shown that aluminum is substantially more flammable in oxygen than brass or other high copper or high nickel alloys.[1]
Sources indicate that commonly used aluminum alloys can easily burn in the presence of high-pressure pure oxygen once an ignition is present. Thus, aluminum will burn in pure oxygen at a pressure of 35 pounds per square inch (psi) (this is about twice the normal atmospheric pressure) whereas some brass alloys require over 5000 psi of pure oxygen to burn. Aluminum will also produce approximately 10 times the amount of heat provided by copper alloys when burning.
One concern of using aluminum in the regulator flow path is the possibility of particle impact and the aluminum not being able to contain the ignition. Particles can be introduced into oxygen resuscitators in many different ways. Experts suggest the presence of a particle or particles in the cylinders is not as problematic as the design of the oxygen flow path and the materials used.
The cylinder has a post valve that closes off the oxygen opening and allows the regulator to attach (Diagram 2). When the post valve is screwed into the aluminum cylinder, there is a possibility that the two metals rubbing together (galling) could create metal particles that would remain enclosed in the cylinder body. Galling is a condition involving smearing and transfer of material from one surface to the other and particles could be introduced by metal-to-metal of seals rubbing which occurs when the post valve is opened and closed. The frictional heat of the galling could lead to ignition of the valve; or the particles generated by the galling could cause malfunction or ignition of another component within the regulator. Therefore, the design of the regulator's flow path should be resistant to ignition if particles should occur. Experts suggest the regulator flow path should be lined with brass, bronze, or a similar material which would resist particle ignition, and that using such a material would shield the particle ignition and provide the opportunity to burn out. Particulate migration from the cylinder can be minimized by the installation of a standoff tube (bayonette) at the inlet of the post valve.
Experts suggest that the design of the regulator involved in this incident allowed for combustion in the high-pressure port to punch through the main seat directly and progress into the piston barrel, leading to rapid involvement of the low-pressure components and venting of combustion byproducts outward (through the vent ports).
In this incident, the regulator (with the aluminum flow path) could not contain the ignition of the particle impact. The significant amount of aluminum in this regulator, directly exposed to the high-pressure environment and oxygen flow, produced a design that was susceptible to an ignition mechanism of this nature. The testing laboratory recommended that the flow path should be constructed of a metal such as brass or bronze to reduce the risk of a flash.
For further information concerning a regulator in use, NIOSH recommends that fire department’s contact the manufacturer of the specific regulator to determine if the regulator contains any aluminum components exposed to oxygen in the high pressure end of the regulator.
Recommendation #2: Fire departments should ensure that the cylinder is placed in an upright position, the cylinder post valve is pointed in a safe direction (away from the operator), and opened then closed before the regulator is attached to the cylinder.[2]
DISCUSSION: After removing the valve protection cap, the cylinder post valve should be opened slowly for an instant to clear the opening of particles of dust or dirt, being careful to point the valve opening away from personnel and in a safe direction. This will help prevent particles from being introduced into the regulator.
Cylinders are generally stored on the fire trucks in a horizontal position, which could allow particles to rest near the neck of the cylinder. Whenever possible, cylinders should be placed and secured in an upright position not only when in storage, but also when in use. If cylinders are stored in an upright manner, any particulate in the cylinder would generally settle in the bottom of the cylinder and not as easily entrain in the oxygen flow discharge.
In this incident, the cylinder was lying in a horizontal position when the victim opened the airway bag. When he placed the cylinder in a vertical position and opened the post valve, the regulator flashed. It is possible that particles inside the cylinder were present near the neck of the cylinder, and when the valve was opened the particles were forced into the oxygen flow path.
