In production, we sometimes find that regulators, pressure relief valves and other throttles generate severe vibration and noise. In fact, the vibration and noise generated at the same time, the valve spool, valve seat and other internal parts were severely washed, resulting in a road wear marks, deep grooves and pits, and some even cause the valve stem fracture, which have a serious impact Valve performance, reducing the service life.
Control valve vibration and noise according to its different evoked factors can be divided into mechanical vibration, cavitation vibration and fluid (vortex) vibration and other reasons.
Causes vibration and noise
Cavitation vibration
Cavitation vibration generally occurs in the liquid medium control valve. The fundamental cause of cavitation is that the liquid in the regulating valve is accelerated by the systolic flow and the static pressure is decreased. The smaller the opening of the control valve is, the larger the pressure difference between the front and the back is. The more the fluid accelerates and cavitation is generated, and the corresponding pressure drop of the blocking flow is smaller.
Mechanical vibration
Mechanical vibration according to its manifestations can be divided into two states. One state is the overall vibration of the regulating valve, ie the entire regulating valve vibrates frequently on the pipe or base, due to violent vibrations of the pipe or base causing the entire regulating valve to vibrate. 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 energy of the forced vibration reaches a maximum value and resonance occurs. Another state is the valve flap vibration, the reason is mainly due to the rapid increase of the medium flow rate, the rapid change before and after the control valve pressure, causing the entire control valve to produce severe oscillations.
Vortex vibration
The fluid is throttled in the valve and due to friction, drag and various disturbances, it inevitably produces a wide range of eddy currents, such as fluid striking a valve stem, passing through gaps, cornering when turning, At the time of diversion, whirlpool flows are generated, and the whirlpool flows interact with the cylinder to induce vibration and generate vortex detached sound. Once the excitation frequency of the gas flow is coupled with the natural frequency of the mechanical element or with the longitudinal gas column standing wave in the pipe, lateral air column oscillation, thermal shock, gas-dynamic compression or other unsteady flow When the vibration increases, the noise increases. If the fluid flows through the control valve to produce flash, a gas-liquid two-phase mixture exists, and the deceleration and expansion of the two-phase fluid will also form noise. In addition, cavitation, the bubble rupture release powerful energy, will produce up to 10000Hz noise, the more the bubble, the more serious the noise.
How to deal with vibration and noise
For cavitation
First of all, should avoid the work of small openings. The opening of the control valve is too small, resulting in increased velocity at the orifice, the pressure decreases rapidly, the fluid flow through the valve is easy to form flash and cavitation. Yijun Jun in this article, "click here" mentioned the small valve opening caused by cavitation damage, we must not ignore.
Second, multi-level distribution pressure drop should be used. To prevent cavitation the most effective way is to make the pressure drop at all levels within the valve are less than the minimum pressure difference occurs cavitation, the critical pressure. When the control valve to withstand the pressure is far greater than the critical pressure, multi-stage structure can be used to reduce pressure. In the design of multi-stage throttling control valve, so that each level of throttle to withstand the pressure difference is less than the allowable pressure, so that each level of energy consumption, making the next level of population pressure is relatively low, reducing The next level of pressure, low pressure recovery, this can reduce the throttle flow rate, to avoid cavitation and reduce the role of cavitation. Of course, if the condition system is not suitable for multi-stage decompression structure, can also use the structure of throttling sleeve.
Finally, you should plan a reasonable driving process. Driving process at the production site is critical to the use of the regulating valve, especially for regulating valves with high differential pressures before and after work pressure.
For mechanical vibration
First of all should be the correct choice of components. If the valve flap changes rapidly, the valve positioner sensitivity is too high, the regulator output small changes or drift, it will immediately converted into a large output signal locator, resulting in valve oscillation. Control valve friction is too small, the external input signal changes or drift slightly, it will be passed to the valve flap, make it vibrate. On the contrary, if the control valve friction is too large, then the action can not be a small signal, the signal is generated by the action of a large phenomenon, will make the control valve hysteresis oscillation. In this case, should reduce the damping of the corresponding part of the control valve to solve, such as the replacement of packing.
Second, pay attention to the valve stem connection. During the normal operation of some process units, the high temperature and high pressure steam continues to pass through the high pressure regulating valve spool, causing the high pressure regulating valve stem and the torque generated between the valve stems, so as to cut the screw pin, Of the reasons that the cylindrical pin was severely affected until the fracture damage, high-pressure regulating valve stem off, threatening the unit safety, if repairs will leave a great potential safety hazard.
Finally, the regulator valve should be installed away from the vibration source. If it is unavoidable, precautionary measures should be taken.
For eddy currents
For eddy currents, first use a well-spaced, small detour access trim. As the fluid flows through the small-bore sleeve or other bypass path with appropriate spacing, a smaller jet flow volume is achieved, which in turn reduces the vortex volume, reduces the conversion efficiency between mechanical energy and acoustic energy, and effectively reduces Vibration and noise. At the same time, smaller vortices move the acoustical energy produced by the liquid to a higher frequency band, the wall of the tube has a good attenuation of the noise in the higher frequency band and the human ear has a lower response to the high frequency sound effect.
The use of stepped trim also reduces vibration and noise. Due to the bending of the stepped path, the fluid flow is sluggish, resulting in the friction in the flow process, resulting in greater pressure loss and consumption of fluid energy, so as to achieve the purpose of reducing vibration and noise.
Other methods
Silencers and wall thickness can also be used to reduce noise and vibration.
Silencer is directly installed in the control valve of the downstream part, and the control valve connected in series, can be used to absorb the sound energy of the control valve in the case of high flow rate, low pressure drop, better reflect its economic characteristics of noise control, In general, it absorbs noise up to 25 dB at right.
Increasing the wall thickness of the pipe downstream of the control valve can effectively reduce the vibration and noise of the control valve. However, the noise will not be weakened by the distance in the pipeline. Therefore, all pipe systems downstream of the control valve must use the same pipe thickness.
