|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.
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