# What boundary condition should I use for the edge of a blind flange?

When testing a pressurized pipe with off-standard flanges, often times the materials available to fabricate a blind flange of the correct size are limited to what is around.

I've made it a habit of referring to Roark's formula's for stresses and strains, and going to the table for large deflection circular plates for these. I use the area inside the bolt circle. They have three boundary conditions available:

1. Simply supported (neither fixed nor held)
2. Fixed but not held (no edge tension)
3. Fixed and held.

Bolted connections are unusual - especially since I have a full face flanges. I've designed to both simply supported and fixed but not held. The simply supported works, the fixed but not held does not. However, due to the flatness out to the OD of the face, it seems like the rotation of the edge should be 0, so it should be fixed but not held. I'd like to make these thinner, so it would be nice if someone can demonstrate an accurate boundary condition (or perhaps that 80% of the fixed + 20% of the simply supported would be conservative via FEA). I'd like to see better research into the mechanics of design for blind flanges (which is rarely covered in the context of the Boiler and Pressure Vessel Code, or elsewhere).

The main question is: what boundary condition for the edge should I use?

## 1 Answer

Given the clamping force of the bolts, I would think that Fixed and Held would be appropriate.

One way to test this is to calculate the edge tension (I don't recall if Roark's has formulations for this) and compare this to the clamping force of the bolts (i.e., the static frictional force between the flanges due to the clamping will need to be greater that the edge tension).

I'm not certain that this case really meets the criterion of "large deflections" for the blind flange. The ones I've seen used are pretty beefy in thickness compared to the span and have almost no deflection.