How to Manage Pipe Support Lift-Off in Stress Analysis

[nikolastrojman]

Hello everyone,

I’m seeking your opinion on managing support lift-off scenarios, particularly in systems where there’s a significant difference between "normal" operating temperature (e.g. 60°C) and design temperature (250°C).

Here’s the situation which I encounter many times when doing stress analysis:
- At ambient (installation) temperature, the rigid support carries the pipe's weight as expected
- At normal service temperature, the stress analysis software shows vertical displacements in the range of 0.5 mm to 2 mm
- for design temperature, the lift-off becomes more pronounced

Should we consider only the design temperature scenario and use variable pipe supports/hangers, which are very expensive and are treated as specialty support items to be used only when required -> looking from practical/economical point of view.

Let's assume hot-sustained has already been checked and stress are in compliance with the code (the system is also ok without the supports which are lifting).

We are talking here only about whether to leave the rigid support in place, remove it (which increases line sagging then) or switch to variable supports.

Which are the strategies you use to avoid overengineering the system from support concept point of view.

 

[RVAmeche]

Ideally the pipe stress should be done at that maximum realistic temperature for the system. So if the operating temperature may be 60C but real upset temps are 150C and the specs/materials of construction are based on a design temp of 250C. In that situation, unless the system could be modified to try to run closer to 250C in the future, 150C would be the upper limit temp to use.

 

[XL83NL]

Ideally the pipe stress should be done at that maximum realistic temperature for the system. So if the operating temperature may be 60C but real upset temps are 150C and the specs/materials of construction are based on a design temp of 250C. In that situation, unless the system could be modified to try to run closer to 250C in the future, 150C would be the upper limit temp to use.

Agreed

Additionally I’d say that, depending on the system and loading conditions, a liftoff of say 1/16” (3 mm.) is ok. (I think that value is also present in Bentley AutoPIPE’s help file, where it refers to some RAGAGEP for liftoff)
Our supports for sliding have a lip that prevents liftoff. The gap to that lip is about 3 mm.
Furthermore it’s good to understand the concept of thermal shakedown and determine if and how a liftoff may fit in the shakedown scenario.