Control valves are used to control or restrict the flow of fluids stream in a closed or open channel. Control valves are an essential part of the most flow process in industrial to increase the productivity, product quality and safety, they are used to control pressure, level and flow.
Control valves alone cannot achieve the purpose, there are many accessories used to work with the control valve to accomplish the task, help to ensure accuracy, There are different types of accessories used for control valves:
Valve positioners are devices used to put the valve in the correct position by increasing or decreasing the air load pressure on the actuator. A positioner is a motion-control device designed to actively compare stem position against the control signal, adjusting pressure to the actuator diaphragm or piston until the correct stem position is reached
Positioners act as control systems within themselves, the valve’s stem position is the process variable (PV), the command signal to the positioner is the setpoint (SP), and the positioner’s signal to the valve actuator is the manipulated variable (MV) or output. Thus, when a process controller sends a command signal to a valve equipped with a positioner, the positioner receives that command signal and applies as much or as little air pressure to the actuator as needed in order to achieve that desired stem position. Thus, the positioner will “fight” against any other forces acting on the valve stem to achieve crisp and accurate stem positioning according to the command signal.
A properly functioning positioner ensures the control valve will be “well-behaved” and obedient to the command signal.
Current to pressure converter (I/P) are device which converts incident current signal into a corresponding standard pressure signal.
An I/P converter in common convert a 4-20mA current signal to 3-15 Psi pneumatic signal that a pneumatic positioner can interpret and send to the control valve.The air supply for the I/P converter must be approximately 20 psi so that the converter can control the pressure between 3-15 psi.
I/P converters are electrical devices, so plant personnel must choose one that has the appropriate electrical characteristics and certifications for the zone in which it will be installed.
Limit switches are indicators, which indicates when a specific position is reached.
Limit switches are used most often to indicate the fully open or fully closed position but can be set at intermediate points if desired. A limit switch also can be connected with alarm or indicator light.
The switch can be connected to any safety circuits. A non-conducting electromagnetic system triggers when a position is reached to activate internal contact of a circuit. Each switch adjusts individually and can be supplied for either alternating current or direct current systems.
The installed limit contacts are either inductive, electric, pneumatic or software-based, depending
on the device version.
The manual override system is to control the valve manually when there is an urgent situation like power shortage or power break. The override is a limited-use device, often designed to counter the built-in spring return within the actuator to push the valve to its fully open or closed position, depending on the fail action.
A volume booster amplifies the air flow, which is a mechanical relay. Every pneumatic instrument associated with the control valve has a limit of flow capacity.
For pneumatic actuator, volume booster works as a relay between a system with low flow volume and one with high flow volume. Volume boosters used on control valves are normally applied with a bypass or gain adjustment to provide stability. The volume booster normally has a built-in deadband where a certain amount of signal change is necessary to activate the volume booster.
The Position sensor senses the valves stem position.
The position transmitter senses the valve stem position continuously and the position is feedbacked to the control system. And the position is compared with the desired position and gives a proper control action by the processor.
The position transmitter is an electrical device. There are, however, no contact switches. Instead, a position sensor is used to follow the valve position and send a signal to an amplification circuit, which then provides the transmitter output, the position transmitter produce a signal in range 4-20mA.
Airset is used to supply constant air to the pneumatic instrument such as valves and positioner. It is also known as regulator.
An airset ensures a clean air supply to keep everything operating as designed. Periodic maintenance is required to ensure the filter is clean and condensate is drained from the dripwell.
Any internal leakage through the control devices could create pressure buildup. Airsets, therefore, typically have an internal relief that will vent any undesired pressure buildup within the system.