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
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
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.
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.
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