Burst disc

[DA Aquamaster]

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!

I used to have a pair of 1800 psi 45 cu ft steel tanks that my son used back when he was shorter than me. It was not uncommon for tank monkeys to treat them like regular AL 50's and fill them to 3000 psi even with the 1800 psi service pressure marked in red. Given that 3000 psi was the hydro test pressure for the tank this never made me very happy.

Nor was I real thrilled on one occasion where I was charged for the first fill to 3000 psi plus the cost of replacing my "old" burst disc as well as the cost of a second fill to 3000 psi with the "new" 3000 psi service burst disc installed. That particular shop employee was pretty much an idiot.

 

[Skipper h]

Still a little confused about the right burst disc for a 2250 steel tank. (2250, 3000, 3750) But I'm determined to learn what the correct burst disc should be. Maybe I will try to contact Sherwood. I'm concerned that even through them I will get conflicting answers. I see here on this site that they offer questions to different reps for a number of manufacturers. Maybe they could give me the answer I'm looking for. Skipper h

 

[Luis H]

Still a little confused about the right burst disc for a 2250 steel tank. (2250, 3000, 3750) But I'm determined to learn what the correct burst disc should be. Maybe I will try to contact Sherwood. I'm concerned that even through them I will get conflicting answers. I see here on this site that they offer questions to different reps for a number of manufacturers. Maybe they could give me the answer I'm looking for. Skipper h

Sherwood valve SVB5000 use the small 3/8" hex 3-piece assembly shown in Fig. 1 (sheet 8 below from the "Sherwood Scuba Cylinder Valves Assembly & Maintenance Guide").

You need Sherwood part number: P/N 4000-850


Note: Most competent dive shop attendants will recognise this part as "the small 3/8" for a 2250 psi tank".

Burst disc P/N 4000-850 will blow at 3750 psi (+0%, -10%). You don't need this last piece of information.
It is for reference only.

 

[Skipper h]

This is the responce I received from the Sherwood Rep at this site. And thank you Luis H. It's starting to make sense. Is the # on the burst disc operating pressure or burst pressure? I think it's opperating pressure.

You want a disc that has an operating pressure of 2250 and a bursting pressure of 3750. The standard for all compressed gases is that the bursting disc is 5/3 of the operating pressure. This means the bursting pressure is 5/3 more than the operating pressure. For example, 5 /3 = 1.666, (1.666 X 2250 = 3749.85). Thus the 3750 bursting pressure.

The 3,000 psi disc is incorrect. Again, you want the disc that has an operating pressure of 2250 and bursting pressure of 3750.

Make sense?

Hope this helps.

 

[Couv]

Skipper, Luis H, et al,

Yes, this makes sense, but never-the-less, that is not how is always works out. I used to have a bust disc blow pretty often on my steel tanks due to some un-attendant putting 3000 psi in them. One old gentleman at Morrison Springs was famous for it.

With the info provided above, it seems very likely that who ever was replacing my disc was putting the wrong bust disc in as 3000 psi should not have blow them out.

It used to clear the shop out pretty quickly though. Looked like a water volcano.

 

[Colliam7]

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. ... 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. ... Either way, I'd find a new dive shop.

I suspect ignorance, rather than malfeasance, is at work here. Probably, there is a bit of conservative, corporate CYA behavior not uncommon to chain operations like SC, at work as well. When in doubt, default to a sure thing (don't fill, or fill only after 'upgrading' to a known standard, be it a new valve or a tumble for flash rust). There have been a number of threads on SB over time expressing concern about tank filling experiences at SC. But, with almost 50 stores spread across 3-4 states, that is to be expected. They hire people who may have little or no real scuba experience, train them in a standardized fashion, and these staff then regurgitate whatever they were told in training. I have seen this in independent LDS operations as well. You did not need a new valve, only a burst disc assembly. You didn't need to have the tank tumbled. If the store manager cannot justify either service action (other than 'It is our policy.'), you should refuse to pay for it. Let the manager, and the organization learn from this.

 

[Luis H]

Skipper, Luis H, et al,

Yes, this makes sense, but never-the-less, that is not how is always works out. I used to have a bust disc blow pretty often on my steel tanks due to some un-attendant putting 3000 psi in them. One old gentleman at Morrison Springs was famous for it.

With the info provided above, it seems very likely that who ever was replacing my disc was putting the wrong bust disc in as 3000 psi should not have blow them out.

It used to clear the shop out pretty quickly though. Looked like a water volcano.

There is a number of things that will weaken a burst disk: the number of over fills, over-tightening, corrosion, reinstalling it, etc.

The burst disk is intentionally working close to its ultimate shear stress, therefore it is susceptible to a number of degrading factors. That is a good reason to replace it every 5 years or so. And make sure whoever installs it use all new matched parts and does not overtighten it. The reason for the match parts is because the bolt is the cutter for the disc; they work as a unit.

 

[Couv]

Thanks Luis,

I was looking at the valve diagram you provided above and noticed a valve that has the knob on the top. (KVA6535X-1) I used to see a few of these on some tanks we had years ago when I worked with a recovery unit. What is the designation of that valve? I called it a "T" valve for no other reason than it's appearance, however, someone in another thread suggested it might be called an "I" valve, but did not know for sure.

couv

 

[Luis H]

That is a 1/2" tapered pipe thread valve. It has not been in production for a while.

It is a non reserve (open /close only, no other special features) valve. So in a way it could also be called a "K" valve, but I have also seen it called an "I" valve (due to its appearance).

Based on the arbitrary nature of how the "J" and "K" valves got there names, I don't feel that there is a formal rule on how to label valves.

 

[Leadking]

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.

I'm sure Lee can step in here to explain

Worthington 3442 psi cylinders are downrated 3500 cylinders (so the use of 200 bar covertable valves with a 3/4-14 thread can be used instead of the 5/8ths) Because these are true 3500 psi cylinders (note the tank stamps of TP5250 which means test pressure 5250 psi). E9791 hydro info requires the cylinder to be hydo'd at 5250 (3/2) not your typical 5/3.

The above is why you need to review the hydro requirements instead of a blanket statement such as 5/3 or 1.7

Hope this helps!