Selection of material grades for medium-pressure steam
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For boiler steam with a design pressure of 5.0 MPa and a temperature of 380°C, should it be rated at PN63 or PN100?
PN100. First, you need to consider what flange material to use. If A105 according to HG/T 20592 is chosen, flanges with PN63 at 380°C can only withstand a pressure of up to 4.77 MPa, which does not meet the requirements. If it is PN100, it can withstand 7.57 MPa at 380°C, which meets the requirements.
I’ve seen that in terms of pipe grades, the material of the pipes can differ from that of the flanges. So, would it be appropriate to use chromium-molybdenum steel for the flanges of my pipes? Will welding flange pipes made of different materials have a significant impact?
This practice does indeed exist; it was first seen in projects within Sinopec’s system, and I have also used it since then. Carbon steel pipes with a temperature rating of 380°C work well, and flanges made of 15CrMo are suitable; a pressure rating of PN63 is sufficient in such cases. Welding 15CrMo and 20 steel is not a major issue at the current level of welding skills; it isn’t considered a particularly difficult case of welding dissimilar steels either.
Yes, I also saw it on Sinopec’s grade chart. When I created my own grade chart, there were cases where the pipe material was 20#, while the flange material was 16Mn; these two materials are quite similar, so welding poses no major issues. But I’m not sure about the situation with carbon steel and chromium-molybdenum steel
I’ve also seen 20 and 15CrMo materials, and I’ve used them myself. The first time I saw them in use was in 2008, and to this day I’ve heard of no problems with them.
What I’m wondering is whether the use of PN100 is intended to avoid problems that can arise from welding different types of steel, or is it due to cost? However, when using PN100, the corresponding outer diameter of the flange, the number of bolts, as well as the specifications of the valve increase compared to PN63. So in this regard, I would like to know: what is the reason for using PN100 in such cases? Now I’ll have confidence in my mind when faced with similar situations in the future