Extinguishers: The Problem with Ratings on High-Flow Extinguishers

By Mark Conroy

On a pound-per-pound basis, high-flow dry chemical extinguishers are more effective on certain Class B fires than conventional dry chemical extinguishers. If high-flow extinguishers are more effective on flowing flammable liquid fires, why do they get the same ratings as smaller conventional extinguishers? At first glance, it seems to make no sense to rate something the same, when one works better than the other. Here is what you need to know about the rating system and why it is not applicable to high-flow extinguishers.

UL 711, Standard for Rating and Fire Testing of Fire Extinguishers requires ratings on all extinguishers as part of the listing and labeling criterion. Extinguisher ratings are applied to extinguishers based on actual test fires as described in this fire test standard. The Class B fires are conducted in square pans (rating numbers increase with bigger pans). Additionally, there is a minimum discharge time requirement.

High-flow extinguishers are intended for special high-hazard fires, like flowing flammable liquids or pressurized flammable liquids and gases. But there is no UL test fire to address listing of high-flow extinguishers for their intended use on flowing or pressurized flammable liquid or gas fires. The objective with designing a high-flow extinguisher is to deliver a higher flow rate during the time required for the rating. A higher flow over the same time as a conventional extinguisher means more agent and larger cylinders. Here is a comparison of the sizes of extinguishers, based on the 17-second discharge time requirement for the 60B rating.

A high-flow extinguisher delivers agent at a much higher rate per second than conventional extinguishers and is therefore more effective per second. As shown in the table, not only is the extinguisher more effective per second, but at the end of 17 seconds there is twice as much agent discharged, which requires a much larger cylinder than a conventional extinguisher. A bigger extinguisher is obviously more effective than a smaller one. A higher flow rate, combined with a larger quantity of agent, means high-flow extinguishers are much more effective compared to conventional extinguishers, despite having the same rating.

Your knowledge of the UL extinguisher rating system and high-flow extinguishers will enable you to explain the ratings and select and install these extinguishers for the most challenging of Class B hazards.