Burst disc

[chrispete]

I would bring this situation to the attention of management (I know, it's a chain-store) and explain that it was their people who told you to put that valve on it in the first place, ergo they are _going_ to pay for any problems caused by that oversight. You might want to toss in that as far as you and the hydro facility were concerned the tank was clean and ready for service and that any flash rust in the tank may have been due to their procedures (or lack thereof) when they replaced your tank valve. I'll bet that if you stand up to them, your bill will be greatly reduced. After that, find a reputable LDS to do business with as everything I've ever heard about CS has been negative.

 

[DennisW]

A bit of flash rust on the inside of a steel tank is normal and actually a good thing. The flash rust will keep the tank from rusting any deeper and causing pitting. They didn't call the old british musket a brown betty because they painted them brown. There was flash rust on the surface that protected the steel from rusting any further. I'll have to look further on the burst disks (I'm not at home with my reference) and get back to this thread. I inspect tanks for family, friends, and for me. So I don't do a lot of them. I always refer to my inspection manual for anything a little different than what I normally see.

 

[captndale]

Either I'm misunderstanding what you're saying here or you're wrong. For example, my 2 sets of twin Worthington 100's burst disks are stamped 5250psi (disks haven't been changed out yet) and the working pressure of the tanks are 3442. I also have on my shelf a brand new ThermoPro valve that has a stamp of 5250psi on the burst disk and it came new with a Worthington 100 @ 3442psi. I'd have to check my twin 80's, but I'm sure they have at least 4000 stamped on the disk. If you're just talking about packaging and not what is stamped on the disk, then I'm sure that you are correct.

There may be a wide variance in the way that burst dick assemblies are marked. I just went back to my parts cabinet to check my stock. I have perhaps 30 to 40 of different sizes and pressures and from various manufacturers. All my disk assemblies are packaged in small poly bags with the tank working pressures printed on the bags. Some are stamped with the tank working pressure; some are stamped with a stock number and some are not marked at all. None is marked with the burst pressure. Many of the disks are printed with letters and numbers that identify the shim stock and have no meaning within the context of our discussion. Based on this, I would be very careful in making the assumption that there is any uniformity in markings throughout the industry.

 

[captndale]

The oldest scuba tanks were made with burst assemblies that consisted of threaded plugs with a hole drilled through them plugged with a piece of lead. The idea was that in a fire the lead would melt and release the pressure before the tank exploded. These plugs are no longer considered safe. A later type of plug consisted of a threaded plug with a hole drilled straight through. It held a small disk of copper or brass in place, sealed with a plastic washer. The disk was supposed to burst on exposure to elevated pressure. The problem with these plugs was that if they blew the reactive force would knock the tank over and start it spinning violently. This problem was solved by redesigning the plug to have the outlet holes cross-drilled in the head so that the forces were balanced in the event of a rupture. Older type plugs with the hole in the center are no longer allowed.

 

[DennisW]

The oldest scuba tanks were made with burst assemblies that consisted of threaded plugs with a hole drilled through them plugged with a piece of lead. The idea was that in a fire the lead would melt and release the pressure before the tank exploded. These plugs are no longer considered safe. A later type of plug consisted of a threaded plug with a hole drilled straight through. It held a small disk of copper or brass in place, sealed with a plastic washer. The disk was supposed to burst on exposure to elevated pressure. The problem with these plugs was that if they blew the reactive force would knock the tank over and start it spinning violently. This problem was solved by redesigning the plug to have the outlet holes cross-drilled in the head so that the forces were balanced in the event of a rupture. Older type plugs with the hole in the center are no longer allowed.

You are absolutely correct, but I believe you can replace the older type plug with a new version. I don't think it requires a replacement of the valve.

 

[captndale]

You are absolutely correct, but I believe you can replace the older type plug with a new version. I don't think it requires a replacement of the valve.

That's right. Although there are at least two thread sizes in current use, the smaller one will fit all the older valves that I am aware of. Unless there is something else wrong with the valve, there should be no reason to replace the valve itself.

