Basic Overview and Sharing of Control Valves (Updated Daily)

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I have been in this industry for 10 years, so I can barely be considered an expert, and I have been actively participating in forums for a long time. My areas of expertise include the research, development, and design of various control valves, temperature and pressure reducers, as well as self-acting valves. Having dealt with control valves for a long time, I’ve taken the initiative these past few days to share some basic knowledge about them. If there are any mistakes or inaccuracies, I hope fellow enthusiasts will point them out and help me correct them. Also, if anyone is facing issues related to control valves, such as actuators or valve selection, etc., I’m here to assist. 1. Basics of automation
a. Control variables: level, pressure, temperature, flow rate
b. Analog signal control circuits
c. Fieldbus control: lower installation costs. 1. Simplified wiring and fewer terminals; 2. Control functions are located at the field instruments, reducing the cost of control systems; 3. Online digital communication enables faster debugging and diagnosis, thereby reducing installation and commissioning costs as well as maintenance costs. Fieldbuses allow monitoring of all aspects of equipment and provide two-way communication, enabling online diagnosis, calibration, and predictive maintenance. This improves performance, as local control becomes faster, more accurate, and safer. HART is compatible with 4-20mA signals, can connect up to 15 actuators, and has a transmission distance of 1500 meters. Profibus can connect up to 126 actuators, with a transmission speed ranging from 9.6 Kbps to 12 Mb/s and a transmission distance of 10 Km. FF can connect thousands of actuators, with a transmission speed of 31.25 Kb/s and a transmission distance of 1.9 Km.

2. Basics of control valves
a. Definition according to IEC (International Electrotechnical Commission): A control valve consists of an actuator and a valve body. The actuator is the control element of the valve; it generates a corresponding force based on the magnitude of the signal, causing the actuator rod to move accordingly, which in turn moves the valve core. The valve body is the part responsible for regulation; it comes into direct contact with the medium, and through the movement of the valve core, the throttling area of the valve is changed, thereby achieving regulation. Control valves are an essential component of automated systems; they receive output signals from controllers and directly regulate energy or materials in order to achieve process parameters such as temperature, pressure, flow rate, and liquid level. b. The importance of control valves in automation
A. Installed at the production site, they are in constant contact with process media and operate under harsh conditions such as high temperatures, high pressures, extreme cold, strong corrosion, wear, blockage, and leakage.
B. They represent the weakest link in the control system; improper selection or poor maintenance can prevent the entire automation system from functioning reliably, and in severe cases, it can lead to plant shutdowns and production halts.
C. They are closely related to the production process, as they directly affect the material balance and energy balance during production.

c. Specifications and pressure ratings
GB/T 4213 (Chinese standard)
Nominal diameter (mm): 6, 10, 15, 20, 25, (32), 40, 50, (65), 80, 100, (125), 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600, 1800, 2000
Nominal pressure (MPa): 0.1, 0.4, 1, 1.6, 2.5, 4, 6.4, 10, 16, 25, 32, 40, 160, 250
ASME B16.34
Nominal diameter (inches): 1/2, 3/4, 1, 11/2, 2, 21/2, 3, 31/2, 4, 5, 6, 8, 10, 12, 14, 18, 20, 24, 26, 28, 30, 32, 34, 36, 38, 40, 48
Nominal pressure (MPa): Class 125, Class 150, Class 300, Class 600, Class 900, Class 1500, Class 2500, Class 4500

d. Basic classification of control valves
e. Valve body types
Straight-through single-seat valve (GLOBE): It has only one valve seat and sealing surface; it features a simple structure and good sealing performance, making it one of the most commonly used valve body types.
Straight-through double-seat valve (GLOBE): It has two valve seats and sealing surfaces; it offers high flow capacity but also higher unbalanced forces and greater leakage, resulting in poor shut-off performance. Its use has declined over time.
Cage valve: The valve element inside the valve body is guided by a cage. Windows on the cage are used to control flow rate and flow characteristics. Balance holes on the valve element help reduce unbalanced forces. Cage valves offer a wide range of tuning possibilities, minimal vibration, low unbalanced forces, and good interchangeability. They can be used in most applications involving single- and double-seat valves, but not for media containing particles or contaminants. They are among the most widely used valve body types. The angular valve (ANGLE) features a simple structure with a single valve seat and sealing surface, resulting in excellent sealing performance. It generally has fluid inlet at the bottom and outlet on the side; in flash evaporation conditions, inlet on the side and outlet at the bottom is commonly used. This valve possesses self-cleaning capabilities, as it does not allow dirt to accumulate inside, reducing the risk of blockages. It is suitable for controlling media with high viscosity, high pressure differences, as well as those containing suspended solids and particles. Three-way valve: A three-way valve has three inlets and outlets connected to pipes; it comes in two types – for combining streams or splitting them. Diaphragm valve: The valve body is lined with corrosion-resistant material, and a diaphragm is used in place of the internal components. It features a simple structure, low flow resistance, and no packing, which prevents leakage of the fluid. Suitable for regulating and shutting off strong acids, strong bases, and highly corrosive media. Butterfly valves have a simple structure; the valve body and butterfly disc can be lined with various corrosion-resistant materials. They come in double-eccentric and triple-eccentric designs and are suitable for regulating and shutting off media with large diameters, high flow rates, and low pressure differences. Ball valves use a spherical element with a cylindrical hole, known as an O-ring ball valve, and are commonly used for on/off control. V-ball valves use a spherical element with a V-shaped notch; they are used in control applications and are particularly suitable for regulating media such as fiber pulp. Eccentric rotary valves use a fan-shaped spherical surface; they are lightweight, compact, and offer good sealing performance, making them suitable for applications that require both regulation and sealing. The actuation devices for control valves can be classified according to the energy source used: pneumatic, electric, hydraulic, electro-hydraulic or pneumatic-hydraulic, and electromagnetic. Control valves can also be equipped with various accessories to enable multiple control functions. These accessories meet the various special requirements imposed by control systems on valves, and their role is to enhance the functionality of control valves, making it more comprehensive and efficient. Valve positioners include pneumatic valve positioners and electrical valve positioners. Smart valve positioner. 1. Improve the linear accuracy of control valves (in high-temperature, high-pressure, viscous, or solid-particle media). 2. Overcome friction and increase actuation speed. 3. Overcome pressure differences to enhance the valve’s closing performance. 4. Enable proportional control. 5. Accept electrical signals. 6. Change the direction of action to alter flow characteristics. 7. Support bus control.

Air filter regulator: Purifies the air supply from air compressors and adjusts the pressure to the desired level, featuring automatic pressure stabilization.

Valve position transmitter: Converts changes in the control valve’s position into corresponding pressure or current signals that are sent to the control room, allowing operators to monitor the valve’s position there.

Solenoid valve: Used to control the opening or closing of air circuits via electrical signals.

Position holding valve: When the instrument pressure air supply is interrupted, it automatically cuts off the connection between the positioner’s output and the actuator, keeping the valve in its position before the air supply was cut off.

Pneumatic switch valve, also known as a pneumatically controlled valve: Switches air circuits when the signal pressure is too low or too high compared to the set value; mainly used for switching air paths.

Pneumatic amplifier (speed amplifier): Overcomes transmission delays caused by long pipelines and large-capacity actuators, increases the output flow of the positioner, and alters the pressure of the control signal.

Limit switch (position switch): Used to set limits for the valve and to transmit signals regarding its position