Selection of exhaust valve on the water pipeline and the role of the exhaust valve
1. Project Overview The project case is the transformation of the water pipeline of a factory. The water of the plant is taken from Xiangjiang River, and a new DN500 water supply pipe is laid from the riverside pumping station to the purification station . From the purification station to the secondary pressurized pumping station, the secondary pressurized pumping room to the production area, two new DN500s are laid. The water supply pipe has a total length of about 3km . In this transformation, the terrain is undulating, and the water pipelines have to cross the entire living area. With the continuous development of production scale, the safety requirements of the water supply system are getting higher and higher, and the difficulty is relatively large.
2. The situation of fault analysis and transformation project is more complicated. In order to do a good job in designing the construction drawings, my designers have gone to the scene twice to conduct the survey. First, check the scene to determine the direction of the water pipeline ; second, listen to the comrades in the power station of the factory. The problems that occurred during the use of the water pipeline and the opinions on this transformation, one of which caused me to attach great importance to it. In recent years, the phenomenon of pipeline bursting occurred frequently, and the tube was blasted more than once a month, seriously affecting the normal. Production. I have the day before to a pipe burst pipe, repair the whole plant without water, I asked to the scene looked, pipes of cast iron pipes, pipe diameter DN300, burst pipes phenomenon socket at the crack length 1m, occurred in the second The secondary pressurized pump room pressure pump is operated by a small pump to change the pump. The comrades of the Power Department said that a tube was blown a week ago. The last time the tube was fired was similar to the situation. It occurred when the whole plant was shut down for maintenance and pumping was started. The exhaust valve on the water pipe line has been in disrepair for a long time. If it has not been replaced in time, the exhaust function has been lost. Through the analysis of the time and phenomenon of the pipe burst, I think that the pipe is not exhausted, and there are a large number of airbags in the pipe. The pump water hammer is the main cause of the tube explosion.
The pump of the secondary pressurized pumping room has no frequency conversion equipment, and the direct start-up speed is too fast. When the large pump is replaced by a small pump or just after the pump is started, the pump can reach the rated pressure and flow rate within a few seconds, and the pressure and flow rate in the pipeline. When the gas is compressed, the volume becomes smaller, the internal pressure rises, the pressure of the water pump port decreases, the flow rate increases, the internal pressure of the gas rises to a certain extent, and then the expansion expands, and the momentum and impulse alternately change, causing a The series of sudden pressures alternately lift and lower the hydraulic impact phenomenon, and transmit it in the pipeline in the form of pressure waves, causing the water hammer to start pumping. When the pump starts, the flow velocity in the pipeline is much higher than the design flow rate. The pressure of the water hammer generated by the pump can be as high as 2 MPa , which is much higher than the test pressure. The dynamic pressure destructive force of this continuous fluctuation is several times that of the static pressure. Longer service life, lower strength, and easy to burst. In order to avoid the recurrence of the squib phenomenon caused by the water hammer of the pump, the exhaust problem of the designed pipeline is very important.
3. The source and operation characteristics of the gas in the pipeline and the hazard of the gas in the pipeline are many. The gas source mainly has two aspects: 1> The pipe network is undulating, and the pump is stopped or the water pipe is partially vacuumed to cause the pipe to be discharged. Gas valve or high water point inhalation ; 2> Natural water body of the suction pump, the large volume gas content of the dissolved air is about 2% ( that is, the dissolved gas in the water is about 20L/m3) , the research shows that when the pressure When reduced to a certain value, the dissolved gas in the water will rapidly precipitate in the form of tiny bubbles, and will accumulate into large bubbles or large air cells as the water flows. In long-distance pipelines, because the design flow rate is generally small, the gas in the pipeline is mostly present in the upper part of the tube in the form of a balloon. In a multi-undulating pipeline, the airbag is mostly present at the convex point of the pipeline ; in a small, flatter pipeline, the gas is dispersed in the form of a plurality of independent large airbags. According to the research theory of the famous American water hammer expert Professor Martin, the relatively flat water supply pipeline presents six gas-liquid two-phase flow states during water filling and operation. Aqueduct water-filled discharge process is quite complicated, In general the beginning of water-filled pipeline flow regime mostly laminar flow, wavy flow, slug flow, or annular flow, the exhaust gas easier; mostly the later In the state of slug flow, air mass flow or foam flow, it is difficult to vent the ordinary exhaust device. The size and number of airbags present in the pipeline depend on the complexity of the pipeline, the size of the pipe, the speed and method of water filling, and exhaust measures.
