Required Pre-Load: Flange with O-Ring instead of Gasket

[Slagathor]

I have a decent understanding of gasketed flange joint (m,y factors / Modern Flange Design 502 Bulletin / ASME Section 8) design, but for some reason I am having a hard time getting my head around an o-ring joint.

Lets say we have a 300# flange pair, flat faced.
Instead of using a gasket, we have an O-Ring, and the O-Ring groove is centered on the 'G' diameter for a Std Dimension Ring Face Gasket. (G = Equivalent Diameter at Gasket Load Reaction, per the 502 Bulleting / ASME methodology)
There is a groove machined in one flange, the other is flat.
We know that with Gaskets, bolt pre-load should be calculated for the specific gaskets used in the joint and the design pressure, etc. (m and y factor calcs provide a minimum start point here...but do not allow for relaxation, temperature effects, etc)

In the case of an O-ringed joint, however, what is the guidance for pre-load?

O-rings are self sealing in that when properly configured, the pressure they are resisting is the mechanism that makes them seal in the groove.
In this case, do we only need enough pre-load to stop separation? (separation would disrupt the self sealing functionality of an O-ring)
Such a minimal requirement (no high multiple of separation forces like you have with gasket m and y factors) will result in dramatically less pre-load than for a gasketed joint.
This would seem to be far too little pre-load, and I assume there must be some standard safety factor (2x, 3x separation force for example...)

Thank you for enlightening me...

 

[KevinNZ]

Class 300 flat face is not standard. These flanges are raised face or ring groove.

You can modify a raised face flanges for an o ring by taking off the gasket face (not all the face, leave it raised) In one flange cut the groove for the O ring.

 

[Slagathor]

My question was not about compliance with ASME B16.5 Table 12 Footnote (9) and flange standard. We have Specs to refer to for that. I can look up those details any time.

The question was about understanding proper practice with regards to BOLT PRELOAD when utilizing a face O-Ring as opposed to a gasket.

 

[btrueblood]

You just need sufficient preload to keep any separation from occurring.

Do realize that the o-ring, unlike a squished gasket, will move within its groove under applied pressure. This means that you may get some weepage/leakage if the pressure reverses (i.e. pull a vacuum on the line then expose it to internal pressure again) as the o-ring gets squeezed from the outside edge of the groove to the inside and back again; gaskets don't.

 

[Slagathor]

btrueblood,

Thanks for the input. I wanted to make sure I was not missing something obvious. Good reminder also on the pressure reversal issue.

 

[racookpe1978]

Not really sure if you have the concept.
A gasket stops the pressure because it does three things:
The internal fluid pressure pushes the gasket "out" - the gasket "shear force" against radial pressure has to be strong enough to resist movement far enough to cause a leak.
The internal fluid pressure tries to squeeze between the gasket face and pipe flange face 1. So, since gasket face 1 always has some ridges and notches ans gaps (and maybe a scar or three), the gasket has to be flexible enough to squeeze into those gaps and notches and create a strong enough (but flexible!) force to prevent any fluid (gas) from getting between face 1 and the gasket. If the gasket is very flexible (see nbr 2) then it won't be strong enough to prevent the blowout forces of nbr 1.)
The internal fluid force has to be resisted between the gasket and face 2 of the second flange.

An O-ring does this too, but it is squished out into the very-specifically-sized O-ring groove in ONE of the two flange faces. Many O-rings have a hollow center that actually pressurized the inside of the O-ring out against the three walls of the O-ring notch in ONE of the two flange faces. try to use an O-ring between the wrong groove sizes and it will fail. Get too small of a groove, and the o-ring gets cut going in. Too big a groove and the O-ring has too much space and it doesn't expand far enough into the three walls to make a seal.

The O-ring must be used the exact flange groove it is intended for at that pressure.