When planning a single compressor station, or a number of connected stations, several items must be studied:

  • Adaptability to future operating conditions;
  • Steady-state and transient capabilities and requirements of the system;
  • Growth requirements and capability;
  • Total cost of ownership; and,
  • Total revenue generated.

The study of operating scenarios suggests certain requirements for the compression system, including a compressor capable of functioning over a wide operating range at high efficiency.

Operational flexibility can be achieved on various levels. The compressor and the driver should have a wide operating range, and using multiple smaller units per station rather than one large unit can be another way.

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Compression units can be set up in series or in parallel. In pipeline applications, the series approach can have advantages in case one of the units fails, but decisions must take into account issues such as pipeline characteristic, further expansion, back-up strategies, operational strategy, and aerodynamic performance.

The failure or unavailability of compression units can cause significant revenue loss, and so standby units must be considered. These can be arranged such that each compression station has one standby unit, or that only some stations have a standby unit, or that the standby function is covered by oversizing the drivers for all stations. Oversizing creates a disadvantage during normal operation, when units would operate in part load. The failure of a compression unit does not mean the entire pipeline ceases operation, but that the pipeline’s flow capacity is reduced.

Many compression systems are not operated at full capacity from the beginning, but rather increase the demand when new customers for the gas are found. The project can start with fewer small units and additional units can be installed later to meet increasing demand on compression, thereby delaying expenses for portions of the equipment. On the other hand, if larger units are used, they must all be purchased at the beginning of the project. A net present value (NPV) analysis reveals that even a higher total cost can render a lower NPV if parts of the cost are deferred.

Any activity has to be judged by its impact on the environment, including NOx, CO2 and UHC emissions. Meeting maximum emission levels is often a requirement for continuous operation. Further, the amount of CO2 produced has to be considered. For a given fuel, the only way to reduce the amount of CO2 produced is to increase the efficiency of operation.

When operating multiple units, the question becomes how to achieve certain objectives, such as minimising running hours of units, optimising load-swing absorption capability, and minimising fuel consumption or emissions. The first requires running as few units as possible, while the second may require running all or most units at part load most of the time. For two identical units, the answer is virtually always to run both of them at equal part load. This is accommodated by operating the compressors at equal surge margin, or at equal gas turbine load (gas producer speed). However, if the units are dissimilar in size or efficiency, it may be best if the larger or more efficient unit carries the base load, while the other powers load swings.