Common quarter-turn (QT) mechanisms used in nuclear plant air-operated valves (AOVs) include scotch yoke, lever, and link-and-lever mechanisms coupled to diaphragms and pistons. QT mechanism efficiency varies as a function of valve position and is a critical design input used to determine AOV margin. Because of the lack of publicly available data of a quality commensurate with “nuclear QA [quality assurance],” Kalsi Engineering, Inc. (KEI), initiated an independent QT-mechanism efficiency test program that includes a number of commonly used actuator manufacturers, models, and sizes based on a survey of U.S. nuclear power plants.

The first test specimen was a diaphragm actuator with a lever QT mechanism. The diaphragm rod of the test specimen was instrumented with strain gauges so that a direct measurement of the net actuator force transmitted to the QT mechanism could be measured. In addition to the net thrust, the output torque, diaphragm pressure, and actuator position were measured. Measuring the net thrust, diaphragm pressure, and position allowed the spring rate, spring preload, and effective diaphragm area to be quantified.

This test specimen was tested using two different types of bearings at the actuator shaft-to-lever connection. Needle bearings were used to provide torque results for a nearly frictionless QT mechanism, and bronze bearings were used to simulate a more realistic QT-mechanism configuration. Predictions made using the first-principles efficiency model are compared to efficiencies extracted from test. The predicted efficiency using a realistic range for the friction coefficient of the bronze bearings is in good agreement with the extracted efficiencies.

Paper published with permission.

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