The Cause And Alteration For Extraction Drain Pipes Rapture In Daya Bay Nuclear Power Staion

Since the 8^th outage in Da Ya Bay, December 8^th ,2001, frequent ruptures and leakage take place in drain pipelines in the system of GSS110/210DI,AHP107/205DI of two units. The ruptures are mainly distributed around the welding joints around valves, elbows, tees and orifice plate from drain trap to flashbox.The drain piping system of GSS & AHP Extraction pipes functions to discharge the condensate water from the bleeding pipeline to the flashbox in condenser, so as to avoid the occurrence of the serious damage to pipelines, which results from the shock due to high-speed fluidity of drop-blended steam or water hammer produced by the seeper. Drain recovery system has the electrical control valve bypass, which is designed for controlling condensate water discharge during the start-up to 30% workload condition. Once the power is over 30%, pipelines of drain trap would take the place to automatically drain water. The medium in drain trap and upper stream pipelines is the saturated water under the working pressure of corresponding bleeding pipelines. After drain trap starts up, condensate water is driven by pressure difference to begin high-speed fluidity, and then the water is throttled for pressure drop in the outlet of drain trap. By absorbing evaporated potential heat, partial saturated water is quickly converted into steam, which afterwards is changed into two phase flow absorbs the. The high-speed water-steam biphasic flow will produce serious erosion to the pipelines.Via performing comprehensive examination, sampling test and root-cause analysis to GSS & AHP pipelines, where leakage and rupture occur, the conclusion can be reached: this case is initial failure led by mal-factors. The erosion of water-steam blended medium is the root-cause of the cracks, which extend due to vibration fatigue, further leading to the leakage and rupture. The fundamental cause is the alternative exertion between erosion and vibration fatigue.Therefore,we put forward the remedial solutions in accordance with the root cause:1. Change the dimension and structure of pipelines and throttle plates,e.g enlarging the inner diameter of pipelines, alternating structure and dimension of throttle plate to alleviate the erosion.2. The erosion region far away from weld jointdownstream can reduce the effect of erosion. But orifice downstream shows high stress because of serious thin and erosion pit. Erosion pit place a premium on flaw, results on rapture in advance under fatigue alternating stress.3. As for elements like tee or elbow, reduction of pipeline stress, elbow ellipticity and curvature radius should also be taken into consideration to reduce additional bending stress.After the drain pipings being modified as the root cause and proposal in DaYa Bay nuclear power station, the pipings leakage and rupture haven't occurred up to now.