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

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