Hydrocarbon Processing April 2013

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PETROCHEMICAL DEVELOPMENTS

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HydrocarbonProcessing.com | APRIL 2013

Shale gas provides a renaissance for North American petrochemical producers and downstream chemicals

HPI FOCUS When does it make sense to build a new unit instead of revamping an existing facility?

REFINING DEVELOPMENTS Upgrading heavy crudes creates new hurdles to be solved with catalysts and better processing methods

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APRIL 2013 | Volume 92 Number 4 HydrocarbonProcessing.com

54

6

32 SPECIAL REPORT: PETROCHEMICAL DEVELOPMENTS 33 Shale energy resources driving resurgence for ethylene industry M. Eramo 37 North American olefin producers riding the shale gas wave R. Klavers and M. J. Tallman 43 Use model-based temperature control for fixed-bed reactors D. Weatherford and J. Ford 47 High-pressure polyethylene: Reemergence as a specialty chemical or not? L. Farrell and J. Virosco

DEPARTMENTS

4 6 9 15 102 105

Industry Perspectives Brief Impact Innovations Marketplace Advertiser index

COLUMNS

HPI FOCUS: NEW VS. REVAMP 51 New vs. debottlenecking projects for the hydrocarbon processing industry

23

Reliability Fact-checking list from recent reliability conferences

BONUS REPORT: REFINING DEVELOPMENTS 55 Evaluate challenges in meeting clean-fuel specifications with heavier crude S. Al-Zahrani, S. Roy, and E. Bright 61 Improve coker efficiency with reliable valve automation B. Deters and R. Wolkart 65 Optimize value from FCC bottoms J. Paraskos and V. Scalco

25

Integration Strategies Industrial considerations for BYOD

27

Boxscore Construction Analysis Ethylene in evolution: 50 years of changing markets and economics

GAS PROCESSING DEVELOPMENTS 73 Take a quicker approach to staggered blowdown M. Sufyan Khan

106

Water Management Update: Online measurement of oxidizing biocides

TURBOMACHINERY DEVELOPMENTS 77 Select the right shaft-riding brushes for turbomachinery T. Sohre and H. P. Bloch GLOBAL TURNAROUND AND MAINTENANCE—SUPPLEMENT T-85 Overcome barriers to proper planning and scheduling J. Wanichko SAFETY/LOSS PREVENTION 99 Conceptually, accidents are a fallacy M. Sawyer Cover Image: In 2006, Technip began construction of the Map Ta Phut Olefins facility located in Thailand. The facility uses seven of Technip’s proprietary GK6 naphthacracking furnaces and one SMK furnace for ethane cracking. The olefins facility was successfully started up in March 2010.The GK6 units are the largest in operation, with an ethylene capacity of 175,000 tpy per furnace.

www.HydrocarbonProcessing.com

Industry Perspectives Key industry officials answer a poll question from HydrocarbonProcessing.com

Are Arctic projects safe? Do global energy companies have sufficient safety protocols in place to deal with the challenges of Arctic projects? The answer, according to hundreds of votes cast in a recent Hydrocarbon Processing industry poll, is an old cliché: it depends. Nearly half (48%) of readers surveyed believe practices vary enough throughout the industry that a single standard has not been adopted, making it dependent on the company in question. Another 28% said they believed the industry does have sufficient safety protocols, while 25% said it does not. The topic became newsworthy after recent incidents involving Shell. That company, for its part, is postponing its planned summer drilling in the Arctic Ocean after a troubled 2012 drilling season marred by bad weather, mechanical failures and regulatory challenges. Shell had been widely expected to push back its contentious, multi-billion-dollar Arctic program after it announced that its rigs needed to be repaired and analysts said replacements would be hard to find. “We’ve made progress in Alaska, but this is a long-term program that we are pursuing in a safe and measured way,” said Shell president Marvin Odum. The Kulluk, a drilling ship owned by Shell and operated by Noble Corp., ran aground on an uninhabited island about 300 miles southwest of Anchorage on Jan. 1 after ships towing it to Seattle for the winter lost control of the rig during a storm (FIG. 1). It suffered damage to the hull and electrical systems. The Noble Discoverer drill ship, which Shell was leasing, had an engine fire in December when it was on its way to Seward, Alaska, prompting a US Coast Guard inspection. Investors and government officials are closely watching Shell’s Arctic plans. The company has spent nearly $5 billion on permits, personnel and equipment over the past six years to assure regulators and native Alaskans that the first drilling in the Arctic Ocean would be safe and environmentally benign. —Additional reporting by Dow Jones Newswires

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SUBSCRIPTIONS Subscription price (includes both print and digital versions): Print—One year $239, two years $419, three years $539. Digital format—One year $239. Airmail rate outside North America $175 additional a year. Single copies $35, prepaid. Because Hydrocarbon Processing is edited specifically to be of greatest value to people working in this specialized business, subscriptions are restricted to those engaged in the hydrocarbon processing industry, or service and supply company personnel connected thereto. Hydrocarbon Processing is indexed by Applied Science & Technology Index, by Chemical Abstracts and by Engineering Index Inc. Microfilm copies available through University Microfilms, International, Ann Arbor, Mich. The full text of Hydrocarbon Processing is also available in electronic versions of the Business Periodicals Index.

