The air valve is a special valve used to prevent the negative pressure in the pipe caused by the pressure reducing wave in the transient process.It is usually installed at a high point where the dynamic water pressure is low during normal operation of the pipeline and liquid column separation may occur during transient flow.When the pressure in the pipeline is lower than atmospheric pressure, air is sucked in, and when the pressure in the pipeline rises above atmospheric pressure, air is discharged.During the exhaust process, the valve can automatically close when the pipe is filled with liquid, and liquid is not allowed to escape into the atmosphere.[1]
Chinese name
Air valve
Foreign name
Air Valve
Alias
Exhaust valve、Breather valve
Role
Prevent negative pressure in pipe during transient process
The air valve is the breathing apparatus of the pipeline and the protector of the pipeline safety. It is also called the breather valve and the exhaust valve, but it has the suction function, so it is more accurate to call the air valve.The air valve is divided into small caliber air valve with single exhaust function, also known asMicro exhaust valve；Large diameter air valves with exhaust and air inlet functions, including high-speed air inlet and exhaust valves, composite air valves, etc;The air valve with single air intake function is also called the suction valveVacuum breaking valve。When water is filled into the pipe, the faster the gas in the pipe is discharged, the faster the water is filled;If the water pressure is too high and the flow rate is too fast, the air valve quickly seals the ball, which is easy to cause water hammer;If air is not sucked into the pipe through the air valve or suction valve (vacuum breaking valve) in time during the drainage process of the pipe, on the one hand, the drainage is not smooth, on the other hand, negative pressure (forming vacuum) is easy to occur.For commonly used pipes, this negative pressure will not cause harm.If the stiffness of some pipes is insufficient, such as large thin-walled steel pipes, it is easy to cause deformation of the pipe geometry, resulting in water leakage. At the same time, sewage and sludge outside the pipe will be pumped into the pipe, endangering the safety of water supply.
There is usually a certain amount of gas dissolved in water, and the degree of dissolution of this gas varies with the temperature and pressure of water.In the process of water conveyance, some gas is also brought into the pipeline along with the water flow, which is gradually released when mixed in the water flow and accumulated at the top of the pipe, compressing the water discharge section and increasing the additional head loss in the process of water conveyance, so it must be timely discharged through the air valve.[2]
working principle
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Fig. 1 Air valve
There are two kinds of air valves: single port air valve and double port air valve.The following takes the double port air valve as an example to introduce its structure and working principle.(As shown in Figure 1)
During the starting of the pump, when the valve body is not full of water, the floating ball in the valve body falls due to its own weight, and the air holes are open.As the starting working head of the water pump increases, the pipeline is filled with water. The air type can automatically discharge the air in the pipeline. When the valve body starts to fill with water, the floating ball floats.
The air in the pipeline and valve body is completely removed, and the valve body is filled with water;If the pressure in the pipe is higher than the atmospheric pressure, the floating ball will block the vent under the pressure in the pipe, and the water in the pipe will not be discharged out of the valve.
During the operation of the water pump, if the air dissolved in water separates and accumulates in the air valve body, the floating ball will lose its buoyancy and fall automatically.If the air volume is small, the floating ball of the small hole will fall to exhaust;If there is a large amount of air, the floating ball of the large hole will fall to exhaust.The drop of the floating ball falls until the jacking force of the water pressure in the pipe is balanced with the gravity of the floating ball.After exhaust, the floating ball will return to the position of closed air hole.
When the pressure in the pipe decreases to form a negative pressure after the pump is stopped in an accident, although the valve body is full of water, because the pressure in the valve is lower than the atmospheric pressure, under the effect of the difference between the atmospheric pressure and the pressure in the pipe, the floating ball drops to replenish air. During the process of replenishing air, the floating ball of the big hole drops first, and then the floating ball of the small hole drops when the pressure drops significantly.Because the falling time of the two floating balls is different, the pressure fluctuation in the pipeline is buffered.
When the water pump enters the second stage braking energy consumption condition, the pressure in the pipe increases, and the water flow begins to flow back. The falling ball floats to open the air hole and exhaust until the air valve is full of water, the ball floats and closes the valve hole.During the closing process, due to the different pressure effects of the large and small exhaust holes, the floating ball of the large hole is lifted first to close, and then the small hole is closed, delaying the full closing process and easing the pressure rise when the water column closes again.[3]
structure
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The left figure in Figure 2 is a structural diagram of an air valve adopted in early China.The lower part is equipped with a larger pontoon to ensure that the water level can be raised better when it rises, so as to reduce water leakage at the sealing surface.The piston cylinder is equipped with a cover, and even if there is water leakage, it will be blocked.The upper part is equipped with two sets of buffer devices, one of which is adjustable and the other is backstop type, to prevent the sealing surface from being excessively impacted.In addition, there is a set of oil filling device to ensure good lubrication between the valve rod and the copper sleeve.
