Suction and Discharge Piping
b) It is usually advisable to increase the size of
both suction and discharge pipes at the pump
connections to decrease the loss of heat from
friction.
c) Install piping as straight as possible, avoiding
unnecessary bends. Where necessary, use
45-degree or long-sweep 90-degree fittings to
decrease friction losses.
d) Make sure that all piping joints are air-tight.
e) Where flanged joints are used, assure that
inside diameters match properly.
f) Remove burrs and sharp edges when making
up joints.
g) Do not “spring” piping when making any
connections.
h) Provide for pipe expansion when hot fluids are
to be pumped. The use of expansion joints is
not recommended.
When installing the suction piping, be sure to
observe the following precautions:
a) Suction piping should be short in length, as
direct as possible, and never smaller in
diameter than the pump suction opening. If
the suction pipe is short the pipe diameter can
be the same size as the suction opening. If
longer suction pipe is required, pipes should
be one or two sizes larger than the opening
depending on piping length.
b) To facilitate cleaning, pump liquid passage
without dismantling pump, an increasing
suction nozzle (available from Allis-Chalmers)
should be bolted to the suction flange. If this
is not done, a short section of pipe 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).
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: 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.
When installing valves in the suction piping,
observe the following precautions:
a) If the pump is operating under static 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.)
b) When foot valves are used, or where there are
other possibilities of “liquid hammer”, close
the discharge valve before shutting down the
pump.
c) 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.
Install gate valves with stems horizontally to
avoid air pockets. Globe valves should not be
used, particularly where NPSH is critical.
d) The pump must never be throttled by the use
of a valve on the suction side of the pump.
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 possi-
ble. 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 grate valve should be installed in the
discharge. The check valve, placed between pump-
and gate valve, protects the pump from excessive
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 can 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 operat-
ing in conformance with the performance curve. If
cavitation, vapour binding or other unstable opera-
tion should occur, widely fluctuating discharge
pressure will be noted.
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