Fatigue Analysis - AD-160

[nashof]

Hello All,
I have two doubts regarding cyclic service / fatigue analysis thay I can't understand, and I'd be very pleased if you could give me a hand:

1- The vessel (horizontal drum, thickness 1/2")has temperature cyclic service. The vessel is at ambient temperature (20ºC = 68ºF), and when the cold fluid gets into it the temperature goes down to -170ºC (-274ºF). This us supposed to happen 500 times during vessel life.
My doubt checking ASME VIII Div.2 - AD.160 is about the "F" number that I have to chose. This number is related with the range of temperature change, and you have to multiply it by the number of cyclics, if the result is more than 1000 it means a fatigue analysis is required.
But, for the situation I have told do I need to consider temperature variation of 190ºC (342ºF)??? This would result a factor of eight (8), so the efective numer of cyclics would be 500x8=4000.
The code talks about difference in wall temperature in meridional and radial direction, but I can't understadn it perfectly.

2) I am talting about AD-160, so you can see I'm with Ed.2004 of Div.2.
I'm asking about this becasue I think new (Ed.2007) of Div.2 is not compulsory at this moment.
But, if the vessel will be designed according Div.1, for which Ed.2007 is compulsory yet, and the fatigue analysis if required should be performed following Div.2, do you think we should use Div.2 Ed. 2007 for this purpose??

Thank you very much!!!

 

[gr2vessels]

1. Div. 1, Clause UG-22;- The loadings to be considered in designing a vessel shall include those from:...
(e) cyclic and dynamic reactions due to pressure or thermal variations, or from equipment mounted on a vessel and (external) mechanical loadings;
Note that the compliance with the above clause can be achieved in Div. 2.
2. ASME VIII Div 1, 2 and 3 ed. 2007 are now compulsory.
Refer to Div. 2, Clause 4.1.1.4:
"A screening criterion shall be applied to all vessel parts designed in accordance with this Division to determine if a fatigue analysis is required. The fatigue screening criterion shall be performed in accordance with paragraph 5.5.2. If the results of this screening indicate that a fatigue analysis is required, then the analysis shall be performed in accordance with paragraph 5.5.2. If the allowable stress at the design temperature is governed by time-dependent properties, then a fatigue screening analysis based on experience with comparable equipment shall be satisfied (see paragraph 5.5.2.2)."
3. I believe your design will include a fatigue analysis.
cheers,
gr2vessels

 

[PVdesigner]

Whether a vessel built per Div 1 or 2, fatigue analysis has to be handled either by running FEA or procedures per div 2.

 

[nashof]

Thank you for your quick responses!
Maybe I was wronk, but I thought ASME VIII Div.2 Ed.2007 was not complusory at this moment.

I also think my vessel will need a fatigue analysis, but I wanted to know how to consider the temperature changes I have previously told (from 20ºC to -170ºC).
In ASME VIII Div.2 Ed. 2007: Table 5.8 "Temperature Factors for Fatigue Screening Criteria", included in Part 5, which Metal temperature differential should be considered? Just the difference between +20 and -170??

I don't find it very well explained in the code.

Thank you

 

[TGS4]

The temperature difference that is important is, as you said, the difference between "adjacent" points. The Code gives guidance on what constitutes adjacent. If you do not understand this concept, then may I suggest that you (and/or your company) contract with an engineer that does understand this concept and can guide you through the intricacies of the fatigue calculation avoidance (AD-160), or a full-blown fatigue analysis.

Because the devil is in the details, the multiplier for AD-160 may be 0, or it may be 8 - hard to say without knowing all of the details and performing a number of calculations (not necessarily FEA calculations, mind you).

 

[nashof]

Thank you all for your help.
I couldn't find any information in order to know which number should be consider for temperature variation.
I will contact an expert about this, becasue I'm not in the position to decide, but I find strange that nobody has experience with this so to know if the difference in temperature to be considered is -157 to + 20 or which one.
The code talks about adjacent points, but, how can this be determined?
The vessel is at 20ºC, cold fluid goes into the vessel, so the vessel starts cooling to -157ºC...
For this, how is it possible to define which points are at which temperature??
If we consider the difference of 177ºC it is clear that we are in safety side, but maybe we are considering too much over design.

If somebody could help I would be very pleased!
But in any case I'll try to contact an expert, and will post here any solution we find.