Page 69 - Reciprocating Compressors Operation Maintenance

56 Reciprocating Compressors: Operation and Maintenance

In general, liners are not used in low- or medium-pressure cylinders
where the gas being handled is non-corrosive, such as air. Liners are
almost always used in medium- to high-pressure cylinders where the gas
being handled is corrosive.
Certain industry users insist on liners in their compressors. Liners have
become standard design for certain industry applications, even though
the gas involved may be non-corrosive and of medium pressure, or with
low differentials. Such a standard exists in natural gas pipeline transmis-
sion service. The API 618 specifications for reciprocating compressors in
refinery service also reflect this standard.
It follows that a liner which increases the initial cost of the compressor
cylinder is more economical to replace than the complex cylinder casting.
Another application of the liner is to reduce the cylinder bore size to
meet certain capacity conditions. By installing liners with different bore
sizes, the same cylinder casting can be used to accommodate a range of
capacity and pressure conditions.
Cast iron is generally used for liners unless the cylinder is high pres-
sure—above 3000 psi on lubricated machines and above 500 psi on non-
lubricated compressors. On these, hardened cast steel may be used.
For corrosive applications a "Ni-Resist" cast iron which contains 20%
nickel is used. In very high pressures where high strength is required,
forged steel, hardened by nitriding, is used.

PISTONS

The design and materials used for compressor pistons will vary with
the make, type, and application of the compressor. They are designed to
take into account a number of conditions:

• Cylinder bore diameter
• Discharge pressure
• Compressor rotative speed
• Compressor stroke
• Required piston weight

Compressor pistons are typically designed as one of three types:
One piece, either solid cast iron or steel, for small bores and high
pressure differential applications, or one piece hollow-cored cast iron or
aluminum, for large diameter and lower pressures (Figure 2-15A).