21
Figure 11 - Unbalanced loading of a double
suction impeller due to uneven flow around
an elbow adjacent to the pump.
When operating on a suction lift, the suction pipe
should slope upward to the pump nozzle. A horizontal
suction line must have a gradual rise to the pump. Any
high point in the pipe will become filled with air and
thus prevent proper operation of the pump. When
reducing the piping to the suction opening diameter
use an eccentric reducer with the eccentric side down
to avoid air pockets.
NOTE: When operating on suction lift never use a
straight taper reducer in a horizontal suction line, as it
tends to form an air pocket in the top of the reducer
and the pipe.
To facilitate cleaning pump liquid passage without
dismantling pump, an increasing suction nozzle or
suction diffuser should be bolted to the suction flange.
If this is not done, a short section of pipe (Dutchman
or spool piece) so designed that it can be readily
dropped out of the line can be installed adjacent to
the suction flange. With this arrangement, any matter
clogging the impeller is accessible by removing the
nozzle (or pipe section).
Valves in suction piping
When installing valves in the suction piping, observe
the following precautions:
1. If the pump is operating under static suction lift
conditions, a foot valve may be installed in the
suction line to avoid the necessity of priming each
time the pump is started. This valve should be of the
flapper type, rather than the multiple spring type,
sized to avoid excessive friction in the suction line.
(Under all other conditions, a check valve, if used,
should be installed in the discharge line.
See Figure 12.)
2. When foot valves are used, or where there are other
possibilities of "water hammer," close the discharge
valve slowly before shutting down the pump.
3. Where two or more pumps are connected to the
same suction line, install gate valves so that any
pump can be isolated from the line. Gate valves
should be installed on the suction side of all pumps
with a positive pressure for maintenance purposes.
Install gate valves with stems horizontal to avoid
air pockets. Globe valves should not be used,
particularly where NPSH is critical.
4. The pump must never be throttled by the use of
a valve on the suction side of the pump. Suction
valves should be used only to isolate the pump
for maintenance purposes, and should always be
installed in positions to avoid air pockets.
Discharge piping
If the discharge piping is short, the pipe diameter can
be the same as the discharge opening. If the piping is
long, pipe diameter should be one or two sizes larger
than the discharge opening. On long horizontal runs,
it is desirable to maintain as even a grade as possible.
Avoid high spots, such as loops, which will collect air
and throttle the system or lead to erratic pumping.
Valves in discharge piping
A check valve and gate valve should be installed in
the discharge. The check valve, placed between pump
and 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 and starting, and when
shutting the pump down.
Pressure gauges
Properly sized pressure gauges should be installed in
both the suction and discharge nozzles in the gauge
taps (which are provided on request). The gauges will
enable the operator to easily observe the operation
of the pump, and also determine if the pump is
operating in conformance with the performance
curve. If cavitation, vapor binding, or other unstable
operation should occur, widely fluctuating discharge
pressure will be noted.
Stuffing box
Contaminants in the pumped liquid must not enter
the packing space. These contaminants may 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 the optimum flushing
pressure that will eliminate contaminants from
the packing. If this pressure is too low, fluid being
pumped may enter the stuffing box. If the pressure
is too high, excessive packing wear will result; and
extreme heat may develop in the shaft causing higher
bearing temperatures. The most desirable condition,
