Page 147 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring kcuum Pumps 129

with this imposed load, most packing will flow and conform to the stuff-
ing box and plunger. It will often be found that after 10 minutes the gland
can be further tightened. This process should be repeated two or three
times, or until the gland cannot be further tightened. The gland should
then be completely loosened, and the packing allowed to expand for 10 to
15 minutes. The gland should then be drawn up only finger tight (no
wrench). Now, the block valves may be opened and liquid allowed into
the pump.
Soaking the packing in oil prior to installation will enhance a proper
break-in and increase packing life.
During the first few hours of pump operation following repacking,
each stuffing box should be monitored for temperature. It is normal for
some boxes to run warmer than others-as much as 50°F above the
pumping temperature. Only if this exceeds the maximum temperature
rating of the packing are steps required to reduce box temperature.
The best lubricant for most installations equipped with stuffing box lu-
bricators has been found to be steam-cylinder oil. This oil is compounded
with tallow, which gives it a tenacity for the plunger surface and makes it
ideal for providing a lubricating wedge between the plunger and packing.
The concepts that a higher discharge pressure requires more rings of
packing and that a larger number of rings lasts longer may have been true
for long-stroke low-speed machines but has been disproven in some
power-pump applications. Unless they are profusely lubricated, the
larger number of rings create additional frictional heat and wipe lubricant
from the plunger surface-thus depriving some rings of lubrication. On
numerous salt-water injection pumps operating at pressures above 4,000
psig, packing life was reported to be only two weeks when twelve rings
of packing were installed in each stuffing box. With three rings in each
box, packing life was approximately six months.

Stuffing Boxes

Stuffing box designs-including the standard nonlubricated types and
various lubrication and bleedoff schemes to minimize leakage and extend
packing life-are shown in Figure 3-8.
The most significant advance in packing arrangements in recent years
is the spring-loading of packing. Although the concept has been dis-
cussed in the literature for decades, and actually put into practice by one
manufacturer for at least twenty years, only recently has this arrange-
ment received general attention.
Spring loading is applied almost exclusively to V-ring (chevron) pack-
ing but also works well with square packing rings. The spring must al-
ways be located on the pressure side of the packing. Springs of various