Tubesheet to shell joint / Leak Detection / Lethal service / ASME VIII-1

[CuMo]

Hi All,
I've been asked to start design on a vertical BEM heat exchanger to ASME VIII-1 / TEMA-R / PED
where the following conditions apply:

Shell side: Cooling water
Tube side: Toxic fluid / Gas (Not 100% sure whether liquid or gas, or mixture of both) - Lethal service
Material both sides: Carbon steel(SA-516-70, SA-350-LF2-1, etc typical grades)

Our client has proposed a design equipped with leak detection using an additional thinner tubesheet welded to the face of the main tubesheet
with a 2mm deep void/cavity between both as per the sketch attached. (Note the seal weld at the periphery).

Correct me if wrong but to my understanding we must subject this 2mm deep void/cavity to the highest test pressure that would be applied to any of the chambers of the HE.
So that we can guarantee the weld connecting both tubesheets is leak free in any pottential condition. There are two openings connected to that void/cavity so we would be able to fill/vent it no problem.

Obviously the thinner tubesheet would be restrained (gets extra support) by the tubes (I haven't tried to design it on its own yet)but it looks to me
as though being much thinner than the main tubesheet - it might not pass calculation if subjected to the same test pressure as the main tubesheet due to bending stress.

Another concern is the weld at the periphery (marked with Continuous weld on the sketch) - how do I size it properly? Any guidance for this?
Do we bolt a channel girth flange against it for extra support during test? It would not be possible to RT this weld so I would assume that only UT/MPI (or both can be applied).

I've been through the forum and at the bottom of this thread:

https://www.eng-tips.com/viewthread.cfm?qid=315071

MJCronin suggests that having Leak detection might allow us to use tubesheet to shell corner joint instead of an extended stub & butt weld for the tubesheet to shell joint (which would typically apply for lethal service).
I have not found provision for this in the code though. It must be company standard practices.

Since we're using seamless tubes and only the tubeside is in lethal service, as per UW-2 (a)(3)(-a) I would assume a corner (fillet weld) joint is permitted.
Also PWHT to shell side would also not be applicable due to MDMT being only -15 deg C (5 deg F) and expected shell thickness is 20mm.
PWHT would apply to tubeside due to the lethal service.

Any comments - welcome.
Apologies for the long post. First time dealing with such configuration.

Best regards,
CuMo

 

[georgeverghese]

Why not an all welded plate heat exchanger made out of CRA materials for both sides, if both sides are clean- non fouling fluids ? Lower interfluid contamination risk if you ask me

 

[r6155]

1)From which parts do you expect leaks?
2)All welds for lethal service shall be full penetration. Welds in your drawing are not allowed.

Regards

 

[SnTMan]

CuMo, first, I agree, a corner joint may be used at the shellside. Although I do think, given letal service the weld connecting the two tubesheets should be full penetration and subject to PWHT.

Second, TEMA Standards contain rules for several configuration of double tubesheet design, you may find them useful. One such configuration is similar to your proposed design. Briefly, TEMA requires the total thickness of the two tubesheets be no less than the thickness of a single tubesheet, as calculated by TEMA rules. There are calculations for the connecting element (between the two) and for tube axial stress as well.

It is not clear to me if the space between the tubesheets is conisdered as a pressurized space or not. It also appers that such construction is not within the scope of Part UHX.

Best of luck,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[CuMo]

Thanks Mike! Appreciate your feedback! Spot on as usual.
I'm giving TEMA RCB-7.124 a go.

r6155,
1) I am not sure what to expect to be honest. Papers suggest that typically if there is something to leak first - it would be the tube to tubesheet connections (joints) as they are the least reliable in a tubular HE.
2) ASME VIII-1 allows to apply the rules for lethal service to only one side of a heat exchanger if certain criteria is satisfied [UW-2 (a)(3)(-a)],
so corner joints are permitted on the non-lethal service side.
Anyway - the client has now asked that we treat the whole unit as in lethal service so the game has changed - full pen welds all around it is.


georgeverghese,
If I understand correctly CRA stands for Corrosion Resistant Alloys.
With the size of this thing - it will probably cost in the range of several million to make out of stainless.

 

[SnTMan]

CuMo, I agree, tube-tubesheet joints are the most likely place to develop a leak. I'd seel seal weld if possible, but your configuration (projecting tubes) may not permit it. Again, good luck


The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[georgeverghese]

Well, there is no harm in asking for prices for this option, if it is suitable for this service; you havent provided any service details to ascertain. You need to provide justification for the option you've chosen, especially since this is lethal service. There are several grades of stainless steel, each with a different PREN value. A compact all welded HX made out of CRA material may occupy less than a 1/3rd the footprint of a shell and tube alternate.

 

[CuMo]

georgeverghese,
unfortunately this is not something we can dictate and is the preference of the client to stick with a tubular heat exchanger.
I understand your point though and I'm sure there are better options for any design.

Mike,
I've been through the TEMA procedures, made some progress, double checked manually quite a few things and it seems that the software which I'm using outputs a lot of
useful data when it does a single fixed tubesheet to TEMA (procedures match), so it looks like I'll end up with a calculation combo )
But of course - priorities changed... :/ so this will have to wait a bit...
Thanks for the idea!