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Blank thickness calc - RTJ Flanges

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kevtsg

Mechanical
Joined
Sep 6, 2005
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3
Location
MY
Hi all,

I am reoriginating an electronic calc for blanks as per B31.3 para 304.5.3. Something that has struck me regarding the calc that I have to work with is that the original author made an allowance for 2x the groove depth on RTJ flanges. Obviously this is giving a pretty large safety margin, (additional 11mm in a 1/2" system), however is this the norm or is it too much? I can see no reference to this allowance anywhere else.

Thanks !
Kevin
 
Kevin-

See also B16.48 which does the calc's for you ased on the B31.3 formula.
Your post is not clear on how the blanks you are dealing with are to be used. If they are used with gaskets on both sides, then you'll need grooves on both ides. If you are using them for something like vessel isolation for entry with the blind inserted in between the vessel nozzle and piping, then you'll only need the gasket on the piping (process fluid) side and thus the groove on the vessel side is irrelevant. In that case, leave the extra groove out of the calcs - but be sure the blind is fabricated with only one side grooved...

jt
 
jt,

Thanks for the reply.

The calculation is merely to determine the required pressure thickness, eg tm=sqrt(3P/16SE) + c. The calc is the same for both raised face and RTJ flanges. However in this calc he has added the thickness of the grooves at both sides. This is what is misleading since what that answer provides is not the required pressure thickness as per ASME B31.3 (as the title suggests) but the required thickness that you would have to make the plate to give to a fabricator. A note explaining this on the calc would have been helpful !

Kevin
 
Kevin-

Yet another argument in favor of erring on the side of too much documentation on engineering calc's vs just enough for the original engineer to understand what he was doing. A few written comments as one works through a calculation can go a long way 20 years later when someone else is trying to understand the logic.

jt
 
We use single blinds and spectacle blinds to API 590 Specification. We use the Spectacle blinds where the piping is ridged where the spacer is left in and is used as place holder to eliminate the use of flange spreaders.
One thing to note is that the blind needs to be machined from stock and not a fabricated piece.

 
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