Cylinders - Bar and Litres

[Cave_Troll]

so doing the mathe with Kelvin,
If P1 = 200 bar
T1 = 30C (hot day in the red sea)
T2 = 15C (gets a bit chilly at 30m)

then P2 = 190Bar

So if you freedived down to the chilly depths of 30m you'll have lost 5% of your breathable air volume.

Of course the other problem is that gas heats up as you compress it. Some places I've seen to pump the tanks until their gaugue says "200" then they unplug them and leave them to cool. you can loose a few bar like this as well.

 

[Johnny]

Sorry. More b*ll*x.

All sounds relevant and helpful to me

Cave troll,
Not sure about the free-dive example, the gas has got to cool, probably wouldnt happen whilst in your lungs.
Letting the cylinders cool and then topping them up should be part of the filling procedure, especially with 300bar cylinders, but all too often it isnt.

 

[Peter Burgess]

To be absolutely sure of getting the sums right, you should use a consistent unit set, which nowadays should be SI. That means temperatures in Kelvin, Pressures in N/m2, Volumes im m3. As in the above examples we were only using ratios, it was OK to use Bars in the equations. We could equally well have used psia. But to be absolutely safe, pure SI is the answer.

 

[Cave_Troll]

PB - > Accepted about using SI, sorry, at least i didn't use cubic feet

Johnny - > the comment about freediving was that you've got to get the cylinder to cool from 30C to 15C. If you breathed from it on the way down to the cold depths of 30m, then you would affect the experiment of seeing what the pressure drop in your tanks was.

 

[AMW]

topping them up should be part of the filling procedure

I quite agree, in small cylinders an under fill can make a big difference's in a dive being completed. Although I am lucky and have my own compressor and gas mixing set up and I will (some times  ) over pump my 232 bar cylinders (by a bit) 250bar ish this will then negate the cooling effect.

Word of warning  :tease:
The down side is continued over pumping will start to affect the steel (cave diving cylinders are invariable steel). This affect will cause the crystal structure to become fatigued losing expansion during filling this can cause a failure in the cylinder (the tap threads can also be put under stress).
In reality the cylinder is unlikely to go bang more likely to fail its pressure test.

Although a cave diver of old once said the marks on the cylinder:

WP= working pressure
TP= test pressure

Actually stood for:

WP=wimps pressure
TP=The pressure



Andrew

 

[Johnny]

Johnny - > the comment about freediving was that you've got to get the cylinder to cool from 30C to 15C. If you breathed from it on the way down to the cold depths of 30m, then you would affect the experiment of seeing what the pressure drop in your tanks was.

Oh right, sos!

I quite agree, in small cylinders an under fill can make a big difference's in a dive being completed.

Absolutely; I have been digging Bagshawe Resurgence recently and, as its a bit tight, I have been using 200bar 4's and 300bar 3's. A short fill on those cuts your digging time considerably!

 

[Pitlamp]

Hello folks,

I've only briefly skimmed through all the above (and someone may already have mentioned this) but Andrew Goddard published an excellent treatise on the behaviour of ideal / non ideal gases a few years ago in the CDG Newsletter. He is an experienced chemical engineer and knows his stuff. He is also a very experienced cave diver (though he has moved on to other activities now). His article contained a number of graphs by way of explanation. It's well worth looking at this; if I get time I'll try and look up the reference but if someone would like to beat me to it, I seem to remember it was in the second half of the 1990s.

 

[peterdevlin]

Does air follow the ideal gas laws at such high pressures, or is the above an approximation?

Francis

On a previous thread on the subject of 300 bar cylinders John Taylor wrote:

A colleague of mine has dug this article up on the internet, its extremely informative.
http://www.babilim.co.uk/pages/gas_laws.html

... I'm not claiming I fully understood it but it definitely helped me on the subject

Ciao,
Peter

 

[peterdevlin]

Word of warning  :tease:
The down side is continued over pumping will start to affect the steel (cave diving cylinders are invariable steel). This affect will cause the crystal structure to become fatigued losing expansion during filling this can cause a failure in the cylinder (the tap threads can also be put under stress).
In reality the cylinder is unlikely to go bang more likely to fail its pressure test.

Andrew,

Given that a) cave divers tend to fill bottles _somewhat_ over working pressure and b) here in the UK our bottles tend to get banged around a bit underground, is there any anecdotal evidence of catastrophic failure of cylinders? ... I'm discounting bottles that have been dropped down pitches or the like.