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President/CEO Vice President Vice President, Production Business Finance Manager

FIG. 1. Shell Kulluk drilling rig in the Arctic.

Part of Euromoney Institutional Investor PLC. Other energy group titles include: World Oil and Petroleum Economist Publication Agreement Number 40034765

4APRIL 2013 | HydrocarbonProcessing.com

John Royall Ron Higgins Sheryl Stone Pamela Harvey

Printed in USA

)/(;,7$//,&6$)(,6025(7+$1$352*5$0,7·6$:$ 99% of the catalyst from the slurry to give a clarified SO product containing less than 50 ppm of FCC catalyst. This model costs about $3 MM installed and is sufficiently robust to handle any of the cases investigated. FIG. 4 shows the estimated payout times for the various cases, which range from three to eight months for all the various cases. Catalyst savings might not be as valuable for a resid unit, but would still be significant. Individual cases involving deep-resid cracking benefits would have to be calculated based on a thorough knowledge of the resid FCCU feed, operating conditions and catalyst characteristics. Smaller catalyst particles returned to the unit have an inherently larger surface-to-volume ratio and provide a considerably higher resid cracking activity than the larger equilibrium catalyst held in the unit. LITERATURE CITED Silverman, L. D. and S. Winkler, “Matrix effects in catalytic cracking,” NPRA annual meeting, Los Angeles, California, March 23–25, 1986. 2 Minyard, W. F. and T. S. Woodson, “Upgrade FCC slurry oil with chemical settling aids,” World Refining, November/December 1999. 3 Guercio, V. J., “US producing, exporting more slurry oil,” Oil and Gas Journal, October 4, 2010. 4 Motaghi M., Shree, K. and S. Krishnamurthy, “Anode-grade coke from traditional crudes,” PTQ , Quarter 2, 2010. 5 Elliott, J. D., “Impact of feed properties and operating parameters on delayed coker petcoke quality,” ERTC Coking and Gasification Conference, 2008. 1

JOHN PARASKOS started his career at Gulf Research and Development Co. after receiving his PhD degree in chemical engineering from the University of Massachusetts. During his 17 year stay at Gulf, he was the recipient of over 35 patents in various processes. VIC SCALCO is the sales and marketing manager for General Atomics’ Gulftronic line.

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Gas Processing Developments M. SUFYAN KHAN, WorleyParsons, Muscat, Oman

Take a quicker approach to staggered blowdown The design of a reliable blowdown system for a large sour gas-processing facility is one of the most important aspects of plant design. Safe design is vital for facility operation. With the development of advanced plant design techniques and the evolution of sophisticated engineering programs, large, complex plants processing highly sour gas are being built that require larger flare systems and more precise blowdown system design. Faster blowdown is also required due to the tightening of design standards in the oil and gas industry to minimize risk— e.g., to blowdown the equipment to 50% of the design pressure, or 7 barg, in 15 minutes. Larger pieces of equipment operating at elevated pressures yield high blowdown loads, and it may be impractical or uneconomical to design a flare system to simultaneously handle the blowdown loads of the entire facility. A staggered blowdown technique is applied in such situations to optimize the flare system design capacity. Staggered blowdown. This process is required when the simultaneous blowdown load of the facility is significantly higher than the largest relief load under governing contingency and when the design of the flare system for simultaneous blowdown load is impractical or uneconomical. The staggered blowdown system should be designed to optimize the flare system design capacity while maintaining the ability to blowdown the facility as quickly and safely as possible. Zonal blowdown approach. The conventional approach to staggered blowdown is to divide the blowdown loads into different zones and carry out the blowdown zone by zone. Zones can be made by two methods: • Physical separation of units • Combining the blowdown loads of adjacent units to make the total zonal blowdown load equal to the flare capacity. Zones by physical separation. Preference should be given to physically segregate zones based on allowable radiation criteria. This is a more efficient way of staggered blowdown, since, during a fire or any other emergency, the entire affected zone will blowdown immediately, followed by the adjacent zones. However, this approach is primarily effective for relatively small facilities where the number of units is less, and where all of the units are placed within a few blocks on the plot. In this case, the largest zonal blowdown load dictates the flare capacity, if it is not challenged by any other relieving rates under simultaneous or individual contingency. However, if the largest zonal blowdown load is still big enough to make the

