LP 95 vs. HP 100

[Luis H]

All this over-filling business sounds more and more like Russian Roulette!

Well…not exactly, in Russian roulette you have one chance in six of a bad outcome.

The odds of a cylinder failure are very small. You can thank an engineer and everyone in the manufacturing plant, the quality control personnel, the code enforcement personnel, etc, for the safety record.

I just find it a bit annoying when an uninformed end user thinks they know better, that the original designer, based on a very limited experience.



The whole thing of over filling, is that as long as you can guaranty that in all conditions you don’t exceed yield it should not be problem. Did you read all the conditional statements?

Even a few time into the low end of some yield should be OK…again more conditional statements.

To avoid yield, you have to not only know the pressure, but also the tank condition, wall thickness integrity, etc.


So...“Do you feel lucky today?”

 

[ucfdiver]

....snip...

Luis, you seem to be antioverfilling, but at least well educated in the subject, so I'll point this question to you. With a tank like faber which is sold overseas as 232bar, do you see an issue filling it to that here? Obviously you're anti overfilling, but it seems odd that the exact same tank has so many different ratings. I don't understand the DOT ratings well enough to wrap my head around how this can be.

 

[Luis H]

Luis, you seem to be antioverfilling, but at least well educated in the subject, so I'll point this question to you. With a tank like faber which is sold overseas as 232bar, do you see an issue filling it to that here? Obviously you're anti overfilling, but it seems odd that the exact same tank has so many different ratings. I don't understand the DOT ratings well enough to wrap my head around how this can be.

I am actually not at all anti overfilling.

I tend to slightly overfill my steel 72s on a somewhat regular basis. Most of the time I fill my steel 72 to about 2700 to 2800 psi (occasionally to just under 3000 psi). These tanks are stamped 2250 and most have recent “+” stamps that put them at 2475 psi. They are hydro at 3750 psi, so I always keep some safety margin to account for unknowns.

Keep in mind the newest one is from the early 70’s, many from the early 60’s. But, even do I bought most of them used, I do all my own visual inspections and I try to keep all my own hydro test data records.

I have also been working on photographic records of the interior of my cylinder to keep track from one year to the next how they are doing.

I wire brush clean the inside of all my steel cylinders (we are talking about 16 steel cylinders, 12 are steel 72 cu ft). And I am very careful when I inspect them.

Two years ago I got my PSI certification (in exchange for doing some consulting work for them), but I started inspecting and hydro testing cylinders in 1971…long before I got my engineering degrees.

I think that you can see that I am taking a very calculated and relatively very insignificant risk when I overfill.


So, what I object is for someone to take totally unnecessary and perhaps uninformed risk when there are substantially better options.

To tell you the truth…if someone in Florida want to take unnecessary risk it doesn’t bother me at all. I don’t mean to be selfish, but what would bother me is if the wrong person those get hurt and some politician decides that we need to change regulations…for something that could have been easily avoided.

If there is any kind of accident (even a motor vehicle accident) and even if the “over pressurized” cylinders are not directly involved, you are already violating codes, which could easily raised the attention to some lawyers (and even politicians).


About the Faber tanks, I am not really familiar with their construction detail, but I am willing to bet they are not the same tanks. They may even be the same alloy, but if just the heat treatment is different the material doesn’t have the same properties.

To meet the 3AA, the material conditions have to meet a very specific allowable stress, which is probably different to the European code requirements. What it looks like the exact same tank may be totally different just by the heat treatment or slightly different steel alloy. And as I mentioned on previous post, higher strength steel is not always better…there are always trade off.

If the stamping on the tanks is different… it means they are meeting different requirements.

You can see tanks that are stamp both DOT and CTC. That means they actually meet both US DOT requirements and Canadian requirements.


I will in general make note that some Europeans and perhaps their codes are more willing to take risk (they don’t have contingency lawyers waiting to sue everyone like we do), but in general that is not the case. Their codes (in most nations) tend to be in parallel and as safety conscious as ours.


Sorry for the long post…I hope it helps.

 

[Stu S.]

With a tank like faber which is sold overseas as 232bar, do you see an issue filling it to that here?

Hey, guys. I checked into it. The Faber distributor informed me that the European and US versions may look like, but can differ in ways that Luis mentioned, such as alloy, heat treat, and thickness.

 

[marchand]

I keep hearing mention of yield strength, while this is a very important number if you are looking at a specific cylinder, it is almost useless from a manufacturing standpoint. What we should be more concerned about is the minimum yield strength of these cylinders. This should be something just over 4000psi +x% -0% when they are new, and shouldn't change a lot if they aren't pressurized past the minimum yield.

I think it is fairly safe to say that the minimum yield is well above 4000psi because the tanks do get filled to 4000 psi on a regular basis and sometime the fill op gets distracted and they get up to 4200+psi. If the minimum yield was just above 4000psi it would only take one of these 4200+psi fills to lower its yield strength to 4000psi or less, and then the next time it was filled the yield strength would get even lower and lower until it blew up. Just one time past the yield strength of a particular cylinder and it will detonate after very few subsequent cave fills. The fact that a 3AA tank has not blown up yet, to me, proves the minimum yield is well above 4000psi, especially with Wayne filling to 4500psi.

