One of the most frequently committed oversights in the commissioning phase of process plants is under-sizing the valve actuator.
This would result in the stalling of the valve actuator, inability of the valve to respond to instructions. Finally, the cause of all of these lies in the torque calculations that did not take into account all the forces.
Getting valve actuator torque calculation right from the start saves rework, downtime, and equipment damage.
What Is Torque and Why Does It Matter?
Torque is the force that causes rotation of the valve stem from one position to another. It can be measured in Newton-meters (Nm) and inch-pounds (in-lb).
Every valve comes with a torque specification provided by the valve manufacturer. The actuator should provide at least the required torque in the worst-case scenario.
The problem is that torque specifications keep changing due to many factors. That includes operating pressure, temperature, viscosity of the process fluid, and age of the valve seat and packing.
An actuator designed just for the nominal torque specification during the commissioning period may not suffice in later operation.
That is why actuator sizing always includes a safety margin above the valve’s published torque requirement.
Three Types of Torque Every Engineer Should Know
Understanding the difference between the three torque types is the foundation of any valve actuator torque formula. Each represents a different point in the valve’s operating cycle.
Breakaway Torque
Breakaway torque is the force needed to start moving a valve from its fully closed or fully seated position. It is always the highest torque value in the operating cycle.
Static friction between the disc or ball and the seat. Combined with any differential pressure across the valve, it creates resistance that must be overcome before the valve begins to move.
For most valve types, breakaway torque is 1.3 to 1.5 times the running torque. In high-pressure applications or valves that have been static for long periods, the ratio can be higher.
Running Torque
Running torque is the force required to continue rotating the valve stem through its travel once it has started moving. It is lower than breakaway torque because dynamic friction is lower than static friction.
Running torque is what the valve manufacturer’s datasheet typically publishes as the rated operating torque at a given differential pressure.
Seating Torque
Seating torque is the force applied at the end of travel to push the disc or ball firmly against the seat. This creates a tight shutoff.
It is relevant primarily for soft-seated valves: butterfly valves with elastomeric seats and ball valves with PTFE or similar seat materials. Metal-seated valves depend more on actuator thrust than rotational seating torque for shutoff.
How to Calculate Torque for Valve Actuator and Three Worked Examples
The standard valve actuator torque sizing formula is:
Required Actuator Torque = Valve Operating Torque x Safety Factor
Where valve operating torque is the maximum value among breakaway, running, and seating torque at the specified differential pressure.
The safety factor is typically 1.25 to 1.5 for electric actuators, per ISA-75.19.01 sizing guidelines.
Example 1: Quarter-Turn Ball Valve, Water Service
A DN100 ball valve in water service has a published operating torque of 120 Nm at 10 bar differential pressure.
Applying a 1.25 safety factor:
Required Actuator Torque = 120 x 1.25 = 150 Nm
An AOX-R quarter-turn electric actuator rated at 160 Nm covers this requirement with an adequate margin.
The “AOX-R” series features a compact structure with excellent weather resistance and vibration resistance.
LEARN MOREExample 2: Butterfly Valve, Chemical Service
A DN200 butterfly valve in a chemical dosing line has a breakaway torque of 280 Nm and a running torque of 210 Nm at 6 bar. The higher breakaway value governs.
Applying a 1.3 safety factor for chemical service due to potential seat degradation:
Required Actuator Torque = 280 x 1.3 = 364 Nm
An actuator rated at 400 Nm is the appropriate selection.
Example 3: Multi-Turn Gate Valve, High-Pressure Steam
A gate valve on a high-pressure steam line has a valve stem torque calculation from the manufacturer’s datasheet showing 850 Nm at 40 bar. Steam service with thermal cycling applies additional packing friction.
A 1.5 safety factor is appropriate:
Required Actuator Torque = 850 x 1.5 = 1,275 Nm
An AOX-M multi-turn electric actuator with a torque range up to 6,000 Nm covers this application with margin for future wear and pressure increases.
The “AOX-M” series is designed to operate valves with 360° multi-turn rotation, such as gate valves and globe valves.
LEARN MOREHow to Find Your Valve’s Torque Requirements
For the most accurate torque information, you need to consult the manufacturer’s datasheet corresponding to your specific valve size and pressure rating.
Most published torque specifications are determined at a certain differential pressure, usually the maximum allowable working pressure of the valve.
If the actual operating pressure is lower, the torque requirement will also be lower.
When a manufacturer’s datasheet is unavailable, torque can be estimated using the valve’s Cv factor and operating conditions. However, this method is less precise and should always include a larger safety margin.
For electric actuator torque sizing, confirm the following before selecting an actuator:
- Maximum differential pressure the valve will see in service.
- Valve type (ball, butterfly, gate, globe) and seat material.
- Number of operating cycles per day.
- Whether the application requires fail-safe spring return.
AOX’s manufacturing team assists with valve-actuator matching as part of their technical support package. This covers wiring diagrams, system integration, and programming guidance.
How Much Safety Margin Should You Leave?
The safety factor bridges the gap between a valve’s nominal torque rating and the real-world conditions it will encounter.
A factor that is too low risks actuator stalling. One that is too high wastes budget on an oversized unit.
| Application Type | Recommended Safety Factor |
| Clean water, low cycle frequency | 1.25 |
| General industrial, moderate pressure | 1.25 to 1.35 |
| Chemical or abrasive service | 1.35 to 1.5 |
| High-pressure steam, high cycle frequency | 1.5 |
| Fail-safe spring return required | 1.5 or higher |
These values align with ISA-75.19.01 actuator sizing standards.
Regarding spring-return actuators, the force of the spring must also be deducted from the torque output capability of the actuator.
The AOX electric actuator family ranges from 30 Nm up to 6,000 Nm torque in their families of quarter-turn, multi-turn, and linear actuators.
All of them have been certified with CE, ATEX, UDEM, and EAC. IP67 is the standard protection class. Standard delivery time is 15 days, with 10-15 days for urgent orders.
FAQs
Q1. What would happen if the actuator size was small?
If the actuator size was small, then it would stall before completing the opening or closing of the valve.
This could lead to the generation of alarm signals, process interruptions or partial valve opening in automated operations.
The actuator motor might even overheat from the constant stalling attempts.
Q2. Can the same actuator be used for water and oil service?
The actuator itself can often be the same unit, since it is not in contact with the process fluid. What changes is the valve torque requirement.
Oil service typically involves higher viscosity and different pressure conditions than water, which affects the valve’s operating torque.
The actuator torque sizing must reflect the actual service conditions for each application separately.
Q3. How do you know when a bigger actuator is needed?
Indicators of an undersized actuator are slow or incomplete valve movement, excessive thermal overload trips, and higher current consumption.
If these symptoms appear, recalculate the valve actuator torque calculation using actual operating pressures. Also, check whether the valve’s seat or packing condition has changed since original commissioning.