flare system design impractical or uneconomical, then layout changes should be considered to keep the blowdown load units in different zones. Zones by blowdown load. If physical separation of zones is not possible, or if the blowdown load of a zone is still high, then the zones can be made on blowdown loads analysis. All unit blowdown loads should be listed, and the adjacent unit loads should be combined to make a reasonable total that will equalize the flare capacity. Consideration should be given to individual relief valve capacities. Of course, one zone load should not be less than the largest relief valve capacity. For an initial guess of the flare system capacity, a comparison should be made between the largest relief valve load (typically the blocked discharge of the inlet separators) and the largest individual blowdown load. If the flare capacity is selected as the largest relief valve/blowdown load, then the next step is to determine how many zones will be needed to blowdown the entire facility and how long this process will take. This evaluation will determine if flare capacity should be increased. It is not necessary to keep the flare capacity equal to the largest simultaneous or individual relief load if the relief load is not excessive. In this case, the flare capacity can be increased to optimize the staggered blowdown design. In some emergency situations, certain units may have preference over others and may need to undergo blowdown first. One example is a simultaneous loss of seal gas to all process compressors. If such a constraint is present, then the combined loads of all preferential units should be evaluated to match the flare capacity. Putting all of these units—which immediately require blowdown under an upset situation—in one zone, and setting the flare capacity equal to the combined zonal load, may help simplify the blowdown and emergency shutdown (ESD) system design. Once the flare capacity is selected, then different zones can be made, preferentially of adjacent units, with each zone totaling to the flare capacity. Case study. A gas-processing facility has several processing

units, with a combined blowdown load of 93 million standard cubic meters per day (MMscmd). The flare capacity is 16.55 MMscmd, of which 3.13 MMscmd is reserved for a relief valve from the trunkline. The relief from the trunkline relief valve is coincidental with the blowdown and continues throughout the entire blowdown duration. The effective flare capacity Hydrocarbon Processing | APRIL 201373

Gas Processing Developments available for facility blowdown is 13.42 MMscmd. Therefore, the combined blowdown load of the facility is about six times the flare system capacity. Developing an effective staggered blowdown sequence for this case is a challenge due to the large combined blowdown load compared to the flare capacity.

sign and calls for a quicker approach to reduce the total facility blowdown time within the UPS backup time. Reducing blowdown time. A quicker approach is adopted

to reduce the total blowdown time of the facility within the UPS backup time. The idea of the quicker approach lies in the fact that the peak depressuring rate occurs in the beginning of the blowdown and then deLarger pieces of equipment operating pletes exponentially. A typical depressuring curve is at elevated pressures yield high blowdown shown in FIG. 2. The depressuring rate falls rapidly in the beginning and then drops to half of the peak loads, and it may be impractical or rate in less than four minutes. uneconomical to design a flare system The quicker approach to staggered blowdown developed for the case study is depicted in FIG. 3. As to simultaneously handle the blowdown can be seen, the last unit starts to blowdown at 45 loads of the entire facility. minutes, which is about half of the blowdown time calculated by a conventional zonal blowdown approach; it is also within the UPS backup time. Unit sizes are bigger due to the high processing capacity of the facility; therefore, the layout includes one processing Methodology of quicker approach. The blowdown load of unit or train per block. Categorizing zones based on physieach processing unit in the facility is listed in TABLE 1. The decal segregation of the units yields numerous zones, and each pressuring curve of each blowdown valve for each processing zone capacity is significantly lower than the effective flare unit is extracted from the simulation model and built into a system capacity. Therefore, zones are made based on blowcalculation program. The blowdown load of preferential units down load analysis. [reinjection compressor 2 and 3, low-pressure (LP) acid gas Based on the calculated blowdown loads for each processing compressor 1 and 2, the flash gas compressor and high-presunit, six zones of 13.42 MMscmd each were made. Adding the sure (HP) acid gas compressor 1] are combined, which gives relief valve load of the trunkline to each zone’s blowdown load nearly 13.14 MMscmd. The relief valve load from the trunk makes each zone’s total load equal to the flare system capacity. line is added, bringing the total to 16.3 MMscmd. Under a plantwide blowdown situation, such as an instruAt zero time, when the plantwide depressurization is acment air failure or a power failure, all the zones will blowdown tivated by the ESD system, the first set of units will start to one by one. Since each zone’s combined load equals the flare blowdown (see peak 1 in FIG. 3). It can be seen from FIG. 3 that, capacity, the blowdown delay between two zones is about 15 after two minutes, the blowdown load has dropped to 10.46 minutes, since the first zone under blowdown will reach its MMscmd. TABLE 1 lists the blowdown load of reinjection comminimal load after about 15 minutes. Only then will blowpressor 1 and HP acid gas compressor 2 at 6.21 MMscmd, at down begin in the next zone, so that the total blowdown load which point the units can safely start to blowdown. does not exceed flare system capacity at any point. The last At 2.17 minutes, the blowdown of reinjection compreszone begins to blowdown at 84 minutes, as shown in FIG. 1. sor 1 and HP acid gas compressor 2 is started, which raises the total blowdown load to 16.4 MMscmd (peak 2 in FIG. 3). In the case of a plantwide power failure scenario, the total blowdown time is found to exceed the given uninterrupted Similarly, after 4.5 minutes, the blowdown rate falls to 11.08 power supply (UPS) backup time and, therefore, a safe blowMMscmd, and inlet separator A can be safely started to blowdown cannot be conducted. This puts a challenge on the dedown (peak 3 in FIG. 3). 18

