Page 204 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

186 Major Process Equipment Maintenance and Repair

stages, but it is difficult for the designer to be sure they are always per-
forming as guaranteed. An inefficient separator will allow water to get
into the cylinder and break down the oil film. However, before any dam-
age is done, the water will emulsify the oil in the packing vent line and
form a yellow emulsion. It is customary to run a test on separators in
every new installation, but this is not necessary if the packing vent is ob-
served as an indication. If emulsion is found at the packing vent of air
machines, or if liquid is found there in the case of other gases, the effects
of the inefficient separator can be temporarily nullified by slightly open-
ing the blowdown line of the cylinder suction drum. The pipe nozzle
extends up into the drum, and since the drum will act as a separator, the
liquid can be drained off before it reaches the top of the nozzle.
If it appears that we have deviated from the immediate subject of pack-
ing maintenance at start-up or post-overhaul, it should be stressed that
water and liquids will also seriousIy affect packing. Once packing seats
and does not blowby, trouble-free service and long life can be expected.
Its expected life depends, of course, on the pressure ratio, the type of
gas being pumped and the amount of dirt or liquids in the gas. The liner
and piston will tolerate and pass small amounts of dirt, but each day a
portion of the dirt will lodge in the packing cups. All of it does not get
flushed out by the oil, and the remaining portion continues to build up.
Generally, this build-up and fouling is the first thing that happens to
packing in the long run; however, by using the packing vent as a PM
measure, the exact time of trouble can be determined.
We have placed a great deal of emphasis here on checking the color of
packing vent oil for PM. At some installations, however, crankcase oil
from the power end is used to fill the compressor lubricators. This prac-
tice is followed for certain reasons and has merit, but this oil naturally is
already darkened. However, a drop from the vent line can be placed on
facial tissue, and if any metallic particles are present the oil will filter
away and leave the particles.
Nonmetallic materials and modern design have made nonlubricated
cylinders commercially sound. As a consequence, the packing vent will
not be available as a PM indicator, and cylinders in this category do not
have any good signs or indications that can be used to determine their
condition or causes of trouble while they are running.
It is true that noises, increase in discharge temperature, fall-off of ca-
pacity and changes in interstage pressures will direct attention to trouble
and will red-flag serious smash ups, but they will not save the materials
involved. Excessive packing compartment vapor and discolored com-
pressor rods will be a good check on packing, but for absolute protection
of liners and pistons, a valve assembly will have to be removed for visual
inspection at specified periods.