Regarding the material grade change for pipelines
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I discussed with the company the issue of changing the material grade for a pipe; although things were explained quite clearly, there are still some points that I don’t understand. I would like to ask for advice from all of you experts. Material changes in pipes usually occur at fittings such as flanges. For example, if the material grade is changed at a certain valve in a PID system, does that mean that both flanges used in piping should be of the same material grade, with one of the flanges being connected to a pipe or fitting of a different material grade? (This also involves the issue of welding different materials; I’m not sure whether different materials can be welded together. Please advise me.)
When changing the grade at the flange, the grades of the two flanges will differ, and the gaskets and fasteners shall meet the requirements of the higher grade.
The flanges on either side of the valve can be made of different materials, as long as each one meets the required design temperature and pressure requirements. Even the pressure ratings of the valve and the flanges can differ, provided that the bolt holes align properly.
When changing the grade of a pipeline, it is necessary that the nominal pressure of the two flanges be the same (according to the higher grade), while the materials should differ (the same material for both high-grade flanges, and the same material for both low-grade flanges); this way, there is no issue with welding different types of materials together； The method mentioned upstairs works too; just be careful with the bolt holes ； Additionally, welding some different materials is actually not a problem, but the selection of electrodes, post-weld heat treatment, inspection, etc., can be somewhat troublesome; it depends on the specific circumstances, and it’s hard to explain in just one or two sentences.
2# lihcsd: One valve and two flanges have different grades, so careful attention is needed; otherwise it’s easy for them not to fit properly...