led to this being the immediate option of choice for aeration of hydropower releases. It is being used
wherever feasible to avoid or reduce costs associated with other aeration alternatives.
Turbine venting is a technique successfully used at many hydropower projects to increase the
DO in reservoir releases. This technique involves the introduction of air into regions of subatmospheric
pressure downstream of the turbine blades. As this entrained air moves through the draft tube, the
increase in the hydrostatic pressure increases the oxygen transfer efficiency, thereby increasing the DO
uptake in the release.
A general approach to evaluation and design has been described which involves the use of a
numerical model and prototype testing. Turbine venting has been successfully implemented by
government agencies and investor owned utilities at many projects, with results that are very site
specific. These turbine venting applications result in some loss in generation efficiency, dependent on
airflow volume and location, and have generally experienced no problems with increased cavitation
damage, shaft deflection, or vibration. Summary information is presented in Table 4.7.1.
4.7.3 AUTO-VENTING TURBINE TECHNOLOGY
For turbines that are being replaced or upgraded, unique opportunities exist to optimize the
turbine's hydraulic and aeration performance. Auto-venting turbine technology is simply turbine venting
applied to a new unit where a "clean sheet" design approach can be used to optimize both turbine
hydraulic performance and aeration. Although aeration considerations and design may delay the turbine
unit upgrade schedule, utilizing an upgrade outage may provide opportunities for better aeration than
would be possible or economical with retrofit systems.
For new turbines, a wide range of design factors, and consequently potential aeration
alternatives exist to improve aeration performance. This is due to the flexibility in selecting the shape
and position of key turbine components during the design and construction of new equipment.
Although some limitations may be imposed by existing structures, new air passageway designs can be
implemented more economically in the periphery of the water passageways while the turbine is
removed, and air passageways can be designed into the turbine itself.