Valve knowledge - how to solve the problem of control valve failure

In the event of a malfunction, focus on which parts to inspect

1. Valve body wall

For the control valve used in the occasion of high pressure difference and corrosive medium, the inner wall of the valve body is often affected and corroded by the medium, and the pressure resistance and corrosion resistance must be checked.

2 seats

When the control valve is working, due to the infiltration of the medium, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and loosens the valve seat, and attention should be paid to it during inspection. For valves working at high differential pressure, the sealing surface of the valve seat should also be checked to see if it is broken.

3 spools

The valve core is a movable part of the regulating valve when working, washed by the medium, corrosion is serious, and the valve core should be carefully checked whether the parts of the valve core are corroded and worn during maintenance, especially in the case of high pressure difference, the wear of the valve core is more serious, (due to cavitation) should be noted. The valve core should be replaced when it is severely damaged. In addition, it should be noted whether the stem has a similar phenomenon, or whether the connection with the valve core is loose.

4 "O" seals and other gaskets

Whether it is aged, cracked.

5 PTFE filler, sealing grease

Whether it is aged and whether the mating surface is damaged, it should be replaced if necessary.

What to do if the control valve leaks

1. Increase sealing grease

For valves that do not use sealing grease, consider adding sealing grease to improve stem sealing performance.

2. Add filler

In order to improve the sealing performance of the packing on the valve stem, the method of increasing the packing can be adopted. It is usually in the form of double-layer and multi-layer mixed fillers, and the effect is not obvious if the quantity is simply increased, such as 3 tablets to 5 tablets.

3. Replace the graphite filler

A large number of PTFE packing, because of its working temperature in the range of -20-+200 °C, when the temperature is in the upper and lower limits, when the change is large, its sealing will be significantly reduced, the aging is fast, and the life is short.

Flexible graphite fillers overcome these drawbacks and have a long service life. Therefore, some factories have changed all PTFE packing to graphite filler, and even the newly purchased control valve will also replace the PTFE packing with graphite filler and use it. However, the use of graphite filler has a large return difference, and there is a creeping phenomenon at the beginning, which must be considered.

4. Change the flow direction and place P2 at the stem end

When △P is large and P1 is large, it is obviously more difficult to seal P1 than P2. Therefore, the method of changing the flow direction can be adopted, and P1 can be changed from P1 at the stem end to P2 at the stem end, which is more effective for valves with high pressure and large pressure difference. Bellows valves, for example, should usually consider sealing P2.

5. Lens gasket sealing

For the sealing of the upper and lower covers, the sealing of the valve seat and the upper and lower valve bodies. If it is a plane seal, under high temperature and high pressure, the sealing performance is poor and causes leakage, so you can use lens gasket sealing instead, which can get satisfactory results.

6. Replace the sealing gasket

So far, most of the sealing gaskets are still made of asbestos plates, which have poor sealing performance and short life at high temperatures, causing leakage. In this case, you can use spiral wound gaskets, "O" rings, etc., which are now used by many factories.

7. Symmetrical screwing bolts, sealed with thin washers

In the control valve structure of the "O" ring seal, when a thick gasket (such as a winding sheet) with large deformation is used, if the compression is asymmetrical, the force is asymmetrical, and the seal is easily damaged, tilted and deformed, which seriously affects the sealing performance.

Therefore, in the maintenance and assembly of this type of valve, the hold-down bolts must be tightened symmetrically (note that they cannot be tightened at one time). It would be better if the thick gasket could be changed to a thin gasket, so that the inclination can be easily reduced and the sealing can be guaranteed.

8. Increase the width of the sealing surface

Flat type spool (such as two-position valve, sleeve valve plug), there is no guide and guide surface in the valve seat, because the valve is working, the valve core is subjected to lateral force, from the flow in to the outflow side, the larger the gap between the valve core, the more serious this unilateral phenomenon, plus the deformation, not centered, or the chamfer of the valve core sealing surface is small (generally 30 ° chamfer to guide), so when it is close to closing, the chamfered end face of the valve core sealing surface is placed on the sealing surface of the valve seat, resulting in the valve core beating when closing, or even not closed at all. The leakage of the valve is greatly increased.

The simple and effective solution is to increase the size of the sealing surface of the spool, so that the small diameter of the spool end face is 1-5mm smaller than the diameter of the valve seat, and there is enough guidance to ensure that the spool is guided into the valve seat and maintain good sealing surface contact

Control valve vibration, how to solve it

1. Increase stiffness

For oscillation and slight vibration, the stiffness can be increased to eliminate or weaken, such as the selection of large stiffness springs, the use of piston actuators and other methods are feasible.

2. Increase damping

Increasing the damping is to increase the friction of vibration, such as the valve plug of the sleeve valve can be sealed with an "O" ring, and the graphite packing with greater friction is used, which still has a certain effect on eliminating or weakening the slight vibration.

3. Increase the guide size and reduce the mating gap

The guide size of the shaft plug valve is generally small, and the matching clearance of all valves is generally large, 0.4-1mm, which is helpful for generating mechanical vibration. Therefore, in the event of minor mechanical vibrations, the vibrations can be attenuated by increasing the guide size and reducing the mating gap.

4. Change the shape of the throttle to eliminate resonance

Since the so-called vibration source of the control valve occurs at the throttle port where the pressure changes sharply at high speed, the frequency of the vibration source can be changed by changing the shape of the throttle, which is relatively easy to solve when the resonance is not strong.

The specific method is to turn the spool surface 0.5-1.0mm within the vibration opening range. For example, a self-operated pressure regulating valve is installed near the family area of a factory, and the resonance noise affects the rest of the workers due to the resonance noise, and the resonance howling disappears after the valve core curved surface is turned off 0.5mm.

5. Replace the throttle to eliminate resonance

Methods include:

Replace the flow characteristics, change the logarithm to linear, and linear to logarithmic;

Replace the spool form. For example, the shaft plug shape is changed to the "V" groove spool, and the double-seat valve shaft plug type is changed to the sleeve type;

Change the sleeve with the window opening to the sleeve with small holes, etc.

For example, a DN25 double-seat valve in a nitrogen fertilizer plant, the connection between the valve stem and the valve core is often broken, and after we confirm that it is resonance, we change the linear characteristic valve core to a logarithmic valve core, and the problem is solved. Another example is a DN200 sleeve valve used in an aviation laboratory, the valve plug produces a strong rotation and cannot be put into use, and the sleeve that opens the window is changed to a sleeve with a small hole, and the rotation disappears immediately.

6. Replace the control valve type to eliminate resonance

The natural frequency of different structural forms of control valves is naturally different, and changing the type of control valve is an effective way to fundamentally eliminate resonance.

A valve resonates very strongly in use - strong vibration (the valve can be destroyed in serious cases), strong rotation (even the valve stem is vibrated and torsional), and produces strong noise (up to more than 100 decibels) of the valve, as long as it is replaced with a valve with a large difference in structure, the effect will be immediately effective, and the strong resonance will miraculously disappear.

For example, a DN200 sleeve valve is selected for the new expansion project of a vinylon factory, and the above three phenomena exist, and the pipeline of DN300 beats out, the valve plug rotates, the noise is more than 100 decibels, and the resonance opening is 20-70%

Valve knowledge - how to solve the problem of control valve failure

Valve knowledge - how to solve the problem of control valve failure

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