An air vessel, as an effective and reliable water hammer protective device, is widely used in long-distance water supply systems. However, the volume of the air vessel must be extra-large to guarantee security of the system, especially in a system with a high head and large flow. In this paper, to reduce the volume of the air vessel, a novel protective method combining an air vessel and over pressure relief valve was proposed and verified using a numerical simulation based on a practical project. In addition, the protection performance under the combined protective method was compared with the traditional method. The results show that the combined protective method can significantly reduce the volume of air vessel and exhibit a better protection performance. Furthermore, after analysis and optimization of the rule of opening and closing over the pressure relief valve, the valve should be opened in a short time to produce fast discharge so as to eliminate the large pressure rise rapidly. The duration of opening the over pressure relief valve has little effect on the maximum pressure. The closing time of the over pressure valve should be slow, as much as possible, to avoid producing large pressure wave.

An appropriate water hammer protective scheme is a significant concern in the operation of water supply projects. According to the special terrain in the water supply project, which forms a siphon breaking structure at the end of the pipeline, three protective schemes were proposed and compared: single vacuum breaking valve (VBV) scheme, VBV and air valve scheme, and VBV and one-way surge tower scheme. Based on the control standards of pipe pressure, three protective schemes were assessed in terms of suppressing the negative pressure caused by a pump trip accident. The results show that the siphon breaking structure with the VBV can achieve good effect protection only in a limited range of pipelines. In the VBV and air valve scheme, the pressure oscillations were obviously caused by repeated inlet and exhaust of the air valves. To avoid supplementing too much gas in the pipe by air valves, which will result in a gas column bridging phenomenon, the VBV and one-way surge tower scheme is proposed and can better meet the requirement of the pressure control standard.

Hydroelectric energy is an increasingly vital and effective renewable energy for modern society. The protective effect on the water hammer in the pipeline, the operational stability of the hydropower system, and the flow regime in the air-cushion surge chamber (ACSC) are three main problems during the design of the hydropower station with an ACSC. Comprehensively comparing the above issues between the horizontal and vertical ACSCs is meaningful. This study established the one-dimensional (1D) model based on the Method of Characteristics (MOC) under large load disturbances (LLD) and the rigid water column theory under small load disturbances (SLD). At the same time, the three-dimensional (3D) model was built based on the Volume of Fluid (VOF) to obtain a more detailed flow regime in the ACSC under the load acceptance condition. The results showed that the vertical ACSC was superior to the horizontal one for its large safe water depth, smaller maximum air pressure, and more stable flow under LLD. In contrast, the horizontal one was better than the vertical one for its extensive water area to calm the SLD during the transient process and smaller fluctuation of the surge under SLD. This study will provide a reference for a future project on selecting the structure of the ACSC.