Optimizing Field Compressor-StationDesigns

K.A. Pennybaker, SPE, River City Engineering, Inc.

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SummaryThe main reasons for optimizing new field compressor-stationsigns and for upgrading existing stations are safety consideratenvironmental impacts, and economics. With a well-desigcompressor station it is possible to improve safety and envirmental conditions while saving capital and increasing income

A well-designed compressor station will be able to reduceventing of heavy hydrocarbons from storage tanks, whichincrease safety by reducing the possibility of forming explosvapor clouds near venting tanks. An optimized design canprove safety by reducing the amount of hydrocarbon inventlocated at the site. This can also save money by eliminatingneed for a site to be covered by OSHA’s procesafety management.

Improving the environmental conditions by reducing the veing of hydrocarbons from the station’s storage tanks is anoadvantage of a well-designed compressor station. The exhpollution can also be reduced by utilizing less compressor hopower and reducing the trucking needs for station suppliesproducts such as methanol and condensate.

The economic advantages of an optimized compressor stainclude reduced operating costs and maximized product recovReduction in operating costs are achieved through fuel savfrom reduced horsepower requirements and reducing truccosts. Maintenance activities are also typically less due tohorsepower, freeze ups, etc. Reducing the venting requiremmeans more gas is recovered as product, resultingmore income.

Evaluating and selecting the best configuration forcompressor-station design is an important decision. Mschemes have been evaluated for different situations and cpared so that the most cost-effective configuration can easilyselected for new or existing station designs.

IntroductionMany compressor stations consist of single-stage to four-sskid-mounted compressor packages linked together by comheaders. Operating pressures for these compressor stationsfrom a slight vacuum at the suction to discharge pressures1,000 psig.

A typical compressor-station design may consist of an inseparator to collect liquids and slugs that may have formed ingathering system pipeline. From there, the gas is sent to the cpressor skid~s! where it is compressed. At the discharge of eastage the gas is cooled, typically, with an air cooler, and thepasses through a scrubber before passing to the next stagethe discharge pipeline.

The liquids collected from the scrubbers are handled a numof different ways. A typical simplified approach is to route thliquids from the scrubber level control valves to a low-press~LP! tank. This process is referred to as the LP tank sche~Fig. 1!.

The LP tank can be a pressure vessel operated at a relatlow pressure~atmospheric to;25 psig! or it can be a simpleindustry standard 210 tank~atmospheric tank with 210 bbls o

Copyright © 2000 Society of Petroleum Engineers

This paper (SPE 66507) was revised for publication from SPE 40006, first presented at the1998 SPE Gas Technology Symposium held in Calgary, Alberta, Canada, 15–18 March.Original manuscript received for review 15 March 1998. Revised manuscript received 28March 2000. Paper peer approved 10 April 2000.

SPE Prod. & Facilities15 ~4!, November 2000

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capacity!. In either case, the vapors produced from the flashliquids are vented to the atmosphere. The low-pressure condenis periodically trucked out and sold.

Efficient and safe handling of the liquids collected from tscrubbers in a compressor station is one of the keys to a gdesign. Poor handling of these liquids can be the major sourcoperating problems and have a significant impact on the staeconomics. These operating problems include things such asdrate formation, control valve failure, etc.

There are four main concerns that should be addressed inliquid handling design for any compressor station: safety, envirmental impacts, economics, and operability.

Process DefinitionsThe goal of this paper is to illustrate to the designer/operatocompressor stations the advantages of good process engineerthe design of compressor stations. There are many proschemes that can be employed for a compressor-station deEach has its distinct advantages over the others dependingthe design conditions~gas composition, pressures, etc.!.

Each process should be evaluated and compared to othedetermine the optimum process for the application. Four proceare presented in this paper and compared showing the proscons for each process. These examples should aid the designtheir initial screening of processes for a new application.

LP Tank. The LP tank scheme discussed above is probablyof the most typical compressor-station designs. It can be refeto as the industry standard and will serve as the basis for mosthe comparisons in this paper.

The advantages of the LP tank scheme are that it is simplecheap to install. It can also typically avoid OSHA’s process safmanagement~PSM! regulation since the bulk storage is operatat atmospheric pressure.

The typical disadvantages are venting, cold temperatures, lshrinkage, and poor compression efficiency. These disadvantdo not apply to all applications. In some cases, however, theybe extreme.

