High temperature pressure vessel *possible* creep-fatigue design required

[jimmyhutmacher]

We're looking at possibly providing a bid to design and build a Section VIII Div. 1 pressure vessel (we don't hold a Div. 2 stamp) and i'm wondering if someone with creep-fatigue experience can confirm if a creep, fatigue, creep-fatigue or thermal ratcheting analysis would be required. If so, we'd have to pass on the work as it's out of our expertise.

Design Temp: 1562F
Design Pressure: 120psi

Cycled daily from atmospheric pressure and temperature to design conditions in 2-3 hours, down to 752F in 6hrs and down to ambient throughout the night.

Estimated to be used for 2 years only.

~1000 full cycles

Inconel 625

 

[TGS4]

You are well into the creep regime with that design temp. In my humble opinion, you would be definitely dealing with creep, creep ratcheting, and creep-fatigue. If you're a fabricator, you don't necessarily need to pass on that type of work just because you don't have that specific engineering expertise. You could always hire a consultant

 

[moltenmetal]

Will the device be monitored routinely? Do they mind throwing it away when it creeps, which it certainly will over time? Is it bigger than a breadbox? What's in it? Is it hazardous, aside from being hot?

People operate equipment in the creep regime on a daily basis. Every steam methane reformer on earth operates there, all day long, every day. Owner/operators of such equipment have to accept the reality that they are operating in a temperature region where a 30 year durability without failure isn't practical, and they have strategies for dealing with it.

 

[jimmyhutmacher]

I would say it could be routinely monitored, but no program has been discussed yet.

They're aware that the design conditions are pushing the limits of available materials and are willing to accept that the design life may be shorter than usual for a pressure vessel.

Contents are non-hazardous.

Quite a coincidence, it's about the size of a breadbox... 10.75" OD x 42" long.

From what you've said, we're going to pass on the design but offer to fabricate. If you know of any outfits that are qualified to design for creep-fatigue, thermal ratcheting, etc., please let me know.

Thanks for the quick responses

 

[davefitz]

Without knowing the full process conditons vs time, I would guess that the thermal stress on the pressure vessel could be reduced by the use of an internal liner ( loose) ,, and the nozzles would use full pen welds with radiused edges. If the process conditions vs time are accurately known at this time, then a finite element model of the vessel would enable you to repetitavely modify the design to provide lower stresses and acceptable life

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[TGS4]

jimmyhutmacher - click on my username and contact me.

 

[MrPDes]

Sounds like a regular monitoring job. Especially for the initial articles.