Valve drive mode selection, to learn the solution of valve leakage

Valve drive mode selection, to learn the solution of valve leakage

Valve drive mode selection is based on:
1) valve type, specification and structure.
2) the opening and closing moment of the valve (pipeline pressure, the relatively large pressure difference of the valve), thrust.
3) Compare high ambient temperature with fluid temperature.
4) Mode and frequency of use.
5) Opening and closing speed and time.
6) Stem diameter, screw moment, rotation direction.
7) Connection mode.
8) Power source parameters: electric power supply voltage, phase number, frequency; Pneumatic air source pressure; Hydraulic medium pressure.
9) Special consideration: low temperature, anti-corrosion, explosion-proof, waterproof, fire prevention, radiation protection, etc
Among all valve actuation devices, electric and film pneumatic devices are the most widely used. Electric devices are mainly used in closed circuit valves; Thin film pneumatic device is mainly used in the control valve. Electromagnetic drive is mainly used for small diameter valves. The embedded bellows drive is mainly used in disc stroke valves and corrosive and toxic media. But its range of use is often limited by the auxiliary pilot device that controls the main transmission.
A special requirement for valve actuation is the ability to limit torque or axial force. The valve electric device uses torque limiting couplings. In hydraulic and pneumatic drive devices, the relative force depends on the effective area of the diaphragm or piston and the pressure of the driving medium. A spring can also be used to limit the applied force.
Solutions to valve leaks
Valve leakage has become one of the main leakage sources in the device, so it is very important to improve the leakage prevention ability of the valve, prevent valve leakage, must master the basic knowledge of the valve sealing parts to prevent media leakage —— valve sealing, this is the top priority.
Sealing is to prevent leakage, so the principle of valve sealing is also from preventing leakage research. There are two main factors causing the leakage, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pair, the other is there is a pressure difference between the two sides of the sealing pair.
The principle of valve sealing is also from the liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pair and other four aspects to analyze.
1. Tightness of liquid
The tightness of a liquid is determined by its viscosity and surface tension. When the valve’s leaking capillary is filled with gas, surface tension may repel or draw liquid into the capillary. And that forms the tangent Angle. When the tangent Angle is less than 90°, the liquid is injected into the capillary tube, and leakage occurs.
The cause of leakage lies in the different properties of the medium. Experiment with different media, under the same condition, will get different results. You can use water, air, kerosene, etc. When the tangent Angle is greater than 90°, leakage will also occur.
Because of the relationship with the oil or wax film on the metal surface. Once these surface films are dissolved, the characteristics of the metal surface change, and the liquid, which was previously repelled, will wet the surface and leak. In view of the above situation, according to Poisson’s formula, the purpose of preventing leakage or reducing leakage can be realized under the condition of reducing capillary diameter and medium viscosity.
2. Gas tightness
According to Poisson’s formula, gas tightness is related to gas molecules and gas viscosity. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and proportional to the diameter of the capillary and the driving force. When the diameter of the capillary and the average degrees of freedom of the gas molecules are the same, the gas molecules will flow into the capillary with free thermal motion.
Therefore, when we do the valve sealing test, the medium must be water to play the role of sealing, with air or gas can not play the role of sealing.
Even if we reduce the capillary diameter below the gas molecule by plastic deformation, the flow of the gas still cannot be stopped. The reason is that gas can still diffuse through metal walls. So when we do the gas test, we have to be more rigorous than the liquid test.
3. Sealing principle of leakage channel
The valve seal is composed of two parts, roughness, which is composed of the roughness of the unevenness spread on the waveform surface and the waviness of the distance between the peaks. Under the condition that the elastic force of most metal materials is low in our country, we need to raise higher requirements for the compression force of metal materials, that is, the compression force of material should exceed its elasticity, if we want to achieve the sealing state. Therefore, in the design of the valve, the sealing pair combined with a certain hardness difference to match.
4. Valve sealing pair
The valve seal pair is the part of the valve seat and shutoff that closes when they are in contact with each other. The metal sealing surface is prone to damage from clamping media, media corrosion, wear particles, cavitation and erosion during use.
For example, wear particles, if the wear particles than the surface roughness is small, when the sealing surface is run in, the surface accuracy will be improved, and will not become bad. On the contrary, it will make the surface accuracy worse. Therefore, in the selection of wear particles, the material, working condition, lubricity and corrosion of the sealing surface should be considered comprehensively. As wear particles, when we choose seals, we should comprehensively consider various factors that affect their performance in order to play the function of leakage prevention. Therefore, materials that resist corrosion, abrasion and erosion must be selected. Otherwise, the lack of any one of the requirements will make its sealing performance ** reduced.
There are many factors affecting the valve seal, mainly the following:
1. Sealing accessory structure
Under the change of temperature or sealing force, the structure of the sealing pair will change. And this change will affect and change the sealing pair between the force, so that the performance of the valve seal is reduced.
Therefore, when choosing seals, we must choose seals with elastic deformation. At the same time, pay attention to the width of the sealing surface. The reason is that the contact surface of the sealing pair is not completely consistent. When the width of the sealing surface increases, it is necessary to increase the force required for sealing.
2. Specific pressure of sealing surface
The specific pressure of the sealing surface affects the sealing performance and the service life of the valve.
Therefore, the sealing surface pressure is also a very important factor. Under the same conditions, too much specific pressure will cause valve damage, but too little specific pressure will cause valve leakage. Therefore, we need to fully consider the specific pressure in the design of the appropriate.
3. Physical properties of the medium
The physical properties of the medium also affect the valve seal performance. These physical properties include temperature, viscosity, and surface hydrophilicity.
Temperature change not only affects the relaxation of the sealing pair and the size of the parts, but also has an inseparable relationship with the viscosity of the gas. The viscosity of gas increases or decreases with the increase or decrease of temperature.
Therefore, in order to reduce the impact of temperature on the sealing performance of the valve, we should design the sealing pair into a flexible seat and other valves with heat compensation.
4. Quality of sealing pair
Seal quality mainly refers to the selection of materials, matching, manufacturing accuracy on the check. For example, the disc fits well with the seat sealing face to improve tightness. The characteristic of more ring corrugations is that its labyrinth sealing performance is good.