Burst discs fail -- that is, the open when they really shouldn't -- with some regularity. They fail during filling. They fail underwater. They fail when the cylinders they're on are outdoors in the sun. They fail in storage, and they fail in cars.
Failures under all these circumstances have been reported in firsthand accounts on ScubaBoard.
This thread is for discussion of risk factors and best practices for preventing failures and mitigating the risk of injury or property damage when failures occur.
Some math
Burst discs for 3AA and 3AL cylinders are supposed to have a rated release pressure of 5/3 the nominal pressure rating of the cylinder. For a typical 3AL cylinder with a 3000 PSI rating, that means the burst disc's rated release pressure should be 5000 PSI.
For a typical LP steel with a 3AA2400 rating, the burst disc's rated release pressure should be 4000 PSI. Since LP steels are usually + rated and authorized for a 10% overfill, the actual ratio between the authorized fill pressure and burst disc's rated release pressure is very close to 3/2 (5 / (3*1.10) = 1.515151). Stated another way, the rated burst pressure is reached when the cylinder pressure exceeds the rated fill pressure by 51%.
HP steels made under the Faber, Worthington, or PST special permits use a 5250 PSI burst disc, which is 3/2 of the design working pressure of 3500 used in the original PST special permit. The 5250 test pressure is called out in the special permit, and applies regardless of whether the cylinder is rated for 3442 PSI or 3500 PSI. In any case, like low pressure steels, the rated burst pressure is reached when the cylinder pressure exceeds the rated fill pressure by about 51% (exactly 50% for 3500, 52.527% for 3442).
Burst disc assemblies are supposed to have a release pressure tolerance of +0%, -10%.
Allowing for this tolerance, a pressure increase of only 35% from authorized fill pressure for any of the steels brings the pressure to the point where the burst disc could fail, even if it is operating within design specifications. (3/2 * .9 - 1)
Aluminum cylinders reach the pressure where the burst disc could fail at 50% above authorized fill pressure (5/3 * .9 - 1)
Gas laws and thermal expansion
On a hot day with no wind and full sun cylinders left outside can be estimated to reach about 140 degrees F. This is roughly similar to the temperature that the trunk of a car would reach under similar conditions. The exact temperature reached is affected by, among other things, the actual air temperature, the angle of the sun, the color and condition of the cylinder's surface, the orientation of the cylinder, and the color and insulating properties of the surface upon which the cylinder is placed.
Cylinder pressure increases by about 1% for every 5 degree (F) increase in temperature. Therefore, in the sun or in a hot trunk, a cylinder filled to its authorized fill pressure at 70 degrees would reach a pressure 14% in excess of the authorized fill pressure ( (140-70)/5 ). This is still considerably below the 35% point at which a burst disc is supposed to stay closed on an LP or HP steel (see preceding section).
In fact, the 35% point is reached at approximately 230 degrees F (70 + 35 * 5), far beyond the temperature a cylinder should be subject to in any conditions other than a fire.
Causes of burst disc failure
Since thermal expansion and manufacturing tolerance, taken together, don't begin to explain why burst discs rupture at the rate they do, there must be other causes.
These are the known causes:
- Reuse of a burst disc that has previously been installed. Considerable strength is lost when this occurs.
- Overtorquing during installation.
- Wrong burst disc. Sometimes a result of inadvertent interchange of valves between LP and HP cylinders or between steel and aluminum cylinders. Many older assemblies and some newer ones are not marked externally, or allow interchange of the disc and the externally marked plug, making a mixup impossible to detect without disassembly (which then requires replacement with a new burst disc assembly)
- Lengthy service history. Due to fatigue effects
- History of overfilling
- Incorrect assembly sequence. Some burst disc assemblies have a washer and a disc, and it is possible to install them in the wrong order.
- Mismatched components. Trying to separate the plug from a one-piece assembly to make it into part of a three-piece assembly will cause failures because the shape is wrong
I would speculate that heat may weaken the burst disc components, also, causing failures to occur in hot conditions with greater frequency than the gas laws alone can explain.
Risks of older systems
Older burst disc assemblies that have a single axial discharge port, rather than the 2, 3, 4, or 6 ports common in modern systems, pose particular hazards. In the event of a burst disc rupture, the thrust from the single axial discharge port is much greater, enough to lift the cylinder off the floor or other supporting surface.
Some much older systems use a lead plug which can pose another hazard as the plug becomes a high-speed projectile in the event of rupture.
Best practices - suggested
Burst discs should be replaced with entire new one-piece assemblies, externally marked with the rated burst pressure, at hydro or any time the burst disc is removed in the course of valve maintenance.
Older systems with a lead plug or a single axial discharge port should be replaced immediately.
Role of the individual diver
Shops vary in their approach to hydro and VIP. Some conservative, safety-oriented shops will not replace burst discs at hydro unless asked to do so. Divers have to be advocates for proper maintenance of personally owned gear.
The risk posed by burst discs "letting go"
Typically cylinders will fall to the ground if upright and unsecured, and will move about and collide with people and objects in their path. The noise is loud enough to create a risk of acute hearing loss. In an enclosed space, such as a car, pressurization may cause damage e.g. from windows breaking or popping out of their moldings.
Related threads
Burst disc failure leads to over $2000 in property damage
Close Call at the LDS and a High Pressure Reminder...
