The differential pressure in a closed water system is the maximum pump differential head. This allows for possible vacuum conditions downstream of the valve when the valve closes. In a steam system, the maximum differential pressure is usually assumed to be the same as the upstream absolute pressure. An actuator coupled to the valve spindle would provide valve movement. Two typical globe valve types are shown in Figure 6.1.1. 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.

Three-port valves can be used to ‘mix’ or ‘divert’ liquid passing through them.

Two-port valves ‘throttle’ (restrict) the fluid passing through them.

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

Rotary types include ball valves, butterfly valves, plug valves and their variants.

Linear types include globe valves and slide valves.

Valve types which have linear and rotary spindle movement.

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.

However, for the sake of clarity at this point, each item of equipment will be considered separately. Interface with plant digital communication systems.Modify the valve movements to alter the characteristics of the control valve.

Interface with a computer to alter the control functions, and perform diagnostic routines.Take input directly from a sensor and provide a control function.For example, the positioner, which traditionally adjusted the valve to a particular position within its range of travel, can now: With the advance of technology, the dividing line between individual items of equipment and their definitions are becoming less clear.

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.Ī basic control system would normally consist of the following components:Īll of these terms are generic and each can include many variations and characteristics.

Valve steam