Jet Pump Air Supply

Draft tube vortex and air supply requirements

When a Francis turbine operates under partial load conditions, vortex bands often appear in the draft tube. This cavity vortex moves periodically in the draft tube, causing strong pressure pulsations and power swings. Adding a certain amount of air to the draft tube is a good measure to destroy the vortex band in the center of the draft tube and eliminate pressure pulsation.

Natural air supply and forced air supply

According to the operating experience of hydropower stations, when the turbine is installed at a depth of more than 3 to 4 m below the tail water level, natural air replenishment will be difficult due to the increased static pressure on the inner wall of the draft pipe. At this time, forced air replenishment must be performed by equipment such as a jet pump or air compressor.

Working principle of air supply jet pump

The working liquid of the air supply jet pump is introduced from the high-pressure water in the volute, and a certain amount of free air is sucked in by the nozzle jet; the water and air are mixed into a jet state and supplied to the vortex zone in the draft tube.

Factors affecting the air supply effect

The air supply effect is related to the suction volume of the jet pump, the outlet pressure of the pump, and the radial distance of the outlet end into the draft tube. When the air intake volume of the jet pump is less than the optimal air supply volume, the pulsating pressure in the draft tube does not decrease but increases.

If the outlet pressure of the jet pump is too small, or the distance into the pipe is not enough, air will not be supplied, and sometimes strong noise will occur. In the early stages of operation of the Wujiangdu Hydropower Station, a test was conducted on jet pump air supply, and similar problems were encountered.

Design considerations

Therefore, when designing an air-supply jet pump, attention should be paid to key parameters such as suction volume, outlet pressure, and extension distance to avoid problems such as insufficient air supply or mismatched parameters that may lead to increased pulsation and noise.

Compressed air supply and jet pump series connection solution

Compressed air is supplied from behind the water guide vanes of the turbine to the runner area, which has been proven to be an effective air supply method. In the past, it was believed that compressed air consumes a lot and is not widely used; in fact, the amount of air required to eliminate unstable operating conditions of Francis turbines is about 5% of the rated flow rate (m³/s) of the turbine (converted to atmospheric pressure), and the air pressure is 0.5 times the working head of the turbine.

If a jet pump is connected in series in a compressed air pipe network, and compressed air is used as the working fluid of the jet pump, allowing it to inhale a certain amount of free air, it will not only improve the air supply effect, but also reduce power consumption by 1/2 to 1/3.

Conclusion

Jet pump air supply is an effective means to destroy the draft tube vortex and suppress pressure pulsation. In actual projects, the suction volume, outlet pressure and extension distance of the jet pump should be specially designed based on the layout of the water head and draft tube of the power station and the optimal air supply volume requirements. If necessary, a series connection scheme of compressed air and jet pumps can be used to balance the air supply effect and operating economy.