LP 80 Tank Question

[scubadan79]

I have an issue I need clarification on.

I believe I have a LP steel 80 cft tank. Here's some of the information on the neck.

Service Pressure 2400 PSI, 3AAA2400

My understanding of these numbers means that if my tank passes hydro, then it will have 80 cubic feet of air inside when pressurized to 2640 PSI. (10% overfill).

When I get it filled at a local dive shop, I always get random numbers on the fill always over 2640. Typically around 2800 PSI.

I recently geared up for a dive, but canceled it last minute because my dive partner felt uncomfortable with the change in conditions.

What alarmed me was when I connected my gear, my dive computer displayed a reading of over 3400 PSI.

Immediately I was freaked out. Am I wrong in assuming that the maximum pressure for this tank should be at 2640 or less?

It sits across the room from me, and I'm always wondering if it's going to blow a pressure relief disk, and why it hasn't already.

Please let me know if my thoughts on this subject are incorrect.

 

[John_B]

Did you get the fill in North Florida?

 

[ucfdiver]

LP tanks aren't rated to 3600psi? Wow, I never knew that. Guess we've all been living under a rock here in cave country.

Anyone who's all that concerned with overfilling should read how tanks are tested by the DOT. It's my understanding that OMS stated that their tanks can survive 10,000 cycles to 4000psi. Of course, they stamp "do not over pressurize" on each tank, which almost sounds like a tongue in cheek type of thing.

 

[mpgunner]

I also have LP tanks (LP 95's) and have been also wondering if the 95 applies to the 2400 rating or the +10 2640 rating.

Also, I often get them overfilled a "little extra". But if your uncomfortable with the current preasure then let some air out.

 

[SparticleBrane]

For LP tanks, the cubic foot rating applies to the + pressure rating of 2640psi.

No, your tank shouldn't blow up at 3400. Hydro pressure is 4000psi on LP cylinders; I've seen some filled to that for diving, and many take 3600psi frequently without blowing up in north Florida (cave country). Personally I'd just be glad they were filled to 3400, and go diving.

Just FYI, the burst disc hasn't blown because it's rated for 4,000psi on LP cylinders.

 

[DA Aquamaster]

The test pressure for a 2400 psi service pressure tank is 5/3rds the service pressure. For the math impaired that is 4000 psi. A burst disc is required to rupture between 90% and 100% of the test pressure, or in this case 3600 to 4000 psi. That is the rated burst disc pressure for a new burst disc, they tend to decline with age and cycles. So if you are getting cave fills to 3500-3800 psi be advised that the busrt disc is not designed for that. Unless you are using a burst disc rated for a 3500 psi service pressure tank you are literally asking for a burst disc failure and nothing will dump the contents of a tank faster.

Many divers in cave country will double up the burst disc, but in some tank valves this can actually cause a burst disc failure. If you want to go the enourmous over fill route, use a single burst disc rated for a 3500 psi service pressure as it will give you a burst pressure in the 4725 to 5250 psi range (special permit steel tanks are tested to 3/2 the service pressure) and be aware that in a fire the tank is likely to explode before any gas is vented by a properly rated and functioning burst disc.

One other thing to consider - a 2400 psi tank is tested at 4000 psi and testing has bene done to 10,000 cycles, but in a hydro test or test cycle, the pressure is maintained for only a few seconds - not continuously. Testing as to how well 2400 psi tanks stand up to 3500 psi pressure on a near continuous basis is essentially on-going with North Florida cave divers/guinea pigs.

With the availability of high pressure tanks rated to 3442 psi that are essentially the same size weight and buoyancy as LP tanks that hold a similar volume at a similar 3500 psi pressure, there is not as much argument for buying and cave filling a low pressure tank.

 

[oxyhacker]

Actually, the 3AA tanks are not required to be tested to 10,000 cycles. 3AL Aluminum tanks are, and steel exemption tanks are, but I will be damned if I can find a requirement for 3AA steels to be.
I know a lot of the OMS literature claimed theirs were, but it is hard to say if this means they or their manufacturer was doing extra testing on them, or if someone at OMS just didn't realize 3AA tanks weren't.

