Hi,

could someone please tell me how to calculate corroded hydrotest pressure?

I don't know if this can be done, because test pressure considers allowable stresses only. So I think I would have to do a kind of fatigue to see how much material would be weaker than it was when the vessel was new? Something like that maybe? I don't know really, and I would appreciate any help on this.

Thanks in advance.

Hi friend,

At the time of the design of vessel generally we are considering the corrosion allowance (C.A.) as specified by our client which means that the thickness calculations are based on that C.A.

Now, if customer specifications are asking for corroded hydrotest at site (cause in case of shop there will not be any corrosion so test pressure can be as per UG99C) & the calculated MAWP is going to be stamped on the nameplate then it means we have to calculate the test pressure as per UG99b & this should be your corroded hydrotest pressure.

Now, if you are talking about the allowable stress, then it can be 90% of yield for hydrotesting as the testing pressure is temporary.
Its like, you are pulling a rubber for a short time upto its yield limit & then again reliving the pulling force which leads to become in its original position without any damage to it.

I saw on some drawings Corroded Hydrotest and value is different than the hydrotest pressure when vessel is new.

Hydrotest is calculated acc to. 1.3*P*Sa/S...I don't see anywhere any corrosion influence in this formula. So the only thing I can figure that could have an effect on this corroded hydrotest pressure is material fatigue. But it is very complicated to determine. Am I right on this? Because, after couple of years in service, material becomes weaker so allowable stresses would be lesser then they were at the beginning of the vessel work life. So this is the only logical explanation for me how hydrotest in corroded condition can be lesser then at the beginning. But determination of theses stresses is very complicated job.

UG99-C says nothing about corroded conditions.

Has anyone ever done corroded hydrotest pressure calculation?

PVelite has an option "Perform corroded hydrotest pressure calculation" but it gives the same results. I'd say that option in pvelite is useless.

Regards

Hi friend,

Please read UG99 carefully. As you said, test pressure is 1.3*P*Sa/S, you are right, but in this equation P is different as per UG99b, UG99b note 34 & UG99c.

Hi Dear,

Can you tell me where in Div 1 it specifies that Hydrotest allowable stress can be 90% of yield for hydrotesting.

Thanks in advance

Dear Anand,

Refer Clause 4.1.6.2(a) of ASME-VIII-2-2007 ED., which says that general primary membrane stress during hyrotest shall not exeed 0.95Sy. In old edition it was 0.9Sy.


u will not find it in Div 1..........

Hi Folks

ASME Section VIII, Division 1, Paragraph UG-99(d) states: 'This Division does not specify and upper limit for the hydrostatic test pressure....'. What are we to make of this as engineers? Well, we know that the hydrotest produced a general primary membrane stress. The principle for containing primary membrane stresses is that the stress must be kept out of the plastic region. In other words, the stresses must be contained within the elastic region (up to the yield point). So the question arises, how far can we approach the yield point? Division 2 say that we can go within 95%, and Division 1 say there is not limit on the hydrotest pressure. So, what must we do? Traditionally, engineers have felt that if the hydrotest goes to within 90% of yield, there is a sufficient margin to keep us on the safe side of the yield point. However, if we go to 95%, that should be safe enough.

Conclusion: You as the engineer must decide if no other information is forthcoming. My favourite method is UG-99(b) so the flanges are less likely to leak during the hydrotest. There is an old English adage goes something like this: You pays your money and you makes your choice. It's up to you the engineer.