Page 209 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 191

Figure 5-8. Cross-sectional view of
reciprocating compressor cylinder and
crosshead guide.

perienced operator. In order to keep from initiating panic, and to create
confidence in the machinery, the operator should become familiar with
the natural beat of the machine by spending a great deal of time around
the unit after it is installed.
A common type of hock is caused when the piston of a double-acting
cylinder hits either the head or crank end of the cylinder. This can happen
if the piston is incorrectly spaced at assembly. The piston may clear dur-
ing the cranking or barring-over check but, due to take-up of all clear-
ances (bushings, rod, and main bearings) by inertia and expansion of rod
and piston due to heat, the piston may strike the head during operation.
The knock is very easy to analyze because it is a distinct metallic thud
and can be felt by placing the hand on the cylinder head. Rapid wear of
the liner and piston also can cause the piston to strike the head. From Figure
5-8 it can be seen that there is a large radius on each end of the piston that
also matches the contour of the heads. If the piston is lowered by wear, it
will strike the radius of the head. Some pistons are bulletnosed and are more
likely to strike other parts when wear sets in.
Note (in Figure 5-8) that the piston rod (14) is threaded into the cross-
head (1 5) and secured by a nut (13). This nut has been known to come loose,
permitting the piston to turn and hit the head.
The knock that has caused more confusion than any other, however, is
the one that results when the piston nut is loose. This is the nut that se-
cures the piston to the rod. If it becomes loose by as little as .003 in., it
will knock very loudly; however the noise will appear to be in the crank-
case of the unit. In many cases the knock has been eliminated merely by
tightening the nut one or two flats. If this nut continues to come loose
after tightening several times, there is good reason to believe that the cyl-
inder is not in alignment. If the cylinder is high on the head end, the pis-
ton will rise as it comes to outer center, thereby setting up a severe vibra-
tion that affects the tightness of the nut. This condition can be further
substantiated by loose grout under the crosshead guide support (Item 10,
Figure 5-8) and perhaps under the frame of the unit. In some cases, the
grout has loosened to such a degree that the guide-support hold-down
bolts break. Vibration at the frame (2) with increasing intensity at the