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Burst Disk for Steel 72?

Discussion in 'Vintage Equipment Diving' started by 73diver, Sep 30, 2017.

  1. 73diver

    73diver Solo Diver

    # of Dives: 200 - 499
    Location: Milwaukee, Wisconsin
    I picked up a couple of steel 72's with mfg dates in the '70's and have ¾" NPS threads. Hydro's are 2017 and are not 'plus' rated. I want to use these for sidemount and purchased left and right modular DIN valves. Of course the burst disks for the DIN valves are too high for these tanks. I would like to know what the desired burst disk rating should be for a tank filled to 2400#. Regards.
  2. Rred

    Rred Manta Ray

    # of Dives: 200 - 499
    Location: In a safe place
    If the tanks were 2400+ rated originally, that would be 2640 psi. (And every subsequent hydro can test for that pressure, and re-stamp the + rating, but the industry doesn't want to be bothered by that.)

    A 2640 working pressure would be hydro tested to 5/3 of that pressure, IIRC, or 4400 psi. So in theory, your tanks should be able to withstand something like 10,000 cycles to 4400psi. (Check that with someone to be sure.)

    A 3000 psi disc would have a wide safety margin for you. 3500 psi would still have nearly a 1000 psi safety margin. Anything between the two should be plenty good enough.

    Now your problem will be to get the burst discs. Shops like to replace them with an "all in one" burst plug assembly, which in theory is more precise. But is also vulnerable to corrosion and way more expensive. So if you do get replacement discs...stash away a dozen while you still can.
    northernone likes this.
  3. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    The, burst discs I have seen are named by the working pressure of the tank, say 2016, 2250, 2400 and so on.

    Store - Vintage Double Hose

    This is the best deal I've seen for the unit. I usually ask any shop if they have any 2250 burst discs available for sale as I haven't run across them online. I have fudged and used a 2400# disc as it is quite close to 2250, but I don't do it as a regular practice.

    2airishuman likes this.
  4. 2airishuman

    2airishuman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Greater Minnesota
    Typically LP72s are rated 3AA-2250. For these, the correct burst disc is 3750 PSI. The burst disc size is based on the 3AA rating, not the 10% overfill pressure permitted by a +. 3750 PSI burst discs have been discontinued by their manufacturers but are still in stock at some dealers and distributors.

    It is common to use 4000 PSI burst discs in these cylinders when 3750 PSI burst discs are unavailable. While technically not in compliance with PHMSA regulations, the slight difference in burst pressure is insignificant from a metallurgical standpoint since the discs will still blow well before the cylinder wall burst pressure is reached (over 5000 PSI).

    It would also be possible to use 3360 PSI burst discs, which are readily available because they are still used on 2016 PSI argon bottles. However these are technically not in compliance with PHMSA regulations either (they're just shy of the 90% minimum), and there is a real risk that they may burst during routine use.

    I would caution you against reusing old burst discs since they may fail early, and since many 3750 PSI burst discs are of the single outlet type that is more prone to turning the cylinder into a missile.

    If you have an LP72 that has a 3AA-2400 rating, then the correct size burst disc is 4000 PSI. These are readily available.
    Last edited: Sep 30, 2017
    northernone likes this.
  5. 2airishuman

    2airishuman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Greater Minnesota
    For 3AA cylinders, hydro test is 5/3 of rated pressure. In your example that would be 2400 * 5/3 or 4000 PSI. The 10% overfill does not change the hydro test pressure.

    There are no burst discs for scuba cylinders that burst at 3000 PSI or 3500 PSI. Burst discs marketed as having these pressures are intended for use on 3000 PSI and 3500 PSI cylinders, and have actual burst pressures of 5000 PSI and 5250 PSI respectively.

    The one piece pressure relief assemblies are safer and are now the only thing I use. The main advantage is that the burst pressure is clearly marked on the outside of the plug and since they are one piece there is no potential for mismatched parts. It is therefore possible to confirm, by looking at the exterior of the burst disc, that it is correct for the cylinder's working pressure. They cost $8 each at DGX and $12 each at my local dive shop. It is an insignificant part of the total cost of diving.

    The burst disc assembly should be replaced every five years. I would be surprised to learn of corrosion problems during this time period.
  6. Eric Sedletzky

    Eric Sedletzky Great White

    # of Dives: 500 - 999
    Location: Santa Rosa, CA
    The old style plug with the hole coming out the front are illegal now. Too many injuries from shrapnel.
    The only ones you can get are the ones that come out the side, or edge where the wrench goes.
    I went in to my LDS and asked for new kits for my 72's and they handed me 4000 psi discs.
  7. Bryan@Vintage Double Hose

    Bryan@Vintage Double Hose Instructor, Scuba

    I have plenty of brand new 3750psi burst disc assemblies in stock.
    northernone likes this.
  8. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    @Rred where did you come up with that information? 2400+ cylinders are tested to 5/3 of their working pressure, their working pressure is 2400, not 2640. Hydro testers can ONLY give a + rating if they have the REE of that bottle. If the bottle doesn't have it stamped on the crown, and the documentation isn't provided to them, they usually don't go hunting for it. It takes less than 5 minutes to hydro a tank, it may take half an hour to go digging up the REE which costs them a lot of money so they won't do it. It doesn't take any longer during the test phase, they just have to have the REE and up until relatively recently, most bottles don't have that stamped onto the crown.

    @73diver the burst discs are set by the DoT based on the cylinders design pressure. There is no "adjustment" based on whether they are + rated or based on what you intend on filling them. DoT says the burst disc goes at 90%-100% ish of test pressure and the discs are usually sold based on the test pressure. In this case 3750. Depending on the valve, those may not be available, so most of us will use 4000psi burst discs from the 2400psi cylinders since they are more readily available.
    You can obviously choose to purchase lower rated burst discs which is not a problem, however unlike what rred said above, you can't just magically find burst discs in whatever increment you want. In this case the DoT says your burst disc must be "rated" for 2250psi, with a rated "blow" of 3750psi. It will usually blow around 3375 which is fine. Depending on the valve though, those may not be available as I said because those bottles aren't made anymore so if you have a new valve you may have to use a 2400/4000psi disc which is fine.
  9. 73diver

    73diver Solo Diver

    # of Dives: 200 - 499
    Location: Milwaukee, Wisconsin
    Thanks everyone. I have the correct burst disks. One more thing. Out of the blue, three more 72's came to me. These have manufacturing dates of 1972. All three brand new in original cardboard boxes. They were sent off for a hydro from this century.
  10. JamesBon92007

    JamesBon92007 Great White

    # of Dives: 2,500 - 4,999
    Location: Southern California...too far from the ocean
    "Came to you?" How do I situate myself so such things come to me?

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