Local Hydro

[bmoorthy]

A pressure vessel(Vertical Column/Tower) was designed and fabricated at shop and Hydro tested at shop. Subsequently due to transportation constraints, the job was split up (that is one category B joint was cut) and the job/Joint was subsequently welded at site with the same configurations, 100% RT, DPT in root pass and final pass, 100% UT with angle beam technique and maintaining dimensions etc within tolerance.

Instead of performing Hydro test of the full tower, is it permissible to perform Hydro test/Pneumatic test of only the Joint in Question by providing temporary cover and then subsequently remove the support and perform DPT at the support location.

Will this be acceptable to AI.

 

[chicopee]

I believe the AI will want a full hydrostatic test at 150% MAWP. For safety sake don't do a pneumatic test at 150% oe even at 100% of MAWP.

 

[garnet1]

Sounds like your vessel was quite tall. It was probably shop tested in the horizontal position. Can this be performed on site? If not you need to see if the design will allow a vertical hydro. I agree, unless stringent precautions are taken pneumatic tests, although allowed, are the last thing you want to consider. Once tested and the AI has signed off on the vessel, it now becomes a "used" vessel and NBIC would take over if in the states. If there is no ligitamate reason other than inconveinience, for not performing a hydro, then you should do one. If for other reasons, a hydro is impractical, you may get away with what you say. I have had to do it in the past. Too many unanswered questions.

 

[deanc]

Look on the back of the U-1 form. Does the shop have a field extension on their Certificate? Does the QC Manual address how to handle this type of job? Ask your AI what the options are. If there will be two different AIs,they need to talk.

This is not uncommon but you must get your AI involved from the start. If the field work and the shop work are thru the same AIA it should be no problem.

 

[jte]

bmoorthy-

I certainly agree with deanc. This is a case which needs to be fully thought out early and agreed upon by all parties prior to expending any significant effort. "Bad news early is good news."

If you do wind up with a hydrotest requirement, the AI can only require a 130% (adjusted for stresses if necessary) of MAWP hydrotest per UG-99(b), assuming this is a VIII-1 vessel. As garnet1 mentioned, the vessel must be checked to be adequate for a full hydro in the vertical position. Normally, wind and seismic are assumed to not be present for a hydrotest calc. If the vessel is installed in a windy area, calc's should be made and a max. allowable wind speed established. Provisions must be made to abort the test if there is a threat of reaching the max. wind speed.

Another item to check is the foundation. What loads were the structural engineers given as a design load? Years ago I was involved with a particularly tall (over 250') column which was field hydro'd. About a week prior to the hydro, the structural engineers made an interesting discovery: The foundation was shared by several vessels, and designed for the hydrotest. However, at the time of the hydro, the other vessels were not yet installed. Thus, the foundation loading was not balanced in the way they had anticipated. The foundation was monitored by surveyors during the hydro to ensure that it was not tipping over, and the test was uneventful.

If your vessel is 60" dia or less, Car-Ber testing might be able to offer a good alternative to a full hydro by providing a tool which would hydrotest only the seam in question:

http://www.carbertesting.com/prod-weld.htm

If your vessel is over 60", they might still be able to help you out. We have just begun to use their services with some of our piping repairs. Again, check with the AI prior to committing to this approach.

jt