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As natural gas moves through a pipeline, the pressure decreases due to friction of the gas along the pipe wall and the gas must be recompressed to maintain the flow. Gas compressor stations are installed at optimum locations along the pipeline as the load profile changes and are sized to sufficiently boost the gas pressure and maintain flow through the pipeline. Compressor stations may be small, situated on gathering lines (Figure 1) or laterals, or large on major trunk line transmission systems. However, all are built up from the same functional blocks of equipment. Each functional element (Figure 2) plays a role in the work of the station and the design and sizing of each is essential to the efficient and safe operation of the plant.
The functional elements include gas scrubbing and liquid removal, compressor and driver units, aftercoolers, pipes and valves. Controls - including Supervisory Control And Data Acquisition (SCADA) system, monitoring and data recording, alarms and shut down procedures, both routine and emergency - are an integral part of the station. Provision also has to be made for venting the compressor and driver housing and buildings, complete with ventilation and fire protection, and safety equipment.
The financial elements include cost-of-service calculations which include the installed cost of the equipment, fuel, maintenance and operating costs for the expected project life (typically 10-25 years). A risk simulated discounted-cash-flow (DCF), rate-of-return (ROR) method of investment (Santos, 2003) analysis is recommended. Fuel cost is the major item in this calculation and the projection for escalation forms an important consideration and must be included in the risk simulation analysis.
Inlet Receiving
The gas in a main transmission line is nominally clean and dry while that in gas gathering lines may contain liquids prior to processing, but in all cases there can be entrained liquids and participates which have to be removed before compression to maintain efficiency and integrity. Efficient and safe handling of the liquids collected from the scrubbers in a compressor station is one of the keys to a good design. Individual scrubbers (with or without standby unit) may be installed for each stage of compression for each compressor unit.
A common scrubber should be considered for multiple units on a common suction line. Filter/separators may...