Design basis capability of safety-related butterfly valve of nuclear power plant should be verified due to the regulation of Korea. Therefore, based on the results from the torque coefficient of thin symmetric plate, the nuclear utility has performed the engineering calculation to determine the required torque for operating a butterfly valve and the margin of operability at design basis condition. However, recent research showed that the torque coefficient of the thin symmetric plate used before is rather larger than newly determined one. Therefore, in the present study, the effect of change in the torque coefficient of thin symmetric plate on the operating margin is investigated. First, the recently determined valve loss coefficient of thin symmetric plate is modified at near valve full open position to reflect a finite thickness of valve disk. Second, the torque coefficient of symmetric disk butterfly valve is determined from the loss coefficient and the torque coefficient of thin symmetric plate and using this, the torque coefficient of butterfly valve with asymmetric disk is also deduced. Then, the variation on the hydrodynamic torque of butterfly valve is quantified and the operating margin variation of safety-related butterfly valve is determined. Calculations of the operating margin are performed for 10 butterfly valves of Uljin 6 nuclear power plant of Korea, which have all asymmetric valve disks. As expected, the hydrodynamic torque which is proportional to the torque coefficient is reduced and the resulting margin of operability is improved for opening safety direction. The margin improvement amounts to from 6.0% to 142.9% for valves considered. For closing safety direction, although the hydrodynamic torque is also reduced, the total dynamic torque due to overall hydrodynamic effect is increased from 1.1% to 63.0% on the contrary. This increase is because the hydrodynamic torque assists valve closing. Despite of these increases in the total dynamic torque, the operability margins of closing direction do not change at all. The reason is that the required torque for operating valve is determined not by the total dynamic torque but by the total seating torque which is more dominating. In conclusion, the margin improvement is significant for opening otherwise the margin deterioration is slight for closing when the torque coefficient is reduced.

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