A contoured plug of stainless steel #316 inserted in a low CV valve has been subjected to erosion tests at a pressure reduction of 20MPa. The test duration on a single plug has extended over 300 hours until the shut-off function becomes incomplete. The fundamental flow pattern inside the valve is almost unchanged at the inlet pressure up to 20MPa in a sense that the pressure reduction has been almost completed at the throat formed by the seat-ring and the characteristic surface of the plug head. The kinetic energy of the flow at the throat describes the energy that is dissipated in the valve. Simulated pressure distribution and cavitation pictures obtained at a low inlet pressure condition indicate that the plug seat-joint-taper is the part that suffers impact of the high kinetic energy flow and collapse of the cavitation bubbles. Then this part is specified as the part of the most heavy erosion. In the erosion test, the process of the erosion development shifts from a phase of pit formation to a phase of rooting out the inside at a stage that the mass loss has reached a certain critical level. In the initial phase, the time averaged loss rate is proportional to a power of the dissipated flow energy. In the second stage of rapid growth, the cumulative mass loss increases with fourth power of the working duration. By using values of the exponent and factor obtained at the full opening test, that provides the shortest test periods, mass loss at an arbitrary condition of CV-value and cavitation number is estimated. Then, a procedure to estimate the valve lifetime is suggested based on a criterion that the seat-leakage flow must not exceed the controllable minimum flow of the valve.

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