1

16

2

3

4

5

6

Flare capacity: 16.55 MMscmd 6 7

5 Depressuring rate, MMscmd

14 Flow, MMscmd

12 10 8 6 4

3 2 1

2 0

4

0 0

10

20

30

40

50 60 Time, minutes

70

80

FIG. 1. Staggered blowdown curve for zonal approach.

74APRIL 2013 | HydrocarbonProcessing.com

90

100

0

60

120 180 240 300 360 420 480 540 600 660 720 780 840 900 Depressuring time, seconds

FIG. 2. Typical depressuring curve.

Gas Processing Developments 18 16

1 2 3

4

5 6

7 8 9 10 11 12 1314 15 16

Flare capacity: 16.55 MMscmd 17

TABLE 1. Blowdown loads for various units Unit

14

Flow, MMscmd

12 10 8 6 4

0

Inlet separator A

5.48

Inlet separator B

5.48

Inlet separator C

1.23

Test separator

0.53

Condensate stabilizer unit

2.21

Flash gas compressor

2 0

10

20

30 Time, minutes

40

50

60

FIG. 3. Staggered blowdown curve for quicker approach.

By this method, the moment that the total blowdown load in the flare system falls to a level where another unit’s load can be added, that unit’s blowdown is initiated. This method utilizes the maximum use of flare header capacity. As soon as the capacity becomes available in the header, the next blowdown is activated. Units with smaller blowdown loads, such as the test separator and the seal gas compressor, will experience smaller peaks if blowdown is initiated independently. The combination of several smaller units is recommended to bring the total blowdown load to a reasonable level (e.g., 2 MMscmd– 3 MMscmd), so that the number of peaks will be less and ESD logic will have a reasonable time delay during which to initiate blowdown at additional units. Making smaller groups to reduce the number of smaller peaks will not increase the total blowdown time of the facility. In this case study, both options (blowing down each unit one by one, and combining smaller units into a reasonable capacity group) were examined. The difference was only a few minutes, with a gain of ESD logic simplicity. ESD design for quicker approach. For the ESD system design of the facility, several staggered blowdown sequences may need to be developed, depending on the ESD philosophy of the project. If the project philosophy is to initiate a plantwide blowdown in case of fire detection in any area of the plant, the fire detection in each area of the facility will need a separate staggered blowdown sequence, as the area under fire will be the first unit in the sequence to blowdown. In the case of common-mode failure scenarios (i.e., an instrument air failure or a plantwide power failure), a set of preferential units will blowdown first. Thus, the ESD system will be provided with several staggered blowdown sequences to accommodate different emergency situations. Takeaway. This case study shows that a quicker approach to

blowdown can reduce the total time by around half, compared to the conventional zonal approach. A faster approach makes good use of the flare header capacity, as unit blowdowns begin the moment capacity becomes available in the flare header. The quicker approach helps to expedite the total blowdown

Peak depressuring rate, MMscmd

1

Trunk line

3.13

Sweetening unit 1

5.42

Sweetening unit 2

5.42

Export gas compressors

2.48

Dewpoint control unit

7.31

Reinjection compressor 1

5.67

Reinjection compressor 2

5.67

Reinjection compressor 3

5.72

Dehydration unit 1

2.15

Dehydration unit 2

1.09

HP acid-gas compressor 1

0.54

HP acid-gas compressor 2

0.54

LP acid-gas compressor 1

0.11

LP acid-gas compressor 2

0.11

Piping segment 1

10.46

Piping segment 2

2.51

Piping segment 3

3.02

Piping segment 4

1.24

Piping segment 5

2.95

Piping segment 6

3.18

Seal gas compressor

0.18

Seal gas buffer vessel

6.53

Fuel gas unit Total

1.71 93.06

time and improves the design of the UPS system for instrumented protective system backup. MUHAMMAD SUFYAN KHAN K.K. is a process engineer with WorleyParsons. He has over seven years of process design and engineering experience in the oil and gas sector, with emphases on process simulation, FEED development and detailed engineering. Mr. Khan has worked on greenfield and brownfield projects at oil refineries and gas processing plants around the world, and he has experience in dynamic simulation. He holds a degree in chemical engineering from the University of Karachi, Pakistan and is an associate member of the Institution of Chemical Engineers (IChemE). Hydrocarbon Processing | APRIL 201375

