Teach you how to see whether the leakage of pneumatic control valve is up to the standard in the valve vibration analysis and solution

Teach you how to see whether the leakage of pneumatic control valve is up to the standard in the valve vibration analysis and solution

Pneumatic control valve structure provides a good balance, and put an end to the possibility of medium leakage, pneumatic control valve sleeve guide, pressure balance type spool, suitable for large pressure difference occasions. The balance seal ring is used to replace the upper seat, so that the traditional sleeve two-seat valve structure into the sleeve single-seat structure, this improvement   improves the leakage level of the sleeve regulating valve. The valve cover is double sealed with bellows and packing, which fundamentally eliminates the possibility of medium leakage. This type of regulator is suitable for the flow pressure control of toxic or precious media.
So we want to detect the pneumatic control valve leakage is not up to standard, can be completed by some tests, the conclusions obtained after the test can also summarize the pneumatic control valve leakage standard.
Pneumatic control valve leakage standard test, the first need to test the pressure of the medium, such as the control valve pressure should be 3.5kg/cm? , when the allowable pressure difference of the regulator is less than 3.5kg/cm? This is the allowable pressure difference.
The test medium should be clean liquid (water or kerosene) and gas (air or nitrogen) at 10-50¡æ. Due to different working conditions, there will be error values in the test of other media, so it is recommended to use the recommended medium to complete the test. When testing the medium, the medium should be added into the valve in accordance with the prescribed flow direction. The valve outlet can be directly connected to the atmosphere or the outlet into the atmosphere of the low pressure loss measurement device, when the valve under the connection pipe part is completely filled with media, this time you can measure the leakage value of the valve.
Test pneumatic control valve actuator, actuator should add a certain signal pressure, must ensure that the regulator is closed state, in the test, for pneumatic actuator, gas open and gas close treatment is not the same, gas open signal pressure must be zero state, The type pressure of the gas close type shall be the upper pressure limit of the input large signal and the lower pressure value. When testing the actuator for the cut-off control valve, the signal pressure of the actuator shall be the pressure specified in the design.
The vibration analysis and solution of regulating valve in valve are introduced
First, the vibration and noise of the regulating valve can be roughly divided into mechanical vibration, cavitation vibration and fluid dynamic vibration and other reasons according to the different inducing factors.
1, fluid dynamic vibration: the medium in the valve throttling process is also the process of friction, resistance and disturbance. The vortex is formed when the turbulent body passes through the regulating valve around the fluid, and the vortex will fall off with the wake of the continuous flow of the fluid. The formation and influencing factors of the vortex shedding frequency are very complex and have great randomness, so the quantitative calculation is very difficult. However, there is a dominant shedding frequency objectively. When the dominant shedding frequency (also including high harmonics) is close to or consistent with the structural frequency of the regulating valve and its auxiliary devices, resonance occurs, and the regulating valve produces vibration, and accompanied by noise. The intensity of vibration depends on the intensity of the dominant shedding frequency and the consistency of the direction of the higher harmonic wave.
2. Mechanical vibration can be divided into two states according to its manifestations. A state is the overall vibration of the regulating valve, that is, the whole regulating valve in the pipeline or the base of frequent vibration, the reason is due to the pipeline or the base of violent vibration, causing the whole regulating valve vibration. In addition, it is also related to frequency, that is, when the external frequency is equal to or close to the natural frequency of the system, the energy of the forced vibration reaches a large value and produces resonance. The other state is the vibration of the regulating valve disc, the reason is mainly due to the rapid increase of the velocity of the medium, the difference pressure before and after the regulating valve changes sharply, causing the whole regulating valve to produce serious oscillation.
3. Cavitation vibration mostly occurs in the regulating valve of liquid medium. The root cause of cavitation is the liquid vaporization caused by the acceleration of fluid shrinkage and the drop of static pressure in the regulating valve. The smaller the opening of the regulator, the greater the pressure difference before and after the valve, the greater the possibility of fluid acceleration and cavitation, and the corresponding blocking flow pressure drop is smaller.
Second, the solution
1, increase the guide size, reduce the fit clearance method; The general guide size of the shaft plug valve is small, and the fit clearance of all valves is generally large, with 0.4 ~ lmm, which is helpful to produce mechanical vibration. Therefore, when slight mechanical vibration occurs, the vibration can be weakened by increasing the guide size and reducing the fit clearance.
2, change the shape of throttle parts, eliminate resonance method; Because the so-called vibration source of the regulating valve occurs in the high-speed flow, the pressure changes sharply at the throttling port, changing the shape of the throttling parts can change the frequency of the vibration source, when the resonance is not strong, it is easier to solve. The specific method is to turn the spool surface 0.5 ~ 1.0mm in the range of vibration opening.
3, increase the stiffness method; For oscillation and slight vibration, the stiffness can be increased to eliminate or weaken, such as the selection of spring stiffness, use piston actuator and other methods are feasible.
4, replace the type of regulating valve to eliminate resonance; The natural frequency of regulating valves with different structural forms is naturally different. Replacing the regulating valve type is an effective way to fundamentally eliminate resonance. A valve in the use of vibration is very severe and strong vibration (serious damage to the valve), strong rotation (even the stem is broken, broken), and produce a strong noise (up to more than 100 decibels) of the valve, as long as the replacement of it into a larger structure of the valve, immediately effective, strong resonance miraculously disappeared.
5, reduce cavitation vibration method; For cavitation vibration caused by cavitation bubble rupture, it is necessary to find ways to reduce cavitation. ¢Ù Let the impact energy generated by the bubble rupture not act on the solid surface, especially on the spool, but let the liquid absorb. Sleeve valves have this feature, so you can change the plug type spool to sleeve type. (2) All methods to reduce cavitation, such as increasing the throttle resistance, increasing the pressure of the inlet, grading or series decompression, etc.
6, avoid the vibration source wave strike method; External vibration source wave caused by valve vibration, which is obviously the normal operation of the valve should be avoided, if the vibration should be taken corresponding measures.
7, increase the damping method; That is to increase the friction to vibration, such as the sleeve valve plug can be sealed with O ring, the use of graphite filler with greater friction, which has a certain effect on eliminating or weakening slight vibration.
8, replace the throttle parts to eliminate the resonance method
(1) Change the flow characteristics, logarithm to linear, linear to logarithm;
¢Ú Replace the spool form.
For example, change the shaft plug shape to V-shaped groove spool, change the double seat valve shaft plug type to sleeve type; Change the sleeve for opening the window to the sleeve for punching holes, etc.