Knowing and minimizing the critical submergence of storage tanks for various operating conditions in power plants is very valuable to engineers. The goal is to maximize the usable emergency water volume and maintain tank operations such that no air (from vortexing or otherwise) is ever withdrawn into tank suction nozzle(s), that then could jeopardize continued operation of system pumps. While empirically derived curves for air withdrawal predictions can provide some general guidance, they cannot predict actual submergence requirements, especially when return flow is present and water level drops continuously. Physical hydraulic models can be used effectively to determine the critical submergence for plant specific geometries at all possible operating conditions. This paper presents a range of results of physical model studies conducted to determine critical submergence at various operating conditions and compares them with empirically derived curves to determine any possible trends or definitive rules. Critical submergence for all points on all studies was below the Hydraulic Institute boundary curve for pump intake design and below the Reddy and Pickford boundary curve for all cases without return flow to the tank and most with return flow to the tank. Both are much more conservative estimates at high Froude numbers as compared with lower Froude numbers. The Harleman curve, which was derived for the selective withdrawal of density stratified fluids, is neither predictive nor conservative.

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