Causes and Treatments of Throttle Valve Vibration

The vibration of the Throttle Valve is generally divided into two states. One is the overall vibration of the Throttle Valve, that is, the entire Throttle Valve vibrates frequently on the pipe or base. The other is the vibration of the Throttle Valve core, which can be seen from the frequent movement of the Valve Stem up and down.

1. Overall vibration of the Throttle Valve

The reasons why the entire Throttle Valve vibrates on the pipeline are roughly as follows: the pipeline or base vibrates violently, which can easily cause the entire Throttle Valve to vibrate; 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, it is forced to vibrate. The energy reaches a maximum value and resonates. These two factors sometimes interact with each other, causing the vibration to become more intense, causing the pipe to jump, accessories or components to loosen, and make a rattling sound. In severe cases, the valve stem will break and the valve seat will fall off, causing the system to fail to work.

Based on this situation, each pipe and base that causes vibration should be reinforced, which will also help eliminate interference from external frequencies.

2. Valve core vibration. Sometimes the flow rate of the measured medium increases sharply, causing the differential pressure before and after the Throttle Valve to change sharply. When the stiffness of the valve is exceeded, the stability of the valve becomes worse, which will also cause serious oscillation of the entire Throttle Valve. But 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 Throttle Valve is poor. Once there is interference from internal or external unbalanced forces that exceed the stiffness of the Throttle Valve, and the Throttle Valve itself does not have the ability to eliminate this interference, oscillation will occur. At this time, it is necessary to increase the stiffness of the Throttle Valve, such as using a spring of 20~100KPa, or increasing its working stability, which will have certain benefits.

The Throttle Valve should be installed away from the source of vibration. If it is unavoidable, preventive measures should be taken. This kind of vibration of the entire Throttle Valve, before reaching resonance, the Throttle Valve can basically be adjusted according to the external given signal. Because the relative displacement of the valve core caused by the external given signal does not change or changes very little due to the vibration of the entire Throttle Valve, the reason is that they are a whole. The stop valves at both ends of the Throttle Valve are suddenly opened or closed, which will cause the rapidly flowing wave medium to produce a strong reflection shock wave, and the reflected wave will impact the Throttle Valve core.

When this force is greater than the downward pressure of the diaphragm on the valve core, it will cause the valve core to move upward and cause vibration. Especially in the case of small signal, because the preload force is small, it is easier for the valve core to vibrate. If the opening of the Throttle Valve is too small, the differential pressure before and after the Throttle Valve will be too large, so that the flow rate at the throttle port increases and the pressure decreases rapidly. If the pressure drops to the saturated vapor pressure of the liquid at this temperature, the liquid will vaporize, form flash evaporation, and generate bubbles. When the bubbles burst, a strong pressure and shock wave will be formed, resulting in an air hammer. This pressure can generally reach tens of MPa. The air hammer impacts the valve core, causing the valve core to form a honeycomb surface and causing the valve core to vibrate. The general reasons for valve core vibration are as follows: the regulator output signal is unstable. Rapid high and low changes. At this time, if the valve positioner sensitivity is too high, the small changes or drifts in the regulator output will immediately be converted into a large positioner output signal. causing the valve to oscillate.

The friction force of the Throttle Valve is too small. For example, the packing of the Throttle Valve is too little, or the gland is not tightened. If 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 emit a signal. A gurgling sound. On the contrary, if the friction of the Throttle Valve is too great, if the packing is too much, the gland is tightened too tightly, or the stuffing box is aged and dried up, it will not operate when the signal is small, and when the signal is large, it will cause another problem once it operates. The excessive phenomenon will cause hysteretic oscillation of the Throttle Valve, and the vibration curve will be approximately a square wave.

In this case, the damping of the corresponding part of the Throttle Valve should be reduced to solve the problem, such as replacing the packing. Fluctuations in the air source cause the positioner output to fluctuate, or the moving parts of the positioner are rusted and inflexible, causing the input and output signals to not correspond, resulting in jump oscillations. At this time, the cleaning positioner of the air source Pressure Reducing Valve should be opened and lubricating oil should be applied to the moving parts to eliminate friction.

Due to the unbalanced force of the Throttle Valve itself, the Throttle Valve core often oscillates. If the zero-point spring tightening force is too small, its ability to resist external interference will be small. When the external signal is small, the valve core will easily vibrate.

To sum up, according to the author`s diagnosis based on practical experience, in general, the impact of the oscillation of the valve core on the measured medium is always greater than the impact of the vibration of the entire Throttle Valve on the measured medium, and the causes and preventive measures of the valve core oscillation are always the same. It is more complicated than the causes and preventive measures of the entire Throttle Valve oscillation. 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.