Production process automation is an important means to ensure efficiency and quality in large-scale industrial production. In the automation of production processes, regulating valves used to control fluid flow capacity have been widely used in industrial automation systems in industries such as petroleum, chemical, power plants, light industry, papermaking, pharmaceuticals, shipbuilding, and municipal administration. Regulating valves play a crucial role in stabilizing production, optimizing control, maintaining and maintaining cost control. Therefore, how to select and apply the regulating valve well to operate at a high level is a key issue. The following mainly analyzes and explores the issues of flash evaporation, cavitation, blockage prevention, and noise in the control valve.

Flash and cavitation of regulating valves

Cavitation is a hydraulic flow phenomenon, and the direct cause of cavitation is flash evaporation and cavitation caused by sudden changes in resistance of pipeline fluids. When the fluid flows through the throttle port of the regulating valve, the flow rate suddenly increases sharply. According to the law of conservation of fluid energy, the static pressure suddenly decreases as the flow rate increases. When the OUTLET PRESSURE reaches or is lower than the saturated vapor pressure of the fluid, some of the liquid vaporizes into gas, forming small bubbles of steam and gas mixture. The phenomenon of gas-liquid two-phase coexistence is the formation of flash evaporation. If the downstream pressure recovers to a saturated steam pressure higher than the liquid, the steam bubble rapidly condenses and ruptures under the action of high pressure. At the moment of the steam bubble rupture, an impact force is formed, which collides with the valve core, valve seat, and valve body, causing plastic deformation on their surface, forming rough honeycomb slag holes. This phenomenon is called cavitation, which is the process of cavitation formation. Therefore, cavitation phenomenon will lead to serious noise, vibration, material damage, etc.

1.1 Selection

(1) Select valves with small pressure coefficient of restitution

If process conditions permit, try to select valves with small pressure coefficient of restitution, such as ball valves, butterfly valves, etc. If the process conditions require the pressure difference △ P of the regulating valve to be greater than △ PT (the critical pressure difference that causes cavitation), two regulating valves can be connected in series, so that the pressure difference △ P of each regulating valve is less than △ PT, and cavitation will not occur. If the pressure difference △ P of the valve is less than 2.5MPa, cavitation generally does not occur, and even if cavitation occurs, it will not cause serious damage to the material.

(2) Select angle control valve

Due to the direct flow of the medium in the angle valve to the downstream pipeline inside the valve body, rather than directly impacting the body wall, the number of saturated bubbles that impact the body wall can be greatly reduced, thereby weakening the flash destructive force.

1.2 Material's anti cavitation performance

From the direct result of cavitation erosion, it can be seen that the damage is caused by the insufficient hardness of the material to resist the impact force released by bubble rupture. Therefore, from this perspective, we can consider using high hardness materials. The commonly used method is to weld or spray weld Stellite alloy on a stainless steel substrate, forming a hardened surface at the site of fluid cavitation erosion. When damage occurs to the hardened surface, secondary surfacing or spray welding can be carried out, which can increase equipment usage and reduce maintenance costs for the enterprise.

1.3 Control Valve Structure

Since cavitation is caused by sudden changes in pressure, and the required pressure drop in the system cannot be achieved, a multi-stage valve core structure can be used to decompose a large pressure change into several times. This structure of valve core can divide the total pressure difference into several small pressure differences, gradually reducing pressure, so that each stage does not exceed the critical pressure difference. Or valve cores and valve seats designed with special structures, such as labyrinth valve cores and laminated valve cores, can make the pressure of high-speed fluid at every point when passing through the valve core and valve seat higher than the saturated vapor pressure at that temperature, or cause the liquid itself to collide with each other, causing high turbulence between channels, causing the kinetic energy of the liquid to become thermal energy due to mutual friction, which can reduce the formation of bubbles.

1.4 Cavitation coefficient

Valves with different structural forms have different cavitation coefficients, and the calculation formula is as follows:

In the formula: H1- downstream (outlet) pressure m;

H2- the difference in saturated vapor pressure corresponding to atmospheric pressure and temperature, m;

△ P - The pressure difference m between the front and rear of the valve.

