Nov.08.2024
Control valves, also known as regulating valves, are control devices used in the field of industrial automation process control. They can automatically adjust based on control signals from the automation system to change process parameters such as flow rate, pressure, temperature, and liquid level.
Control valves consist of three main parts: the valve body, actuator, and accessories.
The valve body is the foundational structure of the control valve, designed with a focus on flow control. The actuator is responsible for moving the valve core to change the flow area, while accessories provide additional functions and performance.
Control valves can be classified based on stroke characteristics into linear and rotary types, and based on function and characteristics into linear, equal percentage, and parabolic types.
Linear Types: single-seat valve, double-seat valve, sleeve valve, angle valve, three-way valve, and diaphragm valve;
Rotary Types: butterfly valve, ball valve, eccentric rotary valve, and full-featured ultra-light control valve.
Driving Operation: pneumatic, electric, and hydraulic types;
Control Form: regulating, shut-off, and regulating shut-off types;
Flow Characteristics: linear, equal percentage, parabolic, or quick-opening.
They are suitable for various media, including air, water, steam, corrosive media, mud, and oil.
Air-open and air-close, direct and reverse action
The air-open and air-close of the control valve refer to the entire valve mechanism. As the air pressure on the diaphragm increases, the valve gradually opens for air-open and closes for air-close. When there is no signal, the air-open valve is closed, and the air-close valve is fully open.
The direct and reverse action of the control valve refers to the actuator of the pneumatic diaphragm control valve. When air is introduced into the upper part of the diaphragm, the stem moves downward, which is direct action. When air is introduced into the lower part, the stem moves upward, which is reverse action.
Flow-open and flow-close are related to the medium. When the medium flows in the direction of valve opening, it is called flow-open type.
Conversely, when it flows towards valve closing, it is called flow-close type.
I. Actuator Action Forms
(1) Direct and reverse action of pneumatic actuator. When the input air pressure of the pneumatic actuator increases, the stem moves downward, which is direct action; conversely, when the input air pressure increases, the stem moves upward, which is reverse action (see Figure 9-16).
(2) Direct and reverse mounting of the regulating mechanism. The valve core has two forms: direct and reverse mounting. When the valve core moves downward and the flow area between the valve core and seat decreases, it is called direct mounting valve; conversely, when the flow area increases as the valve core moves downward, it is called reverse mounting valve. For double-guided direct mounting valves, as long as the valve stem is connected to the lower end of the valve core, it becomes a reverse mounting valve. For valves with a nominal diameter Dg < 25mm, they are generally single-guided, so only direct mounting valves are available.
(3) Actuator action forms. Pneumatic actuators have two forms: air-open and air-close. When the signal pressure increases, the valve opens, called air-open; conversely, when the signal pressure increases, the valve closes, called air-close. Since the actuator has direct and reverse actions, the control valve (with a double-guided valve core) also has direct and reverse actions, so the air-open or air-close of the pneumatic actuator is formed by this combination, as shown in Figure 9-16.
For small-diameter control valves, the output signal is usually changed by changing the direct and reverse actions of the actuator to achieve air-open or air-close; for large-diameter control valves, it is usually by changing the direct and reverse actions of the control valve to achieve air-open or air-close.
II. Positioner
The positioner is used in conjunction with the pneumatic diaphragm actuator.
1) Positive action of the valve positioner: When the input signal increases, the output pressure to the diaphragm increases;
2) Reverse action of the valve positioner: When the input signal increases, the output pressure to the diaphragm decreases;
Positive action actuators and positioners work together to achieve the function of positive action actuators;
Positive action actuators and reverse action positioners work together to achieve the function of reverse action actuators;
Reverse action actuators and positive action positioners work together to achieve the function of reverse action actuators;
Reverse action actuators and reverse action positioners work together to achieve the function of positive action actuators;
III. Control Valve FC (air-open or fail-close) or FO (air-close or fail-open)
The choice of air-open and air-close is considered from the perspective of process safety. When the air source is cut off, whether the valve is safer in the closed or open position. For example, in the combustion control of a heating furnace, the control valve is installed on the fuel gas pipeline to control the fuel supply based on the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to choose an air-open valve.
An air-open control valve means that the valve is fully closed when there is no air, and the valve opens when there is air. When there is no signal, the valve is closed, and the valve opens when there is an input signal. Moreover, the larger the signal, the larger the valve opening. When the signal is at its maximum, the valve is fully open.
