Tema Type 'R', confined gaskets, and nubbins 1

[BigTank]

Again, I'm re-designing a replacement S&T exchanger. This time it's a 2-pass with bolted bonnets on both ends, and must conform to Tema Type 'R' standards. I'm wondering what sealing surface facings are allowed on the bonnet flanges per the Tema standards.

Tema RC-6.31 says that "the min. width of peripheral ring gaskets for external joints shall be 3/8" for shell sizes through 23 inches nominal diameter..."

R-6.5 says "gasketed joints shall be of a confined type."

That and a general arrangement with design data are about all I have to work with to determine what sealing surface facing to use on the flanges and tubesheets.

My specific question is: can I use a 1/8" wide x 1/64" high nubbin on the sealing surface of the flange confined face to reduce the gasket seating stress and thus keep the flanges a reasonable thickness, or does RC-6.31 exclude this possibility? After all, RC-6.31 gives the minimum width of the gasket, NOT the minimum contact width. And then again, this is for EXTERNAL joints. Is a shell-bonnet or tubesheet-bonnet joint really an external joint?

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Fitter, happier, more productive

 

[SnTMan]

BigTank, your exchanger sounds like a fixed tubesheet type, based on bolted bonnets both ends, no?

If the process fluid can end up on the ground it is most certainly an external jont.

TEMA allows the use of any desired facing, including the described nubbin. I generally avoid them except with flat metal gaskets, but that's personal preference.

Regards,

Mike

 

[BigTank]

mike, thank you. you're correct...fixed tubesheet unit. thank you for clarifying what is meant by an external joint. in my section viii thinking, i would've thought an external joint to be perhaps the joint that contains the vessel boundary at the closest.

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[MrBTU]

You can either confine the gasket using a machined step on the tubesheet or bonnet, or by using a spiral wound gasket with an outer gage ring. TEMA is referring to outer confinement. Flange facing and joint design are then to ASME. We used to use nubbins with metal jacketed gaskets, (make sure the nubbing is against the "non-seamed" side of the gasket), but one gasket manufacturer told us that in tests they have not seen a difference with or without nubbins.
What are the press and temp?

 

[BigTank]

Tema=confinement, and ASME=facing/joint design: that's the way i interperted the interfacing of the codes as well. thanks for giving yours as well. i'm glad we agree.

tube side is 150psi @300°F

the gasket manufacturer didn't see a difference in sealing w/ or w/o nubbin?

i would only be using the nubbin to reduce hg and b/bo to keep the flange thickness down. i'm sure the sealing would be fine with these design conditions.

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Fitter, happier, more productive