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feature

November/December 2008
Feature

2008 ASTM International Advantage Award First Place

Preventing Oxygen Equipment Fires

Developing an Effective ASTM International Standard to Prevent Further Devastating Medical Oxygen Fires

On June 12, 1998, a 41-year-old male Florida firefighter performed the daily routine equipment check on his assigned fire engine that he had safely performed hundreds of times before.1 The check included verifying that the emergency medical oxygen equipment (see Figure 1) was sufficiently full for service, following a procedure consistent with most other fire departments and emergency responding units around the United States. The check included verifying that the emergency medical oxygen equipment (see Figure 1) was sufficiently full for service, following a procedure consistent with most other fire departments and emergency responding units around the United States.

When he opened the oxygen post valve, the oxygen equipment instantaneously flashed, emitting two flames, each more than a meter long, from the regulator. The firefighter’s clothing from the waist up ignited as he turned and fell to the ground. Luckily, other firefighters who were washing the engine nearby used a hose to help extinguish the flames.

Figure 1 — Typical portable medical oxygen system

Who would have thought that performing a routine check of emergency medical equipment would put someone in the hospital with first-, second- and third-degree burns on over 36 percent of his body? Figures 2 and 3 show the resulting damage to the regulator and all the equipment. Two months later, on Aug. 27, 1998, after just starting her evening shift, a 24-year-old female emergency medical technician was performing a check on equipment, including the medical oxygen system, during a routine ambulance changeover in South Carolina.2 She opened the top of the oxygen equipment bag and made three attempts to open the cylinder valve to pressurize the regulator. The valve was tightly closed, so she repositioned herself and the cylinder to obtain extra leverage. The valve opened on this fourth attempt and pressurized the aluminum regulator, which immediately flashed, emitting a white fireball.

As the EMT’s clothes caught fire, she pushed the regulator and cylinder back inside the patient compartment of the ambulance and proceeded to run into the station’s bay where other EMTs immediately initiated patient care before she was transported to a local hospital where she was treated for severe burns. The ambulance fire was extinguished by the responding fire department and the ambulance was later declared a total loss with an estimated value of $175,000. Figures 4 and 5 show the remains of the regulator and damage to the ambulance, respectively.

post-incident condition

Figure 2 — Post-incident condition of regulator from June 12, 1998, fire

The Problem

Oxygen medical regulators are devices that convert compressed oxygen at high cylinder pressures to a lower, constant working pressure suitable for patient delivery. They are part of an overall oxygen delivery system that includes a cylinder, cylinder valve, regulator (see Figure 1), delivery cannula and mask or resuscitator. Oxygen regulators are used in emergency medical services, home health care, hospitals and various industrial applications. It was estimated in 2000 that there were approximately 1.5 million medical oxygen regulators in use.

Although essential for life, oxygen is a hazardous substance because it makes materials easier to ignite; their subsequent combustion is more intense than in air. Some materials that are not flammable in air, such as aluminum, become flammable in oxygen. Systems for delivering oxygen must be well designed and properly used to prevent fires.

Figure 3 — Post-incident condition of oxygen medical equipment in fire engine compartment

The two aluminum regulator fires described above were not isolated. The U.S. Food and Drug Administration received 16 reports of other similar fires between 1993 and 1999 involving aluminum regulators attached to cylinder valves of portable oxygen cylinders. Considering the number of devices in clinical use, oxygen regulator fires are relatively rare; however, the consequences of these fires were quite serious. In total, these incidents caused severe burns to 11 health care workers, EMTs and patients, and resulted in the loss of two ambulances and significant damage to a fire station.3,4 What makes this more tragic is that most of the fire victims were first responders, EMTs, firefighters — people who save lives — and the cause of their injuries was life-saving equipment. In February 1999, the FDA and the U.S. National Institute for Occupational Safety and Health released a joint Public Health Advisory titled “Explosions and Fires in Aluminum Oxygen Regulators” to alert routine users of the possible hazards associated with the equipment.4

Because of the severity and increasing frequency of these types of incidents, FDA and NIOSH attempted to identify the causes of these fires by consulting the expertise of organizations such as the National Aeronautics and Space Administration and ASTM International. Members of ASTM Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres had already been involved with the investigation of the medical oxygen regulator fires and were familiar with the cause of the fires. The consensus of the committee was to form a task group to develop a standard for evaluating ignition sensitivity and fault tolerance of medical oxygen regulators with the goal of preventing further fires.

