Technical Water Supply Jet Pump for Hydropower Units

Working principle and efficiency

The jet pump for technical water supply of the unit uses the high-pressure water from the volute or the reservoir in front of the dam as working water, absorbs the river water downstream of the power station, and uses all the water flow passing through the jet pump for the technical water supply of the unit. Since both working water and sucked water are effectively utilized, the maximum efficiency of the water supply jet pump can reach 62% to 65%.

Jet pump efficiency is closely related to area ratio. Usually, the efficiency is higher when the area ratio is smaller; when the area ratio is 4 to 5, the work efficiency can generally reach 45% to 55%.

Water supply safety and scope of application

The outstanding advantage of using a jet pump for water supply is that the cooling water of the unit is not easily interrupted, thus ensuring the safe operation of the unit. In the technical water supply system of large and medium-sized hydropower station units with a water head section of 80 to 150 m, using jet pumps instead of conventional water pumps or gravity-reduced water supply not only has significant energy saving effects, but also operates safely and reliably.

Noise control under high water head conditions

When jet pumps are used in hydropower station technical water supply systems with water heads above 200 m, the operating noise is usually high. Model tests show that when the working head of the jet pump nozzle inlet is 200-250 m, the measured noise is about 101-104 dB. Therefore, when high-head power stations use jet pumps to supply water, sound insulation measures should be taken to meet ergonomic requirements.

Model tests also show that covering the jet pump with about 20 cm of sand can reduce operating noise by about 10 dB, providing a reference technical solution for engineering sound insulation design.

Fixed jet pump series test

According to the requirements of the project "Application of Jet Pumps in Large and Medium-sized Hydropower Stations with Medium and High Heads", a series of model tests of fixed jet pumps for technical water supply of units in the 70-110 m water head section have been completed. The test area ratio ranges from 2.5 to 7.0, with one level set every 0.5. A total of 10 sets of area ratio tests have been completed, which can provide designers with corresponding pressure-flow performance curves.

On this basis, a product series of fixed jet pumps with a water head section of 70 to 150 m can be further compiled to provide a basis for the design and selection of the technical water supply system of hydropower station units.

Effect of water head changes on fixed jet pump

The working conditions of jet pumps in the technical water supply system of hydropower station units are significantly different from those in other industrial applications. Due to the constant changes in water levels upstream and downstream of the power station, the fixed working conditions of the jet pump will be broken, and its flow rate and pressure will also change significantly.

When the actual water head deviates from the design working condition of the jet pump by about 30 to 50 m, the equipment may experience insufficient water supply or severe cavitation noise. For power stations with a large range of water head changes, it is difficult for fixed-nozzle jet pumps to cover all operating conditions; if they rely on frequent replacement of nozzles to maintain stable operation, it will increase the inconvenience of maintenance and repair.

Needle valve regulated jet pump

In order to adapt to changes in water head and load, a movable needle valve can be installed in the jet pump nozzle. By adjusting the needle valve position to change the effective area ratio, the jet pump can adapt to changes in the power station's operating water head and unit load, thereby maintaining stable water supply and achieving energy-saving operation under changing operating conditions.

The Northwest Survey and Design Institute of the Ministry of Water and Power and Wuhan Institute of Hydropower have collaborated to conduct a large number of model tests on needle valve-regulated jet pumps, and have achieved a structural type with good performance and low noise. The needle valve-regulated jet pump has the characteristics of wide working range and high efficiency, and is suitable for the technical water supply system of hydropower station units with large changes in water head and load. In addition to improved performance parameters, its operating noise can also be about 5 to 10 dB lower than similar foreign equipment.

Technology development direction

The model test work of the needle valve regulated jet pump has been initially completed. In the future, it is still necessary to accumulate experience through the actual operation of the power station, continue to improve the design quality, and further study the formation and control methods of operating noise to reduce the equipment noise to the allowable sound level range.

Conclusion

Unit technical water supply jet pumps have the advantages of energy saving, safety and reliability. Fixed jet pumps are suitable for working conditions with small changes in water head; for power stations with large fluctuations in water head and load, needle valve-regulated jet pumps can provide a wider working range and more stable water supply performance. In actual projects, special design and selection should be carried out based on the water head range, water supply flow rate, pressure requirements and noise control objectives.