Causes And Treatment Of Vibration Of Control Valve

- Mar 06, 2020-

Causes of vibration of regulating valve

The vibration of the control valve is generally divided into two states, one is the overall vibration of the control valve, that is, the whole control valve vibrates frequently on the pipeline or the base. The other is the vibration of the valve core of the regulating valve. It can be seen from the frequent movement of the valve stem up and down. The following analysis on the causes of these two kinds of vibration and the treatment measures are as follows:

1. Overall vibration of control valve

The reasons for the vibration of the whole control valve on the pipeline are as follows: the violent vibration of the pipeline or the base is easy to cause the vibration of the whole control valve; in addition, it is related to the frequency, that is, when the external frequency is equal to or close to the natural frequency of the system, the forced vibration energy reaches the maximum value and generates the common vibration. These two factors sometimes influence each other, which will make the vibration more and more intense, make the pipeline run out, the accessories or components loose, and make a clattering sound. In serious cases, it will cause the valve rod to break, the valve seat to fall off, and the system will not work. Based on this situation, the pipes and bases that cause vibration should be reinforced, which also helps to eliminate the interference of external frequency.

2. Sometimes, the flow velocity of the measured medium increases sharply, which makes the differential pressure between the front and back of the regulating valve change sharply. When the rigidity of the valve is exceeded, the stability of the valve becomes worse, which also causes serious vibration of the whole regulating valve. But this kind of oscillation is not necessarily caused by the small opening of the valve. This vibration is usually accompanied by shrieks. The stability of the regulating valve is poor. Once the interference of internal or external unbalanced force exceeds the rigidity of the regulating valve, and the regulating valve itself does not have the ability to eliminate this interference, it will produce oscillation. At this time, it is necessary to increase the stiffness of the regulating valve, for example, to increase the spring of 20-100kpa, or to increase the stability of its operation, which has certain advantages.

The installation position of the regulating valve shall be far away from the vibration source. If it is unavoidable, preventive measures shall be taken. The vibration of the whole control valve can be basically adjusted with the external given signal before reaching the resonance. Because the relative displacement of the external given signal to the valve core does not change or change very little due to the vibration of the whole regulating valve, the reason is that they are a whole. When the stop valves at both ends of the regulating valve are opened or closed violently, the sharp flowing wave measuring medium will produce strong reflection shock wave, which will impact the regulating valve core. When this force is greater than the downward pressure of the diaphragm on the valve core, it will make the valve core move up and generate vibration, especially in the case of small signal, because the preload is small, it is easier to make the valve core vibrate. When the opening of the regulating valve is too small, the differential pressure between the front and back of the regulating valve is too large, so that the flow rate at the throttle increases and the pressure decreases rapidly. If the pressure drops to the saturated vapor pressure of the liquid at this temperature, the liquid will be gasified, flash will be formed, bubbles will be formed, strong pressure and shock wave will be formed when the bubbles break, and air hammer will be produced. This pressure can generally reach tens of megapascals. The air hammer impacts the valve core to form honeycomb and strong pitted surface and vibrate the valve core. Generally, the causes of valve core vibration are as follows: the output signal of regulator is unstable. If the sensitivity of the valve positioner is too high, the output of the regulator will change slightly or drift, and the output signal of the positioner will be very large immediately. Cause the valve to oscillate.

The friction force of the regulating valve is too small, for example, the packing of the regulating valve is installed too little, or the gland is not tightened, and the external input signal has a small change or drift, which will be immediately transmitted to the valve core to make the valve core vibrate and emit a clucking sound. On the contrary, if the friction force of the regulating valve is too large, such as too much packing, too tight gland, aging stuffing box and drying up, it will not act when the signal is small, and it will produce excessive phenomenon when the signal is large, which will cause the regulating valve to produce sluggish oscillation, and the vibration curve is approximately square wave. In this case, the damping of the corresponding part of the regulating valve should be reduced to solve the problem, such as replacing the packing, etc. The fluctuation of air source makes the output of the positioner fluctuate, or the movable part of the positioner rusted and inflexible, which makes the input and output signals not corresponding, resulting in jumping oscillation. At this time, open the cleaning positioner of the air source pressure reducing valve, and apply lubricating oil to the movable part to eliminate the friction force. As a result of the unbalanced force of the regulating valve itself, the regulating valve core often vibrates. When the tightening force of zero spring is too small, the ability to resist external interference will be small. When the external signal is small, it is easy to make the valve core vibrate. To sum up, according to the practical experience, the author diagnosed that in general, the impact of valve core oscillation on the measured medium is always greater than that of the whole control valve vibration on the measured medium, and the causes of valve core oscillation and preventive measures are always more complex than that of the whole control valve oscillation and preventive measures. In practice, it can be seen that the causes of these two vibrations can not be so clear, and sometimes they are mixed and interwoven

The vibration and noise of the control valve can be roughly divided into mechanical vibration, cavitation vibration and hydrodynamic vibration according to their induced factors.

