Page 149 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring Vacuum Pumps 131

types can be used, including a single large coil, multiple coil, wave-
washer, belleville, and a thick rubber washer.
The force required by the spring is small compared to the force im-
posed on the packing by the liquid. The major function of the spring is to
provide a small preload to help set the packing, and to hold all bushings
and packing rings in place during operation.
Spring loading of packings has many advantages. It:

Requires no adjustment of the gland-the gland is tightened until it
bottoms, then is locked. This removes one of the biggest variables in
packing life-operator skill.
Allows expansion-if the packing expands due to frictional heat dur-
ing the initial break-in, the spring allows for the expansion.
Takes up wear-as the packing wears, adjustment automatically oc-
curs from inside the box. The problem of transmitting the force
through the top packing ring during gland adjustment is eliminated.
Provides a cavity-the spring cavity provides an annular space for
the injection of a clean liquid for slurry applications.
Eliminates the need for gland if pump design allows this-the stuff-
ing box assembly (if a separate component) can be disassembled and
reassembled on a workbench.

Disadvantages of spring-loaded packing are associated with the cavity
created by the spring. Since this cavity communicates directly with the
pumping chamber, the additional clearance volume can cause a reduction
in volumetric efficiency if the pumpage is sufficiently compressible. This
cavity also provides a place for vapors to accumulate. If the pump design
does not provide for venting this space, a reduction in volumetric effi-
ciency may occur.
Spring-loaded packing is the reciprocating pump’s equivalent to the
mechanical seal for rotating shafts. Leakage is low, life is extended, and
adjustments are elimhated. Packing sets can be stacked in tandem (they
must be independently supported) for a stepped pressure reduction, or to
capture leakage from the primary packing that should not be allowed to
escape to the environment.

Plunger Material

After the packing, the plunger is the component of a power pump that
requires the most fiequent replacement. The high speed of the plunger
and the friction load of the packing tend to wear the plunger surface.
For longer life, plungers are sometimes hardened. A more popular
method is to apply a hard coating to the plunger surface. Such coatings