This is more an interesting bit of trivia than something to worry about. The 3AA spec has been around a long time, and is wickedly conservative. If the DOT doesn't feel they need the 10K cycle test, it is unlikely its because they can't take it. And many, many cavers have been doing their own informal version of the test in the field.

However it is worth noting that the 6351 bad alloy 3AL tanks WERE subject to the 10K cycle to TP test requirment, and yet this didn't immunize them from developing problems later on, even without overfilling!

One other thing to consider - a 2400 psi tank is tested at 4000 psi and testing has bene done to 10,000 cycles

 

[Luis H]

I posted this recently on another thread.

Basically from the S-N curve I read that ASTM 4130 (the most common steel used for 3AA) is not very susceptible to fatigue life loading. It is relatively ductile steel and therefore it is not surprising for it to have a fatigue life 20 times greater than aluminum or high strength steel.

I think the DOT would be far more concern about fatigue life on an aluminum tank or a high strength special permit steel tank.

In general it is hard to make a very hard strength steel that is also as ductile and fatigue resistance as some of the mid and lower strength material. Think about a wire coat hanger. You can fully bend it several times before it fails. Every time you bend the coat hanger you are going way beyond its yield strength, but it still maintains its ductility.

Note:
I am by no means suggesting that it is safe to overfill any cylinder on a regular basis. This numbers apply only to a perfectly good tank with absolutely no flaws. As I have mentioned in several other threads, there is an important reason why all structural design include some form of safety factor. In an ideal world we don’t need the cushion of a safety factor, but we are in a real world…were we find material imperfections and human error during inspections and testing.

I just looked up an S-N curve (stress versus number of fatigue cycles) for ASTM 4130.
ASTM 4130 is the steel that most 3AA steel tanks are made out of.

With a maximum repeated load of 49,600 psi the maximum number of cycles it can experience is in excess to 10,000,000.

The 49,600 psi is the hoop stress a steel 72 will experience at a fill pressure of 2500 psi.

Even with stresses close to those experienced at hydro, the fatigue life of a steel 72 is close to 200,000 cycles.

The above assumes a cylinder structure that has not being compromised by rust or heat.

All 3AA cylinders are design to the same test stress of 70,000 psi (based on an equation provided on CFR49). The stamped working pressure is 60% of the test pressure (60% is just the inverse of the test pressure of 5/3).

 

[spectrum]

Note:
I am by no means suggesting that it is safe to overfill any cylinder on a regular basis. This numbers apply only to a perfectly good tank with absolutely no flaws. As I have mentioned in several other threads, there is an important reason why all structural design include some form of safety factor. In an ideal world we don’t need the cushion of a safety factor, but we are in a real world…were we find material imperfections and human error during inspections and testing.

I think this goes full circle to the original poster's 3400 PSi incident.

Despite the practices in cave country this was a gross error on the part of the fill operator. If the cylinder capability was closely aligned with the specified fill pressure we would be having a very different conversation right now.

Engineering is great stuff and can predict a lot. The trouble starts when people begin to erode the safety factors and try to opperate things at the actual design limits. I'm not going to get into how significantly a "cave fill" erodes the margins but to suggest that someone size a cylinder purchase based on cave fills is probably not good advice especially when some may not realize that such fill will not be available to them. Board members span a lot of geography and experience levels. There are plenty of examples of products being used "off label" but this must be done from a position of understanding.

Locally I have seen operators go into panic mode after eroneously making a 200 PSI overfill. I'm talking about a fill that when cooled was perfect! Obviously there is some room for latitude.


Pete

 

[scubadan79]

Thank you all for your replies!

For one of the 1st answers, no I do not live in Florida but Los Angeles, CA.

My tank was bought in 2002, and recently passed it's 1st hydro. I guess it's been a while since I've brushed up on my tank physics.

Does anyone have a comment of long term damage at keeping a tank over filled?

Also I make it a practice to fill my tank after (or as soon as I can) I dive, so that my gear is completely ready to dive at a moments notice. I think that I remember, for storage about 1000 psi is recommended kept in the tank. I typically (recently that is), plan on diving at least once a month, hopefully more. Is keeping my tank filled (not precisely overfilled, though often it is) a bad idea, for long term maitenance of the tank's structure or valves?

Thanks again.