 

[WD8CDH]

I was referring to the 5/3 hydroing of 3AA tanks vs. the 1.5 x WP of Exemption tanks. <snip>

The 5/3 hydro pressure of the stamped pressure rating of a 3AA tank is only 1.515 X the WP of the tank at it's +10% fill pressure.

2250 x 5/3 = 3750 hydro pressure

2250 x 110% = 2475 operating pressure


3750/2475 = 1.515 ............. only 1% higher. Not much difference.

 

[DA Aquamaster]

I keep hearing mention of yield strength, while this is a very important number if you are looking at a specific cylinder, it is almost useless from a manufacturing standpoint. What we should be more concerned about is the minimum yield strength of these cylinders. This should be something just over 4000psi +x% -0% when they are new, and shouldn't change a lot if they aren't pressurized past the minimum yield.

I think it is fairly safe to say that the minimum yield is well above 4000psi because the tanks do get filled to 4000 psi on a regular basis and sometime the fill op gets distracted and they get up to 4200+psi. If the minimum yield was just above 4000psi it would only take one of these 4200+psi fills to lower its yield strength to 4000psi or less, and then the next time it was filled the yield strength would get even lower and lower until it blew up. Just one time past the yield strength of a particular cylinder and it will detonate after very few subsequent cave fills. The fact that a 3AA tank has not blown up yet, to me, proves the minimum yield is well above 4000psi, especially with Wayne filling to 4500psi.

You are not wrong in thinking that way, except you are going from a specific case to a general case. All 3AA tanks are made to the same engineering specifications but they can and do differ from manufacturer to manufacturer, from model to model and from batch to batch. The idea of an engineering safety margin is to ensure that the weakest tank with the worst combination of defects in a particular batch will not fail in service between certification and subsequent periodic requalifications (hydro tests).

However, when you start filling to 3800, 4000, 4200 and now it seems for some people even 4500 psi, you are seriously messing with that safety margin - as in you have totally eliminated it. That weakest link tank could be work hardening its way to a failure that will occur not in the safety of a test tank as intended, but rather in the fill station of a shop, in the truck of a car or on the deck of a dive boat. That will be very problematic and wil most likely cost someone life or limb.

Luis brings up a good point that if an overfilled tank is a causative factor or an aggravating factor in an accident, the current loosely enforced laws or going to get seriosuly enforced and when it becomes a felony for me to fill my 3442 psi tanks to reasonable 3800 or so or to fill my steel 72's to reasonable 2800 or so, I'm going to be more than a little peeved at those who took overfills to unreasonable and ultimately fatal extremes that made it impossibel for the rest of us to continue with a reasonable and prudent overfill.

I used to shoot competitively and there were some long range loads I developed that got more an more accurate as they got faster and faster - creating the temptation to keep pushing them faster and flatter still, even though the pressure signs on the cases were saying "back off a bit". The results of yeilding to that temptation was always a separated case or even a rifle with a fractured or gas cut lug due to uncontained gas escaping. Those consequences pale in comparision to an exploding tank, yet I see the cave community inching forward to ever higher overfill on the basis that nothing has blown up - yet. That argument will remain valid right up until the millisecond where it isn't.

A 4000 psi fill in a 2400 psi tank is 167% and 4500 psi is 188% - far higher than the 120% overfills that were the norm 20 years ago, so saying there is 20 years of history of overfilling tanks is correct, but assuming that has any validity or relationship at the 4000 psi / hydro test level of overfill is a gross mis-statement of the facts.

Again, you have to keep the level of overfill at some percentage below the test pressure to ensure that a tank on its way to failing will fail a hydro test before it fails in service and there is just no guarentee that will happen that way when you are filling to or even in excess of the test pressure. If you are gonna do it, perhaps you need to consider having an annual or semi-annual hydro test and then also maintain longitudinal data regarding the temporary and permanent expansion that is occurring because at 4000 psi you are most definitely in the destructive testing business with your tank.

 

[PR_SMITH]

I'm with you there. If one keeps tickling the dragon's tail, sooner or later you WILL get bitten.

 

[D_B]

Thank you DA and Luis for your informed and informative posts

 

[Dive-aholic]

You guys are getting worked up over something that isn't truly happening. While marchand may be getting fills to 4000-4500psi, he has already stated it's usually a distracted tank monkey that's letting that happen. I don't frequent the usual cave shops in central Florida, but when I do go there 3600 is about all I'll get, maybe 3700. I don't know anyone that intentionally fills to 4000psi. So don't think the cave community is pushing things based on one person's comments.

Also, while no tanks blowing up in the past 20 years may not seem like much, how many tanks in cave country have you heard of that have failed hydro? Personally, I haven't heard of any. And if there are some, possibly those are the ones being filled to 4000psi?

Just some more things to think about...