RESEARCH OF BLOCKAGE SEGMENT DETECTION IN WATER SUPPLY PIPELINE BASED ON FLUID TRANSIENT ANALYSIS Ying Xu 2, Yuebin Wu 1, Liang Chen 1, Yun Guo 2, Wei Wang 1 and Zhe Ding 2 1 School of Architecture, Harbin Institute of Technology; Heilongjiang Cold Region Urban-Rural Human Settlements Science Key Laboratory, China, 150090; 2 Harbin University of Commerce, Harbin, Heilongjiang, China, 150028. 1 ybwuhit@163.com ABSTRACT According to the transient analysis and pressure wave method, the mathematical model of blockage segment is established in the water supply pipeline. The control equation is solved by the characteristic line method in the corresponding boundary conditions. The formulas for determining the position of the blockage, the length of blockage and the area of blockage are deduced by the pressure wave method. Through the numerical simulation of calculation example, the scope of application and feasibility of this method for water pipe blockage detection is verified. Through this method the position, the length of blockage and the area of blockage could be estimated accurately. Keywords: Blockage segment detection, Fluid transient analysis, Water supply pipeline 1 Introduction When pipeline transport operates for a long time, there are many factors cause blockage. The blockage will not only increase the operation cost of the pipeline, affect the normal working state of the pipeline, but also cause the explosion of pipe accidents and threaten the safety of human life and property. Effective blockage detection can provide meaningful guidance information, greatly reduce the economic losses. Therefore, it is very important to invent an effective and practical method to locate the blockage in the pipeline and determine the magnitude of the blockage. Most studies of pipeline blockage detection technology based on the theory of transient flow, mainly on the basis of the transient experiments, through the transient wave propagation in the pipeline, research of blockage location and quantitative are carried on according to the propagation rules of transient wave [1]. Most of these researches are for the location detection of blockage. Moreover, most of them are based on simulation, and the testing technology is not very mature. When the fluid in the pipeline is different, the cause of the blockage is very different, so there is a certain complexity in detecting the blockage of the pipe, and the method of judging the blockage of the pipe is difficult, especially for the quantitative of the blockage [2]. In this paper, the transient analysis and the pressure wave method will be combined to detect blockage condition rapidly and real-time, including the location and the magnitude. 2 Theory of pressure transient flow and characteristic line method 2.1 Theory of transient flow In a pressure pipe the flow velocity and momentum of the fluid in the tube suddenly change, which will cause a large fluctuation in the pressure of the liquid that is called water hammer [3]. The water hammer causes the pressure wave to alternate with the supercharged wave, which causes a lot of noise and vibration in the pipe system, damaging the elements in the pipes, and causing the pipes to burst and the accidents. The instantaneous pressure produced by the water hammer is called the water hammer pressure, which is very large, and its several hundred times to the normal pressure inside the pipe. Although the water hammer wave in pipe has more or less damage, but with a
reasonable way to produce water hammer wave, can use the propagation rules of water hammer wave in pipe to solve a lot of problems [4]. Water hammer wave has the characteristics of propagation, reflection and interference [5]. Comparing the size of TZ (valve of the closed or open) and Tr (phase length), water hammer can be divided into two types: direct water hammer and indirect water hammer [6]: T (1) Direct water hammer (Rapid closure):, close the valve quickly, before the water hammer pressure of reflected wave to the valve, the valve is closed totally, so the rise of the valve pressure only depends on the pressure value of the direct wave. T Z T (2) Indirect water hammer (Slow closure):,close the valve slowly, close the valve caused by water hammer pressure is back from a pool of decompression wave partially offset, so the valve of the water hammer pressure value is less than the direct water hammer. Z T In this article, all operating status in numerical simulation adopts the direct water hammer (rapid closure), which causes the water hammer pressure won t exceed the pipeline bearing capacity. 2.2 The solution of characteristic line method With the water supply pipeline running, the blockage of pipeline is inevitable, the blockage can be divided into two types: one is occupying the pipeline segment, the length of the blockage compared with the actual length of pipe can t be neglected, the block is seen as part of a pipeline [7]; The other one is point blockage, the length of the blockage is negligible, assuming that the local water head loss caused by the blockage happens to be concentrated in one point [8]. The point blockage is the limitation of segment blockage; in this paper we use the segment blockage pipeline model. When the pipe is blocked, the pipe is seen as a series of tubes include two sections, which is shown in Figure1. In front of the blockage is the section 1, and behind the blockage is the section 2. Ignoring the local resistance of connection point, the equation C + for the NS step of the section 1 and the equation C - for the first step of the section 2 are solved in the series connection point [9]. Combined with the corresponding initial conditions and boundary conditions, the pipeline blockage can be simulated by program. Programming process of the pipe block in water supply pipeline with MATLAB is shown in Figure2. r r C + t t C 1, NS 2,1 Figure1. The boundary conditions of blockage segment in pipe. 3 Methods of Pipe blockage detection based on pressure wave 3.1 The mathematical model of pipeline segment blockage If the water supply pipeline is blocked, the transient wave is generated by closing valve in the pipeline, then the pressure line will be obtained along with time, which can be used to determine the location of blockage and the magnitude of the blockage[10].the mathematical model is established based on the following assumptions:
Figure2. Program of water pipeline block 1) If the pipe material and diameter are both same, the wave propagation velocity will be same; 2) In the process of transient analysis, the both water level of upstream and downstream are constants; 3) The blockage area from the tube wall evenly to the pipe center increased, blocked the flow area of still is circular. The magnitude of the blockage is determined by the size of the cross section area of the blockage and the range of blockage; 4) The water hammer wave generated in the pipe does not exceed the bearing capacity of the pipe wall. When the pipe is blocked, the pipe is seen as a series of tubes connected to three different pipes, the diameter of the middle part is the smallest, which is seen as blockage segment. The diameters of the front and behind parts are the same. Assume the diameter of blockage segment is d, the length is L2, and the diameter of the other parts is D. The magnitude of the blockage is determined by the ratio of the two diameters d/d and the length L2. The blockage segment model adopts the water tank - pipe - water tank system. The water wave caused by the downstream valve closed quickly, which is shown in Figure 3.
