How to Choose a Valve Actuator

Most actuator specifications go wrong in one of two ways. Either the actuator is undersized, because the torque figure used was the valve's running torque rather than its break-out torque, and it stalls the first time the valve has sat closed for a month. Or the power source is chosen by habit rather than by duty — pneumatic because there is an air line nearby, electric because there is a socket — and the result is an actuator that works but costs more to run than it should, or fails in the wrong position when the supply drops.

This guide covers the four options, how to size against torque properly, what happens on power failure, and what hazardous area classification does to the shortlist. It ends with a decision framework you can work down in order.

The Short Version

Electric, pneumatic, hydraulic and manual, side by side. The rest of the guide explains the reasoning behind each row.

Pneumatic Actuators

A pneumatic actuator converts air pressure into movement, either by rack and pinion or by scotch yoke. Rack and pinion gives constant torque through the stroke and suits smaller quarter-turn valves. Scotch yoke gives its highest torque at the start and end of travel — exactly where a valve needs it, because break-out and seating are the hardest parts of the cycle — which is why heavy-duty valves are almost always scotch yoke.

The Kosaplus range covers both. The R series is rack and pinion, from the compact R32 delivering 4.7 Nm at 6 bar up to the R210 at 1,263 Nm, across thirteen sizes. The A series is scotch yoke, from the A50 at 53 Nm up to the A300 delivering 11,422 Nm at 6 bar. Both are hard anodised aluminium with powder coated end caps, built to DIN 3337, NAMUR and ISO 5211, and both are available double acting or spring return.

What we supply

The Susin HD range in symmetric (IS) and canted (IC) configurations spans 132 to 120,867 Nm on 3.5–10 bar air, rated ISO 5211 with ATEX and SIL 3, working from −20°C to +85°C. The PDS rotary scotch yoke range runs on 3.5–8 bar in double acting (PD) and single acting spring return (PS) versions, mounting to ISO 5211, NAMUR and VDI-VDE 3845. Panam's Kosaplus scotch yoke actuators cover high-torque quarter-turn duty in both double acting and spring return. For linear duty, the PLDS double-acting cylinders run on 3–7 bar to ISO 3322, 4393 and 5597 with multiple corrosion grades, and the compact MPLDS range works at 2.1–6.2 bar (30–90 psig) with IP66 protection where space is tight.

Getting the air supply right

Pneumatic actuators are specified on air pressure, but they fail on air quality. The Kosaplus range operates on 3 to 8 bar with 4 to 6 bar recommended and a 10 bar design pressure — but the more important figure is the air itself: particulates should not exceed 40 microns, equivalent to ISO 8573-1 Class 5, and the supply should be dehumidified. Moisture carried into the cylinder is what destroys seals, and it is the single most common cause of premature pneumatic actuator failure. Where the line is long or the ambient is cold, a dehumidifier close to the actuator is worth fitting: condensate forming in the supply line will reach the actuator whatever the compressor room does.

When pneumatic is right

Where it struggles

Air quality is the usual culprit behind pneumatic failures — moisture and particulates wreck seals, so filtration and drying matter more than the actuator specification. Compressed air is also an expensive utility once you account for generation losses, so a large actuator cycling constantly is not cheap to run. And precise modulating control needs a positioner, which adds cost and a component to maintain.

Electric Actuators

An electric actuator uses a motor and gear train to drive the valve. Compared with pneumatic it is slower and generally more expensive up front, but it needs no air infrastructure, it holds position precisely, and it is straightforward to integrate with a control system for modulating duty.

What we supply

The Susin ITQ rotary series covers 100 to 9,000 Nm with a hard-anodised aluminium housing, IP67 and NEMA 4X/6 protection and a double worm self-locking gear, available on 110/220VAC, 380/440VAC or 24VDC and mounting to ISO 5211. The self-locking gear matters: it holds position without drawing power, and the valve cannot be back-driven by line pressure. For linear duty the IQL series delivers 4–25 kN with IP67 and optional IP68, and the ITL series 1–10 kN on a universal power supply at IP66/67 with a 100% duty cycle rating — the specification to look for if the valve modulates continuously rather than stroking occasionally. The ITM multi-turn series carries IP68 with an optional Ex d IIC T4 rating for hazardous areas. The Danfoss PVE series provides electrohydraulic control on 12V or 24V.

