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.

 

[nikolastrojman]

Regarding to which temperature to use in the calculation my approach matches what you´ve mentioned here and I always try to establish the maximum realistic temperature which also covers the upset cases.

With my question I intended to learn more about how other engineers handle lifting of the supports in their models and what is the max. lift-off which they tolerate. After this limit has been crossed either they use springs, change the position of the support or even change the routing to accommodate for this lifting, or simply delete the support.

 

[RVAmeche]

In my Caesar II training they covered a similar kind of example of where if you follow the rules to absolute letter, on almost any system w/ temps you may come out to all hangers because everythings moving a little bit. They had an equation on "distance to first rigid" based off the expected displacement - so if you have 40' lateral run after a vertical riser and that equation comes out to 25', your first support at say 18' will be a spring hanger and the following support (after 25') would be rigid (clevis).

I can dig up some more notes if there's interest

 

[XL83NL]

I would suggest also reading some literature on this topic. A great resource (apart from the definitive work by the MW Kellogg Company) is Peng & Peng. In their Pipe STress Engineering book, in 6.3 this topic is dealt with.
Perhaps there are similar topics/sections in Peng (or MW Kellogg) that others can refer to, but I would by default resort to such literature first.

 

[nikolastrojman]

In my Caesar II training they covered a similar kind of example of where if you follow the rules to absolute letter, on almost any system w/ temps you may come out to all hangers because everythings moving a little bit. They had an equation on "distance to first rigid" based off the expected displacement - so if you have 40' lateral run after a vertical riser and that equation comes out to 25', your first support at say 18' will be a spring hanger and the following support (after 25') would be rigid (clevis).

I can dig up some more notes if there's interest

Sure! I´m interested if you could find something useful, I would be happy to have a look at it.

XL83NL
I´ll check your suggestion as well, thanks

 

[RVAmeche]

Here's the example from the book - 12" CS pipe at 350 degrees with a good sized vertical riser. The first pass comes up with all spring hangers (8 locations) because every location is moving vertically to some degree.

Using this equation that takes the modulus of elasticity, pipe size, and thermal growth calculated, to determine the location of the first rigid support. In this example Node 46 was changed to a rigid support, so this calculation is the growth up towards Node 40. Since Node 36 falls within that 24.67 ft, it stays a spring hanger, but Node 34 becomes a rigid support. Same process is repeated for lower section.



And final design is this, with three spring hangers instead of eight.

 

[KevinNZ]

B31.3 appendix S has an example of support lift off

 

[GD2]

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.

Typically Max Operating Temperature will be 240C ( a 10C margin with Design temp). Will it make any difference to your stress result?
If you have overhead space, a long rigid spring hanger instead of variable might work.

 

[XL83NL]

Here's the example from the book - 12" CS pipe at 350 degrees with a good sized vertical riser. The first pass comes up with all spring hangers (8 locations) because every location is moving vertically to some degree.
View attachment 4909
Using this equation that takes the modulus of elasticity, pipe size, and thermal growth calculated, to determine the location of the first rigid support. In this example Node 46 was changed to a rigid support, so this calculation is the growth up towards Node 40. Since Node 36 falls within that 24.67 ft, it stays a spring hanger, but Node 34 becomes a rigid support. Same process is repeated for lower section.

View attachment 4910

And final design is this, with three spring hangers instead of eight.

View attachment 4912

Interesting example RVA, thanks for sharing.

Typically Max Operating Temperature will be 240C ( a 10C margin with Design temp). Will it make any difference to your stress result?
If you have overhead space, a long rigid spring hanger instead of variable might work.

I'd also take the most prevailing operating case. remember that for thermal expansion stress, its the range what matters. You might want to look for the temp case that may see the most cycles, as that may affect the shakedown effect perhaps.


Also, Id be VEY Interest in hearing a practical value that the senior pipe stressers here on eng-tips apply as a cutoff for (ignoring) liftoff in the hot sustained case.
I dont consider myself a senior stresses, but Ill go first anything below 3 mm or 1/8" is what I dont consider. One of the reasons is that (since we build all our process pants prefab and modular) all our frames, equipment and piping themselves are never within a 3 mm range accurately assembled.

 

[RVAmeche]

I also agree with ~1/8" threshold for liftoff. I think Caesar's help guide says a typical value is about 0.1" as well.

 

[StressGuy]

As I've gotten older, I've come to realize that springs are better avoided when possible. As for liftoff, if your stresses, equipment loads, and deflections are acceptable without the support active (i.e. lifted), then let it lift. Springs should be used when that support going inactive creates adverse impacts - such as loading up the nozzles on rotating equipment.

