| Service | Initial sizing range | Main condition |
|---|---|---|
| Clean, regular on-off service | 1.20–1.30 | Reliable valve torque data is available |
| Emergency or fail-safe service | About 1.50 | Use only when a separate project margin is required |
| Low-temperature service | Cold torque × required margin | Do not use room-temperature torque directly |
| Dirty, abrasive or crystallizing service | Valve-specific | Obtain service data from the valve supplier |
What the Safety Factor Means
A safety factor adds extra torque above the corrected torque required by the valve.
Trequired = Tvalve × SF
If a valve requires 200 N·m and the selected factor is 1.25:
200 × 1.25 = 250 N·m
The actuator must provide at least 250 N·m at the same valve position where the 200 N·m load occurs.
The following values show how different factors change the required actuator torque. These are calculation examples, not torque values for a specific valve model.
| Safety factor | Valve torque | Required actuator torque | Extra torque |
|---|---|---|---|
| 1.10 | 200 N·m | 220 N·m | 20 N·m |
| 1.20 | 200 N·m | 240 N·m | 40 N·m |
| 1.25 | 200 N·m | 250 N·m | 50 N·m |
| 1.30 | 200 N·m | 260 N·m | 60 N·m |
| 1.50 | 200 N·m | 300 N·m | 100 N·m |
A 20% margin means a factor of 1.20. A 50% margin means a factor of 1.50. The factor is multiplied by the valve torque.
A service correction is different from a safety factor. A service correction covers a known condition such as:
- Low temperature
- Dry gas
- Special seat material
- Long periods without operation
- Sticky or crystallizing media
When the valve supplier uses a multiplication method, the calculation may be written as:
Tdesign = Tbase × Kservice × Kmargin
Do not use this formula automatically. Some suppliers already include the service correction and final margin in their recommended actuator torque. Adding another factor would oversize the actuator.
Factors must also be multiplied correctly. A temperature factor of 1.40 followed by a margin of 1.20 gives:
1.40 × 1.20 = 1.68
The total increase is 68%, not 60%.
Check the Torque Data First
Most sizing errors begin with the wrong valve torque. Before applying any factor, find out what the supplied number represents.
| Torque value | What it usually means | What to do |
|---|---|---|
| Basic valve torque | Torque under stated test conditions | Add the required service corrections and margin |
| Maximum operating torque | Expected maximum under stated conditions | Confirm what is already included |
| Corrected valve torque | Torque after one or more service corrections | Add only corrections that are still missing |
| Recommended actuator torque | Minimum actuator output selected by the valve supplier | Do not add another factor without approval |
Confirm the following before sizing:
- Valve type, model and size
- Pressure class
- Floating or trunnion-mounted construction
- Seat and packing materials
- Pressure used in the torque chart
- Minimum and maximum temperature
- Process medium
- Opening and closing direction
- Break, running and reseating torque
- Whether service factors are already included
- Maximum allowable stem torque
A larger factor cannot correct missing data. If the pressure basis, seat material or temperature is unknown, increasing the factor from 1.25 to 1.50 does not make the result reliable.
For more detail, read the ball valve torque curve guide.
Use the Correct Pressure
Do not assume every torque chart uses differential pressure. Depending on the valve and manufacturer, the chart may use:
- Maximum differential pressure
- Maximum operating line pressure
- Design pressure
- A stated test pressure
Follow the pressure basis printed in the valve torque chart.
Differential pressure is the difference between upstream and downstream pressure. If upstream pressure is 50 bar and downstream pressure is 48 bar, the differential pressure is 2 bar. However, a chart based on line pressure may still require the 50 bar value.
| Upstream pressure | Downstream pressure | Differential pressure |
|---|---|---|
| 10 bar | 8 bar | 2 bar |
| 30 bar | 10 bar | 20 bar |
| 50 bar | 48 bar | 2 bar |
| 50 bar | 0 bar | 50 bar |
Pressure class is not the same as operating pressure. ASME B16.34 covers valve pressure-temperature ratings, materials, dimensions, testing and marking, but it does not give one actuator safety factor for every valve.[2]
ISO 14313:2025 covers the design, manufacture, materials, assembly and testing of pipeline valves and supplements API Specification 6D, 25th edition.[3] API states that API 6D defines manufacturing requirements for pipeline and piping valves.[4]
Check the Valve Design
Floating and trunnion-mounted ball valves carry pressure loads differently. Their torque values should not be exchanged.
