Over the years, fires involving oxygen regulators have been a thorn in the flesh for regulator designers and systems engineers. An oxygen fire is extremely violent and too many times leads to serious injury and even death.
In recent years, there has been an increase in reported incidents (fires) involving small medical and emergency oxygen cylinders. The regulators used on these cylinders are small in size and are routinely constructed of aluminum components for weight considerations. Currently, the FDA, ASTM and UL are all in a mad dash to see who can prohibit the use of aluminum as a material in these regulators first. Are these agencies jumping the gun? To answer this question, let’s first look at the problem.
Adiabatic Compression - For fluids confined to a fixed volume, heat can be transferred in several ways: Warmer fluid can move into colder regions (convection), or the heat can conduct from hot to cold (thermal diffusion). For the inlet cavity of an oxygen regulator, the heat transfer mechanism is "adiabatic compression," also called the piston effect. This is where a hot, expanding outer layer of gas (from quickly opening a cylinder valve) acts like a piston and compresses the interior high pressure cavity of a regulator, heating it up. Gases typically have a very large thermal response to pressure changes. The heat generated from the gas cannot be quickly diffused through the walls of the regulator body. If the engineer has done the proper homework, the regulator body and valve can withstand this high temperature spike, however, if foreign particles are present, such as dirt, dust, oils, insects, etc., ignition may occur. Foreign particulates almost always have a lower combustion temperature than designed components. When a foreign particle is ignited, a kindling chain reaction of other components occurs until the temperature is high enough to ignite metals. With pure oxygen and an ignition, the regulator itself becomes a fuel, burning violently and potentially causing severe injuries to anyone in close proximity.
A large percentage of oxygen regulator fires in the field can be attributed to contamination by foreign particles or substances inside the high pressure cavity of a regulator. Many accidents can be avoided if the end-user takes care to eliminate the possibility of such contamination and uses proper techniques in opening oxygen cylinder valves and changing oxygen cylinders.
Why Use Aluminum Regulators?
Most industrial regulators have predominately brass components. The medical industry, however, prefers small, lightweight aluminum or aluminum/brass regulators more in proportion with the small aluminum or steel cylinders used. Compared to brass, aluminum has a very low ignition temperature and a very large heat of combustion. Because of the explosive violence of burning aluminum, the chance for catastrophic failure for aluminum regulators is greater than in brass designs. This is why we are now seeing manufacturers return to using brass components in medical gas regulators.
Should we just throw in the towel on aluminum designs altogether? That’s the message being sent by the FDA, UL and ASTM. Could one design an aluminum regulator to be more resistant to oxygen ignitions even in the presence of particulates? Perhaps, depending on the circumstances, but let’s first take a practical look at what can be done to improve ignition resistance in aluminum regulators. First of all, the presence of particulates means that there is contamination in the oxygen regulating equipment. The presence of contaminates can be controlled with the use of filters and also using proper techniques when changing cylinders such as cracking the cylinder valve to blow out dust and debris before attaching the regulator. Also, SLOWLY opening the cylinder valve after attaching the regulator can prevent a multitude of accidents. Opening the cylinder valve slowly allows for more time to diffuse the heat associated with the compressing gas as well as decreasing the piston effect and thus the amount of heat generated over time. But aside from using proper safety techniques, what can the engineer do to insure that an aluminum regulator is designed safely?
The Inlet Passage
It is extremely important that engineers take the time to properly evaluate the inlet passage of a regulator and/or system. The inlet passage is the route that the gas takes from the cylinder to the pressure reducing valve. Regulator inlet filters need to have at least a 66 micron filtration capability according to the Compressed Gas Association (E-4, E-7), but the smaller the better.
Ten (10) micron filters are usually not more expensive when purchased in quantity and will improve the ignition resistance due to contaminants. In addition to filters, this passage needs to have a large surface area as it dead ends into the regulator valve. A large surface area allows more heat to be diffused into the walls of the regulator body (see fig.1). Small, blind-hole type passages have the potential to increase the temperature dramatically during adiabatic compression and should be avoided. If the design will not allow for a larger diameter inlet passage, bore the passage in the body deeper than needed past the valve area to increase this surface area. Also, the use of a copper plug may improve ignition resistance.
has a very low heat of combustion compared to aluminum (Nickel 200 = 241
kJ/g•mole vs. Aluminum 6061 T6 = 1676 kJ/g•mole). Typically, nickel and
nickel alloys are much more resistant to ignition than aluminum/aluminum
alloys. It would be impractical to manufacture regulator bodies from nickel
alloys; however, nickel plating could be an effective and inexpensive way to
increase ignition resistance on aluminum regulator bodies and valve
components. The nickel plating would need to be a high quality
electroless process that eliminates flaking of the nickel plating which, of
course, would be counter-productive.
Ignition tests in recent years conducted by independent test facilities have found that in some medical regulators the relief valve can be a source of regulator ignition. Material selection for these types of relief valves is crucial. Designers should use non-metallic materials appropriate for oxygen as well as materials that have a low heat of combustion and high ignition temperature. Other general information regarding selection of materials can be found in the Compressed Gas Association’s publication TB-12 Design Considerations For Non-metallic Materials In High Pressure Oxygen Supply Systems.
In summary, there are some very good reasons to believe that aluminum regulators can be successful in an oxygen environment. Recent efforts by the FDA and UL to prohibit the use of aluminum in oxygen regulating systems seem to be a knee-jerk reaction to this serious problem. With some effort and constructive thought, however, this problem of aluminum oxygen regulator fires could possibly be remedied or, at a minimum, dramatically improved.
David Gailey is the manager for Specialty Products for The Harris Products Group, A Lincoln Electric Co. He has been with Harris for 27 years and served as past chairman of the CGA Industrial Gas Apparatus Committee.