Principle of valve sealing valve vacuum performance test and micro leakage test

Principle of valve sealing valve vacuum performance test and micro leakage test

The principle of valve sealing is to prevent leakage, so the principle of valve sealing is also studied from the prevention of leakage. 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 the pressure difference between the two sides of the sealing pair. Valve sealing principle is also from the liquid sealing, gas sealing, leakage channel sealing principle and valve sealing deputy four aspects
Principle of valve sealing
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 the pressure difference between the two sides of the sealing pair. The principle of valve sealing is also analyzed from the four aspects of liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pair.
1, liquid sealing
The sealing of the liquid is carried out by the viscosity and surface tension of the liquid. When the capillary from which the valve leaks is filled with gas, surface tension may repel the fluid or draw it into the capillary. So that’s the tangent Angle. When the tangent Angle is less than 90¡ÃƒÂ£, the liquid is injected into the capillary and leakage occurs. Leakage occurs because of the different properties of the medium. Under the same conditions, different results will be obtained when different media are used to do experiments. You can use water, air, kerosene, etc. Leakage also occurs when the tangent Angle is greater than 90¡ã. Because of the grease or waxy film on the metal surface. Once the film on these surfaces is dissolved, the properties of the metal surface change, and the liquid, which was previously repelled, penetrates the surface and leaks. In view of the above situation, according to Poisson’s formula, the purpose of preventing leakage or reducing the amount of leakage can be realized on the condition of reducing capillary diameter and medium viscosity.
2, gas sealing
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 is directly proportional to the diameter of the capillary and the driving force. When the diameter of the capillary is the same as the average degree of freedom of the gas molecules, 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 that 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 gas still cannot be stopped. The reason is that gases can still diffuse through the metal walls. So when we do the gas test, we must be more strict than the liquid test.
3. Sealing principle of leakage channel
The valve seal is composed of two parts: the roughness of the unevenness scattered on the waveform surface and the corrugation of the distance between the peaks. In the case that most of the elastic stress of metal materials in Our country is low, if we want to achieve the sealing state, we need to raise higher requirements on the compression force of metal materials, that is, the compressive force of the material should exceed its elasticity. 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 surface of the uneven convex point will appear early, in the first step only with a small load can make these uneven convex point plastic deformation. As the contact surface increases, the surface unflatness becomes plastic-elastic deformation. The roughness of both surfaces is present at the concave. These remaining sizes can be closed by applying a load that can cause severe plastic deformation of the underlying material and by keeping the two surfaces in close contact along a continuous line and in a circular direction.
4, valve sealing pair
The valve seal pair is the part where the seat and the closing part are closed when they contact each other. Metal sealing surface in the process of use, easy to be sandwiched medium, medium corrosion, wear particles, cavitation and erosion damage. Wear particles, for example. If the roughness of the wear particles is smaller than that of the surface, the surface accuracy will be improved rather than deteriorated when the sealing surface runs in. On the contrary, it will deteriorate the surface accuracy. Therefore, in the selection of wear particles, the material, working condition, lubricity and corrosion of sealing surface should be considered comprehensively. As wear particles, when we choose seals, we should consider the various factors affecting their performance, in order to play the function of anti-leakage. Therefore, it is necessary to choose materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any one of the requirements will reduce its sealing performance.
Valve vacuum performance test and micro leakage test Manual valve performance test: The valve is in the open state, the valve chamber is charged to the test pressure, the valve is closed with the specified torque, the pressure is reduced on one side of the valve disc to establish the pressure difference in the unfavorable and direction of opening the valve, and then the valve is opened with the specified torque, so as to carry out at least three times more complete load cycle action, To check whether the valve open and close operation is normal, whether the action is flexible, open and close position indication is correct; Check valve operation performance test, valve opening test under the specified differential pressure, the test number is not less than 3 times…
Performance test
Test medium with shell strength test and sealing test, in the shell strength test and sealing test qualified after.
