Learn About Steam

Block 6 of The Steam and Condensate Loop considers the practical aspects of control, putting the basic control theory discussed in Block 5 into practice.

A basic control system would normally consist of the following components:

• Control valves
• Actuators.
• Controllers.
• Sensors.

All of these terms are generic and each can include many variations and characteristics. With the advance of technology, the dividing line between individual items of equipment and their definitions are becoming less clear. For example, the positioner, which traditionally adjusted the valve to a particular position within its range of travel, can now:

• Take input directly from a sensor and provide a control function.
• Interface with a computer to alter the control functions, and perform diagnostic routines.
• Modify the valve movements to alter the characteristics of the control valve.
• Interface with plant digital communication systems.

However, for the sake of clarity at this point, each item of equipment will be considered separately.

Control Valves

Whilst a wide variety of valve types exist, this document will concentrate on those which are most widely used in the automatic control of steam and other industrial fluids. These include:

• Valve types which have linear and rotary spindle movement.
• Linear types include globe valves and slide valves.
• Rotary types include ball valves, butterfly valves, plug valves and their variants.

The first choice to be made is between two-port and three-port valves.

• Two-port valves ‘throttle’ (restrict) the fluid passing through them.
• Three-port valves can be used to ‘mix’ or ‘divert’ liquid passing through them.

Two-port valves

Globe valves

Globe valves are frequently used for control applications because of their suitability for throttling flow and the ease with which they can be given a specific ‘characteristic’, relating valve opening to flow.

Two typical globe valve types are shown in Figure 6.1.1. An actuator coupled to the valve spindle would provide valve movement.