TM 5-813-9
CHAPTER 6
OPERATING EFFICIENCIES
6-1. General. Water pumping stations will nor-
Depending on the system head encountered by a
mally operate automatically to satisfy the hydraulic
pump, the pump may perform over a wide range of
requirements of the system. Supervisory or remote
efficiencies.
control of electric motors will be provided on the
6-3. Pump Scheduling. Pump scheduling for
larger installations to reduce operator time and to
provide a means for optimal control of energy
optimum energy costs for large pump stations with
costs, and to allow for energy conservation
elevated storage can be established by use of
measures. For optimal control of energy costs,
computer programs where accurate input data
particularly for larger pump stations, the control
reflecting existing system operations can be ob-
system will allow the operators to schedule pump
tained. The total energy consumption charges asso-
operations so that station electrical consumption is
ciated with a pump operation can be decreased by
minimized at the same time adequate storage for
improving the efficiency of individual pumps or
fire protection and system pressures are maintained.
combination of pumps. However, such measures
Energy costs comprise the major component of the
have little impact on reducing the costs associated
operating costs of water supply systems. The
with time of day energy rate schedules. The primary
largest quantity of energy is usually consumed by
way to minimize the cost associated with variable
treated water pumping stations. The overall
electric rate schedules is through use of off-peak
operating cost associated with a particular pump
pumping strategies. This policy should be
station will be dependent upon the following
implemented if energy savings exceed the cost of
factors: the pumps, the distribution system, the
additional storage capacities. The key to imple-
pump drivers, and the governing energy rate
mentation of an off-peak pumping schedule is the
schedule. The design analysis for the distribution
availability of equalizing storage and the develop-
system and pump station and the cost evaluation
ment of an optimal pump operating policy. A pump
will consider these factors.
operating policy is a schedule of water levels that
should be maintained and a series of rules that
6-2. Pump Operations. Three different problem
dictate when different pumps should be operated in
areas are usually encountered in any attempt to
response to different system conditions. The pump
improve pump operations of an existing pump
operating policy might consider time of year as well
station: inefficient pumps, inefficient pump combi-
as time of day. As an example, no pumping when
nations, and inefficient pump scheduling. The effi-
the ambient temperature falls below a preset
ciency of a single pump is the ratio of water
temperature during cold months during the period
horsepower produced by the pump to the input
when electricity usage is the highest. An optimal
horsepower, usually electrical. This efficiency
pump operating policy is that policy which will
should be measured at several flow rates. It is not
satisfy all constraints at a minimum cost.
always physically practical to measure flow rates
6-4. Ease of Operation and Maintainability.
after pumps are installed in existing installations.
However, if this information can be obtained and
The design of pumping equipment and drives will
the pump is shown to satisfy its original perfor-
be evaluated based on several factors: amount of
mance specification, there is still no assurance that
operator attention, frequency of routine mainte-
it will operate efficiently in the system. The
nance and adjustment, energy savings, and avail-
efficiency of a pump running alone can be much
ability of parts and service. Proper alignment of
different than when running in conjunction with
pump and driver, as well as support of suction and
other pumps. With multiple pump operation the
discharge piping will help prevent some noises and
actual flow produced will depend upon the head
vibration. Noise is minimized by choosing pumps to
differences between the suction header and dis-
operate near the point of best efficiency and proper
charge header. The relationship between these
suction conditions. Increased vibration affects the
heads and the flow rate is referred to as the system
life of bearings, stuffing boxes, and mechanical
head curve and is a function of tank water level on
seals. Partially closed valves will not only increase
each size of the pump, pipe carrying capacity near
noise, but will increase radial thrust and resultant
the pump, location of water users with respect to
stresses in shafts and bearings of centrifugal pumps.
the pump, and which other pumps are operating.
6-1
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