Chapter 5 - Publications, US Army Corps of Engineers

EM 1110-2-1701
31 Dec 1985
tested. Therefore, only the term rated head should be used in
planning and design studies.
(1) Cavitation and vibration problems limit turbines to a
minimum discharge of 30 to 50 percent of rated discharge (rated
discharge being discharge at rated head with wicket gates fully open).
This characteristic should be accounted for in power studies, and it
may in some cases influence the size and number of units to be
installed at a given site. For example, if a minimum downstream
release is to be maintained at a storage or pondage project for nonpower
purposes, and it is desired to maintain power production during
these periods, a unit must be selected which is capable of generating
at the required minimum discharge. For run-of-river projects, proper
accounting for minimum discharge is equally important. Streamflows
below the single-unit minimum discharge will be spilled, so flowduration
curves should be examined carefully to determine the size and
number of units that will best develop the energy potential of a given
site. The example in Section 6-6g illustrates the impact of singleunit
minimum turbine discharge on a project’s energy output.
(2) In preliminary power studies, minimum discharge can usually
be ignored, but once a tentative selection of unit size or sizes has
been made, a minimum single-unit turbine discharge must be applied to
the energy computation. For more advanced studies, a minimum
discharge based on the data presented in Table 5-1 (Section 5-6i)
can be assumed. Once a specific turbine design has been selected, the
minimum discharge associated with that unit should be used.
(1) The efficiency term used in power studies reflects the
combined efficiencies of the turbine and generator. Generator
efficiency is usually assumed to remain constant at 98 percent for
large units and 95 to 96 percent for units smaller than 5 ~.
However, turbine efficiency varies with the operational parameters of
discharge and head. The efficiency characteristics of a turbine vary
with type and size of unit and runner design. Figure 5-8 shows
typical performance curves for a Francis turbine.
(2) In reconnaissance level power studies, a fixed efficiency of
80 to 85 percent may be used to represent the combined efficiency of
the turbines and generators. A value of 85 percent can be applied to
installations where the larger custom-built turbines would be used.
The smaller standardized Francis and tubular turbines and units
requiring gearboxes have lower efficiencies, and an overall efficiency
of 80 percent should be used for reconnaissance studies of projects
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