Normally Open vs Normally Closed Solenoid Valves
Figure 1: A normally closed 2-way solenoid valve
Solenoid valves are common in industrial and commercial applications to control the flow of fluids. They are typically normally open or normally closed. In short, a normally open valve allows flow through with no power and a normally closed valve stops flow with no power.
This article overviews the design differences between normally open and normally closed solenoid valves, how they work, and when to use each type.
View our online selection of solenoid valves!
What is a solenoid valve?
The actuator of a solenoid valve has a coil of wire that wraps around a ferromagnetic core. When a current passes through the coil, a magnetic field forms. This field is strong enough to move an armature towards the coil's center, to open or close the valve. When the current stops flowing through the coil, a spring returns the plunger to its original position. Solenoid valves are popular because of their precision, speed, and reliability. To learn more, read our solenoid valve overview article.
Normally open solenoid valve
A normally open solenoid valve is open in its de-energized state. There are various designs, but a basic design has the armature positioned high up in the coil. So when the valve energizes, the armature is pulled down towards the center of the coil. A plunger or piston attached to the armature moves down, closing the valve. When the valve is again de-energized, a spring pushes the armature back up, opening the valve.
Normally closed solenoid valve
A normally closed solenoid valve is closed in its de-energized state. As with normally open valves, there are a variety of designs. A simple design has the armature low in the coil. When the valve is energized, the armature moves up towards the current's center, pulling a plunger or piston. This opens the valve. When the current stops, a spring pushes the armature back down to close the valve.
Choosing between them
Choosing between a normally open and normally closed solenoid valve depends on two conditions:
Failure state: What would happen in the event of power failure that leads to the solenoid valve being de-energized? To prevent damage to the system or environment, in the case of hazardous media, should the valve remain open or closed?
- A normally closed solenoid valve is typically used for emergency safety to stop the flow of media. However, if a pressure-build up is the emergency concern, then a normally open solenoid valve should be used.
- Energy conservation: If a valve is open significantly more than it is closed, then to save on energy cost a normally open valve would be beneficial. Conversely, if the valve is usually closed, a normally closed valve would save on energy cost.
Bi-stable solenoid valves
Figure 2: A latching solenoid valve
Sometimes an application requires a solenoid valve to be open for a long time and then closed for a long time. In such cases, a bi-stable solenoid valve is the best solution due to its energy efficiency. Bi-stable valves use a power supply for a very short period of time to change the armature's position, thus opening or closing the valve. When there is no power, the armature does not change its position.
Latching solenoid valves are found in a number of different industries. A door lock, however, is a specific example of an application that would likely use a bi-stable valve rather than normally open or normally closed. A small amount of power can open or close the lock, which then remains in its position until power is again supplied to the solenoid valve.
Read our article on latching solenoid valves to learn more.
Are solenoid valves normally open or normally closed?
Solenoid valves can be normally open or normally closed. Bi-stable solenoid valves are also available.
What does normally open mean on a solenoid valve?
A normally open solenoid valve is open in the valve's de-energized state. When current flows through the solenoid, a normally open solenoid valve closes.
What makes a solenoid valve open and close?
A normally closed solenoid valve, for example, opens when current flows through the solenoid. The magnetic field formed moves the valve's plunger to open the valve. When the current stops, a spring moves the plunger back to its original position to close the valve.