Goulds Pumps 3408 - IOM - Manual (Page 20)

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3408 IOM 03/99
Suction Piping
When installing the suction piping, observe the following
precautions. (See Fig. 13)
The sizing and installation of the suction piping is
extremely important. It must be selected and installed so
that pressure losses are minimized and sufficient liquid
will flow into the pump when started and operated.
Many NPSH (Net Positive Suction Head) problems
can be directly attributed to improper suction piping
systems.
Suction piping should be short in length, as direct as
possible, and never smaller in diameter than the
pump suction opening. A minimum of five (5) pipe
diameters between any elbow or tee and the pump
should be allowed. If a long suction pipe is required, it
should be one or two sizes larger than the suction
opening, depending on its length.
▲
! CAUTION
An elbow should not be used directly before the
suction of a double suction pump if its plane is
parallel to the pump shaft. This can cause an
excessive axial load or NPSH problems in the pump
due to an uneven flow distribution. (See Fig. 12) If
there is no other choice, the elbow should have
straightening vanes to help evenly distribute the
flow.
Eccentric reducers should be limited to one pipe size
reduction each to avoid excessive turbulence and
noise. They should be of the conical type. Contour
reducers are not recommended.
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 can become filled with air
and 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 concentric reducer in a horizontal
suction line, as it tends to form an air pocket in
the top of the reducer and the pipe.
Fig. 13 shows some correct and incorrect suction
piping arrangements.
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 Discharge Piping.)
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.
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