Internal test cap for hydrotest of new column nozzle?
Internal test cap for hydrotest of new column nozzle?
(OP)
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.
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.





RE: Internal test cap for hydrotest of new column nozzle?
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.
RE: Internal test cap for hydrotest of new column nozzle?
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?
RE: Internal test cap for hydrotest of new column nozzle?
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.
RE: Internal test cap for hydrotest of new column nozzle?
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.