Ensured steady flow

June 20, 2017

Control valves enable safe water transport to a Chinese power plant across mountainous terrain.

The newly constructed Zhijin power plant in China’s Guizhou province sources its water from the Hongjiadu reservoir and transports its 8.1 kilometers across mountainous terrain with an approximate elevation of 1,150 meters.

The water first travels up to a water treatment plant and then into a water tank where supply can be controlled before it is gravity fed to the plant. The high lift and long distance of this water distribution system makes it vulnerable to surges and pipe breaks.

A valve aids the system

To address this vulnerability, a valve company was consulted to determine what technologies to implement to safely transport the water and ensure stable flow regardless of rates that vary power demands.

The first stage involves pumping water from a floating dock and transporting it by two DN600 pipelines to the water treatment plant. To regulate surging during pump startup or shutdown, two surge-anticipating relief valves were installed on the dock. These two valves can be open before the surge comes and relieve the pressure from the pipe. This is especially important if a power failure allows water to rush back down to the pump station. As an added precaution to prevent surges, air valves were installed to release any trapped air within the pipeline.

After the water is treated, it is pumped to a water tank for storage. With a 340-meter pump head, a surge calculation determined the need for a control valve after the pump with a surge-anticipating relief valve mounted in a tee downstream from the pump’s check valve. This is designed to anticipate surges and avoid the severe water hammer often associated with power failure surges. A surge-anticipating relief valve with flow limiting on the solenoid valve opening was selected for the job.

This pilot-operated, pressure relief valve was designed to open when the inlet pressure exceeds a predetermined setting and when the solenoid valve is energized. When the solenoid valve opens the valve, the maximum opening of the valve can be limited. It is typically energized by a controller that senses power failure or low header pressure and uses battery power to energize the solenoid valve for a short adjustable period.

Next, a pump control valve protects the pump and is used as a check valve when the flow comes back. The booster pump control valve was installed after the pump to prevent surging during opening and closing of the pump.

The water tank is 180 meters higher than the power plant, which makes this final transportation gravity fed via two DN600 pipelines across 6.3 kilometers. With this elevation and distance, a pressure-reducing station was required in the middle, and one with an anti-cavitation cage was installed because the pressure drop between intake and outflow was greater than 3-to-1 ratio.

Cavitation consists of rapid vaporization and condensation within a liquid. When local pressure falls to vapor pressure (approximately 0.25 psi/0.018 bar absolute for cold water), vapor bubbles form. When these bubbles travel to an area of higher pressure, they collapse with phenomenal force and great localized stress. The violent collapse of these vapor bubbles near valve components or downstream piping surfaces causes cavitation damage and subsequent performance degradation.

The anti-cavitation technology used contains two heavy stainless steel sliding cages that maximize the full flow capacity. The first cage directs and contains the cavitation recovery, allowing it to dissipate harmlessly, and the second cage allows further control to a level as low as atmospheric pressure downstream. The cages are engineered to meet the flow/pressure differential of each application.

After the pressure reducing station, the water flows into the power plant with one last flow regulating valve installed at the entrance gate to ensure stable flow and pressure.

“We knew it was going to be very challenging to transport the water to the power plant safely with the elevations and distance that it needed to travel”, said Liu Zhizhao, manager of Zhijin Power Plant. “(This) surge-anticipating plan and quality valves gave us the confidence to complete this project.”

The power plant, which employs around 600 people, has capacity to produce 1,320 megawatts (MW) with two units that produce 660 MW that travel 150 kilometers to power the city of Guiyang and the surrounding communities, and its new control valves ensure that the plant efficiently delivers all the power needed.

William Su is head of technical support for Singer Valve China and has been solving problems in the water industry for more than 10 years. For more information, visit http://singervalve.com.

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