Note that the symbols alone in Figure 3 do not provide the reader with enough information to determine whether applying air pressure to the diaphragm opens or closes the process control valve, or whether energizing the solenoid pressurizes or vents the diaphragm. The 3-way valve may supply air to the control valve’s diaphragm or vent the diaphragm to the atmosphere. In this example, air to the process air-operated control valve is controlled by the solenoid-operated, 3-way valve in the air supply line. The intermediate control valve is placed in the line supplying motive force to the process control valve, as shown in Figure 3. In many cases, remote control of a valve is accomplished by using an intermediate, small control valve to operate the actuator of the process control valve. The most commonly found configurations are to manually control the actuator from a remote operating station, to automatically control the actuator from an instrument, or both. Control valves can be configured in many different ways. ![]() ![]() Control valves are symbolized by combining the appropriate valve symbol and actuator symbol, as illustrated in Figure 2. The combination of a valve and an actuator is commonly called a control valve. If no actuator is shown on a valve symbol, it may be assumed the valve is equipped only with a hand-wheel for manual operation. Note that although each is shown attached to a gate valve, an actuator can be attached to any type of valve body. Some valves are provided with actuators to allow remote operation, to increase mechanical advantage, or both.įigure 2 shows the symbols for the common valve actuators. In such cases, information concerning the valve type may be conveyed by the component identification number or by the notes and legend section of the drawing however, in many instances even that may not hold true. ![]() It should be noted that globe and gate valves will often be depicted by the same valve symbol. Figure 1 shows the symbols that depict the major valve types. Valves are used to control the direction, flow rate, and pressure of fluids. To read and understand engineering fluid diagrams and prints, usually referred to as P&IDs, an individual must be familiar with the basic symbols.
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