Cave Fills on LP tanks

[KevinNM]

Agreed, steel is really damn strong. There is no cycle testing required in the 3AA specification for steel tanks, so I couldn't guess the number; but would not be surprised if it were even greater than 10,000.

Aluminum is really damn strong, too. The 3AL specification for aluminum tanks itself requires "Three samples must be subjected to 100,000 pressure reversal cycles between zero and service pressure or 10,000 pressure reversal cycles between zero and test pressure...without failure" also "Three samples must be pressurized to destruction and failure may not occur at less than 2.5 times the marked cylinder service pressure".

Overfilled aluminum tanks make people who routinely fill other people’s LP tanks to 3600 very nervous. I tend to listen to them. The fatigue characteristics of Al is very different than the steel used for tanks.

 

[Rhone Man]

When I lived in the UK, back in the 80s people regularly bragged about filling their tanks to absurd pressures, like 4,500 PSI.

I'd be fine with that for a single dive if you really needed it, but definitely not something you would want to do routinely.

Apart from the strain on the tank, Lord alone knows what that does to a regulator where the first stage is trying to absorb pressures well beyond spec.

 

[Diver-Drex]

My question was what is the justification for recommending someone gets lp85s and overfills them instead of simply getting 100s. For someone buying cylinders that needs cylinders that can hold 100cf is there one reason, other than price, to justify getting Faber LP85s rather than FX100s?
- Whether we like it or disagree with it it is illegal to defeat the over pressure device and overfill cylinders.
- If you don’t live in Florida you may not get overfills. In which case the person needed 100cf and based on bad advice now only has 85 (77 at 2400).

I think the interesting question is: What is a safe fill pressure once you disregard the rating?

"Cave fills" have, from what I understand, crept up in pressure over the course of years. What's a cave fill now on a 3AA2400 cylinder? 3500 PSI? More? Some people say 90% of hydro. The hydro test is at 4000 PSI. Typical yield point is reached at around 4500.

The factor of safety is added to account for less than ideal conditions and wear during the service life of the cylinder. So what happens to a cylinder over time that decreases it yield strength that the owner and fill station operator have no idea about?
How many times has that 30 year old cylinder been tumbled? Heat exposure? Number of cycles at pressures exceeding the service pressure? Inspector miss judged the depth of a pit in an older thin walled cylinder? What condition are the threads in? How about the valve itself? So there is no clear answer until something fails.

72s are tested to 3750psi. Will it permanently deform or fail at 4000psi? Has something happened in the time since the last hydro that could change that?

Someone wants to overfill that is their business. Based on what I’ve seen on this board divers like PfcAJ are in a position to decide for themselves. But this advice is handed out regularly and it does not make sense for all but a select handful of divers. And if you’re asking the question you’re likely not one of them.

 

[PfcAJ]

Lp tanks are a dime a dozen down here in Fl.

If you can easily acquire them, get proper fills, and it’s not dangerous... seems like a pretty good reason to get em.

 

[victorzamora]

My question was what is the justification for recommending someone gets lp85s and overfills them instead of simply getting 100s.

Faber LP85s dive notably better than FX100s in SM, imo. Also, LP85s have 15% more gas for tanks that are the same size. They have about the same gas (slightly more) with a cave fill than rated-filled LP108s....which are notably bigger tanks.

For a lot of my diving, I was carrying 2x cave-filled LP121s and 3xAL80s. To not cave-fill, I'd have to add 2xAL80s to my already-laden setup. Plenty of diving gets done with MANY more tanks than that. And before you say it: My SAC rate is pretty good. I wasn't compensating for a terrible SAC by carrying more gas.

 

[tomfcrist]

Whether we like it or disagree with it it is illegal to defeat the over pressure device and overfill cylinders.

Illegal?
Source?