This paper follows the development of a successful standard — from the need for a new standard to establishing a provisional standard and performing round-robin testing on the current version of ASTM standard G175, Test Method for Evaluating the Ignition Sensitivity and Fault Tolerance of Oxygen Regulators Used for Medical and Emergency Applications. The benefit and impact of this standard is clear and simple yet extremely remarkable: Since the inception of ASTM G175, no fires of oxygen medical regulators that have successfully met the requirements of this standard have been recorded. This is exactly the outcome that Committee G04, FDA and NIOSH were all seeking. The developers of the standard used data from many existing ASTM International standards on oxygen safety.

Figure 4 — Post-incident condition of regulator from Aug. 27, 1998, fire

The Need for a New Standard

By the year 2000, oxygen fire forensic experts from Wendell Hull and Associates Inc., who were also members of ASTM Committee G04, had investigated or inspected evidence from 11 separate incidents, including the first two presented in this paper. To identify specific design or usage problems that were potentially contributing to the fires, they performed an evaluation of adverse event information and forensic analyses of burned regulators. The forensic analyses of failures employ a root cause and origin analysis approach. For oxygen fires, this involves determining the point of ignition and the method of ignition, which is otherwise known as an ignition mechanism.

In order for a fire to occur, three elements (fuel, oxidizer and ignition) are required, as depicted by the fire triangle in Figure 6. Two elements are always present within medical oxygen equipment: The equipment materials are considered the fuel and the pressurized oxygen is considered the oxidizer. Therefore, only an ignition mechanism strong enough to ignite the fuel is required for medical oxygen equipment to catch fire. The strength of an ignition mechanism required for the ignition of materials within a high pressure oxygen environment is much less than the strength required in an air environment because of the relatively low ignition energy of materials in oxygen. ASTM International standard G88, Guide for Designing Systems for Oxygen Service, describes a variety of ignition mechanisms in oxygen systems and how to defend against such heat-generating mechanisms.5

Figure 5 — Post-incident condition of ambulance from Aug. 27, 1998, fire

In all of the investigated fires, the ignition originated within the oxygen-wetted areas of either the regulator or cylinder valve (see Figure 1). Of the 11 fires examined by WHA, a variety of four ignition mechanisms contributed to the fires: heat of compression ignition, contaminant ignition, particle impact ignition and promoted ignition.

Figure 7 shows the assembly of a cross-sectioned cylinder, valve and regulator to better illustrate the flow path and areas of ignition origin. Before the oxygen is introduced into the regulator, the high pressure oxygen in the cylinder is sealed off at the cylinder valve seat. When the cylinder valve is opened by using the valve handle, high pressure oxygen expands across the valve seat, flows into the regulator and rapidly recompresses at the regulator valve seat. The compression of oxygen generates heat within the gas and can be sufficient to ignite nonmetallic materials such as the seat of the regulator. This is known as heat of compression ignition.

fire triangle

Figure 6 — Fire triangle; all three sides are required for a fire to occur.

Other than igniting the regulator materials, heat of compression is capable of igniting any contaminants present. Contaminants (i.e., flammable foreign matter not intended to be present in oxygen components) such as hydrocarbon oils are easily ignited by heat of compression compared to the solid materials. Once ignited, contaminants can release sufficient energy from their heat of combustion to kindle the materials of the regulator. This is known as contaminant ignition.

When the cylinder valve is first opened, the gas flow is extremely fast, with velocities approaching the speed of sound across the seat of the valve (see Figure 7). Accelerated by this high velocity flow, small metallic particles generated during assembly and operation can shoot out of the valve into the regulator. The impact of these particles against the regulator materials causes a transfer of kinetic energy to thermal energy, potentially igniting the particle and the impacted material. This is known as particle impact ignition. Aluminum is extremely susceptible to this ignition mechanism.

The last ignition mechanism of concern for the type of oxygen equipment shown in Figure 7 is known as promoted ignition. This requires another ignition mechanism like the three described above, however, the ignition occurs upstream of the gas flow, and the fire propagates with the flow to kindle flammable materials in its path. If ignition occurred within the cylinder valve, such as ignition of the valve seat, then the gas flow would force the fire toward the regulator and kindle the flammable materials of the regulator.

