Pneumatic vs Electric Valve Actuators: Selection, Sizing and NAMUR Mounting

Quick Answer

A valve actuator opens, closes or positions a valve without an operator. Pneumatic actuators run on compressed air: rack-and-pinion designs give constant torque across the stroke when double-acting and are widely used on small and medium quarter-turn valves, while scotch-yoke designs peak at both ends of the stroke, where ball, plug and butterfly valves need torque most, and are most effective on large valves. Spring-return versions move to a set fail position when air is lost; double-acting ones stay put. Electric actuators need only power and a signal, normally stop where they are on power loss, and are rated by EN 15714-2 duty class: A on-off, B inching, C modulating, D continuous modulating, with at least IP65. Choose pneumatic where instrument air exists and the valve must fail safe or cycles often; choose electric where there is no air, for multi-turn valves and for remote sites. Size from the valve maker's breakaway torque at the minimum air pressure at the valve, add a margin (many buyers ask for 25%), and keep maximum output below the stem's torque limit. Valves and actuators meet at an ISO 5211 flange; accessories mount through the NAMUR (VDI/VDE 3845) interface.

A typical actuator enquiry is one line: "DN100 ball valve, pneumatic actuator, spring return." It gives no valve torque, air pressure at the valve, fail position, solenoid voltage or area classification. A guess on torque is how actuators end up too weak to unseat the valve, or strong enough to twist its stem.

A common result: the actuator strokes the valve on the bench at 6 bar, then stalls months later where the air is 4.5 bar on a cold morning and the valve has not moved for weeks. This guide covers how pneumatic and electric actuators work, how to choose and size them, and what the ISO 5211 and NAMUR interfaces standardise.

What an automated valve package includes

• Valve: quarter-turn (ball, butterfly, plug) or multi-turn (gate, globe)
• Mounting kit: bracket and coupling, or direct mounting on a shared ISO 5211 flange
• Actuator: pneumatic or electric
• Solenoid valve: switches air to a pneumatic on-off actuator
• Limit switch box: tells the control system the valve is open or closed
• Positioner: holds a modulating valve where the control signal asks
• Air filter regulator and a manual override or gearbox

Part-turn actuators turn the valve through less than one revolution; multi-turn actuators drive gate and globe stems through many turns and attach under ISO 5210.

Pneumatic actuators

Rack-and-pinion

Two opposed pistons each carry a toothed rack that engages a central pinion; air between the pistons turns it one way, air on their outer faces turns it back. Torque is piston force × pitch radius × 2, and since the radius never changes, torque is constant through the stroke. On a double-acting unit it is also proportional to supply pressure: 10% less air, 10% less torque. NAMUR faces on the body take a solenoid valve without tubing, and a switch box or positioner on a standard bracket.

Scotch yoke

A piston drives a pin in a slotted yoke on the output shaft. The lever arm is longest at both ends of travel, so torque peaks at 0° and 90° and dips mid-stroke. That suits quarter-turn valves, which need most torque to break off the seat and again as soft seats re-engage, and it is why scotch yokes are most effective on large valves. Double-acting units give different torque on each stroke, so read both columns of the maker's table.

Double-acting or spring-return

Spring-return actuators have four torque values, air start, air end, spring start and spring end, and each must beat the valve's torque at that point. Spring end is often the weak one, since the least-compressed springs must seat the valve.

Electric actuators

An electric actuator is a motor and reduction gearing, with limit switches at the end positions and a torque limit that stops it if the valve jams. Part-turn units turn quarter-turn valves; multi-turn units drive gate and globe valves, and on a worm gearbox can turn very large quarter-turn valves. Integral controls accept open/close commands, a 4–20 mA signal or a fieldbus.

Duty classes

EN 15714-2, the European standard for electric valve actuators, classes them by duty:

Motor, gearing and switching are built for different numbers of starts: in the standard's endurance tests, a part-turn actuator up to 125 Nm does 10,000 on-off cycles in Class A against 1,800,000 modulating starts in Class C. Those are test levels, not guaranteed lives, but a valve cycling six times an hour around the clock passes 10,000 cycles in about 70 days. Duties like that need a higher-rated Class C or D actuator, or a pneumatic one.

