Control valves are used throughout the process to control pressure, level, temperature, or flow. It is beyond the scope of this text to discussthe differences between the various types of control valves and the procedures for their sizing. This section focuses primarily on the functions of this equipment. Figure 2-3 shows a very common single-port globe body control valve. All control valves have a variable opening or orifice. For a given pressure drop across the valve, the larger the orifice the greater the flow through the valve.
Chokes and other flow control devices have either a fixed or variable orifice. With a fixed pressure drop across the device (i.e., with both the upstream and downstream pressure fixed by the process system) the larger the orifice the greater the flow. In Figure 2-3 the orifice is made larger by moving the valve stem upward. This moves the plug off the seat, creating a larger annulus for flow between the seat and the plug. Similarly, the orifice is made smaller by moving the valve stem downward. The most common way to affect this motion is with a pneumatic actuator, such as that shown in Figure 2-4. Instrument air or gas applied to the actuator diaphragm overcomes a spring resistance and either moves the stem upward or downward. The action of the actuator must be matched with the construction of the valve body to assure that the required failure mode is met. That is, if it is desirable for the valve to fail closed, then the actuator and body must be matched so that on failure of the instrument air or gas, the spring causes the stem to move in the direction that blocks flow (i.e., fully shut). This would normally be the case for most liquid control valves. If it is desirable for the valve to fail to open, as in many pressure control situations,then the spring must cause the stem to move in the fully open direction.