Instrumentation and Control Valve Basics - Sizing and Selection

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CONTROL VALVE BASICS – SIZING & SELECTION

Introduction

A control valve is a power operated device capable of modulating flow at varying degrees between minimal flow

and full capacity in response to a signal from the controlling system. Control valves may be broadly classified by

their function as “on-off” type or “flow regulating” type.

A control valve is comprised of an actuator mechanism that is capable of changing the position of flow controlling

element in the valve. The valve modulates flow through movement of a valve plug in relation to the port(s) located

within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator.

The actuator, which can be pneumatically or electrically operated, directs the movement of the stem as dictated

by the external control device. The actuator responds to an external signal which usually comes from a

controller. The controller and valve together form a basic control loop.

There are many types of valves available, each having their advantages and limitations. The basic requirements

and selection depend on their ability to perform specific functions such as:

1. Ability to throttle or control the rate of flow.

2. Lack of turbulence or resistance to flow when fully open - turbulence reduces head pressure.

3. Quick opening and closing mechanism - rapid response is many times needed in an emergency or for

safety.

4. Tight shut off - prevents leaks against high pressure.

5. Ability to allow flow in one direction only - prevents return.

6. Opening at a pre-set pressure - procedure control to prevent equipment damage; and

7. Ability to handle abrasive fluids - hardened material prevents rapid wear.

This course will discuss the selection process and provide the basic principles of sizing the control valves.

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