Page 201 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 183

the oil mist or drops of oil should be inspected. This can be done by col-
lecting it on the tip of the finger. If the color is similar to that of new oil,
the operator can be sure that the parts inside the cylinder are in satisfac-
tory condition. A darkened oil, however, should be further scrutinized
by holding the finger up to the light to check for metal particles, which
will glisten like crystals. If any bronze, cast iron, or steel parts are start-
ing to wear, the metal particles or dust will almost immediately darken
the oil coming out of the vent. Consequently, this line should be checked
immediately after start-up of a new installation, after overhaul, and twice
daily during operation. If discoloration is noticed, it is an indication of
distress, and the compressor should be shut down.
A vast amount of information for evaluating the condition of the cylin-
der can be obtained simply by removing a top compressor valve from the
head and spotting the piston toward the crank end. The liner can then be
seen. The surface of a healthy liner will be bright, and at first glance the
observer may be convinced that the liner is too dry or has insufficient
lube oil. Rubbing a finger across the surface will hardly wet the skin with
oil. The only way the film can be positively checked is by wiping a facial
tissue across the liner (the tissue will be stained with new oil). This is all
the film needed to keep the two rubbing surfaces apart.
There are two vital spots in a compressor cylinder that require an oil
film and one is between the piston and the liner. In cases where the piston
is supported by a wear band, the film is maintained between the wear
band and the liner. The first sign of distress between the piston and liner
shows up in the bottom as a narrow score mark the length of the liner. A
facial tissue wiped on either side of the score mark and held toward the
light will substantiate the presence of darkened oil and small particles of
metal dust or cuttings. The cuttings are a result of oil film breakdown and
they discolor the oil. Contrary to widespread opinion, the discolored oil
and cuttings from the liner and piston find their way through the packing
and out the vent.
The significant point to remember about darkened oil appearing at the
packing vent is that it happens in a matter of minutes after the scuffing
starts. If the indication is not spotted or used as a PM check, the rubbing
contact between piston and liner will make the narrow score mark pro-
gressively wider until the liner and piston are badly scored over a 180"
arc. By that time, the piston may start to knock, dictating a shutdown, but
then the piston, liner, and rings are such a blackened mess that the cause
of failure cannot be determined. If the packing vent is used as a PM pro-
cedure, however, the scuffing will not only be observed before the fail-
ure, but it may be possible to find the cause of scuffing.
For example, excessive liquid in the gas will initiate scuffing because it
dilutes the thin supporting oil film. Sometimes the liquids drop out within