Downside to oversizing an RF pad? And weld details for an an angled nozzle through an RF pad?

[sally31]

Hi all,

First time posting here. I have an unstamped 316/316L vessel, design pressure of 5 psig (MAWP of 15 psig), design temperature of 700 degC, used for reacting material with all sorts of flammables gases. We are following ASME VIII, Div 1 despite the lack of stamping. The modifications requested involve adding two 45deg lateral nozzles at two sections of the pipe. If I follow the reinforcement pad calculations in B31.3, I do not need reinforcement in either location. However, as I am not quite sure how reasonably well I'll be able to manage the externally-induced nozzle loads, I would like to plan for reinforcement to be conservative. For one of the two locations, I'd be welding a 1/2" SCH40 lateral branch into a 6" SCH40 run. I plan on using a latrolet for that location.

For the other location, I'd be welding a 1" SCH40 branch into a 12" SCH10 run. For this location, I've read enough about olets on thin pipe to be scared off from using a latrolet here. I do have an open question out to Bonney Forge to see their thoughts on this (perhaps the small enough branch on the large enough header is enough to minimize distortion concerns?), but they have yet to respond. So I'm looking into adding an RF pad instead and am running into a couple questions.

One, is there any downside to oversizing the repad in the d_2 dimension? We're nowhere close to the D_h dimension max specified in 304.3.3 of B31.3 to be clear. But I need more real estate on d_2 in order to fit a minimum 1/4" NPT weep hole. In case it's at all relevant, the 700 degC design temp of the reactor is not totally in play at this location based on actual/measured data showing this location can expect a max wall temperature of 300 degC.

Two, I'm having trouble figuring out weld details associated with a repad for an angled nozzle. Between Fig 328.5.4D in B31.3 and Figure UW-16.1 in Sec VIII, Div 1, I've got the general idea behind acceptable solutions, but it gets all wonky with the added complication of the nozzle angle. I'm hoping to aim for something like (a-1) in Figure UW-16.1, but because i can't get a constant chamfer on an angled cut thru the nozzle pipe, the geometry of the gap to fill starts getting weird (see screenshot linked below). Are there any other resources I can look to give me more ideas for how I'd specify the welding here?

Or I guess in general, am I barking up the wrong tree? Are their other/better reinforcement solutions for this size nozzle besides olets and RF pads?

Thanks in advance,
Sally

 

[RJW000]

Have you asked the client to supply the nozzle loads?

Your reinforcement design will not have a technical basis as you are just hoping it is adequate.

 

[sally31]

Yes, I know the loads, but just didn't have a good feel for whether I'd be able to stay within those loads realistically without being knowledgable about piping support options out there. But I'm in a better spot now than a week ago after getting help on that question. Appreciate the response though!

 

[VFK]

At 700C you need to consider in-service degradation mechanisms affecting the SS316. If you're unfamiliar, as a starting point you can consult ASME-II Section D Appendix A-200, specifically A-207 475degC Embrittlement, A-208 Sigma-Phase Embrittlement, A-209 Laves phase embrittlement and/or API 571 which give a high-level background explanation of these austentic SS degradation mechanisms. I would ask the end-user the max operating conditions and define more practical design conditions. Then develop the weldment design around the plausible degradation mechanisms.

https://www.youtube.com/watch?v=uo7H_ILs1qc

this CBS video offers some insight about ensuring the mechanical design considers the service conditions - its rather graphic but listen out for the key insights.
Next consult WRC-470 "Recommendations for Design of Vessels for Elevated Temperature Service" for insights on suitable connection details. Ideally you should be looking at a self-reinforced connection detail to eliminate the reinforcing pad.
You mentioned following the B31.3 code rules. For a vessel you need to follow ASME VIII-1 rules in full - the method is essentially the same but the allowable stresses used to calculated the excess thickness available for reinforcing is under-conservative per B31.3 - reason is B31.3 uses lower safety factors when deriving some material allowable stresses compared to ASME II-D. SS316L might be same in B31.3 and ASME-II-D.
As far as detailing the weld preparation angle around the nozzle-to-shell joint, the few vessels that I drafted I provided two cross-section views, side elevation and front elevation. And left it to the workshop teams to figure out the required weld prep angle along to get from one view to the next.
Sorry if you come away with more questions than answers, but hopefully it sets you in a better direction from a design due-diligence perspective.