Do not start actuator selection by debating pneumatic versus electric. That question comes last. Before any brand or model enters the discussion, three things have to be confirmed:
- The breakaway torque the valve needs under its least favorable operating conditions
- Where the valve must go when air or power is lost: fail-open, fail-close, or a controlled, known state
- What the site can actually supply: air pressure, voltage, and control signals
Until all three are confirmed, no actuator can be selected reliably. An actuator that matches the pipe size but not the torque will stall on a sticky seat. An actuator with the wrong fail behavior can leave a line open during a blackout. This guide walks through each decision in order, then ends with a checklist of the details a supplier needs to confirm compatibility.
Torque Is More Than the Valve Size
Two valves of the same size can need very different torque. Size is only one input. The torque a quarter-turn valve demands depends on:
- Valve type and size — a ball valve, a butterfly valve and a plug valve of the same nominal size do not load the actuator the same way.
- Seat material and valve construction — soft seats, metal seats and different stem designs create different friction.
- Differential pressure — higher pressure across the closed valve pushes the ball harder into the seat and raises the torque to break it free.
- Fluid and temperature — media that are viscous, dirty or prone to deposits increase torque, and temperature changes seat behavior.
- Рабочая частота — a valve that sits closed for months needs more force to break away than one cycled daily.
Torque is also not a single number. Breakaway torque starts the valve moving from the seated position, running torque keeps it moving through mid-stroke, and seating torque drives it back into the seat at the end of travel. Breakaway is usually the highest of the three, which is why it sizes the actuator.
Use the torque data published by the valve manufacturer for that specific valve, evaluated at your actual pressure, media and temperature. This guide deliberately gives no generic torque figures and no universal safety factor, because the right margin depends on the valve, the seat and the service. If the valve maker cannot supply torque data for your conditions, that gap has to be resolved before selection, not papered over with an assumed number.
Define the Fail Position Before Choosing the Actuator
The fail position is a process-safety decision, not an actuator feature you pick afterwards. Decide first what the valve must do when the air line or the power feed goes dead, then choose an actuator type that can be verified to deliver that behavior.
| Actuator type | Behavior on loss of air or power | What must be verified |
|---|---|---|
| Pneumatic double acting (DA) | No spring drives it to a predefined position. Its position after air loss cannot be assumed. | Whether the process actually tolerates an undefined position. If a defined fail position is required, DA alone does not provide it. |
| Pneumatic spring return (SR) | The spring drives the valve toward the position the assembly was designed for, either fail-open or fail-close. | Spring action direction, the valve’s rotation direction, and how the actuator is actually assembled onto the valve. A correct spring on a wrongly oriented mount fails the wrong way. |
| Standard electric actuator | Typically stops where it is when power is lost. Actual behavior depends on the specific model; a safe position cannot be assumed. | The manufacturer’s stated power-loss behavior for that exact model, and whether stopping mid-travel is acceptable for the process. |
| Electric actuator with verified battery or stored-energy fail-safe function | Specified models, correctly configured, move the valve to a predetermined position on power loss. | That the specific model and configuration are rated for this function, plus ambient conditions, battery condition, the real valve torque load, and a documented test procedure at commissioning. |
The pattern in the third column matters more than the second. Every fail-safe claim is conditional on the specific model, the assembly and the environment. Treat the fail position as something to be demonstrated during commissioning, not something the catalog settles.
Choose the Power Source and Control Method
Once torque and fail position are fixed, the power source usually chooses itself based on what the site already has. This is the point where the pneumatic-versus-electric question finally becomes worth asking.
- Manual operation fits valves that are cycled infrequently and can be reached by an operator. No utilities, no wiring, lowest cost.
- Pneumatic actuators fit sites that already run clean, stable compressed air and need remote on/off operation.
- Electric actuators fit sites with no air supply but with suitable electrical power, where the valve must be operated remotely.
- Modulating (throttling) control is a separate requirement. A plain on/off actuator does not become a control valve; precise position control needs a specified positioner or a purpose-built modulating actuator, confirmed for the application.
Control signals belong in this decision too: what the control system sends (contact closure, powered signal, analog command) and what feedback it expects back (limit switch contacts, position transmitter) restrict which actuators qualify. For a broader comparison of how manual, pneumatic and electric operation differ day to day, see our overview of manual, pneumatic and electric ball valve operation.
ISO 5211 Helps Mounting, Not Complete Compatibility
ISO 5211 is a standard for the mounting interface between part-turn actuators and valves. It makes the bolt pattern and drive connection predictable, which simplifies pairing hardware from different makers. It does not guarantee that any ISO 5211 actuator will bolt onto any ISO 5211 valve and work.
Before confirming a valve–actuator pair, check each of these:
- Flange pattern — the specific ISO 5211 flange size on the valve and the actuator must match.