A large number of engineering practices have shown that when the airbags in the pipeline flow with the water, it brings great harm to the long-distance water pipeline, mainly in the following two aspects:
1> Poor exhaust gas causes difficulty in water supply to the water pipe, water resistance increases , and the flow rate does not reach the design value.
The slope is not great through water pipes after the initial pass or overhaul again water, water pipes and gas phases is inevitable, if poor exhaust, and causing severe "gas block", no gas discharge, poor water flow It is extremely difficult to fill the water, especially the gravity flow, the slope is small , and the pipe diameter is larger, which is more serious. There is a balloon in the pipeline, and the air resistance increases. When it is serious, it forms a “ gas blockage †. The cross section of the water is reduced, and the flow rate does not reach the design value.
2> The exhaust gas is not smooth, causing water to burst during the operation and operation , and the water supply system cannot operate normally. When the airbag existing in the pipeline flows with water, it is dispersed and polymerized due to the change of the slope of the pipe, the roughness of the pipe wall, and the various pipe fittings of the curved pipe and the variable diameter. The change of the flow velocity in the pipe easily causes the pressure difference between the two ends of the airbag to change. The slight differential pressure change will have no effect on the incompressible water, but the effect on the air is enormous. It can cause the change of the water flow speed and the sharp rise and fall of the pressure in the pipeline, causing the water hammer to cause the burst.
4. Type of Exhaust Valve The exhaust valve is a special product on the water pipeline , and its performance is extremely poor. At present, the common exhaust valves in China can be divided into two types according to the structure , floating ball type exhaust valves ( including combined type, composite type, power combined type ) and cylinder type exhaust valves.
4.1 Float type exhaust valve The floating ball type exhaust valve has a float ball on the valve body of the valve body. When the valve body is waterless, the float ball falls into the protection tube, and the exhaust port opens the exhaust gas. When there is water, the float The ball floats to block the exhaust port and seal the water flow. This type of exhaust valve has a simple structure, but has the following technical defects: 1> The large exhaust port can only discharge * segment of gas during the water vapor phase, and cannot be continuously exhausted. When the slope of the water delivery pipeline is not large, most of the pipelines are in a slug flow state of a section of water. The buoyancy of the float-type exhaust valve is only a few kilograms, and the water is lifted when the water is discharged. The second stage of the gas is pressure , assuming a pressure of 0.2PMa , the towing force of the gas to the float is equal to the area of ​​the exhaust port multiplied by the pressure inside the pipe . The calculated DN100 exhaust valve ball force is about 150kg , and the DN300 exhaust valve is supported . The force is about 1400kg , so it is impossible for the large exhaust port to automatically open the exhaust gas, and the exhaust function is lost in the slug flow state. 2> The small exhaust port diameter is generally 3 ~ 5mm , only a small amount of exhaust gas. If the performance of the large exhaust port is not good , only the micro-hole exhaust is extremely dangerous for the large-diameter water pipe. 3> The long-term non-action during operation , the float is easy to fail due to stickiness. 4.2 Cylinder type exhaust valve Cylinder type exhaust valve is to use the float lever to control the pneumatic diaphragm in the cylinder, so that the large and small exhaust ports on the valve body are opened and closed. It is a full-pressure high-speed exhaust valve, and the valve is in any Under pressure and water flow conditions, as long as the gas in the pipeline enters the exhaust valve, the exhaust valve opens the large and small exhaust ports. When the water enters the exhaust valve, the large and small exhaust ports are closed and there is no water leakage. Or the valve can be exhausted at high speed, and the water is closed, and a large number of exhaust gases can be continuously and repeatedly. The main features of this type of exhaust valve structure are: the effective exhaust diameter of the large and small exhaust ports or only the large exhaust port is not less than 70% ~ 80% of the nominal diameter of the exhaust valve , the exhaust port diameter is large, exhaust It is fast and can be vented at high speed under any circumstances. The general designer calculates the displacement by the nominal diameter of the exhaust valve when selecting the exhaust valve. The cylinder type exhaust valve can fully meet the design requirements , and truly prevents the water hammer and tube from appearing in the pipeline.
