Burst disk plug .....

[scubadiverbob]

Hi everyone!

Haven't been here for awhile ...

I was reading stuff on the internet and found this.

www.cave-exploration.com/papers/kaboom.pdf

Sounds scary, huh? I thought the laws on the plugs was international. I also thought the single hole plugs were no longer good; as, dive shops refused to fill my tanks with them in the valves. Hummm .... wondering how a dive shop would know this had been done before filling a tank?

I wonder what other safety precautions the author of the article fails to take before going into a cave? Maybe, using old batteries in dive lights? Or, not using a line so he doesn't get lost?

Well, I'm late; so, got to go!

Robert

[nikeajax]

I like to cut the seat belts out of my cars so I can get out faster if I have an accident... what doesn't everyone else do this too?

Jaybird

[cnotthoff]

I've had this discussion with some of my tech diving friends. It boils down to risk assessment. Many divers who dive where loss of air supply would be catastrophic, disable the burst disks to prevent that from happening.

I first encountered this with a cave diver in the 70's who used that 3/8" fitting to mate 2 cylinder valves, giving him redundant air supplies that could be isolated and removing any over-pressure relief. This same diver routinely over-filled those aluminum cylinders to increase his air supply. There are other threads on this site concerning sustained load cracking at the necks of early Luxfer cylinders. Suffice it to say that overfilling those cylinders was not a good idea.

Disabling the over-pressure relief presents minimal danger during the dive, so the diver has reduced his risk by reducing chances of a possible loss of air supply. I consider this an informed decision.

I've blown many burst disks. Most of them blew during filling, due to my error. Some have blown during transport due to overheating. By disabling burst disks, these divers assign an increased risk to whoever fills and transfers the cylinders. If they just drop the cylinders off at their local dive shop and walk away, planning to pick them up later, they are putting unknowing shop employees at risk. I believe the technical term for this would be "that sucks".

Many of my tech diving friends fill and transport their own cylinders. They know and accept the risks associated with cylinders lacking over-pressure protection. Again, I have no problem with this informed decision.

Scuba cylinders have failed catastrophically. Proper inspections and testing are the way to reduce the chances of that happening. During hydro tests (every 5 years) for the general diving public, I usually rebuild the valve and replace the entire assembly (gasket, disk, and plug), as recommended by Professional Scuba Inspectors. This provides over-pressure protection that shouldn't fail prematurely, but will prevent over-pressurizing the cylinder.

Good Dives,

Charlie

[SeaRat]

Charlie, you are being too kind. There are several cave diving practices that I cannot condone, this this one tops the list. Note that in the first paragraph of this piece, it states, "US regulations require that all scuba tanks which are to be transported on public roads be fitted with overpressure prevention devices commonly known as “burst disks”..." Actually, this is a DOT regulation, and has to do with interstate commerce. But if someone is transporting a tank with an altered burst disk, and the tank explodes while on the roadway, that tank will be confiscated and examined. If it is found to be modified, then the person who did that can be held liable for any damages that occur. If the tank, for instance, explodes while in a trunk in Florida after having originated in Texas, and the ensuing accident causes injury or death to a third party, that person could see jail time. If someone wants or needs more air, carry or stage more cylinders. In my diving years (since 1959) I have witnessed maybe one burst disk failure, and I think it was in Okinawa in the sun. It is extremely rare, and allows the disk to burst rather than the tank.

Here's some pressure ratings from the article:

200 bar = 2901 psig
210 bar = 3046 psig
250 bar = 3626 psig

The 200-210 bar is not a problem, but I have heard of people filling these to 250 bar (3626 psig). Now, let's do a bit of math with that higher number. Remember:

P1V1/T1 = P2V2/T2

If the volume stays the same (an AL 80), we have:

P1/T1 = P2/T2, or

P2 = P1T2/T1

For the P1 = 3626, at 110 degrees F (filling temperature) that's 43.33 degrees C, or 43.33+273.15 degrees K. T1 is room temperature, or 75 degrees F (23.9 degrees C).

P2 = 3626 psig x (43.33 + 273.15) degrees K / (23.9 + 273.15) degrees K
P2 = 3626 psig x (43.33 + 273.15) / (23.9 + 273.15) = 3863 psig

Now, if we heat this up to 140 degrees F in the back of a car trunk, we get a pressure of 4001 psig. Without a burst disk, the tank is in danger, and the temperature can go higher than that too. At 160 degrees F, it's 4200 psig, for instance.

John

PS, I note that the paper itself has no authorship credit; that is probably to limit liability in case someone uses it and has a cylinder problem.

