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Dynamic Torque Affects on Actuator Sizing

6/15/2015

Houston -- When fluids flow through a rotary control valve, its “dynamic” motion creates unbalanced forces on the surface of the closure element, causing additional torque—or dynamic torque. This change to the flow can eventually stall the actuator.

Severe service ball valve manufacturer, MOGAS Industries, is studying the impact of dynamic torque and its affect on accurate actuator sizing. At the MOGAS R&D facility, modified dynamic torque calculations are being developed to best understand the impact it has on flow conditions, pressure drops, flow rate and torque outcome. The current study aims to customize the torque curve to suit each customer’s specific needs. By altering the torturous path of the MOGAS FlexStream® trim design, not only will accuracy and repeatability be ensured, but potentially a 30% reduction in overall actuator cost can be realized.

Nonuniform static pressure distribution resulting from fluid flow past valve ball causes dynamic torque.

The results from preliminary computational fluid dynamics show the advantage of combining FlexStream’s unique flow characteristic Cv lift curve with variable valve pressure drop systems to reduce dynamic torque. The FlexStream design increases flow resistance, which decreases the dynamic torque coefficient greatly. The trim seat location significantly assists torque reductions, while its configuration has the most favorable dynamic torque coefficient of all positions tested.

More than 50% reduction in dynamic torque coefficient can be obtained by changing the trim location from inside the ball to the seat configuration.