Zero Leakage Tornado Effect Pumps in Carrier Sewage Systems

Abstract

Current centrifugal pumps installed in CVN 68 class sewage collection, holding, and transfer (CHT) systems experience a high rate of seal failures. A CHT pump seal failure requires a significant maintenance effort. In addition, seal leakage releases sewage material into the CHT pump room in violation of Navy health regulations. Efforts to develop improved pump seals have met with less than satisfactory results. An effort was undertaken to identify an alternate pump design that meets the zero external leakage criteria. This effort identified the Eddy Pump, which is gaining increased recognition for its performance in civilian applications. This new pump departs from conventional centrifugal pump design principles in that the pump rotor is able to create and harness a dynamic fluid eddy effect within the pump housing and inlet. This effect is similar to that which occurs in a tornado. A very strong synchronized central column flow develops from the pump rotor to the pump inlet and creates a low pressure reverse eddy flow from the pump inlet to the pump discharge. This results in a region of negative pressure in the vicinity of the pump seal. The seal design allows one way passage of externally supplied low pressure air through the shaft seal toward the pump casing (volute) to oppose the ingress of sewage. This paper is a condensation and combination of two reports. These reports are: (1) Myers, S. and R. Keltner, “CHT Pump Trade‐Off Studies: CVN 76,” prepared by Newport News Shipbuilding under contract N00024‐91‐C‐2108, Task 2318T‐74, January 1993, and (2) Schepis, R. and J. Crew, “Eddy Pump Performance,” Commander, Naval Surface Warfare Center, Carderock Division, Philadelphia Site, Serial 631/110, May 1995.