Each of the above ignition mechanisms leave different fire patterns and clues that help identify the root cause of the fire. The 11 fire incidents that WHA investigated and inspected are listed in Table 1 along with the ignition mechanism causing the fire based on the evidence.

cross section of assembly

Figure 7 — Cross-sectional view of oxygen cylinder, valve and regulator assembly

The major problem identified was the use of aluminum in critical areas of some regulators. Aluminum is used because it is lightweight, however, it is also flammable and highly susceptible to most types of relevant ignition mechanisms. In standard tests, aluminum can burn in oxygen at pressures as low as 170 kPa. In contrast, brass, another commonly used, but heavier material, does not burn in oxygen at pressures up to 70 MPa. Every incident listed in Table 1 involved a regulator constructed mainly of aluminum. Most fires reported to FDA involved aluminum regulators or regulators largely constructed of aluminum, whereas only a few fires involved brass regulators.

But materials selection alone cannot guarantee oxygen regulator safety. It must be coupled with good design practice and proper use. Proper filter design and material selection is essential to mitigating the risk of ignition mechanisms like particle impact ignition, contaminant ignition and promoted ignition. Other design factors contributed to the fires, for example, the lack of protection around flammable components such as valve seals and springs. Finally, user error contributed to some fires, such as the use of multiple gaskets, hydrocarbon contamination, improper maintenance and other practices. However, good components should be designed and tested to safely tolerate reasonably foreseeable user error.

Although there are numerous criteria available to facilitate safe design, those criteria are somewhat subjective and difficult to validate systematically. At the time of these fires, an International Organization for Standardization (ISO) test method was available for assessing regulator vulnerability to heat of compression ignition, however, the majority of the ignition mechanisms that caused the fires in Table 1 were not heat of compression. The other ignition mechanisms were not addressed in the ISO standard. In fact, regulators that pass the ISO test were involved in the fires in Table 1, demonstrating that the existing standard was not adequate to ensure oxygen regulator fire safety.

Therefore, a standard was needed that accounted for all potential types of ignition mechanisms present under normal and reasonably foreseeable abnormal conditions (including user error). The objective was to have a test standard that ensured that regulators were both ignition-resistant and fault-tolerant, having a low probability of ignition and, should it occur, a low consequence of ignition.

Development of the Standard

The task group formed within ASTM Committee G04 involved industry, technical experts, users and regulatory agencies. The committee incorporated the existing ISO test for assessing ignition resistance to heat of compression (Phase 1) and undertook development of a new promoted (forced) ignition test for assessing fault tolerance (Phase 2). Developmental and validation research for Phase 2 was performed at the WHA Test Facility and the NASA White Sands Test Facility, both in Las Cruces, N.M., in cooperation with ASTM International, FDA and NIOSH.

The Phase 2 test subjects regulators to a clinically realistic and reproducible ignition event, simulating real-world conditions such as particle impact ignition, contamination and/or promoted ignition. The regulator is subjected to a single oxygen pressure shock similar to that used in the Phase 1 test that creates compression heating. The major difference is that in the Phase 2 test, an ignition pill is positioned at the regulator inlet where it ignites by heat of compression and promotes ignition of the regulator if the regulator is not fault tolerant. Fault tolerant regulators swallow the ignited pill (dissipate the heat without burning out), posing less of a hazard to users.

The development of a pill that would ignite reliably was the principal technical challenge of this effort. The pill could not release too much energy, causing all regulators to fail, nor could it release too little, leading to an ineffective test. The pill design also had to allow dependable and consistent ignition and complete combustion. Calculating ignition and combustion in oxygen systems is far from an exact science. There are no formulas and no codes or models that accurately predict the ignition of materials by heat of compression. Therefore, experts from ASTM Committee G04, NASA WSTF and WHA relied on experience, guidance and test data from the ASTM International standard tests shown in ASTM Standards Used in Determining Ignition and Combusion for the Development of Standard G175.

elements for G175

Figure 8 — Elements used to fabricate the ASTM standard G175 Phase 2 ignition pill5

It was determined that, at a minimum, the pill required a combination of metallic and nonmetallic materials replicating contaminants that may be found in actual applications and that promote a kindling chain to ignite the metallic component of the ignition pill. Used regulators and cylinders were examined to identify particles typically present in the system. Aluminum and iron particles were found originating from the oxygen cylinders or filling systems, and nylon particles were found originating from the cylinder valve seat. Therefore, the pill was created with this combination of materials. The final design, shown in Figure 8, consists of a small nylon cup that is filled with a mixture of aluminum and iron powders and encapsulated with five layers of polyamide (nylon) sheeting. The aluminum and iron powders replicate the particle impact ignition risk, and the nylon casing replicates the risk of contaminant and promoted ignition. The overall dimensions are 7.1 mm in outside diameter and 3.2 mm in height; total mass is 67 mg, causing a total energy released after ignition of 2,000 kJ ± 200 kJ. This is about half the amount of energy produced when a typical nylon cylinder valve seat ignites, as determined by calculation and verified experimentally using ASTM standard D240, Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter.

