Control valve

How to read it

In the DOE’s handbook on engineering prints, the combination of a valve and an actuator is commonly called a control valve, and its symbol is the valve symbol with the actuator symbol added. A dome on the stem is a diaphragm actuator. A box with M is an electric motor and a box with S a solenoid, and a square box on the actuator’s stem is a positioner. A valve with no actuator drawn can be taken to have only a handwheel. Kuphaldt’s Lessons In Industrial Instrumentation describes a control valve as one that restricts flow on a signal from a loop controller, a logic device such as a PLC, or a human operator, and as two parts: the valve body and the actuator. Some are built for on/off service and others to throttle.

In the DOE’s handbook, a balloon next to a control valve identifies its function, and its signal line shows how the signal reaches it: an electric signal or a pneumatic signal.

Also searched as: control valve p&id symbol, control valve symbol.

How control valves are tagged

In the DOE’s handbook, the first letter of a control valve’s tag is the parameter it controls: F for flow, T for temperature, L for level, P for pressure, H for a hand-operated valve. The second letter is usually C and the third V, so a level control valve is an LCV. Kuphaldt’s account of the ISA 5.1 letters names the valve by its loop’s variable alone: the valve in a level loop is an LV, in a temperature loop a TV.

What the symbol leaves out

The DOE’s handbook points out that the symbols alone don’t say whether air pressure on the diaphragm opens or closes the valve. Diagrams add that in letters: in Kuphaldt’s example, “ATO” under a valve means air-to-open, so the valve opens wider as the air signal rises.

Look-alikes

A relief valve also has something on its stem, but it is the spring, and the process pressure itself opens it; there is no actuator and no signal line.