Air Fill Protocol

[cbrich]

I have personally seen several folks that have overfilled their LP tanks to 3,500 and then go on a 12 hr drive with these tanks sitting in the truck bed exposed to summer time temperatures in the south (along the gulf). I would not say it's safe, but I do agree a couple of hundred psig will not hurt the tank.

 

[rhwestfall]

okay we got there (time to say it):

I WANT TO SEE A TANK EXPLODE!!!!

 

[leabre]

okay we got there (time to say it):

I WANT TO SEE A TANK EXPLODE!!!!

Oxygen Explosion - YouTube

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I have personally seen several folks that have overfilled their LP tanks to 3,500 and then go on a 12 hr drive with these tanks sitting in the truck bed exposed to summer time temperatures in the south (along the gulf). I would not say it's safe, but I do agree a couple of hundred psig will not hurt the tank.

This reply intriqued me and I wanted to understand why this could be. I decided to play around with some numbers to try and understand and/or explain if everything I said earlier was correct or what others are saying is correct. The following is my attempt to explain what's going on during a fill.

When a cylinder is Hydrostatically tested, it is filled to 5/3'rd (or 166%) of its service pressure. This means that AL 80's rated at 3000 PSI would be filled to 5000 PSI (3000 PSI / 3 = 1000 PSI then * 5 = 5000 PSI) at the hydrostatic testing facility. The expansion of the cylinder is measured and recorded before and after the test and if it remains expanded beyond certain tolerances then the cylinder is condemned otherwise it passes hydrostatic testing.

Cylinder valves have a burst disc to help prevent cylinders failing from over pressurization (commonly in-case a fill station operator leaves the fill whip connected and walks away for too long). A properly matched burst disc generally tolerates a 40% pressure expansion beyond the cylinders service pressure before it fails. An AL 80 cylinder rated at 3000 PSI with a properly matched burst disc should fail at 4200 PSI (Service Pressure * 40% = 3000 PSI * 1.4 = 4200 PSI).

NOTE: If an aluminum cylinder filled to its rated service pressure is heated high enough to cause the burst disc to fail, then if the same cylinder were filled with to half or less pressure, it may weaken from structural fatique before reaching burst disc pressure. Aluminum cylinders begin to fatique after about 300* F.

Cylinder volumes/pressures are considered to be at "room temperature" because they spend the majority of their time sitting in a room (not in a fire, not in the trunk of a car in the middle of the Saudi Arabian desert, and not in a Hydostatic testing facility). Room temperature is defined as 72* F. Manurfacturers use this value when establishing a tank's volume/service pressure rating.

There are a few different methods used to fill a cylinder (my definitions may be a bit off)

*) Hot-fill. When a cylinder is rapidly filled to reduce the fill time. Generally this fill is beyond roughly 600 PSI/minute.
*) Slow-fill. When a cylinder is filled at roughly 500 PSI/minute or less
*) Wet-fill. When a cylinder is filled inside a water bin -- usually slow'ish. Wet-fills convect heat away from the cylinder faster than if it was sitting dry at room-temperature.

The very act of filling a cylinder generates heat. This is because molecules generate heat as they are squeezed into a confined space. The more rapid the squeezing, the higher the temperatures. This squeezing is called "compression". Compression is a hot-topic! (bada-boom, bada-bing)

Charles' Law states that (Pressure * Volume)/Tempurature is constant. This means that P1/T1 = P2/T2. When computing pressure changes related to temperature, we must first adjust F to absolute zero which brings us to the Rankine scale -- which is -460* F. This means that zero degrees F = 460* Rankine and 72* F = 532* Rankine (460+72).

I could not easily find an actual measurement or statmeent of the temperature of a cylinder during hot-fill. Let us assume a cylinder reaches 120*F during a hot-fill in a room that is 72* F then we could compute:


P1/T1 = P2/T2
=3000/(460+72) = P2/(460+120)
=3000/532 = P2/580

P2=(3000/532) * 580 = 3384 PSI

What does this mean? This means that if we hot-fill an AL80 cylinder with a service pressure of 3000 PSI to 3384 PSI (with an internal cylinder tempurature of 120* F during fill), we are still well beneath the burst disc pressure of 4200 PSI and signicantly below the Hydrostatic pressure test of 5000 PSI.

