Heat Exchanger Tube Rolling Issues
Metallurgical Consulting has examined numerous heat exchanger tube-to-tube sheet leaks over the past 35 plus years. The three cases highlighted here exhibit instances where tube installation, tube rolling, and seal welding were the proximate causes of extensive leaks at the tube/tube sheet interface. Two of the cases resulted in large financial losses which involved chlorine and sulfur dioxide gases. Any leakage of these gases into the cooling water on the shell side will (and did) produce aggressive acid along with serious corrosion damage.
The chlorine heat exchangers were equipped with heavy wall steel tubes, with no dummy block tests being performed to evaluate the rolling parameters prior to fabrication. After the incident Metallurgical Consulting was called in to do a complete analysis. This required the fabrication of a dummy block to determine the amount of rolling, and the type of mandrel required to achieve a strong, leak proof joint. Failure to achieve adequate rolling was documented with ID tube measurements in the tube sheet, and the ease with which the tubes could be removed from the double grooved tube sheet (Figures 1-4).
The second incident was on a sulfur dioxide gas unit and involved problems with replacement of the tubes and tube sheets in an old unit. There was also an issue with tube sheet warpage during post weld stress relief, and stress corrosion cracking in the tube sheet to shell weld. On site measurements revealed a wide range of tube rolling conditions and a lack of knowledge of how to measure and control the degree of tube rolling on the part of the contractor. Dye penetrant tests revealed numerous pinhole leaks in welds (Figure 5). On site grinding of the leaks and subsequent sectioning of the welds clearly revealed worn hole porosity (Figure 6). It was determined that the large, long tubes had been dragged on the ground through the plant and inserted into the tube sheets without cleaning. Thus, when seal welds were made or repairs attempted, the dirt at the weld root produced worm hole porosity. (Testimony on this case was given in federal court.)
The third case was observed on a fire tube boiler, and was a clear case of over rolling the tubes which resulted in cracks. In addition the tube mandrels were old and not in good condition. These three cases show the results of both under rolling and over rolling tubes into tube sheets. (Figures 7 & 8).
To learn more about these heat exchanger tube rolling issues, contact Metallurgical Consulting directly.
Images From The Metallurgical Services Provided For These Heat Exchanger Issues
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