Thanks,
Peter

 

[AMW]

In reality the cylinder is unlikely to go bang more likely to fail its pressure test.

Peter
As I put in the post about over pumping (see quote above) the fail is unlikely to be in a cylinder bursting. I have never come across this happening in a cave. As quoted again above, the cylinder will most likely fail a pressure test as the required expansion (pressure displacement test) will not be meet. Even this type of failure will require prolonged over pumping. In a cave if dropped the point of failure will probably be the tap I have seen this happen 

In most cases the TP is a lot higher than the WP so a small amount of over pumping will not unduly damage the steel crystal structure.

Andrew.

 

[nickwilliams]

This isn't my particular area of expertise, but I know enough to be dangerous...

I'd be surprised if repeated over-inflation at the sort of levels which you are talking about here has any significant effect on steel cylinders, and I'd be even more surprised if there isn't some definitive data out there somewhere which will prove whether I am right or wrong.

One of the major benefits of using steel for applications which involve the repeated application of high loads (e.g. pressure vessels, lifting gear) is that low steel alloys (i.e. not fancy things like stainless) will not work harden if they are stressed repeatedly below a certain level. This means that (leaving aside problems like corrosion and damage) you can essentially cycle them forever and they do not suffer from fatigue failure. This is by contrast with high alloy steels (e.g. stainless - 316 is particularly bad for this - and aluminium) which will eventually work harden through repeated loads of even low magnitudes.

The guy who introduced me to caving, many years ago was (and remains) an expert on aluminium and worked at the time for one of the world's major producers of the stuff. I remember him telling me about some work he was doing on testing aluminium pressure vessels by listening to the noises them make as they are pumped up. Among the things he told me was that a standard breathing air cylinder will fail at upwards of 600 bar, and when it does, it fails by splitting along the side. While inducing this is not exactly recommended, the biggest danger from things which 'explode' is almost always flying debris and a split which allows the contents to escape without any bits of the cylinder parting company is, relatively speaking, a safe way for a cylinder to fail. While the reactive force of the escaping air in these circumstances would be sufficient to propel the entire cylinder at a considerable speed, it's a danger which worries me a good deal less than the possibility of having to avoid a valve stem which has been sheared off or come away due to thread failure.

Nick.

 

[AMW]

Nick
I will when I have time dig out a few references to over pressure work hardening.

Of course all this time the diving testing industry could have been been taking money for a test we do not need. Then again they could have been doing the pressure expansion test for a good reason.


Andrew.

 

[nickwilliams]

Of course all this time the diving testing industry could have been been taking money for a test we do not need.
Andrew.

While I am in no doubt that regular pressure vessel inspections are a Good Thing, I am also in no doubt that a lot of what the recreational diving industry justifies on the basis of safety actually has rather more to do with making money than it does with the basic needs of safety, or, indeed, the actual requirements of the applicable legislation.

Nick.

 

[peterdevlin]

In most cases the TP is a lot higher than the WP so a small amount of over pumping will not unduly damage the steel crystal structure.

Andrew.

Andrew: many thanks

Peter

 

[Marcus]

What Nick says is right. If I can get around to it I will dig up what spec steel cylinders are made to, and what strength level they are heat treated to. If I recall correctly, they are CrMo, probably with ~0.3%C (maybe more). Cyclic stressing them to the test pressure should be below the fatigue limit (~0.3 x the tensile strength). Though then again, it is extremely low cycle fatigue (defined as below 10^3 to 10^5 cycles, depending on who you read) and the duration of the stress loading at test pressure is very short, so the risk of failure would be very low to non-existant. The yield strength will be at least 0.8 x the tensile strength (for a quenched and tempered low alloy steel).

The bottles will fail either catastrophically (at Nick's 600 bar +) or by fatigue. Either way, the major problem is the kinetic energy of the escaping high pressure gas (which is why they use water for the hydrostatic test). Catastrophic failure requires the yield point to be exceeded, as does "stretching of the threads". This can't theoretically happen for a cylinder in good condition which hasn't been abused, and doesn't have a manufacturing defect. It can happen if the integrity of the cylinder has been compromised, e.g. by localised corrosion such as pitting thinning the wall. This will locally increase the stress due to a reduced wall thickness, and may exceed the yield strength.

