This paper addresses the effect of tube sheet hole enlargement, due to the frequent de-stubbing and re-tubing process during the life of the heat exchanger, on the structural integrity of the tube-tube sheet joint. An experimental study is conducted to evaluate the effect of "clearance" on the strength of roller expanded tube-to-tube sheet joint, the amount of the wall reduction of the expanded tube, and the degree of strain hardening of the tube and its surrounding ligaments. A test block and a special fixture were designed, manufactured, and used to study the integrity of the tube-to-tube sheet joints. To simulate in-service and maintenance hole enlargement, different over tolerance (OT) levels were considered, including and exceeding Tubular Exchanger Manufacturer Association (TEMA) range by more than seven folds. The experimental results show that the increase in the over tolerance level, by up to seven times the maximum value allowed by the TEMA standard, does not affect the level of pull out force. The mean force value remained practically constant in the investigated over-tolerance range. Percent wall reduction (WR) of the expanded tube was found to increase linearly with increasing over tolerance value. However, the maximum value of WR remained well below the 12% limit for which a joint is usually rejected. Roller-expansion in holes with OT levels larger than the TEMA allowable value resulted in an increase in the hardness of the tubes by 22% to 24%, and that of the surrounding ligaments by 2% to 6%. The 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 were calculated using existing methods and were found to compare well with the experimental results.
|Number of pages||5|
|Journal||American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP|
|Publication status||Published - 1 Dec 2000|
ASJC Scopus subject areas
- Mechanical Engineering