API 579 FFS on disbonded clad

[GD2]

Dear friends,

Do anyone have experience in FFS assessment of the SS cladding on Low alloy steel base? The vessel is a Reactor in hydro processing unit. The clad has disbonded at few locations. API 579 does not provide any direction on clad assessment.

GDD
Canada

 

[SJones]

What failure mechanism are you envisaging that will require assessment?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[GD2]

SJones,
Clad disbonding is primarily due to difference in thermal expansion between the clad and base metal during the cooling cycle. This is what most of the studies says.

GDD
Canada

 

[SJones]

Absolutely, but what failure will a disbonded cladding cause?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[GD2]

Clad disbonding will affect the following:
-Full hydrogen partial pressure exposed to base metal - meaning allowable operating temperature reduced by Nelson curve.
-Likelihood of hydrogen/temper embrittlement of base metal.

GDD
Canada

 

[SJones]

If the cladding is not breached are those outcomes more likely?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[r6155]

IMHO the clad does not affect the integrity of the Reactor. But I suggest the repair of these few locations.

Regards

 

[GD2]

r6155,
The weld repair of the clad is something most of users avoid because of the difficulty of repairing the 8 mm thick clad to the massive 200 mm thick base metal reactor in carrying out a PWHT to avoid martensite formation.

Do you have any experience on this?

GDD
Canada

 

[r6155]

Acoustic Emission Examination during normal operation can reveal more information to avoid repairs.

Regards

 

[weldstan]

If this is 347 OL on the hydrocraker, eventually cracking of the overlay will occur and you will have to repair. At this time, I would agree with r6155 recommendation. Any repair procedure would have to be developed and signed off by a Licenced Engineer with experience in Divv.2 design. I have developed and supervized such repairs in the past. The Owner/Operator must agree. What do your company's or Owner's metallurgists have to say?

 

[GD2]

Weldstan,
The reactors are 347 weld overlayed. Customer has not tried any repair welding yet but regularly does NDE surveys.
API 934 specifically API 934-H provides directives on repair welding that the customer may take as a baseline document to develop the WPS.

GDD
Canada

 

[0707]

Customer is in the good way GD2! I would recommend an RBI inspection programme, and NDT in a regularly basis.

luis

 

[DBreyer]

Depending on the dimensions of the reactor I would be worried about hydrogen embrittlement resulting in cracks in the base metal during shut-down/start-up.
API TR 934F (and API RP934F, whenever it will be finally published) have al lot of guidance on calculating safe minimum pressurisation temperatures in order to avoid cracking of the base material during start up of the reactor after a turnaround.
I would recommend contacting one of the specialized engineering companies involved in the report as they have specialized software to assess your case.