Gasket failure in compression

[prem1968]

When a flange is subjected to only external loads (bending moment) without any internal pressure, half of the gasket will be subjected to more compresssive force/pressure than the other half. ASME code convert the bending moment into effective pressure and apply all over the ring. However in reality part of the gasket will be subjected to more compressive force/pressure than the rest of the gasket part. Shouldn't we have to check the gasket for bearing failure due to this compressive force when there is only external forces(bending moments) applied on the flange as this case will occure in installtion condition. Could you please anyone clarify or tell me if there is any method available to check this gasket failure mode. Thanks

 

[Engdoitbetter]

I don't know much about gasket failures (so you shouldn't rely on my opinion really ), nor I know which ASME codes you are talking about, but perhaps local failure is not an important damage as complete failure. I tell you this since it is similar to stress concentration, where local plastic deformation under static load does not really harm material overall strength.
However I suppose that local lack of seal/failure can be an issue.

Hope it helps.

Stefano

 

[jtseng123]

the mechanism of bolting preload plays a roll that even the gasket is under uneven load from bending moment, it will not leak. Because gasket in the moment tension side is still under compression, while the compression side will be compressed a little bit more, but overall, it is still sealed. It is the preload to take care this problem.

 

[SnTMan]

prem1968, seems you are worried about overcompressing the gasket when assembling poorly made pipe spools?

Use spiral wounds with inner & outer rings, no problem.

You can overcompress sheet gaskets for sure, but I don't know of any commom calculations.

Regards,

Mike

 

[jtseng123]

check below to see if that makes sense:
You can calculate bearing stress from the bending moment( similar to the way calculate bearing stress at the skirt base ring or concrete due to wind or seismic), you can also calculate bearing stress due to preloading. Add together now you get the total compressive bearing stress. Then call gasket supplier what is the maximum compressive stress. Do note that gasket is not in elastic mode when bolting for service, that means it can take big compressive load.

 

[TGS4]

prem1968 - yes, what you refer to is a real problem/concern. Every gasket has a limit on compressive stress.

I would second SnTMan's suggestion - with spiral wound gaskets use inner and outer rings. These rings can handle significant additional compressive stresses.

 

[tothepoint]

Spiral wound gaskets may not be the panacea that you think. You have to fully seat them first, and many class 150 flanges do not have adequate bolt load to seat down to the guide rings.

There are several WRC Bulletins and PVRC projects that have looked at the effect of external bending moments on bolted flanges. Also, many ASME PVP (Pressure Vessels & Piping) research papers on this topic.

 

[ricklts]

All,
The gasket is supposedly evenly compressed on assembly, prior to pressure or external loads. As the load moment is introduced, (and I will exclude ANSI 150# flanges), the bolting away from the direction of moment will see increased stresses and the gasket at that point may be subjected to reduced compression if the the bolts are elastically deformed by some percentage. On the other side, a spiral wound gasket with both Inner and Outer metal rings, or with an exteranl only ring, will be subjected to gasket compression equivalent to the bolt elongation/decrease in compression to the opposite side. Having said all of that, in most ANSI/ASME flange designs, the bolt loading is normally in the range of 50-70% of the bolt yield strength (see Appendix S of ASME Sec. VIII), and is normally sufficient to offset flange rotation due to external bending moments.

Rick

 

[rneill]

ASME B16.20 currently requires that spiral wound gaskets (graphite filled) have inner rings unless specified otherwise by the purchaser. In my experience, most purchasers have accepted this change and now purchase with inner rings.