This means that it should be safe to over fill a currently-in-hydro AL80 cylinder to 3384 PSI without fear of structural fatique *IF* that cylinder has not previously been exposed to heat in exceess of 300* F at any point in the past and *IF* it is in good condition (no nicks, scratches, dings, bulges, bows or corrosion beyond prescribed tolerances) and *IF* the burst disc itself is not faulty.

Just for fun, I wanted to see what the pressure would be of a fully filed AL80 cylinder if exposed to a 300* F fire:

P1/T1 = P2/T2
=3000/(460+72) = P2/(460+300)
=3000/532 * 760 = 4285 PSI

Recall earlier that the burst disc of this cyclinder should tolerate up to 4200 PSI? This burst disc should fail just before reaching 300* F if it was filled to 3000 PSI at room temperature before being exposed to the heat source. But what if the same cylinder filled to 1200 PSI was exposed to the same fire?

1200/532 * 760 = 1714 PSI

This means that the cylinder itself would likely fail from heat featique long before the burst disc fails from over pressurization.

Next time a fill operator refuses to fill beyond service pressure alone perhaps you can explain this to them but remember: "their shop, their rules". You are always free to go somewhere else for a fill.

Hope this helps!

EDIT: The DOT does allow a cylinder to be over-filled to a pressure that will cool to its service pressure at room temperature. I was not able to dig in an find an exact reference online to link to but hopefully someone here can (J1Scuba has a Hydro facility perhaps he can point us in the right direction ??... J1... J1?)

---------- Post added ----------

I have personally seen several folks that have overfilled their LP tanks to 3,500 and then go on a 12 hr drive with these tanks sitting in the truck bed exposed to summer time temperatures in the south (along the gulf). I would not say it's safe, but I do agree a couple of hundred psig will not hurt the tank.

Now that I've explained how pressures and temperatures work with cylinder fills, I wish to complete my investigation of the wisdom of people who do such overfills.

Low-pressure tanks are generally 2400/2640 PSI. 3000 PSI is generally considered medium-pressure and 3442+ PSI is considered high-pressure. Because I do not know what cylinders CBRICH observed, I will assume a use an LP95 as an example. An LP95 is a low-pressure steel cylinder rated to 2640 PSI when "+" rated. They are popular in cave-country because they can be overfilled and have characteristics that make them more preferable to use than HP 100's or 130's. For purposes of this investigation, I will assume the cylinder is "+" rated.

The burst disc of such a cylinder should tolerate 3696 PSI before failing to over-pressurization. Let us assume the tanks filled at 3500 PSI heat to 130* while driving through a desert in the summer. The pressure should be:

P1/T1 = P2/T2
=3500/(460+72) = P2/(460+130)
=3500/532 = P2/590
P2 = (3500/532) * 590 = 3881 PSI

What does this mean? Well, first it means that the tanks will overheat causing the PSI to skyrocket beyond the 3696 burst disc tolerance to a whopping 3881 PSI. In order to allow this, a higher rating burst disc must be applied to the tanks. Even a burst disc for a 3000 PSI rated cylinder would not be enough. One would have to use a 3500 PSI service pressure rated burst disc.

What is the hydrostatic testing pressure of the same tank? (2640 / 3 * 5 = 4400 PSI). Doing this would bring the pressure of the LP95, filled at 3500 PSI at a tempurature of 130* F to within 600 PSI of hydrostatic testing limits.

This means that the cylinder could potentially fail and explode before the burst disc.

But what if the cylinder was an AL80? The same formula would apply above to reach 3381 PSI but the burst disc pressure would be 4200 PSI bringing the cylinder pressure to within 400 PSI of burst disc pressure.

I'll leave it up to you to decide if that is a risk worth taking.

 

[Mr Carcharodon]

My LDS fills in water and tends to overfill by 10% which makes the final pressure come out on spec. I'm pretty unconcerned about going 300 psi over. Last summer I was on a dive boat and they did manage to put 3600 psi in a LP steel tank which rather seemed like a lot, but it made for a long dive.

 

[PfcAJ]

Overfilled lp steel tanks simply don't explode. I challenge anyone to find one instance of an overfilled steel scuba tank (within hydro and vip) rupturing. It just doesn't happen.

In regards to heat, at what temp do o-rings melt (or soften enough to extrude)? Europe doesn't even use burst disks at all.

