MIL-HDBK-1003/6
4.10.2
Pump Acceptance Tests
a) All pumps used for HTW should be tested at the factory in a
range of temperatures and corresponding saturation pressures from 250 degrees
F (121 degrees C) up to the design operating temperature of the system. The
pump should be pressure tested at one and one-half times the maximum design
operating pressure of the system. The manufacturer must certify that these
tests were performed and submit them to the Contracting Officer.
b) The pumps should be rated in terms of pounds per minute versus
head requirements at the above mentioned pressures and temperatures.
4.10.3
Constant versus Variable Speed Pumps
a) Variable speed pumps should be used for HTW systems when
economically justifiable.
b) Preferred procedure is to use three pumps, each sized for 50
percent of the load, with one pump for stand-by.
c) Where summer loads are small, an additional pump sized at 25
percent or less may be used.
4.10.4
Net Positive Suction Head
a) The required NPSH of the pump shall not exceed 80 percent of
the available NPSH. The required NPSH is the head that the pump manufacturer
must know for correct operation of the pump. The available NPSH is the head
needed to provide this required head, in the form of either static or pressure
head.
b) If two or more pumps of differing head requirements are used,
NPSH should be based on the pump having the greater head requirements.
4.10.5
One-Pump versus Two-Pump Systems
a) A one-pump system may have many pumps in the system, but they
all serve the combined purpose of delivering water to the generators as well
as the system.
b) A two-pump system may have many pumps in the system, but one
set of pumps delivers water to the generators, and another set of pumps
delivers water to the system.
c) Hot water generators are designed for a specific flow of water
through the generator. A minimum quantity is needed to protect against tube
failure and steam generation. In a one-pump installation, this flow is
usually controlled by an automatic bypass valve which provides a uniform flow
to the boiler regardless of distribution system load variations. In a very
large HTW system it may become necessary to use a two-pump system for control
of water through the generator. The quantity of water circulated through each
generator is kept more or less constant no matter at what rate the generator
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