Jet Pumps for Phase-Modulated Water Pressure in Generator Units

Phase modulation operation and pressurized water requirements

With the development of industry, more and more units are used for phase modulation operation, and some large hydropower stations also need to consider phase modulation operation. In order to make the unit consume less active power during phase adjustment, most power stations use compressed air to press water to dehydrate the runner for phase adjustment operation.

Traditional low-pressure gas systems and their limitations

In the past, phase-modulated pressurized water used an air system with a pressure of 0.5 to 0.8 MPa. Due to the emergence of large-capacity units or several units performing phase modulation at the same time, the equipment and layout of the gas system have encountered difficulties; if the design is improper, the water pressure often fails. Therefore, in recent years, it has been proposed to use high-pressure gas systems for phase-modulated water pressure, and has been successful in practice.

High-pressure gas system phase-modulated water pressure

According to reports, high-pressure gas systems with a pressure of 4 MPa or 6.3 MPa are used abroad to perform phase-modulated water pressure. The purpose of using such a high air pressure is to solve the problem of air system equipment and to share the high-pressure air compressor of the speed control system without the need for additional equipment.

When the high-pressure air successfully presses water for the first time, use a blower to keep the water level in the runner chamber from rising.

Optimal air pressure and risk of air-water hammer

According to the high-pressure phase-modulated water pressure test data, the optimal pressure of phase-modulated water pressure is about 2.5 MPa. When the pressure exceeds 2.5 MPa, not only the actual water pressing time cannot be reduced, but on the contrary, air-water hammer is easily generated.

High-pressure system equipment utilization

At present, the pressure used in the speed control system of large hydropower stations is 4 to 6 MPa or even higher, while the pressure of air compressors is 6 MPa or 15 MPa. In order to give full play to the role of compressed air equipment in high-pressure systems while achieving safety and economical purposes, it is necessary to optimize the phase-modulated water pressure method.

Jet pump series high-pressure gas system solution

At the Second National Academic Symposium on Jet Technology in 1986, it was recommended to install a jet pump on the high-pressure air system pipeline. The high-pressure air should be used as the working pressure of the jet pump, so that the jet pump absorbs a certain amount of free air. After the two are mixed, they are sent to the runner chamber for phase modulation and first water pressure.

When the water pressure is successful, a low-pressure jet pump (outlet pressure is 0.5 to 0.8 MPa) is used to maintain the depressed water level in the runner chamber, replacing the large blower. This not only saves electric energy and obtains the best parameters for phase-modulated water pressure in the high-pressure gas system, but also greatly simplifies the layout of auxiliary equipment and reduces equipment costs.

Conclusion

Using a jet pump in series with a high-pressure air system for phase-modulated water pressure has the potential to simplify equipment layout and reduce investment and operating energy consumption. Of course, using such a phase-modulated water pressure solution still requires a lot of model testing in order to obtain the best combination of the jet pump structure type and water pressure working parameters.