One of our Contractors performed a DESIGN CALCULATION In Accordance with European Code EN-13445, using PV Elite 2015, unknown Build.

I have no access to PV Elite file.
I am interested to understand their results and I need your help:

1. Which is the basis/ effect of the "Use Higher Longitudinal Stresses (Flag) Y" when the calculation is performed under EN-13445?

2. How are calculated the Longitudinal Allowable Stresses for Hydrotest tensile and Hydrotest Compressive Stress- second and fourth (last) columns with numerical values? (one may note that some values are over yield at test temperature, i.e. room temperature)
and
Which is the basis of "CHECKING" in the following sequence?

Longitudinal Allowable Stresses
| | | Hydrotest | | Hydrotest |
From|To | Tensile | Tensile | Compressive | Compressive |
| N./mm² | N./mm² | N./mm² | N./mm² |
-------------------------------------------------------------------
10| 20| 174.637 | 297.012 | -131.717 | -222.962 |
20| Legs| 174.637 | 297.012 | -131.717 | -222.962 |
Legs| 30| 174.637 | 297.012 | -131.717 | -222.962 |
30| 40| 174.559 | 264.611 | -131.661 | -200.252 |
40| 50| 174.559 | 264.611 | -140.433 | -214.538 |

Allowable Stresses = Rm/20 *cs = 390 * 0.8 = 312 N./mm

CHECKING
Allowable Stresses > Hidrotest tensile
312 N./mm² > 297.12 N./mm²

3. Are the results shown in point 2 dependent on the Flag Y in Point 1?

Thank you.

Hello mariog

I looked at the 2015, 2016 and 2017 versions of PV Elite. Only the 2015 version allows the higher axial stress, the 2016 and 2017 versions do not. EN 13445 no longer allows the use of higher stresses.

The axial stress is treated as a General Primary Membrane stress, and as such, during the hydrotest the allowable stress is allowed to reach about yield/1.05. This represent about 95% of yield.

I have always had reservations about the allowable stress during hydrotest, but that is what the code allows. It appears to me to be too close to the yield point.

Thank you for the fast answer.

To summarize my understanding:

1. The option is not longer available in the last versions. In my opinion makes no sense for EN calculations, anyway. Probably was a forgotten option coming from ASME software template.

2a. Material was P275NL1, hence yield is 275 MPa or less, depending on the thickness. The value 264.611 MPa exceeds just a little 0.95*Yield, I have no clue about 297.012 MPa value as allowable exceeding Yield.

2b. No clue either on "Checking:
Allowable Stresses > Hydrotest tensile
312 N./mm² > 297.12 N./mm²" with 312= 80%*390 MPa; 390 is indeed Rm - Tensile strength (MPa) of P275NL1.
In fact this checking says that the allowable exceeding Yield is OK since is less than 80% of Tensile Strength?
From where is (or was in 2015 version) coming up this approach?

3. No suggestion on question 3 for 2015 Version?
In 2015 Version, were (point 2) results sensitive to option of use higher allowable?

About your remark- EN allowable hydrotest is too close to yield. It's true, however in Oil and Gas (not elevated temperature) is current practice to have hydrostatic test as 1.43*Ps and that 1.43 is coming from 1.5*0.95. As a result indeed the hydrostatic stress is near 0.95*Yield in case the design PS has been based on Yield/1.5 and the thickness has been chosen as minimum from calculation. I mean not only the hydrotest allowable is high, but also hydrotest pressure leads to high stress in hydrotest.
By the other hand, often the material has better properties than those in standards and this is helpful in real life. Probably also this fact encouraged EN staff to set 0.95*Yield- theoretically is OK, in practice is better than in theory.


Thanks again.

Hello mariog

1. It seems you are right.

2a. The stress to be generated in accordance with Table 6 is to be Yield/1.05 which would be 275/1.05 or 95.238% of yield. Or to put it another way, 1.5/1.05 would be a hydrotest pressure of about 1.4285 of the MAWP. I find the code a little confusing.

2b. Be careful. When any stress exceeds yield, you are in the plastic region, and you never want to go there with any sustained load at any time. That is dangerous. Strain will be permanent. I am nervous at 95% of yield.

3. The 2015 version of PV Elite has been corrected, so we can forget it.

To be fair, I'm not nervous at 95% of yield- for the reasons here it is current practice, rather I am nervous at 110% of yield as 297 MPa is vs 270 MPa, for the reasons you've mentioned.

Just hope that in the newer versions a value of 110% Yield is not reported as allowable in hydrotest for EN calculations.

Thank you very much and best regards.

Hello mariog

Where did you see a stress of 110% yield in a PV Elite calculation (except of course when you are dealing with secondary stress) which we are not really discussing?

Please see point 2 of my first post.
In fact you would help me explaining what exactly PVElite 2015 calculated there vs. EN requirements.
Thanks.

Hello mariog

In EN 13445, the three stress intensities are computed. Remember, stress intensity is actually the maximum shear stress. There are three symbols used in the table for Stress due to Combine loads. f1 is the hoop tensile stress, f2 is the axial tensile stress. F1 and the hoop tensile stresses are the same. To get the stress intensity, they are f1-f2, f2+0.5p and Hoop(or f1)+0.5p. Does that make it any clearer?

Dear Mr. Delaforce,

Thank you for your answer.
To answer to your direct question- No, I'm afraid it is not clear at all!

1. First, please note that the sequence shown in my post is under the title "Longitudinal Allowable Stresses". In case PVElite 2015 calculated under that title the limits of the stress intensities (also checking the limits not exceed 0.8Rm), how the user can understand this fact?

2. By EN, the limit of hoop stress is Yield/1.05 let's say 0.95*Yield. To get 1.1*Yield as limit of stress intensity (as it seems to be my case), it means that the term 0.5*p_test equals .15*Yield, or p_test equals 0.3*Yield. For Yield= 275 MPa it results a test pressure of 82.5 MPa= 825 barg. In my case, the test pressure is just about 1.43*29= 41.47 barg. I mean something is not matching the values shown, if indeed the intention was to consider the terms 0.5p.

3. Really I don't understand what PVElite 2015 calculated in that sequence. Maybe you can help me improving my understanding, I am not able to identify it in documentation.

4. Is that calculation specific to 2015 version when perform EN checking or is maintained also in the last versions?

5. In fact, to which section of EN you refer? It is true that the effect of radial stress must be counted, it varies from -p at inner wall surface to 0 at outer part. An arithmetic mean of 0.5p is considered by some codes, others ask for 0.6p. Is EN specific in this subject? Is EN establishing a limit of 0.8Rm based on the fact stress intensity is a shear stress? Maybe you can help me identifying the relevant EN section that includes what you've mentioned.

Thanks a lot, I really appreciate your help.

Hello mariog

To get to grips with these questions, I need your PV Elite file. I have sent you an email requesting the file, and perhaps we can deal with this issue via email.