 

[Caveeagle]

My question was what is the justification for recommending someone gets lp85s and overfills them instead of simply getting 100s. For someone buying cylinders that needs cylinders that can hold 100cf is there one reason, other than price, to justify getting Faber LP85s rather than FX100s?
- Whether we like it or disagree with it it is illegal to defeat the over pressure device and overfill cylinders.

Typically around here, 85s command a higher price.

What law are you referring to?

And re "defeating over pressure device".. are you aware that European spec valves don't have burst disks at all?

The factor of safety is added to account for less than ideal conditions and wear during the service life of the cylinder. So what happens to a cylinder over time that decreases it yield strength that the owner and fill station operator have no idea about?
How many times has that 30 year old cylinder been tumbled? Heat exposure? Number of cycles at pressures exceeding the service pressure? Inspector miss judged the depth of a pit in an older thin walled cylinder? What condition are the threads in? How about the valve itself? So there is no clear answer until something fails.

72s are tested to 3750psi. Will it permanently deform or fail at 4000psi? Has something happened in the time since the last hydro that could change that?

Someone wants to overfill that is their business. Based on what I’ve seen on this board divers like PfcAJ are in a position to decide for themselves. But this advice is handed out regularly and it does not make sense for all but a select handful of divers. And if you’re asking the question you’re likely not one of them.

Tanks exploding left and right! Hard to believe we can keep all this hushed up.

Faber LP85s dive notably better than FX100s in SM, imo. Also, LP85s have 15% more gas for tanks that are the same size. They have about the same gas (slightly more) with a cave fill than rated-filled LP108s....which are notably bigger tanks.

...um, we would cave fill the 108s too. So comparing a CF 85 to a 2650 filled 108 doesn't make sense.

For any given cave dive, I am going to carry the maximum amount of gas that I can safely carry with my given gear. When I am > 1,000 linear feet away from surface air, I am not worried even one slight bit about the nominal pressure rating of my tanks.[/QUOTE]

 

[northernone]

Overfilled aluminum tanks make people who routinely fill other people’s LP tanks to 3600 very nervous. I tend to listen to them. The fatigue characteristics of Al is very different than the steel used for tanks.

Here's a data sample:

I've borrowed al80 tanks that were regularly cold filled to 3600psi.

I used with bare worm gear hose clamps as cambands. Tight on the cylinder when full. At 500psi I could fit 5 pennies inside the loose band against the tank had contracted so much... (checked if the band itself had loosened by placing it on another full al80. It was too tight to slide one. This having me conclude it was the tank diameter fluctuating)

Safe to say, I tried other tanks next dive.

Cameron

 

[BRT]

If scuba manufacturers listed an MBS for their tanks it would be easy. Pretty standard in the rope access/climbing/lifting/rigging industry. Wouldn't be too hard for the manufacturers to publish, but I bet they'd run into ratings issues with regulatory bodies. I don't know the regulations but I wouldn't be surprised if they were legally obligated to rate tanks at a certain percentage of their maximum.

Basically, tank ratings are the Working Load Limit, with an unpublished safety factor and an unpublished Minimum Breaking Strength. However because we don't know the MBS with any certainty, we can't calculate the safety factor. Basically, we've all just agreed that somewhere between the rating (WLL) and the hydro (less than MBS) steel tanks are safe to fill.

I think it would be interesting to see the differences between a DOT HP steel, and its European 300 bar volumetric equivalent tank, if there is any.

I might mention that on a chain or a rope the minimum breaking strength is only valid while the item is new. I have had someone I trust tell me that a steel 72 will take north of 7000#.

 

[BRT]

Here's a data sample:

I've borrowed al80 tanks that were regularly cold filled to 3600psi.

I used with bare worm gear hose clamps as cambands. Tight on the cylinder when full. At 500psi I could fit 5 pennies inside the loose band against the tank had contracted so much... (checked if the band itself had loosened by placing it on another full al80. It was too tight to slide one. This having me conclude it was the tank diameter fluctuating)

Safe to say, I tried other tanks next dive.

Cameron

Probably should use more hose clamps.