Page 96 - Reciprocating Compressors Operation Maintenance
P. 96
Design and Materials for Reciprocating Compressor Components 83
FIGURE 2-41. Unloader with Teflon® bushings are used with non-lubricated
compressors (Source: Plant Engineering, October 18, 1979).
ROD UNDER COMPRESSION
Consider first a double acting cylinder, as shown in Figure 2-42. As
the piston pumps toward the head end, the discharge pressure (P 2) on the
piston tends to compress and buckle the piston rod. At the same time, gas
is entering the cylinder behind the piston, exerting suction pressure force
(Pj) on the back side of the piston. Two forces are opposite in direction,
but since the discharge pressure is greater, the net result is a force com-
pressing the rod. This is called rod load compression. It is obvious that as
suction pressure is decreased, or discharge pressure is increased, the net
compression on the rod increases. Therefore, if the operator lets the suc-
tion or discharge pressure deviate too far from the design conditions, the
maximum permissable compressive load may be exceeded.
