How To Choose The Correct Control Valve?

Selection of Control Valve

The Importance Of Control Valve Selection

The regulating valve is an actuator in the automatic control system, and its application quality directly reflects the regulating quality of the system. As a terminal component in process control, people have a newer understanding of its importance than in the past. The quality of the control valve application depends on the quality of the product itself and whether the user installs, uses, and maintains it correctly. Correct calculation and selection are also very important. Due to errors in calculation and selection, the system starts and stops, and some even cannot be put into use. Therefore, users and system designers should understand the importance of valves on-site and must pay enough attention to the selection of regulating valves.

Principles Of Control Valve Selection

  • Select appropriate structural form and materials according to process conditions;
  • Select the flow characteristics of the control valve according to the characteristics of the process object;
  • Select the appropriate control valve caliber size according to the process operating parameters;
  • Select the required auxiliary devices according to the requirements of the process;
  • Reasonably select the execution agency. The response speed of the actuator should be able to meet the process requirements;

Requirements for control stroke time: The selected control valve actuator should be able to meet the requirements of the valve stroke and process for leakage level. In some situations, such as when selecting a pressure control valve (including a vent valve), the actual possible pressure difference should be considered for appropriate amplification, which means that the actuator is required to provide a larger force. Otherwise, when an abnormal situation occurs in the process, the actual pressure difference before and after the control valve is large, and there is a danger that it will not close or open.

application of control valve

Accessories for Control Valves

During the production process, the control system puts forward various special requirements for the valve. Therefore, the control valve must be equipped with various accessory devices (referred to as accessories) to meet the needs of the production process. Accessories for control valves include:

  • The valve positioner is used to improve the working characteristics of the control valve regulation performance and achieve correct positioning;
  • The valve position switch displays the working position of the upper and lower limit strokes of the valve;
  • Pneumatic position-keeping valve: When the air source of the control valve fails, the valve is kept at the opening position before the air source fails;
  • The solenoid valve realizes electromagnetic switching of the gas path to ensure that the valve is in the desired safe opening position when the power fails;
  • The handwheel mechanism can switch to manual operation of the valve when the controller of the control system fails;
  • The pneumatic relay accelerates the action of the actuator and reduces the transmission time;
  • The air pressure reducer is used to purify the air source and adjust the air pressure;
  • The air storage tank ensures that when the air source fails, the springless cylinder piston actuator can move the control valve to a fail-safe position. Its size depends on the size of the cylinder, the valve action time requirements the working conditions of the valve, etc.;

In short, the role of accessories is to make the functions of the control valve more complete, reasonable, and complete.

Maintenance of Control Valve

The control valve has the characteristics of a simple structure and reliable action. However, because it is in direct contact with the process medium, its performance directly affects the system quality and environmental pollution. Therefore, the control valve must be regularly maintained and regularly inspected, especially for important and harsh operating conditions. In some cases, more attention should be paid to maintenance work. Key inspection areas:

  • Valve body wall: For control valves used in high-pressure differential and corrosive media situations, the valve body wall and the diaphragm of the diaphragm valve are often impacted and corroded by the medium, and the pressure resistance and corrosion resistance must be checked with emphasis.
  • Valve seat: When the control valve is working, due to the penetration of media, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and the valve seat becomes loose. This should be paid attention to during inspection. For valves operating under high-pressure differences, you should also check whether the valve seat sealing surface is damaged.
  • Valve core: The valve core is a movable part during adjustment work. It is most seriously washed and corroded by the medium. During maintenance, it is necessary to carefully check whether all parts of the valve core are corroded and worn, especially under the condition of high-pressure difference. The wear is more serious (due to cavitation), so attention should be paid to it. If the valve core is seriously damaged, it should be replaced. In addition, attention should be paid to whether the valve stem has similar phenomena or is loosely connected to the valve core.
  • Diaphragm: “O” ring and other seals. Check whether the diaphragm and “O”-shaped sealing gasket in the control valve are aged or cracked.
  • Sealing packing: Pay attention to whether the polytetrafluoroethylene packing and sealing grease are aged, and whether the mating surface is damaged. They should be replaced when necessary.

The regulating valve, also known as the control valve, is the main type of actuator. It accepts the control signal output by the regulating control unit and uses power operation to change the fluid flow. Regulating valves generally consist of an actuator and a valve. According to the power used by its actuator, control valves can be divided into three types: pneumatic, electric, and hydraulic, that is, pneumatic control valves use compressed air as the power source, electric control valves use electricity as the power source, and liquid medium (such as oil, etc.) pressure-powered electro-hydraulic control valves. In addition, according to their functions and characteristics, there are solenoid valves, electronic, intelligent, fieldbus-type control valves, etc.

Valve Body Type Selection For Regulating Valves

There are many types of valve bodies for control valves. Commonly used ones include straight-through single-seat, straight-through double-seat, angle, diaphragm, small flow, three-way, eccentric rotation, butterfly, sleeve, spherical, etc. When making specific choices, you can consider the following:

Valve core shape and structure

It is mainly considered based on factors such as the selected flow characteristics and unbalanced force.

Wear resistance

When the fluid medium is a suspension containing a high concentration of abrasive particles, the internal material of the valve must be hard.

Corrosion resistance

Since the medium is corrosive, try to choose valves with simple structures.

Temperature and pressure of medium

When the temperature and pressure of the medium are high and change greatly, a valve whose material of the valve core and valve seat is less affected by temperature and pressure changes should be selected.

Prevent flash evaporation and cavitation

Flash evaporation and cavitation only occur in liquid media. In the actual production process, flash evaporation and cavitation will cause vibration and noise and shorten the service life of the valve. Therefore, when selecting valves, valves should be prevented from flashing and cavitation.

Selection Of Regulating Valve Actuator

  • For the regulating valve to work normally, the equipped actuator must be able to generate sufficient output force to ensure a high degree of sealing and the opening of the valve.
  • For double-acting pneumatic, hydraulic, and electric actuators, there is generally no return spring. The size of the force has nothing to do with its direction of operation. Therefore, the key to selecting an actuator is to find out the maximum output force and the rotational torque of the motor. For single-acting pneumatic actuators, the output force is related to the opening of the valve. The force appearing on the regulating valve will also affect the motion characteristics. Therefore, it is required to establish a force balance throughout the opening range of the regulating valve.

Determination Of The Type Of Actuator

After determining the output force of the actuator, select the corresponding actuator according to the requirements of the process environment. When there are explosion-proof requirements on site, pneumatic actuators should be selected. From the perspective of energy saving, electric actuators should be used as much as possible. If the adjustment accuracy is high, a hydraulic actuator can be selected. Such as the speed adjustment of the transparent machine in the power plant, the temperature adjustment control of the catalytic device reactor in the refinery, etc.

Selection Of The Mode Of Action Of The Regulating Valve

The mode of action of the regulating valve is only available when a pneumatic actuator is selected. Its mode of action is formed by a combination of the positive and negative actions of the actuator and the positive and negative actions of the valve. There are four types of combinations, namely forward and reverse (air-closing type), forward and reverse (air-opening type), forward and reverse (air-opening type), and reverse-reverse (air-closing type). The mode of action of the regulating valve formed by these four combinations is air. There are two types: open and gas-closed. For the selection of the mode of action of the regulating valve, three main aspects should be considered: a) process production safety; b) characteristics of the medium; c) ensuring product quality and minimizing economic losses.

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