When a pneumatic valve assembly doesn't perform the way it should — slow to stroke, erratic at end of travel, sluggish on one direction but not the other — the actuator usually gets the blame. More often than most people expect, the real problem is the solenoid valve sitting on top of it.
The solenoid is the component that translates an electrical control signal into pneumatic action. It's what actually lets air into the actuator and vents it back out. Get the solenoid selection wrong and the best actuator in the world can't do its job properly.
What does a solenoid valve actually do on a pneumatic actuator?
A solenoid valve is a small electrically operated valve that directs air supply to the actuator's ports and vents it on command. When the control system sends a signal — whether from a DCS, PLC, or manual switch — the solenoid shifts and routes supply air to drive the actuator open or closed. When the signal drops, the solenoid returns to its resting state, either venting the actuator or holding pressure depending on the configuration.
On most pneumatic valve assemblies, the solenoid mounts directly to the actuator body using a NAMUR interface — a standardized bolt pattern defined by the NAMUR recommendation that puts the solenoid right at the actuator's air inlet. This direct-mount arrangement eliminates the tubing, fittings, and tee connections that a remotely mounted solenoid would need.
3-way versus 5-way: matching the solenoid to the actuator type
This is the first decision, and getting it wrong is surprisingly common on retrofit projects.
A 3-way solenoid has three ports: pressure in, one cylinder port out, and an exhaust. It's designed for single-acting (spring-return) actuators, where air drives the actuator in one direction and the spring drives it back. The solenoid pressurizes one side and vents it — the spring handles the return stroke.
A 5-way solenoid has five ports: pressure in, two cylinder ports, and two exhausts. It's built for double-acting actuators, where air drives the actuator in both directions. The solenoid alternates which port gets supply pressure and which gets vented.
Put a 3-way on a double-acting actuator and one side never vents properly. Put a 5-way on a spring-return and you're paying for ports you don't use and potentially fighting the spring with trapped air on the return stroke. Neither mistake shows up until the valve doesn't behave the way the P&ID says it should.
Orifice size: the hidden cause of slow valve response
This is where the most common performance complaints originate, and it almost never gets traced back to the solenoid on the first service call.
The solenoid's orifice determines how much air can flow through it per unit of time. A larger actuator has a larger internal volume that needs to fill and exhaust with every stroke. If the solenoid orifice is too small for the actuator volume, the actuator starves for air on the fill stroke and can't exhaust fast enough on the vent stroke. The result is a valve that strokes noticeably slower than its rated speed — a problem that looks exactly like an undersized actuator or low supply pressure.
The fix is straightforward: check the solenoid's flow coefficient (Cv) against the actuator's volume and the required stroking time. Most actuator manufacturers publish recommended solenoid sizes for each actuator model. Use them.
Why direct-mount NAMUR solenoids matter
A remotely mounted solenoid connected to the actuator through tubing introduces dead volume, pressure drop across fittings, and additional potential leak points at every connection. Each fitting is a place where air can escape and a joint that has to be checked during maintenance.
A NAMUR direct-mount solenoid bolts straight to the actuator body, putting the solenoid's output port directly over the actuator's inlet. No tubing, no fittings between them. Response is faster because there's no dead volume to fill, and reliability is higher because there are fewer connections to leak. The assembly is also more compact — which matters more than it sounds when you're trying to fit automation packages into tight piping layouts.
Other selection details that matter in practice
Coil voltage has to match what your control system can deliver. Most industrial solenoids are available in 24 VDC, 110 VAC, and 220 VAC. Specify the wrong voltage and the solenoid either won't pull in at all or burns the coil out.
Hazardous area certification — explosion-proof or intrinsically safe — has to match the area classification where the valve assembly lives. This isn't optional and it isn't something you can retrofit after installation.
Manual override on the solenoid itself lets an operator shift the solenoid by hand to stroke the actuator without an electrical signal. Useful during commissioning, loop checks, and troubleshooting. Not every solenoid includes one, so specify it if your operation needs it.
Buy it as a package
The simplest way to avoid solenoid selection mistakes is to not make them separately. When the actuator, solenoid, limit switch, and mounting hardware come as a tested, matched package from a single supplier, the compatibility questions answer themselves.
UniTorq supplies complete valve automation packages — actuators, NAMUR solenoid valves, SeniTec limit switches, and all mounting accessories — sized and assembled to work together. If you're specifying a new project or chasing a performance problem on an existing assembly, contact us at 770-446-7074 or info@unitorq.com.