High hydrocarbon venting rates are a safety and environmeconcern and a primary cause for poor compression efficiency.hydrocarbons that are vented can form vapor clouds which cobe ignited. The vented hydrocarbons are lost to the atmospand result in no value gained as either condensate or gas. Thiscontribute to a large gas shrinkage across a compressor stat

The flashing of liquids from high-pressure~HP! scrubbersacross the level control valve and into the LP tank can resuloperating temperatures significantly below220°F, which is thetypical minimum allowable operating temperature for normal cbon steel. At temperatures below220°F, carbon steel becomebrittle and can easily fail due to vibration or stress.

Level control valves from high-pressure scrubbers can faithe open position either through freeze-up or mechanical prlems. This can result in very large gas blow-by rates which lealarge gas releases and in some cases catastrophic tank failur

HP Tank. The HP tank scheme~Fig. 2! is similar to the LP tankscheme, except that the liquids from the higher-pressure scrubare routed to a high-pressure storage vessel~HP tank!. The HPtank operates at a pressure greater than the inlet pressure~typi-cally, 30 to 200 psig! so that any vapors produced from the Htank can be routed back to the suction of the compressor staBoth a low-pressure and a high-pressure condensate are prodand must be trucked out.

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Fig. 1–LP tank scheme employed in a typical three-stage compressor station.

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The HP tank scheme has many advantages over the LPscheme. It eliminates most of the atmospheric venting andgas losses.

It also maximizes condensate production at the compressortion. This can also be a disadvantage because it leads to mtrucking costs. This is further complicated by having to truck bolow- and high-pressure condensate, which may involve multtrucking companies and market outlets. Single product truckcan be accomplished by pumping the low-pressure condeninto the high-pressure condensate storage. However, this addsfor the pumping equipment.

Handling of the high-pressure condensate does increasesafety risk over handling of just low-pressure condensate.storage of high-pressure condensate can also lead to the sbeing regulated by PSM.

The HP tank scheme is also more expensive than the LPscheme, but can typically easily pay for itself in the reducedlosses. The cost of PSM compliance must also be considereapplicable.

HP Scrubber. The HP scrubber scheme~Fig. 3! is very similar tothe HP tank scheme, except there is no high-pressure condestorage or trucking. This results in a number of advantages othe HP tank scheme, with only a few disadvantages.

Since no high-pressure condensate is stored or trucked, thecern over high-pressure condensate handling is nearly eliminaThe lack of high-pressure storage could result in avoiding Pregulation. This scheme will be cheaper to build and install ththe HP tank scheme, but still slightly more than the LP tascheme. Condensate handling is significantly simplified since oone product must be stored and trucked out.

The HP scrubber scheme does result in some atmosphericing, but significantly less than the LP tank. This does lead to sogas losses that can affect the station economics.

Cascade Recycle.The cascade recycle scheme~Fig. 4! differsconsiderably from the previously described schemes. In thiscess the liquids from each scrubber are routed back to thelowest operating pressure scrubber within the compressionquence. Only the inlet scrubber liquids are then routed to a lpressure condensate storage tank.

224 K.A. Pennybaker: Optimizing Field Compressor-Station Design

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This process has proven to have many advantages in mcompressor-station applications. These advantages includecapital cost, only low-pressure condensate handling, minimizaof liquids produced, and thus maximization of gas output fromstation.

The capital cost is comparable to that of the LP tank scheOnly low-pressure condensate storage and trucking is require

This process typically results in the minimization of liquidproduced, or consequently, a minimization of gas shrinkage acthe station. This means that a richer gas is discharged fromstation and sent via a pipeline to a processing plant downstreThe compression efficiency is typically better than others, as msured in terms of hp/MMSCFD.

Another advantage is that since only low-pressure condenis stored, the station can usually avoid being PSM regulated.

This process can result in atmospheric venting if atmosphtanks are used for the low-pressure condensate storage. Howit is significantly less than either the LP tank scheme or thescrubber scheme. The use of a low-pressure bullet can elimithe venting.

Another disadvantage is the implementation of this schemerental or other existing units. Early planning and involvementnecessary to incorporate the necessary piping changes thadifferent from typical piping configurations.

Process Comparisons Through ExamplesTwo example cases were simulated to illustrate the differenceperformance of the different processes. The first case is a thstage high-pressure compressor station and the second casthree-stage low-pressure compressor station. Both cases arepressing relatively rich gas, which is where typically the liquhandling problems are of the greatest magnitude.

