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Before discussion of reasons, i would like to explain the merits of sequence of completion of expanded and welded joints extracted from Heat Exchanger Hand book (well known author)
Merits and demerits of this method as follows
First expansion followed by welding --
Merits
1.Welds are free from strain induced by rolling operation.
2.The integrety of the tube expansion can be tested.
Demerits
1. Welds may contain porosity due to presence of lubricants used during expansion,if cleaning is not done properly.
2.Expansion may get lossened under the influence of heat during welding.Such tubes would require re rolling.
First welding followed by expansion
Merits
1. Welds are free from porosity due to absence of lubricants.
2.If the tube to tube sheet requires PWHT,there is no point in expanding first.The heat treatment will relax the tubes
3. Tube projection for welding is uniform.
Demerits
1. Welds may be damaged due to strain caused by expansion.
2. There is no fool proof check for the leak tightness of expansion carried out,as the seal welds may help in shadowing the under expansion of the tubes.
3. Weld shrinkage may reduce the tube inner diameter so that it is difficult to insert expanders.This problem is overcome by reaming.
Conclusion: Light expansion followed by seal welding and followed by full expansion is most preferred, since it has merits of the two sequences just discussed.
The purpose of light rolling is to remove the air trap between tube & tube sheet. In normal practice , this requires precise control during light rolling, some times operators are not able to exercise. This results in welding defects. This sequence should be discussed with all concerned, specially the operator and welding supervisor, before proceeding the sequence.
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