Once the ignition pill design was completed in 2000, the provisional version of the standard that would become G175, ASTM PS127-00, Test Method for Evaluating the Ignition Sensitivity and Fault Tolerance of Oxygen Regulators Used for Medical and Emergency Applications, was used to begin interlaboratory testing. Ignition pills fabricated by different laboratories were shown to consistently produce the energy requirements within the tolerance5 using ASTM standard D240. Laboratories from around the world were involved. Using a test article similar to the design of a typical medical oxygen regulator, laboratories performed promoted ignition testing using the ignition pills. A comparison of video and photography that captured the resulting fires showed consistency among laboratories. Round-robin testing of actual medical oxygen regulators also demonstrated consistency among the laboratories. It was critical that damage from laboratory simulations be consistent with that observed in actual fire incidents. The final validation testing for PS127 demonstrated this for several models of problematic regulators, as in Figure 9. After the round-robin testing was completed and analyzed, Committee G04 approved ASTM standard G175 in 2003 and withdrew PS127.

Figure 9 — Comparison of medical oxygen regulators that experienced a fire (a) in service (Springfield, Ore.) and (b) in a laboratory subjected to ASTM standard G175 Phase 2

Effectiveness and Benefits

Since the approval of ASTM G175, no fires have been recorded involving oxygen medical regulators that have successfully met the requirements of ASTM G175. This incredible achievement is further distinguished considering that ignition and combustion is not an exact science; compared to other fields, it is poorly understood. The added difficulty of trying to prevent ignition with high pressure oxygen equipment is that ignition either occurs or it does not. Generally, there is no gray-zone. Oxygen equipment can survive for years just below the ignition threshold without exhibiting any signs of being hazardous. However, if ignition does occur, then the result is typically catastrophic, causing the sudden release of large amounts of energy with the potential to cause major damage to people and objects nearby.

The energy release of a medical oxygen regulator undergoing an ASTM standard G175 Phase 2 promoted ignition test event is better illustrated in Figure 10. This is four frames of high speed video showing the explosion-like fire, which is consistent with the descriptions of the fires investigated by WHA. Initially two flames are emitted out of the regulator (Figure 10a), which rapidly grow in size, reaching 1 to 1.5 m in length, before the entire regulator is engulfed in flames within 50 ms.

Because the ASTM standard closely represented the ignition mechanisms of particle impact, contaminants and promoted ignition, manufacturers began to redesign their regulators to defend against these mechanisms. The result was the new range of safe, ignition and fault tolerant regulators that are currently on the market. Not only has ASTM standard G175 prevented deaths (or even injuries), it has also prevented damage to equipment and facilities. Although significant, the costs of repairs and equipment replacement due to these fires are minimal compared to the multimillion dollar lawsuits and insurance claims that were associated with some of the fires.

ASTM standard G175 has gained wide acceptance throughout the United States in a relatively short period since its promulgation in 2003. Due to the nature of the explosive testing, the facilities capable of performing the test were limited. Approximately three private companies and two governmental organizations (NASA WSTF and NASA Marshall Space Flight Center) perform this testing in the United States. Four or five additional international agencies, predominantly in Europe, have also conducted ASTM standard G175. The number of testing agencies that perform the test now has doubled compared to the number of agencies that were involved with the round-robin testing during the standards development, indicating a significant increase in the use and application of the standard. Out of the three private U.S. companies, two are manufacturers and the third is WHA, who serves the majority of other oxygen regulator manufacturers in the world. WHA estimates that approximately 10 to 15 different regulator types have been tested each year since the standard’s inception for clients in Australia, Canada, China, Europe, Japan and the United States.

ignition test

Figure 10 — Single frames taken from high speed video of an ASTM G175 Phase 2 promoted ignition test. Each frame was taken after the first frame to exhibit flame corresponding to the following milliseconds time periods from this point: (a) 4 ms, (b) 15 ms, (c) 26 ms and (d) 65 ms.

