Page 87 - Reciprocating Compressors Operation Maintenance

74 Reciprocating Compressors: Operation and Maintenance

crosshead guide from the cylinder. This necessitates a longer piston
rod on which a "collar" or oil deflector is installed. This collar pre-
vents oil migration along the rod and into the cylinder.
A Teflon-containing material is used on the piston and in the
pressure packing. Since the pistons of these non-lubricated com-
pressors are fitted with rider bands that contact the cylinder walls,
we call the compressors "conventional."

OPERATIONAL DIFFERENCES

A basic operational difference between the lubricated and non-lubri-
cated cylinder should be explained. The piston works against pressure
and should form a sliding seal so that it can compress the gas without
leakage. So perhaps, in the lubricated cylinder, the simplest piston would
be a plug piston with a very close fit to the cylinder bore. But, because of
temperatures and other engineering and economic reasons, this is not
practical. Piston rings are therefore used for sealing.
These piston rings have many variations, but all follow the basic prin-
ciple of a thin metallic split ring fitted into a groove around the piston.
The ring is made with "spring," or tension, which tends to push out
against the cylinder wall and make a tight sliding fit.
It is important to note that piston rings float in the ring grooves of the
piston and that they only seal. THEY DO NOT SUPPORT THE PIS-
TON, nor is there any other device to support the piston. The piston is
supported off the cylinder wall by the liquid lubrication film only.
In the non-lube or oil-free piston and piston ring assembly there is no
oil film to support the piston, so the metallic piston must be kept off the
cylinder bore by other means or serious damage will result. Note that this
is the difference between lube and non-lube principle.
In the conventional non-lubricated compressor, the piston is kept off
the cylinder wall by a guide ring which is referred to as a bull, wear, or
rider ring. This rider is of a low friction material, such as carbon or
Teflon, and of low unit loading relative to the piston weight.
The outside diameter of the piston is smaller than that of the piston in
the lube compressor; this creates clearance between the piston outside
diameter and the cylinder bore. This clearance allows for rider band wear
before metal contact is achieved with the cylinder bore.