EN-13480 ROHR2 vs CAESAR II fa Calculation

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Patricio
Beiträge: 1
Registriert: Mo 19. Sep 2016, 17:43

EN-13480 ROHR2 vs CAESAR II fa Calculation

Beitrag von Patricio »

Hello everyone,

I am trying to reproduce an analysis realized with ROHR2 using CAESAR II for a cryogenic line operating at -165 °C according EN 13480. I found a difference in the allowable stress range (fa) calculation between the two software.

Fa = U(1.25fc+0.25fh) (Eh/Ec)

CAESAR II  fa = 225 kN/mm2
ROHR2  fa = 253 kN/mm2

The different values are because how CAESAR or ROHR2 considers Eh and Ec. If Eh is for operating or maximum metal temperature and if Ec is for installation or minimum metal temperature.

Input
U=1
Fc=160 kN/mm2
Fh=155.78 kN/mm2
Eh/Ec = 0.943 for CAESAR
Eh/Ec = 1.06 for ROHR2

So in ROHR2 the ratio Eh/Ec for cryogenic is bigger than 1 (1.06) and for this case fa = 253 N/mm2.
We are checking an analysis performed with RORH2 were the compute stress for thermal expansion is 234 N/mm2 so RORH2 verifies and for CAESAR II is overstress.

Do you have some query on this topic? Please could you help me with your interpretation on this topic?
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rohr2support
Beiträge: 455
Registriert: Mi 14. Sep 2011, 08:23

Re: EN-13480 ROHR2 vs CAESAR II fa Calculation

Beitrag von rohr2support »

Hi

The factor Eh/Ec was introduced to "transfer" displacement stress range at hot temperatures to the room temperature at which the fatigue curve were
calculated. In Rohr2 we use the higher Ec modulus at cold temperatures for cryo piping. Logically the displacement stresses should be compared against
higher allowable as well. So the ROHR2 interpretation is:
Ec and fc - At room temperature,
Eh and fh - At operation temperature (even if it is colder than room temperature)
In some cases the allowable stresses may be reduced by reduction factors for cold service if no impact test was done at that temperature.

The problem that the Ec/Eh factor is only sensible at higher than assembly temperatures has been recognized in the EN13480-3 working group.
The next version will have some explanation on this subject. Here is a paragraph that is under discussion (EN13480-3 2012 pr A1):
41 Modification to 12.1.3.2
Below the Equation 12.1.3-1, Ec shall be completed as follows:
Ec is the value of the modulus of elasticity at the minimum metal temperature consistent with the
loading under consideration. In cases where operation conditions with temperatures below assembly
temperature exist, Ec may be taken as the modulus of elasticity at assembly temperature;
I think the decision on this will be taken on the October meeting of the working group.
This would mean that you can take the Youngs modulus at assemby temperature and than use Eh/Ec=1.0.
For the stresses this is equivalent to calculate the stresses based on the Youngs modulus Eh at cryo temperatures (e.g. higher Eh and higher stresses) and
then multiply the allowable stresses by Eh/Ec where Eh is at cryo temperatures therefore Eh/Ec>1.0.
This way we still calculate correct support and nozzle loads based on the higher Eh at cryo temperatures.

Best regards

ROHR2 Support
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