Page 314 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 311

All this discussion on the efficient control of pumps demonstrates
the great capability now available to HVAC water system designers
through the use of digital electronics in the evaluation of a water sys-
tem and the application of pumps.

10.11.2 Emergency backup on pump failure
Equipment failure is always troublesome, and it should be met in a
way that will cause the least amount of trouble for the water system.
Pump failure must be handled so that the system is not momentarily
out of water. The control system must interrogate operating pumps so
that a standby pump is started when pump failure is sensed, not
when the water system reacts to the failure of a pump. This may seem
to be a minor detail to be included in a pump handbook, but it can be
a terrible operating problem if the standby pump control procedure is
not executed properly. Reiterating, pump failure should be sensed
immediately, and the standby pump started then, not waiting for sys-
tem pressures to indicate the failure.

10.11.3 Alternation of operating pumps
Over the years of automatic control, almost every conceivable method
of alternating the lead or operating pumps has been used in the pump
industry. There are systems called “first on, first off” or “last on, first
off.” There are duty cycle or run time cycles. All these alternation
methods were designed to provide equal wear on all the pumps.
Two facts have resulted from these procedures. First, equal wear is
not necessarily the best procedure. As in several cases with large
pumps, all the pumps wore out during the same month and year,
causing an emergency repair program because all the pumps failed
together. Second, automatic alternation in itself allows the operator to
neglect the pumps.
A third factor has arisen, and that is the reduction in wear of HVAC
pumps. The use of variable-speed pumps and factory-assembled
pumping systems where pumps are programmed correctly has
reduced the incidence of pump failure. This has resulted in the elimi-
nation of most of the preceding alternation cycles. The simplest and
cheapest procedure is manual operation by the operator. Equipping
the pump control center with an elapsed time meter for each pump
motor enables the operator to maintain around 2000 to 4000 operat-
ing hours between pumps so that they do not all fail at the same time.
More on this subject is included in Chap. 26.

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