Hi,
I´m doing a stress analysis @ 1000 °F. I´m using Hot elastic modulus for loads and cold elastic modulus for stress. My question is about "hot sustained". When I use hot elastic modulus the pipe is not lifting up and I don´t have problems with hot sustained case. But If i use cold elastic modulus the pipe is lifting up, so I have problems with hot sustained stresses. According to the codes I shall use cold modulus for stress, but I´m not sure how to deal with hot sustained in my case.

So, you're looking at two discrete points in "time."

You see your pipe being at max temperature and wanting to lift off due to thermal expansion, but because it becomes more pliant, it "sets back down" onto the support.

However, there may be intermediate temperatures where the pipe heats up, lifts off, but doesn't have adequate flexibility to set back down.

If you model for intermediate temperatures, you could demonstrate a trend of whether or not the pipe ever actually picks off the supports and demonstrate adequacy of the design with fewer supports removed (or none).

Without that intermediate check, however, you'll be stuck with the basic requirements as called out within the code, which are often designed to protect you with large brush strokes.

Thank you Mike. Very well explained.
Regards,

Adding to what Mike said, if you are using CII version 2017 which complies with B31.3 2014, the lift-off check is now mandated by code and accordingly,CII has facilitated Alt+Sus case for this check. A separate check for this is no more required if you are working with B31.3.

Remember App P in one of the versions which was optional and now a compulsory check!