THE EXPECTED. We provide solutions. Smith & Burgess is the expert in flare and relief system design. Our methodology is cost-effective and we pride ourselves on using realistic assumptions that meet industry standards, yet don’t burden the owners with excessive concerns. We are committed to our customers. At Smith & Burgess we are passionate about protecting your assets. We insist on quality. If that’s beyond what you expected, we did our job right.

www.smithburgess.com Select 72 at www.HydrocarbonProcessing.com/RS

Turbomachinery Developments T. SOHRE, Sohre Turbomachinery, Monson, Massachusetts; and H. P. BLOCH, Reliability/ Equipment Editor, Westminster, Colorado

Select the right shaft-riding brushes for turbomachinery Since the mid-1950s, much progress has been accomplished in the development, design and production of shaftriding brushes for turbomachinery applications. The machinery includes steam and gas turbines, gears, turbocompressors and ship propulsion systems. The brushes assist in controlling shaft current problems, especially under conditions in which the shafts operate at high-surface velocity, oil-splash and other susceptible environments. Shaft currents are often generated by nonelectric machines. Of course, they also occur in equipment trains with electric motors and generators. Shaft currents can be of electrostatic or electromagnetic origin. They can cause severe damage to bearings, shafts, seals, gears and other machine elements. FIGS. 1 and 2 are typical examples of such damage. The application of brushes (FIGS. 3–5) can assist in monitoring shaft voltage and determining how much shaft current is being developed. Brushes and proper monitoring will provide warnings of dangerous current buildup. This buildup can be caused by self-magnetization and self-excitation, among other reasons. Strongly magnetized machines must be demagnetized. Residual electromagnetic currents of reasonable strength can be grounded to protect the machine against discharge damage. Typical electrostatic currents can always be grounded through a brush because the amount of current is usually low in comparison to magnetically induced currents.

NEW SOLUTIONS Another application for modern brushes is signal transmission from strain gages or other instrumentation located on the rotor. For example, the measurement of torque, torsional and lateral vibration, blade vibration, temperatures and pressures on rotor components can be facilitated by brushes. Good brushes provide a very low electrical noise level, even at high-surface velocity and in an aggressive environment. Together with a low-wear rate and easy replacement during operation, a low electrical noise level permits continuous, long-term monitoring and removal of modest amounts of current during both normal and abnormal equipment operation. Competent brush manufacturers provide standardized products, and many parts are interchangeable. Models of

FIG. 1. This thrust bearing damage could have been prevented by a well-engineered shaft-riding brush.

FIG. 2. Spark damage of the type that can be prevented by using shaft-riding brushes for early detection of developing problems. Hydrocarbon Processing | APRIL 201377

Turbomachinery Developments varying lengths are available from experienced vendors to accommodate requirements of various turbomachinery manufacturers, and to meet the requirements dictated by emergency-retrofit field installations. Many installations are in hydrocarbon processing industry (HPI) facilities and often in aggressive environments. It is vital to inform vendors of the processing conditions in which the brush will operate. It is very important to define if an installation will be in a hazardous, chemical attack-prone or otherwise severe environment. Materials. Shaft material is also an important item, as most shaft-riding brushes will run best on carbon steel or low-alloy carbon steel. Other shaft materials should be avoided for HPI installations. For instance, operation of typical metal-fiber (bristle) brushes on aluminum, titanium, high-alloy austenitic steels, copper, brass or any material with poor wear characteristics can result in shaft grooving, rapid bristle depletion, or some other undesirable, perhaps catastrophic, consequences. Never run a brush on a highly stressed surface, especially if vibratory stresses are present. Examples of such cases include thin-wall hollow shafts or spacers, areas of stress concentration and quill shafts. Running brushes on a shaft can introduce stress concentrations. For example, frosting can occur due to unusually high current above the brush rating. On a highly stressed surface, this can lead to catastrophic failure.

indicator. Brush elements can often be inspected or replaced within a few minutes while the machine is in operation. Therefore, replacement is not restricted to periods of shutdown or plant turnaround. Brushes installed by the original equipment manufacturers (OEMs) during initial construction of the machine often have longer bristle element life. Those installed as an emergency field retrofit typically have shorter wear lives—for reasons of much less than ideal shaft surface finish, out-of-roundness, cleanliness, and lubrication conditions. Also, the excessive shaft currents often found in field retrofit situations will drastically reduce bristle element life. Well-designed shaft brushes work with shaft surfaces dry or wetted with oil, or the brush may operate in an oil-splash or submerged surrounding. Applications operating with a reasonable oil-splash or oil-spray work better and last somewhat longer than brushes running in completely dry applications. The reasons are found in the effects of lubrication and the removal of dirt, gum and wear debris. The axial shaft space requirement for many brush installations can be of real importance because of the very restricted areas in typical machines. One brush manufacturer’s standard version requires about ¾ in. (20 mm) of axial shaft space. The larger brushes require a minimum of approximately 2.5 in. (63 mm).

