Goulds Pumps 3409 - IOM - Manual (Page 22)

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3409 IOM 11/04
Do not “spring” piping when making any connections.
Provide for pipe expansion when hot fluids are to be pumped.
SUCTION PIPING
When installing the suction piping, observe the following
precautions. (See Fig. 12)
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.
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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 neveruse 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 mainten-
ance purposes, and should always be installed in
positions to avoid air pockets.
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