Experimental investigation of heat exchanger tubesheet hole enlargement

An experimental study has been conducted to evaluate the effect of roller expansion of heat exchanger tubes in enlarged tubesheet holes on joint strength, tube wall reduction (i.e., thinning), and strain hardening of the tubes and surrounding tubesheet ligaments. Different levels of tube-to-tubesheet hole clearance, or overtolerance (OT), were investigated to simulate tubesheet in-service, and during-maintenance, hole enlargement process. The experimental results have shown that, in spite of increasing overtolerance level by up to seven times the maximum value allowed by the Tubular Exchanger Manufacturer Association (TEMA) standard, the average strength of the test joints has not declined below the strength of joints in the allowed TEMA OT range. Percent wall reduction (WR) of the expanded tube was found to increase linearly with increasing overtolerance value; although its maximum value remained well below the wall reduction limit for which a joint is rejected in industry. An observed increase in hardness of the expanded tube material is evidence of the high degree of strain hardening to which the expanded tubes are subjected compared with the surrounding ligaments. Pull-out forces, calculated using existing analytical methods, were found to compare well with the experimental results.