Head is simply the work expressed in foot pounds per pound of gas or N-m/kg. When inlet and discharge pressures and temperatures are known, the polytropic exponent can be determined from the relationship Because the value of n changes during the compression process, an average value is used. The polytropic exponent n is experimentally determined for a given type of machine and may be lower or higher than the adiabatic exponent k. Dynamic compressors generally follow the polytropic cycle as defined by the formula Nevertheless, the adiabatic compression cycle is rather closely approached by most positive displacement compressors and is generally the base to which they are referred.Ī polytropic process is one in which changes in gas characteristics during compression are considered. Where k = C p/ C v = ratio of specific heats, dimensionless.Īlthough compressors are designed to remove as much heat as possible, some heat gain is inevitable. This section defines terminology and discusses the operating principles essential for understanding compressor design, operation, and maintenance.Īn adiabatic process is one in which no heat is added or removed from the system. Dynamic compressors are further divided into:īoth positive displacement and dynamic compressors are governed by a few basic principles derived from the laws of thermodynamics. Positive displacement compressors are further divided into:ĭynamic compressors are continuous-flow machines in which a rapidly rotating element accelerates the gas as it passes through the element, converting the velocity head into pressure, partially in the rotating element and partially in stationary diffusers or blades. Like vapor recovery compressors, casinghead compressors operate with low suction pressures, high compression ratios, and low gas throughput rates.Ĭompressors are classified into two major categories: Often the compressor discharge is routed to either a booster or flash gas compressor or to a low-pressure gathering system. Low suction pressures, high compression ratios, and low gas throughput rates characterize these compressors.Ĭasinghead compressors are usually used with electric submersible pumps and rod pumps where formation gas is required to be separated downhole and then transported through the annulus. Often the gas from a vapor recovery compressor is routed to a flash gas, gas lift, or booster compressor for further compression. Vapor recovery compressors are used to gather gas from tanks and other low-pressure equipment in the facility. Many booster applications can be configured in a single-stage centrifugal compressor. Booster compressors typically are designed for high throughput rates and low compression ratio. Low-speed integral gas engine reciprocating compressors also are used for gas transmission applications. Most centrifugal pipeline booster compressors are gas turbine driven, although the use of variable-speed motor drives is becoming more prevalent. Booster compressors also are used in fields that are experiencing pressure decline. Selection of these compressors involves evaluating the economic trade-off of distance between pipeline boosting stations and life-cycle cost of each compressor station. Booster compressors are used to restore the pressure drop from these losses. Gas transmission through pipelines results in pressure drop because of friction losses. Compressors, applied to these services, have large compression ratios, high power requirements, and low volume flow rates. Reinjection compressors also are used for underground storage of natural gas. Reinjection compressors can be required to deliver gas at discharge pressures in excess of 10,000 psi. The reinjection of natural gas is employed to increase or to maintain oil production. Many gas lift compressors are installed on offshore facilities. Gas lift compressor duty is frequently of low to medium throughput with high compression ratios. Gas lift compressors are frequently used in oil handling facilities where compression of formation gases and gas lift gas is required. Flash gas compressors typically handle low flow rates and produce high compression ratios. These are:įlash gas compressors are used in oil handling facilities to compress gas that is “flashed” from a hydrocarbon liquid when the liquid flows from a higher pressure to a lower pressure separator. Installation, safety, and maintenance considerations also are discussed in their erspective pages.Ĭompressors used in the oil and gas industry are divided into six groups according to their intended service. Major components and construction features of centrifugal and reciprocating compressors are emphasized. Specific topics relating to compression theory include:
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