼ Cr – ½ Mo), Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service.  Corrosion allowance is 0.0625 in.

Required Wall Thickness for Internal Pressure of Straight Pipe (ASME B31.3)

t = Required thickness for internal pressure, in. P = Internal design pressure, psig S = Allowable stress in tension (Table A-1), psi E = Longitudinal-joint quality factor (Table A-1B) Y = Wall thickness correction factor (Table 304.1.1)

Corrosion allowance (CA) is an additional thickness that is added to account for wall thinning and wear that can occur in service. The corrosion allowance is based on experience and data for the particular pipe material and fluid service. tm = Total minimum required wall thickness, in Mill tolerance accounts for the difference between the actual manufactured pipe wall thickness and the “nominal” wall thickness specified in the relevant pipe dimensional standard. The typical pipe mill tolerance is 12.5%. This means that the as-supplied pipe wall thickness can be up to 12.5% thinner than the nominal thickness and still meet its specification requirements. Use the following equation to determine the minimum required nominal thickness to order. tnom = Minimum required nominal pipe wall thickness, in.

Source : ASME B31.3 (Table A-1) Basic Allowable Stress in Tension for Metal

Source : ASME B31.3 (Table A-1B) Basic Quality Factor for Longitudinal Weld Joint

The following equation applies:

Based on the given information: Design pressure (P) = 1,380 psig. Pipe Outside Dia. (D) = 14 in.

For the A335, Gr. P 11 material: S = 16,150 psi. [Table A-1 of ASME B31.3 at 650°F] E = 1.0 [Table A-1B of ASME B31.3] Y = 0.4 [Table 304.1.1 of ASME B31.3], since the material is ferritic and the temperature is below 900oF.

In this case, a 0.0625 in. corrosion allowance has been specified.

tm = t + c = 0.577 + 0.0625 tm = 0.6395 in.

 Pipe flanges that are made to standards called out by ASME B16.5 or ASME B16.47 are typically made from forged materials and have machined surfaces.  ASME B16.5 refers to nominal pipe sizes (NPS) from ½" to 24“ and ASME B16.47 covers NPSs from 26" to 60".  Each specification further delineates flanges into pressure classes: 150, 300, 400, 600, 900, 1500 and 2500 for ASME B16.5; and ASME B16.47 delineates its flanges into pressure classes 75, 150, 300, 400, 600, 900. Flange strength increases with class number.

Materials for flanges are usually under ASME designation: • SA-105 (Specification for Carbon Steel Forgings for Piping Applications), • SA-266 (Specification for Carbon Steel Forgings for Pressure Vessel Components), • SA-182 (Specification for Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service).

New piping system to be installed at existing plant. Determine required flange class. • Pipe Material : 1 ¼ Cr - ½ Mo • Design Temperature : 700°F • Design Pressure : 500 psig

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Determine Material Group Number (Table 1) → Group Number = 1.9 Find allowable design pressure at intersection of design temperature and Group No. Check Class 150. (Table 2)  Allowable pressure = 110 psig < design pressure  Move to next higher class and repeat steps For Class 300, allowable pressure = 570 psig Required flange Class: 300