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 effect on accurate actuator sizing. At the MOGAS R&D facility, modified dynamic torque calculations are being developed to understand the impact it has on flow conditions, pressure drops, flow rate and torque outcome. The current study aims to customise 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.
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 favourable dynamic torque coefficient of all positions tested.