Figure3. The model of blockage segment in pipeline The diameter changing at both ends of the blockage is treated as boundary condition, regardless of the local resistance loss, but the central part of the tube flows with pressure normally. The basic control equation is the transient flow continuity equation and the motion equation, and the Vardy- Brown friction resistance model is selected, and the method of characteristic line is used to solve the problem. 3.2 The effect of blockage length and area on reflected wave In the simulation condition, the basic parameters of the pipeline are the same as above. The blockage diameter d is 0.1 m, the other section diameter D is 0.2 m, and the block segment length is changed for 1m, 5m and 10m, respectively. Under the above three conditions, the average flowrate in the pipeline are 0.0102m 3 /s, 0.0101m 3 /s and 0.00995m 3 /s, respectively. After closing the downstream valve to create the water hammer wave, the pressures at the end point are shown in Figure4, Figure5, and Figure6 with different flowrates. Figure4.The pressure valve with the blockage length of 1m Figure5. The pressure valve with the blockage length of 5m
Figure6. The pressure valve with the blockage length of 10m From Figure4 to Figure5, it is shown that with different blockage lengths, the change of pressure is similar before the wave reflected at the right edge. With the increase of blockage length, the initial water hammer wave constantly reduced. When the blockage lengths are 5m and 10m, the reflected wave is not too big, but bigger than the length is 1 m. The reason is that the length of the blockage becomes long and the corresponding frictional head loss increases, the initial velocity of the pipe is smaller, so the water hammer wave is smaller. Moreover, the positive reflected wave on the right side of blockage doesn t increase to the maximum immediately, and on the other side there will be a negative reflected wave. If the length of blockage is longer enough, the positive reflected wave will be the maximum and propagate to the valve before the negative reflected wave travels from the left to right of blockage. If the length is short enough, the negative reflected wave goes back to the right side of blockage before the positive reflected wave becomes the maximum, the two reflected waves will offset each other. To sum up, when the area of blockage is the same, if the length of blockage is so short that the negative reflected wave can t propagate to the right side of blockage after the positive reflected wave to maximum, the reflected wave will become bigger and bigger as the length of blockage increases. If the length of blockage is so long that the negative reflected wave can propagate to the right side of blockage after the positive reflected wave to maximum, the reflected wave is nearly the same. Using the quasi-steady state friction model and Vardy-Brunone unsteady friction model to simulate with blockage length 1m and 10m, the results of pressure wave at the end point are shown in Figure7 and Figure8. Figure7.curve at the end valve of the two models when the plug length is 1m. Figure8.curve at the end valve of the two models when the plug length is 10m
From Figure7 and Figure8, the pressure with the blockage length of 1m has some differences under the quasi-steady state friction model and Vardy-Brunone model, respectively, especially the peak of the pressure. However, the pressure curve with the block length of 10m is almost the same under the two unsteady friction models. When the blockage length is 1m, the simulation result of the Vardy-Brunone friction model is closer to the reality, which is indicating that when the numerical simulation is carried out to detect the blockage, the influence of instantaneous wall shear stress should be considered. The various reflections of transient pressure waves are based on the previous reflection, and in general, the pressure waves of each reflection are decreasing. In summary, when the length of the blockage is certain, the reflected wave will increase with the enlargement of the blockage magnitude. 4 Conclusions In this paper, the method of detecting blockage is studied, the transient flow theory and the pressure wave method are combined, and the blockage segment detection model is established. According to the time the reflected wave generated to determine the pipeline blocking location, the size of the reflected wave determines the magnitude of blockage. The feasibility of the pressure wave detection pipeline is verified by the example and the influence of the plugging amount on the reflected wave size is studied. The conclusions are shown as following: (1) The method of applying transient flow theory and pressure method to detect the blockage of water supply pipeline is proposed. After the pipe is blocked, the position and the magnitude of blockage are determined according to the regularity of pressure wave propagation and reflection in the pipeline; (2) The blockage detection model is established, and the Vardy - Brown friction model, on the basis of the block model is solved by characteristic line method, the stress wave method is verified by a numerical example the feasibility of detecting segment blockage. The research results show that the pressure wave method for segment blockage detection is feasible, but only when the block length is longer. The pressure wave method can be used to detect the blockage when the duration from the negative reflection wave is generated to its first reflection back to the end of the blockage segment is longer than the time of closing valve; (3) Based on the blockage model of the pipe segment, the effect of the reflected wave is observed by changing the length and area of the blockage segment. It is shown that when the blockage area is constant, with the increase of length, the magnitude of reflection wave increases then becomes constant. When the negative reflection wave propagates to the right edge of the segment faster than closing valve, the negative reflection wave becomes bigger and bigger. But when the negative reflection wave propagates to the right edge of the segment slower than the closing valve, the reflected wave is almost unchanged. When the length of the blockage is constant, the magnitude of the reflected wave becomes bigger as the blockage area increases. Compared the Vardy - Brown friction model and the quasi-steady friction model in blockage detection model, it is expressed that considering the influence of the instantaneous wall shear stress is necessary when detecting the segment blockage.
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