Kosaplus covers the smaller end of electric quarter-turn duty across four families: the KE series from 8 to 2,000 Nm in spur gear and scotch yoke versions, the KP series explosion proof from 25 to 500 Nm, the KS series for quick operation where a one-second stroke at 10 Nm or three seconds at 25 Nm is needed, and the KR safe-return series from 20 to 60 Nm where an electric actuator has to reach a defined position on power loss. That last family is worth knowing about, because it answers the usual objection to electric actuators without bolting a battery pack onto a standard unit.

When electric is right

Where it struggles

Fail-safe is the weak point. On loss of power an electric actuator stops where it is unless a battery or capacitor backup is fitted, which adds cost and a component that ages. If the process demands the valve reach a safe position on power loss, spring-return pneumatic or accumulator-backed hydraulic is the more honest answer. Electric is also slower, and in a hazardous area the Ex rating adds cost that a pneumatic actuator simply avoids.

Hydraulic and Electro-Hydraulic Actuators

Hydraulic actuators use fluid under pressure rather than air. Because hydraulic fluid is effectively incompressible and runs at far higher pressures, they deliver much more torque from a given physical size — torque density is the reason to choose them. Electro-hydraulic units package a pump, reservoir and controls with the actuator, so they need only an electrical supply. You get hydraulic force without plant-wide hydraulic infrastructure, which is what makes them practical on isolated valves.

What we supply

The Susin PANDA and PA electro-hydraulic rotary series covers 46 to 10,000 Nm. PANDA operates at low pressure (8 bar), PA at high pressure (180 bar). Both mount to ISO 5211 and NAMUR, carry ATEX Zone 1 and 2 approval, SIL 2 and SIL 3 rating, and IP67/IP68 ingress protection. Fail-safe options include spring return and hydraulic accumulator, so the safe position can be reached even when the electrical supply is lost — confirm which arrangement applies to the specific series and configuration at enquiry.

When hydraulic is right

Where it struggles

Cost, first and foremost — this is the most expensive of the four. Hydraulic systems also need maintaining: fluid condition, seal integrity and leak management are ongoing tasks, and a leak is both a housekeeping and an environmental problem in a way an air leak is not. For straightforward on/off duty within pneumatic's torque range, hydraulic is usually over-specified.

Manual Gearboxes: The Option Worth Checking First

Before specifying anything powered, it is worth asking how often the valve actually moves. A large valve operated twice a year does not need an actuator, a power supply, a control signal and a maintenance regime. It needs a gearbox and a handwheel. A gear operator multiplies operator effort so a large valve can be turned by hand, and self-locking worm gearing holds the valve wherever it is left. Cost is a fraction of a powered actuator, there is nothing to fail on power loss, and there is no certification question in a hazardous area.

What we supply

The Susin MAW quarter-turn worm gear series covers 100 to 32,000 Nm — self-locking, ISO 5211 direct mount, with ±5° fine stroke adjustment and IP54 through IP67M, working from −20°C to +85°C. It is designed for ball, butterfly and plug valves and is MOV compatible, so it can be motorised later if the duty changes. The MAB multi-turn bevel gear series covers 250 to 18,000 Nm with thrust to 2,500 kN, in a ductile iron housing to ISO 5210 with IP55 through IP68 and marine coating options, for gate and globe valves. The ITG quarter-turn industrial gear series spans 60 to 28,000 Nm to ISO 5211/5210 with a declutchable manual override.

Sizing an Actuator: Torque and Safety Factor

This is where most specifications go wrong, and it is worth getting right before comparing technologies at all.

Break-out torque, not running torque

A valve does not need the same torque throughout its travel. On a quarter-turn valve the demand typically peaks in four places: break-out (unseating from closed), running (mid-travel), ending (approaching the seat) and seating (final closure). Break-out is usually the highest, and it rises the longer a valve has been left closed, as the seat takes a set against the ball or disc. Size on break-out torque. An actuator sized on running torque will operate perfectly on the test bench and stall on a valve that has been shut for six months.

