Hi all,

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 applications.

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

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

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 if for installation or minimum metal temperate.

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

Do you have some query on this topic? Please could you help me with your interpretation on this topic?


Thanks in advance, regards,

Eh is obtained from the maximum metal temperature associated with the load case. Ec is obtained from the minimum metal temperature associated with the load case (which is not necessarily ambient). If this isn't sufficient, please open a Support Request on Smart Support and send in your job file.

I made a mistake with the units, here go the corrections:

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

Input
U=1
Fc=160 kN/mm2
Fh=155.78 kN/mm2

Dear Richard,

Thanks for your answer, in principle we agree with your interpretation. But we continue with doubts because, we ask both SIGMA (ROHR2) and COADE the same query and we received different interpretations.

SIGMA authorized us to publish their answer here, please find it below:

“Dear Patricio

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 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.
where the allowable stresses may be reduced by reduction factors for cold service.

When you do the calculation with CEASAR, do you use the Y-Modulus at cold or at assembly temperature for your calculation?

The problem that the Ec/Eh factor is only sensible at higher than assembly temperatures has been recognized in the EN13480-3 working group.
Please have a look at the presentation I have given there some time ago (page 30 ++). See attached.
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.
Once it it decided we will implement it accordingly.”

Are you aware about the clarification to Ec definition?

We are checking an analysis performed with RORH2 were the compute stress for thermal expansion is 234 N/mm2 so with RORH2 verifies (allow 253 N/mm2) and for CAESAR II is overstress (allow 225 N/mm2).