Manual valve action performance test: The valve is in the open state, the valve chamber is charged to the test pressure, the valve is closed with the specified torque, the pressure is reduced on one side of the valve disc to establish the pressure difference in the unfavorable and direction of opening the valve, and then the valve is opened with the specified torque, so as to carry out at least three times more complete load cycle action, To check whether the valve open and close operation is normal, whether the action is flexible, open and close position indication is correct;
Check valve operation performance test, valve opening test under the specified pressure difference, the test number is not less than 3 times;
Electric and pneumatic valve actuation performance test, according to the provisions of the technical specification for the valve, the valve has no specific provision, technical specification should be rated operating valve actuator to complete the three full load cycles, throughout the trial, the valve must be smooth operation, flexible, the figure, the valve must be in place, position indicator must be correct.
Vacuum seal test
Vacuum sealing test (or vacuum leak detection) is a highly sensitive sealing test method. Aerospace and atomic energy industry valves and valves with high sealing requirements are generally vacuum sealed test.
Vacuum test is usually carried out after the valve strength and seal test is qualified. In order to ensure the accuracy of the test, the valve to be tested should have a high cleanliness and fine processing sealing surface. And the valve body, valve cover should generally be used forgings.
The usual method of vacuum seal test is helium mass spectrometry leak detection: the valve to be tested is pumped to the specified vacuum degree by vacuum pump, and helium gas is applied outside the measured part of the valve (helium cover method or helium spray method). If there is a leak, helium enters the measured part of the valve, which is shown by the helium mass spectrometer leak detector in the system, from which the leak rate can be calculated. Test equipment pressure test table blind plate
When pressure test, one side flange of the valve is pressed under the pressure test platform with bolts, and pressed from the bottom side, the upper side is observed to seal, or the upper side is sealed with blind plate, and the lower side is pressed to check strength. As the valve body at both ends of the test directly under the compression force and easy to cause sealing deformation, so as to affect the accuracy of the test. Therefore, the compression force should not be too large, under the premise of ensuring that the valve end face does not leak, the smaller the compression force, the better.
Microleakage test
In recent years, with the strengthening of people’s environmental awareness, various organizations in the world put forward more stringent requirements for the sealing of valves, especially for the use of media for strong corrosive, strong radiation, highly toxic. The valve’s requirements for fugitiveemission are one of these. Valve micro leakage detection (FETEST) is mainly to check the valve flange and packing box in the micro leakage degree, belongs to a valve shell sealing test.
The basic principle of valve microleakage detection is: when the valve is half-open and half-closed, helium gas with specified pressure is injected into the valve, and a helium mass spectrometry leak detector with suction probe with adjusted leakage rate is used to detect the cavity and stuffing box parts to see whether the parts meet the leakage rate specified by the user. The valve microleakage requirement is a kind of direction of valve development nowadays, this kind of microleakage requirement is very accord with the requirement of nuclear power valve.
Test equipment
Flip test table and hanging blind plate
When pressure test, one side flange of the valve is pressed under the pressure test platform with bolts, and pressed from the bottom side, the upper side is observed to seal, or the upper side is sealed with blind plate, and the lower side is pressed to check strength. As the valve body at both ends of the test directly under the compression force and easy to cause sealing deformation, so as to affect the accuracy of the test. Therefore, the compression force should not be too large, under the premise of ensuring that the valve end face does not leak, the smaller the compression force, the better.
Flip test table and hanging blind plate
The flange on the side of the valve is tensioned by three oil-pressed hook plates on the pressure test table, and the flange on the other side is sealed by the lifting cylinder against the blind plate for the strength test of the valve. Because both ends of the valve are not subjected to axial force, the sealing surface of the valve will not be deformed, so the test is more accurate.
Flange pump test plate
Butt welding end pump test head
Pre-installation inspection
Pressure testing machine
Press after valve is closed
The pressure does not drop within 10min
Rubber gaskets for flange connections
Lead plate for welding groove