Xylem AC2008B Series HSC3 Base Mounted Centrifugal Pump - Manual (Page 10)

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Suction piping should be short in length, as direct as possible,
and never smaller in diameter than the pump suction opening.
If the suction pipe is short, the pipe diameter can be the same
size as the suction opening. If longer suction pipe is required,
pipes should be one or two sizes larger than the opening,
depending on piping length.
Suction piping for horizontal double suction pumps should not
be installed with an elbow close to the suction flange of the
pump, except when the suction elbow is in the vertical plane.
A suction pipe of the same size as the suction nozzle,
approaching at any angle other than straight up or straight
down, must have the elbow located 10 pipe diameters from
the suction flange of the pump. Vertical mounted pumps and
other space limitations require special piping.
There is always an uneven turbulent flow around an elbow.
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
Illustration 7) This results in high unequalized thrust loads that
will overheat the bearings and cause rapid wear, in addition to
affecting hydraulic performance.
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 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 suction pres-
sure 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 for 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 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 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
Illustration 7 – Unbalanced loading of a double suction impeller
due to uneven flow around on elbow adjacent to the pump.