Pneumatic vs hydrostatic in a rerate

[robsalv]

Folks,

In simplistic terms, if one needed to rerate a line that was originally hydro tested [1.5xDP old], but it couldn't be hydrotested today due to water contamination issues... and the original hydrotest pressure was GREATER than the calculated new pneumatic test pressure [1.1xDP new]... and the line condition was sound such that the original hydro test could be considered a valid test all these years later... could the line rerating be treated as a paper excercise and not require any leak testing at all??

With some due considerations, I'm leaning towards yes since the original hydro was greater than the calc'd pneumatic.

References I've been using are B31.3 and API570.

Much appreciate your thoughts.

Thanks.

Rob

 

[eyec]

things change (condition of original materials/service) and time takes its tole on everything. what may have withstoor 1.5X may not even meet a 1.1X.

however, you AI should be the answering authority on this question.

 

[robsalv]

Eyec, I'm based in Victoria Australia. We do not have an AI regulatory situation like America, but we're an American managed Petrochem company. The decision rests with me.

The line condition is sound - but the service precludes a hydro pressure test. The new DP is only moderately higher than it is today making the pneumatic test 1.1xDPnew LESS THAN the old hydrotest 1.5xDPold.

It would seem I have a case to waive the pressure test alltogether. But this situation is not explicitly catered for in B31.3. What do you think?

The equivalent Australian piping codes do not have any guidance in this situation. API 570 section 8 might be hodl the answer...

Cheers

Rob

 

[eyec]

with an eye on safety i would do at least an operational pressure plus 20% and hold it for at least an hour.

even though the line may be sound enough for the test it is usually other components within the system that may fail the pressure test.

finally, whatever you recommend i would pass it up the line to the risk manager for their signoff.

 

[StoneCold]

robsolv
The only reason anyone does the hydrotest is because it is much safer. When you do the calcuations of how much stored energy is in the pipe using compressed air vs water it is astounding. So if you go with a pneumatic test just make sure you are keeping people back from the pipe so that no one is hurt if it lets go. The diameter and the length both contribute to the amount of energy stored in the pipe, so obviously the bigger it is the more the risk.

Goodluck
StoneCold

 

[robsalv]

Thanks for your input Eyec and Stonecold. The dangers of pneumatic tests are well understood. I'm a champion of avoiding pneumatic tests as a search on my posts will reveal. Australia's inservice inspection standard has an appendix for determining the exclusion zone based on the amount of stored energy in a pneumatic test. Handle with care!

I'm trying to get a consensus opinion on whether I can avoid ANY KIND of pressure test based on the fact that if I had to pressure test this rerated line, I'd have to do a pneumatic test. Time does not permit dealing with the water which would contaminate the process. BUT the thing is, the pneumatic test value today would be less than the original hydro. I'm failing to see the value of the test.

I'm sorry I'm labouring the point.

Eyec, what medium are you suggesting the operational + 20% test is undertaken with? And what would you consider the benefit of such a test given it will be less than the original pressure test??

API 570 8.3d seems to suggest I have a point.

8.3 RERATING
Rerating piping systems by changing the temperature rating
or the MAWP may be done only after all of the following
requirements have been met:
a. Calculations are performed by the piping engineer or the inspector.
b. All reratings shall be established in accordance with the requirements of the code to which the piping system was built or by computation using the appropriate methods in the latest edition of the applicable code.
c. Current inspection records verify that the piping system is satisfactory for the proposed service conditions and that the appropriate corrosion allowance is provided.
d. Rerated piping systems shall be leak tested in accordance with the code to which the piping system was built or the latest edition of the applicable code for the new service conditions, unless documented records indicate a previous leak test was performed at greater than or equal to the test pressure for the new condition. An increase in the rating temperature that does not affect allowable tensile stress does not require a leak test.
e. The piping system is checked to affirm that the required pressure relieving devices are present, are set at the appropriate pressure, and have the appropriate capacity at set pressure.
f. The piping system rerating is acceptable to the inspector or piping engineer.
g. All piping components in the system (such as valves, flanges, bolts, gaskets, packing, and expansion joints) are adequate for the new combination of pressure and temperature.
h. Piping flexibility is adequate for design temperature changes.
i. Appropriate engineering records are updated.
j. A decrease in minimum operating temperature is justified by impact test results, if required by the applicable code.

Cheers

Rob

 

[PZas]

d. Rerated piping systems shall be leak tested in accordance with the code to which the piping system was built or the latest edition of the applicable code for the new service conditions, unless documented records indicate a previous leak test was performed at greater than or equal to the test pressure for the new condition. An increase in the rating temperature that does not affect allowable tensile stress does not require a leak test.


This seems fairly clear to me.

However, you must consider the time from the last test to the present. I find it odd that the above section does not state a time frame.

Do you feel that there is any possible way that this line will fair the test you are considering. If you do then test it. Code or not make things safe first.

 

[robsalv]

Thanks PZas.

In the specific case that brought up this question, the line's integrity is not in question. If I had my choice, there'd be a hydro without a 2nd thought, but the hydro meets the requirements for being waivered.

So far, the only really valid objection I've been given [from other sources] is what if there's a line contingency that causes the pressure to rise greater than 110% of DP? In the case where the old hydro and new pneumatic test are the same pressure, a contigency that takes the line to >110%of DP now puts the whole line in no mans land from a tested pressure envelope point of view.

Anyway, I'm fast developing a conservative view to the whole scenario. I'll waiver the pneumatic test IF some strict guidelines are met, including an assessment that the line condition today is equivalent to the condition of the original hydro, extra NDE, a risk assessment and probabilities for contingencies that may exceed the original hydro pressure, and a flexi study that shows no net increase in imposed stresses from the new operating conditions...

Cheers

Rob

 

[PZas]

robsalv, in the api code copied above how is the term "leak test" used. After letting this sit over the weekend I find that a little odd to use "leak test" for a hydro test. However I am not intimately familiar with the code in question and therefore am not up to speed on the symantics.

IMHO that line should be tested at greater than 1.1x operating pressure. With that small of a safety margin you could be into slow crack propogation or if not a little corrosion and you might be.