High Temperature Allowable Stresses for Bellows

[flexy]

I am attempting to design a 16" bellows expansion joint for a 2000F application @ 80 psig. At this temperature, it is enormously difficult to select an appropriate bellows material. Even if I am able to do this, at 2000F any material I select would be in the creep range. Should I decide that I DO NOT want to refractory line my EJ, what material would be most suitable (...the media is hot gas exhaust from a aircraft engine)? How could I determine an appropriate allowable stress based on the 2000F temperature? Does the fact that the bellows operates in the plastic range affect how I would select an appropriate allowable stress? Please note that this project is ASME B31.3. I am aware that the code genrerally does not recognize temperatures this high, but, assuming ASME did, how would I approach determining the allowable stress?

 

[metengr]

Flexy;
At 2000 deg F you are very limited in materials that could be used without some type of cooling or refractory coating to keep the metal temperature of the bellows at a maximum of 1800 deg F. Creep rupture data is typically not reported beyond 1800 deg F for most metals.

I was able to locate some creep rupture data at 2000 deg F for Haynes 214 (UNS 07214), which is a Ni-Cr-Al-Fe high temperature alloy. From a practical standpoint, you would have to maintain an allowable stress value below 140 psi at 2000 deg F to avoid creep rupture, and to have 0.5% creep deformation in 10,000 operating hours. This rather low stress value would require such a thick-walled bellows (over 2" wall thickness) that it would be impractical to manufacture.

If you can get the metal temperature down to 1800 deg F thru cooling or by refractory, the Haynes HR-120 (instead of the Haynes 214) would work well because the allowable stress value for Haynes HR-120 would increase to 1.9 ksi.

 

[flexy]

Thank you for your response. I was actually considering the use of Haynes Alloy 230 which has some very remarkable properties. Unfortunately, it is the creep end of things that I am not that familiar with. If I had creep data, how would I use it to postulate an allowable stress? Would ASME recognize an approach like this considering that the allowable would not be based on the Section IID?

 

[metengr]

The allowable stress would be based on either the stress value that results in creep rupture (failure) and/or the stress value that results in some level of creep deformation (like 1% creep strain) - no failure. You would need to find creep rupure and creep deformation data for the alloy at the desired service temperature. Since your intended service temperature exceeds the maximum ASME B&PV code metal temperatures for these materials, ASME B&PV code would not recognize your approach.

You could assign your own safety factor or design margin, following ASME guidelines in Section I, Part D, Appendix 1 -for example, take the stress value that is reported for rupture or creep deformation in so many hours and divide by 2.5 or 3 (depending on your level of conservatism), this becomes your allowable stress value.

Please be aware that when you have service temperatures over 1800 deg F, many of the Haynes alloys have warnings regarding extrapolation of creep data. This is the reason why, if you can provide a thermal barrier on the ID surface of the bellows to reduce exposure to radiant heat and drop the metal temperature from 2000 to 1800 deg F or even lower, you would be better off in design.

 

[flexy]

Thank you for time and your help. I will try to get some creep data from Haynes for Alloy 230.

 

[unclesyd]

Being try to see if I still had the literature on a Flexible connection we used at 1850°F-1900°F.
We had a 18" and 20" flexible connection used in lieu of an expansion joint that was made by one of the companies that made flexible connectors and clamps for the aircraft industry. The flexible were made from Inconel 625 convolutions with 3 layers of Inconel 625 braid and had welded ends. I don't have the pressure, but remember it being around 80 PSIG. These joints were installed on an emergency basis but were still in operation 4 years later when the unit was shutdown.
If I remember correctly all flexible connectors on airplanes at the time were hoses not expansion joints. The operating conditions determined the type and number of external plys.

We had several high temperature expansion joints and all were made from multi plies, normally 3, of Inconel 625.

 

[rmw]

Have you considered a fabric expansion joint. 2000F joints are not unheard of. The correct design will have enough stand off to get the outer fabric layers away from the radiated and/or conducted heat of the duct, and there will be sufficient layers of insulation between the process and the vapor barrier, and the outer cover. Piece of cake, and lots of companies manufacture them.

rmw

 

[rmw]

Is there a way to red flag your own posts??? I completely missed the 80 psig part of the question, hence my recommendation of fabric was erroneous by a bunch. Mia culpa.

rmw

 

[flexy]

I have serious reservations about the use of Inconel 625. It's grain structure can be seriously compromised when it is used above 1000F and then brought down to room temperature repeatedly. Even the 625LCF material is not recommended over 1200F.

I believe that refractory lining is my only solution. I will approach it from this angle. Thanks to all for your help and your input. It appears that the magic bullet does not exist.

 

[GenB]

I do not know much about hi-tem flex conns.
I know of a mfr "senior flexonics", Burbank CA makes all kinds of them. If that helps!
ER

 

[unclesyd]

Here is one approach to fabrication of a high temperature expansion joint from a reference posted by imok2

http://www.flexcomonline.com/flatbelts.htm