PWHT on Pressure Vessel

[sadminto6]

Hi Guys,

I want to know about reference of requirement of RT ( radiography test), we have to perform before or after PWHT?.
Material is SA 537 Class 1., thickness = 31 mm (PWHT is user requirement)
Applicable Code is ASME sec.VIII Div. II.

Thanks in advance

 

[metengr]

ASME Section VIII, Div. 1 does not have a specific requirement for NDT after PWHT. If NDT is required, it is after welding is completed. If a client specifically requests NDT after PWHT, which is after welding, it shall be done.

 

[metengr]

I see that you referenced Div 2 requirements. A quick check of Div 2 reveals no specific requirement for sequence other than when the weld is completed.

 

[DeleteriousPhases]

I recommend doing RT before PWHT. If a welder has put some slag surprises in there, you'll be able to grind it out and reweld and RT without redoing the PWHT. and as metengr says, always follow the customer spec, and sort out any NDT after PWHT as required.

 

[Shmulik]

thread794-140304
thread292-210980

 

[CoCoE]

I think this can be either way. If you need to take the risk of doing PWHT again if the joint fails in RT, then you may go for PWHT before RT. I think PWHT is costlier.

 

[racookpe1978]

Several things come into play: You cannot make a single "rule" for everry weld.

If you are in the plant, and have RT and PWHT "continuously" available without stopping every other activity going on the plant (regularly batch RT on weld joints in the test facility by moving the pipe sub-assemblies to the RT machine) then the RT should be done before and after the PWHT. PWHT can affect joint integrity - it doesn't in 98-99% of the cases because PWHT reduces welding stresses and stabilizes the pipe/joint weld - but it can. So your final NDT (RT, MT, PT, VT) MUST be done after the PWHT and cleanup.

BUT! In the field, when re-assembly for the outage completion IS the critical path, then the 24-36 hour PWHT cycle is much, much more expensive than an RT shot by calling in RT specialists. HOWEVER! That RT shot may require everybody in the nearest 50-100 feet clear out for 3-4 hours during the RT shots. So, that also delays the project schedule.

Doing an RT shot before PWHT requires the RT film be "cold enough" to take the heat from the pipe. So, if you've pre-heated the pipe and vessel to say 300 deg F, then you might not be able to get the RT done until you cool down.

Say you have a 4 inch thick weld requiring 2-4 days of welding. A single flaw deep inside the 4 inches is very possible, right? So a preliminary RT shot at the 2 inch point will tell you early about a flaw that might requrie a very, very deep groove just to expose the slag. But then you need a preliminary RT, and a second RT. Maybe even a third.

BUT!

Assume you have to begin the PWHT cycle, or you need to run a martinsite cooldown to 150 degrees, hydrogen bakeout AND the PWHT cycle all within 8 hours of final welding. So, you don't have time to wait 4 hours for the RT shots to be run before the PWHT time clock closes out.

Usually, a preliminary RT (before PWHT) may not be acceptable as final RT.

HOWEVER, even if it is not "a final shot" at least you have very, very good confidence that there are no submerged slag or inclusions under the final weld passes. SO, if at any way possible, try to get a preliminary RT shot before PWHT starts. Then, if something is wrong, you are already at pre-heat temperatures for the repairs, your welder and scaffolding are in place, and you can do the repairs quickly. (You know exactly where the flaw is and only have to fix a single point.)

If you are running an open root pipe joint in stainless, you need an internal Argon purge all the time for the root pass and hot passes. A "thin" pipe weld still requires the long PWHT cycle, but heatup times are a little shorter than for a fat thick casting. A preliminary RT and thus explicit knowledge of purge cycle requirements for the joints let me shave 6 days off of a recent schedule. At the expense of shutting down other work for 3 hours.

So because you can find a flaw BEFORE a 36 hour PWHT cycle, a cooldown cycle, then the re-heat and the re-wrap for a second PWHT cycle and a second cooldown cycle, you may find - as I just did - that your final schedule has cut 4-6 days off of your repair period. At the expense of a single 4 hour RT shutdown period.