Enclosure, hazardous areas and power loss

EN 15714-2 calls for at least IP65 to EN 60529. Above IPX6 the codes are not cumulative: IPX7 is temporary immersion (30 minutes with the lowest point 1 m down) and IPX8 continuous immersion on the maker's stated terms, and neither proves jet resistance. For a pit that floods and is hosed down, specify IP66/IP68; outdoors, add an anti-condensation heater.

In explosive atmospheres the actuator must be certified for the zone, gas group and temperature class: under the ATEX Directive 2014/34/EU in the EU, and often under the IECEx scheme elsewhere. The same applies to electric motors.

On power loss an electric actuator stops where it is. Fail-safe versions with a spring or stored-energy module cost more and come in narrower torque ranges; where air exists, spring-return pneumatic is usually simpler.

Pneumatic vs electric compared

Three questions usually decide it: Is there reliable instrument air at the valve? Must the valve fail to a set position? How often does it move? For the air supply, see our guide to compressed air systems; actuator makers state air quality as an ISO 8573-1 purity class.

Mounting interfaces: ISO 5211 and NAMUR

ISO 5211: valve to actuator

ISO 5211 fixes the flange and drive dimensions between a part-turn valve and its actuator, so a compliant actuator fits any valve with the same flange code and drive. The fourth edition was published in 2026; check the edition your specification calls up. Each flange code carries a maximum flange torque:

The limits assume a stated bolt stress and friction; shock and inertia add load. Drives are square, diagonal square, flat head (two flats) or keyed: state type and size. Dual-drilled valve tops, such as F07/F10, accept either actuator size.

NAMUR: actuator to accessories

NAMUR is the user association of automation technology in the process industries. Its name is used loosely for the accessory interfaces in VDI/VDE 3845, the German guideline for interfaces between valves, actuators and auxiliary equipment:

• Solenoid valve face: in the common 1/4 in size, a 32 × 24 mm pattern with M5 screws and G1/8 or G1/4 ports, so the solenoid valve bolts straight onto the actuator's air ports; the 1/2 in size uses 45 × 40 mm and M6 screws
• Top face: an 80 × 30 mm bracket pattern (130 × 30 mm on larger actuators) and the pinion top that drives a switch box or positioner

"NAMUR" also names a two-wire inductive sensor for intrinsically safe circuits, so ask whether a "NAMUR switch box" means the mounting, the sensors or both.

Accessories: solenoids, switch boxes, positioners and gear operators

Solenoid valves

• 3/2 (three ports, two positions) fills and exhausts one chamber: for spring-return actuators
• 5/2 (five ports, two positions) fills one chamber while exhausting the other: for double-acting actuators. Many NAMUR solenoid valves convert between the two with an interface plate
• Also specify normally closed or open, coil voltage, certification, a manual override and exhaust restrictors for stroke speed

Limit switch boxes

A switch box mounts on the NAMUR top face and reports open and closed through microswitches or inductive sensors, with a visual indicator: proof the valve moved, not just that it was told to.

Positioners

A positioner compares the control signal, typically 4–20 mA, with actual position and adjusts the air until they agree; rotary types suit quarter-turn valves, linear types sliding stems. See our guide to control valves and Cv sizing.

Manual gear operators

A worm gearbox cuts handwheel torque on quarter-turn valves and holds position; bevel gearboxes serve multi-turn valves. Input torque = valve torque ÷ (ratio × efficiency). A 1,200 Nm butterfly valve on a 40:1 worm gearbox at 30% efficiency needs 100 Nm: 500 N at the rim of a 400 mm handwheel, 333 N on a 600 mm one. Use a chainwheel for valves out of reach.

Sizing an actuator

1. Get the valve torque from the valve maker for your differential pressure, fluid, temperature and operating frequency, at points such as break-to-open (BTO), run, end-to-open (ETO), break-to-close (BTC) and end-to-close (ETC).
2. Add a margin. Many buyers ask for 25%. One valve maker's published method adds factors for fluid (0 clean lubricating, 0.1 clean non-lubricating, 0.3 dry gas, 0.5 abrasive slurry), frequency (0.2 weekly, 0.5 monthly or yearly) and seat material, so a weekly-operated valve on clean non-lubricating service with a PTFE seat gets 1 + 0.1 + 0.2 = 1.3: valves that rarely move stick harder.
3. Use the minimum pressure at the valve, not the compressor setting.
4. Check every torque point on spring-return actuators.
5. Check the top end: output at maximum supply pressure must stay below the valve's maximum allowable stem torque (MAST) and the ISO 5211 flange limit.

