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Large low-pressure flange/ blind flange

Large low-pressure flange/ blind flange

Large low-pressure flange/ blind flange

(OP)
I have an ASME Section VIII vessel with a 36" flanged manway, with total pressure at the manway around 25-35 psi. It looks like the choices are to use a standard flange, which in that size, would be a weld-neck flange about 3" thick. Or, I can design a plate flange or hub-type flange per App. 2, but they come out around 2"-3" thick. It seems like major overkill, given that a similar manway with similar hydrostatic pressure on an API oil tank would be much lighter. Am I overlooking any options there?

Also, in applying the App. 2 design, it is the flange rigidity that governs the design. Yet, I can't tell that it applies to cover plates. So I am required to have a 2"-3" thick flange to avoid leakage, but can use a 1" blind flange, which seems a bit off. Any input there?

RE: Large low-pressure flange/ blind flange

Firstly, I am surprised at the size of manway, the normal size would not exceed 24", unless it is an access opening for internals handling of large size.
Secondly, the vessel may be designed to ASME VIII-1, even if the design pressure doesn't warrant the ASME jurisdiction (you don't mention if the design temperature is high enough to justify the ASME design).
However, for a manway of this size, at this design pressure, I would investigate the option of Appx 2, instead of B16.47 flange, which tends to be grosly oversized for your needs. Obviously, you should look to use lighter gasket for your application, since the cover design will be determined by the gasket seating forces, not the internal pressure. That will allow you to minimise the damage (to the pocket) from this large manway. I don't know about 1" cover being unreasonable, but that's coming from crunching few numbers, rather than 'gut feeling'. Give it a try.
Cheers,
gr2vessels

RE: Large low-pressure flange/ blind flange

JStephen, agree w/ gr2vessels, use a soft gasket, many small bolts, gasket as close to the bolt circle as possible to minimize the moment arm(s), maybe look at an intergral slip-on design.

You are correct, rigidity does not apply to blinds, but unfortunately does seem to drive most lower-pressure flange designs these days.

Regards,

Mike

RE: Large low-pressure flange/ blind flange

I agree with gr2vessels that you should use Appendix 2 and adjust your m and y factors for softer gasket material. Expanded PTFE works very well for manway applications that require chemical resistance. For water, there are many low durometer rubber compounds that offer low m and y factors.

RE: Large low-pressure flange/ blind flange

Do the real calc's - I'm making estimates off of the top of my head here from some projects many years old now: What I've done before for a large hatch in the sides and ends of low pressure (oil service) oil reservoirs needed for eqpt movement is buy a "conventional" RFSO flange and weld it to a pipe stub of the right diameter for the eqpt (30, 36, 42 inch).

Then fab a cover of thinner plate with reinforced gussets across the back (outside) of the blind flange to stiffen the cover against bending. Take very careful efforts to make sure the plate does not warp under the welding stress: Those welds do not need to be continuous, and stitch welds reduce heat input..

RE: Large low-pressure flange/ blind flange

JStephen,

Can't you use a 36" ASME B16.47, Class 75 Series B flange? The weld neck is 1.38" thick and the blind is 1.61" thick.

If standard flanges like the Series B flanges are available, why go with the complexities of Appendix 2?

RE: Large low-pressure flange/ blind flange

(OP)
Dcot, I ran across those a bit earlier, and looks like the solution, checking into availability now. Thanks, everyone!

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