 

[macado]

While I don't agree with the practice of filling tanks in a water bath that is another conversation. I would rather not have a possibility that allows water to enter my tanks. (Note: If the tank has any pressure, water is not going to get in there however if the fill whip falls in the water or the tank falls over in the water bath then there is now water in/on the scuba valve.) It only takes a fine mist of water to coat the inside of a tank which is not very good for steel tanks.

With that being said, a lot of shops do this but you certainly DONT need to do this to get a good fill as long as the shop fills the tanks slowly and waits for them to cool down.

300psi per minute is actually recommended. It also my understanding the CGA regulations allow for a slight overfill to compensate for the gas loss when tanks cool. I'm sure someone can point out the CGA regulation but there should be nothing limiting you from getting 3000psi fills.

Most shops will generally let your tanks cool down a bit and top them off as long as you're not in a hurry. I always make a point to tell the fill person to take their time and let them know i'm not in any sort of rush.

At 2800-2900psi, I wouldn't complain but 2200psi I would not be happy.

 

[KD8NPB]

This is part of the reason I like running LP72s. The shop is nice enough to give a slight overfill at a very low speed so the tank doesn't heat up very much. I'm usually around 2100-2200 when I hit the water.

 

[D_B]

Doesnt matter if it's a LP tank, or a HP tank if the LDS wont fill it so it's at it's rated pressure when at 72F

Why do some LDS not understand It's not an over fill or overfilling a tank if when the tank cools to 72F it's at the rated pressure?

 

[supergaijin]

My experience in filling is a bit different I guess.

I fill to 200 bar or so and then change. These I consider to be hot fills which I try to keep aside to top up the next day after filling. The temp will reduce the pressure to approx 175 to 180 bar depending on the gauge. Using my Bauer Poseidon 250, 2 tanks take 10mins to go from 100 to 200 bar. Thus a single tank should fill from 0 to 200 in ten mins, or at a rate of 20 bar per minute. Or approximately 290 psi/min.

Thus these are not 'fast fills' but the temperature does make a good difference in the pressure the next day. Many compressors have a 'blow off preventing filling beyond a certain pressure. My Bauer is set to 225bar. To intentionally fill to that pressure on every fill, every day will create undue wear and tear on the valve, requiring eventual replacement.

Firing up the compressor to only do the top up is a waste of time and money. For those people who operate compressors and count the minute scavenging the compressor of CO2 and after 15hrs having to replace filters, the cost is real. I consider anything under 30mins run time a 'waste' as it requires 2 mins for me to fire it up, scavenge and blow off the condensation valves. Thus I'm losing somewhere around 7% of the running lifetime of the filter to use the compressor for under 30mins.

I do not have a commercial filling station but the cost to do this for one or two tanks would be high over time.

Just about any shop near me thinks that a 3000 PSI rated tank cannot be filled past 3000 PSI even during a fill. For the same reason one cannot every actually receive a fully rated fill -- even after a top-off.

An 80cf rated volume of a tank rated at 3000 at room temperature (72*) would acutally be about 77 CF but lets just say 80 for this argument. If that volume warms up by some degrees you may have 3200-3300 PSI but it is still the rated volume and will cool off to about 3000 PSI. Even during Hydro they fill the tank to 5/3's of the volume (meaning 166%of its rating which means a very high PSI) and if it passes hydro then the tank can handle a few hundred extra PSI during a hot fill even when that extra PSI is the same volume rating. If you have a full 3000 PSI fill and leave it in the summer heat it may expand to about 3200 PSI but it is fine -- it will cool off when you hit the water and contract back to 3000 PSI.

When I do my own fills I always overfill a tad so it cools off to its rated pressure but no shops around me will do that. Sometimes I attest it to not fully undersanding the laws/rules.

The rules are there for a reason. Not all tanks have burst disks and not all burst disks should be trusted.

That's why the fill station should really put the tank into water while filling it. It will help keep the tank cool and that will give you more air and more pressure at the end of the fill.

That one has been debunked according to this: http://www.fillexpress.com/library/dryfills.pdf
Basically it states that the tank doesn't sit in the water long enough for it to have an impact on the temperature inside the tank.

 

[phuonguyen]

When working in my LDS, I fill the tanks to 235 Bar straight from the compressor, which then, when left overnight, cools down to 210 Bar. When filling from the banks, I tend to fill according to the sound or flow rate through the filling whips. For those fills, I tend to go up to 215-225 Bar.

Mind you these are with AL80, with the faber steels, I prefer to fill via the banks, as the compressor heat up the tanks too much.