2-Way vs 3-Way Solenoid Valve

Quick Answer

A 2-way solenoid valve has one inlet and one outlet; it opens or closes a single flow path. A 3-way solenoid valve has three ports (pressure, exhaust, and cylinder/output); it switches flow between two paths. 2-way valves are used for simple on/off isolation. 3-way valves are used as pilot valves for single-acting pneumatic actuators, diverting air between the actuator and exhaust.

Detailed Comparison

Parameter	2-Way Solenoid (2/2)	3-Way Solenoid (3/2)
Number of ports	2 (inlet + outlet)	3 (pressure + cylinder + exhaust)
Function	Open or close one flow path	Switch flow between two paths
States	Open / Closed	Pressurize-cylinder / Exhaust-cylinder
Typical application	On/off fluid isolation	Pilot for single-acting actuator
Default state	NC (closed) or NO (open)	NC or NO (determines actuator rest position)
NAMUR mount	Not typically	Yes (NAMUR standard interface for actuators)
Body style	In-line pipe connection	In-line or NAMUR-mount
Response time	10-50 ms	10-50 ms

2-Way Solenoid Valve (2/2)

The 2-way valve has two ports: inlet (P) and outlet (A). In the de-energized state, a normally closed (NC) valve blocks flow from P to A. Energizing the coil lifts the plunger and allows flow from P to A. De-energizing closes the valve and stops flow.

Applications:

• Fuel gas shut-off
• Water and steam on/off control
• Chemical dosing and metering
• Instrument air supply isolation
• Fire suppression system activation

3-Way Solenoid Valve (3/2)

The 3-way valve has three ports: pressure supply (P), cylinder/output (A), and exhaust (R). It performs two functions simultaneously: connecting one path while blocking the other.

State	Port P to A	Port A to R	Actuator Effect
De-energized (NC)	Blocked	Connected	Cylinder exhausts; spring drives actuator
Energized	Connected	Blocked	Air pressurizes cylinder; actuator strokes

This makes the 3-way solenoid the standard pilot valve for single-acting pneumatic actuators. Energizing the solenoid pressurizes the actuator to open (or close) the process valve. De-energizing vents the actuator, and the return spring drives the valve to its fail-safe position.

4-Way and 5-Way Solenoid Valves

For double-acting actuators (no return spring), 4-way (4/2) or 5-way (5/2) solenoid valves are used:

Type	Ports	Actuator Type	Function
2-way (2/2)	2	None (direct isolation)	Open/close one path
3-way (3/2)	3	Single-acting (spring return)	Supply or exhaust one actuator port
4-way (4/2)	4	Double-acting	Supply one side, exhaust the other
5-way (5/2)	5	Double-acting (separate exhausts)	Independent exhaust control per side

NAMUR Interface

NAMUR (VDI/VDE 3845) defines a standard mounting interface for solenoid pilot valves on pneumatic actuators. A NAMUR-mount 3-way solenoid bolts directly onto the actuator body, with internal passages connecting the solenoid ports to the actuator cylinder. This eliminates external tubing, reduces potential leak points, and simplifies installation.

Feature	NAMUR Mount	Remote Mount
Tubing	None (internal passages)	Required (copper, SS, or plastic)
Leak points	Minimal (2 gasket joints)	Multiple (fittings, tube connections)
Response time	Faster (no tubing volume)	Slower (tubing adds dead volume)
Maintenance	Easy swap (bolted to actuator)	Requires tubing disconnection

Pro Tip

For ESD (emergency shutdown) applications, specify a 3-way NC solenoid on single-acting spring-return actuators. On power or signal loss, the solenoid de-energizes, exhausts the actuator air, and the spring drives the valve to the safe position; no control system action required.

Read the full guide to valve types