MIL-HDBK-1003/6
is fired. There need be no relationship during operation between the
quantities of water circulated by the system pumps and the generator
circulation pumps in this arrangement except that system flow must always be
less than generator flow. With this double pump arrangement no instruments or
other equipment are needed to control the flow through the generator because
the circulation pumps have a constant capacity and are entirely independent of
the system circulation.
d) Where two or more generators are installed, the generator
having the greatest pressure drop should be used in pump head calculations.
Single- versus Multiple-Stage Pumps. Single-stage pumps should be
4.10.6
used for HTW systems. Where the head requirements exceed the capabilities of
a single-stage pump, the preferred procedure is to use single-stage booster
pumps within the distribution system.
4.10.7
Location
a) On steam-pressurized systems, pumps should be located in a
central position with respect to the generator(s) and expansion vessel. The
lines connecting the expansion vessel and pump should contain no unnecessary
turns or restrictions, and the pump location should provide the maximum NPSH
possible. The pump should be accessible for maintenance, and provisions
should be made to prevent misalignment due to external piping at elevated
temperatures. Usually the pump sucks from the hottest water in the system.
On some systems, a blending mechanism is utilized to reduce the temperature to
the pump to prevent flashing or cavitation.
b) On gas-pressurized systems, the expansion vessel should be
located on the inlet side of the generator and on the suction side of the
pump. The pump shall be accessible for maintenance and provisions made to
prevent misalignment at elevated temperatures.
4.10.8
Miscellaneous
a) Where separate generator pumps are used, the pump
characteristics should be in accordance with the recommendations of the
generator manufacturer.
b) In order to reduce the costs of meeting increased future head
requirements of the system, all pumps and casings should be designed to take
the next larger impeller size.
c) The pump motor and accessories should be selected on the basis
of the larger impeller size.
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