 

[Skipper h]

Thanks again for the information, I talked to another dive shop near my home and he showed me the valve that I have and said that he could have changed the disc and serviced the valve for alot less than the $39 that I paid for the valve at the other shop. Boy now I'm p#**^#. When they get my tank back I'm going to have a long talk with the store Manager. More like a short talk, and they will know how I feel about being ripped off. I told them not to fill the tank until I inspect it myself. With all the help I have received here I think I have learned something about tanks and burst discs. From what I understand the burst disc should be a 2250, and they Knew that when they sold me the valve that I probably didn't need. And the tank probably didn't need tumbling either. Thats the last time I do bussiness with them and they will know that when I leave. Again thanks to everyone Skipper h

 

[captain]

Your bad experience with the LDS is all too common everywhere.

 

[DA Aquamaster]

For 3AL (aluminum) 3AA (steel) tanks the DOT requires the burst disk rupture at a pressure within a range of 5/3rds the service pressure to 10% less than 5/3rds the service pressure. So for a tank with a 2250 psi service pressure, you need a disc that will fail between 3750 psi and 3375 psi. The 5/3rds figure is based on the same 5/3rds pressure requirement for hydrotesting of 3AA and 3AL tanks and essentially ensures the tank will not be subject to more than its test pressure.

Other tanks certified under DOT exemptions may have less conservative test pressures of 1.5 to 1.7 times their service pressure and woudl consequently need a burst disc to match the test pressure.

A disc that would rupture at 3000 psi would have been intended for a 3AA steel tank with an 1800psi service pressure. Over time the disc flexes with each fill cycle and the pressure they will actually fail at decreases, so the concern with using a 3000 psi rated burst disc in a 2250 psi tank would be that it would fail due to age/corroision or cycles slightly sooner than the correct 3375 to 3750 psi rated disc. It is safe from an explosion standpoint, but would slightly increase the potential for a burst disc related OOA situation.

Valves usually come marked with the service pressure of the tank they are intended to be used in (3000 psi for example for the average AL 80) and the disc would need to be changed for 2250 psi service as the disc for a 3000 psi tank will not burst until a pressure of between 4500 and 5000 psi is reached -well over the test 3750 psi test pressure for a 2250 psi steel tank.

The saving grace here is that for the most part all tanks with a given service pressure are all certified under the same standards or exemptions so marking the packaging for the burst disc asssembly by tank service pressure works and it saves the average tank monkey from having to do higher math like multiplying the service pressure by 5 and then dividing by 3 or more legitamately, having to understand the hydro test requirement for various exempt tanks that may be encountered.

What bothers me most here is the LDS taking advantage of the diver by suggesting the old valve needed to be replaced at all when a $6.00 burst disc assembly would have worked fine. Sometimes if the valve is old and needs to be rebuilt as well as have the burst disc replaced, it cna be more economical to just replace the valve, but the choice to do that should remain with the customer and the LDS should give them all the options.

It is less galling that LDS insisted on tumbling the tank - but not much. Part of it might be price gouging, but some of it may have just been ignorance about what level of flash rusting is acceptable. If it is not heavy enough to interfere with a visual inspection by potentially hiding an underlying pit and if it is not clumping together in heavier spots that could hold moisture and lead to pitting, tumbling is not required. Either way, I'd find a new dive shop.

 

[fweber]

The burst disk you describe sounds like the sort that should be replaced. Regardless of it's exact type and whether it expelled any shrapnel or not, the single orifice burst devices are capable of causing a tank to spin. The newer desings as I'm sure you've noticed, have two unaligned holes so as not to provide any propulsion. If the threads for your old burst disk are the same as any of the more modern ones, that's all you needed. If not, then a new valve would be required.

With regard to burst disk rating. A tank should have a burst disk that ruptures at no more pressure than it's hydro test pressure. For yours a 2250 disk that should burst at 3750. However, most shops don't notice low pressure cylinders and end up filling them to 3000 like everything else. Subsequently, it's not unusual to see people use 3000/5000 disks. While this might be o.k., it's not approved!