Floating ball valves: Pressure moves the ball slightly toward the downstream seat. This helps the valve seal but can increase contact load and operating torque. The effect depends on ball size, pressure, seat area, seat material and packing friction.
Trunnion-mounted ball valves: The ball is supported by trunnions. The seats move toward the ball. Seat spring load, cavity pressure, pressure direction and seat design affect the torque.
A torque value for a small forged soft-seated floating ball valve should not be applied to a large trunnion valve. See the floating and trunnion ball valve comparison for the main structural differences.
Read the Torque Curve
A ball valve does not require the same torque throughout its 90-degree movement.
| Torque point | Meaning | Why it matters |
|---|---|---|
| BTO | Break to open | Overcomes static seat and packing friction |
| RTO | Run to open | Moves the ball toward the open position |
| ETO | End to open | Completes the opening stroke |
| BTC | Break to close | Starts the closing stroke |
| RTC | Run to close | Moves the ball toward the seat |
| ETC | End to close or reseat | Compresses the seat and completes shutoff |
The following example shows how torque can change through one opening and closing cycle. The values are illustrative.
| Valve position | Valve condition | Base torque | Torque with 1.25 factor |
|---|---|---|---|
| 0° | Break to open | 180 N·m | 225 N·m |
| 15° | Early opening | 100 N·m | 125 N·m |
| 45° | Mid-opening | 70 N·m | 87.5 N·m |
| 90° | End to open | 60 N·m | 75 N·m |
| 45° | Mid-closing | 75 N·m | 93.75 N·m |
| 15° | Closing approach | 90 N·m | 112.5 N·m |
| 0° | End to close | 130 N·m | 162.5 N·m |
Break-to-open torque is often high because the actuator must overcome static friction before the ball moves. End-to-close torque is also important because the seat must be compressed to achieve shutoff.
Always compare valve and actuator torque at the same angular position and in the same direction.
Match the Actuator Type
Double-acting pneumatic actuator: Use the manufacturer’s output at the lowest dynamic air pressure available while the actuator is moving. Do not use only the pressure shown at the air header when the valve is stationary.
The following values illustrate how lower air pressure can remove an apparent torque margin. Actual actuator output must come from the manufacturer’s data.
| Air pressure | Example actuator output | Required torque | Result |
|---|---|---|---|
| 6 bar | 330 N·m | 250 N·m | Pass |
| 5 bar | 275 N·m | 250 N·m | Pass |
| 4.5 bar | 248 N·m | 250 N·m | Fail |
| 4 bar | 220 N·m | 250 N·m | Fail |
Spring-return pneumatic actuator: Check the air and spring strokes separately. For a spring-to-close valve, compare:
| Actuator position | Valve load |
|---|---|
| Air start | Break-to-open torque |
| Air end | Opening run or end torque |
| Spring start | Closing torque at the matching position |
| Spring end | End-to-close torque |
Scotch-yoke actuator: Output changes through the stroke. It may be high near the ends and lower near the middle. Compare the full actuator curve with the valve curve.
Electric actuator: Check starting, running, seating and maximum output torque. Also check the torque-switch setting, minimum voltage, motor duty and permitted starts per hour.
Hydraulic actuator: Check the lowest dynamic hydraulic pressure, maximum relief pressure, accumulator pressure at the end of travel and oil viscosity at the lowest temperature.
The pneumatic, electric and hydraulic actuator guide explains the main differences between these drive types.
Normal Service
A factor of 1.20 to 1.30 is a useful starting range when:
- The valve torque data is reliable
- The pressure basis is known
- The medium is clean
- The standard seat and packing are used
- The temperature is within the torque-chart range
- The valve is operated regularly
- The air pressure, hydraulic pressure or voltage is stable
- The valve is used for on-off isolation
- The valve is not part of a safety instrumented function
Use the lower end only when the valve and service are well defined. Do not use a larger margin to hide unknown pressure, temperature or valve-condition data.
Example: The following values are examples only and are not standard values for a particular valve size.
| Valve torque | Base value | Value with 1.25 factor |
|---|---|---|
| Break to open | 180 N·m | 225 N·m |
| Run to open | 70 N·m | 87.5 N·m |
| Run to close | 75 N·m | 93.75 N·m |
| End to close | 130 N·m | 162.5 N·m |
A candidate double-acting actuator provides:
| Position | Required torque | Available torque at minimum pressure | Result |
|---|---|---|---|
| Opening start | 225 N·m | 245 N·m | Pass |
| Opening run | 87.5 N·m | 230 N·m | Pass |
| Closing run | 93.75 N·m | 230 N·m | Pass |
| Closing end | 162.5 N·m | 245 N·m | Pass |
If the stem limit is 400 N·m, the mounting-kit limit is 380 N·m and the actuator produces 360 N·m at maximum supply pressure, the selection passes both the operating and strength checks.
