Full vacuum case for ASME B16.5 Standard flanges

[ksquare]

can anybody tell how to select the rating of standard flange if there is Full vacuum case. there was no external pressure in pressure-temperature rating table. any help is appreciated.

 

[waskillywabbit]

You select the flange based on pressure-temperature for internal pressure.

Brian

 

[ksquare]

waskillywabbit,
I believe the standard flange should be able to withstand the full vacuum pressure without any failure. but is there any paragraph or code (evidence) stated that we just can selected it like this. is that rule of thumb?

 

[jte]

ksquare-

Perhaps you could look at it from a loading perspective: As long as you seat the gasket, the operating (external) pressure will tend to add to the forces closing the flange pair together rather than opening them up. All you have to do is look at what the longitudinal stress is doing.

Seems to me that if the flange pair can handle 285 psig internal, its not too much of a stretch to believe that it can handle 15 psi external... though I searched for any official statement in B16.5 and couldn't find any...

jt

 

[prex]

ASME VIII D.1 App.2 has a paragraph on external pressure design of flanges.
From the formulae of that paragraph it is easy to demonstrate that a flange designed for an internal pressure P will always withstand with margin the same pressure when external.
This result is quite obvious if you realize, as jte states with a different wording, that the bolts act as a support with external pressure, while their load add up to pressure load with internal pressure.

To give another reference:

11.5.5 Narrow face flanges subject to external pressure
If the flange is subject to both internal and external pressure it shall be designed for both conditions,
except that external pressure need not be considered where the external calculation pressure Pe is
less than the internal calculation pressure....

prex

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[rhg]

the gasket shall be selected for vacuum service

rhg

 

[ksquare]

rhg
What will be the gasket that you think suitable for vacuum service regardless of its temperature and pressure.

 

[rhg]

ksquare

See precision rubber O-ring. Also see Flexitallic catalog

rhg

 

[labastidasjose]

I have feced this question before, you need to check your flange using Appendix to for Div 1 vessels in particular if your design temperature for both conditions is different. B16.5 does not provide any rating for external pressure