The right figure in Figure 2 shows the air valve designed in the 1960s in China, which is simple in structure but has the disadvantage of large water leakage.
Figure 2
Figure 3 is the latest design, which not only has the advantages of simple structure, but also has the function of preventing water overflow.
Figure 3
The following points shall be paid attention to in the design of air valve: ① There is a large enough pontoon, and the buoyancy generated by it shall be greater than the weight of the moving part; ②The valve rod shall be made of stainless steel to prevent rusting and blocking which may affect the flexibility of action; ③The opening of the valve under its own weight shall ensure the necessary air supply area; ④The sealing surface shall be carefully ground, and there shall be no water leakage after closing.[4]
Air valves in the engine
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The air valve is a device used to overcome the friction of engine rotation when the engine is started and warmed up. It can provide the engine with the necessary amount of air for warming up.When the engine temperature is low, in order to make the engine warm up more smoothly, open the air valve to allow additional air to be supplied to the engine through the bypass channel of the throttle valve.Since the air passing through the air valve at this time is fromAir flow meterRear entry ofIntake manifoldIt is also detected by the air flow meter, so the effect of these additional air volumes is the same as that of slightly opening the throttle valve, which will also increase the engine speed, thus reducing the warm-up time.
The air valves are either installed together with the throttle valve body or separately installed on the engine.When the engine is restarted, in order not to waste gasoline due to too high idle speed, heat capacity is used for control.When the heat capacity of the air valve is smaller than that of the engine, because the air valve cools earlier than the engine, the opening area of the air valve is large, and there is more air passing through the air valve,Engine idlingWill improve.In order to prevent this phenomenon, it is necessary toBimetallic sheetOr paraffin and engine cooling water maintain the same temperature, so the temperature detection part of the air valve is installed on the engine.
working principle
There are two kinds of air valves: one is bimetallic;The other is wax column type.The bimetallic type uses the hot wire coil to heat the bimetallic to generate displacement to drive the air valve.The wax column type uses the engine water temperature to expand and contract the wax column to drive the air valve.
structure
1. Bimetallic air valve
Figure 4 Bimetallic air valve and structure diagram
Figure 4 shows the structure of the bimetallic air valve.It consists of bimetal elements and heating coils. When the engine starts to run, the temperature is low, the air valve is open, and the air enters the intake manifold through the bypass channel of the throttle valve and the air valve.At this time, although the throttle is fully closed, the air intake volume is large, the idle speed is high, and the current flows into the heating coil, causing the temperature of the bimetal element to rise, causing deformation and slowly closing the valve, thus reducing the engine speed.After the engine warms up, the bimetallic elements are not only heated by the coil, but also by the heat of the engine, so that the valve is kept closed and the engine is in normal idle state.When the hot engine is restarted, the valve shall be kept closed to avoidEngine idlingThe speed is too high.
2. Wax column air valve
The wax column type air valve is installed in the throttle valve body, and its structure (as shown in Figure 5) and working principle are as follows:
Figure 5
The wax column type air valve controls the passage area of the air valve depending on the temperature change of the engine cooling water.The driving force source of control is the change of engine cooling water temperature, which makes the wax column expand and drive the valve.The wax column type air valve is composed ofTemperature regulating valve, valve springs A and B (as shown in Figure 6), and the temperature control valve is filled with paraffin.
Figure 6 Structure of wax pillar air valve with throttle valve
When the temperature is low, the temperature regulating valve will shrink and the valve will be opened by spring A to allow air to pass through the air inlet manifold of the air valve.When the temperature of cooling water rises, the temperature regulating valve expands, and the spring B extends to close the valve.Since spring B is harder than spring A, the valve is gradually closed, so that the engine speed slowly decreases to normal idle state.When the cooling water reaches 80 ℃, the valve is always closed and the engine runs at normal idle speed.ifengine cooling When the water temperature is higher than 80 ℃, the temperature regulating valve will continue to extend and compress the spring B to increase the spring force, and the valve will close tightly.[5]
Idle air valve
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The function of the idle air valve is to realize the fast idle of the engine in cold condition.During the warm up of the engine during cold start, the idle air valve is opened to allow some air to bypass the throttle valve through the idle air valve and bypass passage and enter the intake pipe behind the throttle valve directly, so as to ensure stable fast idle operation of the cold car.When the engine reaches normal temperature, the amount of additional air decreases with the gradual closing of the idle air valve until the idle air valve is completely closed.[6]