If the actuator can produce 450 N·m, it exceeds the 380 N·m mounting-kit limit and should not be used without an approved change.
Emergency Service
For emergency or fail-safe service, about 1.50 may be used as an initial value when the project or valve supplier requires a separate margin. It is not a universal ESD rule.
A safety instrumented system includes sensors, logic and final elements such as valves that move the process to a safe condition.[5] IEC 61511-1 covers the specification, design, installation, operation and maintenance of process-industry safety instrumented systems.[6]
For an emergency valve, check:
- Maximum possible pressure condition
- Lowest actuator inlet pressure
- Lowest stored-energy pressure
- Lowest temperature
- Longest expected idle period
- Required fail-open or fail-close direction
- Weakest point in the fail stroke
- Required opening or closing time
- Reset torque after the emergency
- Simultaneous demand on shared utilities
Torque alone does not prove that the safety function will work. Pilot valves, positioners, power supplies and utility systems must also be reliable. HSE notes that actuator performance can be checked through travel time, position, speed and torque monitoring.[7]
A valve may have enough torque but still move too slowly because of small tubing, restricted exhaust, a blocked filter or low accumulator pressure. Excessively fast closing can cause pressure surge, pipe movement or heavy seat impact.
Check reopening as well as emergency closing. A fail-close valve may be unable to reopen if full pressure remains across the closed ball. Some systems need a bypass to equalize pressure before reset.
A fire-tested valve does not mean that its pneumatic, electric or hydraulic actuator has passed the same fire test. ISO 10497:2022 excludes powered actuator fire testing from its scope.[8]
More actuator torque also cannot correct a damaged spring assembly. HSE has reported a spring-return emergency shutdown valve that failed open after corrosion damaged parts of the spring housing.[9]
Low-Temperature Service
Do not apply a normal safety factor directly to room-temperature torque when the valve must operate at low temperature.
Low temperature can change the behavior of:
- PTFE, PEEK and other polymer seats
- Stem packing
- O-rings
- Grease
- Bearings
- Actuator seals
- Hydraulic fluid
ISO 28921-1:2022 covers metallic isolation valves used from −50°C down to −196°C, including ball and plug valves.[10]
When the valve manufacturer supplies a temperature factor:
Tcold = Tambient × Ktemperature
If the corrected torque does not already include the required margin:
Trequired = Tcold × Kmargin
The following table shows how temperature correction and final margin affect a base torque of 250 N·m. The factors are calculation examples only.
| Condition | Calculation | Required torque |
|---|---|---|
| Room temperature, normal margin | 250 × 1.25 | 312.5 N·m |
| Cold factor 1.20, margin 1.25 | 250 × 1.20 × 1.25 | 375 N·m |
| Cold factor 1.35, margin 1.25 | 250 × 1.35 × 1.25 | 421.9 N·m |
| Cold factor 1.35, fail-safe margin 1.50 | 250 × 1.35 × 1.50 | 506.25 N·m |
When verified torque at the actual minimum temperature is available, use that value instead of applying another general cold factor.
Also check the actuator at the cold condition. ISO 28921-2 states that low-temperature valve type testing does not evaluate the actuator unless the actuator is an integral part of the valve.[11]
Three temperatures may need to be checked:
- Process temperature: the fluid cools the valve body, seats and stem.
- Ambient temperature: cold air affects the actuator, solenoid and switches.
- Cold-soak temperature: the assembly stays cold long enough for internal parts to reach a similar temperature.
A valve may work during a short cold-flow test but fail after a long cold soak.
Moisture in instrument air can freeze in solenoids, exhaust ports and small control passages. Increasing actuator size will not solve an icing problem. Check the air dew point, seal materials, grease, accessory ratings and hydraulic-oil viscosity.
See the PTFE, PEEK and metal seat comparison when selecting seat materials. Severe cryogenic applications may also require an extended stem and other features used in forged cryogenic ball valves.
