Regarding the interchangeability of flanges
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This post was last edited by yugs on 2022-5-19 16:10. In the HG/T20592-2009 flange standard, there are flanges with the same pipe diameter and different grades, but identical parameters. If parameters such as the connection dimensions of necked butt-weld flanges for PN25 and PN40, DN80 are identical, why are they classified as different grades? If the material is the same, what is the difference then? If it is to be connected to PN40 valves or instruments, can a PN25 flange be used as a substitute? I would appreciate it if the experts could give me some advice.
Different grades can be matched with each other as long as the connection dimensions are the same.
They have the same dimensions, but their model numbers, i.e., PNs, are different; thus, it is not recommended to interchange them. Consider the traceability and compatibility of the parts. In other words, a PN25 valve should be equipped with a PN25 flange; using a PN40 flange is unreasonable from the perspectives of design, operation, and maintenance.
Even when flanges of different pressure ratings have the same dimensions, care must be taken: the pressure rating of the flange used must be higher than the system pressure
The connection dimensions are the same, but it’s not the case for the strength dimensions; the thickness may vary. Low pressure cannot replace high pressure
Cross-matching is not recommended; identical connection dimensions do not imply the same voltage resistance
The connection dimensions are the same, but the thickness of the flanges varies; as a result, their pressure resistance capabilities differ. It is not recommended to use them for high-pressure applications when they are designed for low pressure.
When the external dimensions, connection dimensions, thickness, and type are all exactly the same, it is indeed possible to use them interchangeably. However, as the designer, it is not permitted in principle for such components to be used together; at the very least, the markings must be made consistent – that is, all fittings marked as PN40 should have that designation only, with PN25 markings not being allowed. Similarly, fittings rated PN25 must not have PN40 markings, as this is a precaution for the future. However, it’s not the contractor’s fault if they can’t control it.
If the pressure rating, external dimensions, connection dimensions, and thickness are all the same, it is usually the flanges with smaller pipe diameters that are not made thinner; due to their relatively low overall thickness, a larger proportion of the thickness is allocated to corrosion allowance, and there are also requirements regarding minimum stiffness. Therefore, the thickness of flanges for low-pressure applications is not reduced. Theoretically, they can be used together. The difference between flanges lies only in the steel stamp. However, for the sake of future document management, maintenance, and spare parts management, as well as clear assignment of responsibilities, this practice is generally not recommended. If it must be done, it is advised to remove the pressure rating stamp (this applies only when all dimensions, including thickness, are identical).