Tray support ring cracking

[robsalv]

Folks,

We have a carbon steel distillation column/tower with cracking in the tray support ring fillet attachment welds. The service is not one where the environment promotes cracking, so the cracking appears to be related to welding technique/practice. Most cracking is on the toe of the weld on the tray ring side which we're not worrying about, but some are on the toe of the weld on the shell side.

We've been blending the shell side toe weld indications and have been finding that the cracks follows the fillet weld HAZ. One or two were followed all the way back to the tray support ring - effectively meaning we removed the weld.

These indications could be LOF or they could genuinely be cracking in the HAZ - single run fillets can have high hardnesses in the HAZ so it's not inconceivable. So far none of the crack like indications have wandered off into the parent metal of the shell.

Ther problem I have is that there are some tray ring shell side crack indiciations we can't get to as the ring is "behind" welded downcomers. I'm pretty confident that these crack like indications are exhibiting all the same signs/morphology as the cracks we've already blended out.

API 579 and our own Australian codes do not really seem to treat this particular scenario very well. These standards are more focused on cracks with some depth into the pressure retaining component.

Any thoughts on the leaving in of these hard to get to cracks???

My thoughts are: based on the finding of all other blended cracks, I'm considering them as non deleterious original manufacturing highly restrained one run fillet weld defects and providing that angle ultrasonic UT scanning from inside with a full skip confirms that no cracking veers off into the parent metal - I'll leave these hard to get to indications in. Further, I'll record the crack length and register these locations in the inspection database to ensure we have a tight repeatable and regular inspection of these specific locations.

Any thoughts on this whole scenario would be greatly appreciated.

Thanks in advance.

Rob

 

[unclesyd]

Can you comeback with more information about the welding procedure and the materials involved?

 

[arto]

It's not differential thermal expansion causing the cracks, is it?

 

[metengr]

Your argument regarding leaving the shell side fillet weld cracks based on past precedent sounds reasonable provided you periodically monitor these areas of the vessel with ultrasonic testing (UT), not just a one time test, but during scheduled outages.

I suspect that the cracks on the shell side could possibly be underbead cracking related to either a lack of preheat OR delayed cracking from hydrogen in the weld electrode when the fillet welds were deposited.

 

[robsalv]

Metengr - The regular UT scanning is a given.

I've asked the techs to go overboard on the set up and equipment details so that the testing will be repeatable - UT can be such an operator dependent test.

The preheat and hydrogen issues crossed my mind too with further thinking and discussion after posting my query.

All pressure containing welds are ok, it's just the structural type welds which appear dodgy and if the MDR is to be trusted, were carried out using E-7010 and E-7020 electrodes. There are no inspection reports on the tray ring welding. The shell material is A212-B so you can guess the vintage of the tower.

The laying of a one run fillet weld onto a cold tower would go some way to explaining what we're seeing. If they didn't use hot boxes in those days - then that would just about seal what we're seeing.

The tower has a history of tray ring defects over its life - but past engineers blended out all the indications they were concerned about. This is my first time in this tower and the first time we look like leaving in some pressure side defects...

It's clear the operating service and temperature gives cause for the shabby welding to keep showing up its defects, so we're never going to be "out of this tower" during it's operating life... the life cycle cost of this tower must be enormous.

I'll post how the "inaugral" scanning goes.

Cheers

Rob

 

[robsalv]

Correction -

"The laying of a one run fillet weld onto a cold tower would go some way to explaining what we're seeing...."

should read

"The laying of a one run fillet weld without preheat would go some way to explaining what we're seeing..."

"Cold tower" could be interpretted as the service.

Cheers

Rob