Low-temperature fail-safe example:
- Room-temperature break-to-open torque: 250 N·m
- Room-temperature end-to-close torque: 180 N·m
- Approved temperature correction: 1.35
- Project fail-safe margin: 1.50
- Stem limit: 850 N·m
- Mounting-kit limit: 780 N·m
Required air-start torque:
250 × 1.35 × 1.50 = 506.25 N·m
Required spring-end torque:
180 × 1.35 × 1.50 = 364.5 N·m
The actuator must provide at least 506.25 N·m at air start and 364.5 N·m at spring end under the specified cold and minimum-pressure conditions. Its maximum output must not exceed the 780 N·m mounting-kit limit.
Check Maximum Output and MAST
MAST means maximum allowable stem torque. It is the highest torque the valve stem is allowed to transmit.
The actuator must satisfy both checks:
Tactuator minimum ≥ Tvalve required
Tactuator maximum ≤ Tlowest assembly limit
Use minimum air pressure, hydraulic pressure or voltage to check whether the valve will operate. Use maximum pressure, maximum spring output or maximum motor torque to check whether the actuator can damage the assembly.
The following example shows why the stem MAST is not always the controlling limit.
| Part | Allowable or available torque | Check |
|---|---|---|
| Valve stem MAST | 800 N·m | Not the lowest limit |
| Coupling limit | 650 N·m | Controls the assembly |
| Mounting bracket limit | 720 N·m | Above coupling limit |
| Gearbox limit | 700 N·m | Above coupling limit |
| Maximum actuator output | 680 N·m | Fail: exceeds 650 N·m |
In this example, the assembly limit is 650 N·m because the coupling has the lowest rating. An actuator with a maximum output of 630 N·m would pass this strength check, provided it also has enough torque at minimum supply conditions.
ISO 5211:2026 specifies attachment dimensions, drive dimensions and reference torque values for part-turn actuator interfaces and couplings.[12] ISO 5640:2024 provides requirements for metallic mounting kits that transmit torque between part-turn valves and actuators.[13]
A pressure regulator may reduce pneumatic actuator output, but its setting tolerance and failure condition must be considered. Do not assume a regulator alone always provides enough protection against excessive torque.
Other Conditions That Increase Torque
Long idle periods: Static seat friction, packing adhesion, corrosion and deposits can increase the first movement torque. Use the valve supplier’s frequency correction when available.
Dry gas: Some seat and packing combinations have higher friction when the medium provides little lubrication.
Sticky or polymerizing media: Material can bond to the ball, seat or cavity while the valve remains still.
Crystallizing media: Crystals can form between the ball and seat and prevent full movement.
Slurry and solids: Particles may increase torque or stop the valve from fully closing. A larger actuator can also force particles into the seat and cause damage.
Do not use one general “dirty-service factor” for all these conditions. The valve supplier should review the actual medium.
When adding an actuator to an existing manual valve, check the valve’s age, leakage, corrosion, deposits, stem wear and current operating torque. A no-pressure workshop test does not prove that the valve will operate at maximum field pressure.
If the valve sticks or jumps during movement, inspect it before fitting a larger actuator. The field troubleshooting guide explains how to separate seat, stem and installation problems.
After actuator installation, check alignment, travel stops and full-stroke operation using a suitable installation and testing checklist.
ISO 5208:2015 covers valve pressure-boundary integrity, closure tightness and the structural adequacy of the closing mechanism. A successful pressure test does not replace actuator sizing for the worst field condition.[14]
Selection Checklist
- Identify whether the torque is basic, corrected or recommended actuator torque.
- Confirm the pressure basis used by the torque chart.
- Obtain break, running and reseating torque.
- Confirm the seat, packing, medium and temperature.
- Apply only service corrections that are not already included.
- Add a separate margin only when required by the supplier or project.
- Compare valve and actuator torque at matching positions.
- Use minimum dynamic power or pressure for the operating check.
- Use maximum actuator output for the strength check.
- Check the stem, coupling, bracket and gearbox limits.
- For fail-safe service, check the fail stroke, travel time and reset condition.
- Record every torque value, correction and assumption.
For pipeline valves, ISO 12490:2011 covers actuator and mounting-kit sizing for valves made under ISO 14313 and API 6D. It remains current but is expected to be replaced by a new edition.[15]
Conclusion
Use 1.20 to 1.30 only as a starting range for clean, regular ball-valve service with reliable torque data. A value near 1.50 may suit some fail-safe applications, but it is not a universal ESD rule. At low temperature, correct the valve torque first and confirm that actuator output is valid at the same temperature. Check break, run and reseating torque at minimum supply pressure. Then compare maximum actuator output with the lowest limit of the stem, coupling and mounting kit. For example, an 800 N·m stem does not make a 680 N·m actuator acceptable when the coupling is limited to 650 N·m.
