How is the pipe material grade determined?
Thread Content
Today, I was faced with a question regarding the classification of pipe materials that puzzled me. I designed a steam pipe with a design pressure of 1.0 MPa and a temperature of 200°C. When determining the appropriate material grade for this pipe, I used “N1B”. The “N” designation came from the 25 bar value associated with the flanges used; however, my instructor told me that this was incorrect, as the material grade should not be determined based on the flange specifications, but rather on the pipe’s design pressure, which would correspond to “M1B”. I’ve searched through many sources yet couldn’t find any authoritative references regarding the correct classification for pipe materials. I hope experts can help me figure out what criteria should be used to determine the material grade, along with the relevant sources. Thank you!
If the flange selected is PN25, it should be N1B； M and N correspond to the nominal pressure codes; refer to HG/T20646 and HG20519 for details
I don’t know why the flange pressure is set at 25 bar while the design pressure for the pipeline is set at 1.0 MPa; generally, shouldn’t these two values be the same?
First, determine the operating pressure of your pipe, and then calculate the design pressure based on your institute’s design specifications. For example, the design pressure might be calculated as 1.1 times the operating pressure (different design institutes use different values for the design pressure, so it’s important to check this first). If the operating pressure of your steam is 1 MPa, then the design pressure will be 1.1 MPa. This value of 1.1 MPa represents the maximum allowable pressure for your pipe; various parameters such as the pipe wall thickness and the setting pressure of the safety valve must all be calculated based on this value. As for the flanges to be used subsequently, in principle their allowable stress should be higher than the value specified in your design; therefore, a flange with an allowable stress of 1.6 MPa will meet the requirements of your current pipeline. For a flange rated at 1.6 MPa, the corresponding grade is M grade. For example, if the design pressure you calculate is 1.5, a flange of 1.6 MPa will still meet your requirements; however, for safety reasons, design institutes usually choose a higher grade than that required by the flange standard. (This optimal value varies among different design institutes.) So when raising the grade to 2.5 MPa, the grade becomes N.
The temperature is not at room temperature; the pipe material must be selected based on temperature adjustments
The temperature is not at room temperature; the pipe material must be selected based on temperature adjustments
It needs to be determined comprehensively based on pressure, temperature, allowable stress, etc
It depends on how you interpret this question. I’ve encountered my teacher’s version of that interpretation as well, but I lean toward your understanding. First of all, pipes do not have such a concept as a pressure rating; the determining factors for pipes are the design temperature, design pressure, and material. Only flanges have pressure rating specifications. If you write M in this field, it will be impossible to tell from the flow diagram that you are using PN25 flanges. Secondly, Chapter 2, page 814 of the \"Manual for the Installation Design of Process Piping in Petrochemical Plants\" states that the letters used represent the pressure rating of the flanges. . .
At the design temperature and pressure you specified, carbon steel grade 1.6 is sufficient, so it’s fine to use M.
The flange class does not indicate the pipe class; it depends on the calculation process, after which a margin is taken into account. The grade of the flange should be at least as high as that of the pipe; generally, it is the same as or slightly higher than the pipe’s grade.