Jet pumps for unit phase-adjusting dewatering

Phase-adjusting operation and dewatering requirements

With industrial development, more and more units are used for phase-adjusting operation, and some large hydropower stations also need to consider phase-adjusting operation. In order to reduce active power consumption during phase adjustment, most power stations use compressed air to depress water from the runner so that the unit can operate in phase-adjusting mode with the runner dewatered.

Traditional low-pressure air systems and their limitations

In the past, phase-adjusting dewatering used air systems with a pressure of 0.5 to 0.8 MPa. With the emergence of large-capacity units or several units performing phase adjustment at the same time, the equipment and layout of the air system encountered difficulties; if the design was improper, dewatering often failed. Therefore, in recent years, high-pressure air systems have been proposed for phase-adjusting dewatering and have been successful in practice.

High-pressure air system for phase-adjusting dewatering

According to reports, high-pressure air systems with a pressure of 4 MPa or 6.3 MPa are used abroad for phase-adjusting dewatering. The purpose of using such high air pressure is to solve air system equipment problems and to share the high-pressure air compressor of the governor system without adding new equipment.

After the first successful dewatering with high-pressure air, blowers are used to keep the water level in the runner chamber from rising.

Optimal air pressure and air-water hammer risk

According to high-pressure phase-adjusting dewatering test data, the optimal air pressure for phase-adjusting dewatering is about 2.5 MPa. When the pressure exceeds 2.5 MPa, the actual dewatering time cannot be reduced; on the contrary, air-water hammer is easily generated.

Utilization of high-pressure system equipment

At present, the pressure used in governor systems of large hydropower stations is 4 to 6 MPa or even higher, while air compressors operate at 6 MPa or 15 MPa. In order to make full use of compressed air equipment in high-pressure systems while achieving safe and economical operation, it is necessary to optimize the phase-adjusting dewatering method.

Jet pump series connection with high-pressure air system

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. High-pressure air should be used as the working pressure of the jet pump so that the jet pump draws in a certain amount of free air. After mixing, the air is sent to the runner chamber for the first phase-adjusting dewatering.

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

Conclusion

Using a jet pump connected in series with a high-pressure air system for phase-adjusting dewatering has the potential to simplify equipment layout and reduce investment and operating energy consumption. Of course, such a dewatering scheme still requires extensive model testing to obtain the best jet pump structure and the optimal matching of dewatering operating parameters.