Flanges have pressure ratings defined by a standard, most commonly ANSI/ASME B16.5 for which the pressure ratings decrease as temperature rises above 38ºC. However the basis for the rating is conservative and AS 2885 allows some exemption from the de-rating requirements: pressure de-rating of B16.5 flanges is not necessary until the temperature exceeds 120ºC (see Clause 3.4.3, including the useful notes that follow it).
The effects of de-rating can be very significant. For example, the NT Gas pipeline (1400 km to Darwin from fields west of Alice Springs) is nominally a Class 600 (10.2 MPa) pipeline but has an MAOP of 9.65 MPa. As far as I know this MAOP was selected to meet the flange de-rating requirement at some mildly elevated design temperature. (It was designed in 1985, before the de-rating exemption was introduced.) This looks to me like a severe case of the tail wagging the dog. MAOP of 1400 km of pipeline reduced by 550 kPa because of a few flanges? That 5% pressure reduction would have been worth about 10% of pipeline capacity. (No criticism of the designers implied here – there may have been other factors I’m not aware of. And as it turns out the gas fields are declining so maximum capacity is unlikely to become an issue anyway.)
These days the usual practice in response to flange de-rating is not to reduce MAOP but to require specification of flanges from the next higher pressure class. So a 10.2 MPa (Class 600) pipeline would be fitted with Class 900 flanges (15.3 MPa). This reaches its extreme on things like filter vessels. I’ve seen filters on a Class 900 (15.3 MPa) system fitted with Class 1500 flanges that were considerably larger than the vessel itself. In the overall scheme of things that might not matter, but it offends my sense of proportion. Unfortunately the AS 2885 de-rating exemption doesn’t help in this case because he vessel design is governed by AS 1200, although a design check of the lower-rated flange might allow it to pass the AS 1210 requirements.
Standards other than B16.5 are not so restrictive – MSS SP44 and ISO 15590.2 don’t require de-rating until above 120ºC.
In summary: For piping flanges (not vessel flanges) at ordinary pipeline temperatures there is no need to de-rate the allowable pressure, or to use a higher pressure class in order to maintain the desired rating. Vessel flanges may be more complicated – do the design check, use non-B16.5 flanges, or just grit your teeth and accept flanges of the next higher pressure class.