Selection of pressure class for pipe flanges
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There is a device made of material S30408, with a design pressure of 0.44 MPa and an operating pressure of 0.4 MPa. The operating temperature is 150°C. HG/t20592 standard pipe flanges are to be used; according to this standard, the maximum allowable operating pressure for flanges of PN6 grade at this temperature is 0.41 MPa. Since 0.41 MPa is less than 0.44 MPa (the design pressure), is it appropriate to use PN6 grade flanges?
It’s not appropriate; generally, the pressure obtained from reference tables should be greater than or equal to the design pressure (or the maximum allowable operating pressure). Flanges rated at 6 kilograms are usually used for normal pressure conditions. Even if the value falls within the acceptable range, it’s still not advisable to use them. The cost difference between flanges rated at 6 kilograms and those rated at 10 kilograms is actually very small!
Fittings should be selected based on the design pressure; therefore, PN6 pressure is insufficient.
Why is it chosen based on the design pressure? The flange standard clearly specifies the maximum allowable operating pressure. It is also acceptable to choose based on the design pressure; PN6 is itself a flange of the 6-kilogram class, and 0.6 MPa > 0.44 MPa, so that should be fine too, right?
The design pressure is the maximum allowable operating pressure of the pipeline; under your current operating conditions, this pressure is 0.44 MPa, whereas PN6 flanges can only be used in conditions where the pressure is less than 0.41 MPa.