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Subject: Gap Observed Between 6BX Flanges

Question:

We have a customer that is concerned about a stand-off on mating BX flanges. He has checked both the flange grooves and the ring joint gasket and both are within tolerances.
I have enclosed a photo of the flange make up. I wonder if the flanges are fully torqued this gap will be eliminated. Can you offer any advice or recommendation to resolve this issue?

  

Answer:

We don’t know your customers experience, but the photos show two flanges stacked up with a gasket in between without bolts. The gap shown is consistent with our experience on our test rack when making assemblies for test. 6BX gaskets are designed to compress radially before beginning to coin in the grooves as the bolts are tightened. Without bolts, we would consider a 1/8” approx. stand-off to be normal. Even with the bolts tightened to 50% of yield strength a small gap will often remain (API mentions in Spec 6A that a “small” gap is acceptable). API Spec 17D for subsea applications requires the flange bolts be tightened to 70% of yield to remove any remaining gap (this doesn’t always close the gap if you look closely).

Perhaps your customer has experience with flanges sealing with R ring gaskets and has not used BX gaskets before. If your customer is going to make a test, we feel sure the experience of tightening the connection will relieve his concerns.

For more information about ring gasket seal performance and flange make-up, see:

https://www.woodcousa.com/field_appr.htm

https://www.woodcousa.com/flg_make-up.htm

 

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