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Burst disc failure leads to over $2000 in property damage

Discussion in 'Accidents and Incidents' started by 2airishuman, Jun 5, 2017.

  1. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
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    Last week, the burst disc in one of my LP72s failed prematurely while the cylinder was flat in the bed of my pickup truck. The truck was parked on a warm, sunny day. The force of the air being discharged was sufficient to lift the end of the cylinder from the truck bed, causing the cylinder to strike the front of the bed with considerable force, as well as striking the rear window of the truck. The bed will require repair to allow the tonneau cover to seal, and the glass will have to be replaced. Estimated repair costs are $2150.

    20170601_152158.jpg

    There were no injuries, since there was no one in the immediate vicinity of the cylinder when this happened. Witnesses heard the noise but there was nothing to see but the aftermath by the time they arrived to investigate.

    I had loaded a number of cylinders in the truck in anticipation of a day of diving for 3 people in the Brainerd/Crosby area over Memorial Day weekend. Due to bad weather, we never went, and I left the cylinders in the truck in anticipation that we would dive them somewhere else over the coming week.

    The cylinders were not overfilled. In fact, the affected cylinder had been used briefly on a dive that was called within the first few minutes and only had about 2200-2300 PSI remaining.

    Disassembly of the valve showed that the burst disc had sheared completely:

    20170602_170232.jpeg

    Analysis

    This cylinder was one of 12 LP72s that I use for shore dives. I had purchased these cylinders from various sources over the last two years. I have replaced most of the valves with Thermo Pro valves, but a handful of the nicer original valves remain in service. LP72s have a service pressure of 2250 PSI and require a burst disc of 3750 PSI. This size of burst disc has been recently discontinued, but some old stock is still available from dealers, all of it with the newer 1-piece construction.

    I did not replace the burst disc in this valve when reconditioning the cylinder because the newer 1-piece burst discs, the only ones available in the proper pressure rating, would protrude from the valve body making the cylinder awkward to lift and carry by the valve. However, the valve had been disassembled for cleaning, with the burst disc removed and re-installed. I believe that the removal and reinstallation of the burst disc, combined with the unknown prior service history of the burst disc assembly, were the main cause of the accident.

    1496678085275.jpg

    While the cylinders had been left in the sun on a warm day (85 degrees), I do not believe that the cylinder temperature would have exceeded about 120 degrees, which would lead to only a 10% increase in pressure over that at 72 degrees, not enough to contribute meaningfully to a burst disc failure.

    The failure of the burst disc may have been uneventful had the newer style been used. A known limitation of the flush style burst disc assembly on this valve is that it discharges air in a single high-pressure stream, unlike newer assemblies that have 3 or 6 discharge holes arranged radially. The single discharge resulted in a large enough amount of force to cause the cylinder to lift upright and cause damage.

    DOT and SCUBA industry guidelines

    The valve was in place when the cylinder was hydro tested earlier this year. The hydro facility passed the cylinder and also issued a VIP sticker. While DOT requirements mandate that requalifiers confirm that the correct size PRD (pressure relief device) be installed, if the valve is installed in the cylinder at the time of requalifiaction, replacement of the PRD is not required.

    PSI procedures require that a SCUBA shop providing a hydro test service (even if the actual hydro is subbed out as is commonly the case) replace the burst disc at time of hydro if a new burst disc is available. PSI shops differ in their interpretation of this rule.

    Conclusions

    Though known, the hazards posed by burst disc failure, and the safety problems specific to older single-outlet burst disc assemblies, are not considered serious and are not widely communicated to divers or dive shops. Accidents usually result in property damage (only) or minor injuries, and are not treated as "diving accidents," leading to underreporting and a lack of appreciation for the risks.

    The use of burst discs in the USA and in jurisdictions that have copied its regulatory framework is largely an accident of history, because the wording of legislation intended to cover steam boiler safety devices in the late 1800s was broad enough to apply to compressed gas cylinders. Pressure relief devices are not used on high-pressure cylinders in Europe, which instead relies on tighter controls of the filling process for protection against accidents resulting from overfilling. It is unclear whether a balance-of-harms analysis would come out in favor of the U.S. approach.
     

    Attached Files:

  2. RayfromTX

    RayfromTX Student Of Gas Mixology Staff Member ScubaBoard Sponsor

    # of Dives: 200 - 499
    Location: Hill Country of Central TX
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    I'm sorry this happened to you but thanks for the great write-up and analysis. You make quite a habit of sharing good info. Too bad this time it came at your expense.
     
    2airishuman likes this.
  3. guruboy

    guruboy Divemaster ScubaBoard Supporter

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    Sucks that this happened.

    This is one reason why you should replace the burst disc if it gets removed.
     
  4. rhwestfall

    rhwestfall Woof! ScubaBoard Sponsor

    # of Dives: 200 - 499
    Location: "La Grande Ile"
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    sucks when things get messed up....

    ...as well as subscribe to updating to accepted devices. Just because it is "available as NOS", doesn't mean it is a best practice to use. A captured style also has bi-directional discharge ports which (theoretically?) disperse escaping gasses so as not to concentrate a stream to make the vessel a propelled object...
     
    2airishuman likes this.
  5. CuzzA

    CuzzA Solo Diver

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    2airishuman likes this.
  6. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    major bummer. Removing and reinstalling those discs definitely weakens them severely. If you're replacing them, go with the new 4000psi burst discs for the 2400psi+ low pressure steels. The extra 250psi isn't going to be an issue for the tank structurally and at least you know it's got modern parts in it.
     
  7. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
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    @CuzzA , based on what you saw, do you think the newer burst disc assemblies with the 3, 4, or 6 radial holes would allow an accident like this to occur? In your post you mention the cylinder leaving the ground.
     
  8. northernone

    northernone Great White Rest in Peace

    # of Dives: 1,000 - 2,499
    Location: Currently: Cozumel, from Canada
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    Sorry to hear this happened to you.

    Thank you for your thoughtful analysis and providing the facts public for our benefit.

    Seen one go off underwater and the results of a couple in storage but I haven't seriously considered the potential damage. I have one in the back seat of my car at the moment.

    So thank you,
    Cameron
     
    2airishuman likes this.
  9. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
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    the new burst discs shouldn't allow this since the gas comes out the side of the plug. I've seen a couple fail and haven't seen them lift off of the ground...
     
    Johnoly and 2airishuman like this.
  10. guruboy

    guruboy Divemaster ScubaBoard Supporter

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    A newer assembly would not have caused the same damage.
     
    2airishuman likes this.

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