Air-open type (Air to Open) means that when the air pressure on the diaphragm increases, the valve moves in the direction of increasing opening, and when it reaches the input air pressure limit, the valve is fully open. Conversely, when the air pressure decreases, the valve
moves in the closing direction, and when there is no input air, the valve is fully closed. Therefore, the air-open type valve is sometimes called fail-close (FC).
Air-close type (Air to Close) moves in the opposite direction to the air-open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or is absent, the valve moves in the opening direction or fully opens. Therefore, it is sometimes called fail-open (FO). During use, common failure positions are (FO, FC, FL), where FO means the valve fails to open/close due to air source failure.
For pneumatic valve failure positions, they are mainly divided into several situations:
1) Under interlocking action of pneumatic valve device, the valve position should have the following situations:
FC-When the air source is lost, the valve is in the closed position;
FO-When the air source is lost, the valve is in the open position;
FL-When the air source is lost, the valve is in a momentary position and remains;
FLC-When the air source is lost, the valve holds its position but tends to close, and the valve is in the closed position (the gas in the cylinder is exhausted);
FLO-When the air source is lost, the valve holds its position but tends to open, and the valve is in the open position (the gas in the cylinder is exhausted).
2) Under interlocking action of regulating or shut-off valve device, the valve position should have the following situations:
FC-When the air source is lost or the solenoid valve loses power, the valve is in the closed position;
FO-When the air source is lost or the solenoid valve loses power, the valve is in the open position;
AFL/EFC-
1) When the air source is lost and the solenoid valve does not lose power, the valve holds its position;
2) Regardless of whether the air source is lost or the solenoid valve loses power, the valve is in the closed position;
AFL/EFO-
1) When the air source is lost and the solenoid valve does not lose power, the valve holds its position;
2) Regardless of whether the air source is lost or the solenoid valve loses power, the valve is in the open position.
Pneumatic valves achieve valve cut-off, connection, and regulation functions through output signals, with relatively fast opening and closing speeds. They are often used for quick two-position shut-off and can also be used for flow regulation. By matching different accessories, various control methods can be achieved.
Air-open control valves increase flow area with increasing signal pressure; while air-close valves decrease flow area with increasing signal pressure.
Selection of Control Valve Type
There are many types of control valve bodies, with commonly used types including straight-through single-seat, straight-through double-seat, angle, diaphragm, small flow, three-way, eccentric rotary, butterfly, sleeve, and ball.
When making specific choices, the following considerations can be made:
(1) Valve core shape and structure
Mainly consider factors such as selected flow characteristics and unbalanced forces.
(2) Wear resistance
When the fluid medium is a suspension containing high-concentration abrasive particles, the internal material of the valve should be hard.
(3) Corrosion resistance
Due to the corrosive nature of the medium, it is preferable to choose a valve with a simple structure.
(4) Medium temperature and pressure
When the medium temperature and pressure are high and fluctuate significantly, the material of the valve core and seat should be selected with minimal temperature and pressure changes.
(5) Prevent flashing and cavitation
Flashing and cavitation only occur in liquid media. In actual production processes, flashing and cavitation can cause vibration and noise, shorten the service life of the valve, so when selecting a valve, it is necessary to prevent the valve from generating flashing and cavitation.
Selection of Control Valve Actuator
To ensure the normal operation of the control valve, the selected actuator must generate sufficient output force to ensure high sealing and valve opening.
For double-acting pneumatic, hydraulic, and electric actuators, there are generally no reset springs. The magnitude of the force is unrelated to its operating direction. Therefore, the key to selecting an actuator lies in determining the maximum output force and motor torque. For single-acting pneumatic actuators, the output force is related to the valve opening, and the force appearing on the control valve will also affect the motion characteristics, so it is required to establish a force balance within the entire opening range of the control valve.
Determination of Actuator Type
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 used. From an energy-saving perspective, electric actuators should be used as much as possible. If high control accuracy is required, hydraulic actuators can be selected, such as speed control of transparent machines in power plants, temperature control of catalytic reactors in refineries, etc.
Selection of Control Valve Action Mode
The action mode of the control valve is only applicable when selecting a pneumatic actuator. The action mode is formed by the combination of the positive and reverse actions of the actuator and the valve. There are four combination forms, namely positive-positive (air-close type), positive-reverse (air-open type), reverse-positive (air-open type), and reverse-reverse (air-close type), which form two types of control valve action modes, air-open and air-close.
When selecting the action mode of the control valve, consider mainly from three aspects:
a) Process production safety;
b) Characteristics of the medium;
c) Ensure product quality, with minimum economic loss.
