Principle of valve sealing valve sealing those things! Valve packing gland and thread strength strength checking method
Valve seal performance requirements, to prevent leakage Angle. According to the different parts and degrees of leakage, the leakage of the valve is different, so it is necessary to put forward different leakage prevention measures. 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 main 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
Valve seal performance requirements, to prevent leakage Angle. According to the different parts and degrees of leakage, the leakage of the valve is different, so it is necessary to put forward different leakage prevention measures.
Principle of valve tightness
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 main 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.
Tightness of liquids
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.
Tightness of gas
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.
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, under the action of pressure, it will produce a certain degree of plastic deformation sealing effect.
If the sealing surface is metal material, then the uneven convex point of the surface will appear early, at the beginning of the need to use a small load can make these uneven convex point plastic deformation. When the contact surface increases, the surface unevenness will become plastic – elastic deformation. Then the roughness of the two surfaces in the concave place will exist.
These remaining paths can be made to fit when a load that causes severe plastic deformation of the underlying material is applied and the two surfaces are in close contact, along the continuous line and in the ring direction.
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. Such as wear particles.
If the wear particles are smaller than the surface roughness, the surface accuracy will be improved when the sealing surface is run in, and will not deteriorate. 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.
Main factors affecting valve seal
There are many factors affecting the valve seal, mainly the following:
Seal pair construction
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.
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.
The 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. Viscosity is related to the permeability of a fluid.
Under the same conditions, the greater the viscosity, the less permeable the fluid. Surface hydrophilicity means that when there is a thin film on the metal surface, the film should be removed. Because of this thin film of oil, it will destroy the hydrophilicity of the surface, leading to blockage of fluid channels.
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.
Valve leakage is common in the life and production, light can cause waste, or bring danger to life, such as tap water valve leak, or causing serious consequences, such as chemical industry of poisonous and harmful, inflammable, explosive and corrosive medium leakage in the nature of, serious threat to personal safety and property safety, and environmental pollution accidents.
A valve that depends on external force rotation drive to open and close is designed with a sealing device that is used in the packing culvert with a certain number of packing rings, so as to achieve the sealing effect, but what is the sealing situation?
Valve packing leakage is one of the most vulnerable parts of the valve leakage fault, but there are roughly two reasons.
Type of valve seal
Seals are also critical components in valves. The sealing performance of the valve refers to the ability of the sealing parts of the valve to prevent media leakage, it is the most important technical performance index of the valve.
There are three sealing parts of the valve:
The contact between the opening and closing parts and the sealing surface of the seat; The fit of packing and valve stem and stuffing box; Joint of body and bonnet. The FORMER leak IS called an ENDoleaker, commonly KNOWN as a loose CLOSE, AND WILL AFFECT THE VALVE’S ABILITY TO cut OFF THE medium. For the cut-off valve class, internal leakage is not allowed. The last two leakage is called leakage, that is, the medium leakage from the valve to the valve.
Leakage will cause material loss, pollution of the environment, and serious accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have a reliable sealing performance.
How to solve the sealing problem is not careless, the valve run, risk, drop, leakage phenomenon, most of the department happened here. Below we will consider the valve dynamic sealing, static sealing problem.
Dynamic seal
Valve dynamic seal, the main finger valve stem seal. Do not let the valve medium with the stem movement and leakage, is the valve dynamic seal central topic.
Packing box form: valve dynamic seal, mainly packing box. The basic form of stuffing box is:
1, gland type: this is in many forms.
Unified form can also distinguish many details. For example, in terms of compression bolts, separable T-bolts (for low pressure valves with pressure ≤16 kg/cm2), double head bolts and movable joint bolts, etc. From the gland, can be divided into integral and combined.
2, pressing nut type: this type of form, the external size is small, but the pressing force is limited, only used in small valves.
Packing: in the packing box, the packing is in direct contact with the valve stem and filled with the packing box to prevent the leakage of the medium. There are the following requirements for packing:
Good sealing;
Corrosion resistance;
Small friction coefficient;
Comply with medium temperature and pressure.
Post time: Aug-20-2022