Worked example: DN100 ball valve at 5.5 bar

A DN100 (4 in) floating ball valve on clean hydrocarbon liquid. The valve maker gives BTO 180 Nm, run 110 Nm, ETO 120 Nm, BTC 120 Nm and ETC 150 Nm at full differential pressure, and a MAST of 450 Nm. The air header runs at 6 bar, falls to 5.5 bar (80 psi) at the valve at worst and can reach 7 bar (102 psi). The buyer asks for a 25% margin.

Required torques: BTO 180 × 1.25 = 225 Nm (1,991 lbf·in); ETO and BTC 150 Nm; ETC 187.5 Nm.

Double-acting rack-and-pinion. Torque scales with pressure, so read the catalogue at 6 bar and correct:

At 7 bar, size B gives 260 × 7 ÷ 6 = 303.3 Nm: below the 450 Nm MAST, but above the 250 Nm that ISO 5211 allows an F07 flange, so mount it on the F10 interface (500 Nm).

Spring-return, fail-closed. The same valve on a spring-return actuator whose air section gives 600 Nm at 5.5 bar, with 330 Nm at spring start (springs fully compressed) and 200 Nm at spring end:

Two lessons. The spring-return actuator is about 2.5 times the double-acting one (600 Nm against 238 Nm), and the spring end, not the breakaway, sets its size. At 7 bar its air start rises to 600 × 7 ÷ 5.5 − 200 = 563.6 Nm, above the 450 Nm MAST, so fit a filter regulator set at 5.5 bar.

An electric actuator for the same valve needs at least 225 Nm, a torque limit set below the MAST, Class A duty if the valve only opens and closes, and a stroke time that suits the process.

Common Mistakes

1. Sizing at the compressor setting. The pressure at the valve is lower, especially on cold mornings and at peak demand.

   Prevention: Size at the minimum pressure measured or calculated at the valve.

2. Using clean-service torque for real service. Dirty fluids, dry gas, low temperatures and infrequent operation all raise breakaway torque.

   Prevention: Get service torques from the valve maker and add a margin for the duty.

3. Oversizing without a top-end check. At maximum air pressure it shears keys, twists stems or overloads the flange.

   Prevention: Check output at maximum supply pressure against the MAST and the ISO 5211 flange torque; fit a regulator where needed.

4. Checking only the breakaway on spring return. A spring end below the seating torque leaves the valve passing after air failure.

   Prevention: Check air start, air end, spring start and spring end against the matching valve torques.

5. The wrong solenoid valve. A 3/2 valve cannot drive a double-acting actuator back; an uncertified coil breaks a hazardous area's protection.

   Prevention: Use 3/2 for spring return and 5/2 for double-acting, and match coil voltage and certification to the site.

6. An on-off electric actuator on frequent-cycling or modulating duty. The motor overheats, trips on its thermal protection and wears early.

   Prevention: Specify the EN 15714-2 duty class from the real cycling rate.

7. Assuming an electric actuator fails safe. On power loss it stays where it is.

   Prevention: Use spring-return pneumatic, or a fail-safe electric actuator, where the hazard analysis needs a fail position.

8. Mismatched mounting. The wrong flange code, drive size or orientation, or a loose coupling that makes the switch box misread.

   Prevention: Send the valve's top-flange code, drive type and size, and the required actuator orientation.

What to send for a quotation

• Valve: type, maker and model, size and pressure class, quarter-turn or multi-turn, ISO 5211 top flange, drive type and size, orientation
• Torque data: BTO, run, ETO, BTC and ETC at maximum differential pressure, and the MAST
• Service: fluid, temperature, differential pressure, how often the valve moves
• Function: on-off or modulating, fail position (closed, open or in place), stroke time
• Pneumatic: minimum and maximum air pressure at the valve, double-acting or spring-return, solenoid function (3/2 or 5/2) and coil voltage
• Electric: supply voltage and phases, control signal (open/close, 4–20 mA or fieldbus), duty class, enclosure rating
• Accessories: switch box and sensor type, positioner and signal, filter regulator, manual override or gearbox
• Site: ambient temperature range, indoor or outdoor, corrosive atmosphere, hazardous-area zone, gas group and temperature class

