Application of the most fire gas cylinder exhaust

Application of air cylinder exhaust valve in the water transmission pipeline of a plant

1. project overview

this project is located near Funa, Hengyang City, Hunan Province, and is the reconstruction of the water transmission pipeline of a plant. The water for the plant is taken from the Xiangjiang River. A new DN500 water supply pipeline is laid from the water intake pump room on the river side to the purification station, and two new DN500 water supply pipelines are laid from the purification station to the secondary pressurization pump room, and from the secondary pressurization pump room to the production area. The total length of the service life of the optical fiber is extended by about 3km. The terrain of this reconstruction is undulating, and the water transmission pipeline needs to pass through the whole living area. With the continuous development of production scale, the safety requirements of the water supply system are higher and higher, and it is more difficult

2. Fault analysis

the reconstruction project is relatively complex. In order to do a good job in the construction drawing design, our designers have visited the site twice. First, check the site conditions to determine the direction of the water transmission pipeline; Second, I listened to the comrades of the power department in the factory report the problems of the old water transmission pipeline in the use process and their opinions on this transformation. One of them has attracted my great attention. In recent years, pipe bursts have occurred frequently, on average once a month, seriously affecting normal production. The day before I arrived, there was a pipe burst, and the water supply of the whole plant was cut off for emergency repair. I asked to go to the site to have a look. The pipe was cast iron pipe with a diameter of DN300. The pipe burst phenomenon was that the crack at the socket was 1m long, which occurred when the booster pump in the secondary booster pump room was changed from a small pump to a large pump. The comrades in the power department said that a pipe burst occurred a week ago. The last time the pipe burst was similar to this time, it occurred when the pump was turned on after the whole plant was shut down for maintenance

the exhaust valve on the water transmission pipeline has been in disrepair for a long time and has not been replaced in time, which has lost its exhaust function. Through the analysis of the time and phenomenon of pipe explosion, I think it is the poor exhaust of the pipeline, the existence of a large number of air bags in the pipeline, and the water hammer caused by starting the pump is the main cause of pipe explosion. There is no frequency conversion equipment for the water pump in the secondary pressurization pump room. The direct start-up speed is too fast. When the large pump is replaced by the small pump or the pump is just started, the water pump can reach the rated pressure and flow rate within a few seconds. The pressure and flow rate in the pipeline increase sharply. After the gas is compressed, the volume becomes smaller, the internal pressure increases, the pressure at the water pump outlet decreases, the flow increases, and the internal pressure of the gas rises to a certain extent, then it expands, and the momentum and impulse change alternately, A series of hydraulic shock phenomena with sharp alternating pressure rise and fall are caused in the pipeline, which is transmitted in the form of pressure wave in the pipeline, causing pump start-up water hammer, and finally leading to pipe explosion. When the water pump is started, the flow rate in the pipeline is much higher than the design flow rate, and the pressure that produces the water hammer to start the pump can be as high as about 2MPa, which is much higher than the test pressure. The destructive force of this continuous fluctuating dynamic pressure is several times that of the static pressure. The old pipeline has a long service life and reduced strength, which is very easy to burst. In order to avoid the recurrence of pipe explosion caused by water hammer when starting the pump, the exhaust problem of the pipeline in this design is very important

3. The source, operation characteristics and harm of gas in the pipeline

there are many cases of gas in the pipeline, and the gas source mainly includes two aspects: 1. The pipe fluctuates greatly, and the vacuum is generated locally when the pump is stopped or the water is stopped, causing the pipe to inhale from the exhaust valve or high water point; 2. The maximum volume gas content of dissolved air in the natural water body that is sucked into the pump is about 2% (that is, the dissolved gas in the water is about 20l/m3). The research shows that when the pressure decreases to a certain value, the dissolved gas in the water will quickly separate out in the form of tiny bubbles and accumulate into large bubbles or air bags when it runs with the water flow. Mobile/wechat: 13649160039

in long-distance transmission pipelines, because the design flow rate is generally small, the gas in the pipeline mostly exists in the upper part of the pipe in the form of air bags. In the multi undulating pipeline, the air bag mostly exists at the convex starting point of the pipeline; In the small slope and flat pipe, the gas is dispersed in the form of many independent large air bags. According to the research theory of Professor Martin, a famous American water hammer expert, the relatively flat water supply pipeline presents six gas-liquid two-phase flow states during water filling and operation (Fig. 1)

Fig. 1 various gas-liquid two-phase flow states

the water filling and exhaust process of the water transmission pipeline is quite complex. Generally speaking, the pipeline flow state at the beginning of water filling is mostly laminar flow, wavy flow, slug flow or annular flow, and the exhaust is relatively easy; In the later stage, it is mostly in the state of slug flow, air mass flow or foam flow, and it is difficult for ordinary exhaust devices to exhaust. The size and number of air bags in the pipeline depend on the complexity of the pipeline, the size of the pipe diameter, the speed and method of filling water, exhaust measures, etc

a large number of engineering practices show that when the air bag in the pipeline flows with water, it brings great harm to the long-distance water transmission pipeline, mainly in the following two aspects: 1. Poor exhaust makes it difficult for the water transmission pipe to pass water, the water resistance increases, and the flow cannot reach the design value. If the water delivery pipe with a small slope is opened for water again after the initial opening or maintenance, it is inevitable that the water and gas in the pipe are alternated. If the exhaust is not smooth, it will cause serious "air blockage". The air cannot be discharged, the water flow is not smooth, and it is extremely difficult to fill water, especially when the gravity flow has a small slope and a large pipe diameter. There are air bags in the pipeline, the air resistance increases, and in serious cases, an "air blockage" is formed, the water passing section decreases, and the flow cannot reach the design value. 2 the verification regulations of "tensile force, pressure and universal material testing machine" and "electronic universal testing machine" play a leading role. The poor exhaust causes the pipe explosion during the water supply and operation, and the water supply system cannot operate normally. When the air bag in the pipeline flows with water, it is dispersed and aggregated due to the change of pipe slope, pipe wall roughness, elbows, and various pipe fittings with variable diameter. The change of flow velocity in the pipeline is very easy to cause the change of differential pressure at both ends of the air bag. This small differential pressure change will have no impact on incompressible water, but it will have a great impact on air, which can cause the change of water flow velocity and the sharp rise and fall of pressure in the pipeline, Cause water hammer to cause pipe explosion

4. Type of exhaust valve

exhaust valve is a special product on water pipeline, and its performance is very poor. At present, the common exhaust valves in China can be divided into two types according to their structural forms, floating ball exhaust valves (including combined, composite and dynamic composite) and cylinder exhaust valves

4.1 floating ball exhaust valve

there is a floating ball on the casing in the valve body of the floating ball exhaust valve. When there is no water in the valve body, the floating ball falls into the casing, and the exhaust port is opened for exhaust. When there is water, the floating ball floats to block the exhaust port and seal the water flow. This type of exhaust valve is simple in structure, but has the following technical defects:

1 when water and gas alternate, the large exhaust port can only discharge the first section of gas, and cannot exhaust continuously