Cave Filling LP versus HP tanks

[Compressor]

This is probably a dumb question but I will ask it anyway. Been following the thread below on cave filling tanks.
What is the difference between LP tanks and HP tanks. If we assume that LP (2600 psi) tanks can be "Cave Filled" to say 3400 (24% increase) why can't a HP tank (3442) be filled to 4268 psi (24%)(assuming compressor can handle it) ? Or maybe it is already and I don''t know.

 

[RyanT]

A lot of compressors just won't pump that high.

Second (and I'm speculating a bit here), I also suspect that the strength and metal fatigue characteristics of LP vs. HP tanks does not scale linearly. What I mean by this is just because a LP steel tank can easily handle a 24% overfill does not necessarily entail that HP tank would also continually handle 24% overfills. Perhaps someone with more knowledge of metallurgy can comment on this.

 

[northernone]

I'm neither, but I have caver friends who pump hp tanks to 5000 psi for extenuating circumstances. So I can confirm some people cave fill hp as well.

 

[Lobzilla]

Was wondering about the same. However, my PST104s (rated 2400 PSI) are actually a little bit heavier than my X8-130s (rated 3442 PSI) without manifolds and bands. Metallurgy may be different but the HP tanks are definitely not much beefier.

The PST104s have a long history of being cave filled without issues. It would be interesting to learn more about 'experiments' with HP tanks.

 

[Compressor]

Perhaps the HP tanks have either different alloy or thickness. I don't know.

 

[Storker]

Over here, tanks are 200, 232 or 300 bar (2900, 3365 or 4350 psi). Hydro test pressures are 50% above service pressure. As far as i understand, test pressure to service pressure is a bit higher in the US than they are in Europe. Maybe that's why I've only heard about overfilling that much here on SB.

I have caver friends who pump hp tanks to 5000

That's 345 bar. And your HP tanks are rated for 3442 psi (237 bar), right? That would be close to test pressure over here. If I ever visit your waters, I'd appreciate a tip about who it is who does that (so I can make sure to be a decent distance away when they're doing it).

And just FTR, non-ideal behavior starts to set in around 250 bar. At 300 bar, the pressure increase due to increased temperature is quite a bit more than what would be expected from ideal gas behavior. So if I ever were to overfill my tank like that, I'd make pretty darned sure I didn't leave it in the trunk of my car on a warm summer day.

 

[northernone]

Over here, tanks are 200, 232 or 300 bar (2900, 3365 or 4350 psi). Hydro test pressures are 50% above service pressure. As far as i understand, test pressure to service pressure is a bit higher in the US than they are in Europe. Maybe that's why I've only heard about overfilling that much here on SB.


That's 345 bar. And your HP tanks are rated for 3442 psi (237 bar), right? That would be close to test pressure over here. If I ever visit your waters, I'd appreciate a tip about who it is who does that (so I can make sure to be a decent distance away when they're doing it).

And just FTR, non-ideal behavior starts to set in around 250 bar. At 300 bar, the pressure increase due to increased temperature is quite a bit more than what would be expected from ideal gas behavior. So if I ever were to overfill my tank like that, I'd make pretty darned sure I didn't leave it in the trunk of my car on a warm summer day.

Thankfully it's a rarity.

...but of course they adopt no burst disk like your side of the pond.

Funny how often I hear the argument in the Americas that it's Europe that does "cave fills" as regular service pressure.

I get nervous diving around the 100$ Chinese carbon fiber tanks I'm seeing on some rebreathers. (That's another story)

A healthy respect for gasses under pressure is good for our longevity.

 

[Storker]

Funny how often I hear the argument in the Americas that it's Europe that does "cave fills" as regular service pressure.

We do? Interesting. Do those people also provide any basis for those claims?

A healthy respect for gasses under pressure is good for our longevity.

Amen.

 

[northernone]

We do? Interesting. Do those people also provide any basis for those claims?


Amen.

I've heard the story go a dozen (rough numbers) times "This LP cylinder is identical to the one they ship to Europe for 300 bar service and they just stamp it different here because the manufacturers are scared of law suits."

...no literature or sources.

 

[Charles2]

Most LP tanks are based on the DOT 3AA specification which is very conservative. HP tank specifications represent exemptions to the 3AA specification and as a result, allow service (working) pressures that amount to a slightly higher percentage when compared to burst pressures. There is ample anecdotal evidence regarding the safety of "Cave Filling" (50% overfill) LP cylinders. In reading one manufacturer's exemption specifications, it appears that their HP cylinder could sustain a 35% overfill and not exceed the same percentage of overfill to burst pressure that seems to be common on LP cylinders in 'Cave Country'. Of course, overfilling any cylinder means operating outside of the design parameters and accepting its concomitant risk. I do not recommend the deliberate overfilling of any cylinder. I also do not get too concerned about a slight overfill that cools to service pressure, nor do I get too concerned about a properly filled cylinder that lies on hot asphalt and rises to uncomfortable levels. Again, I do not recommend deliberate overfilling of any cylinder.