Installing Spiral Wound Gaskets

Categories: Technical Information

The outer ring is not a compression stop
While some manufacturers call the outer ring a compression stop, it IS NOT. If your gasket has enough seating stress to compress to the outer ring, you essentially have a flat metal gasket, which gives you little to no recovery for when your plant cycles and the flange faces move apart due to thermal expansion.

Flange faces do not come down on the sealing element as flat as most people think.
Flange faces actually rotate a little bit. What do we mean by rotate? If you look at the flange faces of a raised face flange the raised face is actually pivot point to a lever. The end of the lever is where the nuts are actually clamping down together.

So what you do is you rotate your flange — i.e. you “bird mouth” your flange a little bit. Now they should bird mouth equally, but sometimes they don’t. One flange face typically bends to the other.

This leads to what we call dishing. Some people have called it cupping, but the technical term is dishing. That’s when the outer ring bends down.

Now there are a lot of conspiracy theories on this. Some people say, “Oh, It gets caught up in the bolt threads.” That’s not what happens at all.

What happens is since the flange is rotating, it rotates maybe a degree or two. Depending on the length of your outer ring, that determines how much deflection you see at the end of it.