Knife-Edge Flanges
Knife-Edge Flanges
(OP)
I tried this post over in the Pressure Vessel forum, but all I heard back was the wax building up in my ears. So I thought I'd give it a try over here.
I've got a question that I haven't been able to answer, so I thought I'd throw it out to the experts in the field. I'll preface my question by saying that I am not doing this, I just am unsure why it isn't done. More curious than anything else. So here's the question (hopefully it isn't too idiotic): why shouldn't flanges that use knife-edge seals (such as con-flat flanges) be used in pressure vessel designs?
I understand that CF Flanges are designed specifically for vacuum applications, and the manufacturer hasn't tested them for pressurized applications. From one manufacturer's website: "Positive pressures are inherently dangerous and failure comes without warning. In a vacuum system, failure depends on various strength properties of bolts, clamps, chamber walls, welds, feedthroughs, valves, viewports, etc. The only safe answer is that the internal absolute pressure cannot exceed the external absolute pressure." The real crux of my question is where will the CF flange fail? Is it the copper gasket? I'm hoping for suggestions other than "Because the manufacturer said so," or "Review the Boiler and Pressure Vessel Code."
I tried to contact the engineers at the manufacturer, but I just got a lot of CYA statements instead of actual physical reasons that this type of flange is a no-no.
I've got a question that I haven't been able to answer, so I thought I'd throw it out to the experts in the field. I'll preface my question by saying that I am not doing this, I just am unsure why it isn't done. More curious than anything else. So here's the question (hopefully it isn't too idiotic): why shouldn't flanges that use knife-edge seals (such as con-flat flanges) be used in pressure vessel designs?
I understand that CF Flanges are designed specifically for vacuum applications, and the manufacturer hasn't tested them for pressurized applications. From one manufacturer's website: "Positive pressures are inherently dangerous and failure comes without warning. In a vacuum system, failure depends on various strength properties of bolts, clamps, chamber walls, welds, feedthroughs, valves, viewports, etc. The only safe answer is that the internal absolute pressure cannot exceed the external absolute pressure." The real crux of my question is where will the CF flange fail? Is it the copper gasket? I'm hoping for suggestions other than "Because the manufacturer said so," or "Review the Boiler and Pressure Vessel Code."
I tried to contact the engineers at the manufacturer, but I just got a lot of CYA statements instead of actual physical reasons that this type of flange is a no-no.





RE: Knife-Edge Flanges
-handleman, CSWP (The new, easy test)
RE: Knife-Edge Flanges
the practice may be something prohibited by your code due to bad experience in the past. it could also be something that you can prove (by analysis and test), particularly if it is designed to be fail-safe (lose one fastener).
RE: Knife-Edge Flanges
-handleman, CSWP (The new, easy test)
RE: Knife-Edge Flanges
RE: Knife-Edge Flanges
Here's a link to a PDF with a brief explanation:
www.
-handleman, CSWP (The new, easy test)
RE: Knife-Edge Flanges
RE: Knife-Edge Flanges
I guess my question to Dan would be, why would you want to use these flanges against internal pressure?
-handleman, CSWP (The new, easy test)
RE: Knife-Edge Flanges
If you are going to see positive pressures you need a better design than trying to use CF fittings.
RE: Knife-Edge Flanges
I can see how the CFs would be attractive in laboratory situations: high temperature rating, compactness and chemical resistance. My guess is that why they are avoided in industry mostly has to do with leakage as you mentioned above. I was just wondering what the other potential problems might be.
Dan
www.eltronresearch.com
Dan's Blog