[drado]

Noting two things:
1. The requirement for burst disks is not a universal one. European diving cylinders are not required to have pressure release devices.
2. CGA S-11 states that in general, PRDs should not exceed 5/3 of the service pressure of the tank, which at 3000 PSI, would be 5000PSI - also the test pressure a tank undergoes during hydro.

I think Charlie summed it up nicely: That plugging the burst disk by the author of the article was an informed decision taking into account the risks of gas loss in a cave vs catastrophic tank failure - the chances of which can be reduced by regular hydrostatic testing, visual inspections, and valve rebuilds.

It would be interesting if we could get data re the service status of tanks wherein the burst disk ruptured in the sun - I bet the probability is high that those valves had not undergone a rebuild in a while.

[duckbill]

Mar 22, 2015 8:18:17 GMT -8 scubadiverbob said:

I also thought the single hole plugs were no longer good; as, dive shops refused to fill my tanks with them in the valves.

They're good if the filler says they're O.K. I don't think there's any law or regulation regarding how many holes there are, or their placement. Most dive shops don't like them because the cylinder can become a heavy, spinning top should it blow, causing damage and possibly injury. But, those old single hole burst disc nuts still do the intended job.

[SeaRat]

Here is what Bill High, who used to be President of PSI (which is now PSI/PCI Inc., and I think Bill is now retired, but still consults with them), says about burst discs:

Burst discs should be inspected every time the tank is inspected. Some agencies say they should be replaced yearly, others only if they show signs of corrosion. There are four major problems with burst discs:

--Age and corrosion
--People stacking burst disks, i.e., butting two or three in the valve so it will not pop
--Plugging the burst disc (unfortunately a regulator LP plug is the same size as most burst discs)
--Older style "lead plug" burst discs - these are literally bullets and should be replaced. Is should be common sense that you don't mess with burst discs and don't plug them - why would anyone want to carry around a potential bomb?

Burst discs should be inspected every time the tank is inspected. Some agencies say they should be replaced yearly, others only if they show signs of corrosion. There are four major problems with burst discs:
www.tdconline.com/articles/cylinder.html

Undercurrent carried this article about burst disks, and stated,

Disks are designed to burst at pressures well above the tank's working pressure rating -- as much as 40 percent above nominal. For a 3,000 psi aluminum 80, that would be 4,200 psi. To ensure the proper margin of safety, each burst disk must be matched to the working pressure of the tank on which it's installed.

In fact, one dive shop owner told us that they received 2,400 psi LP steel cylinders with 3,000 psi DIN Valve upgrades. "When we brought this issue to sellers' attention they sold us the correct burst disks; then we had to bring it to their attention each time we ordered more."

Many shop retailers order tanks and valves separately -- often from different manufacturers -- and assemble them in the shop. That leaves lots of room for human error, according to Jack Kuhn, owner of Harbor Dive Center in Sausalito, CA. The shop could mis-order, the manufacturer could ship the wrong valve, or the shop technician could confuse, say, a valve rated at 3,300 psi and install it in a tank rated at 3,000 psi working pressure.

That unsafe combination could allow a negligent fill technician to overfill the tank dangerously. While a tank might survive such an overpressurization, repeated overfilling will induce cylinder wall fatigue that could cause a catastrophic rupture. Even if a rupture doesn't occur, cylinders that have been frequently overpressurized consistently fail their first hydrostatic test and therefore must be replaced.

Either way, the risk seems hardly worth a few extra minutes of breathing time. So it's a potential problem you ought to address.

On new valves, the burst disk pressure rating should be noted on a small sticker. So before you buy a valve, be sure it matches the working pressure of the tank you're planning to use it with. But on older valves the disk pressure is impossible to detect. Even worse, burst disks can fatigue. Some older disks were made with lead slugs that could fire out of an overpressurized tank like a .22 bullet.
www.undercurrent.org/UCnow/dive_magazine/2003/BurstDisk200306.html

The 2000 Emergency Response Guidebook has these bits of information:

--Containers may explode when heated.
--Ruptured cylinders may rocket...

Fire is the greatest danger, followed by sustained load stress on the cylinder, especially the neck and threads. Heating in a car trunk, where on a hot day the temperature can get extreme, appears to be enough (see the calculation at 160 degrees F above) to heat the over-pressurized cylinder to the burst plug's limit of about 4200 psig. But it is much better to have a burst plug give way than to have the cylinder explode. There have been dive boat fires, and you really want the burst disc to give rather than the tank. The burst discs have a release through two holes, which cancel the force of the escaping gas so that the cylinder doesn't rocket (the old ones, of lead, did not, and they melted in a fire at much lower heat). Here is a video of flammable gas cylinders (probably hydrogen) involved in a fire:


John

John C. Ratliff, CSP, CIH, MSPH

[scubadiverbob]

I read today that the single hole burst disk are just a safety problem and aren't required to be replaced. I was previously told they had to be replaced, before filling the tank, with the three hole plug. I can't find any DOT or gov. regulations on this. Anyone know of any? Or, did the shop I worked at just liked to sell burst disk sets?

