Compressors are used whenever it is necessary to flow gas from a lower pressure to a higher pressure system. Flash gas from low-pressure vessels used for multistage stabilization of liquids, oil treating, water treating, etc., often exists at too low a pressure to flow into the gas sales pipeline. Sometimes this gas is used as fuel and the remainder flared or vented. Often it is more economical or it is necessary for environmental reasons to compress the gas for sales. In a gas field, a compressor used in this service is normally called a “flash gas compressor.” Flash gas compressors are normally characterized by low throughput rate and high differential pressure.
The differential pressure is expressed in terms of overall compressor ratio, RT, which is defined as:
Flash gas compressors typically have an overall compressor ratio in the range of 5 to 20.
In some marginal gas fields, and in many larger gas fields that experience a decline in flowing pressure with time, it may be economical to allow the wells to flow at surface pressures below that required for gas sales. In such cases a “booster compressor” may be installed. Booster compressors are typically characterized by low overall compressor ratio (on the order of 2 to 5) and relatively high throughput.
Booster compressors are also used on long pipelines to restore pressure drop lost to friction. The design of a long pipeline requires trade-off studies between the size and distance between booster compressor stations and the diameter and operating pressure of the line.
The use of large compressors is probably more prevalent in oil field facilities than in gas field facilities. Oil wells often require low surface pressure and the gas that flashes off the oil in the separator must be compressed in a flash gas compressor. Often a gas lift system is needed to help lift the oil to the surface. As described in Volume 1, a “gas lift compressor” must compress not only the formation gas that is produced with the oil, but also the gas-lift gas that is recirculated down the well. Gas lift compressors are characterized by both high overall compressor ratios and relatively high throughputs.
Often, other forms of artificial lift are used to produce oil wells such as downhole submersible pumps and rod pumps that require that most of the formation gas be separated downhole and flowed up the annul us . between the tubing and the casing. When it is economical to recover this gas, or when the gas must be recovered for environmental reasons, a “casinghead gas compressor” will be installed. These are sometimes called “casing vapor recovery (CVR) units” or just “vapor recovery units (VRU).” Casinghead compressors are typically characterized by low suction pressure (0 to 25 psig). They often discharge at low pressure (50 to 300 psig) into the suction of a booster or flash gas compressor or into a low-pressure gas gathering system that gathers gas from several locations to a central compressor station.
Vapors from tanks and other atmospheric equipment may be recovered in a “vapor recovery compressor” (VRU). Vapor recovery compressors have very low suction pressure (0 to 8 ounces gauge) and typically have low flow rates. They normally discharge into the suction of a flash gas
compressor.
This chapter presents an overview of the types of compressors, considerations for selecting a type of compressor, a procedure for estimating horsepower and number of stages, and some process considerations for both reciprocating and centrifugal compressors.
