Page 202 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

184 Major Process Equipment Maintenance and Repair

the cylinder and escape the attention of the operator. In addition, liquid
being carried in the pipe to the cylinder is difficult to detect by checking
blow-downs, but is likely to be spotted after the unit is shut down. The
liner can now be observed through a valve port. Although the liquid may
not be noticed at first, as the diluted oil drains down the sides of the liner
and leaves a small stream of liquid and oil in the bottom of the liner. An
inspection of this sort is very likely to disclose small amounts of harmful
liquid. In cases of large quantities, detection is easy, because the cylinder
may knock and, if allowed to continue, the heads or pistons may crack.
The liquid also will come out the packing vent and may spit out between
the rod and flange of the packing.
The other vital spot that requires a lubricating oil film is between the
face of the piston rings and the liner. As in the case of the piston-to-liner
interface, any scuffing of the rings will discolor the oil coming out the
packing vent. This problem can be differentiated from that of the piston-
to-liner interface because the valve-port inspection will show the liner to
be marked or scuffed over the entire 360" contour, even in the early
stages of failure.
The value of using the packing vent as a PM check is that it catches the
scuffing in the initial stages. Generally speaking, once two mating sur-
faces start to scuff, they never heal, even if the oil film is reestablished,
and the scuffing will continue to destruction. But if detected early, it may
be stopped merely by wiping the discolored oil from the cylinder and
spot-honing the scuffed area of the liner. The unit then can be put back on
the line and the vent carefully watched. It will not take too long for the
discolored oil left in the vent piping to be purged. If the cause has been
corrected the oil will clear up; if not, it will be necessary to remove the
piston from the cylinder and do a thorough job of cleaning and honing the
liner.
Piston distress always occurs at the bottom of the piston. Therefore,
the piston can be smoothed and rotated. Piston rings likewise can be
saved, but their edges should be carefully checked-a small chamfer, or
radius, was machined on these edges at the time of manufacture, but
wear may have made the edges sharp. A metallic ring with a sharp face
will start cutting and wearing and will progress. The sharp edges should
be broken with a three-cornered scraper. Another critical area for scuf-
fing is between the packing rings and piston rod. As mentioned before,
the amount of vapor coming from the packing vent depends upon the ra-
tio and pressure of the cylinder. Of course, excessive blowing is an indi-
cation the rings are not seating. Packing problems are common at the
start-up of new installations and after overhauls. Dirt and liquid are the
reason for problems at start-up, and, on rare occasions, there has been an
improper choice of packing materials and design. Generally, packing