Page 135 - HVAC Pump Handbook

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Physical Description of HVAC Pumps

132 HVAC Pumps and Their Performance

and inexpensive device is a manual petcock installed on top of the
pump volute that will allow manual removal of air that collects
there. If air continually enters the pump and cannot be stopped, this
pet cock can be replaced with an automatic air vent. The automatic
air vent should have a discharge line near to a floor drain, since they
are notorious for leaking water. If the water carries grit or other
small particulate matter, a cyclone separator can be installed on each
flushing line to the seals or packing. Methods of venting air from
water systems are included in Chap. 9.
There is much argument over the value of strainers on HVAC sys-
tems other than condenser water flowing to chillers. Many similar
industrial applications are not equipped with strainers. The normal
location of the strainer is on the suction of the pump. In the case of
cooling tower water feeding a chiller, the strainer should be installed
on the discharge of the pump, as shown in Chap. 11. The strainer is
there to protect the tubes of the condenser, not the pump. The cooling
tower sump strainer is adequate to keep rocks and other debris out of
the pump. Installing the strainer on the pump discharge prevents the
strainer from being obliterated with algae and ruining the pump. If
the strainer becomes clogged when located on the pump suction, a
vacuum will be generated on the pump suction, and it will be
destroyed by steam and heat. For other HVAC applications, some
engineers prefer to use strainers for start-up service only. After the
system has been flushed and operated for a while, the strainer
screens or baskets are removed.
Coupling guards conforming to Occupational Safety and Health
Administration (OSHA) or particular state codes should be located
over the rotating coupling and pump shaft where it is exposed.
OSHA’s requirements should be checked from time to time to ensure
that their requirements are met.
Balance valves should never be installed on the discharge of any
pump, let alone variable-speed pumps. Some designers feel that they
are a safeguard to prevent overpressuring a system; this is a terrible
waste of energy. On constant-speed pumps, the amount of overpres-
sure should be determined and the impeller trimmed to eliminate as
much of the overpressure as possible. See Chap. 29 on the procedure
for trimming an impeller. On variable-speed pumps, the pump con-
trols should be designed to prevent this overpressuring by limiting
the maximum speed of the pump.
Multiple-duty valves that include the features of a check valve, a
balance valve, and a shutoff valve should never be used on the dis-
charge of any pump and particularly on the discharge of a variable-
speed pump (Fig. 5.21). Some multiple-duty valves can be closed

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