Xylem 8100 Series Centrifugal Pumps AC2515 REV.C - Manual (Page 16)

WATER VELOCITY INCREASES
HERE, CAUSING A GREATER
FLOW TO ONE SIDE OF THE
IMPELLER
PUMP CASING
CASING
RINGS
SUCTION
ELBOW
PUMP SUCTION
FLANGE
IMPELLER
FIGURE 12 – UNBALANCED LOADING OF A DOUBLE SUCTION IMPELLER
DUE TO UNEVEN FLOW AROUND AN ELBOW ADJACENT TO THE PUMP
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.
Figure 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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