Page 231 - Reciprocating Compressors Operation Maintenance

216 Reciprocating Compressors: Operation and Maintenance

push out against the cylinder wall, making a tight sliding fit. The piston
rings float in the ring grooves of the piston and seal only* They do not
support the piston off the cylinder wall. The piston is supported off the
cylinder wall by only the liquid lubrication film.
The conventional non-lubricated piston and piston ring has no liquid
oil film to support the piston off the cylinder wall; therefore, the metallic
piston must be kept away from the metallic cylinder wall by other means,
or serious damage could result.
The piston is kept off the cylinder wall by using guide rings, which are
referred to as "bull" rings or rider rings. The outside diameter of the pis-
ton is smaller relative to the lubricated piston to allow clearance between
the piston OD and the cylinder wall. This allows the rider ring to wear
somewhat before approaching metal-to-metal contact.
Conventional non-lubricated compressors require more attention to
installation and maintenance than oil-lubricated ones. When servicing a
non-lubricated compressor, be aware of its weaknesses. The greatest
drawback of the non-lubricated compressor is its sensitivity to dirt, abra-
sives and moisture.
Contamination and failure risks are reduced by the following:

1. Air (or gas) filtration. An efficient filter capable of filtration down
to 10 microns is required, and it must be carefully maintained. It is
typically sized larger than one for the lubricated compressor.
2. Suction and interstage piping. These must be maintained clean of
rust and scale. If a problem of rusting persists on typical air compres-
sors, the piping should be replaced with aluminum or stainless steel.
3. Moisture. Moisture may act as a lubricant, however, more care is
required to assure that the system remains corrosion-free. Cylinder
ports of air compressors are treated with phenolic or epoxy paints to
prevent rusting.
4. Cylinder cooling. Teflon is a poor conductor of heat. It softens, thus
extruding more easily, when hot. Friction heat is generated during
operation and is not transferred through the Teflon. It is important
that good jacket water cooling is maintained and scale deposits are
minimized. It is also strongly recommended that an automatically
controlled solenoid-operated valve be placed in the piping to shut
off water flow when the compressor is out of service for an extend-
ed period of time.