Substantiation of fac rate and service life estimation under operation control data
The paper deals with development of procedure of FAC rate estimation. Flow accelerated corrosion – complex mechanism of NPP equipment elements damage. Its rate depends on such factors as metal corrosion, stream hydrodynamic, equipment elements geometry (straight parts of piping, welding area, bending and etc.), operating conditions, operation time, metal composition, thickness of corrosion product deposits and others. Decrease of element wall thickness is due to metal corrosion, increase ofelement wall thickness is due to corrosion product deposits. Arising of accuracy of service life prediction it is necessary to reduce uncertainties connected with control means and data quality. At present trust in procedure of FAC rate estimation is absent. Estimation of residual service life for NPP equipment elements also needs in above-mentioned procedure.
Development of FAC rate estimation procedure is based on analysis of control data. There are calculating main indexes of FAC such as numbers of both increasing wall thickness and decreasing one, rate of wall thickness decreasing and rate of corrosion product deposing.
Wall thickness decreasing due to FAC is accompanied by corrosion product deposing on the inner surface of piping. Due to this process the calculated residual service life formally is increasing. However, real wall thickness under corrosion product deposit is unknown as well as initial wall thickness.
To correspond to the facts, the new procedure of FAC rate estimation must be well-grounded. Research performed leads to original procedure taking into account tolerance of manufacturing on piping wall thickness and corrosion product deposit influence on both initial and minimal measured wall thickness (in all, three coefficients). To estimate the residual service life new index – safe coefficient - is using. This coefficient is recommended by international practice of NPP operation. Additional coefficients allow increasing of FAC rate calculation conservatism about 2,5 times in comparison with FAC rate calculation based on nominal wall thickness.
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