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
A one-piece safety assembly eliminates the possibility of mismatched components and reduces the likelihood of accidental incorrect installation of the safety assembly. This PRD (Pressure Relief Disk) assembly fits all brands of valves sold by Dive Gear Express including DGX Premium, Dive Rite, Thermo and Blue Steel. 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.
|Type of Cylinder||Service||PRD|
|Low Pressure Steels||2400||4000|
|High Pressure Steels||3442||5250|
Pressure Release Devices aka Burst Disk
All US 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 in scuba valves is a non-reclosing rupture type, also called a burst disk assembly. The thickness of a small metal disk, captured in an assembly between a metal or plastic gasket and a hollow plug, determines the pressure at which it will rupture and release the cylinder pressure more slowly than if the cylinder itself were to rupture with explosive force.
With loose piece PRDs, a common assembly error is installing the gasket between the disk and the plug. Also if the assembly is installed in the scuba valve too tightly it will rupture prematurely, and if installed not tightly enough it will leak. The assembly should be installed with a very precise amount of force using a special tool known as a torque wrench, calibrated in a unit of measurement known as inch-pounds. The safety assembly should be installed using torque wrench at 100 to 120 inch-pounds. In addition to ruptures due to being over tightened, if the burst disk is ever removed from the valve the disk and gasket must be discarded because reusing a disk will also cause it to rupture prematurely.
In practice, the presence of PRD's 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 prematurely. Sometimes a valve with burst disk intended for a low pressure cylinder is inadvertently installed in a high pressure cylinder. If the PRD does not fail immediately upon filling, the mismatched PRD is almost certain to eventually fail prematurely. 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 considered a safety device required by law on all pressure vessels in the US, the safety of PRDs on scuba valves is debatable. PRD failure typically occurs while the scuba cylinder is being filled, or often within a few minutes after being filled. The sudden and unexpected rupture of the PRD creates 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, causing rapid loss of breathing gas for life support. For these reasons, Dive Gear Express recommends that the cylinder valve be overhauled and an appropriately matched 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.