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 vacuum-breaker system to allow more air to enter through the vacuum-breaker ports or the addition of
another venting system. Such modifications usually include changes to the vacuum-breaker valve,
which often must be maintained in an open position or replaced with an new operational system.
To increase subatmospheric pressures at aeration outlets, deflectors are often added to the
turbine just upstream of the vacuum-breaker ports. These deflectors create a low-pressure region in
their wake, thereby increasing air flow into the draft tube. Research by TVA has indicated that such
"hub baffles" are very effective in extending the hydroturbine operating range over which air can be
drawn into the water flow as shown in Figure 4.7.2 (Carter and Harshbarger, Sep 1997).
Tenkiller Ferry
80
70
With Hub
Baffles
60
50
40
30
20
Without Hub
Baffles
10
0
0
10
20
30
40
50
60
70
80
90
100
Gate, % open
Figure 4.7.2 Effect of Hub Baffles on Air Flow
Modifications to the venting system should be tested to determine their effectiveness. In conjunction
with these tests, turbine performance should be monitored because a loss in generation efficiency
occurs during turbine venting (Almquist, Hopping and March, 1991, Wilhelms, Schneider, and
Howington, 1987).
4.7.2.4 Applications
Several applications of turbine venting have been made at CE projects. The report by
Wilhelms, Schneider, and Howington (1987) includes a comprehensive analysis of a turbine venting
applications at CE projects. In tests on Clarks Hill Dam powerhouse (now J.S. Thurmund), hub baffles
were added to the vacuum-breaker system to increase air flow. Turbine efficiency monitored during
the tests, indicated an efficiency loss of approximately 2- to 3-percent was observed as a result of the
4.7-3
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