MATERIAL SELECTION IS IMPORTANT All brush materials used must have good stability in hightemperature service. The standard brush design is suitable for operation in environments to 400°F (205°C). Even the vendor’s standard brushes should be equipped with a wear

Description of typical models. There are two basic types of

brushes: The “toothbrush” type, shown in FIGS. 3 and 4, and the “plunger” type, as illustrated in FIG. 5. Different sizes and minor changes accommodate a very wide range of turbomachines. Plunger applications. In general, “plunger” types are used where available space requires a radial- or axial-brush installation. It is especially suited for field retrofit applications, in which the mounting possibilities are usually very limited, and Bearing case or coupling guard

Brush raising screw Wear indicator Lead wire

Bristle element, (replaceable) silver/gold Internal cartridge can be removed while in service

FIG. 4. Typical mounting arrangement of a “toothbrush” type of shaftriding brush. Bristle element silver/gold (replaceable in service)

Wear indicator Brush raising screw

Lead wire

Spring assist

FIG. 3. A “toothbrush” type of shaft-riding brush.

78APRIL 2013 | HydrocarbonProcessing.com

FIG. 5. Cross-section view of a plunger type of shaft-riding brush.

Turbomachinery Developments the axial positioning in an outboard bearing case end cover may be especially attractive. Retrofits can sometimes be accomplished while the machine is in operation. A temporary cover plate has been used occasionally while the permanent cover is removed for machining and mounting of the brush. For turbine generators, one manufacturer suggests experience-based sizing guidelines, as described in TABLE 1. These guidelines are approximations and actual figures will depend on particulars of a given installation. Some particulars may not be fully known in advance and may need to be established at the time of brush installation. There are many factors that can have a strong influence on the life expectancy of the sacrificial brush elements. Examples include residual magnetism in shafts and casings, or the design and condition of a motor or generator and exciter. At higher frequencies of current, the rate of bristle burn-off increases significantly, but its complexities are not yet well understood or predictable. Installations for retrofits are, by necessity, often difficult. OEMs have an opportunity during the design stage to select the most favorable arrangements. These manufacturers can modify bearing housings and other components to provide good mounting conditions. Usually, OEMs design and manufacture their own mounting flanges, to which the brush casing is then welded at the assembly stage. Typically, these flanges should be about ¼-in. (6-mm) thick and should be made of 300 series, nonmagnetic, stainless steel. Frequently asked questions. Because details tend to vary considerably, a user may wish to consult a competent brush manufacturer’s guidelines. Manufacturers may refer to relevant instruction and installation manuals for additional information. Frequently asked questions include: Q1. Why use shaft-riding brushes? To measure stray electrical currents (“shaft currents”) on the rotating shafts of machinery, and to ground modest amounts of electrical rotor currents. This should prevent or minimize electrical damage to bearings, seals, gears and other critical components. Q2. What are the orientations in which well-designed brushes can be installed? Installation can be in any position with respect to the shaft: tangential, radial, axial or skewed, as well as vertical, horizontal or upside down. Q3. What is the correct method of permanently installing the brush? There are six installation steps. A general summarized procedure is: • Prepare a stainless steel mounting flange • Mount flange to brush with temporary brackets • Bolt to machine and adjust • Weld flange to brush casing • Bolt and dowel brush and flange unit to machine • Make electrical connections, and check out everything. Q4. What conditions are required on the shaft? A surface finish of 63 micro-inches root-mean-square (RMS) is acceptable, but 32 RMS is preferred. Shaft surface finish is an important factor in bristle element life. A rough shaft will result in an unacceptably high rate of bristle element replacement. On a smooth shaft, the rate of bristle depletion due to mechanical wear will be close to zero. Electrical burn-off will still be a factor. The shaft needs to be free of irregularities such as rust, nicks, dings, scratches and match marks. Some oilsplash, spray or mist is ideal but not essential.

