Gasket cupping question

[airmaxximus]

Hi,

This is my first post here at eng-tips.com, so if I have missed any information that you need to help me, please ask and I will do my best to find answers.

We are in the midst of commissioning a new facility and as part of the hydrotest walkdown team we have encountered numerous cases of "gasket cupping" or "radial buckling" of gaskets.

Our primary concern is figuring out whether or not this buckling is acceptable. Beyond that, we are trying to understand what is causing the buckling; is it a result of 1)improper torquing 2)misalignment of the flange faces, 3)incorrect gaskets, 4) incorrect installation etc etc.

Please see the attached pictures and provide any input you may have. Again, if you require additional information regarding the specs we are following and that sort of stuff, please ask and I will post that information.

 

[airmaxximus]

additional picture

 

[btrueblood]

"Our primary concern is figuring out whether or not this buckling is acceptable. "

Do the joints leak? Did they pass hydrotest? That's the only real criteria for a gasket.

Radial buckling? Are the gaskets deflecting radially? I see axial displacements, but your photos don't show radial displacements.

Why does it happen? You are compressing a ring axially, the rubber has got to go somewhere, so it squeezes out of the joint radially. The gasket has a lot of stiffness radially, but very little axially, so it deforms (squirms or buckles) by axial displacements. The "free" or unconstrained rubber outside the seal faces does nothing for the job of of the gasket, which is to seal between the contact faces. You could cut the free rubber away and the joints will still seal; the gasket will stay put (if properly torqued) by friction forces alone.

 

[airmaxximus]

btrueblood,

Much appreciate your response.

To quickly answer your first two questions, of all the joints that have been tested, everything is ok. The untested joints, joints that were used as test limits during hydrotesting, have yet to be tested and we will have to wait until startup.

You are correct, we are not experiencing any radial buckling just axial deflection. My mistake.

Thanks for the input! Much appreciated

 

[unclesyd]

One can see this a lot with spiral wound gaskets where the OD is a couple of thousand's too big. While tightening the stud you can actually jack the outer edge of the gasket around. We would always let this go if it wasn't in critical application.

This problem got so bad at on time we actually had our sheet metal shop set up to notch out for the studs.

 

[airmaxximus]

Unclesyd,
Very good point and I believe that suggestion was brought forward. I will take a closer look tomorrow myself to see if that is contributing to our problems.

Also, you mentioned that you had your sheet metal shop set up to notch out the studs. Was this a result of a request from the client or were you just tried of dealing with the oversized gaskets.

 

[unclesyd]

We used thousands of spiral wound gaskets every year during overhaul of our polymer process lines. We were at the mercy of the gasket manufacturer so any problems with size or construction would cause many headaches. The size problem really hit us when our primary supplier went south. They had trouble form the get go with sizing and for us, leaving out the inner ring. Even by notching this caused us problems as the mechanic had to orientate the gasket instead of just dropping one in. The last time this showed up was about a year ago, one or two lot were oversize.

This problem doesn't normally affect the sealing ability of the spirals as in some applications we don't have the outer ring. With the proper flange a spiral wound gasket is more forgiving than one would think.

 

[JJPellin]

If you do a thread search, you will find that this issue has been discussed several times. There is very good information in some of these previous threads. We see cupping of the outer metal ring often. It can also be cause by any mismatch between the two flanges. We see it sometimes when a standard pipe flange is bolted up to a flange face that is machined integrally to a piece of rotating equipment. If the two raised faces are not exactly the same size or not very will lined up, the outer ring is more likely to cup. Or, simply over-tightening can result in increased cupping. It is not always caused by interference between the metal ring and the studs. I have never seen this cause a leak or any other problem. We have gaskets with cupped outer rings running in everything including high pressure hydrogen.

Johnny Pellin

 

[rconner]

I am likewise having difficulty understanding the exact "problem" as described (latest words used are "axial deflection", and of course the axis at least of the pipeline runs along the left to right of the picture). I do think I see in th epictures what appears to be some outer area/volume of gasket that has apparently extruded radially outward between the bolt holes. While it appear others may know or have assumed exactly what kind of gaskets are involved, could you confirm what exactly is the material is that has apparently extruded from within the original confinement of the flange faces, and either before or after may have also been somehow colored/painted? Is it just some kind of rubber, metal, something else, or some combination of materials etc.? Some more information may be helpful.

While I will not profess to be an expert in all types of proprietary flange gaskets and applications/installations, if it is just rubber I would think some degree of outward extrusion might be normal as others have stated, and I think the degree to which this extrusion occurs around a joint could be influenced by many factors, including but perhaps not limited to the following:
Specific proprietary or type of gasket (sheet or molded “rubber” of some type and hardness, is it “reinforced” or not, and if so with what and where is the reinforcement placed oriented on the area of the gasket etc.?)
The nature and mating orientation of the both flange and gasket contact faces (are they basically flat, smooth, serrated, and has any kind of lubricant been applied in assembly by the contractor etc. – if the gaskets are basically just rubber, I in general kind of like “clean and dry” assembly myself, though you might find many differences of opinion here among at least mechanics?)
The effective bolting force level, and uniformity, of bolt tightening accomplished by the installer (if the extrusion is reasonably uniform around the joint, one would be suspicious of at least uniform tightening!)
Presence of extreme beam load on the piping (e.g. due to settlement or transverse movement/deformations of the piping system under pressure?)

I hope you can tell by the nature of my questions the kind of conditions (perhaps among others?) that might influence the degree and location of outward gasket extrusion.

 

[airmaxximus]

All,

I just wanted to acknowledge your replies and let you know that I am tracking down as many answers to your questions as I can and will post later.

 

[airmaxximus]

Here is some information that I was able to take from the spiral wound gaskets we are using.

- 1/8" thick
- 316SS windings w/ flex graphite filler
- Carbon steel outer ring