Overfilling LP Steel Tanks -- How bad is it?

[SparticleBrane]

Holy smokes, guys. Here we go again - ignorance is bliss until it goes "BANG!"

Your 3AA and 3AL cylinders are designed for unlimited fills AT SERVICE PRESSURE (or for 3AA, the 110% fill, if marked with the "plus" sign). Once you go over service pressure , you are in danger of entering into the fatigue-cycle life of the cylinder. There are a limited number of cycles that a cylinder can withstand at test pressure (typically 5/3 times service pressure) - that's how they're designed. When you exceed service pressure, you are on your way up the cycle life curve to that limit, (at burst pressure, there is only ONE cycle!). For example, paperclip works in the range of a few pieces of paper indefinitely, but try bending it back and forth a few times and see what happens.

You will NOT see this in hydro!! If you cycle your cylinder to higher pressures and fatigue it beyond it's cycle life, the result will be a sudden and catastrophic failure - NO WARNING. I don't care what any of the "armchair experts" have said here. I do this for a living, I've watched cylinders fail after exceeding cycle life - it isn't pretty. In fatigue failure the cylinder has lost the properties that were built into it. It will most likely fragment - something that it's not supposed to do. Failing the hydrostatic test is a ductile failure. Fatigue failure is not.

If you're an engineer, you can go find some of this information in the design spec's in 49 CFR, section 178, or download and read the Special Permits for the E/SP cylinders. Otherwise, keep your speculation to yourself. Most of what I read here is so far off base I really didn't even want to reply.

"Leadking" got it right. Manufacturers do both cycle testing AND burst testing. These are two separate and distinct tests.

Unless you have read AND UNDERSTAND the specifications, you are playing Russian Roulette.


Darrell
CTC Seminars
(We teach cylinder regulations in more than a dozen countries.)

On the other hand, years and years and years of anecdotal evidence says the risk of overfilling LP cylinders to 3600 is quite small.

 

[bleeb]

On the other hand, years and years and years of anecdotal evidence says the risk of overfilling LP cylinders to 3600 is quite small.

If you believe that that type of catastrophic failure is possible with LP steel tanks, or are even just concerned about it, what level of "small" probability are you (or the shop worker doing the fill) willing to accept? Consider that it might not be possible to reduce this risk to zero, and diving isn't a zero-risk activity anyway.

One example of quantifying this risk: From the previous sub-discussion about the number of overfills, it sounds like "years and years and years" amounts to significantly less cylinders or fills than one Al 6351 explosion. The latter was enough of an issue for regulatory authorities to get involved for society as a whole, but maybe for an individual's personal compressor, they're willing to accept more risk.

 

[jmorriss]

If you believe that that type of catastrophic failure is possible with LP steel tanks, or are even just concerned about it, what level of "small" probability are you (or the shop worker doing the fill) willing to accept? Consider that it might not be possible to reduce this risk to zero, and diving isn't a zero-risk activity anyway.

One example of quantifying this risk: From the previous sub-discussion about the number of overfills, it sounds like "years and years and years" amounts to significantly less cylinders or fills than one Al 6351 explosion. The latter was enough of an issue for regulatory authorities to get involved for society as a whole, but maybe for an individual's personal compressor, they're willing to accept more risk.

Considering that for all intents and purposes I have a better chance of being killed in a car accident on the way to the dive shop/boat/quarry/ 7-11 and that everyday risks appear infinitely higher....many of us have deemed it an acceptable risk.

 

[JDMerk]

Does anyone have a picture of a steel SCUBA cylinder of any kind that failed due to reasonable overfilling (ie not 10000psi or physical damage)?
I spoke with the owner of a hydro facility in the area that handles most if not all of the local hydros and he said he has never seen a steel SCUBA cylinder of any kind explode.

Im not advocating either argument here just looking for pics of the aftermath if one has ever actually let go under pressure.

 

[jmorriss]

Does anyone have a picture of a steel SCUBA cylinder of any kind that failed due to reasonable overfilling (ie not 10000psi or physical damage)?
I spoke with the owner of a hydro facility in the area that handles most if not all of the local hydros and he said he has never seen a steel SCUBA cylinder of any kind fail.

Im not advocating either argument here just looking for pics of the aftermath if one has ever actually let go under pressure.

Trying to get some proof on failure mode?

 

[JDMerk]

Yes, I want to see if they split or if the fragment like the AL cylinders appear to. Im also curious to see if a properly maintained steel cylinder has ever failed in the history of SCUBA.

 

[Luis H]

On the other hand, years and years and years of anecdotal evidence says the risk of overfilling LP cylinders to 3600 is quite small.

I really…really didn’t wanted to get dragged into this thread, but...

All these years of experienced mentioned are what 20 years? If the tanks are going though a daily (every day) filled cycle for all twenty years we are ONLY talking about 7300 cycles. That may or may not be a significant number.

It is very possible that we will never see a large number of failures from overfilling practices, but it is just as possible that we will start seeing a bunch in…let’s say 10 more years or maybe 20… or maybe 5 years or less. I would not attempt t guess.

Just because they survived the last 20 years give you no guaranty that they will survive the next 20. The years are accumulating, but the rate is very unpredictable since the fills and stress levels are not being carefully logged.

Also be aware that the number of cave divers and cave dive fills is statistically relatively small.

Fatigue life curves are drawn in a logarithmic chart as a function of stress level. The closer the stress levels to the yield strength the lower the number of cycles. The number of cycles change exponentially with stress, therefore the significance of 7300 is a wild guess based on the limited information gathered by a few cave fills.

The important point is that the closer the overfill gets to the hydro test pressure (and therefore yield stress) the number of cycles drop exponentially.

Also a tank that is stressed to its yield strength has no residual yield strength to handle other loads, for example those that could be encountered in a serious car accident.

There are many engineering reasons why we design a safety factor into any structure. You may find some of the reasons in some of my previous posts.

Note: I sometimes do cut somewhat into the design safety factor of pressure vessels, but I always leave what I consider a reasonable safety factor. IMHO it is OK to take a calculated risk, but it helps to know how to calculate the risk.

 

[rjack321]

"Leadking" got it right. Manufacturers do both cycle testing AND burst testing. These are two separate and distinct tests.

And neither one looks anything like cylinder use in the real world.
Cycle testing is way too rapid compared to the filling, sitting around, use and then refilling done by divers.
Burst testing is not done by repeated marginally sub-hydrotest pressurization in the ~3000-3800psi range either.

In my experience, the urban definition of a "good' LP overfill has gone up over the past 5 or 6 years. From what seems like 3200psi to 3600 or more. So any guesstimates of life of a 3AA tank based on what was done 20yrs ago are a WAG. Regardless of lifespan, they are not failing catastrophically like the AL6351 tanks have. The concept of drastically overfilling an E series steel tank is really outside of any historical experience so I'm not sure where that got dragged in here.

 

[H2Andy]

Holy smokes, guys. Here we go again - ignorance is bliss until it goes "BANG!"

or so you hear and dutifully report

of the hundreds of thousands of steel tanks in use over the past 20 years, do you know how many have cracked due to overfilling?

no?

are you sure you know what you're talking about?

 

[Leadking]

When I was in business and researched the 6351 aluminium alloy issue, I found that despite all the hype, more steel cylinders had catastrophically failed than aluminium.
Take it for what it's worth.