OH TWO Can Be Oh So Deadly

John Sampson

After astronaut Jack Swigart first uttered the famous words, "Houston we've had a problem here," later investigation revealed that one of the two oxygen tanks in the Apollo 13 service module had suffered damage during testing before launch, and that an electrical failure caused the tank to explode and damage the second tank during the flight of April 1970.

Oxygen by itself is a non-flammable oxidizing gas, however oxygen vigorously supports combustion. The main concern with oxygen concentrations above 23% is the possibility that materials which would usually not ignite at normal oxygen levels will be quite prone to combustion. Oxygen tanks and cylinders have been exploding or catching fire throughout the history of aviation and the causes are largely unchanged.

In 1987 an Australian P3 Orion was destroyed at RAAF Edinburgh by an oxygen fire in its cockpit. The cause was found to be contaminants and aluminum swarf (drill shavings). Aluminum and oxygen are best kept segregated. Other contaminants are grease and lubricants. Oil and oxygen are a deadly combination; so too is arcing or sparking wires. In the 1967 Apollo 1 accident, in which three astronauts were killed in a ground test of the capsule atop the rocket, faulty wiring and a pure oxygen environment led to the deadly conflagration.

Contaminants in oxygen systems are potential fire or explosion hazards. When a gas is compressed, it releases energy in the form of heat. When the gas is compressed quickly inside a closed system such as a container or piping, temperatures inside the system can rise sharply. In an oxygen system, this rise in temperature can be high enough to cause explosive ignition of contaminants such as oil, grease, solvents and materials such as dust, lint, metal chips and many organic materials.

Oxygen flowing at high speed through valves and piping systems can also propel contaminants with such force that friction or impact between particles can raise their temperature to the ignition point. The ignition of these particles may be enough to cause heavier metal sections to ignite and cause a major accident. Most oxygen regulators are made of brass or aluminum. Aluminum and its alloys are more likely to ignite than brass. In standard tests, aluminum can burn vigorously at pressures as low as 25 pounds per square inch (psi), while brass does not burn at pressures below 10,000 psi. Over the past five years, the Food and Drug Administration (FDA) has received 16 reports of aluminum regulators used with oxygen cylinders burning or exploding. These incidents caused severe burns to 11 health care workers and patients. Many of the incidents occurred during emergency medical use or during routine equipment checkout. FDA and The National Institute for Occupational Safety and Health (NIOSH) believe that the aluminum in these regulators was a major factor in both the ignition and severity of the fires. Liquid oxygen (or LOX), as used in some aircraft systems, is a further step up the hazard ladder ... but the gas is bad enough.

In July, a Swearingen Metro 23 of Sunshine Express was written off by a cockpit explosion inside a hangar in Brisbane, Australia. The incident is still under investigation but thought to have been an oxygen explosion during scheduled maintenance. The technician involved narrowly escaped with his life.

Aluminum swarf isn't good for wire-bundles and it's deadly once oxidized by an oxygen gas-flow. "Golfers, replace your divots" and "Drillers, please remove your swarf." The lesson is evident: life giving oxygen can be deadly under pressure, and extreme caution during maintenance is the watchword.

A bank of oxygen cylinders was connected to a pipe leading to a high temperature furnace. When the tap from the cylinders was turned on there was a loud explosion and the pipe from the cylinders was ruptured. The explosion was attributed to the spontaneous combustion at room temperature of a spider which had crawled into the pipe.

Source: caspaerospace.com

A cylinder that has been fully depleted may allow the ingress of moisture and/or contaminants so oxygen cylinders should be maintained at a positive pressure at all times. Theoretically, any pressure differential is sufficient to prevent contamination, but cylinders that have been depleted to zero PSI must be internally inspected and purged.

Source: caspaerospace.com

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