Water hammer protection for pressurized water supply system with gravity branch

The inter-basin water transfer project is an essential instrument for achieving better water resource distribution. The hydraulic parameters will fluctuate significantly while the water supply system is transitioning, endangering its stability and security. Therefore, it is necessary to investigate the transition process of the hydraulic system and the water hammer protection measures.For a typical long-distance pumping station pressurized water supply system with a gravity flow branch in a large-scale water supply project in Northeast China. A mathematical model of the hydraulic system was established based on the characteristic line method, and the hydraulic transition process simulation was carried out.The water supply system did not have negative pressure under steady-state operating conditions and met the requirements of overflow capacity. The calculated values of water level at each water diversion were in good agreement with the engineering values, which verified the credibility of the simulation results. Under unprotected accidental pump stopping conditions, the pump went into flyaway reversal and the maximum reversal speed was exceeded. The pressure drops after the pump led to vaporization in several sections of the pipeline. Gravity flow branches necessitated the regular use of flow regulating valves at the pipeline's end to adjust the flow rate, and it was quite possible to cause serious water hammer accidents if the regulating valve did not function properly. It was generally considered that the worst operating condition was when all branch regulators were closed at the same time. Therefore, this condition was usually used to verify whether the protection measures could still meet the water hammer protection requirements under the most severe conditions. The condition in which the valves closed one after the other, generated the greatest increase in water hammer pressure. And the calculation formula when each valve started to operate successively under the successive shut-off condition was given. With the combined protection of the 4.5 s one-stage linear fast closing of the pump outlet valve and the air valve layout, the maximum reversing speed of the unit and the pressure extremum in the main pipeline was effectively lowered. Under steady-state operating circumstances, the estimated values of the water level of each water diversion were in good agreement with the engineering values (the calculated error of the water level of each water diversion was less than 0.5%), confirming the trustworthiness of the simulation findings. Under an unprotected accidental pump stopping circumstance, the pump entered a flyaway reversal and exceeded the limit reversal speed. Pressure drops caused vaporization in numerous portions of the pipeline. Therefore, water hammer protection measures must be taken. The successive shutdown scheme of the regulating valve can generate the maximum water hammer pressure. By comparing the regulating valve successive shutdown operating conditions and simultaneous shutdown operating conditions, it was discovered that the successive shut-off scheme derived induced a larger water hammer pressure (92.91 m), which was 4.4% higher than that of the simultaneous shut-off scheme (88.99 m). The calculation formula when each valve started to operate successively under the successive shut-off condition could provide a theoretical basis for the calibration of water hammer protection of water supply systems with gravity flow branches. The joint water hammer protection scheme was proposed and the hydraulic instability characteristics of the pump outage process were effectively mitigated. With the combined protection of the 4.5 s one-stage linear fast closing of the pump outlet valve and the air valve layout, the maximum reversing speed of the unit was reduced by 94.95%; meanwhile, the maximum positive pressure in the main pipeline was effectively lowered by 33.82% and the maximum negative pressure was decreased by 89.24%. This water hammer protection solution can provide a reference for water hammer protection of the same type of water supply system.

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