12.10.2 Reciprocating Compressors
Reciprocating compressors are utilized for high compressions. They have long been the machines used for compression to the high pressures required in ammonia plants and on offshore platforms. Accounts of reciprocating compressors are given by Whittaker (1975), Barnes (1976), Messer (1979), and Woollat (1993). Relevant standards are BS 7322: 1990 Specification for the Design and Construction of Reciprocating Type Compressors for the Process Industry and API Std 618: 1986 Reciprocating Compressors for General Refinery Services.
Reciprocating compressors can be provided with capacity control, or turndown, by the use of volume pockets and by drive speed control. Some principal malfunctions on reciprocating compressors are: (1) valve leakage, (2) cylinder/piston scoring, (3) piston ring leakage, (4) gasket failure, (5) tail rod failure, and (6) vibration, as well as the general compressor failures such as those caused by liquid slugs or loss of lubricating oil or cooling water.
Leakage of the suction or discharge valves is one of the commonest failures. There are a number of symptoms of valve malfunction. The valve may become unusually hot, the cylinder capacity may fall, the discharge temperature may rise, and the interstage pressures may be abnormal. The suction pressure may rise and the discharge pressure fall unless automatically controlled.
On some machines, there is nothing to prevent a suction valve being fitted on the discharge, or vice versa, by mistake. If this happens, it is possible to create very high pressures in the cylinder, particularly the high pressure cylinder, and so cause failure of the cylinder, the piston, or the drive system. Preferably, this feature should be designed out. Where this is not the case, there should be procedures to minimize the probability of error.
The piston rod in high pressure cylinders is sometimes balanced by a tail rod on the other side of the piston. There have been some serious accidents in which the tail rod has broken off and flown out like a projectile. There should be a ‘catcher’ of sufficient strength to prevent escape of the tail rod if it does break. Tail rods are frequently surface hardened and it is important for them to be free of surface cracks. They should be regularly inspected. A tail rod failure allows the escape of high pressure gas.
The reciprocating movement of the piston inevitably causes some degree of vibration. This vibration may be transmitted to, and cause failures in, the process pipework. Small auxiliary piping on the machine tends to be particularly vulnerable to fracture from this vibration. It should be anchored to reduce vibration and inspected regularly.
Changes in the discharge temperature are often a sign of malfunction on a reciprocating compressor. High discharge temperature may be associated with valve failure, piston ring leakage, increased compression ratio, gas composition change, or loss of cooling water.
On air compressors lubricated with oil, a high discharge temperature can result in an explosion. Such air compressor explosions, and the discharge temperature limits necessary to avoid them, are discussed in Chapter 17. Some compressors are required to produce compressed air which is free of oil. It is very desirable, for example, that instrument air be oil-free. Carbon ring compressors are often used for this purpose.
The reciprocating compressors were one of the potential sources of the gas leak investigated in the Piper Alpha Inquiry (Cullen, 1990). The questions of the tolerance of such machines to ingestion of liquid slugs and of the bolt tightening practices used were considered in evidence by Bett (1989).