There are two basic types of reciprocating compressors distinguished by the style of piston and the linkage between the piston and crankshaft. One type covers the lighter-duty machines having trunk-type (automotive-type) single-acting pistons lubricated by crankcase oil, with no crossheads. These machines are typically used for air compression to 125 psig, although cylinders are available for working pressures to 6000 psig, for various gases. They operate at speeds in the range of 1200 to 1800 RPM and have ratings to about 125 HP. This type of machine is not often used for petroleum gas service, therefore, it is not described further in this manual.
The other type of reciprocating compressor is a heavy-duty crosshead-type machine where each piston is usually double-acting, and is connected to the crankpin by a piston rod, crosshead, and connecting rod. The cylinders are lubricated by a force-feed lubricator. Refer to Figure 300-1 for a cross-section of a typical single-cylinder reciprocating compressor.
Crosshead-type machines may be categorized into four broad classes:
• Class A – API 618 type of heavy-duty process gas machine.
• Class B – Often called “separable,” are normally used in oil and gas field operations.
• Class C – Similar to Class A, but the frames are smaller. Used in both process and oil field services.
• Class D – Smaller, light-weight versions of Class B. Usually applied in the oil field.
The manual concentrates generally on Classes A and B, but much of the information is also generally applicable to Classes C and D.