Page 145 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring kcuum Pumps 127

A set of square of V-type packing rings will experience a pressure gra-
dient, during operation, as indicated in Figure 3-7. The last ring of pack-
ing adjacent to the gland-follower bushing will experience the largest ax-
ial loading of all rings-resulting in greater deformation, tighter sealing
and, therefore, the largest pressure drop. Hence, the gap between the
plunger and the follower bushing must be small enough to prevent pack-
ing extrusion. Most packing failures originate at this critical sealing
point.
Because this last ring of packing is the most critical, does the most seal-
ing, and generates the most friction, it requires more lubrication than do
the others. In the nonlubricated arrangement (Figure 3-7), this ring must
rely on the surface of the plunger to drag some of the pumpage back to it
in order to provide cooling and lubrication. To maximize packing life in
this situation, the overall stack height of the packing should not exceed
the stroke length of the pump. Short packing life has resulted from nonlu-
bricated operation of stuffing boxes equipped with lantern rings, espe-
cially on short-stroke pumps (approximately 2 in. stroke length). The
lantern ring located in the center of the packing sometimes causes the
overall stack height of the packing to exceed the stroke length.

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