How Solenoid Valves Work in Industrial Automation
Walk through any automated plant, whether a refinery, water treatment works, food factory or packaging line, and you will find thousands of solenoid valves. They are the link between the electrical world of PLCs and control systems and the mechanical world of valves, cylinders and actuators. Each time a control system opens a process valve, moves a pneumatic cylinder or trips a plant to a safe state, a solenoid valve is usually doing the work.
This article explains how solenoid valves work in industrial automation, how they control pneumatic actuators and cylinders, the main configurations, and what to consider when selecting them for reliable and safe operation.
What Is a Solenoid Valve?
A solenoid valve is an electromechanical valve that converts an electrical signal into mechanical movement. It consists of a coil of copper wire, a movable ferromagnetic plunger (armature), a return spring, a valve body with ports and seals. When current flows through the coil, the resulting magnetic field pulls the plunger, opening or closing flow paths. When the current stops, the spring returns the plunger to its rest position.
The Role of Solenoid Valves in Automation
In automation, solenoid valves rarely control the main process flow directly. Instead, they act as pilot valves, directing compressed instrument air to pneumatic actuators that operate much larger process valves, or to pneumatic cylinders that move machinery. This allows a small, low-power electrical signal from a PLC output, typically 24 VDC and a few watts, to control very large forces.
Typical automation tasks include opening and closing on/off ball and butterfly valves, operating dampers and gates, driving pneumatic cylinders in packaging and assembly machines, controlling air blow-off and cleaning, and dosing liquids and gases in small lines.
How a Solenoid Valve Controls a Pneumatic Actuator
Single-Acting (Spring-Return) Actuators
A spring-return actuator has air on one side of the piston and springs on the other. A 3/2-way solenoid valve (three ports, two positions) controls it:
- Energised: The valve connects the air supply (port 1) to the actuator (port 2), pressurising the actuator and driving the process valve to its operated position, for example open.
- De-energised: The supply is blocked and the actuator is vented to atmosphere through the exhaust (port 3). The springs drive the process valve to its fail-safe position, usually closed.
This arrangement provides inherent fail-safe action: any loss of power or air returns the valve to its safe state.
Double-Acting Actuators and Cylinders
A double-acting actuator uses air to move the piston in both directions. A 5/2-way solenoid valve (five ports, two positions) supplies air to one side while exhausting the other, and reverses when switched. Single-solenoid versions return to a default position by spring; double-solenoid (bistable) versions hold their last position until the opposite coil is energised, which is useful where the valve must stay in place on power loss. A 5 port solenoid valve is the standard choice for double-acting cylinders and actuators. 5/3-way valves add a centre position, closed, exhaust or pressure, for stopping a cylinder mid-stroke.
NAMUR Solenoid Valves
NAMUR is an interface standard (VDI/VDE 3845) that defines a common mounting pattern for solenoid valves on rotary pneumatic actuators. A NAMUR solenoid valve bolts directly to the actuator’s air ports without tubing, which reduces leak points, installation time and space. Many NAMUR valves can be converted between 3/2 and 5/2 function with a reversible plate, so a single valve type suits both spring-return and double-acting actuators.
Direct-Acting, Pilot-Operated and Semi-Direct Designs
- Direct-acting: The plunger opens the orifice directly. Works from zero pressure, fast and simple, but limited to small orifices.
- Pilot-operated (internally piloted): The coil operates a small pilot that uses line pressure to move a larger diaphragm or spool. Handles large flows with low power, but needs a minimum differential pressure, typically around 2-3 bar for pneumatic spool valves.
- Externally piloted: Pilot air from a separate supply allows operation at low or vacuum main line pressures.
- Semi-direct (assisted lift): Combines both, operating from zero pressure with higher flow.
Integration with Control Systems
- PLC and DCS outputs: Usually 24 VDC digital outputs, sometimes 110/230 VAC.
- Valve manifolds and valve islands: Groups of solenoid valves on a common manifold with fieldbus (PROFINET, EtherNet/IP, Modbus) or IO-Link communication, common in machine automation.
- Position feedback: Limit switch boxes on the actuator confirm open and closed positions back to the control system.
- Low-power coils:5 W to 1.8 W intrinsically safe coils allow direct connection to barriers and reduce panel heat.
Solenoid Valves in Safety Systems
Solenoid valves play a critical role in emergency shutdown (ESD) and safety instrumented systems (SIS). In a typical ESD valve, the solenoid is energised during normal operation (energise-to-run). On a trip, the logic solver de-energises the solenoid, the actuator vents and the valve moves to its safe position. For high-integrity applications:
- Use SIL-certified solenoid valves with documented failure rates (IEC 61508 / IEC 61511).
