Control Valve Operation and Design Criteria for Beginners 1

Control valves are used throughout the process to control pressure, level, temperature, or flow. All control valves have variable opening or orifice. For a given pressure drop across the valve, the larger the orifice the greater the flow through the valve.

















Fig 1: Typical single-port body control valve (courtesy of Fisher)

Fig 2. Typical Pneumatic actuator (courtesy of Fisher)

Figure 1 shows the orifice is made larger by moving the valve stem upward and the orifice is made smaller by moving the valve downward. The most common way to affect this motion is with a pneumatic actuator (shown in Fig 2). Instrument air or gas applied to the actuator diaphragm overcomes the spring resistance and either moves the stem upward or downward.

Control valves are shown on the P&ID. However, the types are selected by a process engineer without the detailed process data. Accordingly, it is important to verify whether the type shown on P&ID are properly selected in accordance with the given process data.

Type Selection

Globe valve, Double P-port shall normally be considered as the first choice. Control valves type and sizing selection process is shown on “Flow Chart For Valve Selection” below

Flow Chart For Valve Type Selection

Control valve (1/3)

References:

1. Maurice Steward , Surface production operation
2. Fisher, Control valve handbook catalogue