Actuator Sizing Information

There are three torques to be considered when selecting the proper actuator for a butterfly valve.

(1) Seating Torque – Torque required to displace a resilient seat and effect shutoff.

(2) Bearing Torque – Torque required to overcome friction forces on the valve shaft bearing surfaces.

(3) Dynamic Torque – Torque due to fluid forces that tend to close the valve.

The torques for resilient seated valves tabulated in this section are the sum of (1) and (2) above for various shutoff pressures. These tabulated values include a safety factor large enough to ensure proper valve operation in most general butterfly valve applications. Where unusual service conditions exist (such as the likelihood of seat swelling or low and high-temperature seat hardening), an additional safety factor may be applicable.

Dynamic torque is the torque on the valve shaft due to the fluid forces on the valve disc. This torque is a function of valve diameter, pressure drop, and a torque coefficient (Ct) which varies with angle opening. Torque is calculated by the equation:

Where

Dynamic torque is not usually of major concern in resilient seated butterfly valves unless the line velocity exceeds 20 fps. If line velocity exceeds this, a check should be made to ensure that the actuator output exceeds the calculated dynamic torque. Dynamic torque should be checked at 80° open for on-off applications.

Dynamic torque is of prime consideration in situations where line velocity is not recovered downstream of the valve. This situation exists on installations where there is an unlimited source and less than six diameters of pipe downstream of the valve. If a valve discharges to the atmosphere, the pressure drop across the valve will be equal to the height of water above the valve for all angles of valve opening. This pressure drop must not exceed the pressure drop tabulated in the Maximum vs. Angle Opening tables for any angle. If it does, provisions must be made for velocity recovery by adding downstream piping.