PWHT of CS Channel with 316L SS Cladding or Weld-Overlay 1

[thermmech]

We have a CS tube exchanger (both SS and TS) where CS tubes have corroded.

TEMA type: CKU (channel butt-welded to tubesheet, originally PWHT for Code reasons, not service)
Shell side: hydrocarbons boiling (corrosive agents)
Tube side: high pressure saturated steam
Channel = SA-516-70N, thks = 75mm
Tubesheet: SA-266-2N, forging, thks = 250 mm

To address shell side corrosion, we plan to purchase a new tube bundle c/w channel with:
- 16BWG 316L SS tubing in lieu of 14BWG CS tubing
- 3/8" min. 316L SS cladding or weld-overlay on shellside of tubesheet (to expand 316L SS tubing into the 316L SS layer on TS and prevent corrosive agents from contacting CS)
- 1/4" min. 316L SS cladding or weld-overlay on tubeside of tubesheet (to accommodate strength welding of SS tubes to tubesheet).

I have discussed PWHT of the channel with in-house metallurgists, and they have advised that
- the CS tubesheet forging could be clad with 316L (explosion bonded), or could be weld-overlaid
- than 316L SS layers would be machined
- tube holes pre-drilled (not finished),
- 75mmthk channel but-welded to the tubehsheet,
- channel PWHT'd @ 1150°F c/w 316LSS cladding / weld-overlay
- tube holes finished
- tube-to-tubesheet joints welded.

The have no concerns with sensitizing of 316L SS cladding/weld-overlay. Any advantages of cladding vs. weld-overlay? (cost is not an issue).

Do you see any issues with our plan?
Any comments would be appreciated.

 

[metengr]

I am in agreement with your in-house metallurgists on the plan. I would go with weld overlay of the tubesheet versus cladding. I prefer cladding in pressure retaining locations where you do not have multiple openings (tubesheet holes). Have you evaluated a solid 316L tubesheet?

 

[thermmech]

metengr,

Thanks on your feedback. We also prefer weld-o'lay We do not want to go with solid SS tubesheet as it is a CKU type of exchanger (channel integral with TS) and:
1. 316L SS has lower allowable stresses than CS, TS thks would grow, tube-side nozzles c-lines would move.
2. Big and thick H-type channel nozzles would be hard to get in SS (I think) and we need the replacement bundle urgently.

Paramount is to get the sequence of fab. operations correctly. We just want to avoid any surprises after placing the PO as vendors are not responsive at bidding stage. If you could comments of 2 "last" things:
a) We intend to heat-treat U-bends of 316L tubes after forming {that would have been required by the SA-213 spec after forming anyways. Additionally, our internal specification calls for: Non-stabilized low-carbon grades shall be solution heat treated by the electric resistance method (1095°C [2000°F] for 15 seconds, followed by rapid air quench).
b) We will strength-weld SA-213-TP316L with 2 passes to 316L weld-o'lay, and then additionally roller expand the tubes into 2 grooves in CS, and 1 groove in the shell-side overlay. Care will be taken to stay away at least 3/8" from the strength welds, and 3/16-1/4" away from the TS shell-side face.

Thanks again.

 

[metengr]

thermmech;
Here are my comments as you requested;

) We intend to heat-treat U-bends of 316L tubes after forming {that would have been required by the SA-213 spec after forming anyways. Additionally, our internal specification calls for: Non-stabilized low-carbon grades shall be solution heat treated by the electric resistance method (1095°C [2000°F] for 15 seconds, followed by rapid air quench).

Agree,with your internal specification.

b) We will strength-weld SA-213-TP316L with 2 passes to 316L weld-o'lay, and then additionally roller expand the tubes into 2 grooves in CS, and 1 groove in the shell-side overlay. Care will be taken to stay away at least 3/8" from the strength welds, and 3/16-1/4" away from the TS shell-side face.

I would roller expand first to 8 to 10% wall reduction, 2-pass weld the 316L tube followed by a light tube roll.

 

[thermmech]

Thanks again!

I would roller expand first to 8 to 10% wall reduction, 2-pass weld the 316L tube followed by a light tube roll.

Would it be better to expand a bit less since this is a SS tube which work hardens?

 

[metengr]

Yes, you can use the lower bound value.

 

[SnTMan]

thermmech, something to think about....

The weld / roll process is often done opposite of what metengr describes, i.e. light contact roll to set the tube, weld, full roll. The purpose is to allow gases generated by the welding to escape out the backside of the tubehole to help prevent defective welds.

Regards,

Mike

 

[thermmech]

SnTMan;
thanks on your comment. I am aware of the issue leading to porosity. We have lots of food for thought on this one.

 

[SnTMan]

thermmech, you're welcome, worth what you paid for it, hey?

Regards,

Mike

 

[thermmech]

It will work out, I am sure. If everything was straightforward oil companies wouldn't need engineers but just secretaries and technicians. ;-)

Cheers,
Sean

 

[SnTMan]

If it was EASY, anybody could do it