Page 196 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

178 Major Process Equipment Maintenance and Repair

Figure 5-2. Reciprocating compressor connected to electric motor.

Valve breakage.
Knocks, noises and vibration.

Exceeding assigned rod load. It is essential that operators and mechanics
understand rod loads before they attempt to start a compressor. Most ma-
jor casualties, such as broken rods, damaged crossheads, pin bushings
and frame failures, are caused by exceeding the maximum rod load. The
frightening aspect of this is that the wreck can happen within just a few
revolutions after the infraction. By way of explanation, let us consider a
double-acting cylinder. As the piston pumps toward the head end (Figure
5-3) the discharge pressure force (P,) on the pistons tends to compress or
buckle the piston rod. At the same time, gas is entering the cylinder
behind the piston, putting a suction pressure force (P,) on the back side of
HEAD EN0 -

Figure 5-3. Reciprocating compressor piston-rod-crosshead system in compression.