Screen effects on turbine efficiency16 June 2000
Acoustic scintillation flow meters played an important role in assessing turbine performance at the McNary dam in the US
ASL’s acoustic scintillation flow meter (ASFM) played an important role in assessing the effect of fish diversion screens on turbine performance at a US Army Corps of Engineers’ hydroelectric dam
on the Columbia river. The Corps is currently engaged in a programme of improving fish passage through Kaplan turbines. Collecting efficiency information for existing turbines is an important part of this work. A series of measurements were made in unit 5 at McNary dam, Oregon in January 1998 for that purpose.
McNary dam is located at river mile 292 on the Columbia river. The power house contains 14 Kaplan turbine/ generator sets. The generator nameplate ratings are 70MW each and they can be operated continuously at 115% of rated capacity with a maximum power output of 80.5MW. Each of the Kaplan turbines (five-blade, 7.1m diameter runner,
85.7rpm) develops 111,300hp at a design head of 24m.
An individual turbine intake consists of 3x6m wide bays. Each bay contains slot openings for an operating head gate (emergency closure) and an upstream slot for bulkheads. To protect downstream migrating juvenile salmonids, fish diversion screens are installed in the bulkhead slot. The screens divert juvenile fish and water up the bulkhead slot ,
where the juvenile fish can enter a
system designed to bypass the fish safely to the tailrace of the dam.
Desirable near-uniform flows into
a turbine intake are disrupted when
fish diversion screens are installed.
The diversion screens create large
scale eddies within the intake, causing
a decrease in turbine performance.
This results in less power production
and may also create a more harmful environment for some of the juvenile
fish, which are not intercepted by diversion screens and pass through the operating units.
The ASFM’s ability to measure absolute discharge under the conditions prevailing in low head plants was the reason for
its use in the unit 5 tests at McNary
dam. The ASFM uses a technique called acoustic scintillation drift to measure the flow speed of water perpendicular to a number of acoustic paths established across the intake to the turbine. Fluctuations in the acoustic signals transmitted along a path result from turbulence in the water carried along by the current. The ASFM measures those fluctuations (known as scintillations) and from them computes the lateral average (ie along the acoustic path) of the flow perpendicular to each path.
The acoustic sensors were installed on frames in the intake gate slots,
leaving the flow passage unobstructed. Installation of the equipment required three days, without the necessity of dewatering the turbine. After that, flow measurement required approximately 20min per condition.
The ASFM is an effective method
for determining the effects of intake modifications on turbine performance. The presence of the diversion screens
in the intake caused a loss of 2-3% in turbine operating efficiency within the normal operating range. Their presence also decreased the full load power production by 6%.
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