Xylem AC8584C Series HSCS Base Mounted Centrifugal Pumps - Manual (Page 11)

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Suction piping should be short in length, as direct as possible,
Ê
and never smaller in diameter than the pump suction opening. Ê
If the suction pipe is short, the pipe diameter can be the same Ê
size as the suction opening. If longer suction pipe is required,
Ê
pipes should be one or two sizes larger than the opening Ê
depending on piping length.
Ê
Suction piping for horizontal double suction pumps should not Ê
be installed with an elbow close to the suction flange of the
Ê
pump except when the suction elbow is in the vertical plane. Ê
A suction pipe of the same size as the suction nozzle Ê
approaching at any angle other than straight up or straight
Ê
down must have the elbow located 10 pipe diameters from the Ê
suction flange of the pump. Vertical mounted pumps and other
Ê
space limitations require special piping. Ê
There is always an uneven turbulent flow around an elbow and
Ê
when it is in a position other than the vertical it causes more Ê
liquid to enter one side of the impeller than the other (See
Ê
Figure 8). This results in high unequalized thrust loads that will Ê
overheat the bearings and cause rapid wear in addition to Ê
affecting hydraulic performance.
Ê
When operating on a suction lift, the suction pipe should slope Ê
upward to the pump nozzle. A horizontal suction line must
Ê
have a gradual rise to the pump. Any high point in the pipe will Ê
become filled with air and thus prevent proper operation on Ê
the pump. When reducing the piping to the suction opening
Ê
diameter use an eccentric reducer with the eccentric side Ê
down to avoid air pockets.
Ê
NOTE: When operating on suction lift never use a straightÊ
taper reducer in a horizontal suction line, as it tends to form an
Ê
air pocket in the top of the reducer and the pipe. Ê
To facilitate cleaning pump liquid passage without dismantling
Ê
pump, a short section of pipe (Dutchman or spool piece) so Ê
designed that it can be readily dropped out of the line can be
Ê
installed adjacent to the suction flange. With this arrangement, Ê
any matter clogging the impeller is accessible by removing the Ê
nozzle (or pipe section).
Ê
V alves in Suction Piping
When installing valves in the suction piping, observe the fol -
lowing 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 possi-
bilities 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 hori-
zontal 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 dischar ge 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.
V alves in Discharge Piping
A triple duty valve should be installed in the dischar ge. 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.
Pr essure Gauges
Pr operly 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 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 ar e insulated. As
part of this practice, the pump bearing housings should not be
insulated since this would tend to “trap” heat inside the hous-
ing. This could lead to increased bearing temperatures and
premature bearing failures.
WATER PRESSURE
INCREASES HERE CAUSING
A GREATER FLOW TO ONE
SIDE OF THE IMPELLER
PUMP CASING
CASING RINGS
IMPELLER
PUMP SUCTION
FLANGE
SUCTION
ELBOW
FIGURE 8 – UNBALANCED LOADING OF A DOUBLE SUCTION
IMPELLER DUE TO UNEVEN FLOW AROUND ON
ELBOW ADJACENT TO THE PUMP.