Apply a safety factor

Valve torque figures come from the valve manufacturer under ideal conditions. Real service brings process deposits, corrosion, temperature effects and ageing seats. A safety factor over the calculated break-out torque covers that — a common range is 25% to 50%, with the higher end for dirty, corrosive or infrequently operated service. Kosaplus's own selection tool offers 30% (a multiplier of 1.3) as a standard option, which sits squarely in that band. Your valve manufacturer's guidance takes precedence where it differs.

Size on the worst-case supply

A pneumatic actuator's output torque is proportional to the air pressure applied. Sizing on 7 bar and running the plant at 5.5 bar leaves you short by more than a fifth. Size on the lowest supply pressure the actuator will actually see, not the nominal figure. The same applies to voltage on an electric actuator.

For spring return, check both directions

A spring return actuator has two torque profiles: air stroke, where air compresses the spring, and spring stroke, where the spring alone drives the valve. The spring stroke is the weaker of the two and it is the one that has to work when everything else has failed. Check it against break-out torque, not just the air stroke figure. The gap is larger than people expect, and it varies with the spring specified and the position in the stroke. Manufacturer torque tables give the spring stroke separately from the air stroke for exactly this reason — take the figure from the spring column, at the weakest point in the travel, and size against that.

What to send us

If you would rather we sized it, we need: valve type and size, break-out torque from the valve manufacturer, available supply pressure or voltage, required fail position, stroke time if it matters, ambient temperature, and the hazardous area classification if there is one.

Fail-Safe: Decide This First

What the valve should do when power, air or signal is lost is a process safety decision, not an actuator one — and it should be settled before the technology is chosen, because it rules options in and out.

Pneumatic spring return is the simplest and most reliable route to a genuine fail-safe action: nothing has to work for it to function, only the air has to go away. Electro-hydraulic achieves the same with a spring and accumulator. Electric actuators need added components and those components need testing, so if fail-safe is a real requirement rather than a preference, that is a point in favour of the other two.

Hazardous Areas, SIL and Ingress Protection

In a zoned area, certification narrows the shortlist before performance does — so check it early.

ATEX and IECEx

Equipment for potentially explosive atmospheres must be certified for the zone it sits in. ATEX is the European regime; IECEx is the international equivalent, often preferred on export projects. Our Susin HD pneumatic range carries ATEX, and the PANDA and PA electro-hydraulic series are approved for Zone 1 and Zone 2. Where an electric actuator is required in a zoned area, the ITM multi-turn series is available with an Ex d IIC T4 rating. A pneumatic actuator with no electrical components has a simpler path here, which is a practical reason it remains the default in oil and gas.

SIL

Where a safety instrumented function is assessed at SIL 2 or SIL 3 under IEC 61508, every element in the loop must be justified against the target — the actuator included. Our Susin HD pneumatic range carries SIL 3, and the PANDA and PA electro-hydraulic series carry SIL 2 and SIL 3.

Ingress protection

IP ratings across the range run from IP54 on indoor gear operators to IP68 on submersible-rated units. Outdoor installations should be IP66 or better; anything that may be submerged or washed down at pressure needs IP67 or IP68. The ITQ electric series is IP67 with NEMA 4X and 6; the ITM is IP68; the electro-hydraulic range is IP67/IP68.

Mounting: ISO 5211, ISO 5210 and NAMUR

Mounting is the detail that turns a correct actuator into one that will not fit.

Check the flange code and the drive size, not just the standard. Two actuators can both be ISO 5211 and still not fit the same valve. Across the Kosaplus range the codes run from F03 on the smallest R32 and A50 bodies through F05, F07, F10, F12 and F14 to F16 and F25 on the A300 — with several sizes offering more than one flange as standard or option, and stem squares from 9×9 mm up to 55×55 mm. The flange and the stem square both have to match.

How to Choose: A Decision Framework

Work down in order. Each step removes options, and by step five there is usually one sensible answer.

Frequently Asked Questions