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 venting. However, there was an increase in the release DO of approximately 2.5 mg/L or a reduction
of the DO deficit of approximately 30 percent. A numerical model of turbine venting was developed
from this work to predict the impacts of various air flow rates on release dissolved oxygen.
J. Percy Priest Reservoir on the Stones River in the Nashville District has operated with turbine
venting for a number of years. At this project, the vacuum-breaker system is blocked open so that air
can be entrained in the draft tube at all generation cycles. This results in a deficit reduction of
approximately 27 percent, with an increase in the release DO of up to 2.0 mg/L (Price,1988).
Turbine venting has been applied at a number of hydroplants in the Alabama Power system.
(Bohac et. al, 1983). At some of these sites, existing air piping used for tailwater suppression was used
to supply air. At these sites, DO levels were raised about 0.5 to 1.0 mg/L and generation efficiency
was decreased by up to 2%, depending upon the amount of air induced. At other sites, deflector baffles
were added to the draft tube walls to cause local negative pressures for air induction. Air was supplied
to openings just downstream from the deflectors by specially installed air supply piping. These systems
were found capable of increasing the DO by 0.5 mg/L up to 1.8 mg/L, but required retrofitting piping
systems and also reportedly caused about 2 % loss in generation efficiency.
Duke Power Company applied turbine venting at the Wylie Station using the existing vacuum
breaker system (Bohac et. al., 1983). Significant aeration was obtained at reduced turbine loads, but
at high turbine loads, there was no negative pressure to induce air.
A summary of the early turbine venting designs and hub baffles tested by TVA is included in
Bohac et al. (1983). Since 1983, TVA has implemented turbine venting at 9 TVA hydropower pro-
jects (Carter 1995) and has recently assisted in the implementation of turbine venting at 5 CE projects
such as Norfork, Tenkiller Ferry and Hartwell (Carter and Harshbarger, 1997; Carter and
Harshbarger, 1998), and an investor owned utility project at Saluda (Kleinschmidt Associates, 1996).
These turbine venting systems are used as a stand alone aeration solution or as a part of several
aeration systems combined to meet downstream water quality targets.
TVA's successful implementation of turbine venting thus far has applied been exclusively to
Francis units that exhibit some negative pressure under the headcover during operation. Site specific
modifications are made to the turbine to increase the suction at the vacuum breaker outlet and to the
supply piping to reduce pressure losses for air entering the unit. Together, these changes increase the
air aspirated into the turbine and the amount of DO uptake. The most successful TVA installations
provide 2 to 3 mg/L uptake and do not require limitations on the turbine gate operation. Turbine
efficiency losses have been reported in the range of negligible to 1.5% with the majority of the losses
occurring only during air entrainment. Turbine shaft deflections during air entrainment do not change
significantly or tend to increase slightly at some units but have not been a problem. Cavitation damage
to the TVA turbines has not increased significantly although wear patterns have sometimes been
changed due to the air entrainment and hub baffles. Overall, TVA's experience with turbine venting has
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