m & y factors in bolted connection design

[SilverMani]

My understanding of m & y factors of gasket:
1) "Factor m": In order to ensure leak proof flanged joint in operating condition, residual compressive stress in gasket shall be minimum "m" times internal pressure.
2) "Factor y" is interpreted as minimum gasket seating stress required to create an initial seal with essentially no pressure in the vessel.

Question:
If "m" factor take cares of sealing of joint in operating condition, what is the necessity checking sealing at no pressure condition using factor "y"?

 

[NBrink]

Certain gaskets require very high seating stresses to "flow" into the seating surface.


That said, I think you'll find the m&y factors to be of limited use beyond initial design.

Nathan Brink

 

[SnTMan]

SilverMani, to add to NBrink's post, the bolted joint must be designed for both conditions, operating and gasket seating. It is not always obvious which will govern.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[rtmorgan]

Both Lamons and Flexitallic's Gasket Handbooks have pretty descriptive explanations of 'm', 'y', and even 'Sg'. These are all pretty much design inputs for the minimum loads required to seat and seal a gasket under design operating conditions. If you are Sec.8 Div. 1, take a look at Appendix S, Div. 2 is slightly different but PCC-1 also gives some guidance.

 

[DSB123]

If you want a good reference have a look at Tuckmantel's book (available from Kempchen) on gasket sealing - optimum gasket seating stress/ minimum seating stress/maximum seating stress.

 

[SilverMani]

Thanks for your response....
I would like to elaborate my query with example.
A flanged joint, with spiral wound gasket, is subjected to an internal pressure of 1.5 MPa.
1) While designing for operating condition, if compressive stress in gasket is maintained at "m" times internal pressure (i.e. 4.5 MPa); it won't allow leakage.
In that case, flange will be designed for bolt load corresponds to compressive stress in gasket (i.e. 4.5 MPa) plus hydro-static end load due to internal pressure.
2) While designing for gasket seating condition, minimum seating stress of 69 MPa is required to be considered that to avoid leakage at almost no pressure condition. {which is contradictory with first condition}...

Now the query is,
During operation, joint will be leak proof by maintaining just 4.5 MPa compressive stress on gasket. Then what is the need of considering 69 MPa as min seating stress??

In other words, in this situation, gasket seating condition will govern bolt load & subsequently flange will be over-designed.

 

[SnTMan]

SilverMani, no the flange will not be overdesigned, because if you cannot provide the needed initial load to seat the gasket in the first place you cannot be assured that it will not leak in operation.

The bolted joint must be designed for both conditions. This is explained fairly well in Apx 2 and elsewhere.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[NBrink]

Once again, M&Y aren't REALLY that applicable to a leak free joint. I've never met the spiral wound that will seal at 4.5 MPa operating stress, in fact more than one gasket manufacturer has suggested bolt pre-loads to me sufficient to maintain 10,000 psi (69 MPa) at operating conditions.

You're not designing for a leak free joint at no pressure, you're designing for a minimum seating stress. The gasket actually needs to be deformed to work. That's how it seals, your metal/graphite/Teflon/etc deforms to match the seating surface.

Nathan Brink

 

[A8yssUK]

Already well answered but I'll just put it in more simple terms, the y stress is the stress required to get the gasket to initially work. If you don't squash it enough it doesn't press into the surfaces enough to form a seal or isn't dense enough to prevent the fluid from simply going through the material. Spiral wound gaskets generally need a lot of squashing due to having metal components.
The "m" is the the factor used to work out the very bare minimum you must keep on the gasket while it is in service to retain a seal.

In your example you need a minimum 69MPa effective gasket stress at install, then during service as long as it remains above 4.5MPa the gasket "should" not blow out. (I wouldn't try that...)

That is the very basic m and y theory, which basically works for 75% of cases, everything else you really need to use better calculations... The EN-1591 is the current top of the range, but as you can imagine much more complicated.