AHP Compressor Selection

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By using expert-choice software or Excel spreadsheets for calculation, the project parties can processthe consistency and sensitivity analyses to evaluate the decision’s reasonability. The AHP is simpleand relatively easy for all authorized persons in a project team to understand, and it uses both

qualitative and quantitative criteria for compressor type evaluation.1

Key assumptions

A sample sales gas compression process is shown in Fig. 1. The question to be answered is: Whichcompressor type is better for sales gas compression? The key assumptions for this study aresummarized in Table 1.

Fig. 1. Process flow for a sample two-stage sales gas compression process.

Decision-making methodology

The engineering of the sales gas compressor includes type selection, which is always a problem for project parties at the front-end engineering phase of the project, due to two factors. The first factor isthe need to gather, identify and make decisions on the number of criteria, and the second factor ishow to select the most suitable compressor between the two types.

The AHP goal is to select the most suitable compressor type. To determine the decision variablesand selection criteria for the type selection process, the author prepared a questionnaire based on anextensive literature review and background research. In the questionnaire, the industry professionalswere asked two simple questions:

1. What are the main decision variables and selection criteria for the compressor type selectionprocess?

2. What is the importance of each variable or criterion?

More than 30 decision-making variables and selection criteria were identified. The top 15 mostinfluential factors for decision-making can be determined using the AHP method ( Table 2) for a plantsize of 50 MMscfd–100 MMscfd. The AHP model can be developed for all or some of the selectioncriteria. Fig. 2 shows an example of a simplified AHP model that includes six criteria.

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of 4Use a hierarchical process to evaluate sales gas compressor selection

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Fig. 2. Sample AHP h ierarchy structure with six criteria used for compressor type decision-making.

Data in the circles in Fig. 2 are a percentage of the project party’s decision regarding each criterion.For example, a score of 0.34 for reliability means that 34% of the project parties’ decisions regardingcompressor selection type is related to this criterion. Finally, the overall score for each compressor type is calculated as a total sum of each criterion priority vector multiplied by the respectivecompressor type’s priority vector related to that criterion.

Other selection factors

Additional factors considered in the study are explained in Table 3. These factors are used todetermine compressor selection for a plant size of 50 MMscfd–100 MMscfd.

Takeaway

The AHP method considers both qualitative and quantitative criteria to evaluate sales gascompressor selection. The compressor type selected depends on many factors in a specific project.The project engineer should consider the project model (time, cost, quality and safety) and fitness-for-purpose concepts during the compressor selection process, and not concentrate only on technical

subjects. Each project is unique.8

A reciprocating compressor may be a good fit for one project, but, in a similar project, the centrifugaltype may be the best and most proper choice, or vice versa. A systematic approach should beconsidered when the hierarchy of the decision criteria depends on managerial or technical power,personal preference and/or emotion. GP

Nomenclature

PS=Suction pressure

PD=Discharge pressure

IA=Instrument air

PA=Plant air

ISO=International Organizationfor Standardization

kW=Kilowatt

MTBF=Mean time between failures

N2=Nitrogen

Note

This article presents the author’s own views and research and is not affiliated with any company.

Acknowledgments

The author heartily thanks Dr. Janaka Y. Ruwanpura, professor of project management in theSchulich School of Engineering at the University of Calgary, Canada, for his guidance and for hisbrilliant lecture on the AHP method. The author is also very grateful to those who offered support andinput for gathering the data and finalizing the findings for this article, including clients, consultants,practitioners and vendors. Finally, the author is also very grateful to his family for their keen,committed and warm encouragement during the preparation of this article.

LITERATURE CITED

1Ruwanpura, J. Y., Material offered for second program of project management at Sharif University of Technology, Tehran, Iran, January 2010.

2 American Petroleum Institute, API 617, 7th Ed., Reaff irmed: January 2009, Ch. 1, Sec. 2, Cl. 2.1.2.

3 American Petroleum Institute, API 616, 4th Ed., January 2011, Cl. 4.1.1.

4 American Petroleum Institute, API 618, 5th Ed., December 2007, Cl. 6.1.1.


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5 American Petroleum Institute, API RP 686, 2nd Ed., December 2009, Ch. 1, Cl. 2.51.

6Bloch, H. P., “Consider a low-maintenance compressor,” Chemical Engineering, July 18, 1988.

7Griffith, W. A. and E. B. Flanagan, “Online continuous monitoring of mechanical condition andperformance for critical reciprocating compressors,” Proceedings of the 30th TurbomachinerySymposium, Texas A&M University, Houston, Texas, 2001.

8Project Management Institute, A Guide to the Project Management Body of Knowledge, NewtownSquare, Pennsylvania, 1996.

Shahab Zardynezhad is a senior mechanical engineer with more than 21 years of experience at many of the world’s largest oil, gas and petrochemical projects. His areasof specialization and industrial experience include engineering, design, procurement,

inspection, installation, startup, maintenance and troubleshooting of general andspecial-purpose rotating equipment, with special concentration on project managementand fitness-for-purpose. He holds a BS degree in mechanical engineering from theUniversity of Petroleum in Ahwaz, Iran; an MSc degree in industrial engineering from

Iran University of Science and Technology in Tehran, Iran; and an MEng degree in projectmanagement from the University of Calgary, Canada.

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AHP Compressor Selection

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