5. Selection and installation of exhaust valve The pipeline of this project has pressure flow, gravity flow, large slope, small slope, and the operating state of gas in the pipeline is more complicated. The state can effectively discharge the gas in the pipeline. A cylinder type exhaust valve was selected for comparison. The purification station was built on a hill, taking the pump house from the river to the purification station. The pipeline is 800m long , the pressure flow is large, and the slope is large. The exhaust valve is installed at the starting point of the middle ridge. In order to evenly distribute the water into each treatment structure, before purification stations into the ultrasonic transit-time flow meter installed, if a large airbag conduit, easily lead to tolerance or the flow meter can not be measured, it is greater than 10 pipe diameters prior to the ultrasonic transit-time flow meter installed exhaust valve To ensure accurate measurement. From the purification station to the secondary pressurized pumping station, the pipeline length is 1600m , gravity flow, the slope of the downhill section is large, and the slope of other pipelines is relatively flat. The gas in the downhill pipeline can be directly discharged into the clear water pool on the mountain. An exhaust valve is installed at three places. In order to protect the water pump in the secondary pressurized pump room, an exhaust valve is installed on the pipe at the secondary pressurized pump room.
From the secondary pressurized pumping station to the production area, the pipeline is 600m long , the pressure flow is smooth, and the exhaust valve is installed at the starting point of the intermediate ridge. A total of six exhaust valves are installed on the water supply line to effectively discharge the gas in the pipeline, ensuring that there is no problem with the gas in the pipeline.
6. Conclusion Because the smooth discharge of the water pipeline is an important indicator of its safe operation, the engineering practice proves that if the performance of the exhaust valve is not installed or the installed exhaust valve does not meet the requirements, it may cause the tube to burst , stop the water and stop production, resulting in A certain degree of economic loss. Therefore, the use of exhaust valves with good exhaust performance and reasonable installation is the key to safe operation of the pipeline. The exhaust valve should have good performance, especially when the gas in the pipeline is slug flow and air mass flow, not only can quickly discharge * airbags, but also can continuously discharge any airbag. That is, under certain pressure conditions, the exhaust valve must be filled with air to open the exhaust port, and the water enters the valve body, and the valve is closed without draining. For the exhaust valve that meets the above requirements, the diameter of the large exhaust port shall not be less than 70% to 80% of the nominal diameter, and the diameter of the small exhaust port is not limited. The bidder must present the certificate that the exhaust valve has passed the inspection by itself or the inspection agency. The diameter of the exhaust valve is generally 1/8 to 1/5 of the diameter of the main pipe . If you choose an exhaust valve without a slow closing, you can install one large and one small exhaust valve in one place. If you choose a venting valve with a slow closing, install only one at the top of the pipe. If the pipe is flattened, when the slope ≤D / 1000, should be installed at intervals of 0.5 ~ 1.0km one exhaust valve. If the slope ≤D / 100, should be installed at intervals of 1.0 ~ 1.5km one exhaust valve. When the slope is large , consider installing an exhaust valve every 1.5 to 3.0 km . When the slope ≥D / 10, consider installing only at high exhaust valve (D is the pipe diameter). For multiple undulating pipelines , an exhaust valve should be installed at each high point. According to the relevant specifications and technical requirements, scientifically select the exhaust valve, perform performance test on the selected exhaust valve to identify the performance of the exhaust valve, and then use it after passing the test.
Source: Network of valve: Valve Information Network
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