Butterfly Valve Torque Higher Than Expected
Butterfly torque is not one number. It depends on where the disc is, what the pressure is doing, and which way the flow is going.
Butterfly Valve Torque Higher Than Expected
Butterfly torque is not one number. It depends on where the disc is, what the pressure is doing, and which way the flow is going.
What it looks like on site
The actuator stalls mid-stroke, or the handwheel effort is far above what the datasheet suggested.
The causes, in order of likelihood
- Torque peak away from the closed position. On many designs the maximum occurs part-way through the stroke, not at the seat.
- Differential pressure higher than quoted. The torque figure was given at a lower pressure drop than the installed condition.
- Seat interference. A resilient seat is compressed by design, and the interference adds a friction component throughout travel.
- Offset geometry. Concentric and offset designs have different torque profiles, and a comparison between them is not like for like.
- Flow direction. Some designs are sensitive to which side the higher pressure is on.
Ask for the curve, not the number
A single torque figure is only meaningful if the position and the differential pressure it applies to are stated with it. The useful document is the torque curve across the stroke at the expected differential.
That is what an actuator should be sized from. Sizing from a single number produces an actuator that works on the test bench and stalls on the line, which is the most common butterfly actuation problem there is.
The contributions, in order
Seat friction acts through the whole stroke on a concentric valve, because the disc stays in contact with the seat. On an offset design the disc clears the seat early and that component falls away.
The hydrodynamic component grows with the square of the flow and with the differential across the disc, and it peaks where the disc presents the largest unbalanced area to the flow. Bearing friction and packing friction add a roughly constant term.
What to change
Where the shortfall is modest, a larger actuator or a higher supply pressure closes the gap. Where the shortfall is large, the valve configuration may be the better change: an offset design removes a large part of the travel friction.
Where the valve is used for throttling, remember that the peak torque may be in the throttling zone rather than at either end. That is a specification issue, not a fault, and it is cheaper to resolve before the actuator is ordered.
Torque Components
| Item | Detail | Note |
|---|---|---|
| Seat friction | Concentric designs, through the whole stroke | Offset geometry reduces it after the disc clears |
| Hydrodynamic | Grows with differential pressure and flow | Peaks where the unbalanced area is largest |
| Bearing and packing | Roughly constant | Increases with age and with tight packing |
| Flow direction | Some designs are sensitive | Confirm the preferred direction |
| The figure to specify | Torque at the worst point in the stroke | A single number is not enough for sizing |
The checks, in order
Obtain the torque curve
At the expected differential, across the stroke.
Confirm the actual differential
Installed pressure drop, not the design figure.
Locate the stall point
Mid-stroke suggests a hydrodynamic peak; at the seat suggests interference.
Check the supply at the port
Rule out an air supply shortfall before re-sizing.
Review valve geometry if the shortfall is large
An offset design changes the profile rather than the actuator.
From the works and the projects
Questions this page answers
Because a single figure has to state a position and a differential pressure to mean anything, and the installed condition rarely matches. The torque curve is the document that answers the question.
On some designs, yes. The preferred direction should be confirmed against the manufacturer's data for the specific valve.
Reducing the operating differential pressure reduces the hydrodynamic component. Beyond that, changing the valve geometry is the more effective route.