POWER April 2021 - 29

HYDROPOWER
costs. In addition, the upstream end of
the draft tube is armored. That is, a steel
lining is embedded to protect the concrete
from erosion by cavitation until the
maximum velocity of the discharge in
the draft tube is less than about 20 feet
per second.
The original hydraulic turbines were
small machines and the original draft
tubes were conical diffusers that pointed
straight down. However, as turbines
grew larger, such configuration for a
draft tube required ever-increasing excavation.
Consequently, designs were
introduced that put right-angle circular
bends in the draft tubes so the exits
could discharge horizontally.
However, the overriding design feature
of draft tubes has remained consistent,
even accommodating the aforementioned
round to circular transitions, intermediate
lateral piers, and right-angle vertical bends.
That is, the cross-sectional flow area perpendicular
to the centerline of the draft
tube is formed to expand at a constant
angular rate so that as the flow velocity decreases
the pressure will increase, or stated
another way, there will be a pressure
recovery. In this manner, any cavitation
downstream in the draft tube is avoided
and the velocity at the draft tube exit can
be designed to match an open channel river
velocity and avoid scouring bottom sediments.
The U.S. Bureau of Reclamation's
Engineering Monograph No. 20, " Selecting
Hydraulic Reaction Turbines, " has both
plan and profile designs recommended for
draft tubes.
Whirl Component
Reaction turbines convert the energy in
a fluid column by using the specific energy
in the incoming flow to first impart
a whirl or angular momentum to the incoming
flow. This is done through the
centrifugal action of the spiral or semispiral
case. The turbine runner is then
designed to straighten this flow out by
removing the whirl component.
The reaction to removing the whirl
component is to impart the equivalent
mechanical energy, minus losses, into
the turbine-generator shaft. Upon leaving
the runner, if all the whirl component is
removed, the flow would have a natural
tendency to expand at a conical angle of
about 12 degrees. To avoid any flow reversal
and increased losses, the rate of
2. The runner shown here is from a Francis
turbine. Source: Creative Commons / Qurren
cross-sectional area increase with draft
tube length would need to be less than
that angle. This would require a long draft
tube with high frictional losses and degrade
the efficiency of the turbine.
Consequently, it has been a U.S.
practice to leave about 5% of the whirl
component in the flow when discharged
from the runner. This allows centrifugal
action to cause the discharged flow to
expand at a conical angle greater than
12 degrees. Thus, draft tubes can be
formed of shorter length, cause less frictional
loses, and cost less to construct.
The design standard in some European
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POWER April 2021

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