POWER April 2021 - 28

HYDROPOWER
Understanding Hydro Turbine
Draft Tubes and Their Importance
Significant engineering goes into every hydropower facility. Among the items that
require detailed design work are draft tubes, which are basically conical diffusers
installed at the outlet of turbine runners. This seldom-mentioned feature is highly
important to turbine efficiency.
Lee H. Sheldon, PE
T
oday's hydroelectric
reaction-type
turbines are usually constructed
with draft tubes. These civil structures
have no moving parts. Their design
is usually the responsibility of the turbine
manufacturer and they are invariably included
as part of a turbine model test, if
such a test is conducted.
The word draft has many meanings.
As a noun, Webster's defines it as, " air
artificially put in motion by any force or
action. " Since air and water are both fluids,
it was a natural extension to apply
that same definition to the discharge
of hydraulic turbines. Draft tubes may
be considered analogous to volutes on
centrifugal pumps.
First Hydraulic Turbines
and Draft Tubes
The first machine that could be properly
called a turbine was initially devised in
1824 by Claude Burdine, a professor in
the Ecole des Mines at Saint Etienne.
He adopted the name turbine from the
Latin word for " spinning top. " However,
he never succeeded in constructing
a working model. That challenge was
picked up by one of his students, Benoit
Fourneyron, who did develop an experimental
unit, which showed the basic
idea of a free efflux discharge from a
centrifugal runner. A free efflux discharge
refers to water entering the center
of a rotating element (runner) and
discharging freely into the atmosphere
from the outer periphery.
After Fourneyron had built more
than 100 such turbines, he recognized
the possible advantages of submerging
the free discharge into a diffuser. In
1855, he patented a diffuser that looked
somewhat like a modern scroll case. He
explained the benefit a diffuser provided
his turbine design by slowing the velocity
of discharge, after the outer periphery,
and thus recovering the pressure further
downstream from a new point of dis28
charge.
In his patent application in 1855,
he explained, " By this means the water
will be stripped, by the shape of the passage,
of all the velocity over and above
that needed to barely flow out. Thus, an
artificial head will be created, greater that
the natural head ... provided that it does
not exceed the column of water which
represents the atmospheric pressure. "
The draft tubes of today's design still
operate on that same principle. That is,
a conical diffuser seals atmospheric air
pressure from access to the underside
of the turbine runner. This increases the
net head across the runner and, just as
with any orifice, provides for a greater
discharge, and consequently more power
and efficiency. Fourneyron's last reference
in his quote refers to not lowering
the pressure under the runner too far
below atmospheric, lest the inception of
cavitation can occur.
Reaction Versus Impulse Turbines
The two basic types of hydraulic turbines
are the impulse and reaction. Impulse
turbines operate in the highest head
(specific energy) ranges and convert the
energy in a fluid column into mechanical
energy solely by changes in velocity
across the runner. The Pelton turbine
(Figure 1) is an example of an impulse turbine.
Reaction turbines are used in lower
and medium heads and convert energy
by changes in both velocity and pressure
across the runner. Consequently, it is the
performance of reaction turbines that is
enhanced by draft tubes. Francis (Figure
2) and Kaplan (Figure 3) turbines are examples
of reaction turbines.
In hydraulic turbines, if there is any
pressure change across the runner, that
is sufficient to classify it as a reaction turbine.
However, steam and gas turbines
are classified as reaction machines only
if 50% or greater of the energy conversion
is due to pressure changes.
A crossflow or Banki turbine is a hybrid.
It is often manufactured with a draft
www.powermag.com
1. The runner shown here is from a Pelton
turbine. Source: Creative Commons / Dietmar
Rabich
tube. However, even though the runner
may operate in sub-atmospheric pressure,
there is no pressure change across
the runner, and therefore, by definition, it
is an impulse machine.
Draft Tube Length
and Cross-Sectional Area
Draft tubes are expensive civil features
to construct. They require a considerable
amount of concrete forming. First, they
must start upstream on the underside of
the runner in a perfect circular shape, but
transition so that at their downstream
end they have a rectangular shape. That
shape is needed to accommodate rectangular
draft tube gates or stoplogs for
unwatering purposes.
Further, for large, vertical-shafted turbines,
the width at the draft tube exit
becomes excessive. Therefore, one or
more intermediate lateral piers must be
designed to provide structural support
in the downstream portion of such draft
tube barrels, further increasing forming
POWER | April 2021
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POWER April 2021

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