Introduction to Main Accessories of Control Valves
A valve positioner is the primary accessory for pneumatic actuators. Matched with pneumatic actuators, it improves the positioning accuracy of valves and overcomes the impacts of valve stem friction and medium unbalanced force, so as to ensure the valve can be positioned accurately according to signals transmitted from the controller.
A positioner shall be equipped under the following conditions:
The medium pressure is high and the pressure differential is large;
The nominal diameter of the control valve is large (DN＞100);
High-temperature or low-temperature control valves are adopted;
Faster response speed of the control valve is required;
Standard signals are used to operate actuators fitted with non-standard springs (spring range outside 20~100 kPa);
Used for split range control;
Reverse valve action needs to be realized (switching between air-to-close and air-to-open modes);
Modification of the valve flow characteristic is required (by replacing the cam of the positioner);
Proportional action needs to be realized for springless actuators or piston actuators;
When an electric signal is adopted to operate a pneumatic actuator, an electro-pneumatic valve positioner must be installed.
Solenoid Valve
When program control or two-position control is required for the system, a solenoid valve shall be configured. When selecting a solenoid valve, besides AC/DC power supply, voltage and frequency, attention must be paid to the matching relationship between the solenoid valve and the action type of the control valve; either normally open type or normally closed type can be selected.
If it is required to increase the capacity of the solenoid valve to shorten the action time, two solenoid valves can be used in parallel, or the solenoid valve can be used as a pilot valve combined with a large-capacity pneumatic booster relay.
Pneumatic Relay
The pneumatic relay is a power amplifier. It can transmit air pressure signals over long distances and eliminate signal delay caused by extended signal pipelines. It is mainly installed between field transmitters and control instruments in the central control room, or between controllers and field control valves. It can also amplify or reduce signal pressure.
Converter
Converters are divided into pneumatic-electric converters and electro-pneumatic converters. Their function is to realize mutual conversion between pneumatic signals and electric signals according to a certain relationship. They are mainly used to convert 0~10 mA or 4~20 mA electric signals into 0~100 kPa pneumatic signals, or convert 0~100 kPa pneumatic signals into 0~10 mA or 4~20 mA electric signals when electric signals are used to operate pneumatic actuators.
Air Filter Regulator
The air filter regulator is an accessory for industrial automation instruments. Its main function is to filter and purify compressed air supplied by the air compressor and stabilize the pressure at the required set value. It can serve as the air supply source and pressure stabilizing device for various pneumatic instruments, solenoid valves, cylinders, spraying equipment and small pneumatic tools.
Lock-up Valve
The lock-up valve is a device for maintaining valve position. When air supply fails, this device cuts off the air supply signal of the pneumatic control valve, and keeps the pressure signal inside the diaphragm chamber or cylinder at the state immediately before the fault. In this way, the valve position is maintained to realize position retention.
Valve Position Transmitter
When the control valve is far away from the control room, a valve position transmitter needs to be installed to accurately obtain the valve opening status without on-site inspection. It converts the mechanical displacement of valve stroke into an electric signal and transmits the signal to the control room. The signal can be a continuous signal reflecting any valve opening, and it can be regarded as the reverse operation of a valve positioner.
Limit Switch (Valve Position Feedback Switch)
The limit switch is a device that reflects the two extreme positions (fully open and fully closed) of the valve and sends out indication signals simultaneously. Based on these signals, the control room can judge the open/closed status of the valve and take corresponding measures.
A positioner shall be equipped under the following conditions:
The medium pressure is high and the pressure differential is large;
The nominal diameter of the control valve is large (DN＞100);
High-temperature or low-temperature control valves are adopted;
Faster response speed of the control valve is required;
Standard signals are used to operate actuators fitted with non-standard springs (spring range outside 20~100 kPa);
Used for split range control;
Reverse valve action needs to be realized (switching between air-to-close and air-to-open modes);
Modification of the valve flow characteristic is required (by replacing the cam of the positioner);
Proportional action needs to be realized for springless actuators or piston actuators;
When an electric signal is adopted to operate a pneumatic actuator, an electro-pneumatic valve positioner must be installed.
Solenoid Valve
When program control or two-position control is required for the system, a solenoid valve shall be configured. When selecting a solenoid valve, besides AC/DC power supply, voltage and frequency, attention must be paid to the matching relationship between the solenoid valve and the action type of the control valve; either normally open type or normally closed type can be selected.
If it is required to increase the capacity of the solenoid valve to shorten the action time, two solenoid valves can be used in parallel, or the solenoid valve can be used as a pilot valve combined with a large-capacity pneumatic booster relay.
Pneumatic Relay
The pneumatic relay is a power amplifier. It can transmit air pressure signals over long distances and eliminate signal delay caused by extended signal pipelines. It is mainly installed between field transmitters and control instruments in the central control room, or between controllers and field control valves. It can also amplify or reduce signal pressure.
Converter
Converters are divided into pneumatic-electric converters and electro-pneumatic converters. Their function is to realize mutual conversion between pneumatic signals and electric signals according to a certain relationship. They are mainly used to convert 0~10 mA or 4~20 mA electric signals into 0~100 kPa pneumatic signals, or convert 0~100 kPa pneumatic signals into 0~10 mA or 4~20 mA electric signals when electric signals are used to operate pneumatic actuators.
Air Filter Regulator
The air filter regulator is an accessory for industrial automation instruments. Its main function is to filter and purify compressed air supplied by the air compressor and stabilize the pressure at the required set value. It can serve as the air supply source and pressure stabilizing device for various pneumatic instruments, solenoid valves, cylinders, spraying equipment and small pneumatic tools.
Lock-up Valve
The lock-up valve is a device for maintaining valve position. When air supply fails, this device cuts off the air supply signal of the pneumatic control valve, and keeps the pressure signal inside the diaphragm chamber or cylinder at the state immediately before the fault. In this way, the valve position is maintained to realize position retention.
Valve Position Transmitter
When the control valve is far away from the control room, a valve position transmitter needs to be installed to accurately obtain the valve opening status without on-site inspection. It converts the mechanical displacement of valve stroke into an electric signal and transmits the signal to the control room. The signal can be a continuous signal reflecting any valve opening, and it can be regarded as the reverse operation of a valve positioner.
Limit Switch (Valve Position Feedback Switch)
The limit switch is a device that reflects the two extreme positions (fully open and fully closed) of the valve and sends out indication signals simultaneously. Based on these signals, the control room can judge the open/closed status of the valve and take corresponding measures.
