Bell & Gossett P5002169C Series VSCS - Manual (Page 15)

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Figure 15: 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 can become filled with air and thus
prevent proper operation of the pump.
To facilitate cleaning the pump’s liquid
passage without dismantling the pump, a
short section of pipe (Dutchman or spool
piece), designed so 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 spool
piece (or pipe section).
Valves in Suction Piping
When installing valves in the suction piping,
observe the following precautions:
Suction piping valves should be placed right
before the run of recommended straight pipe.
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. The foot
valve and pipe should be sized to maximize
NPSHa to the pump by minimizing suction
line losses.
Check valves are placed in the discharge
piping in normal applications. Before using a
check valve in the suction piping, consider
the added pressure drop to the pump,
potential water hammer, and the chance of
allowing the entire pump volute to be
exposed to the discharge pressure.
When foot valves are used, or where there
are other possibilities of “water hammer,”
close the discharge valve slowly before
shutting down the pump.
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
The maximum velocity in the discharge piping
should not exceed 15 feet per second. If a
check valve is present in the outlet piping,
this value shall be reduced. System losses,
life-cycle costs, and process considerations
will usually drive the size of discharge piping
and fittings. Some high energy pumps are
sensitive to flow disturbing devices mounted
close to the pump outlet. Consult Bell &
Gossett when in doubt of the minimum
required straight pipe length.