Selection of Accessories for Pneumatic Control Valves

Pneumatic control valves constitute a crucial component in the instrumentation of automated control processes. Proper selection is of paramount importance for the normal operation of the system. In addition to commonly considered parameters such as pressure, temperature, port size, material, and structural form of the control valve, attention should also be paid to relevant accessories. The main accessories for pneumatic control valves include valve positioners, solenoid valves, valve position feedback devices, air filter pressure regulators, lock-up valves (position-holding valves), and pneumatic boosters. Below, the selection principles for the main accessories are described respectively:

The valve positioner is a primary accessory for pneumatic actuators. Used in conjunction with pneumatic actuators, it enhances the positional accuracy of the valve, overcoming the effects of valve stem friction and unbalanced media forces, thereby ensuring the valve is correctly positioned according to signals from the controller. The valve positioner converts 4 - 20 mA control signals into pneumatic signals for the pneumatic actuator, driving the control valve to perform its regulating function. Valve positioners are classified into mechanical positioners and intelligent positioners. Intelligent positioners offer higher control accuracy and lower air consumption compared to mechanical ones. Additionally, they can be equipped with HART protocol functionality.

When program control or two-position control is required in the system, a solenoid valve must be employed. When selecting a solenoid valve, in addition to considering AC/DC power supply, voltage, and frequency, it is essential to note the relationship between the solenoid valve and the operating mode of the control valve. Solenoid valves can be "normally open" or "normally closed." If an increase in the solenoid valve's capacity is required to shorten the actuation time, two solenoid valves can be used in parallel, or the solenoid valve can be used as a pilot valve in combination with a high-capacity pneumatic booster.

The pneumatic booster is a power amplifier that transmits pneumatic signals over long distances, eliminating the lag caused by extended signal lines. It is primarily used between field transmitters and central control room regulating instruments or between controllers and field control valves. Another function is to amplify or reduce signals.

Converters include air-to-electric and electric-to-air converters, whose function is to achieve a specific relationship of mutual conversion between air and electric signals. They are mainly used to convert 0 - 10 mA or 4 - 20 mA electric signals into 0 - 100 kPa air signals, or vice versa, when using electric signals to operate pneumatic actuators.

The air filter pressure regulator is an accessory in industrial automation instrumentation. Its primary function is to filter and purify compressed air from air compressors and stabilize the pressure at the required value. It can be used as a gas supply source and pressure stabilizer for various pneumatic instruments, solenoid valves, pneumatic cylinders, spraying equipment, and small pneumatic tools.

The lock-up valve is a device that maintains valve position. When a pneumatic control valve experiences a failure in the air supply, this device cuts off the air supply signal, keeping the pressure signal in the diaphragm chamber or cylinder at the state just before the failure. This maintains the valve position at the pre-failure state, serving as a position-holding function.

When the control valve is located far from the control room, a valve position transmitter is required to accurately determine the valve's open/closed position without going to the site. It converts the mechanical displacement of the valve opening into an electric signal according to a certain law and sends it to the control room. This signal can be a continuous signal reflecting any valve opening or can be considered as the reverse operation of a valve positioner.

The limit switch is a device that reflects the two positions (open and closed) of the valve and simultaneously sends out an indication signal. Based on this signal, the control room can determine the valve's open/closed state and take corresponding measures