High Temperature Pipe Stress 2

[mrmoffitt]

I am a ME with a consultant firm and I have built a few CAESAR models but still feel like every project is my first one because they have all been so different. This time I am working on a combustion gas system, and we are reusing part of the combustion gasses from the furnace to preheat sweep gas for the process side of a piece of equipment. It is a NPS4 316L SS line and my design temperature is 1300°F and low pressure (<1 psig). My CAESAR II pipe stress model is failing in the SUS case because the hot allowable stress is very low, 3.5ksi according to B31.3 Appendix A. I have supports placed fairly close together, but the model is still failing in the SUS case just from the weight of the pipe+insulation because Sh is so low. Has anyone worked with this high temperature line before and could provide some insight here on how to support high temperature gas lines. My line is NPS 4 with 6.5" of insulation, so a pretty small line. Thanks in advance!

 

[1503-44]

Your problem is the 316SS
As you know, it has severe high temp stress limits.
The high temperature expansion stress alone is greater than the allowable stress at 1300F.
Spacing supports at 100mm will not solve that issue.
You must change the material, or revise the pipe configuration until there is virtually no temperature stress.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

I would agree. At 720C that's beyond most "normal" materials.


See e.g. these suggested materials

https://www.lbf-technik.de/en/galle...ne-in-high-temperature-stainless-steel-1-4828

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NovaStark]

Your best bet may be to improve the metallurgy.

You can also look into increasing the pipe thickness however that can only get you so far.

Did your stress model pass the EXP load case ?

At one of the facilities that I have worked at, there were 304H SS piping designed to 1200 degF. However, those lines were NPS16 and NPS8 with thicknesses around 1".

 

[XL83NL]

If this is real 316L, you have an issue, as that grade is limited to below 500 deg C in most codes. Straight 316 may give you some slack, but as 1503-44 mentioned, not much. 316H is better, and I think also required in most - if not all - ASME codes, due to the minimum required carbon content. Per ASME IID you'll have about 25 MPa, which is not much - but that not bad. Metallurgy-wise Id suggest something like 321H instead though, provided the MoC is resistant to the fluid service ..

Without knowing the pipe routing and constraints, it's hard to tell what's going wrong and what can be improved. However, not using any spring hanger (at least, you dint mention) seems like a first hint your supporting idea may be incorrect.

Ive recently done a stress analysis of system at 2000 deg F (almost 0 pressure), and sustained was an issue as well. The allowable I have was < 1% of the ambient allowable for some load cases.

Also, high temp pipe stress is not an easy task. You mentioned you've built a few CII models, which seems to indicate you're not a senior piper yet? There;s a lot of interesting reading here on ET, but also in literature, on high temperature stuff. Google this, and read a.o. these;

https://www.eng-tips.com/viewthread.cfm?qid=383570

https://www.eng-tips.com/viewthread.cfm?qid=358750

https://www.eng-tips.com/viewthread.cfm?qid=182922

A lot of JohnBreen's post are a huge treasure of knowledge (link).

Furthermore I can highly recommend a copy of "Analysis of ASME Boiler, Pressure Vessel, and Nuclear Components in the Creep Range", which also discusses high temp pipe stress. Bear in mind that high temp pipe stress is not a read and understand topic, but at least this will give you some food for thought and will get you started.

Huub
- You never get what you expect, you only get what you inspect.

 

[mrmoffitt]

The metallurgy matches what the furnace vendor selected for their flue gas pipe, ASTM 240 316L - which is plate. We have ASTM 312 TP316L which is pipe but has the same allowable stress derate at temperature. So I will need to go back to the vendor and see if they know something I don't. B31.3 allows 3.5 ksi at 1300F so it sounds like I will need to get under that. The expansion case has higher allowable stress due to the way its calculated in the code, so it passes. Its the sustained and Alt SUS cases that are failing. I am close though (~5ksi at max).

Is B31.3 the appropriate code for this application? I can't think of something else that would govern, but I have been looking at STS-1 Steel Stacks or Boiler External Piping under BPVC? Only reason I am looking at that is becasue my pressure is so low (< 1psig).

 

[EdStainless]

I don't have the Code tables in front of me, but I would suggest looking at 310H.
It is commonly used at high temperatures.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[GD2]

OP,
Try using a higher schedule and A312 TP 347 pipe and see what result you get. No matter what material you use, you will be in the creep. One needs to make sure that the material needs to comply to the requirements given under Notes in A-1 Table.
You are covered by B31.3. Read para 300.1.3 for exclusion.

Not sure which Code edition your Caesar has values for A 1 table stress. Often the stress values get modified with new revisions.

GDD
Canada

 

[StressGuy]

NPS 4" pipe with 6.5" of insulation is already a tough order. All that insulation creates a lot of weight, but contributes no strength. Pyrogel would certainly help as you get a much thinner and lighter weight of insulation in that case.

1300°F with 316L is pretty extreme, as the others have noted for you. I'm surprised that the heater vendor is using that kind of material.

I would get absolute verification on that 1300°F temperature. What's driving that value? Push back on your process folks and make sure they are rounding up - as you are finding out, even 25°F in that temperature range will make a big difference to allowable stresses.

Also, what schedule are your running? I'm sure at effectively no pressure, 10S probably passes the wall thickness calc, but it's going to do you little good trying to span. 10S is a terrible thing at ambient, much less 1300°F

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[mrmoffitt]

Thank you all for the input! This is very beneficial. I think I have a path forward now. Looking at Table A-1 in B31.3 Appendix A, 316L is about as good as anything at that temperature in terms of allowable stress, so I think I'll stick with that. I found a more experienced stress engineer in my company who is going to help me as well, but all of your comments have been beneficial. There are some questions I have back to the furnace vendor and my process engineer on design temps. I will look at other options for insulation that might help me as well.

 

[TiCl4]

Depending on how long this pipe run is and what your budget is, I would suggest you look at 309H or 310H stainless steels. These are commonly used in high temp furnace applications, with good creep resistance and oxidant resistance.