Spiral Wound Gasket with External Pressure
Spiral Wound Gasket with External Pressure
(OP)
I am investigating a problem with a possible internal leak across a spiral wound gasket. After digging deeper, I found that the spiral wound gasket is oriented such that the pressure in on the outside. I realized that I could not recall ever seeing a spiral wound gasket with OD pressure. I attempted to research this on-line, but did not find any relevant information. I am hoping someone here has some experience in this area.
Just to clarify, I am referring to a standard spiral would gasket of the same configuration as a typical flange gasket for a raised face pipe flange or a head gasket in an API pump. In cross-section, the gasket is composed of wraps of 316 SS with a chevron cross-section. The filler between the wraps is flexible graphite composite. Normally, the pressure would be on the inside of this type of gasket such that the pressure would tend to spread out the chevron, forcing the edges harder against the sealing surfaces on either side. It seems to me that applying external pressure would tend to collapse the chevron shape inward and cause the sides to pull away from the sealing surfaces. But, this is just speculation on my part.
In my particular application, the gasket is subjected to approximately 2000 psid of 350 F gasoil.
Just to clarify, I am referring to a standard spiral would gasket of the same configuration as a typical flange gasket for a raised face pipe flange or a head gasket in an API pump. In cross-section, the gasket is composed of wraps of 316 SS with a chevron cross-section. The filler between the wraps is flexible graphite composite. Normally, the pressure would be on the inside of this type of gasket such that the pressure would tend to spread out the chevron, forcing the edges harder against the sealing surfaces on either side. It seems to me that applying external pressure would tend to collapse the chevron shape inward and cause the sides to pull away from the sealing surfaces. But, this is just speculation on my part.
In my particular application, the gasket is subjected to approximately 2000 psid of 350 F gasoil.
Johnny Pellin





RE: Spiral Wound Gasket with External Pressure
The spiral wound gaskets are rated to 2000 psig - but that is very definitely inside pressure to outside pressure (usually zero!). Each spiral wound "V" is nested from inside to outside so the "V" resists leakage from inside to out. The other way, across a small psid, a standard spiral-wound will help by eliminating bad fitups and slight waves in the two faces. maybe some small scars and imperfections as the gasket faces settle in and get compressed.
But NOT 2000 psid.
RE: Spiral Wound Gasket with External Pressure
Johnny Pellin
RE: Spiral Wound Gasket with External Pressure
The combination of temp and cyclic pressure loading during operation resulted in the joint losing load over time and leaking. The location was accessible during operation and didn't lend itself to maintenance tightening very easily. The solution we tried was to use Belleville springs to try to maintain load as the gasket went through creep.
If you have pressure on the outside I'd recommend you analyze the joint to see if pressure is compressing the gasket further during operation (so that it may be loose when off-line). I'm not sure if Bellevilles would help in every application, there is an active thread in this forum right now that some people have relayed their experience.
I would also evaluate your joint: Is there a gage ring to limit compression? Is the gasket in a groove that will be limiting regarding compression? I would also check what you should expect from your bolt load vs your gasket rating and whether the bolt load (and pressure load if pressure loaded) is a good fit for the gasket. Gasket manufacturer's change the winding density to accommodate different bolt loads dependent on gasket rating. I've seen specs that have load vs deflection requirements for spiral wound gaskets so that they're matched to bolt load.
RE: Spiral Wound Gasket with External Pressure
RE: Spiral Wound Gasket with External Pressure
Is your pump gasket simply a sealing element in a tongue & groove recess or does it have an inner ring etc. - If it was not fully compressed initially so the flanges come metal-metal, then additional external hydrostatic forces could have added to compression and thereby loosened the bolts perhaps? Worth also checking gasket to recess clearances - when compressed the recess can help support the outside of the windings and increase density. You need to analyse the joint as a whole of course.
RE: Spiral Wound Gasket with External Pressure
Thank you for the very helpful replies.
Johnny Pellin
RE: Spiral Wound Gasket with External Pressure
So, in addition to my concern about the external pressure, I am concerned about the fact that 7 gaskets with 6 spacers and two sealing faces create 14 potential leak paths in parallel. The compression on this stack is intended to properly compress the gaskets, but I still need to take all the measurements necessary to prove that the compression is adequate for this gasket.
Any other suggestions or comments will be welcome.
Johnny Pellin
RE: Spiral Wound Gasket with External Pressure
The part that troubles me, beyond the expense and extra leak paths, is physical stability of the stacks, given the apparent thin-ness of the inter-stack spacers. Do they come out nice and planar, or wrinkled and warped?
Mike Halloran
Pembroke Pines, FL, USA
RE: Spiral Wound Gasket with External Pressure
I would very much like to eliminate these and replace them with an o-ring arrangement that can fulfill the first and last of these purposes. It will take some work to come up with something to satisfy the second purpose. I may end up leaving one stack of these gaskets to act like spring and replace the second stack with an appropriately designed spacer ring with o-rings. The process temperature never exceeds 375 °F.
Johnny Pellin
RE: Spiral Wound Gasket with External Pressure
If you are talking to your local gasket manufacturer, then maybe worth asking if the stainless winding strip they use is available as "half-hard" material (as an alternative to fully annealed), as this should add a fraction of extra recovery to the gasket behaviour, or maybe try Inconel windings etc.
RE: Spiral Wound Gasket with External Pressure
Thank you for the suggestion. If I am unable to eliminate this stacked-gasket arrangement, I will certainly consider material changes to make it more effective. For these particular pumps, I have found three sources of inefficiency from internal recirculation. I believe that this gasket configuration is a significant source, but probably not the primary source of our losses. But, I plan to attack all three.
Johnny Pellin