I. Introduction Water pipelines, mainly refers to the water source to the water purification plant or water purification plant to the pipe network. Because of the water supply pipeline system-wide water supply, and high pressure, so its safe operation has always been the water sector and the design department attention. Common accidents are water pipe burst pipe, pipe explosion caused by the main reasons: temperature stress, pipe quality, construction quality, geological structure and water hammer and so on. Although the airbag in the pipeline can not directly cause water hammer, it can cause harm by means of water hammer. This article on how to set the exhaust valve on the water pipeline to avoid the gas gathered into the airbag to explore. Second, the examples and analysis In the terrain from the lot, requiring the highest point of the water pipe set the exhaust valve, but in actual operation, many burst pipe did not occur in the high or low, but occurred after the high point Bend sections, and even low-pressure piping, occur with such squibs. Hegang, Heilongjiang, a pipe burst pipe is a typical example. Hegang City is a hilly area, water purification plant and pumping station built, both 5 km apart, purified water by gravity flow to the pumping station, water purification plant elevation of 210m, send water pump station clear pool elevation 185m, DN800 continuous cast iron pipe, the average flow rate of 1.0m / s, pumping station has a high post 500m, elevation 185 (see Figure I), the highest point of a post a vent valve, but the exhaust valve 50m , Burst pipe accident occurred many times, and later installed at the burst tube at a pair of mouth exhaust valve, in the past few years, there is no pipe burst between the two rows of valves, only in the new exhaust valve 10m after a burst pipe. From this example, the squib is related to the gas in the tube (no squib after valve arrangement). The following analysis of the formation process of the airbag in the pipeline and its stress conditions: 1. Gathering and balancing of gas Under normal circumstances, the water flow in the pipe can be approximated as a constant flow (pressure, flow rate, temperature constant ). In this state, the water in the gas to be gradually precipitated, the formation of bubbles of varying sizes rise to the tube wall, the bubbles move forward by the water flow rate. In the uphill section, the bubble flow rate may be greater than the water flow rate due to the buoyancy effect. Due to the tube wall has a certain roughness, the same direction of movement of the bubbles, it is difficult to gather into large bubbles. Small bubbles along the pipe wall flow forward a certain width, after the highest point of the exhaust valve, the exhaust pipe bubbles within the conditions of the conditional discharge, and other bubbles by the thrust of the water flow downstream. Due to the turbulence and flow rate at the pipe wall and the tangent characteristics, some small bubbles passing through the exhaust pipe flow downstream through the vent (see Figure 2). The air bubbles across the exhaust valve run smoothly along the slope. The movement direction is opposite to that of the component P1 of buoyancy force (see Figure 3). The resistance generated by this buoyancy force inevitably slows the movement of the bubble and the post-order bubble is easy Hit the front of the bubble and full into large bubbles, large bubbles produce large buoyancy. Buoyancy component P1 = PSinα (1) where: p - buoyancy of bubble by water (P = 1 / 6π d3 · P) P - bulk density of water d - bubble diameter α - Of the thrust P 'V2 p' = -----. S (2) 2g10 V2 Where: ----- --- Flow pressure (Kg / cm2) 2g10 S - bubble maximum cross-sectional area (S = 1 / 4πd2) V - water flow rate (m / s) When P1 = P `, the bubble force balance and remain in the pipeline. (1), (2) Formula 3 d.Sinα = ----------- 10.4pg C - equilibrium constant equation (3) shows that under a constant flow conditions, the bubble diameter and the pipe Depression sine inversely proportional. When d · Sinα> C, the bubble moves upward. 2. Shape of Airbag in Pipe When the bubble balance problem described above, it is assumed that the bubble is spherical with diameter equal to d, which can only approximate micro-bubble. In fact, when the bubble reaches a certain volume and rises to the wall to become the airbag, due to the surface tension It will have a semi-elliptical shape (see Figure 4). As the bubble grows, the shape of the bubble will be controlled by the flow of water, gravity and the shape of the pipe, with a greater elongation in the length direction and arching in the transverse direction. According to the simulation measurement, the relationship between the length L and the height h of the airbag is: 1≈15h 3. Analysis of the airbag force and its critical position According to the principle of hydraulics, the thrust of the air flow received by the airbag in the pipeline equals to the section perpendicular to the flow (See Figure 5 (1)), this section is a bow with h height (see Figure 5 (2)) n V2 p '= Σ pi = -----. S (5) 1 2g10 S - bow in the bursts can cause a squib, due to the rapid switching valve or pump start and stop, the pipeline out of a large pressure value, the compressibility of the gas, so that the stress concentration to the balloon to produce high pressure and burst pipe. According to some squib experience, the balloon height reaches a quarter of the pipe diameter is the danger of squib, which is the critical point of the balloon. At this time the size of the balloon and the cross-sectional area is simply calculated as: V ≈ 0.5πr3, S ≈ 0.2πr2. For the DN800 pipeline in the previous example, when V = 1.0m / s, the thrust received is calculated by equation (5): 1.02 p '= ------ 0.2π.402 = 5.124 (Kg) 1) Formula P1 = PSin α = 0.5π0.43 × 1000Sinα = 100.5Sinα When P '= P1 5.214 α = arcSin = ----------- = 2.92 ° 100.5 The depression angle α and the measured explosion Pipe point depression basically consistent. Third, the conclusion 1. The descending slope of the water pipeline must be an additional exhaust valve, the specific location determined by the formula P `= PSinα, calculate the average flow rate is appropriate; 2. The actual pipeline depression angle is less than the calculation, the exhaust valve should be set At the intersection of the lower curve and the straight line; 3. In order to make the gas discharged as early as possible without the formation of airbags, bend pipelines and straight pipelines should be set at the intersection of the exhaust valve; 4. In this paper, the exhaust valve can not replace the highest Point exhaust valve.
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