ISA TR75.25.02 PDF

This technical report applies to throttling control valves in closed loop control applications. The concept has some application to open loop control applications. It does not address control valves used in on-off control service. The "control valve" in the context of this document includes the following components:

Valve: A valve is a device used for the control of fluid flow. It consists of a fluid containing valve body assembly, one or more ports between connection openings and a moveable closure member, which opens, restricts or closes the port(s) (see ANSI/ISA-75.05.01-2000, "Control Valve Terminology").

Actuator: An actuator is a device that supplies the force and causes the movement of the valve closure member. Commonly these are fluid or electrically powered (see ANSI/ISA-75.05.01-2000). Actuators often use air but other types use electric, hydraulic and electro-hydraulic power.

Motion conversion mechanism: A mechanism installed between the valve and the power unit of the actuator to convert between linear and rotary motion where required. The conversion may be from linear actuator action to rotary valve operation or from rotary actuator action to linear valve operation.

Accessories: Additional devices used in the operation of the control valve. As described in ANSI/ISA-75.05.01-2000, typical examples include a positioner, transducer, signal booster relay, air set, snubber, etc.

Purpose

This technical report describes the characteristic response of a control valve to step input signal changes. It considers the factors that affect this response, the impact of the response on the quality of process control, and the appropriate control valve specifications. In this document, a control valve is the complete control valve body, with actuator and any accessories required for normal operation assembled and ready for use. This document supports standard ANSI/ISA-75.25.01-2000 "Test Procedure for Control Valve Response Measurement from Step Inputs". See the standard for the test procedures.

Users and manufacturers have developed a better understanding of the effects of control valve response characteristics on process control. This document identifies and defines four regions of control valve response to step input changes of varying sizes. Existing standards do not include the definitions and methods to measure certain valve characteristics now understood to be important. This technical report provides guidance that can be used to relate the control valve performance to process control.