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Fighting Fire with Fire

New Test Standard Benefits Medical Oxygen Regulator Designers and Users

by Lori Kubinski

A new test standard developed by ASTM Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres, in collaboration with the U.S. National Institute of Occupational Safety and Health (NIOSH), Food and Drug Administration (FDA), National Aeronautics and Space Administration (NASA), and Wendell Hull and Associates (WHA), was brought to full-consensus status this spring and will be of great benefit to medical oxygen regulator manufacturers and their customers.

Background

G 175, Standard Test Method for Evaluating the Ignition Sensitivity and Fault Tolerance of Oxygen Regulators Used for Medical and Emergency Applications, includes two test phases that will allow oxygen regulator designers to screen designs that could fail under use conditions. This standard covers more than do existing test standards, such as ISO (International Organization for Standardization) 10524, Pressure regulators and pressure regulators with flow-metering devices for medical gas systems, which examine only one ignition mechanism (see sidebar).

The FDA requested that Committee G04 develop the standard in February 1999, after the issuance of a joint FDA/NIOSH public health advisory warning health care workers and institutions of the risks associated with using aluminum regulators in high-pressure oxygen systems. At that time, NIOSH had reported 14 incidents involving occupational injuries to 15 firefighters, emergency medical technicians, and health care workers from fire flashes related to aluminum oxygen regulators. The FDA had received reports of 16 incidents of aluminum regulators used with oxygen cylinders burning or exploding, causing severe burns to 11 health care workers and patients. Many incidents had occurred during emergency medical use or routine equipment checkout, but fires also occurred in other venues such as hospitals and home health care situations.

WHA, a forensic consulting and safety engineering firm with a longtime presence on Committee G04, had been called in by several fire departments to investigate accidents involving oxygen systems. Results from those investigations, many of which are detailed at www.cdc.gov/ niosh/firehome.html, suggested several possible causes for those incidents, including:

• Ignition of particles impacted onto regulator inlet filters, which kindled ignition to the regulator body and caused a breach;
• Cylinder valve seat ignition caused by frictional interaction with the metallic nozzle; and
• Ignition of an inlet gasket contaminated by oil or grease.

When NIOSH became aware of the failure analysis results, they joined with the FDA to issue a public health advisory on explosions and fires in aluminum oxygen regulators. The FDA, which regulates pressure regulators for medical use under 21 CFR 868.2700 (a), was considering an outright ban on the use of aluminum exposed to high-pressure oxygen in regulators and mandating the use of brass or an equivalent material. Sensitive to the economic and design impact that such a restriction would have on users, and the fact that using brass still would not completely ensure a safe regulator design, the FDA then approached Committee G04, long known for its expertise in oxygen system safety, to develop a standard test method for the government to use in ensuring product safety.

Test Method Development

Until the development of the new ASTM standard, regulator manufacturers had only one type of test method available for testing regulator performance. ISO 10524, an adiabatic compression test, deals only with regulators in pristine condition – not necessarily the condition found in many emergency medical or firefighter situations, which tend to be extreme and not reflective of laboratory conditions. Indeed, regulators that had passed the ISO test were in fact failing in the field, which prompted the regulatory agency to pursue development of a more stringent test method.

G04 agreed to take on the task of developing such a method. Early work examined contaminating regulators both upstream and downstream of the regulator inlet filter with a known amount of possible contaminants – metal particles typically resident in gas cylinders, nylon valve seat material, and even a hydrocarbon contaminant. It was important to expose regulators to usage conditions that had been shown would cause regulators to ignite and burn. After some preliminary testing, it was decided to abandon the idea of contaminating a regulator downstream of the inlet filter, due to the complexity of designing a repeatable igniter for those conditions. Instead, an igniter, called an “ignition pill,” was developed by WHA and refined by NASA to provide 500 calories per gram (cal/g) of energy and, when pneumatically impacted, damage vulnerable regulators very similarly to those examined from explosion incidents.

