Normally open solenoid valves is the valve stays open unless energized. When energy is applied, they immediately close, and stay that way as long as energized. Because of the way they close, these are not intended for continuous use of more than a few hours. They are a great choice for situations where a valve needs to only close for short spurts, and should remain open most of the time.
The Normally Open valve operation is useful in situations where it is necessary to keep the fluids flowing at all times. A good example is the cooling tower of an industrial system that should always be on to avoid overheating. In that case, the solenoid is only energized (to close the valve) when there is a real need to close it, for example, during scheduled maintenance or repairs.
