Terms & Definitions: Digital Valve Positioners

ACTUATOR: A mechanical device that drives a control valve open or closed via a command signal from a positioner or I/P transducer.

AIR SET: An adjustable pressure regulator that ensures a stable source of air to the positioner.

BOOSTER: A device that amplifies the amount of air (gas) going to a pneumatic actuator, producing a faster response. Having a separate air supply than the positioner, a booster typically is used with larger actuators that have larger volumes to fill.

CALIBRATION: The process of removing errors from a device or system during setup or at different time intervals, ensuring the accuracy of the device.

DIAGNOSTICS: Information extracted from a device or sensor that can be used to troubleshoot or diagnose a problem.

DOUBLE-ACTING: A type of actuation that uses air on both sides of a piston to drive a control valve open or closed.

ELECTROPNEUMATIC: A type of digital valve positioner that receives an analog signal and converts it to a pneumatic output. That output is used to drive an actuator, which in turn opens or closes a control valve.

EXPLOSION-PROOF: A certification that ensures a device has the ability to contain an explosion within itself so it does not ignite its hazardous surroundings.

HART: HART (Highway Addressable Remote Transducer) is a communications protocol that converts a 4-20mA analog signal into a digital signal, making communication to smart field devices possible.

INTEROPERABILITY: The ability of different types of devices from different manufacturers to communicate and interact with one another because they adhere to a common set of standards.

INTRINSIC SAFETY: A set of standards that ensures a particular device is unable to create enough energy to ignite a flammable gas-rich environment.

LIMIT SWITCH: Prevents a valve from opening or closing once a specified electrical or mechanical condition has been met.

LOOP-POWERED: A loop-powered device draws its power from the system and does not require a separate power supply.

MOUNTING KIT: A mechanical bracket that attaches a positioner to an actuator. The mounting kit is a vital part of the valve assembly because the position of the positioner relative to the actuator directly affects the performance of the system.

PROFIBUS: A digital communications protocol used primarily in Europe for digital devices.

REMOTE-MOUNT: An installation in which a positioner is not mounted directly on the actuator, but in a location some distance away. This can be done for convenience or to protect the electronics of the device from harsh conditions, such vibration or radiation.

SMART: Refers to the intelligence of a digital positioner that is capable of assessing the health of the control valve it is controlling and providing diagnostic information to the end-user.

VALVE SIGNATURE: The graphical representation of a valve’s performance. This includes, but is not limited to, friction, travel, actuator pressure, and such errors as hysteresis, overshoot and linearity.

WIRELESS HART MODULE: A wireless adaptor used in conjunction with digital positioners, particularly in wired analog (4-20mA) systems. The module uses HART communication to extract from the positioner diagnostics and process data that would otherwise be trapped in a wired analog system.

The terms and definitions for this glossary were contributed by Michael Patterson, director of marketing of Digital Solutions for Dresser Masoneilan, a provider of process control valves and solutions for more than 125 years. Mr. Patterson’s responsibilities include digital positioners, emergency shutdown devices, and other smart instruments. He can be reached at michael.patterson@dresser.com.