Definitions and Terms of Control Valves

In the article "Different Types of Industrial Valves", we introduced various types of valves in several parts, and in the article “What is a control valve? Types of control valves”, we discussed the classification of valves based on their physical operation. We even addressed how to select the control valve we have in mind in the article “Valve selection guide; How to choose the best functional valves”. Now, in this article, we will cover the definitions and common terms related to control valves so that you can become familiar with the technical language of this field.

Air Set:

This is referred to as a "Supply Pressure Regulator." It is a device used to reduce the pressure of the air supply for use in positioners and other control devices. (The pressure is usually around 20-35 psi).

Av:

This refers to the flow coefficient of the valve in the SI system.

ANSI Class:

It is an indicator and display of the pressure tolerance of a valve under fluid pressure. For this purpose, tables have been prepared by the American Society of Mechanical Engineers for each alloy, which list the maximum pressure that the valve can withstand at specific temperatures.

Booster:

A booster is a pneumatic relay used to reduce the time lag in pneumatic circuits by generating high-volume or high-pressure output signals. This device can be used as a volume booster or a pressure booster.

Cage:

A hollow, cylinder-like member that is part of the valve trim. This part can be used as a guide to direct the closure element onto the seat. In some types of valves, the cage may have holes of various shapes that determine the flow characteristics of the control valve. The cage can also serve as a noise reducer and as a device to prevent cavitation.

Control Valve Gain:

The relationship between the movement and the flow rate passing through the valve, which is expressed through curves on a graph as the valve characteristic curves.

Face to Face:

The distance between the inlet surface and the outlet surface of the valve. These dimensions are specified by the standard ANSI/ ISA.

High Recovery Valves:

A type of valve that loses a small amount of flow energy and creates minimal turbulence in the flow. In these valves, the pressure after the area of “Vena Contracta” is lower than that of high recovery valves. Globe valves fall into this category.

Nominal Pipe Size or NPS:

The diameter of the pipe in inches, indicated by writing a number next to the term NPS.

Nominal Size or DN:

The metric size designation of a pipe (measured in millimeters), indicated by the letters DN followed by a specific number.

Plug:

A cylindrical member that moves linearly in the flow path to adjust the direction of fluid flow. This part can also be a cylinder or a frustum that allows fluid to pass through it and regulates the fluid flow with its rotational movement.

Range ability:

The range associated with a control valve in which the valve performs its control function, defined as the ratio of the maximum to the minimum controllable flow. Range ability is typically considered within 10 percent to 100 percent of the valve's movement.

Reverse Acting Valve:

It is a control valve that opens when the signal applied to the actuator increases.