the manufacturer, and the most probable ignition mechanism was particle impact on the filter during the initial flow transient after the fire fighter opened the cylinder valve. This ignition led to the burning of the regulator’s aluminum body which caused the flash. Particle impact has been shown to be one of the most efficient mechanisms for directly igniting metallic components in a high-pressure oxygen environment. Particle impact ignitions occur when a metallic particle contained in the oxygen flow contacts a rigid surface and ignites. The ignition of the particle then promotes ignition of the target material. With the exception of aluminum, testing indicates that the particle itself must ignite during the impact event for an ignition of the target material to occur. For aluminum, however, even inert particles, such as grains of sand, have been shown to cause ignition of the target material. Testing also indicates that aluminum particles, such as would be produced from the aluminum cylinder, are susceptible to ignition when they are in an oxygen flow stream. Aluminum has been proven to ignite by particle impact at low temperatures and at sonic flow rates similar to the conditions that exist in the valve and regulator assembly. The aluminum is flammable at pressures as low as 35 pounds per square inch gauge (psig) and has been shown to ignite by particle impact at the flow and temperature conditions present in the valve at the time of the incident.
In this incident, tests show that the particle traveled with the oxygen downstream (into the regulator) and ignited as it made contact with the bronze sintered-inlet filter (identified as a stainless steel screen on Diagram 1). Testing indicates that while bronze has been shown to resist ignition and sustained combustion at these pressures, the thin cross-section of the filter and the very close proximity to the aluminum body provides for a kindling path to the aluminum body for particle impact ignitions on the surface of the filter. While bronze is resistant to ignition, experience has shown that sintered filters can melt and break apart when exposed to a strong ignition mechanism like particle impact.
The regulator involved in the incident was an aluminum body regulator with a bronze sintered-inlet filter housed inside an aluminum downstream flow path. The design of the high-pressure section provides minimal protection of the highly flammable aluminum to promoted ignition mechanisms. Further, the significant amount of aluminum in this regulator, directly exposed to the high pressure environment and oxygen flow, produced a design that is susceptible to an ignition. The design also allows for combustion in the high-pressure port to punch through the main seat in the regulator (Diagram 1) directly and progress into the piston barrel leading to rapid involvement of the low-pressure components and venting of combustion by-products outward (through the vent ports), potentially towards the operator. Some regulators used by the fire and EMT services have aluminum bodies that are equipped with brass or bronze components in the downstream flow path, and these components are very resistant to particle impact. Bronze or brass, which are both non flammable at the pressures in the regulator and do not ignite by particle impact, act as a shield between the particle that ignites and the aluminum body. In this type of design, a particle ignition usually will burn itself out before kindling the ignition and combustion of surrounding materials.
INJURY RESULTS
The victim received first-, second-, and third-degree burns over 36% of his upper body, including his hands, arms, chest, back, neck, and head area.
RECOMMENDATIONS/DISCUSSION
Recommendation #1: Fire departments should use oxygen regulators constructed of materials having an oxygen compatibility equivalent to brass.[1][2]
DISCUSSION: Aluminum alloys are attractive candidate materials for pressure vessels because of their high strength-to-weight ratios. High pressure oxygen system components for portable or flight use must be lightweight, so it may appear to be desirable to build their housings from such lightweight metals as aluminum. The use of aluminum alloys in lines, valves, and other components should be avoided whenever possible because they easily ignite in high-pressure oxygen, burn rapidly, and have very high heats of combustion. Aluminum is ignited exceptionally easily by friction because the wear destroys its protective oxide layer; it should not be used in systems where frictional heating is possible.[2]
Aluminum is easily ignited by particle impact, and aluminum particulate is a far more effective ignition source than many other metal particulate tested to date (titanium particulate has not been tested). High-pressure oxygen systems fabricated from aluminum must be designed with extreme care to eliminate particulate. Testing has
- ↑ Newton, BE. Report to State of Florida Fire Marshal’s Office concerning oxygen resuscitator fire. Wendell Hull & Associates, Inc., Las Cruces, NM, September 11, 1998.
Canadian Standards Association [1987]. Pressure regulators, gauges, and flow- metering devices for medical gases. Ontario, Canada, CAN CSA-Z305.3-M87.
ASTM. Standards related to flammability and sensitivity of materials in oxygen-enriched atmospheres [1997]. West Conshohocken, PA, ASTM PCN 03-704097-31.
National Aeronautics and Space Administration [1983]. Design guide for high pressure oxygen systems, Washington, DC, Publication 1113.
Newton, BE., Personal communication regarding the review of NIOSH FACE report 98-F23, January 13, 1999. - ↑ 2.0 2.1 National Aeronautics and Space Administration [1996]. Safety standard for oxygen and oxygen systems. Washington, DC, Publication NSS 1740.15.