- Consider redundant 1oo2 or 2oo3 solenoid arrangements for higher safety integrity or availability.
- Use stainless steel bodies and high-flow exhaust for fast stroking times.
- Perform partial stroke testing and proof testing at defined intervals.
Hazardous Area and Environmental Requirements
- Flameproof (Ex d) coils: Contain any internal ignition within the enclosure.
- Intrinsically safe (Ex ia) coils: Limit energy to levels incapable of ignition; used with IS barriers.
- Encapsulated (Ex m) coils: Common in Zone 1 and Zone 2.
- Ingress protection: IP65, IP66 or IP67 for outdoor and washdown environments.
- Materials: Stainless steel 316 bodies for offshore, marine and corrosive atmospheres; aluminium or brass for indoor utility service.
- Temperature: Low-temperature seals for cold climates and high-temperature seals near hot equipment.
Stroking Time and Flow Capacity
The speed at which an actuator opens or closes depends largely on how quickly air can be supplied to and exhausted from it. A solenoid valve with too small a flow coefficient (Cv) will make large actuators slow to stroke, which can be unacceptable for ESD valves that must close within a few seconds. For large actuators, designers increase solenoid valve size, add quick exhaust valves or use the solenoid to pilot a larger volume booster. Always check the required stroking time against the actuator volume and the solenoid valve flow capacity during selection.
Benefits of Solenoid Valves in Automation
- Fast response, typically tens of milliseconds
- Simple interface with PLC and DCS digital outputs
- Low power consumption, especially with pilot-operated and low-power coils
- Inherent fail-safe action with spring return
- Compact size, easy installation and low cost
- Long service life with clean, dry instrument air
Selecting a Solenoid Valve for Automation
- Function: 2/2, 3/2, 5/2 or 5/3, and normally closed or normally open.
- Actuator type and size: Required flow (Cv) to achieve the specified stroking time.
- Supply pressure and medium: Instrument air quality, pressure range, minimum differential for pilot-operated valves.
- Electrical: Voltage, power, duty cycle and connector type; manual override requirement.
- Mounting: NAMUR, inline, sub-base or manifold.
- Certifications: ATEX / IECEx, SIL and project specifications.
- Materials: Body, seals and coil enclosure suited to the environment.
Maintenance and Troubleshooting
- Supply clean, dry instrument air; contamination and moisture are the leading causes of sticking spools.
- Check coils for overheating, insulation damage and correct voltage.
- Listen for continuous air exhaust, which indicates worn seals or a leaking actuator.
- Verify manual override operation and stroke times during scheduled tests.
- Replace seal kits and coils according to the manufacturer’s recommendations or test results.
Solenoid Valves from Bliss Flow Systems
Bliss Flow Systems supplies a complete range of solenoid valves for industrial automation, including NAMUR, 2-port, 3-port and 5-port, sub-base mounted, engineered and customised solenoid valves, along with pneumatic actuators, limit switches and positioners. Explore our solenoid valve range for hazardous-area, SIL and offshore-ready designs.
Frequently Asked Questions
How does a solenoid valve control a pneumatic actuator?
The solenoid valve directs compressed air to the actuator when energised and vents it when de-energised, so a small electrical signal moves a large valve or cylinder.
What is the difference between 3/2 and 5/2 solenoid valves?
A 3/2 valve has three ports and is used for single-acting spring-return actuators. A 5/2 valve has five ports and is used for double-acting actuators and cylinders.
What is a NAMUR solenoid valve?
It is a solenoid valve with a standard mounting interface that bolts directly onto rotary pneumatic actuators without external tubing.
Why are ESD solenoid valves normally energised?
So that any loss of power, wiring fault or trip signal de-energises the valve and drives the process valve to its safe position.
Which solenoid valves are used in hazardous areas?
Valves with ATEX or IECEx certified coils, such as flameproof (Ex d), intrinsically safe (Ex ia) or encapsulated (Ex m) designs.
Can solenoid valves be used for liquids as well as air?
Yes. While automation valves mostly pilot instrument air, 2/2-way direct-acting and pilot-operated solenoid valves are widely used to control water, oils, gases and chemicals directly in smaller lines, with body and seal materials chosen for the medium.
Conclusion
Solenoid valves are the essential link between control systems and the mechanical movement that runs industrial plants. By understanding how they pilot actuators and cylinders, choosing the right function, mounting, certification and materials, and maintaining a clean air supply, you can build automation systems that are fast, reliable and safe.
Contact Bliss Flow Systems
For product selection, technical datasheets or a quotation, reach our engineering sales team at the office nearest to you, or send an enquiry through our contact page.