Tank buying time: aluminum vs. steel

[rescuediver009]

FredT once bubbled...
hydros at 4000 psi.

Burst disk must be rated to fail above working pressure and below hydro, I'd generally use one rated for about 3800 for this bottle.

Remember, the burst disk as defined in US DOT rules is there to protect firefighters. It's definitely NOT there to protect divers as their presence in the rules predate both sport and military SCUBA diving by several decades. Much of the rest of the world does not use burst disks.

FT

I hate to be the one to break it to you but hydros are not done with the valve on so this eliminates the valve along with the burst disc. So no they do not have to be rated high enough for hydro. second hydro pressure is 5/3 of the working pressure o fthe tank. Wrong on both iounts good lad.

 

[nyresq]

Fred said the disk needs to blow BELOW the hydro pressure... this is so if the pressure build up from heat, or a dumba$$ overfills by too much it will vent BEFORE it reaches the hydro pressure and stresses the tank.
... read before pulling the trigger my friend... and by USDOT regs, what fred said about btw rated pressure and hydro pressure is dead on.... for the techno geeks it is a specific % range of the rated pressure as listed in the DOT regs.

Hated to be the one to break it to you Lad....

 

[nyresq]

4000psi IS 5/3 of a working pressure of 2400psi....

the 10% over fill (2640 psi)doesn't count toward a hydro calculation as it is an "overfill" pressure.

sorry Lad, both accounts were dead on....

;-0

 

[Genesis]

Steels are typically hydroed at 5/3rds working pressure.
Aluminums are typically hydroed at 3/2s working pressure.

The EXACT test pressure is either stamped on the tank (in the case of PST HP steelies) or is looked up depending on the material. For Exempt tanks (e.g. "E-9791s") it should be stamped on the tank. For the "non-exempt" ones its in the tables that the hydro shop has based on the material.

Burst disks must be, according to DOT rules, be installed to vent at +0/-10% of Hydro pressure. Installing ones that vent too low is not a good idea as they can blow before you get to the fill pressure. Installing ones that vent too high is dangerous.

As FredT posted, burst disks are there for firefighter protection more than anything else. In a fire, without them, a full tank can easily reach pressures that will cause the tank to rupture without a burst disk. If a firefighter is near the tank when that happens he will be shredded.

There is one other thing to be aware of. Aluminum tanks are quite dangerous in case of fire if they are partially full. AL tanks will be permanently annealed at about 350F; a temperature easily and quickly reached in a fire. The problem is that when this occurs the metal's crystal structure is permanently altered and greatly weakened. If the tank is completely full, then the burst disk will fail and all is well. If the tank is almost completely empty (e.g. 100 psi) then there is insufficient stored energy in the tank to produce a rupture and serious injury.

But a tank that is HALF full has more than enough stored energy to do serious damage, and its pressure will not rise high enough to vent through the burst disk. In a fire, such a tank is quite likely to fail explosively, even when properly disk'ed.

So if you're storing AL tanks, do so either nearly empty (e.g. 50-100 psi) or completely full. For older (6351 alloy) ALs, storing full is ill-advised due to sustained-load cracking problems; those tanks should be stored nearly empty. 6061-T6 alloy tanks have no history of having SLC problems, and can be safely stored full.

This is not an issue for steels.

 

[FredT]

rescuediver009 once bubbled...
Wrong on both iounts good lad.

Bite the bullet and read 49 CFR 173.301.F.3

"(3) For a DOT 3, 3A, 3AA, 3AL, 3AX, 3AXX, 3B or 3BN cylinder, from the first requalification due after May 30, 2003, the set pressure of the pressure relief device must be at test pressure with a tolerance of -10% to +0."


The tank is "know good" to hydro pressure, to put a plug in less than WP is an excuse to change plugs. Previous versions of the CFR did not contain the -10% limit.

FT

 

[rescuediver009]

nyresq once bubbled...
4000psi IS 5/3 of a working pressure of 2400psi....

the 10% over fill (2640 psi)doesn't count toward a hydro calculation as it is an "overfill" pressure.

sorry Lad, both accounts were dead on....

;-0

I knew that the specs for the hydros were 5/3, maybe I didn't word it right I mentioned the ratio because there are still the odd 2250 tanks and there are aloso the HP PST 3500 (my favourite... I guess FredT missed something.. i was never going that deep. Onlyu saying that there was 5/3 of the working pressure=-)

 

[ColdH20diving]

scubasean once bubbled...


One thing you might consider is that shops sometimes miss that you have a high pressure tank, if you have one. It is easier to get a low pressure (2450psi) tank filled to rated capacity than a high pressure (3450psi) tank. Its only the awareness of the LDS that makes that difficult. Some LDS's do it right every time, and others are hit and miss.

Good luck!

Sean

I found this to be the case and the reason why I switched from HP steel to low pressure steel tanks. With lp I always get a full fill if not a slight over fill.