Tank storage question - in case of fire

[Wookie]

What vapor? What liquid in a scuba tank?

My bad, I should have stuck to the subject at hand. Exploding scuba cylinders have nothing to do with a BLEVE.

 

[elan]

 

[elan]

Why would the tank fail before the tank o ring/ valve seat... Both melt at relatively low temperatures...

Exactly...

 

[Jax]

Why would the tank fail before the tank o ring/ valve seat... Both melt at relatively low temperatures...

I believe he is saying, with the half-fill pressure (let's say 1500 in a 3000 tank), that it will take longer for the flames to heat the gas to burst disk pressure . . . and since the flames are also heating and melting the whole bottle, the bottle will fail and give way at a lower pressure than the burst disk will.

A full tank, at 3000 psi, will rapidly reach burst disk pressure, but the tank won't have been softened, yet.

 

[Jax]

Okay, another question:

Condemn all AL cylinders that have been exposed to heat over 350^oF. If the cylinder was heated, do not send it to hydro. It can pass hydro and explode in the dive shop!

Okay, I don't get it. Hydro pressurizes the tank to 5/3s of its rated pressure. Hydro passed. What makes it explode in the dive shop?

Anyone?

:huh:

How would a tank pass hydro and fail at the dive shop?

 

[Shftrdog]

A half filled tank will explode because as it heats up the pressure builds and the metal weakens due to the heat. Before the burst disk ruptures the tank is to weak and will explode. With a full tank the burst disk will rupture before the tank itself gets to weak, and an empty tank will not explode becuase it is empty.

 

[knotical]

Is it time for a mythbusters type of experiment?

 

[Jax]

Is it time for a mythbusters type of experiment?

Wookie's and Shftrdog's explanation makes sense. I'm sure there are lots of data about tanks in fires, too.

Now, about that "Heated tank can pass hydro and fail in the dive shop . . ."

 

[Wookie]

Wookie's and Shftrdog's explanation makes sense. I'm sure there are lots of data about tanks in fires, too.

Big cylinders (T's and K's) have burst discs and they go up spectacularly during a fire.

Now, about that "Heated tank can pass hydro and fail in the dive shop . . ."

Alright, let's try this theory. Only a theory, though I don't teach this class, nor am I a metallurgist, except in my spare time. So, the max temp to heat a aluminum cylinder is 350F, and steel can go to 450F before you have to condemn them. Lets say you heat aluminum hotter than that. Welders do it all the time because it makes the aluminum weld better. the welds look nicer. The reason is that it changes the lattice of the aluminum so that the molecular bonds line up. In welding on aluminum vessels you are only allowed to weld a joint a couple of times before you have to replace the aluminum. Thats because you overheat and embrittle the aluminum. All joints must be at least 6 " from another weld, except where welds cross. So, you heat the aluminum and embritle it. The cylinder passes hydro because the pressure is applied slowly. Or at least relatively slowly. The cylinder passes, and off to the LDS it goes, where Litehedded overfills it to 3500 PSI. No harm, no foul, we all fill our aluminums to 3500 PSI hot. He pulls it out of the water bath he filled it in, and Superlyte27 comes along and clumsily knocks it over. It hits on the shoulder where it had gotten too close to the sun on its last orbit and shatters.

DOT allows certain cylinders a 10 year hydro interval if you can hammer test them at every fill. I'm guessing that the hammer test (I don't know the hammer test procedure, as I have none of these cylinders) tests for that very brittleness.

Anyone have a better theory?

 

[Jax]

Alright, let's try this theory. Only a theory, though I don't teach this class, nor am I a metallurgist, except in my spare time. So, the max temp to heat a aluminum cylinder is 350F, and steel can go to 450F before you have to condemn them. Lets say you heat aluminum hotter than that. Welders do it all the time because it makes the aluminum weld better. the welds look nicer. The reason is that it changes the lattice of the aluminum so that the molecular bonds line up. In welding on aluminum vessels you are only allowed to weld a joint a couple of times before you have to replace the aluminum. Thats because you overheat and embrittle the aluminum. All joints must be at least 6 " from another weld, except where welds cross. So, you heat the aluminum and embritle it. The cylinder passes hydro because the pressure is applied slowly. Or at least relatively slowly. The cylinder passes, and off to the LDS it goes, where Litehedded overfills it to 3500 PSI. No harm, no foul, we all fill our aluminums to 3500 PSI hot. He pulls it out of the water bath he filled it in, and Superlyte27 comes along and clumsily knocks it over. It hits on the shoulder where it had gotten too close to the sun on its last orbit and shatters.

DOT allows certain cylinders a 10 year hydro interval if you can hammer test them at every fill. I'm guessing that the hammer test (I don't know the hammer test procedure, as I have none of these cylinders) tests for that very brittleness.

Anyone have a better theory?

So - am I correct in that when people "bake on" a finish, it is at 350^oF or more?

And when they do that, the metal softens enough to line up the molecular bonds, creating a more brittle aluminum . . . Interesting.