CMG Pipe Flange Gaskets

[Krausen]

Curious the forum’s opinions/experience with using CMG gaskets in pipe flanges in oil & gas industry. In my experience, CMGs should be avoided in all pipe flanges & typically are not even allowed in 300# pipe flanges & higher by most companies & consultants. However, company I work for allows them in 300# & higher pipe flanges as an option “if approved by engineer”. My understanding is this was allowed in company specs in cases like having existing slip-on flanges & replacing the gasket, where caution must be taken for using a spiral-wound gasket with inner ring (or prohibited from using a spiral-wound with inner ring per B16.20). I don’t agree with this being an allowable option in company’s standards regardless of situation, but wanted to see what the forum’s thoughts were?

 

[LittleInch]

CMG is Compressed Metal Gasket?

Seems to be a HX type thing whee it's difficult to get a lot of bolt stress?

If you don't like it don't approve it.

But if there is an occasional use for it then why ban it?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Krausen]

CMG = corrugated metal gasket. These are often covered with graphite or similar layer. Sometimes referred to as CMGC (corrugated metal, graphite covered) gaskets. Semi-metallic gaskets under B16.20. And yes, used more often with HXs.

I would never approve these in pipe flanges, but my claim is there should not even be an option for occasional use of these in any pipe flange (especially 300# & higher), due to the high failure rates seen in industry over time & the relatively low radial/shear strength of this gasket type.

 

[XL83NL]

ASME conf. proceedings contain some useful reading on this matter as well

https://asmedigitalcollection.asme....?page=1&q=Corrugated gasket&fl_SiteID=1000161

Huub
- You never get what you expect, you only get what you inspect.

 

[Krausen]

Correction to my above statement on industry code used for CMG/CMGC gaskets in standard pipe flanges. These are made per the dimensions in B16.21 for RF flanges instead (not B16.20)

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[BJI]

I wouldn't say the failures are overly common, and it takes a combination of factors (low bolt loads, high external loads, etc) to develop sufficiently high leakage rates to get blow out in these gaskets types. The CMCL gaskets are actually pretty good for radial shear, but that is generally more of an issue in heat exchanger applications where they are more commonly used.

 

[Krausen]

I wouldn't say the failures are overly common, and it takes a combination of factors (low bolt loads, high external loads, etc) to develop sufficiently high leakage rates to get blow out in these gaskets types. The CMCL gaskets are actually pretty good for radial shear, but that is generally more of an issue in heat exchanger applications where they are more commonly used.

Yes I think you’re correct on them being good for radial sheer, I think their weakness is instead the ability to recover from thermal cycling. Which makes me wonder why they are used so often in heat exchangers instead of pipe flanges? Likely due to the lower m & y values required to achieve adequate bolt stress with exchangers is my guess. But this can be achieved with a Kammprofile (Grooved Metal, Graphite Covered) type gasket just as well, if not better, for exchanger girth flanges. And Kammprofiles will recover better & also have a capacity for higher bolt stress if needed

 

[TugboatEng]

I think their weakness is instead the ability to recover from thermal cycling.

That may be a specification issue. The corrugated metal core is universally a 300 series stainless steel in these gaskets which is subject to creep at fairly low temperatures. I see the same problem with spiral wound gaskets in diesel engine exhaust systems. The manufacturer specs the highest rated filler, Thermiculite, but also specs 316 stainless for the wire when it should have been Inconel 718. In this application the gaskets take compression set which is a failure. We experience the same compression set issues with u-ring gaskets as well. The manufacturer chased the problem with more bolt load on the flanges. The leaks never went away.