1. Mechanical vibration

Mechanical vibration can be divided into two states according to its manifestation. One state is the whole vibration of the control valve, that is, the whole control valve vibrates frequently on the pipeline or the base. The reason is that the vibration of the whole control valve is caused by the violent vibration of the pipeline or the base. In addition, it is also related to the frequency, that is, when the external frequency is equal to or close to the natural frequency of the system, the forced vibration energy reaches the maximum value and generates resonance. Another state is the vibration of the regulating valve disc, which is mainly caused by the sharp increase of the medium flow rate, which causes the sharp change of the differential pressure before and after the regulating valve and causes the serious vibration of the whole regulating valve.

2. Cavitation vibration

Cavitation vibration mostly occurs in the control valve of liquid medium. The basic cause of cavitation is the liquid vaporization caused by the acceleration of fluid contraction and the decrease of static pressure in the control valve. The smaller the opening of the control valve is, the greater the pressure difference between the front and the back, the greater the possibility of fluid acceleration and cavitation, and the smaller the pressure drop of the corresponding blocking flow.

3. Hydrodynamic vibration

The throttling process of medium in the valve is also the process of friction, resistance and disturbance. When the turbulent fluid passes through the regulating valve which is not good around the fluid, it forms a vortex, which will fall off with the continuous flow of the fluid. The formation and influencing factors of the vortex shedding frequency are very complex and random, so it is very difficult to calculate quantitatively, but there is a dominant shedding frequency objectively. When this dominant frequency (including high-order harmonics) is close to or consistent with the structural frequency of the control valve and its accessories, resonance occurs, and the control valve produces vibration, accompanied by noise. The strength of vibration depends on the strength of dominant shedding frequency and the consistency of the direction of higher harmonic wave.

Butterfly-valve

Treatment of vibration of regulating valve

1. Increase stiffness

For oscillation and slight vibration, the stiffness can be increased to eliminate or weaken. For example, it is feasible to select spring with large stiffness and use piston actuator instead.

2. Increase damping

Increasing the damping means increasing the friction to the vibration. For example, the plug of sleeve valve can be sealed with "O" ring, and graphite packing with larger friction force can be used, which has certain effect on eliminating or weakening the slight vibration.

3. Increase guide size and decrease fit clearance

Generally, the guide dimension of the axial plug valve is small, and the fit clearance of all valves is generally large, 0.4-1mm, which is helpful to produce mechanical vibration. Therefore, in case of slight mechanical vibration, the vibration can be weakened by increasing the guide size and reducing the fit clearance.

4. Change the shape of throttle to eliminate resonance

Because the so-called vibration source of the regulating valve occurs in the orifice with high-speed flow and rapid pressure change, changing the shape of the throttle can change the frequency of the vibration source, which is easier to solve when the resonance is not strong.

The specific method is to turn the surface of valve core within the range of vibration opening by 0.5-1.0mm. For example, a self operated pressure regulating valve is installed near the family area of a factory. The squeal caused by resonance affects the rest of the staff. After the valve core curved surface is dropped 0.5mm, the squeal disappears.

5. Replace throttle to eliminate resonance

The methods are as follows:

Change flow characteristics, change logarithm to linear, linear to logarithm;

Change the valve element form. If the plug shape is changed to "V" groove valve core, the plug type of double seat valve is changed to sleeve type;

Change the sleeve with a window to the sleeve with a hole.

For example, a DN25 double seat valve in a nitrogenous fertilizer plant often vibrates and breaks at the connection between the valve stem and the valve core. After we confirmed that it is resonant, we changed the linear characteristic valve core to logarithmic valve core, and the problem was solved. Another example is a DN200 sleeve valve used in the laboratory of an aeronautical college. The valve plug has strong rotation and cannot be put into use. After changing the sleeve with open window to the sleeve with small hole, the rotation disappears immediately.

6. Method of reducing cavitation vibration

For cavitation vibration caused by cavitation bubble rupture, it is natural to find a way to reduce cavitation.

The impact energy generated by bubble rupture does not act on the solid surface, especially on the valve core, but allows the liquid to absorb. The sleeve valve has this characteristic, so the plug type spool can be changed to sleeve type.

All measures to reduce cavitation are taken, such as increasing throttling resistance, increasing the pressure at the necking port, grading or series decompression, etc.

7. Avoiding vibration wave attack

The vibration of the valve caused by the wave strike of external vibration source is obviously what should be avoided during the normal operation of the regulating valve. If this vibration occurs, corresponding measures should be taken.