Is a machining allowance left on the flange sealing surface of equipment that has heat treatment but no partition plates?
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Generally, in the tube side or shell side equipped with partition diaphragms, a machining allowance is left on the sealing surfaces of the equipment flanges; after welding these components together, the sealing surfaces are finely machined (even if heat treatment is not required for that section). There should be no doubt regarding this. But if the shell side or tube side has heat treatment but no partition diaphragm, is it necessary to leave a margin on the flange sealing surfaces of the equipment in that side, and to perform finish machining after heat treatment?
This post was last edited by wanlirn on 2020-10-3 at 16:01. The reason for leaving a margin is that the structure deforms after annealing. . For those that have heat treatment, of course, a margin needs to be left. . . . Our company has a decent machining capability; per our internal rules, regardless of whether heat treatment is applied or not, for large-diameter heat exchangers with high pressure, after welding the pipe joints and the large flanges of the tube sheet, the sealing surfaces must also be processed ； It’s easy for workers to tighten the flange bolts, and the quality of the steel is excellent
Oh, because I saw that some didn’t leave any machining allowance
It depends on the situation and the equipment; it needs to be determined based on the specifics of one’s own manufacturer!
This standard is not mandatory and is determined based on the manufacturing capabilities of each producer.
In the absence of a split diaphragm, for standard Type A and Type B flange sealing surfaces, a margin needs to be left and it is machined after heat treatment; long-neck flanges can be machined before heat treatment without the need for such a margin.
It depends on each company’s manufacturing capabilities and experience; the key is to ensure that there are no leaks after pressurization. Just as we did for a batch of tube sheet seals with large-diameter flanges, the drawings specified that the tube sheet seal surfaces needed to be processed after welding. Based on past experience, those tube sheets were quite thick, so the deformation of the seal surfaces after welding should have been minimal. However, since the drawings required it, we ended up leaving 0.5 mm of margin on one side of the seal surface. After welding, we checked the levelness; since there were no issues with pressurization, we didn’t proceed with further processing – any problems would be addressed later.
Overall heat treatment should leave a machining allowance on the flange, as heat treatment-induced deformation can affect sealing; if no such allowance is left, any resulting deformation cannot be corrected.