Page 200 - Reciprocating Compressors Operation Maintenance

186 Reciprocating Compressors: Operation and Maintenance

rod. As the rod progresses into or out of the packing, the other edge
comes into contact. This abnormal condition causes two separate contact
surfaces to exist in the ring bore. In either position, there is a gas passage
along the bore from one radial cut to the other and gas escape is made
possible.

UNDERSIZED PISTON RODS

If a piston rod is undersized but concentric and there is no taper, the
packing can form an effective seal within reasonable limits because bore
contact will be in the center of each segment and the cut in each ring is
overlapped by its mate. Nominally sized rings can be used if the piston
rod undersize dimension does not exceed .002". Whenever certain under-
size conditions exist, wear-in time is required before the packing will
properly seal. If the piston rod is greatly undersized, leakage will occur at
the tangent cut of the packing rings.

OVERSIZED PISTON RODS

Where the packing rings have a somewhat smaller bore than the piston
rod diameter, the segments will ride at their respective ends. Since the
center part will now be away from the rod at the exact point of the cut in
the other member, gas leakage will result along this line.
This condition will possibly be corrected by wear-in, although in the
case of either undersized or oversized rods, lubrication is obviously
being blown away by the gas passage. Hence, dangerous overheating and
dry contact may occur to the point of destruction. If the condition is
severe, however, a break between the tangential cuts of the packing rings
is likely to occur.
Figure 3-46 shows packing ring breaks that can be caused by under-
sized and oversized piston rods.
GARTER SPRING AND/OR RING BREAKAGE

As already mentioned, all packings operate somewhat on the labyrinth
principle so that there is penetration of gas into the assembly.
As will be noted from Figure 3-47, considerable time exposure exists
on the compression line. It is during this period that the gas enters the