Burst discs: Yes they burst when they shouldn't

[tbone1004]

@Storker
This is a "normal" fill whip end over here. It's identical to the 300bar regulator ends. The same yoke adapters spin on and off we go. It's quite nice. I have never seen a 200bar fill end, and I have only seen a handful of 200bar regulators so it isn't a common thing over here. We use a lot of 200bar valves, but they are identical to our 300bar valves with the face shaved back 2 turns so we can use a yoke plug to convert it
Din Filler with Screw Bleed - Upgraded

 

[Akimbo]

It's physically impossible to mount a properly designed 300 bar fill whip to a 200 or 232 bar tank valve.

I'm confused. I understood that the 300 Bar din connections had more threads (longer) than 200 Bar. That would prevent a 200 Bar regulator or fill whip from making a seal on a 300 Bar valve but a 300 Bar fill whip would screw into and make a seal on a 200 Bar valve. True?

It has been years but I seam to recall that DIN dominated Northern Europe and Scandinavia but yokes were very common in southern Europe, especially around the Med. Is that still the case? It is interesting here. Nobody actually checks blowout disk ratings here, even during VIPs and hydros. XS-scuba's valves are the only ones I have seen that clearly marks the pressure on the valve body.

 

[2airishuman]

I recently got a bunch of "new" steel 72s hydroed (new to me) so now I am going to replace the burst discs. Some of them are the old three-piece type. I believe they are all 3/8-24 but one listing I read says "UNF." Does this sound right?

3/8" UNF is 24 threads per inch (chart).

Also, is there any reason why I can't put a one-piece unit in place of a three-piece unit?

No, in fact, it is a recommended practice to do that.

I normally fill my tanks to 2700 psi and they cool down to about 2650. Is the 3AA-2400 psi = 3600-4000 psi the correct choice? The hydro guy said they were tested to 3750 psi so 4000 seems a bit high however 3375 (the first choice on the list) seems a bit low. Unfortunately the best deal I can find right now is for 5000 burst pressure which seems way too high for my steel 72s.

All (oddball exceptions aside) LP72s are 3AA-2250, not 3AA-2400 like most newer LP steels. The correct choice is a 3750 burst disc, which has been discontinued. Some dealer/distributor stock still exists but they are getting hard to find. The usual choices are 3360, which is what you probably encountered, or 4000. Which of those is safer is debatable. I have 3750s in mine so I don't have to choose.

 

[2airishuman]

@Storker
This is a "normal" fill whip end over here. It's identical to the 300bar regulator ends. The same yoke adapters spin on and off we go. It's quite nice. I have never seen a 200bar fill end

The photo shows a fill end that is deliberately designed to fit either 200 bar or 300 bar DIN.

300 bar DIN fill whips have a post at the end. If you look at some 200 bar and 300 bar valves, you'll see that the 300 bar valves have a recess that will fit the post, but 200 bar "provalves" do not.

https://www.nuvair.com/store/media/...9df78eab33525d08d6e5fb8d27136e95/a/1/a101.jpg

I'm confused. I understood that the 300 Bar din connections had more threads (longer) than 200 Bar. That would prevent a 200 Bar regulator or fill whip from making a seal on a 300 Bar valve but a 300 Bar fill whip would screw into and make a seal on a 200 Bar valve. True?

The connection design is intended to accommodate interlocking for both filling and usage, so there are 2 possible valve-side connections:

a) 200 bar
b) 300 bar

And 3 possible connections for the reg/fill side:

1) 200 bar only
2) 300 bar only
3) either

Nobody actually checks blowout disk ratings here, even during VIPs and hydros.

Some do. PSI requires it, from what I understand, and I've run into PSI inspectors who replace burst discs at every hydro.

XS-scuba's valves are the only ones I have seen that clearly marks the pressure on the valve body.

The new strategy is to mark the burst disc rating on the one-piece burst disc itself, since in theory there's no way to mis-assemble valves so that the wrong rating is displayed. Prior to that, with the 3 piece burst discs assemblies, XS scuba and maybe others marked the valve with the service pressure, so that at least the common scenario of valves being mixed up between cylinders of varying service pressures could be addressed.[/quote]

 

[BRT]

Interesting. IIRC our yoke adapters can only be fitted to a 232 bar fill whip (or adapter).

There's also a difference in the number of threads. A 300 bar DIN fitting has 7 threads, a 200/232 bar has only 5. Not an issue for regs, since they're designed to fit both 200/232 bar valves and 300 bar valves, but the fill whips are designed to make it physically impossible to screw a 300 bar fill whip into a 200/232 bar tank valve.

How do they do that? (I found the answer above)

 

[Storker]

I understood that the 300 Bar din connections had more threads (longer) than 200 Bar. That would prevent a 200 Bar regulator or fill whip from making a seal on a 300 Bar valve but a 300 Bar fill whip would screw into and make a seal on a 200 Bar valve. True?

You've got it right about the threads. But a 300 bar fill whip also has a longer protruding thingy than a 200 bar whip, effectively preventing the whip from sealing in a 200/232 bar valve. The 200/232 bar protruding thingy is shorter and fatter.

 

[Storker]

I seam to recall that DIN dominated Northern Europe and Scandinavia but yokes were very common in southern Europe, especially around the Med. Is that still the case?

The last years I've dived abroad in Italy, France and the canaries. All the ops used DIN.

 

[NAM001]

2air

a little correction . I believe the burst disk used is required to blow between 90-100% of test pressure. an al80 tests at 5k so the disk should blow about 4600 psi a little over the 90%. Picky perhaps but i have had one blow. It was on a lp95 pumped to 3200 on the gage , however teh gage was in error and off about 250 psi. It actually had about 3500 in it and the heat raised it to a little over 3600 EST. and it blew. tp was 4000 on a lp tank. 35 degree increase in temp at 5 psi per is about 175 psi increase from temp. I have never crossed paths with a blown disk since. Over filling was the cause of the rupture. BTW i believe the vis and hydro is to include verification that the burst disk is undamaged and the correct size for the tank application. I have 3000 psi valves on my lp's (they have burst disks for 4500#) and I do not give the valves to the hydro shop. In the above diagram item 11 is not always an item the the disk can be separated from the bolt. If they insist on valves i put on the original 2400 psi thermo valves. Item 11 is not a cheap item (5-7$ a piece) when you vis a dozen tanks a year.

 

[tbone1004]

@2airishuman I require a full valve rebuild including burst disc replacement at hydro if I'm going to sticker a tank for someone else

 

[NAM001]

You've got it right about the threads. But a 300 bar fill whip also has a longer protruding thingy than a 200 bar whip, effectively preventing the whip from sealing in a 200/232 bar valve. The 200/232 bar protruding thingy is shorter and fatter.

I think you have it backwards. a 300 bar whip will seal on a 300 and a 200 bar valve. the 200 bar regulator will not seal on the 300 bar valve, insuring the lower rated reg is not connected to the higher pressure source.