Luxfer Aluminium Tank, pre 1980
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jbrians
Contributor
Luis, (or whoever)
I know that the burst disc must be designed to blow at X amount over the tank's working pressure, but is there a formula? Years ago, at Morrison Springs, FL. a gentleman blew 2 disc in 2 days on my steel 72s. He seemed to be in a hurry and would forget to reduce the fill pressure after filling AL 80ies. Of course this would clear the shack out as people would scatter in a hurry when the water volcano would blow .except for the fill guy, who seemed to be quite used to it
At any rate, back then, most fill stations would hardly get much over 3000psi, so it makes me wonder why those disc gave way so easily.
Thanks,
Couv
The hydro pressure for any 3AA is 5/3 of the working pressure.
For a steel 72:
2250 * 5/3 = 3750 psi
The burst disk is supposed to let go at the hydro pressure with a tolerance of + 0, - 10%
In other words if it had a proper burst disk it should had let go somewhere from 3375 psi to 3750 psi.
The fact that it didn’t let go lets me to believe that it probably had the wrong burst disc.
In any case at 4000 psi that tank was exposed to 250 psi higher that a normal hydro test and 150 psi higher that a hydro test redone due to equipment malfunction.
That tank has seen some plastic (permanent) deformation. Plastic (permanent) deformation is what occurs when a metal is stressed beyond its elastic limit (yield point).
When steel is taken into the plastic deformation range it can work harden and become more brittle.
Is it likely that the amount of over stress caused to become very brittle… probably not? That is why you can permanently bend a coat hanger a few times before it fracture, but it doesn’t take many times and it will fracture. That is why I do recommend to have it retested (at the LDS expense) and if it fails they should replace them with some other nice steel 72’s.
Higher strength steel as the type found on 3AA steel tanks are more susceptible to work hardening and becoming brittle than the very mild steel found in a coat hanger.
The main purpose of a hydro test is to test for the material elasticity to make sure it has not become brittle. A brittle pressure vessel could in theory fracture like a hand grenade.
On the other hand if you stay below the yield strength then the number of stress cycle that steel can endure increases exponentially (as the level of stress goes down). I tend to laugh when some people talk about overfilling (to 90% of hydro test pressure) for 20 years is significant proof on anything…it is not, but this is a separate subject.
For a steel 72:
2250 * 5/3 = 3750 psi
The burst disk is supposed to let go at the hydro pressure with a tolerance of + 0, - 10%
In other words if it had a proper burst disk it should had let go somewhere from 3375 psi to 3750 psi.
The fact that it didn’t let go lets me to believe that it probably had the wrong burst disc.
In any case at 4000 psi that tank was exposed to 250 psi higher that a normal hydro test and 150 psi higher that a hydro test redone due to equipment malfunction.
That tank has seen some plastic (permanent) deformation. Plastic (permanent) deformation is what occurs when a metal is stressed beyond its elastic limit (yield point).
When steel is taken into the plastic deformation range it can work harden and become more brittle.
Is it likely that the amount of over stress caused to become very brittle… probably not? That is why you can permanently bend a coat hanger a few times before it fracture, but it doesn’t take many times and it will fracture. That is why I do recommend to have it retested (at the LDS expense) and if it fails they should replace them with some other nice steel 72’s.
Higher strength steel as the type found on 3AA steel tanks are more susceptible to work hardening and becoming brittle than the very mild steel found in a coat hanger.
The main purpose of a hydro test is to test for the material elasticity to make sure it has not become brittle. A brittle pressure vessel could in theory fracture like a hand grenade.
On the other hand if you stay below the yield strength then the number of stress cycle that steel can endure increases exponentially (as the level of stress goes down). I tend to laugh when some people talk about overfilling (to 90% of hydro test pressure) for 20 years is significant proof on anything…it is not, but this is a separate subject.
…..so in the case of Jbrians, the bust disc was for a higher rated tank, and most likely in my case the bust discs was either old, too low a burst pressure or the fill station was really cranked up-not likely as this happened in the mid 1980ies if memory serves me correctly.
Thanks Luis,
couv
Thanks Luis,
couv
The burst disk could have been corroded, over torqued, faulty, the screw/ cutter could have been faulty, etc., etc.
A buddy of mine, in PR back in the 70's, had a set of double US Divers manifold that I had to change the burst disc about 1/2 dozen times before I got it to work... I never figured that one out. It could have been something on the valve interface surface or a bad batch of burst disc... I never figured it out.
jbrians
Contributor
We know why it held now.
the burst disk was replaced with one from an AL80 early this year. I started a thread back then on VDH about having a tank hydroed assuming it was a 3000psi tank. It passed then but now with this HP fill, i'm now thinking it's luck is running out... and mine too.
Over the years, "I was told" the hot fills were causing disk failures...they'd pump them over so they'd cool down to the right PSI.
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