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 4.7 IN-STRUCTURE TECHNIQUES: TURBINE VENTING, AUTO-VENTING TURBINE
TECHNOLOGY, AND FORCED AIR
4.7.1 PROBLEM ADDRESSED
Low dissolved oxygen is a common problem with releases from hydropower projects during
periods when the upstream reservoir is stratified. Most Francis turbines and some Kaplan turbines
offer an opportunity to reaerate the release flow as it passes through the turbines. This can be
accomplished at some sites by supplying air to openings in the turbine where the pressure is
subatmospheric (turbine venting, auto-venting turbines), and at other sites by application of blowers or
compressors (forced air systems).
4.7.2 TURBINE VENTING
4.7.2.1 Description
For many existing turbines, the vacuum breaker system can be modified or a separate venting
system can be retrofitted to induce air into the discharge. These types of systems achieve, at most,
about 30 percent reduction of the penstock DO deficit1. This generally means a maximum oxygen
uptake of about 2.5 - 3.0 mg/l for a penstock DO of 1.5 mg/l and water temperature of 18oC.
Advantages of this technique include (a) it is a passive system, often requiring only valves and
piping to implement, and (b) installation and maintenance costs are relatively small compared to other
DO enhancement alternatives. Two disadvantages of this system are (a) the absorption of nitrogen
from aspirated air bubbles, resulting in total dissolved gas supersaturation, which may pose a
downstream water quality problem, (b) a reduction in generating efficiency and capacity, and (c) the
potential for increased noise in the powerhouse.
4.7.2.2 Theory
Turbine venting is the introduction of air, typically into the region of subatmospheric pressure
downstream of the turbine blades. The pressures in this region are controlled by many factors,
including the geometry of the turbine and draft tube, flow rate, tailwater elevation, and the elevation of
the turbine. Therefore, turbine venting will not be applicable to all turbines. The magnitude of
subatmospheric pressure and hydraulic characteristics of the air passageways determine the rate of air
flow into the draft tube. Once the air is aspirated into the flow, the intense turbulence and transport
DO deficit is the difference between the penstock oxygen concentration and the oxygen saturation
1
concentration.
4.7-1
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