Gasket Selection and Minimum Required Bolt Preload 3

[LargeTuna]

Hello All,

Although my background is in engineering, I'm definitely "green" when it comes to pressure vessels, so I thought I'd reach out to the experts for a little guidance. Basically, I have been tasked with getting a very large horizontal pressure vessel ready for re-certification. I'm mostly looking at replacing all of the blind flanges, gaskets, nuts, bolts, etc... The vessel is very large (~30k gallons) and is only actually filled and pressurized (with water @ 150 psi) during testing. Most of the flanges already welded to the vessel are 2" 150# SA 105 RF, although a lot of the blinds I have been pulling off are actually FF. So my first question is, shouldn't RF flanges also be mated with other RF flanges? Also, is it recommended to only use ring-type gaskets with RF flanges instead of full face gaskets? The only reason I'm considering full face is that they seem like they will be easier to align and keep in place during assembly. Also, how do I go about determining what type of bolting configuration to use (studs, bolts, lock washers, etc..)? Is the normal process to select the type of gasket that will best suit your system's needs first, then to calculate the minimum preload required to seat that gasket along with containing the internal pressure (Wm1)?

Thanks in advance. Any and all information is appreciated!

 

[SnTMan]

TGS4, yeah if the flanges cup, I believe it. So, assumnmg they are flat and properly aligned at rest, what cups 'em? Excessive bolt load. Of course then there are operating effects that may cause a leak as well. At that point what do you do? Tighten the bolts some more

Regards,

Mike

 

[TGS4]

My papers are here:

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1596083

This first paper outlines the theory and calculation basis.

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1600170

This second paper provides an application based on the theory.

For some of the Class 150 flanges, they have, by design, insufficient bolts. So, you need "excessive bolt stress" to generate enough bolt force to seal on the gasket. But, the flanges aren't "designed" for the high bolt loads.

You can get flange rotation without cupping. I would define rotation as what results from pivoting around the end of the raised face, which is in contact with the centering ring. It is caused by rotation around the hub-to-pipe connection. Cupping, on the other hand, results from deformation in the flange ring itself. Often it is observed as plastic deformation (permanent), but there is an elastic component to it - both of which is captured in the EN-1591 calculation methodology.

The general approach by those who troubleshoot flange leakage for a living, is that more bolt load is better. Yes, some flanges may deform (even permanently), but the failure mode of flange is leakage - it takes an awful lot of bolt load to actually "break" a flange. Some flange rotation can be good, because it increases the contact pressure. Likewise, some cupping can be good. It becomes detrimental when the flanges don't actually seal. Whether a flange seals on the spiral windings, or the centering ring (and actually, the spiral windings are designed to compress down to the height of the centering ring, so you need a very minimal amount of rotation or cupping to engage the centering ring). There's been lots of good papers at the PVP Conference by Dr. Warren Brown on the subject.

 

[EngAddict]

Thanks for the links.

I was thinking relatively small displacement. Assuming large displacement (permanent plastic) how much would be acceptable (even beneficial) and when would you classify as too much cupping (potential for undetected leakage) that should be replaced?