Abstract
The work in this paper relates to an improved tube to tubesheet joint in a shell and tube type heat exchanger that have dissimilar metallurgies of tube and tubesheet. This improved tube to tubesheet joint configuration will not only impart reliability and strength to the mechanical integrity of the joint but also reduce the costs of the manufacturing without compromising the functional performance of the exchanger. In a shell and tube heat exchanger, generally the material of tubes may be similar to the tubesheet or of a higher corrosion resistant grade in comparison to that of the tubesheet. Conventionally, shell and tube heat exchangers that involve dissimilar materials of tube and tubesheet are either:
i. Tubes welded directly to the tubesheet with matching grade weld metal
or
ii. The tubesheets are clad / weld overlaid with the higher corrosion resistant material or material similar to the tube material
For purpose of this study, we have considered the material of tube as stainless steel type 304L and that of tubesheet as carbon steel.
The above case 1 wherein the SS-304L tubes are directly welded to the CS tubesheet, may have the following advantages:
i. Manufacturing time is less.
ii. Tubes are attached to the base metal and hence free from defects of weld overlay or cladding, if any.
In case 2, wherein the SS-304L tubes are welded to the SS-304L clad / weld overlaid portion of tubesheet, the advantages are:
i. Welding takes place between two austenitic stainless steel grades, hence PWHT is not required.
ii. The tube to tubesheet joint and adjacent material is of austenitic stainless steel grade, hence has better corrosion resistance.
The main objective of the work done and reported in this paper is to combine the advantages of the above two cases thereby:
i. Reducing the cost of manufacturing
ii. Increasing the mechanical reliability of the tube to tubesheet joint.
Extensive trials were carried out on sample tube to tube sheet joints, the results and conclusions of which are reported in this paper.
