Goulds Pumps 3498 - IOM - Manual (Page 28)

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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. 11). 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. 12 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.
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CAUTION
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.
3498 IOM 12/04
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Fig. 12