In the most commonly used ASME B31.3 code it is written in 302.3.5 that: "(c) Stresses Due to Sustained Loads, SL. The stresses due to sustained loads, SL, in any component in a piping system (see para. 320), shall not exceed Sh, where Sh is the basic
allowable stress provided in Table A-1 or Table A-1M at the metal temperature for the operating condition being
considered.”
In reality, we do not approach these values. We try to keep to 60-70% Sh. This is because permanent stresses belong to the basic stresses (ang. primary stress). Their impact is permanent, so any errors in assembly are irreparable. Additionally, exceeding the yield point by permanent loads is unstoppable due to the phenomenon of positive feedback.
The second, much more important issue is the phenomenon of lifting supports due to temperature expansion. If we have, for example, 90% permanent stresses, this is definitely too much. You have to be aware that if support F03 is lifted from the temperature (here T3), then it is switched off for GR+Max (P3). In such a situation, we have increased permanent stresses.