Q5. How long will bristle elements normally last? On a smooth shaft, the sacrificial bristle element will typically last from one to three years. Actual performance depends very much on how much current is flowing through a brush, as well as, the shaft surface finish. The amount of current flowing through a brush is difficult to predict and can change significantly during operation, even on a daily basis. So, then, the primary factors affecting the life of the sacrificial bristle element are the amount of current through a brush, and the shaft-surface finish. Bristles deplete due to electrical “burn off ” caused by the current, and the shaft surface finish affects the rate of bristle depletion attributable to mechanical wear. An experienced manufacturer typically tries to select a model type and then a number of brushes that will result in about one year of bristle element life. (Note: In many cases, it will be possible to replace the bristle element while the unit is TABLE 1. Brush rating or “sizing” guidelines Up to 25-MW generator rating per brush, or up to 1 amp DC continuous for one year of bristle element life. Up to 50-MW generator rating per brush, or up to 4 amp DC continuous for one year of bristle element life. Up to 500-MW generator rating per brush, or up to 100 amp DC continuous for one year of bristle element life.

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Turbomachinery Developments often possible to reduce the burn-off rate by installing a larger brush, or by installing more than one brush. For instance, the time between brush insert replacement will be about four times as long if two brushes are used instead of just one. Q6. Why pay more for silver/gold composite bristles when copper or carbon brushes work fine? Solid copper or carbon brushes are not suitable for high-shaft surface velocities; they will groove the shaft, spark or quit working altogether. Solid copper or carbon brushes will also stop performing at low levels of current. For instance, copper straps and carbon brushes do not function well in oil or in contaminated or dirty environments. Well-designed brushes are, by comparison, self-cleaning even in dirty environments; they actually benefit from an oil environment. Q7. What is the advantage of silver/ gold bristles over other materials? The noble metals are corrosion-resistant, even in very hostile environments. Also, silver/gold bristles have exceptionally good electrical contact characteristics at the shaft surface and produce the lowest possible residual shaft voltage. Q8. How many brushes should be installed, and what type of brush should be used? The quantity and 1967 Nova Pro Street model of grounding brush best used on a particular unit will depend on the peak current flowing to ground, as well as, on physical constraints. For this reason, reliability professionals should stay in touch Process Maxum with an experienced manufacturer and ask for competent guidance. Realistic Do you have flows up to professionals are prepared to pay for good 9,900 GPM (2,000 m3/hr), counseling and quality products. The heads up to 720 Ft (220 M), amount of current is difficult to predict, speeds up to 3,500 RPM, and since it depends on many factors such as temperatures up to 500°F (260°C)? Then you the strength of residual magnetism in the need Carver Pump Process Maxum Series muscle! machine (particularly in the rotor, stator, With an extended range of hydraulic coverage and rugged foundation, piping, etc.). With electrical construction, the Process Maxum Series is ideal for equipment, generator and exciter designs Industrial Process applications. Manufactured in 35 sizes, or conditions are also very important. standard materials include WCB, WCB/316SS, 316SS and In some situations, it may be necesCD4MCu, with others available upon request. A variety of options include various types of mechanical seals and sary to install more than one brush. For bearing lubrication/cooling arrangements, auxiliary example, if a turbine is driving a generator protection devices and certified performance testing. through a gear, then current could be genWhatever your requirements, let us build the erated either by the generator, turbine, muscle you need! gear or any combination of the three. In case only one grounding brush is installed (on the turbine, for instance), current from the generator could be drawn Creating Value. across the gear teeth. This would result Carver Pump Company in damage to the gear teeth or bearings 2415 Park Avenue of the gear set. Installing brushes at both Muscatine, IA 52761 ends of the gear would eliminate this pos563.263.3410 Fax: 563.262.0510 sibility. It would ground the gear, as well, www.carverpump.com and prevent it from magnetizing the rest of the train. Gear-type couplings are susceptible to damage in a similar manner.

online.) However, there are many factors to consider, not all of which will be known at the time of brush installation. For that reason, it is not unusual for bristle elements to be depleted significantly sooner, or to last much longer than expected. When considering the brush burn-off rate, remember that the brushes are sacrificial components. The bristles burn down to prevent spark damage to far more expensive parts of the machine (bearings, governor and gears). Consequently, a high burn-off rate would simply mean that the brush is doing what it was supposed to, i.e., preventing expensive damage, which would otherwise occur inside the machine. It is

