Welding flat covers to Slip-ON Flanges

[workermonkey]

Hello,

I'm a young engineer who’s recently been thrown into a heat exchanger design job. When designing flat covers to be welded to standard slip-on flanges I’ve come across an odd problem. The ASME code dictates that the weld thickness should be at least 1.25 times tmin but it does not specify which component would be considered the minimum thickness. Also the minimum thickness would be almost 1/2" on the flange and 3/4" on the flat plate. When beveling the plate and flange for welding a 1/2" x 1/2" bevel is used but this seems like over kill for the weld. It’s a simple weld but I’ve found no refrence to flat covers being welded directly to flanges in the code. Any thoughts or comments would be greatly appreciated. Thank you.

 

[unclesyd]

I don't think you will find a references other than in the design of a flat head.
The only time I’ve seen a flat cover welded to a flange, weldneck, was for capping CTW lines.

On heat exchanges that require a flat cover we use a flat plate, like blind flange.

Come back with some sizes of the flanges with pressure and temperatures.

Are you talking about a hub on the flange being 1/2" or is this a hubless?

 

[GenB]

your tmin is ussually the thinner member
in this case your .5 x 1.25
more information is needed as members size, fitting desing and welding desig ,
ER

 

[workermonkey]

ok, in this case its a 6" 150# RFSO flange and welding a 3/4" carbon steel plate to cover its hub. pressure 150psi @ 400deg F.

 

[donf]

workermonkey,

Please explain the advantage or necessity for using a slip-on with a plate welded to the hub versus a standard blind flange.

Thanks,

Donf

 

[abeltio]

workermonkey...
it's been a while since i stopped doing ASME calculations and i am just guessing what u are trying to do...
seems like you are trying to build a return header for a very small HE...
and instead of using a formed head u are using a piece of flat plate to do something that will look like the hats worn by the banjo players - flat top
vs.
the hat worn by the brits (e.g. mr. john steed's hat - the avengers)- formed top

if that's the case...
check the ASME glossary but as far as i remember the min thickness referred to by the ASME code is the min thickness required for pressure containment, and not the physical thickness of the parts to join
The extra thickness could be a customer requirement for CORROSION ALLOWANCE which the code does not consider in the stress calculations at all.
YOU ARE VERY CORRECT TO THINK THAT 1/2in WELD THICKNESS FOR THOSE CONDITIONS IS AN OVERKILL...

Then for 150psi @ 400 degF the min thickness required for press containment at the internal diameter of the SORF hub exclusive of any corrosion allowance is the min thickness to consider.

with those conditions what i would do is to set a gap of 1/16 in between the flat head and the hub.
weld from the inside with a 3/16 weld depth (should be more than enough). Do a girth weld from the outside.
i.e. no beveling required.

pressure test the whole thing with 600psig... u should be ok.

Remember that u need FULL PENETRATION so you must have a 1/16 gap between the flat head and hub.

May be someone else has a different opinion... but shooting from the hip... with those process conditions seems like you should be ok.

HTH



saludos.
a.

 

[workermonkey]

abeltio,

thanks, you hit it right on the button. that my situation exactly. we weld nozzles to the plates and in this case its nessecary to weld the plate on the outside since the customer wants 2" 3000# cplg's attached for the inlet/outlet of a 2 passs HX.

 

[GenB]

workermonkey,
In the above case you may have to use a weld fillet
equal to the size t of the cover, and only if if you groove it (as you state) you still have to put a smaller fillet equal to .7 tmin

look at UW-13.3-m sec VIII-1 where a= 3 times the cover t,
This will excede sec I requirements,
It looks a lot of weld but it is what you need, the t of the attaching cover, plus the same size groove weld (groove and fillet combination),
hope this helps. I do not see another way of compensating the type of fitting design.
ER