I have never come across a report of an incident which was attributed to fatigue - the toughness is pretty good in low alloy steels used for pressure vessels (i.e. 'hammer stoppers'), and I would expect the cylinder to fail hydro test before it went due to fatigue. Having said that I have never actually looked at the failure initiation point on a failed cylinder, but there have been enough investigations done that I would have expected a noise to have been made if fatigue were an issue.

I will see if I can get my brain into gear and do a few rough and ready sums to see what the numbers come out like.

Marcus

PS: overblowing cylinders is, of course, not a good idea, and I wouldn't dream of doing it, especially if the TP is relatively low, as it is on some older cylinders!

 

[paul]

More interesting facts (and photos!) can be seen here.

 

[nickwilliams]

More interesting facts (and photos!) can be seen here.

Very interesting. Note that these are aluminium cylinders - as I indicated in my original post, aluminium is not a good material to use for high stress applications (unless it is subject to a rigorous in-service inspection regime, which is how aircraft manufacturers get away with it).

The summary, for anyone not following the technical details, is that steel cylinders eventually fail because they corrode or because the threads wear out when the valve is removed for inspection too many times. Aluminium cylinders just wear out.

Nick.

 

[AMW]

Nick
Your conclusion is interesting although , the most common mode of failure in a tap thread is stretch this can be cause by 
over tightening ,but a tap is removed only every two years and now only every two and a half years. The other mod of failure is pressure movement on te tap. Stress in the cylinder is also a factor as you say more so in aluminum cylinders but not exclusively. As said a common mode of failure is down to internal rust pockets that fail the ultrasound thickness test. 

Peter (Devlin) Did you switch motors on your compressor Ok ? if not let me know and we can do it in the work shop for you. 

I will now duck out of this thread as I have no more to say 

Andrew.

 

 

[Marcus]

I have, for the first time, actually thought about cylinder failure in detail, and even managed to do a few sums. Going through the posts above, I would comment as follows (sorry if it is too “techno-babbly”):

• The hoop stress (greatest stress) on a 12l cylinder at WP is ~410 MPa (assuming a wall thickness of 5mm). This is barely at the lower limit for high cycle fatigue, which would require you to fill and empty a cylinder every day for more than 28 years! And then it only just counts as high cycle fatigue. Even at the test pressure (610 MPa) fatigue on its own is still not really an issue.

• The only way to affect the crystal structure is to really belt the cylinder (drop it down a pitch), and that will generate local deformation, or heat it to >750°C or so. Heating it to >500°C for a period will temper it back and drop the strength. Then all bets are off.

• The hydrostatic pressure test is designed to work within the elastic region of the steel’s properties, and as such there will be no lasting effect from the test. Basing on a consistent test pressure (from one test to the next) the only way you can get a change in the degree of expansion during the test is by a thinning of the wall (either local or general).

• ‘Stretching’ (i.e. plastic deformation) can only occur if the yield strength of the steel is exceeded. The hoop stress on the neck (threaded region) of the cylinder is lower than the rest of the cylinder, and so circumferential stretching is even less likely. Over-tightening of the tap in the threads seems a little strange to me for modern, parallel threaded valves. They have a shoulder which I would expect to prevent over-tightening. This is not the case with old taper threads. I do not doubt that this is the main cause of hydro test failures, but I do wonder if it is sometimes associated with manufacture – I had two taps that failed the go / no-go gauge on their first test.


The problem with all of the above is that it relates to a cylinder in good condition with no localised wall thinning, corrosion or defects. Manufacturing techniques are not perfect; corrosion is usually localised at an inherent weakness in the steel (and there will always be inclusions); a localised weakness (whether corrosion induced or residual from steelmaking / cylinder manufacture) will act to concentrate the stress, and may significantly reduce the pressure at which the cylinder with fail hydro test / go bang. As Andrew pointed out (and we all should know), the tap is the weakest point anyway.

Also, from a safety point of view, I think all of us would prefer a cylinder to fail an over-cautious hydro test than actually go bang underground.

Just as an aside, using the same assumption as I used for the above calcs, a 12l blown to 600 bar would approach / exceed the yield point of the steel and fail catastrophically.

Sorry, but this is very much the “short version”.

Marcus

 

[nickwilliams]

Thanks, Marcus - very interesting.

One day I hope you'll grace us with the long version - I wouldn't like to think that UKCaving.com was a victim of dumbing down!

Nick.