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Turbomachinery Developments Generally speaking, a particular brush category is suitable for mechanical-drive equipment and small turbine generators. Another category is designed for central station power plants with units of 100 MW to over 1,000 MW. Of the smaller brushes, a “premium” type will carry about four times as much current as a certain related type will do for the same bristle “burn-off ” rate. For this reason, one type may distinguish itself as the most economical of the small brushes in terms of ampere-hours per dollar and have the longest bristle element replacement interval in a type or configurational grouping. Working with a competent brush manufacturer is valuable and provides economic sense. Q9. Why is residual magnetism in rotors and casings important and what should be done about it? Residual magnetism in rotors and stators is a common and troublesome cause of stray-electrical shaft currents. For this reason, it is appropriate to do a careful and thorough check for residual magnetism whenever a machine is disassembled. It is mandatory that not only rotors should be demagnetized, but also the casings, piping, base plate, foundation and so on. If the demagnetization step is omitted, remagnetization will occur immediately when a rotor is placed in its casing. In fact, the entire unit, its mounting, and accessories will start to be magnetized as soon as the rotor is turned. Here are very general rules-of-thumb for turbomachinery and similar equipment: • Rolling-element bearings are much more vulnerable to shaft current damage than hydrodynamic bearings. In the case of residual magnetism-induced stray electrical current damage to rolling-element bearings, the best remedial action would be to reduce all residual magnetic field levels as far as possible (1 gauss–2 gauss), especially in the bearing, its surroundings and the nearby shaft. • Measure each component and de-gauss as necessary before installation, and as the machine is being assembled. Magnetic field levels will often increase as the parts are assembled and installed into the machine. The suggested maximum allowable levels of residual magnetism for typical turbomachinery with hydrodynamic bearings are summarized in TABLE 2. Q10. What is the suggested electrical arrangement for shaft-grounding brushes and voltage-sensing brushes? Experienced manufacturers will be pleased to supply drawings or schematics. The purpose of a shaft-grounding system is to bypass stray currents around the parts to be protected (for example, a thrust bearing). Connecting the brush to the lower half of the casing, close to the part to be protected, is suggested. This is as close as possible to the current path, assuming it had traveled through the component. It is also desirable to have the shortest and simplest connection from the brush to ground. Connecting directly to a plant grounding grid (rather than the lower half of the casing) can cause a high rate of bristle depletion and is not advisable. Also, grounding a brush to the frame of an electrically active machine, such as a motor or generator, will often result in a very high rate of bristle burn-off. Q11. Are there additional issues to consider? Stray electrical shaft current situations and their remediation can become surprisingly complex. Consequently, reliability en-

TABLE 2. Maximum suggested levels of magnetism allowed with hydrodynamic bearings Bearing components, including pads and retainers, journals, thrust disc, seals, gears and coupling teeth

2 gauss

Bearing housings

4 gauss

Mid-shaft and wheel areas, diaphragms, etc.

6 gauss

Components remote from minimum clearance areas, such as casings, piping, etc.

10 gauss

gineers are encouraged to contact a competent provider for all new installations, as well as for installations exhibiting unexpected behavior. It should be recognized that, in some situations, simply installing a shaft-grounding brush without taking additional investigative and corrective action will not eliminate the problem. For example, if a machine has become highly magnetized, grounding brushes will not be able to protect the machine. The brushes will not draw off all the current generated, and the rate of bristle burn-off will be unacceptably rapid. At worst, the unit could become electromagnetically selfexcited, resulting in a catastrophic failure. This phenomenon is described in the technical information kits supplied by highly experienced brush manufacturers. The correct action in the event of unusual behavior is to thoroughly investigate the situation. The equipment should be carefully monitored and surveyed for both residual magnetism and shaft-current activity. Again, the equipment may need to be carefully demagnetized without undue delay. Brushes should then be installed for the purpose of grounding and monitoring any remaining electromagnetic activity. Certain electrical problems with generators and motors can create shaft current magnitudes far beyond the capacity of any shaft-grounding device. Electric motors or generators driving from both ends deserve special consideration, as will variable frequency drives. Bottom line. There is no substitute for understanding preventive and proactive measures needed to preserve both physical and human assets. Reliability professionals are urged to stay abreast of shaft-current-elimination technologies and to work with a competent vendor-manufacturer. Both actions will pay great dividends over the long term. TOM SOHRE has been professionally involved with turbomachinery for approximately 40 years and is the general manager of Sohre Turbomachinery, a manufacturer of shaft riding brushes for turbomachinery. His prior positions have included design and field service engineering at Westinghouse, GE, Brown Boveri, and the Hartford Steam Boiler Inspection and Insurance Co. Mr. Sohre is a graduate of the University of Connecticut. HEINZ P. BLOCH resides in Westminster, Colorado. His professional career began in 1962 and included long-term assignments as Exxon Chemical’s regional machinery specialist for the US. He has authored over 520 publications, among them 18 comprehensive books on practical machinery management, failure analysis, failure avoidance, compressors, steam turbines, pumps, oil-mist lubrication and practical lubrication for industry. Mr. Bloch holds BS and MS degrees in mechanical engineering. He is an ASME Life Fellow and maintains registration as a Professional Engineer in New Jersey and Texas. Hydrocarbon Processing | APRIL 201381

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