Page 85 - Reciprocating Compressors Operation Maintenance

72 Reciprocating Compressors: Operation and Maintenance

Carbon rider rings

Carbon compression rings with
expander ring in back
FIGURE 2-33. Early version of non-lubricated compressor piston.

The pressure packing was either a soft braided asbestos yarn, some-
times filled with animal fat lubricant, or rings made of graphite or seg-
mented carbon.
Carbon has a great disadvantage; it is an extremely brittle material and
requires extreme care when installing to prevent chipping and breakage.
The carbon dust generated as a result of wear is somewhat abrasive
and accelerates ring wear. Ring slap, caused by the resulting excess side
clearance of the rings in their grooves, tends to chip or fracture the rings.
Plain carbon rings are thus banned from non-lubed reciprocating com-
pressors. Prior to the advent of high performance polymers, the process
industry had adopted a standard of ordinary Teflon construction. Piston
and packing rings are often fabricated from a group of materials based on
DuPont's polytetrafluroethylene (PTFE). Various fillers are used such as
glass (fibre), carbon, bronze or graphite.
Compressors are often manufactured as either oil-free (totally non-lubri-
cated) or mini-lube, where are a reduced amount of lubrication (usually
10% of the amount of lubrication used in a lubricated compressor) is used.