Solenoid valves are among the most widely used components in industrial automation and compressed air systems. Whether you are controlling the flow of air to a pneumatic cylinder or managing process fluids in a factory, understanding how a solenoid valve works is fundamental to selecting and maintaining the right equipment.
What is a Solenoid Valve?
A solenoid valve is an electromechanically operated valve that uses an electromagnetic solenoid coil to control the flow of a gas or liquid. When an electric current passes through the coil, it generates a magnetic field that moves a plunger or pilot sleeve, opening or closing the valve. Remove the current, and a spring returns the valve to its default position.
The basic principle is straightforward: electricity in, mechanical movement out. This makes solenoid valves ideal wherever you need to control fluid flow remotely, automatically, or with precise timing.
Direct-Acting vs Pilot-Operated Solenoid Valves
There are two main operating principles you will encounter:
Direct-acting solenoid valves use the electromagnetic force directly to open or close the main orifice. The plunger physically seals or unseals the valve seat. These valves work at zero differential pressure, making them suitable for low-flow applications and systems where inlet and outlet pressure are equal.
Pilot-operated solenoid valves use a small pilot orifice controlled by the solenoid to direct line pressure, which then opens the main valve. This gives greater flow capacity from a relatively compact coil. However, pilot-operated valves require a minimum differential pressure (typically 0.5 bar) to operate correctly, so they are not suited for zero-pressure applications.
Valve Configurations: 2/2, 3/2 and 5/2
Solenoid valves are described by the number of ports and positions they offer:
- 2/2 valves have two ports and two positions — simply open or closed. They are used for on/off flow control.
- 3/2 valves have three ports and two positions. They are the standard choice for driving single-acting pneumatic cylinders — one port pressurises the cylinder, one exhausts it, and the third connects to the air supply.
- 5/2 valves have five ports and two positions and are used to control double-acting cylinders. They can direct air to either end of the cylinder while exhausting the other side.
5/3 valves add a third centre position — typically open centre, closed centre, or pressure centre — giving you additional control states for more complex applications.
Normally Closed vs Normally Open
Every solenoid valve has a “normal” or de-energised state. A normally closed (NC) valve blocks flow when no current is applied; it only opens when energised. A normally open (NO) valve passes flow by default and closes when energised. For most safety-critical applications, normally closed is preferred — if power is lost, the valve shuts off flow automatically.
Choosing the Right Voltage and Sealing Material
Solenoid valves are available in a wide range of coil voltages, typically 24V DC, 110V AC, or 240V AC. 24V DC is standard in most modern industrial controls because it is safer and more compatible with PLCs and control systems.
Sealing materials affect compatibility with different media. NBR (nitrile) is the most common general-purpose seal, suitable for air and many oils. FKM (Viton) handles higher temperatures and aggressive chemicals. EPDM is used for water and steam applications.
At Captivair we stock a comprehensive range of solenoid valves from leading brands including ASCO. Browse our ASCO solenoid valves to find the right configuration for your system.
Common Applications
Solenoid valves appear in an enormous range of industrial and commercial applications:
- Controlling pneumatic cylinders and actuators on production lines
- Automating irrigation and fluid dosing systems
- Managing compressed air in food and beverage processing
- Safety shut-off in gas supply lines
- Automotive paint spray booths and test equipment
Understanding the basic operating principles described above will help you choose the correct valve type, configuration, and voltage rating every time.
Frequently Asked Questions
What is the difference between a direct-acting and pilot-operated solenoid valve?
A direct-acting solenoid valve uses the electromagnetic coil force to directly open or close the valve orifice and works at zero differential pressure. A pilot-operated valve uses the solenoid to open a small pilot orifice, allowing line pressure to actuate the main valve u2014 giving higher flow capacity but requiring a minimum inlet pressure (typically 0.5 bar).
Can a solenoid valve work at zero pressure?
Yes, but only direct-acting solenoid valves can operate at zero differential pressure. Pilot-operated valves require a minimum differential pressure across the valve u2014 usually 0.3 to 0.5 bar u2014 to function correctly.
What does normally closed mean on a solenoid valve?
A normally closed (NC) solenoid valve is closed in its de-energised state and opens only when electrical current is applied to the coil. This is the most common configuration for safety applications, as losing power automatically shuts off flow.
Browse our pneumatic components: