Valve Actuator Torque Calculation: Breakaway, Running & Seating Torque

Quick Answer

Valve actuator torque has three components: breakaway torque (to start moving from closed), running torque (to keep moving), and end/seating torque (to reach the seat at the end of travel). Select an actuator whose output torque covers the maximum of these, multiplied by a safety factor (typically 1.25–1.5 for pneumatic, 1.5–2 for electric).

Understanding the Three Torque Values

Torque Component	When It Occurs	Typical Value vs. Breakaway
Breakaway torque	Start of opening/closing	Baseline (highest usually)
Running torque	Mid-stroke movement	40–70% of breakaway
End / seating torque	Final seating travel	Often 100–130% of breakaway

Basic Torque Calculation

For a ball or butterfly valve, total required torque is approximately:

T_required = T_valve × safety_factor

Where T_valve is the manufacturer’s torque at the maximum differential pressure (breakaway), and the safety factor depends on the actuator type and service:

• Pneumatic on/off: 1.25–1.5
• Pneumatic modulating: 1.5–2
• Electric: 1.5–2 (electric actuators cannot stall safely)
• Manual gearbox: 1.3–1.5

Factors That Increase Torque

• Higher differential pressure across the valve
• Soft seats (PTFE) vs. metal seats
• Low temperatures (packing stiffness)
• Aging, dirt, and scale buildup on seats
• Larger valve size

Example Calculation

A 6-inch butterfly valve has a manufacturer breakaway torque of 250 Nm at operating pressure. For a pneumatic on/off actuator with a 1.5 safety factor:

Required output = 250 × 1.5 = 375 Nm

Select a pneumatic actuator rated ≥ 375 Nm (at the available air supply, typically 5.5-8 bar).

Frequently Asked Questions

What is the difference between torque and thrust?

Torque is rotational force (used by ball, butterfly, plug valves). Thrust is linear force (used by gate and globe valves, and linear actuators).

Should I size the actuator for breakaway or running torque?

Size for the highest value — usually breakaway or end torque, then multiply by the safety factor. Running torque is lower and not the sizing case.

Can I use a manual gearbox instead of an actuator?

Yes, if the valve is operated infrequently. Gearboxes reduce handwheel effort but add turns; they are sized with the same torque with a slightly lower safety factor.

Tags: Actuators