Reciprocating Compressors—Process Considerations

Most gas lift, flash gas, and vapor recovery compressors require a recycle valve because of the unsteady and sometimes unpredictable nature of the flow rate. Indeed there may be periods of time when there is no flow at all to the compressor. 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

As flow rate to the compressor increases, the suction pressure rises until the volume of gas at actual conditions of temperature and pressure compressed by the cylinder equals the volume required by the cylinder. A flare valve is needed to keep the suction pressure from rising too high and overpressuring the suction cylinder, creating too high a rod load or increasing the horsepower requirements beyond the capability of the driver (see Chapter 11 for further discussion). 

The flare valve also allows production to continue momentarily if a compressor shuts down automatically. Even in booster service it may be beneficial to allow an operator to assess the cause of the compressor shutdown before shutting in the wells. In flash gas or gas-lift service, it is almost always beneficial to continue to produce the liquids while the 

LC LEVEL CONTROL LSH HIGH LEVEL SHUT DOWN PC PRESSURE CONTROL PSHiHIGH pressure SHUT DOWN PSLLOW PRESSURE SHUT DOWN S.O SliUT DOWN 

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The previous section discussed the various types of compressors, their selection, and process flow. This chapter presents greater detail concerning the major components, performance, operational and installation considerations, and standard specifications for reciprocating compressors. 

Some process equipment such as reciprocating pumps, compressors, and conveyors must be started while fully loaded other drives such as centrifugal pumps, fans, and agitators have light loads at starting. Many applications require considerably more torque to start and accelerate the load than to run it the reverse is also true. Starting torque, maximum or breakdown torque, inertia of the load, and the accelerating time must be considered. 

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