Cleanout Door PWHT Problem

[MohdYaseen2010]

Dears;

we are constructing new storage tank having the following data;
Design Code = API 650
Materials = A 283 C (Group I)
Design Temperature = 85C
Stressed Components include:
Shell = 12 mm, 10 mm, 8 mm
RF of Cleanout Door = 16 mm
RF of Flush-Type Shell Connections = 12mm (N9), 12mm (N10), 12mm (N11) & 12mm (N13)
Welded bottom plates to Shell =10mm
Appendix A didn't specified by the Purchaser

API 650 Clause 5.7.4 specify that Thermal Stress Relieve shall be applied either;

(1) Prior installation into the Tank to the Assembly.
Assembly = Clean-out Door + Attached Shell + bottom reinforcing plate (or annular plate) + flange-to-neck weld
(2) After installation into the Tank if the entire Tank is going to be Thermally Stress Relieved.

point (2) can't be applied, so point (1) (PWHT) applied on the factory.

Problem:
After dispatching the Cleanout Door to site, the Contractor did cutting/welding works on its reinforcement plat and neck (where little mismatch with the annular plate) without informing the Client.
The Contractor is suggesting to apply PWHT on the modified parts only (not to the assembly where is no furnace to occupy it).
What do you think? is it acceptable? if yes what is the reference? if not, are the available solutions?







Mohd Yaseen

 

[Dhurjati Sen]

Dear MohdYaseen2010,

What is the thickness of RF plate and neck where the contractor did cutting & welding without your knowledge?

If the thickness does go above the limits to carry out PWHT, ask contractor to fill up a "deviation" form and suggest remedial measures.

If they suggest localized PWHT, you review and decide.

Regards.

DHURJATI SEN
Kolkata, India

 

[Geoff13]

I would be very reluctant to do a local PWHT of just where the repair welding was done. Local PWHT (meaning just a small zone surrounding the repair weld) will most likely result in a distorted assembly from the diferential thermal growth between the hot and cold areas. Much safer to PWHT the entire assembly.

However, was the repair welding significant enough to justify re-PWHT? As an example, on an ASME pressure vessel that was PWHT'd per Code rules, you could do a weld repair that stayed below the PWHT thickness limit, and not have to re-PWHT the weld. However API 650 does not address your situation. This would require engineering judgement regarding the extent of the welding, as well as final owner approval of any decision.

It is too late now, but there are controlled welding techniques that could have been used as an alternative to PWHT. See API 653 section 11.3 for details.