Origin of the fact that, in version 99, for the 151 medium-strength weld, the fillet weld height of the heat exchange tube is greater than or equal to 1.4 times the wall thickness of the heat exchange tube
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I recently reviewed version 14 of 151, and it shows that the height of the fillet welds on the heat exchange tubes has been modified. There is no longer a requirement that this height be at least 1.4 times the thickness of the tube wall; instead, the height of the weld fillets is determined based on pull-out force calculations, with the requirement that it be no less than 1 time the thickness of the heat exchange tube wall. This is a good technical improvement. I remember having seen an explanation in the older version of 151 regarding the fact that the weld corner height should be more than 1.4 times a certain value, but I can’t recall it now. Does anyone else still remember how this 1.4 times figure was determined? It would be best if theoretical derivations or calculation procedures could be provided, thank you.
The weld roots at the pipe and tube sheet are subjected to shear stress. For the shear stress at fillet welds, the maximum value occurs at 45°, and the area subject to shear stress is equal to the length of the line at 45° multiplied by the outer diameter of the pipe (I’m sorry, I can’t draw a diagram; it’s available in various reference materials). This value is 0.717 * weld root height * pipe circumference. To make the tensile stress acting on the tube roughly equal to the shear stress at the weld leg, the height of the weld leg is set to 1.4 times the wall thickness of the tube; as a result, the shear area becomes 0.717*1.4*wall thickness*circumference of the tube = wall thickness*circumference of the tube, which is exactly equal to the tensile normal stress acting on the tube.
The standard definition of 151 in version 99 also lacks relevant explanations.