Supply from Kasko Makine

Kasko Makine supplies valve automation products and accessories:

• Pneumatic actuators: convert compressed air into rotary motion for quarter-turn valves, or into linear motion through piston-and-cylinder designs, for automated valve control systems
• Electrical actuators: electromechanical actuators for AC, DC or inverter-based supplies, producing rotary or linear motion
• NAMUR valves: NAMUR solenoid valves that mount directly on pneumatic actuators without additional fittings, in 3-way versions and in 4-way versions for double-acting rack-and-pinion actuators, normally closed or normally open
• Limit switch boxes: open and closed position feedback, with visual and electrical indication, for valves and actuators
• Rotary and linear type positioners: for on-off and modulating control valve applications, with rotary and linear valve designs
• Gearboxes for valves: cast iron body, stainless steel stem, electrostatic powder epoxy coating and EN ISO 5211 mounting, for part-turn and quarter-turn valves from DN40 to DN400 in PN10 to PN16, with handwheel or chainwheel operation

Engineering support: Send the valve datasheet with its torque values, the air pressure range or power supply at the valve, the fail position and the area classification. We will review actuator type and size against your data and quote the actuator and its accessories together.

Send the valve type, size, torque data, service and air or power supply for an actuator proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Control Valves & Actuators: Types, Cv Sizing & Selection Guide
• Ball Valves: Floating vs Trunnion, Types & Selection Guide
• Ball Valve vs Butterfly Valve: Detailed Comparison & Selection Guide
• Compressed Air Systems: Compressors, Dryers & System Design Guide
• Industrial Electric Motors: IE Efficiency Classes, Enclosures and Hazardous-Area Selection

Frequently Asked Questions

Q: What is the difference between pneumatic and electric valve actuators?
A: A pneumatic actuator uses compressed air to drive a piston; an electric actuator uses a motor and reduction gearing. Pneumatic actuators are fast, suit frequent cycling and fail safe with spring return, but need instrument air. Electric actuators need only power and a signal, suit remote sites and multi-turn valves, and normally stay in place when power is lost.

Q: Which actuator is better for fail-safe service?
A: A spring-return pneumatic actuator is the usual choice: its springs drive the valve closed or open whenever air or the solenoid signal is lost. Double-acting pneumatic actuators stay where they are on air loss, as standard electric actuators do on power loss. Fail-safe electric models with a spring or stored-energy module exist, but they cost more.

Q: What is the difference between rack-and-pinion and scotch-yoke actuators?
A: In a rack-and-pinion actuator, two pistons drive racks on a central pinion, so a double-acting unit gives constant torque through the stroke; on spring-return units it falls as the springs work. In a scotch-yoke actuator, a piston drives a pin in a slotted yoke, so torque is highest at both ends of travel and lowest mid-stroke. That matches the break-out and seating demand of large ball, plug and butterfly valves.

Q: How do you size a pneumatic actuator for a ball valve?
A: Take the valve maker's torques for the actual service, add a margin (many buyers ask for 25 percent), and choose an actuator that exceeds them at the minimum air pressure at the valve. A spring-return actuator must pass four checks: air start, air end, spring start and spring end. Then check output at maximum pressure against the stem torque limit.

Q: What is a NAMUR solenoid valve?
A: A NAMUR solenoid valve has a standard face that bolts directly onto a pneumatic actuator's air ports without tubing. The interface comes from VDI/VDE 3845; the common 1/4 inch size uses a 32 by 24 mm pattern with M5 screws. Use a 3/2 version on spring-return actuators and a 5/2 version on double-acting ones; many convert between the two.

Q: What is ISO 5211?
A: ISO 5211 is the international standard for attaching part-turn actuators to industrial valves. It fixes flange dimensions, drive dimensions and maximum flange torques, so an actuator fits any valve with the same flange code and drive. An F07 flange has a 70 mm pitch circle and a 250 Nm rating; an F10 flange has a 102 mm pitch circle and 500 Nm.

Q: What are the duty classes for electric valve actuators?
A: EN 15714-2 defines four classes: Class A for on-off duty, driving the valve fully open or closed; Class B for inching or positioning, occasionally moving it to any position; Class C for modulating, frequently moving it to any position; and Class D for continuous modulating. Match the class to the real cycling rate. The standard also requires at least IP65 enclosures.