A position transmitter is similar to a limit switch in that it is connected to the valve or actuator stem (shaft) to detect valve position and is often housed in a similar enclosure. The transmitter’s function, however, is quite different.
While a limit switch triggers at one defined position, a position transmitter provides a continuous output signal proportional to the actual valve position. That data can then be fed into a control system to compare the valve’s actual position to the desired one, or the data may be used to only provide a visual indication on a display panel.
Like a limit switch, a 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. That output is usually a 4-20 mA signal, though other ranges are available.
Position transmitters are often loop-powered devices (two-wire) in which the electrical supply is derived from the control loop power. Externally powered (four-wire) versions also are available for use when there is insufficient power available within the 4-20 mA loop. The sensors can range from simple potentiometers to high-tech optoelectric and other non-contacting technologies.
COMBINED POSITIONER ASSEMBLIES
A number of positioners on the market can be supplied as combined assemblies, incorporating the function of the positioner, limit switch and position transmitter in the same housing and using the same feedback connection.
Solenoid valves are often found in applications in which a control valve under certain conditions must be quickly driven to the fail position.
Solenoid valves are usually electrically actuated on-off or three-way valves installed into the air system and designed to take a specific action when tripped. They range from small, single-coil units to large, high-volume piloted designs, depending on the desired function and required capacity.
The solenoid can be thought of as an on-off switch for a pneumatic system. The signal to the solenoid controls the action of the internal valve assembly, allowing flow through the valve in one position and either isolating or venting the pressure in the other.
For example, an application might require that the operator in an emergency situation be able to manually drive the valve to its fail position instantaneously. A switch is connected to a three-way solenoid valve. When the switch is in the normal “on” position, power is supplied to the solenoid, which in turn opens its internal valve, allowing the positioner output pressure to flow to the actuator. When the switch is turned to “off,” power to the solenoid is removed, which opens the solenoid, venting the control valve and moving the valve to the fail-safe position.
Because of the wide variety of solenoid valve actions, voltage ratings and current ratings available, an almost infinite number of possible system configurations exist. When specifying a solenoid valve, it is important to describe the desired action under loss of electrical power, as well as the required voltage and current ratings.
Airsets, also known as filter regulators, are small pressure-reducing regulators that manage the air supply to pneumatic instruments and valves. They perform two critical functions: providing a constant air supply pressure to the instrument or valve, and filtering the instrument air.
The pressure-reducing function is essential to a plant’s performance and safety. Most plant instrument air systems operate at pressures of 100 psi (6.9 bar) or higher, while most control valves and other instruments are designed to run at much lower air supply pressures—as low as 20 psi (1.4 bar) in some cases. Exceeding the rated supply pressure can lead to early failure, mechanical damage, system shutdowns and potentially unsafe conditions.
Air supply pressure requirements can vary significantly from device to device, and having separate instrument air systems to meet the demands of each control system component would be impractical. Therefore, the standard practice is to have a single air system and install airsets on individual devices to reduce the instrument air pressure to the appropriate level.
Control valve actuators are designed to be sealed devices with no air leakage. As a result, 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.
Finally, clean instrument air is critical for consistent instrument performance. Valve positioners are made of precisely machined and fitted components, so even a small debris particle or droplet of condensate can affect performance. 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.