- Stem coupling dimensions — stem height, shape and dimensions must fit the actuator’s drive socket, directly or with a properly machined adapter.
- Actuator output torque — must exceed the valve’s required torque across the whole stroke, not just at one point.
- Rotation angle — the actuator’s travel must match the valve’s quarter-turn operation, including stop adjustment.
- Mounting alignment — misalignment between actuator and stem causes side loading, stiff operation and early wear.
- Аксессуары — manual override, limit switches, solenoid valves and position feedback each add their own compatibility requirements.
A shared standard shortens the checklist. It does not delete it.
Where Battery-Backup Electric Actuators Fit
One recurring gap in the table above: a site needs a confirmed action on power loss, but has no compressed air, so a pneumatic spring-return unit is not an option. This is the situation a battery-backup electric actuator is built for.
Наш сайт electric valve actuator with fail-safe battery backup is an on/off unit whose built-in lithium battery drives the valve back to its designated safe position when main power is lost. Published specifications include wide-voltage input (AC 110V / 220V / 380V single phase, or DC 24V / 48V), dry-contact open/close feedback, an aviation plug connector for wiring without removing the cover, battery protection against overcharge, over-discharge and over-temperature, and an operating temperature range of 0°C to 45°C.
It is not a universal answer. The same verification rules from the fail-position table apply: confirm that the ambient temperature stays within the unit’s published range, that the battery’s condition is covered by a maintenance routine, that the actuator’s output matches the real torque load of your valve, and that the fail action is tested on the installed assembly during commissioning. For service conditions outside its published specifications, a different arrangement should be engineered rather than assumed.
RFQ Checklist: What to Send for a Reliable Match
An actuator quote is only as good as the information behind it. Sending these details up front lets a supplier confirm compatibility instead of guessing:
- Valve model, size and manufacturer
- Required breakaway, running and seating torque, if available
- Medium, operating temperature and differential pressure
- Operating frequency (cycles per day, week or month)
- Required fail behavior: fail-open, fail-close, or no defined fail position
- Available air supply (pressure, quality) or power supply (voltage, phase)
- Control signal type and feedback requirements
- Ambient environment: temperature range, indoor or outdoor, washdown, corrosive atmosphere
- Mounting interface and stem dimensions, or the valve drawing that shows them
- Any manual override, limit switch or certification requirement
Items you cannot supply are worth flagging rather than skipping. A missing torque figure, for example, changes the conversation from quoting to confirming data with the valve maker first, and that order of operations prevents an undersized actuator from ever reaching your line.
Часто задаваемые вопросы
What torque data do I need before selecting an actuator?
You need the breakaway, running and seating torque for your specific valve at your actual operating conditions: differential pressure, medium, temperature and operating frequency. This data should come from the valve manufacturer, not from generic tables, because seat material and valve construction change torque significantly between otherwise similar valves. The actuator’s output must exceed the required torque across the full stroke, with a sizing margin appropriate to the service. If torque data for your conditions is unavailable, obtain it before selection rather than substituting an assumed figure.
Переходит ли пневматический привод двойного действия в безопасное положение в случае отказа?
No. A double-acting actuator uses air pressure for both opening and closing and has no spring. When supply air is lost, nothing drives the valve to a predefined position, so its position after failure cannot be assumed. If your process requires the valve to reach a defined fail-open or fail-close position, a double-acting unit alone does not provide that. Use a spring-return actuator or another arrangement whose fail behavior has been verified for the specific assembly.
When is spring return the better choice?
Spring return is the better choice when the site has clean, reliable compressed air and the process requires a defined position on air failure. The spring drives the valve toward the position the assembly was designed for, either fail-open or fail-close. Before relying on it, verify the spring action direction, the valve’s rotation direction and the actual mounting orientation, because a correctly specified spring installed on a wrongly oriented assembly will fail toward the wrong position. Spring-return units also need more air capacity than double-acting units of similar output.
Does ISO 5211 guarantee direct actuator fit?
No. ISO 5211 standardizes the mounting interface between part-turn actuators and valves, which makes pairing easier, but it does not make every combination compatible. You still need to confirm the specific flange pattern on both sides, the stem coupling dimensions, the actuator’s output torque against the valve’s requirement, the rotation angle, and mounting alignment. Accessories such as manual overrides, limit switches, solenoid valves and position feedback add further requirements. Treat ISO 5211 as a shared bolt pattern, not a compatibility guarantee.
Can an electric actuator provide a fail-safe action on power loss?
Not by default. A standard electric actuator typically stops wherever it is when power is lost, and its exact behavior depends on the model, so a safe position cannot be assumed. A defined action on power loss requires a specified model with a battery or stored-energy fail-safe function, verified for the application. That verification covers the ambient environment, battery condition, the real torque load of the installed valve, and a test of the fail action during commissioning on the actual assembly.