MIL-HDBK-1003/6
c) Incorporate a pressure differential switch across the pump
headers or provide a minimum flow switch in each flow meter to cut out
combustion when there is insufficient flow through a generator.
d) Interlock the pump starters with combustion control to prevent
generator operation without pump operation.
e) Variable speed control of pumps may be used if savings justify
additional costs of wound rotor motors and speed control equipment or variable
speed clutches.
f) Control flow of blending water from return line to pump suction
to maintain a constant supply water temperature to the distribution system and
to prevent cavitation of pumps due to flashing. This should be done manually
for plants with generators under 30,000,000 Btu per hour (8790 kW) and
automatically for plants with larger generators.
4.11.3.2
Plants with Separate Generator Circulating Pumps in Use with System
Circulating Pumps
a) Measure water flow at each generator and provide manual control
of this flow so that all generator flows can be equalized.
b) Incorporate a minimum flow switch in each generator flow meter
to interrupt combustion when the flow drops below a safe value.
c) Interlock the generator circulating pump starters with
combustion controls of all generators to prevent any generator operating
without a pump operating.
d) Generator circulating pumps should always operate at constant
speed. The system pumps may operate at variable speed only if the additional
costs of variable-speed equipment can be economically justified.
e) Control the flows of blending water from the return line to
system pump suctions by a manual valve for plants with generators under
30,000,000 Btu per hour (8790 kW) and by an automatic valve for plants with
larger generators. Install a manual throttling valve in the return line to
the suction side of the generators circulating pumps to create a pressure drop
so that the control valve for the blending water will be effective and to stop
steaming conditions.
4.11.3.3
Makeup Water Flow Control
a) For small flow loss, use manual start and stop for normal
makeup pumps when expected loads are steady. If loads fluctuate widely,
provide for automatic start and stop of these pumps by float controls in the
expansion tank.
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