The four process schemes were simulated using theHYSIM™process simulator. The inlet gas was saturated with water atinlet conditions. The inlet temperatures were 60°F and the instage temperatures were 80°F. This is relatively cool for intstage temperatures, but is typical in some winter-time climawithout good louver control. These low temperatures also increthe magnitude of the example cases.

s SPE Prod. & Facilities, Vol. 15, No. 4, November 2000

Fig. 2–HP tank scheme employed in a typical three-stage compressor station.

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In all cases the amount of water condensed through the cpression train is the same. The amount of water is a functionthe operating pressures and temperatures and is not affected bprocess.

Three-Stage High Pressure.The inlet gas rate was 45 MMSCFDof relatively rich gas (8.70 C21gal/MCF). The inlet pressure wa100 psig, and the discharge pressure was 2,200 psig. The refor the four schemes are compared inTable 1. This exampleclearly highlights the differences of each scheme. A careful ansis is required that considers the economic contracts in placecapital costs, and the safety and environmental effects.

Although the LP tank scheme utilizes the least amounthorsepower, it also results in the least amount of outlet~residue!gas ~39.17 MMSCFD! entering the downstream pipeline. It alsproduces the second lowest output of condensate~1,772 BPD!.These two factors lead this scheme to have the highest aspheric venting rate~3,410 MSCFD! of all the schemes~over fourtimes higher than any other!.

TABLE 1– THREE-STAGE HIGH-PRESSURECOMPRESSOR STATION

LPTank

HPTank

HPScrubber

CascadeRecycle

Horsepower 7,553 7,952 7,952 7,874Discharge rate(MMSCFD)

39.17 41.32 41.32 42.35

Discharge composition(gal/MCF)

6.77 6.89 6.89 7.33

Efficiency(hp/MMSCFD)

192.8 192.5 192.5 185.9

Atmospheric venting(MSCFD)

3,410 0 783 497

LP condensate(BPD)

1,772 0 2,113 1,628

HP condensate(BPD)

¯ 2,605 ¯ ¯

Total condensate(BPD)

1,772 2,605 2,113 1,628

K.A. Pennybaker: Optimizing Field Compressor-Station Designs

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The HP tank scheme results in no atmospheric venting anthis case no low-pressure condensate. Therefore, only HP consate would typically have to be trucked.

The HP scrubber scheme is similar to the HP tank schemethere is some atmospheric venting losses. Both the HP tankthe HP scrubber scheme have the highest horsepower reqments of the four schemes.

The cascade recycle scheme has the highest outlet~residue!rate ~42.35 MMSCFD! and the lowest condensate rate~1,628BPD!. This can be an advantage if the goal is to get the gasplant to recover the liquids. It reduces the amount of trucking ahandling required.

The cascade recycle scheme also results in the best comprefficiency requiring only 185.9 horsepower per MMSCFD of dcharge gas.

Metallurgy temperature and hydrate problems are anticipafor the LP tank. The HP tank, HP scrubber, and cascade recschemes avoid the metallurgy problems and the hydrate probare also minimized.

Three-Stage Low Pressure.The inlet gas rate was 5 MMSCFDof relatively rich gas (7.95 C21gal/MCF). The inlet pressure wa14.5 psia and the discharge pressure was 275 psig. The resulthe four schemes are compared inTable 2. This example againhighlights some of the differences between the four schemAgain, a careful analysis is required to determine which schebest meets the needs for a new compressor-station.

The cascade recycle scheme utilizes the most horsepowethough it is only slightly higher~3 hp, 0.3%! than the otherschemes. However, the cascade recycle process produces theamount of condensate~3.8 BPD!, and thus the least amount oshrinkage across the compressor station. Also, due to the lowpressure, there is no atmospheric venting from this scheme.

The LP tank scheme has the highest atmospheric venting~5.6 MSCFD! by a factor of almost 3.

The HP tank scheme has the highest condensate produ~7 BPD!. Again, in this example, no LP condensate was productherefore, primarily only one condensate product has totrucked out.

SPE Prod. & Facilities, Vol. 15, No. 4, November 2000 225

Fig. 3–HP scrubber scheme employed in a typical three-stage compressor station.

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The HP scrubber scheme is similar to the HP tank schemeslightly less condensate~5.6 BPD of LP condensate! productionand a small amount of atmospheric venting~1.9 MSCFD!.

Field ExperienceSome past experience is used to highlight how two comprestations were optimized through a process evaluation. In one cit was an existing compressor station that was experiencing aof operating problems. The other example shows how a quinvestigation resulted in an optimized station design.

