Design per Div.1 Fatigue per Div.2

[Paulettea]

Dear All

I have designed a vessel as per ASME BPVC Sec.VIII-1. The owner has asked to perform fatigue calculations for the vessel and I want to do the calculations as per Div.2. For the fatigue screening criteria I have faced some difficulties.

First, in screening Method B I have to use allowable stress in the formulae 5.18 and 5.19.
N_ΔFP<=N(C₁S) 5.18
Δ_PN<=P/C₁*(S_a(N_ΔP)/S) 5.19
Do I have to use allowable stress as per Div.1 or Div.2?

Second, for four hours the vessel has been exposed to a flow of gas with 230 ^oC temperature and the vessel can be assumed to have this temperature uniformly. Then a cooling gas at 40 ^oC starts to flow through the vessel. I want to know that for the maximum range of temperature difference, shall I consider the transient temperature difference or steady state difference. And if I am supposed to consider transient temperature, do I have to run a transient temperature finite element analysis or just simply say that the gas is at 40 ^oC and the vessel at 230 ^oC therefore: delta T=230-40=190 ^oC. You see the temperature on the surface of the vessel is not necessarily the same as that of the gas.

Warm Regards

 

[TGS4]

Good question on the first question.

Regarding the second, the standard approach is to perform a transient thermal analysis. But it would be conservative to assume that maximum temperature difference.

 

[Paulettea]

Thank you TGS4 for the reply.

I have decided to use Div.1 allowable stress based on my justification as follows:

Normally, when we use allowable stresses as per Div.1 we are being more conservative in the sense that the value for allowable stress is lower as per Div.1 in comparison with Div.2.
However, in this especial case, it is more conservative to use Div.2 allowable stress. Looking at equation 5.18 it can be seen that if the allowable stress increases then “N(C₁S)” decreases (due to the negative slope of SN curve) and this allows less full-range pressure cycles to exempt the vessel from detailed fatigue calculations.
In equation 5.19 allowable stress appears in the denominator and if the value of allowable stress increases this equation allows less significant pressure cycles to exempt the vessel from detailed fatigue calculations.
Now, if I am supposed to choose between one of these allowable stresses, I go for Div.1 allowable stress. The reason why I am using Div.1 allowable stress is that if I had chosen to use Div.2 for my design, then definitely the designed thicknesses for the vessel would have been lower hence less allowable cycles in 5.18 and 5.19.

TGS4, do you think my justification makes sense?

Warm Regards

 

[TGS4]

Your logic seems reasonable.

 

[MrPDes]

For Fatigue Screening method A, if cyclic Pressure range is less than 20% of MAWP then there is an exemption from conducting a detailed fatigue analysis (ΔP/(0.2×MAWP)). MAWP is calculated using the Div 1 Allowable Stress S.

I think the result of 5.18 is not so important because its result always leads on to either a detailed fatigue analysis or the use of Eqn 5.19.

Eqn 5.19 is where the action is. It is tempting to just focus on S however it is the fraction P/S (or MAWP/S) is more important. This fraction is similar to ΔP/MAWP in fatigue screening method A. As P (or MAWP) is dependant on the Div 1 allowable stress S, I don't see how it makes sense to introduce the different Div 2 allowable stress S into to denominator of Eqn 5.19.