管线末端含虹吸断流结构的输水工程水锤防护

Water hammer protection in water conveyance project with siphon breaking structure at end of pipeline

基于瞬变流计算的特征线法,建立真空破坏阀和单向调压塔数学模型,应用于管线末端含虹吸断流结构的输水工程。针对管线系统水锤防护问题,提出真空破坏阀单独防护、真空破坏阀和空气阀联合防护、真空破坏阀和单向塔联合防护等3种防护方案。结合管道压力控制要求,分析对比危险工况下水泵事故掉电时3种防护方案对管线系统水力过渡过程的影响。结果表明:针对管线末端含虹吸断流结构的输水工程,在真空破坏阀单独防护方案下,泵后部分管段压力降至汽化压力;在真空破坏阀和空气阀联合防护方案下,虽然管道压力没有降至汽化压力,但是不能满足空气阀防护下-5m最小压力控制标准要求;在真空破坏阀与单向塔联合防护方案下,单向塔防护范围内管线最小压力为0.55m,满足单向塔防护下管线不出现负压的控制标准要求,该方案有较好的水锤防护效果。研究结果可为管线末端含虹吸断流结构的输水工程水锤防护提供参考。

In the pressurized water supply project,the power failure accident will lead to the rapid decrease of the flow after the pump and the sudden change of the flow velocity in the pipeline,which will form the pump-stopping water hammer.Its specific characteristics are reflected in the significant decrease and increase of water pressure in the pipeline,which seriously endangers the safety of water supply projects.The water conveyance project with siphon breaking structure at the end of the pipeline is replacing the regulating valve in the conventional water conveyance project with siphon structure.When the pump is powered off,the siphon structure can quickly isolate the water flow between the pipeline and the downstream outlet pool,and suppress the water flow backward to the pipeline.Simultaneously,this structure can avoid large valve-closure water hammer,and the control of pipeline negative water hammer is mainly considered in water hammer protection.Based on the method of one-dimensional characteristics,the mathematical models of vacuum breaking valve,air valve,and one-way surge tank were established and applied to the water conveyance project with siphon breaking structure at the end of the pipeline.Given the problem of water hammer protection in pipeline systems,three protection schemes were proposed:single vacuum breaking valve scheme,vacuum breaking valve-air valve scheme and vacuum breaking valve-one-way surge tank scheme.Combined with the control requirements of pipe pressure,when pumps are in power outage accident at dangerous conditions,three protection schemes were assessed in terms of the influence of the hydraulic transition process of pipeline systems.In the protection scheme of a single vacuum breaking valve,theoretically,the minimum pressure of the pipeline can reach-25 m,which is far lower than the water vaporization pressure of-10 mand may cause the water column bridging phenomenon,which seriously endangers the project safety.Under the combined protection scheme of vacuum breaking valve-air valve,the minimum pressure of the pipeline is-9.94 m.Although the water vaporization pressure is not reached,it can not meet the requirements of minimum pressure control standard of-5 munder air valve protection.In the combined protection scheme of vacuum breaking valve-one-way surge tank,the pressure at each point of the pipeline can meet the requirements of water hammer protection,which can effectively ensure project safety.In summary,the combined protection scheme of vacuum breaking valve and other water hammer protection measures can often achieve a better water hammer protection effect.For the water conveyance system with a large flow rate and gentle terrain,the protection method of a one-way surge tank by water supplement is better than that of an air valve by the gas supplement.Besides,when water hammer protection is carried out in the pipeline system with a large flow rate and fast flow rate,it is necessary to avoid gas being mixed into water pipelines.Given the water hammer protection problem in water conveyance project with siphon breaking structure at the end of the pipeline,the pipeline characteristics are analyzed,and different water hammer protection schemes are proposed.The protection schemes are analyzed and compared,and the following main conclusions are obtained:(1)The vacuum breaking valve has a good effect on the stable operation and water hammer protection of the siphon structure,but the water hammer protection of the whole pipeline system should be combined with other water hammer protection measures.(2)For the water conveyance project with a large flow rate,flat terrain and siphon breaking structure at the end of the pipeline,the water hammer protection measures combined with the air valve and vacuum breaking valve will cause a large amount of gas intake in the pipeline.After the valve is closed after the pump,a large number of water will impact the air valve,resulting in the acceleration of the exhaust speed of the air valve,making the gas column bridging pressure larger,and adversely affecting the positive and negative water hammer protection of the pipeline.(3)The water hammer protection measures combined with a one-way surge tank and vacuum breaking valve are adopted.The water and gas are separated by the siphon breaking structure through the way of water supplement by one-way surge tank and gas supplement by vacuum breaking valve,which prevents the mixing of gas in the pipeline water body,avoids the generation of gas column bridging phenomenon,and effectively ensures the safety of the pipeline.