Fisher Fisher - Why do control valves vibrate? Cause analysis

The vibration of the control valve is generally divided into two states, one is the overall vibration of the control valve, that is, the entire control valve frequently vibrates on the pipeline or base. The other is the vibration of the spool of the regulating valve, which can be seen from the frequent movement of the valve stem up and down, and the following are the causes of these two vibrations and their treatment

The analysis of the measures is as follows:

1. The overall vibration of the regulating valve

The reasons for the vibration of the whole control valve on the pipeline are roughly as follows: the pipeline or the base vibrates violently, which is easy to cause the vibration of the whole control valve; 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 maximum value and produces resonance. These two factors sometimes affect each other, which will make the vibration more vibrate, make the pipe runout, loose accessories or components, and make a clattering sound, and in serious cases, it will also cause the valve stem to break, the valve seat to fall off, and the system can not work. In this case, the pipes and bases that cause vibrations should be reinforced, which also helps to eliminate interference from extraneous frequencies.

2. The vibration of the valve core sometimes increases sharply in the flow rate of the measured medium, which makes the differential pressure before and after the regulating valve change sharply, and when the stiffness of the valve is exceeded, the stability of the valve becomes worse, which will also cause the whole regulating valve to produce serious oscillation. However, this oscillation is not necessarily caused by the small opening of the valve. This vibration is usually accompanied by a piercing scream. The stability of the control valve is poor, and once there is interference from internal or external unbalanced forces and the stiffness of the control valve is exceeded, and the control valve itself does not have the ability to eliminate this interference, oscillation occurs. At this time, it is necessary to increase the stiffness of the control valve, such as the spring of 20-100KPa, or increase the stability of its work, which has certain benefits.

The control valve should be installed away from the source of vibration, and precautions should be taken if unavoidable. This vibration of the entire control valve can basically be adjusted with a given signal even before the resonance is reached. Because the relative displacement of the spool by a given signal does not change or changes very little due to the vibration of the entire control valve, the reason is that they are a whole. The shut-off valves at both ends of the regulating valve are opened or closed suddenly, which will cause the rapid flow of wave measurement medium to produce a strong reflected impulse wave, and the reflected wave will impact the regulating valve core. When this force is greater than the downward pressure of the diaphragm on the spool, it will cause the spool to move upwards and produce vibrations, especially in the case of small signals, because the preload force is smaller, it is easier to make the spool flutter. The opening of the control valve is too small, so that the differential pressure between the front and back of the control 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 that temperature, the liquid can be vaporized, flash evaporation can be formed, bubbles are generated, and strong pressure and shock waves are formed when the bubbles burst, and air hammers are generated, which can generally reach tens of megapascals. The air hammer strikes the spool, causing the spool to form a honeycomb surface and vibrating the spool. Generally, the cause of spool vibration is roughly as follows: the output signal of the regulator is unstable. Rapid high and low changes, at this time, if the sensitivity of the valve positioner is too high, the regulator output small changes or drifts, will be immediately converted into a large output signal of the positioner. causes the valve to oscillate.

If the friction force of the control valve is too small, if the packing of the control valve is too small, or the gland is not tightened, and there is a slight change or drift in the external input signal, it will be immediately transmitted to the valve core, causing the valve core to vibrate and make a gurgling sound. On the contrary, if the friction force of the regulating valve is too large, if the packing is too much, the gland is too tight, or the packing box is aged and dried up, it will not be able to act when the signal is small, and the phenomenon of over-heading will be produced once the signal is large, which will cause the regulating valve to produce hysteresis oscillation, and the vibration curve will be approximately a square wave. In this case, the damping of the corresponding part of the control valve should be reduced, such as replacing the packing. The fluctuation of the air source makes the output of the positioner fluctuate, or the moving part of the positioner is corroded and inflexible, so that the input and output signals do not correspond, resulting in a jumping oscillation. At this time, the cleaning positioner of the air source pressure reducing valve should be opened and the movable part should be lubricated to eliminate friction. As a result of the unbalanced force of the control valve itself, the control spool often oscillates. If the zero point spring clamping force is too small, the ability to resist external interference is small, and it is easy to vibrate the valve core in the case of small external signal. To sum up, according to the author's diagnosis of practical experience, under normal circumstances, the influence of the oscillation of the valve core on the measured medium is always greater than that of the vibration of the entire control valve on the measured medium, and the causes and preventive measures of the oscillation of the valve core are always more complex than those of the entire control valve. In practice, it can be seen that the causes of these two vibrations cannot be so clearly distinguished, and sometimes they are mixed and intertwined

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

1 Mechanical vibration

Mechanical vibration can be divided into two states according to its manifestation. One state is the overall vibration of the control valve, that is, the entire control valve frequently vibrates on the pipeline or base, and the reason is that the whole control valve vibrates due to the violent vibration of the pipe or base. 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 maximum value and produces resonance. The other state is the vibration of the disc of the regulating valve, which is mainly due to the sharp increase in the flow rate of the medium, which makes the differential pressure before and after the regulating valve change sharply, causing serious oscillation of the entire regulating valve.

2 Cavitation vibration

Cavitation vibrations mostly occur in the regulating valve of a liquid medium. The root cause of cavitation is the vaporization of the liquid caused by the acceleration of fluid shrinkage and the drop in static pressure in the regulating valve. The smaller the opening of the control valve, the greater the pressure difference between the front and back of the valve, 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 the medium in the valve is also the process of its friction, resistance and disturbance. A vortex is formed when a turbulent fluid passes through a regulating valve that poorly winds around the fluid, and the vortex falls off as the fluid continues to flow in the wake of the fluid. The formation and influencing factors of the vortex shedding frequency are very complex, and there is a great randomness, and it is very difficult to quantitatively calculate, but there is a dominant shedding frequency objectively. When this dominant shedding frequency (including higher harmonics) resonates when it is close to or consistent with the structural frequency of the control valve and its accessories, the control valve vibrates and is accompanied by noise. The intensity of the vibration depends on the strength of the dominant shedding frequency and the degree of consistency in the direction of the fluctuation of the higher harmonics.

Fisher Fisher - Why do control valves vibrate? Cause analysis

Fisher Fisher - Why do control valves vibrate? Cause analysis

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