Double Your Burst Disk - Arguments For & Against, Please...

[wedivebc]

Ive seen disks go for 2 different reasons. One was an overfilled tank, the second was a burst disk not tightened to specs.(80-100 inlbs on a thermo valve)

Now assume you have a 40cuft deco bottle that has been filled with 100% O2 to 3000 psi and the burst disk goes. I wouldnt want to be anywhere near that when it happens. No matter how clean the valve is!! I could see how some people would want to double disk a tank like this.

-Tom

You have seen burst disks go under 2 easily corrected conditions.
The one not tightened to specs did not burst it leaked right, How would double disking fix that problem? It would not.
Don't overfill your tanks and there is no problem.

 

[hlsooner]

Almost any pressure vessel will be designed in accordance with a Leak-Before-Break criterion.

http://www.tech.plym.ac.uk/sme/Inte...ractureMechanics/StressIntensity/KTheory4.htm

The result of this type of design is that your tanks are made so that, if suffering from damage or corrosion, they will release pressure before catastrophically failing.

However, in a very rapid overpressurization like a fire, the burst disk is there to save you since Leak-Before-Break only works with small enough flaws in the pressure vessel or overpressurizations. Shoot a fully pressurized tank with high powered rifle, and Leak-Before-Break is not likely to save you.

 

[scubafool]

Shoot a fully pressurized tank with high powered rifle, and Leak-Before-Break is not likely to save you.

Please, PLEASE, do not drag that old nag back out of the grave that is underneath the barn. The very thought of it gives me nightmares.

 

[PaulChristenson]

Not commenting on the practice in particular, but DOT only governs cylinders "offered for transportation". I believe this refers to commercial operations conducting interstate transports. I don't think there is a legal issue for privately owned cylinders for personal use.

DOT MOT(Materials Of Trade) does cover it...440 lbs gross weight is allowed on a single vehicle

http://hazmat.dot.gov/pubs/hm200/mot42c.pdf

 

[Tommygun53]

You have seen burst disks go under 2 easily corrected conditions.
The one not tightened to specs did not burst it leaked right, How would double disking fix that problem? It would not.
Don't overfill your tanks and there is no problem.

You are correct, both of these situations were easily preventable. I do not double disk my valves. I dont see a need to. But I do know people that do,most of whom fill thier own tanks.

The burst disk that let go from improper torque specs actually burst,it did not leak first. We believe it was actually overtightened to the point that the metal ring cut into the disk instead of just seating against the plug.

-Tom

 

[fire_diver]

First of all for all of those using the law argument who cares about the law. Its illegal to overfill but that hasn't stopped that all the cave divers and everyone else who does it.
Secondly how often do tanks that are in Vis and hydro spontaneously explode when getting filled to a pressure that is something close to what it is rated to? Pretty much never.
Thirdly I HAVE seen burst disks hit in an overhead environment and go that way, but not just spontaneously.
Lastly if Europe isn't worried about thier firemen dieing from exploding tanks then I don't think we should be. I think the risk to you is much greater if you hit the disc on the back of your manifold against a cave ceiling then start losing all your gas and have to isolate and then exit a cave system than it is to a firefighter in this hypothetical fire.

Wanna go down to your local fire station and explain your views to the fire marshal?

FD

 

[fire_diver]

So do you walk your tanks to the dive site? I agree, and have, double disked tanks, but ponder this:

Faber, the manufacturer, warranties their tanks for 10,000 fills to 4,000 PSI. In no place where I get my tanks filled are they capable of filling beyond 3800 PSI. Now with that in mind, if you were to have a house fire, a fireman would really only be able to enter the house with temperatures reaching 150-200 degrees, at which point a rescue would be pointless. At 200 degrees, pressure would reach 1.34 times the pressure previously in the tank. I personally won't leave the tank in my home with more than 2640 in it, and that would reach 3432 psi. Now the average house fire reaches around 500 degrees at floor level (where most tanks would be stored). At this level, a tank stored with 2640 would be holding 5781 PSI, which most tanks would be tested to between 4000 and 5000. It would be high, but the probability of failure at that point is still not great.

Boy do you need an education! Try more like 1500 degrees in a house fire! And yes, we DO go in in those temps!

FD

 

[fire_diver]

Oh, and I can't forget, the metals used in burst disks are thin and fairly maleable (look at a burst disk when you replace them, its like a little bowl). When metals heat up, they loose some of their strength and integrity. I would bet money the burst disks loosen up and blow in a house fire situation rather than the tank blow, since there is more metal surrounding the tank than there is in two burst disks! Maybe there's a myth busters episode idea in all of this...

Well, you'd loose that bet! Fire department SCBA bottles are one of the main reasons for burst disks.

Fd

 

[PaulChristenson]

Boy do you need an education! Try more like 1500 degrees in a house fire! And yes, we DO go in in those temps!

FD

1500 degrees F...I don't think so...

Steel trusses start failing at 1000 degrees...

You might be going in...but we'd be backing out...Nomex thread that holds our gear together starts failing at a little over 900 degrees...

Firedex's outer shells degradation temperature vary between 700 for the cheap Nomex to 1300 degrees F for the top of line gear...
http://www.firedex.com/admin/docume...aspx?UID=462146A4-7D9D-499F-88D7-F7DC623304E2

 

[DiverBuoy]

I've only heard one burst disk blow ... it was on a dive boat ... its owner blamed it on poor maintenance. I'd say the only concern in increasing the burst point is if the rating on the (tankpressure + 10%) < BurstPressure ... that would cover a fire.