Xylem 9100 Series Base Mounted Centrifugal Fire Pumps AC8585 REV.C - Manual (Page 17)

There is always an uneven turbulent flow
around an elbow and when it is in a position
other than the vertical it causes more liquid to
enter one side of the impeller than the other
(See Figure 8). This results in high
unequalized thrust loads that will overheat the
bearings and cause rapid wear in addition to
affecting hydraulic performance.
17
FIGURE 8 - UNBALANCED LOADING OF A
DOUBLE SUCTION IMPELLER DUE TO
UNEVEN FLOW AROUND ON 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 will become filled with air and thus
prevent proper operation on 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 straight taper reducer in a horizontal
suction line, as it tends to form an air pocket
in the top of the reducer and the pipe.
To facilitate cleaning pump liquid passage
without dismantling pump, a short section of
pipe (Dutchman or spool piece) so designed
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 nozzle (or pipe section).
Valves in Suction Piping
When installing valves in the suction piping,
observe the following precautions:
a. 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
“Valves in Discharge Piping” below.)
b. When foot valves are used, or where there
are other possibilities of “water hammer,”
close the discharge valve slowly before
shutting down the pump.
CASING RINGS WATER PRESSURE
INCREASES HERE CAUSING
A GREATER FLOW TO ONE
SIDE OF THE IMPELLER
c. 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.
PUMP CASING
d. 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 or maintenance
purposes, and should always be installed
in positions to avoid air pockets.
e. A pump drain valve should be installed in
the suction piping between the isolation
valve and the pump.
Discharge Piping
If the discharge piping is short, the pipe
diameter can be the same as the discharge
opening. If the piping is long, pipe diameter
should be one or two sizes larger than the
discharge opening. On long horizontal runs, it
is desirable to maintain as even a grade as
possible. Avoid high spots, such as loops,
which will collect air and throttle the system or
lead to erratic pumping.
Valves in Discharge Piping
A triple duty valve should be installed in the
discharge. The triple duty valve placed on the
pump protects the pump from excessive back
pressure, and prevents liquid from running
back through the pump in case of power
failure.
Pressure Gauges
Properly sized pressure gauges should be
installed in both the suction and discharge
nozzles in the gauge taps (which are
provided on request). The gauges will enable
the operator to easily observe the operation
of the pump, and also determine if the pump
SUCTION
ELBOW
PUMP SUCTION
FLANGE IMPELLER