Internal test cap for hydrotest of new column nozzle?

[treemkr]

A client is modifying an existing CS column by removing an existing nozzle and replacing with a larger nozzle. When previous work of this kind was performed on this vessel in 1999 they welded a larger OD pipe cap to the inside of the column in order to perform a local hydrotest of the nozzle. Afterward they cut out the cap and then performed PWHT of the nozzle and adjacent tray clips. They want to follow the same procedure this time around.

Is this an accepted practice for hydrotesting?
Typically hydrotest is performed AFTER PWHT - what issues arise when following this procedure?

Thanks in advance for any responses received.

 

[Duwe6]

That technique is indeed "Standard Practice". Typically only used if the AI is not 'comfortable' with waiving the hydro in favor of some NDT testing. Hydro 'usually' gets waived if the AI has seen an unbroken run of excellent welds from the same welders doing the work, and if the new nozzle is an exact match to an existing [already hydro'd] nozzle, and if the new nozzle is well away from existing welds, penetrations, and features like head knuckles.

The hydro it to test the new nozzle's design, in addition to the welds. And after the hydro, the temp. cap has to be removed. The PWHT is to de-stress both the nozz welds and the cap weld area. If a hydro has been required, the only other alternative would be to PWHT first, then hydro, and cold-cut the cap off post-hydro.

 

[treemkr]

Duwe6: Thanks for the quick reply.

I should have also mentioned that part of the problem is that the pressure test cap interferes with the internal tray clips. They actually will want to cut out old nozzle, weld in new, install cap, pressure test, cut out cap, install new tray clips, then PWHT both nozzle and clips. In their mind this removes the need to PWHT twice.

What are the hazards that arise when PWHT is performed after hydrotest?

 

[Duwe6]

None, 'real world'.

In the 'as-welded & still stressed' condition, the material is more likely to crack. Just like a falling meteor is more likely to hit a 12-bedroom mansion than a 2-room bungalow. If the steel was actually 'vessel quality' - met appropriate SA-spec - the probability of cracking during a 1.3/1.5 x hydro are about as low as my rediculous example.

 

[jtseng123]

This kind of nozzle local hydrotest is not mandatory and if performed, only to find pin hole on the new welds. Hydrotesting entire vessel is mandatoty however, it can be waived by AI and owner with certain conditions as written in NBIC.
Local PWHT may induce crack if not done properly. My recommendation is to do PT or MT on all new welds after PWHT and completely cool down, not to concern which one(local hydrotest or local PWHT) been performed first.