Effective Gasket Width in ASME VIII-1 App2 Calcs
Effective Gasket Width in ASME VIII-1 App2 Calcs
(OP)
Hello there. I am trying to do the calculations in ASME VIII-1 App 2, however, I do not understand how to calculate "N" (based upon the possible contact width of the gasket). On what aspect of the gasket do we base it : the sealing element of the gasket or the actual surface contact of the gasket on the raised face?
Thanks.
Thanks.





RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
Regards,
Mike
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
Thanks
Antonio
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
Having said that, I have seen a type of arrangement (not often) which uses a nubbin against the outer ring of a spiral wound. For this arrangement the calculaton used the loadings for spiral wound portion plus the loadings for a solid metal gasket with a nubbin. [A nubbin is as shown in sketch (1c).] It was not straight additive, each portion was "weighted". Very complicated, I don't recommend it.
With a flat face as shown in sketch (1a), left side, N will nearly always be just the nominal gasket width.
Regards,
Mike
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
May help.
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
And SnTMan, thank you very much. That was the answer I was looking for. But this leads me to my other question and the reason for asking the previous question. Lets use a spiral wound for a 24 inch ANSI 600 flange as an example. The sealing element width for such a gasket would be 1.125 inches. But the “effective gasket width” of it turns out to be 0.375 inches!
This is not a problem by itself, however, when calculating the bolt load required to seat the gasket, Wm2, in App 2, they use the “effective width” instead of the actual width to calculate the required bolt load. (Wm2 = pi*b*G*y) Doesn’t this basically replace the actual gasket by a smaller, narrower one (by quite a bit), which is easier to crush, which in turns leads to very low bolt load?
So why would you use the effective width when calculating the bolt load required to seat the gasket and Hp during the operating condition, instead of the actual width?
Thanks
Antonio
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
The methods of Appendix 2 were developed in the 1940's (?) and certain parts of it are empirical. You may be able to find a reference to the original work in something like the Taylor Forge Bulletin "Modern Flange Design", I suppose the original work itself is still available somewhere, although I've never read it.
Similar to your question, why is the flange design bolt load W calculated from the average of Ab(min) and Ab(actual)? Don't know, it just is.
As a designer, nothing prevents you from increasing the bolting if you think it necessary. I often do so if I think it is advisable, one thing I have learned is when you're out of bolting, you're out of bolting.
Regards,
Mike
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
And App S says “However, a distinction must be kept carefully in mind between the design value and the bolt stress that might actually exist of that might be needed for conditions other than the design pressure.” I thought the point of designing something was to try and predict the conditions the object will be in during operation so that it will be able to withstand them. However here, we clearly know that the stresses will be much greater, yet we still design it to lower stresses?
Thanks
Antonio
RE: Effective Gasket Width in ASME VIII-1 App2 Calcs
I have been thinking this same problem. I have been using higher seating stresses for gaskets to get higher bolt loads because I know that bolting loads will be higher than App 2 calculations predicts.
However, increase of bolt loads doesn't affect thickness of the flange too much. So App 2 gives you acceptable flange thickness although bolt loads and stresses are underestimated.