Reverse engineering of a centrifugal turbine housing

Abstract

The Garrett GT 42 turbine housing is a spheroidal ductile cast iron casting, which according to the ASME B31.3 (1996) Code for Pressure Piping, does not satisfy the Pebble Bed Micro Model (PBMM) design requirements. By using Geometrical Reverse Engineering (GRE) methods, a 3-D virtual model, and if required, a mould of the existing design can be created to recast a turbine housing in a suitable material. The aim of this research was to study, evaluate, identify and implement the most promising method(s) to reverse engineer the Garrett GT 42 turbine housing without destroying it in the process. GRE advantages and limitations influenced by practical contact (tactile) and non-contact data acquisition problem areas provided evaluation guidelines that assisted in the allocation of Computed Tomography (CT) as the most promising method to reverse engineer the turbine housing. X-ray and neutron CT were implemented. Neutron Computed Tomography (NCT) using CADKEY® V21 CAD modelling software at the SAFARI-1 nuclear reactor, NECSA, produced the most favourable 3-D NURBs model, which had a geometrical external accuracy of 95% and a volumetric internal accuracy of 94%. It was verified by comparing the results with an 87% geometrical external- and 85% volumetric internal accurate 3-D NURBs model created by the implementation of an alternative contact GRE method, i.e. geometrical inspection and measurements.