Study on Water Hammer Prevention in Pumping Water Supply Systems by Multi-valves

Transcription

1 Study on Water Hammer Prevention in Pumping Water Supply Systems by Multi-valves Manlin Zhu, Xiaohong Zhang, Yanhe Zhang, Tao Wang Institute of Water Resources and Hydro-electric Engineering Xi an University of Technology Abstract Air valves are widely used in water supply systems for their simple structure, low cost and easy installation. Many water hammer studies had proved that in order to effectively protecting water hammer, air valve should have both large air inflow area and small outflow area, i.e. the air inflow speed is fast while outflow slow. But research on how to implement that idea can hardly be found. Based on a real pumping water supply system, this paper presented a water hammer protection method using characteristic method of water hammer calculation, and the concept of air valve set proposed. The calculation results show that area ratio between air valve inlet and outlet has great influence on water hammer protection effect, and the ratio has an optimal value. Using both air valve set and butterfly valve two-phase closing procedure can effectively prevent water hammer in pumping water supply systems. In a pumping system, opening/closing of discharge valves and starting/closing of pumps can cause sudden change of water velocity inside, and in turn sharp change in water pressure. While water velocity and pressure change, parameters of pump, like capacity, head, revolution and torque etc. also instantaneously change. This instantaneous transformation is called hydraulic transient of pumping stations, or pumping station water hammer. Water hammer of pumping stations has great impact on normal and safe operation of pumping facilities, and many pumping stations were seriously damaged by water hammer [1-4]. Therefore research on hydraulic transient has been focus of many studies. The objective of this research is to analyze a water hammer prevention approach by jointly using butterfly valve and air valve set by mathematic modeling. 1. Pumping station in the study One pumping station includes four units with each two of them connected in parallel by sharing a single outlet pipe, and each pump outlet has a two-phase closing butterfly valve. The type of pump is of 8S76, and its corollary motor Y71-8. The rating head is 73.9m, capacity 1.64m 3 /s, efficiency 89% and revolution 73rpm. The total static head of pumping station is 62.79m, and pipeline length 172m, water hammer wave speed 16m/s and Manning coefficient of roughness of pipe Boundary conditions of air valve When pressure inside the pipe section where air valve is installed is lower than local atmospheric pressure, the air valve will open and let air into the pipe; otherwise the air inside the pipe would escape. Air valves are widely used in water supply system for its simple frame, low cost and easy installation [3, 4, 5]. Assuming that: (1) the air into or out the air valve has the same entropy; (2) the air temperature inside the pipe remains constant; (3) the air inside the pipe stays around valves where it can be extracted; (4) the surface height of pipe water basically keeps constant, while the ratio of air volume to liquid inside the pipe is very negligible. Then, the mass flow passed the air valve dm/dt is dependent on absolute atmospheric pressure P and absolute temperature T outside and inside the pipeline. These can be classified into 4 cases as following [6]: dm P P Cin Ain 7Pρ = P P.528P dt P P (1) 26 International Conference on Hybrid Information Technology (ICHIT'6) /6 $2. 26

2 dm C A.686 P P P dt = in in.528 (2) RT dm 7 P P P = Cout A out P P dt RT P P.528 (3) dm.686 P P = Cout Aout P (4) dt RT.528 Where C in is air valve discharge coefficient of inflow, C out is air valve discharge coefficient of outflow; A in is area of inflow air valve opening, and A out is area of outflow air valve opening; ρ is mass density of atmospheric air, and R is the gas constant. For air valve showed in Fig. 1, relationship between pressure and head of a typical line can be derived according to general gas law: CP + CM 2 P P V + 5. dt Qi Qpxi + + Z H B B ρg = m + dt dm/dt + dm/dt RT [.5 (( ) ( ) )] i (5) Where V is beginning volume of cavity; Q i is initial outflow from cavity, and Q pi is final inflow from cavity; Q pxi is initial inflow to cavity, and Q pp is final inflow to cavity; m is initial mass of air in cavity; (dm/dt) is initial rate of air mass flow into or out of cavity, and (dm/dt) i is final rate of air mass flow into or out of cavity.; B is the specific coefficient of pipeline; Z is the height that air valve overtops base level; H is local barometric head. Equation (1)~(5) form the boundary conditions of air valve, in which only P is unknown and can be solved [6]. Q pxi Q ppi C + air valve C base level Q i Q pi Figure 1. Boundary conditions of air valve Z 3. Analysis of water hammer prevention effects of butterfly valve and air valve set Method of characteristics was used to calculate water hammer. If air valve is not installed and only two phase shutdown butterfly is used, maximum and minimum intrinsic pressure curves of pipeline based on pure theory [9] when shutdown by accident are showed in Fig. 2. It shows that the most minimum pressure of the pipe is lower than vapor pressure of water, therefore water column separation will takes place in many pipe sections, and in turn water hammer pressure rises due to collapsing of discontinuing column. Pressure head H /m asial line of pipeline Hmax Hmin Distance L /m Figure 2. Maximum and minimum pressure envelop curves of water hammer prevention by butterfly valves An effective way to prevent negative pressure from occurring is to install air valves on pressure pipelines. Here consider the situation that air valves are installed at six distances to the pumps: 382m, 575m, 965m, 115m, 134m and 153m. Air valve discharge coefficients of inflow C in and outflow C out are.95 and.65 respectively, and areas of inflow and outflow air valve opening are A in and A out. Previous researches showed that the area of air inflow and outflow and the ratio of them A in /A out had great impact on water hammer prevention [7, 8, 11, 12]. Therefore, it is possible by optimizing the shutdown procedure of butterfly installed right after pump outlet and the areas of air valve inflow and outflow, water hammer can be effectively protected, and pressure of pipe and pump unit rotate speed met the requirements [1]. Table 1 shows the calculation results of outlet pressure of butterfly valve under different ratios of air valve inflow area to outflow area (A in /A out ), and for fast close in 4s at an angle of 65º, slow close in 8s. The results indicated that ratio of air valve inflow area to outflow area A in /A out had an optimum value, which should vary for different pumping stations, and for the case study, A in /A out was about 1. Table 1. Relationship between A in /A out of air valve and pressure at butterfly valve outlet A in /A out International Conference on Hybrid Information Technology (ICHIT'6) /6 $2. 26

