3angles Non Destructive Testing
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Removal and Installation of Rotor Bore Plugs
Before inspecting bored rotor forgings the bore plug(s) will need to be removed as the inspections are conducted from the bore-side. Bore plugs have been installed in bored rotors to seal the internal cavity from the external ambient atmosphere. 3angles deploys two techniques for bore plug removal:
Applying the collapsing technique will collapse the bore plug and relieve the interference fit that holds the plug in place. This process utilizes a drilling technique to structurally weaken the bore plug which will in turn yield to the stresses produced by thermal expansion when the plug is properly heated; combined with thermal differentials between the plug and rotor coupling. The machining or milling technique will machine the bore plug and reduce the material to chips, leaving a thin shell that can be extracted from the plug bore cavity without galling of the bore surface.
After the inspections have been performed new bore plug(s) will need to be installed. A bore plug provides a gas tight mechanical seal by incorporating an interference fit in the order of 0.0013 to 0.0015 inches per inch of bore diameter. For example, a 4.000” plug bore opening would receive a bore plug that is 0.006” larger than the bore opening. Therefore, the diameter of the plug must be reduced to allow insertion into the bore.
In general, bore plugs are shrunk in diameter by immersing the plug in a bath of liquid nitrogen. Since the steel plug has a positive coefficient of linear thermal expansion, it will contract linearly as the temperature of the metal is reduced. Given sufficient reduction in diameter, the cold plug may be inserted into the bore cavity and held in place until it expands due to heat transfer from the surrounding metal shaft. Plug installation by shrinkage of the plug alone is the preferred method. However, under some conditions the clearance between the shrunk plug and the bore opening may still remain to tight for a safe insertion. In this case, it may be possible to gain additional clearance by heating the coupling flange to expand the bore diameter as well.