Reciprocating compressorReciprocating compressors are positive displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder. The discussion onthe this page on reciprocating compressors includes a description of process configuration for multistage units, as well as an explanation of the concepts of: Speed control Inlet throttling Recycling Pressure relief Blowdown istance piece venting and draining Types of reciprocating compressors There are two types of reciprocating compressors: !igh speed "separa#le$ %ow speed "integral$ The high&speed category also is referred to as 'separa#le,( and the low&speed category also is )nown as 'integral.( The *merican Petroleum Institute "*PI$ has produced two industry standards, *PI Standard 11P and *PI Standard 618, which are fre+uently employed to govern the design and manufacture of reciprocating compressors. Separable compressors The term 'separa#le( is used #ecause this category of reciprocating compressors is separate fromits driver. ,ither an engine or an electric motor usually drives a separa#le compressor. -ften a gear#ox is re+uired in the compression train. -perating speed is typically #etween .// and 0,1//rpm. Integral compressors The term 'integral( is used #ecause the power cylinders that drive the compressor are mounted integrally with the frame containing the compressor cylinders. Integral units run at speeds of #etween 2// and 3// rpm. They are commonly used in gas plants and pipeline service where fuelefficiency and long life are critical. Integral compressors may #e e+uipped with two to ten compressor cylinders with power ranging from 04/ to 02,/// hp. Integral compressors offer high efficiency over a wide range of operating conditions and re+uire less maintenance than the separa#le units. !owever, integral units usually must #e field&erected and re+uire heavy foundations and a high degree of vi#ration and pulsation suppression. They have the highest initial installation cost. Fig. 3 is a cross section of a typical integral compressor. Fig. 4 shows an integral compressor pac)age. Fig. 3Cross section of an integral compressor (courtesy of Dresser-Ran!. Fig. 4Integral reciprocating compressor pac"age (courtesy of Dresser-Ran!. #a$or components Reciprocating compressors are availa#le in a variety of designs and arrangements. 5a6or components in a typical reciprocating compressor are shown in Fig. %. Frame The frame is a heavy, rugged housing containing all the rotating parts and on which the cylinder and crosshead guide is mounted. 7ompressor manufacturers rate frames for a maximum continuous horsepower and frame load "see the section on Rod %oad #elow$. Separa#le compressors are usually arranged in a #alanced&opposed configuration characteri8ed #y an ad6acent pair of cran) throws that are 01/ degrees out of phase and separated #y only a cran) we#. The cran)s are arranged so that the motion of each piston is #alanced #y the motion of an opposing piston. Integral compressors typically have compressor and engine&power cylinders mounted on the same frame and are driven #y the same cran)shaft. 7ylinders in integral compressors are usually arranged on only one side of the frame "i.e., not #alanced&opposed$. Cyliner The cylinder is a pressure vessel that contains the gas in the compression cycle. Single&acting cylinders compress gas in only one direction of piston travel. They can #e either head end or cran) end. ou#le&acting cylinders compress gas in #oth directions of piston travel "see Fig. &$. 5ost reciprocating compressors use dou#le&acting cylinders. 7hoice of cylinder material is determined #y operating pressure. 7ast iron is normally used for pressures up to 0,/// psi. 9odular iron is used for pressures up to 0,:// psi. 7ast steel is usually used for pressures #etween 0,:// and 2,:// psi. ;orged steel is selected for cylinder operating pressures greater than 2,:// psi. * cylinder