through submerged, high pressure regions of the draft tube and tailwater contribute to the oxygen
transfer (Figure 4.7.1).
Figure 4.7.1 Vacuum Breaker Venting System
220.127.116.11 Design Methodology
Successful implementation of this technique requires a combination of both hydraulic analyses
and field tests to determine the most effective method for introducing air into the turbine or draft tube.
The following paragraphs describe a general methodology based on previous investigations and
applications of turbine venting.
The analyses should begin with a description of existing hydrologic and water quality conditions.
This can be accomplished by investigating the seasonal characteristics of in-reservoir temperature and
DO and the resulting oxygen released without turbine venting. The observed water quality data can
verify the application of a numerical model, such as SELECT (Davis et al. 1987), by comparing
predicted and observed release quality. Once verified, the SELECT model can be used to estimate
impacts of turbine venting by conducting simulations using the venting subroutine in the model. The
venting subroutine assumes that a maximum of 30 percent of the upstream DO deficit can be satisfied
by turbine venting.
If the results of the SELECT simulations indicate that an acceptable increase in release DO is
predicted, field tests of an appropriate venting system are recommended. This testing should consider
the specific turbine design and operating conditions to determine if venting of air is feasible and to
identify potential locations for introducing air into the flow. This may involve modification of the