Questions regarding the determination of pressure ratings for valves and flanges in methanol pipelines
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I have a question regarding the determination of the pressure ratings for the valves and flanges in methanol pipelines. Everyone, I have some doubts: for methanol pipelines at 0.3 MPa and 98°C, with diameters of DN250 and DN400, the contractors have chosen a design pressure rating of PN10, and valves and flanges corresponding to 1.0 MPa have been used. However, the design institute now claims that this pressure rating is incorrect; it should be PN50 or Class 300. They mentioned factors such as stress as reasons for this change. Currently, all materials are being purchased and supplied based on PN10 standards. Is there any standard or guideline that can support the correctness of choosing PN10? That would be great Or what is the standard used by the design institute to select PN50? Which clause does it correspond to? Are there any reasons to reject this basis? Personally, I think the temperature and pressure of this medium are not high, so I assume that PN10 would also be sufficient; I’m not very familiar with this matter. Thanks for helping to clear up the doubts.
1 MPa is definitely sufficient; PN50 is not necessary. As for why, you should ask the design institute to provide you with a formal answer and explanation
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The temperature-pressure curve must be taken into consideration; that is, at the corresponding temperature, the maximum allowable operating pressure for flanges of that grade, and whether this pressure is higher than the operating pressure.
The pressure ratings of the valves and flanges for the pipeline are indeed
PN10 follows the European standard, while PN50 follows the American standard; there is a significant difference between the two. Additionally, a strength level of 1.0 is sufficient, but usually at least PN16 is required
The use of two different flange systems in one design device is quite poor quality on the part of this design institute. Write a letter to contact them and ask.
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The requirement by the design institute for Class 300 may stem from Table 3.3.1 of HG/T 20635-2009 (hex head bolts cannot be used with flammable media, with a nominal pressure ≤ Class 150), but this is a misunderstanding of the standard.