Nubbins 6

[JohnBreen]

Hello all,

I would be very interested in reading informed opinions regarding the pros and cons of "nubbins" on exchanger channel body flanges. Can they be machined off and if so what would be the gasket of choice?

http://www.lamonsgasket.com/html/styles1.html

Can Appendix 2 calculations really predict the effectiveness of sealing after the nubbins are gone? I have had my fill of nubbins.

Regards, John

 

[jte]

John-

In my experience, "the only good nubbin is a machined off nubbin!" More seriously, most of the time I ran across nubbins on exchangers which were designed for double jacketed gaskets. If we were not upgrading the gaskets, we did nothing. If we were upgrading the gasket - usually to a kammprofile (GMGC) style - I would call for the nubbin to be machined off. Never had a problem. On the other hand, another engineer I knew made a point of not machining off nubbins when upgrading to kammprofile type gaskets unless the gasket surface was being remachined anyway. As far as I know… never had a problem with that approach either.

Might be a different story with a corrugated metal graphite coated (aka "Graphonics") gasket: Lamons specifically calls for the lack of nubbins when using their CMG style gaskets on heat exchangers… See Guideline #2 in

http://www.lamonsgasket.com/html/cmg-ex.html

jt

 

[rzrbk]

I agree that getting rid of nubbins is the right way to go, although in many cases you will find that the mfg has used the nubbin to reduce the available bolting and you will not have enough bolting to install a full width double jacketed gasket.

In several of these cases, we have used a CMG style gasket, after milling off the nubbin, with excellent results. Although it has fairly low seating stress, you will need to work with the gasket mfg to optimize gasket width with available force to seat the gasket.

For high pressure applications a spiral wound or kammprofile gasket works well, although they will have much greater seating stress.

 

[doct9960]

John,

I don't know if you came across this ASME paper about the sealability behavior of double jacketed gaskets installed on flanges with and without nubbin.

http://www.teadit.com.br/new/images/publicacoes/asme_pvp2005.pdf

The conclusion was flanges without nubbin sealed better and the cost of machining off the nubbin is justified by the increase in sealing efficiency.

 

[doct9960]

FYI, there are other good "stuff" about gaskets and bolted joints in the teadit website. Some of them are in English (Inglis)

http://www.teadit.com.br/new/geral/publicacoes.php

 

[JohnBreen]

Hello all,

My sincere thanks for your valued opinions and for your references. The nubbins are about to be history.

Thanks again, John

 

[SnTMan]

The only gasket I would use a nubbin with is a flat metal. Otherwise I design without, and if anybody insists I can then add one without chnging bolting. rzrbk is right, some designs will not have "enough" bolting when the nubbin is eliminated.

I would think they would be a huge pain if re-machining a flange.

FYI, API 660, 7th does not permit unless approved by the purchaser. Another good reason not to.

Regards,

Mike