When installing valves in the discharge piping,
you should install a check valve and gate valve in the
discharge piping. The check valve, placed between the
pump and the gate valve, protects the pump from
excessive back pressure and prevents liquid from
running back through the pump in case of power
failure. The gate valve is used in priming, starting, and
shutting down the pump.
pressure gauges should be installed in both the suction
and the discharge nozzles in the gauge taps. The
gauges enable the operator to observe the operation of
the pump easily and also to determine if the pump is
operating according to the performance curve.
cavitation, vapor binding, or other unstable operation
occurs, widely fluctuating discharge pressure will
Contaminants in the pumped liquid must not enter
the packing space. These contaminants can cause
severe abrasion or corrosion of the shaft, or shaft
sleeve, and rapid packing 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
packing. The most important consideration is to
establish optimum flushing pressure to eliminate
contaminants from the packing. If this pressure is too
low, the fluid being pumped may enter the stuffing
box. If the pressure is too high, excessive packing wear
can result; also, extreme heat may develop in the shaft,
causing higher bearing temperatures. The most
desirable condition, therefore, is to use the lowest
possible flushing pressure that the operating
conditions will permit. If the pump system pressure
conditions vary during the day, the packing problem
becomes difficult. Consideration should be given to
using a mechanical seal. (See Mechanical Seals
One recommended method to minimize error in
regulating flushing is a controlled pressure system
(fig. 6-35). Most important is the pressure-reducing
valve adjusted to a valve slightly exceeding the
maximum stuffing box operating pressure (assuming it
is reasonably constant). A flow-indicating device
serves to indicate a failing of the bottom packing rings,
allowing leakage into the pump. With this
arrangement, the packing gland must be tightened only
against the lowest necessary pressure. Longer packing
life is possible with the controlled pressure system,
if it is properly installed and operated.
The actual stuffing box operating pressure may be
obtained by installing a pressure gauge on the box.
This is done with an extra seal cage temporarily
replacing the two rings of packing in the bottom of the
box to obtain accurate gauge readings. Take gauge
readings with the pump running under various head
and capacity conditions. Then set the pressure of
flushing or lubricating liquid at a value of 5 to 10 psi
above the maximum expected stuffing box operating
Even under the best conditions, a properly packed
stuffing box should be watched closely. When
pressure conditions change slightly, there is a resultant
change in packing (seating) which should be
compensated by a change in gland adjustment.
Consideration should also be given to the lubrication
pressure. A wide variation in pressure indicates a need
for a mechanical seal.
Standard pumps are normally packed before
shipment. If the pump is installed within 60 days after
shipment, the packing should be in good condition
with a sufficient supply of lubrication. If the pump is
stored for a longer period of time, it may become
necessary to repack the stuffing box. In all cases, you
should inspect the packing before starting the pump.
INTERNAL LIQUID LUIBRICATION.
Pumped liquid may be used to lubricate the packing
when the following conditions exist:
Figure 6-35.Controlled pressure system for stuffing box.