Actuator Torque for Butterfly Damper

[simar187]

I am trying to determine the actuator torque required for my metal seated, eccentric (meaning offset) butterfly damper. I am using formulae from Appendix A of AWWA C504-87, titled "Suggested Method for Calculating Torques Required to Operate Rubber-Seated Butterfly Valves".

The formula for SEATING TORQUE: Ts = Cs D^2;
where Cs is the SEATING COEFFICIENT,
D is the valve diameter in feet.
If I were to use the SEATING COEFFICIENT of METAL SEATING it should allow me to use the formulae to size the torque of my damper (despite the method being for rubber seated valves). My sealing system is 316SST on 316SST, but I do not know the seating coefficient or where I might find it.

The formula for DYNAMIC TORQUE: Td = Ct D^3 ?P;
where Ct is the DYNAMIC TORQUE COEFFICIENT,
D is the valve diameter in feet,
?P is the pressure drop across the valve in PSI.
The DYNAMIC TORQUE COEFFICIENT varies as the DISC OPENING ANGLE varies from 0° open to 90° open. I have been unable to come across values for this coefficient for a butterfly valve, and specifically an eccentric butterfly valve. From an example in the AWWA document it appears that Ct has values greater than 1.

These dampers are a conceptual design intended for manufacturing.

If anyone can help me at arriving towards values for these coefficients I would really appreciate the help.

Thanks, Simar

 

[gerhardl]

Size and details are essential for further comments, but:

A damper, as well as a metalseated BFL valve, has an actual torque depending on the real working condition at site. A teoretical calculation might give you the normal torque for a given valve at given conditions (flow, pressure, delta P, liquid, temperature, construction, valve dimension and materials given)

The best approximation check would probably be to find an actual valve on the market approximate to the one you are constructing and ask the supplier/producer for the 'normal torque for actuator sizing'. The supplier will then also include or give you the normal safety factor.

Most important is not the teoretical torque but starting torque if and when the damper/valve is stuck or set into the seat.

Both loosening and closing into seat would normally be considerably/several times higher than operating torque for a damper. (Gas? Fluid? Temperature? Size?)

If the valve is stuck on seat necessary torque could be as high as 1 to 10 times more as torque under 'normal' closing and opening conditions.

Your primary task is actually to decide on the necessary safety factor for this.

You do not describe your selected way of actuating the damper. Is this a part of your own construction?

Normally I would recommend to use an electrical multiturn actuator with handwheel, manual operation possible. Depending on size of damper and closing /opening time required (select as slow as possible to downsize actuator) - I would also recommend evaluating the possibillity to mount a separate gear unit between the actuator and the valve, with gearing for instance 1:2 or 1:4. A gear unit could also be added later or gearing increased later if the first solution shows to be selected with too weak torque.

All large suppliers of actuators will also have suitable gears in their range. (For instance Auma).

Electric lever-arm solutions with threaded stem is also available on the actuator market (Auma).

For large-sized dampers hydraulic oil actuators could be a solution but normally at higher price, normally much higer, especially if automation degree is high.

(I would normally not recommend air operation for this task, unless special conditions, for instance small and many dampers easily actuated under ideal conditions.)

Good luck!

 

[simar187]

Offset Butterfly Damper Specifications
- 48" diameter
- 0.75 PSI pressure drop
- 100 degrees F
- intended for GAS service
- velocity is approximately 0.5 ft/s

Once again, the sealing system is 316SST on 316SST. The seating torque will be minimal, but since its for gas service, I would like to find a coefficient of metal seating for a butterfly valve.

I intend to use a Pneumatic Scotch Yoke Spring Return Actuator.

Thanks, Simar