Easing back into writing after a long period of zero inspiration has been helped by some advice from Phil Venton in response to a query he received about the temperature de-rating rules in AS 2885.1:
I was going through the pipeline temperature de-rating section in AS2885.1. Can you please explain the data/reasoning behind this equation and why there is a different maximum temperature which requires de-rating for the pipelines and flanges to AS2885 and flanges to ASME B16.5?
Very reasonable question! The relevant part of AS 2885.1 is Clause 3.4.3. I’ve touched on this before but in a vague sort of way. Phil’s response is much clearer and went like this (slightly edited):
The matter relates to the basis by which pipes and flanges are designed, and the change in properties of materials in response to changes in temperature.
The yield strength of steel declines more or less continuously with temperature, whereas the tensile strength does not start to degrade until perhaps 200°C.
Pipe is designed using yield strength. In principle the yield strength should be reduced for any temperature higher than the reference temperature, however given the complexity that this introduces to typical pipelines, most of which have a design temperature less than 60°C, it seemed reasonable that the nameplate yield strength should be used for design at temperatures <65°C, while for temperatures ≥65°C the yield strength should be reduced at the nominated rate, assuming a starting temperature of 23°C. (The note at the end of Clause 3.4.3 points out that 65°C is a convenient temperature for this purpose since it covers the discharge temperature from most gas pipeline compressor stations.)
Flanges are designed to a pressure equipment standard (ASME VIII or AS 1210). These standards require the use of the lesser of the tensile strength/3.5 (/2.35 in some instances) or the yield strength/1.5. It happens that the factored tensile strength governs the flange design to about 120°C. Above this temperature (approximately) the factored yield strength is less than the factored tensile strength.
Consequently the basis for each temperature de-rating requirement is entirely rational provided you appreciate the principles, and recognise that the authors of the Standard have made rationalisations to provide practical rules for design.
(Thanks to Phil Venton for this clear and useful explanation of rules that at face value can appear arbitrary.)