Compressors

Figure 2-14 shows the configuration of the typical three-stage reciprocating compressor in our example flowsheet. Gas from the FWKO enters the first-stage suction scrubber. Any liquids that may have come through the line are separated at this point and the gas flows to the first stage. Compression heats the gas, so there is a cooler after each compression stage. At the higher pressure more liquids may separate, so the gas enters another scrubber before being compressed and cooled again.

In the example, gas from the intermediate pressure separator can be routed to either the second-stage or third-stage suction pressure, as conditions in the field change. Gulf of Mexico accident records indicate that compressors are the single most hazardous piece of equipment in the process. The compressor is equipped with an automatic suction shut-in valve on each inlet and a discharge shut-in valve so that when the unit shuts down, or when an abnormal condition is detected, the shut-in valves actuate to isolate the unit from any new sources of gas. Many operators prefer, and in some cases
regulations require, that an automatic blowdown valve also be installed so that as well as isolating the unit, all the gas contained within the unit is vented safely at a remote location.

Compressors in oil field service should be equipped with a recycle valve and a vent valve, such as shown in Figure 2-14. Compressor operating conditions are typically not well known when the compressor is
installed, and even if they were, they are liable to change greatly as wells come on and off production. The recycle valve allows the compressor to be run at low throughput rates by keeping the compressor loaded with its minimum required throughput. In a reciprocating compressor this is done by maintaining a minimum pressure on the suction. In a centrifugal compressor this is done by a more complex surge control system.

The vent valve allows production to continue when the compressor shuts down. Many times a compressor will only be down for a short time

three-stage-compressor

and it is better to vent the gas rather than automatically shut in production. The vent valve also allows the compressor to operate when there is too much gas to the inlet. Under such conditions the pressure will rise to a point that could overload the rods on a reciprocating compressor.

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The two basic types of compressors used in production facilities are reciprocating and centrifugal. Reciprocating compressors compress the gas with a piston moving linearly in a cylinder. Because of this, the flow is not steady, and care must be taken to control vibrations. Centrifugal compressors use high-speed rotating wheels to create a gas velocity that is converted into pressure by stators.

Reciprocating compressors are particularly attractive for low horsepower (< 2,000 hp), high-ratio applications, although they are available in sizes up to approximately 10,000 hp. They have higher fuel efficiencies than centrifugals, and much higher turndown capabilities. Centrifugal compressors are particularly well suited for high horsepower (>4,000 hp) or for low ratio (<2.5) in the 1,000 hp and greater sizes. They are less expensive, take up less space, weigh less, and tend to have higher availability and lower maintenance costs than recips. Their overall fuel efficiency can be increased if use is made of the high temperature exhaust heat in the process.


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