Existing Station. An existing three-stage high-pressure comprsor station configured in the LP tank scheme was experiencilot of problems. The condensate was routed and stored in amospheric 210 tank. The station was experiencing high venrates creating a visible vapor cloud at times around the tank.venting rates were so high at times that liquid carryover hadcurred and was visible as stains on the side of the tank.

TABLE 2– THREE-STAGE LOW-PRESSURE COMPRESSORSTATION

LPTank

HPTank

HPScrubber

CascadeRecycle

Horsepower 1,056 1,056 1,056 1,059Discharge rate(MMSCFD)

4.99 4.99 4.99 4.99

Discharge composition(gal/MCF)

7.90 7.90 7.90 7.93

Efficiency(hp/MMSCFD)

211.7 211.6 211.6 212.1

Atmospheric venting(MSCFD)

5.6 0 1.9 0

LP condensate(BPD)

4.8 0 5.6 3.8

HP condensate(BPD)

¯ 7.0 ¯ ¯

Total condensate(BPD)

4.8 7.0 5.6 3.8

226 K.A. Pennybaker: Optimizing Field Compressor-Station Design

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A look at operating conditions via simulation also showed ththe operating temperatures were well below220°F at times, ex-ceeding the minimum temperature rating of carbon steel. Themary concern at this station was that of safety. Of course,environmental problems were clearly visible.

A quick study was completed that evaluated different schemThe study showed that any of the other schemes would sigcantly reduce the venting and metallurgy problems, while increing the revenue streams. Ensuing discussions, however, enated the cascade recycle process since the compressionwere rental units and it would be difficult and costly to get trental company to modify the skids. The HP tank scheme wasfavorable due to the added burden of making the site a Pregulated facility. Therefore, the HP scrubber process waslected and installed.

The HP scrubber process significantly reduced the atmosphventing. It also eliminated the excessively cold temperatuwhich jeopardized the carbon steel metallurgy. As an added pit reduced the hydrate problems associated with such cold oping temperatures when the liquids flashed. All these itemsproved the safety and environmental aspects of the station. Aadded bonus, the modifications paid for themselves with a morate of return, by increasing the condensate recovered andfrom the station and reducing the gas shrinkage across the staresulting in more gas available for processing and sale.

New Station.A new three-stage high-pressure compressor stawas proposed. Initially, the client had proposed a LP tank desAfter some simulation study work the client opted for the cascarecycle process due to significant advantages.

The simulation results indicate that the cascade recycle proutilizes slightly less horsepower~0.2%!, increases the discharggas rate~1.2%!, and increases the discharge BTU content~2.7%!.In addition, this process reduced the atmospheric venting sigcantly ~98.7%! and the condensate produced and trucked sligh~36%!.

In addition, further comparison showed that separator ascrubber sizes within the compression train were not affectherefore, confirming a minimal cost of implementation.

s SPE Prod. & Facilities, Vol. 15, No. 4, November 2000

Fig. 4–Cascade recycle scheme employed in a typical three-stage compressor station.

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Conclusions and RecommendationsOptimizing the design of compressor stations can result in somajor benefits. A good compressor-station design generally leto an increase in safety and revenue, and a reduction in envmental impact and operating costs. All of these are positivepects for any operating company.

With all these positives potentially to be gained, it makes gosense to examine the design of new facilities and even to reevate existing facilities. Each case must carefully be evaluatedall factors considered: safety, environmental, and economics.

AcknowledgmentsThe author wishes to thank Terryl Filip and David ScarboroughConoco, Inc. for their help over the years in this area. Also,author wishes to thank his colleagues, Scott Wolverton and SChafin for comments and suggestions on this paper, and to CPritchard for his help in preparing this paper.

K.A. Pennybaker: Optimizing Field Compressor-Station Designs

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SI Metric Conversion Factorsbbl 3 1.589 873 E201 5 m3

ft3 3 2.831 685 E202 5 m3

°F (°F232)/1.8 5 °Cgal 3 3.785 412 E203 5 m3

hp 3 7.460 43 E201 5 kWpsi 3 6.894 757 E100 5 kPa

SPEPF

Kent A. Pennybaker is a consulting process engineer withRiver City Engineering Inc. in Lawrence, Kansas. e-mail:rvrcityeng@aol.com. He previously worked in Conoco Inc.’supstream process engineering group. He holds BS and MS de-grees in chemical engineering from the U. of Kansas.

SPE Prod. & Facilities, Vol. 15, No. 4, November 2000 227