This is quite similar to how the word “normally’ is used to describe switch contact status: a normally-open (NO) electrical switch is open when unactuated (at rest) a normally-closed (NC) electrical switch is closed when unactuated (at rest). In this context, the word “normally” refers to the unpowered state of the solenoid valve. Recall that a normally open solenoid valve allows fluid to pass through when it is de-energized.Ī normally closed solenoid valve, by contrast, shuts off fluid flow when de-energized. Unfortunately, this use of the word “normal” is altogether different from the use of the word “normal” when describing a solenoid valve’s open/close characteristics. Under “normal” conditions, where the turbine is not needed, the solenoids remain energized and the steam valve remains shut. Therefore, this steam turbine control system, which serves as a back-up to an electric motor-driven pump, relies on either (or both) of the solenoid valves de-energizing to make the turbine start up. In this case, “NE” stands for normally energized. The only indication of the solenoids’ typical status (energized or de-energized) comes from the letters “NE” next to each solenoid coil. My first inclination when sampling this real P&ID for inclusion in the book was to correct the errors, but I think an important lesson may be taught by leaving them in: documentation errors are a realistic challenge you will contend with on the job as an instrumentation professional! A more minor error in this diagram snippet is the labeling of SOV-590A: it actually reads “SOV-59DA” if you look closely enough! The output tube of PY-590 should only connect to the bottom of the piston actuator, not to the bottom and the top. Point 2 : If you examine this diagram closely, you will notice an error in it: it shows the top and bottom of the piston actuator connected together by air tubing, which if implemented in real life would prevent air pressure from imparting any force to the valve stem at all!Ĭonnecting the top and bottom of the actuator together would ensure the piston always sees zero differential pressure, and thus would never develop a resultant force. Point 1 : This solenoid valve arrangement would be designated 1oo2 from the perspective of starting the turbine, since only one out of the two solenoids needs to trip in order to initiate the turbine start-up.
This much is evident from the curved arrows showing air flowing to the “Vent” ports in a de-energized (DE) condition.Īn additional valve (PY-590) guarantees the piston actuator’s upward motion by simultaneously applying air pressure to the bottom (point 2) of the actuator if ever air is vented from the top.Īs an additional feature, the left-hand solenoid valve (SOV-590A) has a manual “Reset” lever on it, symbolized by the letter “R” inside a diamond outline. If either (point1) of the two solenoid valves de-energizes, instrument air pressure will vent from the top of the piston actuator to atmosphere, causing the steam valve to “fail” to the full-open position and send steam to the turbine. Take the following segment of an actual P&ID for a steam turbine-driven pump control system for example, where a pair of 3-way solenoid valves control instrument air pressure to a piston-actuated steam valve to start the turbine in the event that an electric motor-driven pump happens to fail: The choice to use a solenoid’s energized or de-energized state to perform a specific function is left to the system designer, but nevertheless it is important for all maintenance personnel to know in order to perform work on a solenoid-controlled system. Solenoid valves may be used in such a way that they spend most of their time de-energized, energizing only for brief periods of time when some special function is required.Īlternatively, solenoids may be maintained in an energized state, and de-energized to perform their design function.