The allowable cavitation coefficient of various valves due to their different constructions δ  It is also different. If the calculated cavitation coefficient is greater than the allowable cavitation coefficient, it indicates that it is available and cavitation will not occur. If the allowable cavitation coefficient of the butterfly valve is 2.5, then:

When δ＞ 2.5, then cavitation will not occur.

When 2.5> δ＞ At 1.5 hours, slight cavitation will occur.

When δ＜ At 1.5 hours, vibration occurs.

When δ＜ When the condition of 0.5 continues to be used, it will damage the valve and downstream piping. The basic characteristic curve and characteristic curve of the valve cannot be seen when cavitation occurs, let alone at which point it is reached. Through the above calculation, it is clear at a glance. From the above calculation, it is not difficult to see that there is a great relationship between cavitation and the pressure H1 behind the valve. H1 will obviously change the situation, and improvement methods are as follows:

1. Install the valve at the lower point of the pipeline.

2. Install an orifice plate on the pipeline behind the valve to increase resistance.

3. The valve outlet is open, directly storing water in the reservoir, increasing the space for bubbles to burst and reducing cavitation erosion

Blockage of the regulating valve

Valve blockage is one of the common faults in applications such as mud, pulp, mineral pulp, and caustic soda. In addition to clogging caused by unclean media, welding slag, iron filings, etc. in the pipeline can also cause valve clogging. Therefore, the selection of regulating valves under these working conditions must consider the anti clogging function of different valve types. Generally, the following aspects should be considered:

(1) The smoother the flow path, the smoother the transition;

(2) According to calculations, if necessary, the valve seat diameter should be reduced to increase the throttling speed to improve the "self-cleaning" performance;

(3) An actuator with sufficient stiffness and thrust (torque);

(4) Angular stroke valves are much better than straight stroke valves. Angular stroke valves overcome the problems of complex flow paths and easy blockage in the upper and lower directions of straight stroke valves. When the medium flows through angular stroke valves, it seems to flow directly in and out. A typical "O" type ball valve, like a straight pipeline, has good anti blockage performance; Secondly, there are fully functional ultra light valves, butterfly valves, etc.

The noise of the regulating valve

The noise on the regulating valve is the main source of pollution in petrochemical production. The prevention of regulating valve noise should be done from three aspects.

1. Noise generated by vibration

The noise generated by vibration generally comes from the vibration of the valve core. When the valve core moves horizontally within the sleeve, the gap between the valve core and the sleeve can be minimized or a hard surface sleeve can be used. If the valve core or other components have a natural vibration frequency, the characteristics of the valve core can be changed through specialized casting or forging treatment. If necessary, other types of valve cores can also be replaced. If noise is generated due to fluid pressure fluctuations caused by oscillating displacement of the valve core, this situation is generally caused by damping factors such as adjusting the circuit actuator. Therefore, the damping coefficient can be readjusted or vibration reduction facilities can be added in the direction of valve core displacement.

2. Aerodynamic voice generated by high-speed airflow

There are currently three methods to avoid aerodynamic noise. Firstly, the noise source should be controlled to limit the fluid velocity passing through the regulating valve; Secondly, a special structure of the valve body is used to gradually decelerate the fluid through the tortuous flow path of the valve core and valve seat; Thirdly, a multi hole flow limiting plate should be used, which absorbs the partial pressure drop behind the regulating valve, thereby achieving noise reduction by regulating the flow rate of the valve.

3. Preventing hydrodynamic noise

At the same time as cavitation occurs, noise and vibration also occur. This type of noise, also known as hydrodynamic noise, how to avoid cavitation phenomenon has been explained earlier.

In short, the selection of regulating valves should be tailored to local conditions, and it is necessary to continuously summarize and innovate in the process of practice, so as to better control the adjusted parameters and greatly increase the use of regulating valves.