In February 2007, the FDA proposed a new rule in the Federal Register that involves the development of a special controls guidance for medical oxygen regulators.6 The main component of the special controls guidance is ASTM G175. It is not a requirement for manufacturers to conform to the standard; however, manufacturers who do not conform must demonstrate that the alternate measures were followed to address the risks identified in the guidance document and provide equivalent assurances of safety and effectiveness, which, based on WHA’s experience, is a more challenging and expensive measure than conforming to G175.

Due to the success of G175 for medical regulators, the application of the standard to other oxygen components is increasing in interest. The FDA’s proposed rule is also applying the special guidance control to medical oxygen conservers, which are essentially a regulator with an extra mechanism to conserve oxygen while the patient is exhaling. This is expanding the use of ASTM standard G175 since conserver manufacturers are already testing according to the standard. In addition, Committee G04 is revising G175 to incorporate the testing of a similar device known as a valve integrated pressure regulator, which combines the cylinder valve and the regulator into one component. Because the ignition risks remain, the standard applies, and new sections in the standard are being proposed to address the different design of a VIPR in comparison to a stand-alone regulator. The application of this standard to VIPRs is anticipated to significantly increase the use of ASTM G175 because of the increasing popularity of VIPRs in the United States and the already predominant use of VIPRs in Europe.

Conclusion

The effectiveness of ASTM standard G175 has clearly prevented any further devastating fires with conforming medical oxygen regulators. The development of the standard required the expertise of many large and distinguished organizations, including ASTM International, FDA, NIOSH and NASA, and the use of multiple existing ASTM International standards related to oxygen fire safety. The use and applicability of the standard is increasing internationally and for other types of oxygen components. The standard has and will continue to ensure that rescue personnel and the people they serve are benefiting from safe medical oxygen equipment.

References

1. NIOSH, “Oxygen Regulator Flash Severely Burns One Firefighter – Florida,” Firefighter Fatality Investigation and Prevention Program Report 98-F23, February, 1999.

2. NIOSH, “Emergency Medical Technician Receives Serious Burns from an Oxygen Regulator Flash Fire – South Carolina,” Firefighter Fatality Investigation and Prevention Program Report 98-F24, September, 1999.

3. Newton, B. E., Hull, W. C., and Stradling J. S., “Failure Analysis of Aluminum-Bodied Medical Regulators,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Ninth Volume, ASTM STP 1395, T. A. Steinberg, B. E. Newton, and H. D. Beeson, Eds., American Society for Testing and Materials, West Conshohocken, PA, 2000.

4. Miller, T. H., “Fires Involving Medical Oxygen Equipment,” Special Report, United States Fire Administration, Federal Emergency Management Agency, Emmitsburg, MD, March 1999.

5. Smith, S. R., and Stoltzfus, J. M., “Preliminary Results of ASTM G175 Interlaboratory Studies,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Tenth Volume, ASTM STP 1454, T. A. Steinberg, H. D. Beeson, and B. E. Newton Eds., American Society for Testing and Materials, West Conshohocken, PA, 2003.

6. Medical Devices; Anesthesiology Devices; Oxygen Pressure Regulators and Oxygen Conserving Devices, 21 CFR Part 868, Food and Drug Administration, Department of Health and Human Services, Federal Register, Vol. 72, No. 38, February 27, 2007, Proposed Rules, pp. 8643-8652.

Gwenael Chiffoleau, B.Eng., Ph.D., Aerospace Engineering, joined Wendell Hull Associates in 2002 and is the test facility manager and senior flammability scientist at WHA. He oversees the testing activities and coordinates the standard testing, special projects and test support groups. Chiffoleau leads research and test development involving flammability and ignition studies of materials and components in oxygen and other oxidizers such as nitrogen trifluoride (NF3) and nitrous.

Barry Newton, BSME, P.E., Ph.D. candidate, is the vice president of research and development at Wendell Hull Associates. He consults with private industry and government in the evaluation of pneumatic system/component failures, structural and component fires, fuel gas explosions, and oxygen fire risk analysis on industrial and medical oxygen systems. Newton also teaches the WHA Oxygen Fire Hazards Training courses worldwide.