[SeaRat]

Bob, I'll look into your question.

In the meantime, my thoughts on compressed gas tanks without burst discs exploding because of heat in a car apparently has happened.
www.divebuddy.com/forum_topic.aspx?ForumID=28875

John

[drado]

Bob, I used to have single-hole PRDs in my 72s. One of them gave way soon after filling, and since it only had a single point of exit, the tank started spinning and knocking other things about. I've since changed to the multi-hole PRDs.

[SeaRat]

Bob,

Here's what I could find in the regulations:

(d) Pressure relief device systems. No person may offer a cylinder charged with a compressed gas for transportation in commerce unless the cylinder is equipped with one or more pressure relief devices sized and selected as to type, location, and quantity and tested in accordance with CGA Pamphlet S-1.1 (compliance with paragraph 9.1.1.1 of CGA Pamphlet S-1.1 is not required). The pressure relief device system must be capable of preventing rupture of the normally charged
cylinder when subjected to a fire test conducted in accordance with CGA Pamphlet C-14, or in the case of an acetylene cylinder, CGA Pamphlet C-12. Cylinders shall not be shipped with leaking safety relief devices. Safety relief devices must be tested for leaks before the charged cylinder is shipped from the cylinder filling plant; it is expressly forbidden to repair leaking fuse plug devices, where leak is through the fusible metal or between the fusible metal and the opening in the plug
body, (except by removal of the device and replacement of the fusible metal.) Exceptions are as follows:

(1) Except as provided in paragraphs (d)(1) (i) through (iii) of this section, a pressure relief device is not required on a cylinder 12 inches or less in length, exclusive of neck, and 4\1/2\ inches or less in outside diameter.

(i) A pressure relief device is required on a specification 9, 39 (§178.65 of this subchapter), 40, or 41 cylinder. A metal pressure relief valve is required on a specification 39 cylinder used for a liquefied flammable gas. A fusible pressure relief device is not authorized on a specification 39 cylinder containing a liquefied compressed gas.

(ii) A pressure relief device is required on a cylinder charged with a liquefied gas for which this part requires a service pressure of 1800 psi or higher.

(iii) A pressure relief device is required on a cylinder charged with a nonliquefied gas to a pressure of 1,800 psi or higher at 70 °...
www.c-f-c.com/gaslink/docs/dot_cylinder.htm

The duel or three-hole outlet does not appear to be required, but it is a very good idea, as Ed described above.

The Compressed Gas Association may be referenced, and may have something to say about it in their standard. Here's a manufacturer's publication on burst discs.
www.airproducts.com/~/media/Files/PDF/company/safetygram-15.pdf

John

[scubadiverbob]

Thanks John,

Guess what I was told was wrong. I replaced quite a few of the single hole plugs, at a cost of $35 - $40 (if I remember right) to the customers. Sometimes I had to take the valve to the machine shop to get the plugs removed; so, I didn't mess up the threads on the valves. Well, they got the three hole plugs (which are safer than the two hole plugs; air escapes faster) and the dive shop made a few bucks ...

Had a friend who's tank was in a fire (Terry (duckbill) remember Jack's Al63?). It had a three hole plug. Guess it didn't work. We never found the valve !!!

Robert

[cnotthoff]

Bob,

You're correct on all counts. There is no regulation about single hole burst plugs. It is I won't pass or fill a tank valve that has that old style. They are unsafe for a number of reasons. As previously noted, the single jet of air can set the cylinder spinning. I know of one incident where someone was carry a cylinder by the valve when a single-hole burst plug blew. It shot a jet of air into his hand inflating it like Popeye. I can't guarantee that newer hex burst plug wouldn't have done the same, but at least the air jet wouldn't have been aimed directly at his hand.

Another factor is that any burst assembly with a single-hole plug is very unlikely to have been changed in this millennium. I stated previously that valves used for scuba should be serviced when cylinder is hydroed, including replacing the burst assembly. This is a PSI standard.

To be clear, I will not pass or fill any cylinder with a valve that has an altered or non-existent over-pressure relief. I've had a few European systems come in with no burst assembly.

I support the decision of divers who dive in situations outside of normal recreational diving to assess and accept the risk of altering their equipment, as long as they assume all those risks themselves. This includes filling and transporting cylinders whose valves have been altered themselves.

Good Dives,
Charlie Notthoff

[scubadiverbob]

Oh my! .... and he didn't have to eat his spinach ??

Anyone else notice the word "kaboom" in the .pdf file?