PRD Safety Assembly (Burst Disk Kit)
PRD Safety Assembly (Burst Disk Kit)
- One-piece assembly for safety and convenience
- 4 options for multiple PSI ratings from 3360-5250
- Ratings are release pressure, not service pressure
- Compatible with DGX, Dive Rite, Thermo, Blue Steel
- NOT compatible with Sherwood or Genesis valves
A one-piece safety assembly eliminates the possibility of mismatched components and reduces the likelihood of accidental incorrect installation of the safety assembly. If the PRD is installed in the scuba valve too tightly it will rupture prematurely, and if installed not tightly enough it will leak. This PRD (Pressure Relief Disk) assembly requires 100-120 in-lb of torque and fits all brands of valves sold by Dive Gear Express including DGX Premium, Dive Rite, Thermo and Blue Steel; as well as most other brands. However this assembly does NOT fit Sherwood or Genesis valves.
Safety assemblies are described by their release pressure, not the service pressure, of the cylinder. Dive Gear Express recommends a new safety assembly be installed by a qualified technician whenever the cylinder is hydro tested and annually if the cylinder is known to have been overfilled.
Pressure Release Devices aka Burst Disk
In the U.S. at the turn of the twentieth century, a series of boiler explosions caused significant loss of life. The most famous of the accidents, the Grover Shoe Factory disaster in 1905 resulted in stringent safety laws and a national code related to safe operation of boilers but is inclusive of most pressure vessels, including modern portable compressed gas cylinders. All U.S. cylinder valves are required by the Department of Transportation (DOT) to have a pressure release device (PRD) that operates if the pressure exceeds the cylinder test specifications. The design of the PRD used is a non-reclosing rupture type, also called a burst disk assembly. The thickness of a small metal (usually stainless steel or copper) disk in the assembly determines the pressure at which it will rupture, releasing the compressed gas.
In practice, the presence of PRDs in SCUBA valves has proven to be problematic. Over time the thin metal disk can weaken or corrode due to exposure to moisture, causing it to fail and release in the range of the service pressure. Sometimes a valve with burst disk intended for a low pressure cylinder is inadvertently installed in a high pressure cylinder. A PRD also can be incorrectly over torqued during installation. If the PRD does not fail during or shortly after filling, the under rated or over torqued PRD is almost certain to eventually fail. Mismatched PRDs and cylinders are common, in part because most older PRDs on SCUBA valves are not clearly marked or not marked at all as to their release pressure.
While the PRD is required by law on SCUBA cylinders in the US, the PRD is not present on SCUBA valves in Europe because they are considered a hazard. A PRD failure can be especially dangerous with small scuba cylinders such as pony, bailout, and deco bottles. The sudden and unexpected rupture of the PRD releases a powerful jet of high pressure gas which can propel the relatively small metal scuba cylinder like a rocket and cause it to careen about the immediate area with enormous force. PRDs could, in theory, fail during a dive and cause rapid loss of breathing gas for life support. Some divers, particularly those who over fill their cylinders and/or dive in overhead environments, deliberately defeat the PRD using a practice known as 'double disking', installing a disk rated for a pressure much higher than the test pressure of the cylinder, or even installing an ordinary bolt in place of the PRD.
Proper installation of the PRD requires a torque wrench tool and proper removal of the disk may require an extractor tool, along with correct technique to avoid damage in either case. For these reasons, Dive Gear Express recommends that the cylinder valve be overhauled and a new burst disk be installed by a qualified Valve Repair Technician whenever the cylinder is hydro tested (i.e., every five years) and annually if the cylinder is known to have been overfilled.