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The phenomenon of void formation in a piping system due to heat transfer and subsequent collapse due to pump start has been explored. The analysis is based on a computer model developed for the RELAP5 computer program [Ref. 1] and is applied to a cooling water system in a nuclear plant. This evaluation has been performed to respond to a generic concern in the industry. The concern arises from the fact that if the cooling water pump fails and there is significant heat transfer from the process side into the system, there is a potential for void formation in the system. Subsequently, as the pump restarts the void collapses due to pressurization and condensation, and consequently, may result in significant pressure surges. It has been observed that for a closed system the pressure surges are greatly influenced by the presence of a surge tank connected at the pump suction piping. During the initial period of the restart of the pump the fluid is drawn from the surge tank at a greater proportion until the fluid in the main line is accelerated to a steady state value at which time the flow in the surge line ceases. The relative speeds of the two fluid fronts, one approaching to the void and another departing from the void, at the time of the complete collapse determine the surge pressure. Factors affecting the surge pressure include the resistance in the surge line, the size of the void and the quality of contained steam, temperature of water surrounding the void, and the amount of non-condensable present in water. The forces calculated for the piping segments could be significant compared to other loads on the supports. The most effective ways of reducing excessive pressure surges include pressurization and/or increasing the elevation head of the surge tank and increasing the surge line resistance by installing an orifice or throttling of an existing valve. |
