22

2020

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04

The uses and characteristics of pneumatic control valves! Worth a look!

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Pneumatic control valve is one of the industrial process control instruments widely used by industrial enterprises such as petroleum, chemical industry, electric power, and metallurgy. According to the working principle of the pneumatic control valve, compressed air is used as the power source, the cylinder is used as the actuator, and the valve is driven with the help of electrical valve positioners, converters, solenoid valves, positioning valves and other accessories to achieve switching or Proportional adjustment receives control signals from the industrial automation control system to adjust various process parameters of the pipeline medium: flow, pressure, temperature, etc. The characteristics of pneumatic control valves are simple control, fast response, and intrinsic safety, without the need for additional explosion-proof measures.

Pneumatic control valve is one of the industrial process control instruments widely used by industrial enterprises such as petroleum, chemical industry, electric power, and metallurgy. According to the working principle of the pneumatic control valve, compressed air is used as the power source, the cylinder is used as the actuator, and the valve is driven with the help of electrical valve positioners, converters, solenoid valves, positioning valves and other accessories to achieve switching or Proportional adjustment receives control signals from the industrial automation control system to adjust various process parameters of the pipeline medium: flow, pressure, temperature, etc. The characteristics of pneumatic control valves are simple control, fast response, and intrinsic safety, without the need for additional explosion-proof measures.

 

1. What are the characteristics and uses of pneumatic control valves in the manual of pneumatic control valves?

 

1. Purpose of pneumatic control valve : It is a right-angle reversing structure, which is used in conjunction with the valve positioner to achieve proportional adjustment ; the V-shaped valve core is most suitable for various adjustment occasions, with a large rated flow coefficient and a large adjustable ratio , good sealing effect, flexible adjustment function, small size, and can be installed vertically or horizontally. Suitable for controlling gas, steam, liquid and other media.

2. Features of the pneumatic control valve : It is a right-angle reversing structure, consisting of a V-shaped valve body, a pneumatic actuator, a positioner and other accessories; it has an inherent flow characteristic of approximately equal proportions; it adopts a double-bearing structure and has a starting torque of Small, with excellent sensitivity and sensing speed.

3. The pneumatic piston actuator uses compressed air as the power source, and the movement of the piston drives the crank arm to rotate 90 degrees to automatically open and close the valve. Its components are: adjusting bolt, actuator box, crank arm, cylinder block, cylinder shaft, piston, connecting rod, and universal shaft.

4. Working principle of pneumatic regulating valve : Pneumatic regulating valve is composed of an actuator and a regulating mechanism. The actuator is the thrust component of the regulating valve. It generates corresponding thrust according to the control signal pressure to promote the action of the regulating mechanism. The valve body is a component of the pneumatic regulating valve. It is in direct contact with the regulating medium and regulates the flow of the fluid.

2. Briefly describe the solutions to noise and cavitation problems in the use of pneumatic control valves:

In application, the noise on the pneumatic control valve is the main source of pollution in petrochemical production. Cavitation and noise are the two main problems of regulating valves in controlling high-pressure differential fluids. So what are the reasons for these two major problems? How to solve it?

1. Mechanical vibration - for example, when the pneumatic regulating valve valve core moves horizontally in the sleeve, the gap between the valve core and the sleeve can be made as small as possible or a sleeve with a hard surface can be used.

2. Mechanical fluidity of the medium - when the medium flows in the pipeline or pneumatic control valve, it will also make noise. We will not elaborate on this situation. Of course, cavitation will also cause noise.

3. Natural frequency vibration - such as pneumatic control valve spool or other components, they all have a natural vibration frequency. For this, the characteristics of the spool can be changed through special casting or forging processing, and they can be replaced if necessary. Other types of spools.

4. When the causing factor is the instability of the valve core - if the noise is caused by the pressure fluctuation of the fluid caused by the oscillatory displacement of the valve core, this situation is generally caused by the damping factors of the regulating loop actuator, etc. For this The damping coefficient can be readjusted or a damping facility can be added in the direction of the spool displacement.

3. What should we pay attention to when selecting pneumatic control valves?

Fundamentally speaking, there is not much difference between pneumatic control valves and electric control valves , except for the difference in actuators. The former requires a compressed air source, while the latter only requires power supply, which is more convenient. Pneumatic control valves need to be equipped with compressors and other equipment. The advantage is that they are explosion-proof, simple to maintain, and have a low failure rate. Therefore, in chemical, petrochemical and other industries, the scope of use is much larger than that of electric control valves.

Pneumatic control valves can be divided into pneumatic single-seat control valves, pneumatic sleeve control valves, pneumatic two-seat control valves, pneumatic three-way control valves, pneumatic cage control valves, pneumatic angle control valves, and pneumatic V-shaped control ball valves according to their structural forms. , pneumatic regulating butterfly valve and other categories. The actuators mainly include diaphragm type, piston type, etc. Among them, diaphragm type actuators have higher control accuracy than piston type. According to the different electric valve positioners, they are divided into mechanical and intelligent pneumatic control valves. The intelligent type has better control accuracy than the mechanical type.

Depending on the medium, the valve body material also has different choices. Commonly used carbon steel, corrosive ones choose stainless steel 304 or 316, 316L and other materials. You can also choose fluorine-lined regulating valves. There are also several special types of pneumatic control valves: low temperature control valves, high temperature control valves, high pressure control valves, small flow control valves, etc. The single-seat regulating valve is used in working conditions with a relatively small pressure difference. Once the pressure difference is relatively large, you can choose a sleeve regulating valve or a double-seat regulating valve. Because the leakage of the double-seat regulating valve is relatively large, if the leakage is required to be relatively small , it is not suitable to choose a double-seat control valve.

The three-way regulating valve is used to combine two fluids into one or to divide one fluid into two. For situations where the pressure difference is particularly large and cavitation or flash evaporation is prone to occur, a multi-stage cage control valve can be selected to reduce the pressure of the high-pressure fluid in stages to achieve stable control. Pneumatic V-type regulating ball valve is suitable for media containing particulate matter, such as pulp, etc. Because the valve internal parts have shear left and right, its flow characteristics are approximately equal percentage, and the adjustability ratio is much larger than that of ordinary single seats. , sleeve control valve, so the scope of use has a trend of becoming larger and larger. The electric regulating butterfly valve is more suitable for large-diameter occasions and has a relatively high cost performance.

 

In short, users should correctly select pneumatic control valves according to their actual needs in order to achieve good results.