European Sealing Association - Gasket inner bore intrusion question 1

[A8yssUK]

Hi there,
I am a gasket engineer for a gasket manufacturer but also part of a technical working group for the European Sealing Association (ESA), it's with this hat on I am posting this.

You see last year an important flange gasketing standard in Europe (EN 13555) was updated. This standard looks into the testing of gasket materials for various physical attributes, one of them being the maximum allowed compressive stress the gasket material can withstand.

Last year the gasket industry added a new limitation to the maximum allowed stress to include a limit to how much the gasket can intrude into the pipe inner bore after compression. We had heard from customers that problems can arise with pigging and/or flow disruption, so we set the limit to zero intrusion (based on a DN40 PN40 raised face EN flange). This in some materials significantly lowered the maximum allowed compressive stress, meaning the materials cannot be used in as many applications. Which many members of the ESA are obviously not happy about.

Therefore the ESA is currently looking into if we were a little too aggressive with this limit and I have taken it upon myself to ask some pipeline engineers what they think about it, please if you or anybody else has a bit of time could you let us know if you have problems with pigs getting stuck due to gaskets intruding into the pipelines and how frequent these problems may be ? do you think an intrusion limit into the pipe by a gasket is a sensible measure for a gasket material? Any help for me researching this issue from the pipeline engineers side would be a big help.

Thank you very much for your time.

 

[racookpe1978]

I would limit the depth into the pipe that the gasket material can extrude into different gasket classes:

For example,
Class A extrusion tolerance means 0.0 extrusion is allowed. (If you demand no extrusion, you live with the limits on the gaskets and gasket forces that 0.0 extrusion requires, or you change gasket materials and live with the requirements that decision forces on your material selection and pipe expense.)
Class B extrusion tolerance = 1-2 mm of depth into the pipe. This may be acceptable for some applications.
Class C extrusion tolerance = 4-6 mm into the pipe. For coarser pipelines or pipelines where a pig is not expected to slide perfectly through a perfectly round pipe that has no weld remnants at all.
Class D extrusion allowance means the gasket material may, or may not, extrude into the pipe at all. Extrusion is not measured, not limited, but not prohibited.

 

[gasketguru]

You must be talking about materials that can flow quite a lot - e.g. uniaxial PTFE, or perhaps displace such as rubber, as mostly at "normal" bolt loads and stresses a CNAF or graphite gasket would not spread into the bore. Note that the I/D of most cut joints is the same as the pipe O/D (N.B. = O/D over 12" of course), so you might displace into the area at the end of the pipe of a SOW flange, or a very soft material might displace in on a very low schedule pipe, but over the years I've had almost no complaints of serious pigging problems from gasket intrusion.

Note that since the Germans influence the EN standards, they traditionally make gasket O/D's to the edge of the bolt holes rather than the actual IBC dimension (thus EN1514-1 are a couple of mm smaller than the old BS4865 etc.) so the gaskets can slop about sideways a bit more, but in ASME the 16.21 gaskets are proper IBC outside size. There were some problems in spiral-wounds where the old API 601 sizes (now ASME 16.20) used to have some very narrow inner rings which would buckle inwards, but that is another problem altogether, and this standard has been modified quite a few times over the last 20 years or so BS 3381 never gave those problems).

The 13555 characterisation test can give quite a high stress on the typically used test rig but there are only a handful of flanges where you would achieve those stresses realistically of course

 

[A8yssUK]

Thank you very much for the replies!

gasketguru, yes this problem is only really in soft materials which have high creep, especially under temperatures i.e. high elastomer content and low grade PTFEs. Fibres/graphites will normally fail in other ways under load before spreading enough to intrude, and this is captured in other criteria of the testing.
On your final point, yes this is why we added the new criteria to stop this misreporting...but maybe we have been too aggressive. The old way would allow say a virgin PTFE material at much too high a stress, it would block 50% of the pipe... clearly nobody wants that.

I do like racookpe1978's suggestion of differing classes and will bring it up with the standardisation group.