Xylem 8200 Series Base Mounted Centrifugal Fire Pumps AC2675 REV.C - Manual (Page 16)

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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”)
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.
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.
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 slow closing check valve and an isolating
gate valve should be installed in the
discharge line. The check valve (triple duty
valve), placed between pump and gate valve,
protects the pump from excessive back
pressure, and prevents liquid from running
back through the pump in case of power
failure. The gate valve is used in priming and
starting, and when shutting the pump down.
Pressure Gauges
Properly sized pressure gauges should be
installed in both the suction and discharge
nozzles in the gauge taps provided. The
gauges will enable the operator to easily
observe the operation of the pump, and also
determine if the pump is operating in
conformance with the performance curve. If
cavitation, vapor binding, or other unstable
operation should occur, widely fluctuating
discharge pressure will be noted.
Pump Insulation
On chilled water applications most pumps are
insulated. As part of this practice, the pump
bearing housings should not be insulated
since this would tend to “trap” heat inside the
housing. This could lead to increased bearing
temperatures and premature bearing failures.
STUFFING BOX LUBRICATION
Contaminants in the pumped liquid must not enter
the stuffing box. These contaminants may cause
severe abrasion or corrosion of the shaft, or shaft
sleeve, and rapid packing deterioration; they can
even plug the stuffing box flushing and lubrication
system. The stuffing box must be supplied at all
times with a source of clean, clear liquid to flush
and lubricate the packing or seal. It is important to
establish the optimum flushing pressure that will
keep contaminants from the stuffing box cavity. If
this pressure is too low, fluid being pumped may
enter the stuffing box. If the pressure is too high,
excessive packing wear may result; and extreme
heat may develop in the shaft causing higher
bearing temperatures. The most desirable
condition, therefore, is to use a seal water
pressure 15-20 psig above the maximum stuffing
box pressure.
PACKING
Standard pumps are normally packed before
shipment. If the pump is installed within 60 days
after shipment, the packing will be in good
condition with a sufficient supply of lubrication. If
the pump is stored for a longer period, it may be
necessary to repack the stuffing box. In all cases,
however, inspect the packing before the pump is
started.
NOTE: Packing adjustment is covered in the
MAINTENANCE section of this manual.
On some applications, it is possible to use internal
liquid lubrication (pumped liquid) to lubricate
packing. Only when all of the following conditions
prevail, can this be done: