RT and PWHT in Pressure Vessel Inspections
RT and PWHT in Pressure Vessel Inspections-which NDE techniques must be performed before PWHT and which techniques after PWHT? What about RT? Doing RT before PWHT is correct? What is the best practice for purchasers?
Bob's Reply: ASME Code VIII Div 1 is not specifying any NDE just because our vessel must be subjected to the PWHT.
After completion of each weld unit, the required NDE must be done so if the work was unsatisfactory then corrective steps can be taken to improve the welding in the subsequent units.
So either our vessel is subjected to PWHT or not subjected, there is no differences in the NDE technique and scope.
If our vessel is subjected in spot or full radiography and also PWHT is mandatory. We must do radiography when welding completed and all probable defects removed, repaired and reradiographed, then it is the time to send the vessel to PWHT shop.
I have seen personally some vessel which had been cracked in weld or even in base metal after PWHT and I understand why you are concern, but ASME Code has not mandated radiography after PWHT.
So you may ask why code has not mandated this. It might because we have another check point in hydrostatic testing so any crack which developed in PWHT most probably will fail in the hydrostatic testing.
We do not must forget, based the ASME code the full responsibility for pressure vessel is on the shoulder of manufacture not purchaser or other parties.
You may note before start of the fabrication, the manufacture provided his WPS and PQR, so if all welding controlled properly based WPS, this cracking must not be happen.
I have seen some pressure vessel inspections and test plan which the purchaser mandated RT before and after PWHT, so definitely the manufacture will charge the purchaser because this is not code requirement.
I have also seen some pressure vessel purchasers stated this requirement in their purchase order and manufacture know from first day of fabrication which has to do two times radiography.
I believe this practice is not correct in pressure vessel inspections, the manufacture is responsible for quality on his own work and this responsibility must not be transferred to the purchaser, engineering companies and etc.
If the pressure vessel failed under hydrostatic test, then manufacture must make repair or even rebuild the vessel.
So to prevent this bad incident, the manufacture will make all effort to control the welding process precisely.
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