3 H/H Although there are many different kinds of air valves, such as floating, level and gas jar air valve, the requirement of A in /A out =1 is generally difficult to be satisfied. Here proposed an approach of air valve sets comprising two valves as showed in Fig. 3, and this problem could be solved. The air valve set is composed of air valve 1, air valve 2 and a vertical pipe, which air valve 1 has large area of valve opening and mainly be used to let air in, air valve 2 has small area of opening and mainly to let air out. By selecting the two air valves with required parameters, the condition of A in /A out =1 could be readily met. The volume of vertical pipe can be determined by inflow air volume from hydraulic transient calculation. If the inflow air volume is small, the maximum inflow air volume can be used as volume of vertical pipe. Fig. 4 shows the maximum and minimum intrinsic pressure curves of the pipeline when two pumps were shutdown by accident at the same time, and both butterfly valves and air valve sets were jointly used. The pipe pressure and backup rotating speed of pump units met the requirements. Fig. 5 shows curves of air volume against time in places where air valves are installed. Only the place which is 382m distant from butterfly valve has a large air volume, the other places all have very small air volumes, so that air valve sets and butterfly valves can be used. Because air volume in place which is 382m distant from the butterfly valve is very large, using one air valve unit do has some difficulties. Therefore the place is designed as a local peak point when arrange the pipeline. air valve 1 air valve 2 pressure pipeline vertical pipe Pressure head H /m axial line of pipeline Hmax Hmin Distance L /m Figure 4. Maximum and minimum pressure envelop curves of water hammer prevention by butterfly valve and air valve set Figure 5.(a) Cavity volume vs. time (382m) Figure 5.(b) Cavity volume vs. time (575m) Figure 3. Air valve set.12.1 Air volume /m Figure 5.(c) Cavity volume vs. time (965m) 26 International Conference on Hybrid Information Technology (ICHIT'6) /6 $2. 26

4 Figure 5.(d) Cavity volume vs. time (115m) Figure 5.(e) Cavity volume vs. time (134m).6 valve is installed on the local peak point. After pumps are shutdown by accident, water flow inside the pipe will remain continued. This agrees with the assumptions of the math model, and can ensure the calculation valid. 4. Conclusion Air valves are widely used in water supply project for their simple structure, low cost and easy installation. In long distance water transport work, the air valve serves as air exhaust when pipe is being filled, air intake and exhaust when pump is shutdown normally or accidentally, and thus water hammer prevention. Research on jointly using air valves and other protection methods has great significance in practice using characteristic method of water hammer calculation, this study analyzed water hammer protection methods by mathematical model, and proposed the approach of air valve set. The calculation results show that the ratio of air valve inflow area to outflow area has great influence on water hammer protection, and there exits an optimum value; water hammer can be effectively prevented by jointly using both air valve set and two-phase shutdown butterfly valve. Acknowledgments Supported by the key laboratory scientific research program of the Education Department of Shaanxi Province, China (Grant No. 5JS35) Figure 5.(f) Cavity volume vs. time (153m) For pressure pipeline that has local peak point, installing air valve at this point can generally meet the 4 assumptions of air valve boundary. If an air valve is installed on a pipeline that has a certain gradient, the assumption that the air inside the pipe stays around valves where it can be extracted is difficult to be satisfied, and the calculation precision is hard to ensure. When air valve set is used, if inflow air capacity is small, the air will stay inside the vertical pipe section, and if inflow air capacity is big, the pipe section where air valve set is installed can be arranged as local peak point. In such case, air inlet valve is installed below the centerline of pipeline; air exhaust References [1]Liu Baohua, Cause Analysis of Water Hammer Accident for a Water Plant, Design of Hydroelectric Power Station, Vol.16 No.1 (2), pp [2]Zheng Daqiong, Shen Kang, and Wang Nianshen, The conditions and precautions on emergency water hammer occurrence, Electric Power,Vol.35 No.9 (22),pp [3]Xu Qiaoquan, Accident analysis of outlet pipe water hammer of water supply pumping station, China Water & Wastewater,Vol.16 No.5(2),pp [4]YU Ling-Hong, WU Sheng, and ZHANG Qi, Analysis and countermeasures of actuating pump water hammer fault, Science & Technology of Baotou Steel (Group) Corporation, Vol.26 No.4 (2), pp [5]JIA Wei-zhi, Study on Aeration/Deaeration of Pressured Delivery Conduits, Yellow River, Vol.26 No.9 (24), pp International Conference on Hybrid Information Technology (ICHIT'6) /6 $2. 26

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