In This Article
- 1.Torque Components
- 2.Calculating Breakaway Torque
- 3.Dynamic Torque at Partial Opening
- 4.Actuator Supply Pressure and Torque
Actuator torque sizing is one of the most common sources of field problems with automated butterfly valves. Undersized actuators fail to open or close the valve; oversized actuators damage the valve stem, disc, or seat through over-torquing. Correct sizing requires calculating all torque components and applying appropriate safety factors.
Torque Components
| Torque Type | Symbol | Definition | When Maximum |
|---|---|---|---|
| Breakaway torque | BTO | Torque to start moving from fully closed position | Closed with high differential pressure |
| Running torque | RTO | Torque during movement (open or close) | At 70° open (maximum hydrodynamic torque) |
| End-of-travel torque | ETO | Torque to reach full open or fully closed stop | At full open or closed seat compression |
| Seating torque | STO | Torque required to achieve bubble-tight shut-off | At fully closed, varies with seat design |
| Unbalanced pressure torque | ΔPT | Torque from differential pressure across disc | Highest ΔP during opening |
Calculating Breakaway Torque
Valve manufacturers publish breakaway torque data at rated differential pressure (ΔP) for each valve size. The key formula: T_actuator ≥ T_valve_BTO × Safety_Factor. AUMA and Rotork recommend a minimum safety factor (SF) of 1.25 for pneumatic actuators and 1.5 for electric actuators on shut-off duty. For modulating (throttling) service: SF ≥ 2.0 due to continuous torque cycling.
Dynamic Torque at Partial Opening
Butterfly valve discs experience maximum dynamic (hydrodynamic) torque at approximately 70–80° open (10–20° from fully open) in high-flow conditions. This occurs because the disc is partially obstructing flow — the velocity distribution across the disc creates a net torque trying to close (or open) it. In modulating service, the actuator must overcome this dynamic torque throughout the stroke — this is why modulating butterfly valve applications require larger actuators than simple on/off service.
Actuator Supply Pressure and Torque
Pneumatic actuator output torque is proportional to supply air pressure: T = P × A × r (pressure × piston area × lever arm). At 5 bar supply air: a typical DN200 butterfly valve actuator produces 200–400 Nm. If supply pressure drops to 3 bar (common in older compressed air systems), output torque drops by 40% — the actuator may fail to operate. Always size for minimum available instrument air pressure (typically 5.5 bar working, minimum 4.5 bar).
- Step 1: Get valve BTO (breakaway torque) at rated ΔP from valve manufacturer datasheet
- Step 2: Multiply by safety factor — 1.25 for on/off pneumatic, 1.5 for electric on/off, 2.0 for modulating
- Step 3: Verify against actuator output torque at minimum supply pressure (not nominal)
- Step 4: Check stem torsional strength — actuator maximum torque must not exceed valve stem rated torque
- Step 5: If fail-safe (spring return) required — verify spring torque is adequate to close/open against differential pressure
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