Once a repeatable igniter was developed in a single laboratory, round robin testing among several stateside and overseas laboratories commenced. It was verified that all laboratories could reproducibly make a pill that would provide ±500 cal/g of energy as determined by heat of combustion testing (in accordance with ASTM D 240, Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter). Next, test systems in participating laboratories were compared using test fixtures specially designed to witness the post-nozzle flame jet so that a subjective measure of test system performance could be made. When it was evident that the test systems were performing similarly enough to commence with testing various types of regulators, the laboratories then tested six different types to compare regulator performance among laboratories. The results were sufficiently alike to convince G04 members that the test method was ready to be used worldwide.
Even before the standard reached its full-consensus status, a provisional version was used by a number of regulator manufacturers to verify that safety design changes were adequate to reduce or preclude further incidents. The design changes often involved simply replacing vulnerable parts that had previously been made of stainless steel or aluminum with brass, and not replacing the entire regulator with brass, thereby preserving the manufacturers’ ability to offer a product that was lightweight and relatively inexpensive to produce. It is anticipated that this test method will substantially reduce the incidence of fires associated with the use of medical oxygen regulators. //

If you would like to learn more about oxygen system safety, ASTM offers two Technical and Professional Training courses to meet both designers’ and operators’ needs. Contact Scott Murphy, ASTM director of Educational Services (phone: 610/832-9685).

Copyright 2003, ASTM

Lori Kubinski has worked at the NASA White Sands Test Facility for eight years as a technical editor and project coordinator. She has been a member of Committee G04 since 2000 and serves as their publicity subcommittee chairman.

Common Potential Ignition Mechanisms

While it is impossible to prevent fires in oxygen systems, it is possible to mitigate them through maximizing the use of good materials, following safe operating practices, and minimizing known ignition hazards.

Particle Impact Ignition: Heat generated when small particles strike a material with sufficient velocity to ignite the particle and/or the material.

Adiabatic Compression Ignition: Heat generated when a gas is compressed from a low to a high pressure.

Flow Friction Ignition: Oxygen leaking across a polymer in such a way that enough heat is generated within the polymer to cause ignition.

Friction Ignition: Heat generated by the rubbing of two or more parts together.

Promoted Ignition/Kindling Chain: Heat from the ignition and combustion of a more flammable material igniting a less flammable material.

Static Discharge Ignition: Accumulated static charge on a nonconducting surface discharges with enough energy to ignite material receiving the discharge.

Electrical Arc Ignition: Electrical arcing through ungrounded or short-circuited powered components causing ignition of a flammable material.

Mechanical Impact Ignition: Single or repeated impacts on a material with sufficient force to ignite it.

Resonance Ignition: Acoustic oscillations within resonant cavities that cause rapid temperature rise – more severe if particles are present.

A History of Fruitful Collaboration

Acceptance of this standard by Committee G04 is just one example of the fruitful partnership that has existed over the years between ASTM, government agencies, and private industry. The FDA has participated extensively in the development of standards with different ASTM committees to facilitate regulation of medical devices.

Tests from NASA Standard 6001 that have become ASTM standards include:

• Test 13 (A), Mechanical Impact for Materials in Ambient Pressure LOX, and Test 13 (B), Mechanical Impact for Materials in Variable Pressure GOX and LOX, which now comprise ASTM G 86, Test Method for Determining Ignition Sensitivity of Materials to Mechanical Impact in Ambient Liquid Oxygen and Pressurized Liquid and Gaseous Oxygen Environments;

• Test 14, Pressurized Gaseous Oxygen Pneumatic Impact for Nonmetals, now ASTM G 74, Test Method for Ignition Sensitivity of Materials to Gaseous Fluid Impact; and

• Test 17, Upward Flammability of Materials in GOX, now ASTM G 124, Test Method for Determining the Combustion Behavior of Metallic Materials in Oxygen-Enriched Atmospheres.

NASA has also adopted several ASTM standards related to the aerospace industry, including:

• ASTM G 72, Standard Test Method for Autogenous Ignition Temperature of Liquids and Solids in High-Pressure Oxygen-Enriched Environments;
G 125, Standard Test Method for Measuring Liquid and Solid Material Fire Limits in Gaseous Oxidants; and
E 595, Standard Test Method for Total Mass Loss and Collected Condensible Volatile Materials from Outgassing in a Vacuum Environment.

Finally, Committee G04 has begun discussing developing a similar test method for oxygen regulators used for industrial gas applications that would be similar to G 175.