Chapter 5 - Publications, US Army Corps of Engineers

EM 1110-2-1701
31 Dec 1985
structure, and penstock. Hydraulic losses between the entrance to the
turbine and the draft tube exit are accounted for in the turbine
efficiency.
(2) For projects where the intake is integral with the powerhouse
structure, the losses across the trash racks are the major
consideration. For most planning studies, a trash rack head loss of
1.0 feet can be assumed. This value is based on a typical entrance
velocity of about 5.0 feet per second. For more detailed information
on trash rack losses, reference should be made to the Bureau of
Reclamation’s Engineering Monograph No. 3 (62).
(3) Steel penstock head losses can be derived using the Scobey
equation:
where: ‘f =
D=
v=
ks =
‘f
~l.g
=ks— D1.l
friction loss in feet per thousand
feet of penstock length
penstock diameter in feet
average velocity of flow in penstock
in feet per second
a friction loss coefficient
(Eq. 5-6)
The friction loss coefficient k. is a function of the rouEhness of
the penstock wall. For steel p~nstocks, a value of 0.34 ;an usually
be assumed for k . Additional information on estimating penstock
losses (includin~ estimating losses for concrete-lined ~wer tunnels)
can be obtained from standard hydraulic design references, including
the Bureau of Reclamation$s Engineering Monograph No. 7 (61).
(4) For preliminary studies and for analysis of projects with
short penstocks, it is usually satisfactory to use a fixed penstock
head loss, based on the average discharge. For projects with longer
penstocks, it is preferable to use a head loss versus discharge
relationship. Where a fixed value is used, it would be based on the
average daily discharge for a run-of-river plant, but for a peaking
project, it should be based on the average discharge when generating.
(5) For Projects with long penstocks, the size of the penstock
will have a major impact on project costs, and to minimize costs it is
desirable to minimize penstock diameter. However, smaller penstock
diameters lead to larger losses in potential power benefits due to
penstock friction losses. For projects where penstock costs are
large, it is usually necessary in advanced stages of planning to make
an analysis to d~termine the optim~ penstock diameter considering
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