It says right on my tanks - DO NOT OVERPRESSURIZE

[Ted Judah]

A year ago, I bought tanks for my wife and I. In that year we have used them on 16 dives and had them filled at 5 different dive shops. The pressures after each fill seem to vary wildly and is often overfilled.

Here are the markings on the tanks:

FABER MADE IN ITALY M8303 21/0154/ 073 02•21+
TC - 3AAM - 184/DOT - 3AA2400 DO NOT OVERPRESSURIZE REE67 BS85S


Below are the start fill pressures for each dive:

2400 lbs.
3400 lbs.
2600 lbs.
2500 lbs.
2500 lbs.
2500 lbs.
2200 lbs.
2800 lbs.
2800 lbs.
2800 lbs.
2950 lbs.
2400 lbs.
3000 lbs.
2750 lbs.
3250 lbs.
3525 lbs.

Am I missing something? should I be concerned? Should I request a certain pressure from dive shops?

 

[rob.mwpropane]

As I already posted, here in Italy we had a couple of cases of natural gas tanks on top of a bus exploding due to overfilling and overheating.
This was proofed by the technical investigation following the accidents.
The technical consultant on one of these accidents was prof Groppetti, a fellow colleague in my department, who discussed with me the details of his report before releasing it. So I have first hand knowledge of one of these two accidents.
Of course you need to BOTH overfill and overheat for this to happen...
One hint: at very high pressure, the compressibility factor Z plays a role. For a fixed volume, the increase of Z makes the increase of pressure to become more than proportional to the Kelvin temperature of the gas, as the perfect gas law would suggest.

Do you know the pressure rating on the NG containers?

 

[Superlyte27]

Uh, natural gas is flammable. Air is not. You do see the difference right?

While you need oxygen for a fire. You also need the other two parts of the triangle.

Your example is providing all three parts of the triangle.

You can heat a scuba tank with air in it all the way to infinity. It’s not going to catch fire. The tank MIGHT explode.

You can heat NG up a little bit and it will catch fire and thus explode. C’mon man. Use your head. Apples and oranges.

 

[100days-a-year]

Perhaps we should post speed limits on motorways soon. I'm sure that will reign in dangerous driving.
Maybe even make driving intoxicated a crime.
We can recommend against unprotected sex, particularly on dive meet ups while were at it.
People are gonna people and even the laws of physics are not safe from some of them.
Divers tend towards thrill seeking behaviors, overpressure is not high on the list as far as danger goes.

 

[Angelo Farina]

Do you know the pressure rating on the NG containers?

They were rated at 200 bar and pressure-tested at 300. One of them gave up at an estimated pressure close to 400 bar.

 

[Angelo Farina]

Uh, natural gas is flammable. Air is not. You do see the difference right?

While you need oxygen for a fire. You also need the other two parts of the triangle.

Your example is providing all three parts of the triangle.

You can heat a scuba tank with air in it all the way to infinity. It’s not going to catch fire. The tank MIGHT explode.

You can heat NG up a little bit and it will catch fire and thus explode. C’mon man. Use your head. Apples and oranges.

Wrong. There is no oxygen in a tank containing natural gas. It must first explode (due to ripture of the tank), then it can catch fire.
So the rupture of the tank was caused by the excessive pressure, not by the fire...

 

[inquis]

They were rated at 200 bar and pressure-tested at 300. One of them gave up at an estimated pressure close to 400 bar.

Assuming it was 200 bar at room temperature (21 C), to get to 400 bar would require the tanks to reach a temperature of 315 C. Did the report give any indication how that might have happened? I'm skeptical that solar loading would do it.

 

[Superlyte27]

Assuming it was 200 bar at room temperature (21 C), to get to 400 bar would require the tanks to reach a temperature of 315 C. Did the report give any indication how that might have happened? I'm skeptical that solar loading would do it.

Thanks for the post. I didn’t want to do the math. But yeah, exceptionally hot that day. Lol.

 

[Angelo Farina]

Assuming it was 200 bar at room temperature (21 C), to get to 400 bar would require the tanks to reach a temperature of 315 C. Did the report give any indication how that might have happened? I'm skeptical that solar loading would do it.

The tanks were overfilled at low temperature (roughly around 250 bars) and then left under the sun in July, being black-painted.
Due to the compressibility factor Z being well below 1 below 250 bar and well above 1 at 350 bars explains the more-than proportional pressure increase when heathing...

 

[mcohen1021]

I'm coming into this conversation a little bit late, but funny that just 2 weeks ago on the aggressor they mentioned the term "hot fill..."

The simpleton explanation that I received was that they overfilled the tank to about 3200, and when the tank cools off it would be right at 3000

 

[inquis]

Due to the compressibility factor Z being well below 1 below 250 bar and well above 1 at 350 bars explains the more-than proportional pressure increase when heathing...

Thanks for that. Iteratively evaluating using the natural gas Z curves, 10 liter cylinder, 250 bar initial pressure, and 21 C (70 F) initial temperature, I can confirm the temperature required to reach 400 bar is 83 C (181 F) -- so not totally impossible.

If a similar LP tank of air was cave-filled (250 bar/3600 psi) and heated to the same temperature, the pressure would rise to 329 bar/4770 psi (considering compressibility), which is about 8% over hydro pressure. Good thing I use galvanized LP85s and not black ones.
Angelo, thank you for persisting. The difference when using a real gas is more than I thought it would be.