TUBE-To-TUBESHEET -- Welding Before or After Expanding

Tube to tube sheet joint (Heat Exchanger) -> Welding prior to or after expanding? Please describe what is feasible if joint for tube to tube sheet is expanded and strength welded. (Consider fixed tubesheet heat exchanger) 1-Welding after expanding 2-Welding before expanding

There are certain merits and demerits of both the sequences you mentioned. Codes including ASME Sec. VIII Div. 1 and TEMA do not say anything regarding the recommended sequence to be followed. But the preferred and recommended sequence is 2, that is welding before expanding based upon following reasons: 1. Welding shall be performed in an atmosphere where welding gases can escape; welding after expansion will surely result in bad tube-to-tubesheet joints having porosity or weld inclusions by lubricants (if cleaning not done properly) 2. Welding after expansion would cause adverse effects on expansion (can get loosened) due to welding heat. Such tubes would be requiring re-rolling. On the other hand, the de-merits of above mentioned recommended solution include: a. No foolproof check would be in place for leak tightness of expansion done b. Chance of welding getting damaged during expansion process. The later can be overcome by increasing the starting distance of expansion towards shell side (recommended is > 15 mm from the toe of the weld joint, used by manufacturers)

The leak tightness of expansion generally done only through Mock-up test.After mock-up test the test piece is cross sectioned and leak path is measured under microscope. The % of expansion is strictly done as per TEMA Rules and it is measure by GO and NOGO gauge and Bore dial gauges. To carryout full expansion after welding also induce more stresses into the tube to tubesheet weld joint. Generally initial rolling i.e 25% to 50% expansion done initially to make sure that the tube is in position and does not move inward or outward during welding and complete the final expansion after welding and this will reduce stress on the tube to tubesheet weld joint.

Specification for the Tube to Tubesheet welding you may find in ASME SEC III indetail. I have followed both section III and Nuclear Power Corporation of India specification which

will be more stringent.