Torque sizing is where actuator projects succeed or fail quietly. An undersized actuator does not work slowly or wear out early — it simply fails to open a valve on the day you need it, usually during a plant upset when everything else is also stressed.
This guide is the sizing worksheet logic our own engineers use, written out so you can check a supplier's quotation yourself.
Know your three torques
Valve torque is not one number. Suppliers who quote against one number are cutting a corner.
- Breakaway (seating) torque: the peak needed to unseat a closed valve — the sizing case for most actuators.
- Running torque: typically 50–70% of breakaway; sets the continuous motor load.
- End (reseat) torque: the closing peak, which can exceed opening on some seat designs.
Apply margins for what will change
Valves age. The torque you measured on commissioning day is the minimum you will ever see.
- Standard margin: 125–150% of worst-case breakaway torque.
- Toward the high end for: aged packing, high temperature, slurry or scaling media, infrequent cycling (valves seize when idle).
- Toward the low end only for: clean gas service with frequent cycling and soft seats in good condition.
- Never size below 125% to hit a price point. The actuator that cannot open the valve is a 100% loss.
Worked example
A DN200 butterfly valve in cooling-water service: measured breakaway 180 Nm, running 110 Nm.
- Worst case = aged packing + occasional scaling → apply 150%: 180 × 1.5 = 270 Nm.
- Select the actuator frame whose rated torque ≥ 270 Nm at your voltage and ambient.
- Verify travel speed at 270 Nm, not at the frame's nominal rating.
- Confirm duty: 4 starts/hour isolation duty → S2 motor is fine; a modulating loop → S4/S5 class.
Quick margin guide.
How SJMC sizes
Send us your valve torque data (or just the valve make, size and service) and our engineers return the sizing calculation with the platform recommendation — the same worksheet logic, with the numbers shown.
- JVX-01 Quarter-turn — 50–100,000 Nm covers butterfly and ball valves from HVAC to ore gates.
- JVX-02 Linear — 10–500 kN thrust for gate and globe valves.
- Sizing worksheet — Part of the free SJMC Technical Pack — valve torque to actuator selection in one sheet.
Frequently asked questions
Is 125% margin enough for a butterfly valve?
For clean water service with regular cycling, yes. For slurry, scaling or infrequently cycled valves, use up to 150%.
What if the valve manufacturer gives only one torque figure?
Treat it as breakaway torque and size against it with margin. Ask them to confirm whether it is measured or calculated — measured data deserves the lower margin.
Does voltage affect delivered torque?
Yes. Verify rated torque at your actual supply voltage including cable drop; undervoltage reduces speed and can stall the motor at peak torque.