What do the abbreviations “NO” and “NC” mean on valve drawings?
“NO” means normally open; “NC” means normally closed. The “normal” position for a valve is the position it is in at rest, with no electropneumatic actuation. An “Air Open / Spring Close” valve would be normally closed, since without any air pressure to open the valve, the spring would force the valve closed. Valves that are “Air Open / Air Close” can be set for either normally open or normally closed, depending on the user’s requirements. “Air Open / Air Close” valves always have air pressure being applied to one side or the other of the air piston, unless the facility has a catastrophic failure where all electricity and air pressure is lost. Without activating the solenoid, if air is directed to the top of the air piston, the air pressure will hold the valve closed – this would be a normally closed configuration. If the air lines are reversed so that the air is directed to the bottom of the air piston when the solenoid is not activated, the air pressure will hold the valve open – this