As for temperatures, I've regularly found the case with many clients that "design" temperature may just be whatever the selected design temperature for an attached piece of equipment is. Particularly if it is a pressure vessel, that "design" temperature may bear little resemblance to anything that will happen with the process fluids. There is often not much cost in terms of wall thickness, but all know that extra temperature can force added flexibility for no good reason. I've tended to find the data sheets and PFD's to be better sources for understanding what a system needs to be able to handle from a flexibility basis.

 

[XL83NL]

Excellent feedback Edward.

I've tended to find the data sheets and PFD's to be better sources for understanding what a system needs to be able to handle from a flexibility basis.

How do the projects you work on (say) 'allow' you to use other, lower temperatures than design? Do you have a standard or technical spec you apply for each - or most - projects that spell out the rules applied for pipe stress, and justify the use of data sources that spell out lower temperatures?

 

[ilovechickens]

Often you can find that correct positioning of pipe supports can eliminate support lift offs. I don't let my junior engineers just blindly accept any lift offs. They need to be discussed with the senior pipe stressing engineers first, otherwise there is this desire to just ignore and accept all lift offs and carry on. I want the piping engineers working for me to understand where in the stress model the piping is lifting off and why it is lifting off. Some lift offs can then be easily fixed by strategic placement of pipe supports and some are viewed as noted but acceptable and the system is checked to ensure the piping remains code complaint with these gravity supports removed from the system.

Very small lift offs (perhaps 0.1mm) under design conditions we do not get too worried/stressed about because in normal operating conditions it is likely these will not happen, but the design is being signed off to meet a range of design conditions, so if I saw a large lift off under the design conditions (say 10mm for example) then I would want to make sure that the piping was checked and remained code complaint after this pipe support had been removed.

One problem with visible lift offs in the field is that people can see the gap under the pipe shoe, not understand the system has been designed with this in mind, and then come and wedge something into that gap. Then the pipe is not able to return to its natural cold position.

Lift offs are a natural part of piping designs and just need to be managed.

 

[RVAmeche]

XL, in my world (general heavy industry consulting), we typically discuss this overtly with the client on projects. Like okay we understand the equipment and piping is rated for 500F but it operates at 300F and would need serious modifications to get there - do we run the analysis at a reasonable real temperature or 500F? I don't think there's any standard or rule of thumb for saying something like "assume 90% design temperature as long as it exceeds max operating temps" or "max operating temp + 100F" or similar.

 

[XL83NL]

One problem with visible lift offs in the field is that people can see the gap under the pipe shoe, not understand the system has been designed with this in mind, and then come and wedge something into that gap. Then the pipe is not able to return to its natural cold position.

Lift offs are a natural part of piping designs and just need to be managed.

Excellent points.

XL, in my world (general heavy industry consulting), we typically discuss this overtly with the client on projects. Like okay we understand the equipment and piping is rated for 500F but it operates at 300F and would need serious modifications to get there - do we run the analysis at a reasonable real temperature or 500F? I don't think there's any standard or rule of thumb for saying something like "assume 90% design temperature as long as it exceeds max operating temps" or "max operating temp + 100F" or similar.

I understand that approach, but by the time I get into such discussions, the project has already been sold (usually at fixed price), and technical requirements are already fixed. Changing those requirements (say reducing the acceptance criteria) would likely result in clients asking for a return of project scope (i.e. cash) since we now have lowered the bar at their cost.
The sweet spot for us is to be able to spell out something in advance in our (technical) contract that allows some remedy without including an overkill in the ultimate design.

 

[RVAmeche]

Yeah in my world (general industrial consulting) our contracts and scopes aren't super technical in terms of locking down specific variables and inputs like that, so we normally have to establish those kinds of parameters after we get the job.

 

[ilovechickens]

As I've gotten older, I've come to realize that springs are better avoided when possible. As for liftoff, if your stresses, equipment loads, and deflections are acceptable without the support active (i.e. lifted), then let it lift. Springs should be used when that support going inactive creates adverse impacts - such as loading up the nozzles on rotating equipment.

As for temperatures, I've regularly found the case with many clients that "design" temperature may just be whatever the selected design temperature for an attached piece of equipment is. Particularly if it is a pressure vessel, that "design" temperature may bear little resemblance to anything that will happen with the process fluids. There is often not much cost in terms of wall thickness, but all know that extra temperature can force added flexibility for no good reason. I've tended to find the data sheets and PFD's to be better sources for understanding what a system needs to be able to handle from a flexibility basis.

Yes, when the operating temperature and design temperature are miles apart it can certainly cause some issues. Technically we have to design everything to be code compliant at design conditions, (including seismic design, nozzle loads etc) to be able to state the system meets the design conditions. However if you then rerun the stress model at the lower operating conditions I have seen scenarios where the model is actually not then code compliant at the operating conditions.