Impact of expansion pressure on wall thinning and contact pressure for hydraulically expanded tube-to-tubesheet joints: Numerical analysis

The design of the tube-to-tubesheet joints is very critical to the functioning of the heat exchangers. In an expansion process, the pressure that is applied on the inner tube surface causes the tube to deform radially outward. Further plastic deformation of the tube, while tube in contact with the tubesheet, creates a contact pressure at the interface. Therefore, an optimum expansion pressure adequately develops sufficient contact pressure at the tube-to-tubesheet interface. Wall thinning percentage is generally used in practice to estimate the right amount of expansion. This paper focusses on the wall thickness reduction expressed in percentage, radial displacement of the inner tube surface, contact pressure and theoretical pull-out force for a various expansion pressure range. Results show that an expansion pressure lower than the yield stress of the tube does not fully close the gap between the tube and tubesheet. There is an evident spring back of radial displacement and contact pressure due to bauschinger effect upon retrieving the expansion pressure. However, the maximum radial displacement and contact pressure increase as expansion pressure increases. All the expansion pressures above the tube yield stress were sufficient to close the initial gap of the tube-to-tubesheet joint. A sudden increase in the radial displacement was observed for expansion pressures above tubesheet yield stress. Expansion pressure from 240 MPa to 340 MPa resulted in wall thinning percentage of 1.15% to 2.97%. Since the pullout strength and contact pressure are interrelated, high expansion pressure resulted in greater pull-out strength.