Discharge Piping | Stuffing Box
EXTERNAL-INJECTION LIQUID-SEAL PIPING
a) A tapped hole is supplied in the stuffing box
directly over the seal cage to introduce a clean,
clear sealing liquid. The stuffing box must at
all times be supplied with sealing liquid at a
high enough pressure to keep the box free from
foreign matter, which would quickly destroy the
packing and score the shaft sleeve. The sealing
liquid must be left on even when the pump is
shut down, unless the casing is isolated from
the discharge pressure.
b) Only a sufficient volume of sealing liquid to
create a definite direction of flow from the
stuffing box inward to the pump casing is
required, but the pressure is important.
c) We recommend that piping supplying sealing
liquid of stuffing box be sized to supply a
sufficient volume of water at the required
pressure, based on the location of the pump
(or pumps) with respect to the liquid source. A
small pipe can be utilised for the connection to
the stuffing box. A valve should be installed to
adjust and regulate sealing liquid and a gauge
installed to check pressure.
d) External sealing liquid should be adjusted to
the point where packing runs only slightly
warm, with a very slow drip from the stuffing
box. Excess pressure from an external source
can be very destructive to packing. More
pressure is required, however, for abrasive
slurries than for clear liquids. Examination of
the leakage will indicate whether to increase
or decrease external pressure. If slurry is
present in the leakage, increase the pressure
clear liquid drips from the box. If the drippage
is corrosive or harmful to personnel, it should
be collected and piped away.
e) A common error is to open the external piping
valve wide and then control the drippage by
tightening the packing gland. Actually, a
combination of both adjustments is essential
to arrive at the optimum condition. The life of
packing and sleeve depends on this careful
control more than any other factor.
STUFFING BOX
Contaminants in the pumped liquid must not enter
the stuffing box. These contaminants may cause
severe abrasion or corrosion of the shaft, or shaft
sleeve, and rapid packing or mechanical seal
deterioration; they can even plug the stuffing box
flushing and lubrication system. The stuffing box
must be supplied at all times with a source of
clean, clear liquid to flush and lubricate the pack-
ing or seal. The most important consideration is to
establish the optimum flushing pressure that will
keep contaminants from the stuffing box cavity. If
this pressure is too low, fluid being pumped may
enter the stuffing box. If the pressure is too high,
excessive packing or seal wear may result; and
extreme heat may develop in the shaft causing
higher bearing temperatures. The most desirable
condition, therefore, is to use a seal water pressure
slightly above the maximum stuffing box pressure.
STUFFING BOX OPERATING PRESSURE
- The
actual stuffing box pressure operating pressure
may be obtained by installing a pressure gauge on
the box. This should be done with an extra seal
cage temporarily replacing the two rings of pack-
ing in the bottom of the box to obtain a more
accurate gauge reading. Gauge readings should be
taken with the pump running under various head
and capacity conditions. The pressure of the
flushing or lubrication water can then be set at a
value 5 to 10 psi above the maximum expected
stuffing box operating pressure. Even under the
best conditions, a properly packed stuffing box
should be watched closely. If pressure conditions
change slightly there will be resistant change in
packing “seating” which should be compensated
by a change in gland adjustment.
FLUSHING AND LUBRICATION WATER
– The
recommended method to minimise error in
regulating flushing water is the “Controlled
Pressure System”. Most important is the pressure
reducing valve which may be adjusted to a value
slightly exceeding the stuffing box operating
pressure. A flow indicating device that will serve to
indicate a failing of the bottom packing rings
allowing leakage into the pump. is advisable eith
this arrangement, the packing gland need s to be
tightened only against the lowest necessary
pressure. The “out” connection from the stuffing
box should be closed with this system. Longer
packing life and less frequent adjustment are
possible with the “controlled pressure system”
properly installed and operated.
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