High Temperature Pressure Vessel

[bpick]

I am working on a preliminary design of a pressure vessel with design paramaters of 100 psi and 1600F. Once I get something that seems workable then I will turn it over to a Code shop to have them run the ASME Sect. 8 Div. 1 calcs on it.

I am looking at Nickel based super alloys for corrosion resistiance in Sect. 2 Table 1b, specifically UNS N06230. This alloy has note G5 which pertains to creep and note T16 which deals with time-dependent properties. I have also heard that there are allowances for analyzing flanges at temperatures lower than what is stamped as long as the U1 papers detail it and the flange is controlled at the lower temp.

Three questions:
1) How is creep analysed in the code?
2) What is the significance of time-dependent properties?
3) Is there a procedure for derating flanges in the code as mentioned above?

Thank you for your time in reviewing my application.

 

[metengr]

bpick;
I can help with questions 1 and 2

1) How is creep analysed in the code?
2) What is the significance of time-dependent properties?

For ASME B&PV Code, Div 1 design by rule applications, creep is handled by determination of allowable stress values where a certain amount of creep deformation is assumed to occur over time without consequence of failure from creep rupture. If you elect to design by analysis, the designer of the vessel now uses a specific creep deformation rate that is either known or assumed by testing. Here one has the flexibility to have increased creep deformation under design by analysis assuming this is factored into the design.

 

[bpick]

Sound like my next task is to find some info on creep for nickel alloys.

Note G5 states creep fatigue and thermal ratcheting set in at 1500F and must be considered in the design. Is there another part of the code that deals with this? or does the code leave it up to the engineer to apply good sound analysis beyond the code?

Note T16 states "Allowable stresses for temperatures of 1150F and above are values obtained from time-dependent properties." I'm not sure what time-dependent properties are other than creep but I would have to assume it is refereing to something different since the temperatures are lower. Does this mean that the vessel should be allowed to cool for a period to releive these time-dependent properties?

Thanks for the help.

 

[TGS4]

The allowable stresses listed in Section II, Part D, with note T16 INCLUDE the effects of creep, as described by metengr above. The damage due to creep is irreversible, so your suggestion to allow the vessel to cool periodically will NOT have impact on the creep life. In fact, the heating and cooling will created thermal stress issues and coudl cause other problems.

Note G4 relates to creep-FATIGUE, thermal ratcheting, and environmental effects that should be considered in the design, but there are no rules indicating how to accomplish that. Therefore, you need to refer to U-2(g), Section VIII, Division 1.

Analyzing flanges at that temperature and low allowable stresses (and low modulus of elasticity) will be a challenge. I hope that you find an engineer who has significant experience in doing that.

Have you dealt with ASME Section VIII, Division 1 pressure vessels before?

 

[bpick]

I would be the first to say that I am over my head, and will greatly rely on the experience of the vessel designer. The task for me is to take the operation requirements and fit them into a vessel and get something close to what the end product will look like. On this project this intermediate step has saved quite a bit of effort. The client can run through differing process scenarios before getting a detailed vessel design.

Thank you for the advice.

 

[EdStainless]

Is there a budget for this project? Using 230 will allow you to use the least amount of metal. That will help minimize thermal stress issues.
But have you looked at the cost. There aren't many alloys more expensive.
My other comment would to talk about fabrication. What size is the vessel and what size of plate can you get. You may better off selecting an alloy that you can get is larger plates in order to minimize fabrication.

Yes, design in the creep range is an art. There are a lot of shops with experience in high temp petrochem equipment that have this experience.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[bpick]

I would really like to stay under the creep threshold so is this 1500F per note G5 or 1150F per note T15? I am getting more concerned with the T15 because it is somewhat ambiguous and affects that vessel at a lower temperature.

Yes, I have learned quite quickly the cost of alloy 230! The availability is very thin, too. But we are dealing with Bromine and quite a few materials react with it.

 

[jte]

bpick-

I realize you are doing preliminary work, and apparently with good reason. You mentioned that you'll have the Section VIII work done by a shop. I would point out that there are precious few shops in the world which have the engineering know-how and experience to evaluate your vessel. Your ordinary mom-and-pop shop and mid-grade shops do not have the engineering skills to work this. So - be sure to go to a premium shop or, probably better, get an independent consultant involved.

Having said all that, though I prefer "hot wall" designs from an owner/user perspecive, have you considered refractory lining for a "cold wall" design? I would still keep the design temp as high as possible (i.e. don't go to 500 deg F) since hot spots will happen during operations due to refractory loss (which puts food on my table) so I guess I'd advocate a "warm wall" approach just low enough to be out of the creep range. This will reduce the concern with hot spots (the difference between the hot spot and design temp will be lower) and may reduce the likelyhood of refactory spalling due to thinner refractory. This also reduces future repairs and replacement of refractory. If you stay in the creep range, at the end of your 20 year (or whatever) design life some poor slob of an expert will be challenged by the plant manager to prove that the vessel can't be run for another 5 years. And again, and again... Finally, with a "warm wall" vessel, you will be able to use your usual shops since they won't have to be proficient in creep issues.

jt