Amen brother.Originally Posted by cavediver27
Welcome to the Cave Diver's Forum.
Amen brother.
Chris Hill
www.ocda.org
"Every man dies, but not every man really lives." William Wallace - Braveheart
Yup, the first time you convert to no-blows you have to pull the original copper disc off.
Dave Grimm
Now there was only me and this timeless, eternal cave. I felt part of things, a tolerant guest.
by Rob Palmer from "Deep Into Blue Holes"
There is a huge difference between plugging or double discing versus installing single burst discs of a range higher than the rated value from a fireman's perspective:
Plugging a burst disc port with a bolt or molten lead does not allow for the escape of gas in a fire. This is potential bomb for sure. Given a fire where the heating of the tanks is predominately due to convectional heat exchange, a 5000psi rated burst disc will blow on a set of doubles filled to 3000psi at room temperature, at approximately 400 deg F (assuming minimal expansion leading to an increase in volume) which is still 200 deg F below the melting point of lead and much much less than that of a steel bolt or disc (1200 deg F or more). Hopefully, the nylon seats in the valve and viton or rubber Orings would melt somewhere around 370-660 deg F and release the gas through the valve stem, neck threads or manifold.
Moral of this story: If you plug or double disc your tanks, there is no safe haven if they get caught in a fire and are heated faster than the lower melting point materials can fail and harmlessly release the tank constituents.
What is the blow pressure of double discing.....? Is it as simple as serial addition...? i.e. 2 x 5000psi discs = 10K blow?
Nobody really knows. You could do some tests
Nope. It is way more than that so the tank would fail long before the double disks would.
I like the nylon washer & SS disk. In the event of a fire the nylon washer would melt at a fairly low temp (approx. 350-400 F) and it would get soft and begin to leak before that. I'm hoping not to encounter a fire but I sure don't want tanks letting go while guys are trying to put the fire out.
Scott
It's a nice theory but I don't think it wil hold up in actual practice. The nylon washer in most old style burst disc assemblies is intended to seal between the valve and disc. The disc itself is actually supported by the bolt in the burst disc assembly and over time the pressure in the tank will seal it very tightly to the face of that bolt.
If the nylon washer melts, you may get a comparatively minor leak past the bolt and the threads, but it won't vent the tank all that quickly and it may not prevent an explosion.
If the heat source is on the base of the tank and the valve is staying cooler, then again the tank could fail before the nylon washer even melts to start even a small leak.
The truth is, if you are using a SS disc or are using a burst disc assembly that ruptures significantly over 5000-5500 psi in a tank uperating at 3442-3600 psi fill pressures, you are pretty much not using any safety mechanism at all in the event of a fire. if that is the case, at least have the courtesy to warn the fire fighters who show up.
Personally, I've never had issues with a 3000 psi service disc (4500-5000 psi burst pressure) failing at 3600 psi fill pressures. I did have a failure this spring right after the initial fill, but ironically enough it was a brand new burst disc (just VIP'd the tank and serviced the valve) intended for 3500 psi service.
Actually the experiment could be devised using hydraulic fluid to safely achieve the pressure needed to blow a SS "no blow" or a double copper discing. I have several old valves and manifolds that could serve as test articles.
All the copper discs I have ever seen after being placed in use (even ones that have not failed) have a concave deformation in the face of the copper disc suggesting a degree of malleability prior to failure of the disc due to compressive force. Double discing may limit this malleability effect over the small surface area of the escape orifice. Material science is not my background but I suspect a great deal of pressure would be required to deform a double layer of copper over the tiny orifice. Most likely other "soft goods" type components in the valve will have failed prior to this occuring.
It is a good idea, but it would be adviseable to try it with various burst disc assemblies and do multiple trials.
I suspect what you'd find is a wide degree of variability.
Hydraluic fluid, water, etc, just reduces (almost eliminates) the expansion that would occur with compressing a gas. But even that would not be a big deal if the volume is low and nothing else in the valve or system fails first.
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I agree that a concave shape appears on a used burst disc. I am pretty sure you get that effect with permanent deformation on the very first fill as the disc itself is by no means strong enough to resist the pressure without bending. After that, there is probably some very minor flexing as would occur in a diaphragm, but not much movement overall as that would rapidly result in a failure of the copper disc within a small number of cycles.
A higer rated SINGLE burst disk is much safer than doubling up two lighter burst disks.
There is a sharp edge on the burst disk "nut", for lack of a better description, that is designed to cut the disk at a specific pressure range. When you add a second disc you "cushion" that second disc from that cutting edge. At that point you may as well just plug it which of course is not a good idea.
It's bad luck to be superstitious.
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