We take it for granted that many small-scale approval tests use heptane as a substitute for gasoline around the world, but might this have significant concerns for public safety?
Historically, fluorinated C8 and C6 foams have exhibited very similar control and extinction times on both heptane (a tightly defined fuel ideal for testing) and the varied formulations of gasoline. The main reason fluorinated foams are successful on many different hydrocarbon fuels is because they possess inherent fuel repellency on all hydrocarbon fuel. In contrast, F3 foams have no fuel-repellent properties and their fire performances are more fuel specific, e.g. F3s designed for aviation jet fuel may be significantly less effective on heptane or gasoline.
It has been widely assumed this heptane read-across also applies to Fluorine Free Foams (F3), but does it? Are we misleadingly increasing potential danger to life safety where heptane-based approval test standards are being used for gasoline hazards?
Heptane is not a ‘read-across’ for gasoline with F3s but is for C6AFFFs
Heptane has been used for good reason by several major approval test standards. Using a closely defined test fuel made good sense, enabling fair comparison between different foam agents, irrespective of the time of year or test location. It conveniently avoided seasonally and geographically varying problems of gasoline specifications around the world. A convenient AFFF ‘read-across’ for gasoline hazards made it the ideal choice. But assuming this read-across capability also holds for F3s is incorrect. Using such F3 approvals for gasoline hazards may be placing us all in unexpectedly increased danger – right now.
Important recent research by US Naval Research Laboratories (June 2019) has shown this traditional approach may be misleading us, particularly as more organizations are beginning to rely on F3 agents for their future fire protection.
Few, if any, actually use heptane as a significant fuel source, so gasoline and other similarly volatile hydrocarbon fuels remain the major hazards from which these approval tests are designed to adequately protect us. They still provide suitable read-across on volatile hydrocarbons including gasoline for virtually all fluorinated foams like AFFFs, FPs, FFFPs, AR-AFFFs, AR-FFFPs but probably not for any Fluorine Free Foams (F3).
Significantly divergent results with F3s on gasoline
An anticipated revision of MIL-F-24385F specification to include a pool fire fuel change to heptane from gasoline provided the impetus for conducting this research and publishing NRL’s important findings. NRL confirmed: ‘The focus [of this paper] is on the hydrocarbon pool fire suppression effectiveness of the C6AFFF and F3 formulations and, in particular, on a divergence in extinction effectiveness of the F3 formulations when the pool fire fuel is heptane vs gasoline. Advancement of this experimental F3 formulation to the MilSpec 28 ft2 gasoline pool fire suppression testing resulted in no extinction instead of the anticipated 30–60 second extinction time. Experimentation with commercial F3 formulations revealed a similar but variable and less pronounced effect.’
This latest research makes it very clear that heptane does NOT provide any read-across for F3 fire performance on gasoline, unlike C6 AFFFs, with the possibility of some F3s even failing to extinguish gasoline fires. This also dangerously erodes the test’s safety factors, supposed to handle many variables that happen at a real fire scene. While this includes fuel type, it must be based on a successful test fire that correlates to the hazards being faced.
How could we not know this after nearly 20 years of F3 developments trying to mimic AFFF’s unique capabilities? Why have we been making such a fundamentally dangerous assumption, seemingly untested – until now? Is this symptomatic of our complacency elsewhere concerning the criticality of life safety (or lack of) in the modern world? Examples include many recent high-rise cladding fires in Dubai, Melbourne and the tragic Grenfell Tower in London. How could we assume flammable cladding without any firebreaks in windy locations ‘wouldn’t happen’? Surely our job as fire professionals and regulators is to assume it probably will happen but adequately mitigate against such potential disasters occurring, using fire-protection selection, informed management decisions, materials choices, swift evacuation, collection and remediation of run-off etc., all helping to minimize that danger to life and firefighter safety, plus critical infrastructure. Just because we experience fewer major-life-loss disasters doesn’t make their imminence any less likely, or potentially catastrophic, if we fail to adequately prepare for those realistic worst-case scenarios.
Disturbing implications for common foam approval standards
The implications are equally disturbing as several major international fire test standards (e.g. EN1568-3, UL162, FM5130, Lastfire) use heptane when assessing F3s as the sole test fuel representing all non-miscible hydrocarbons (including gasoline) as recognized by NRL: ‘With the exception of MilSpec [which uses gasoline], heptane is also the test fuel prescribed by most international standards.’
Following in the light of UK Fire Research Station’s Briggs & Webb 1988 findings, which confirmed: ‘Increasing vigor of application can change a promising-looking foam into an ineffectual one. Output velocities from commercial nozzles are mostly of the order of 10–20m/s (as measured by high-speed photography). By contrast the measured 4–5m/s of Mil F24385 tests for AFFF are less demanding, particularly when it is considered that kinetic energy (of impact) is related to the square of the velocity. With the laboratory nozzles readily available, the output velocities were restricted to 6–8m/s. Though this falls short of impact velocities likely to be found in practice, it will be seen that the increase of impact velocity was sufficient to show significant loss of performance.’
They also found: ‘Other test methods, including the widely used Underwriters Laboratories [UL162] use heptane, where although consistency should be good, severity and realism are open to question. … In the case of firefighting foams, reproducing severe conditions calls for much higher impact velocities than those found in existing standards. … Realism also calls for fuels in common use or fuels formulated to simulate them. Final judgement of a foam’s effectiveness should be made on a full-scale “severe” version of a test, which in turn bears limited resemblance to the typical demonstration.’
Such incorrect assumptions of equivalency, both between gasoline and heptane, and test nozzle and commercial nozzle velocities, have worrying implications for public safety, where decisions already made are entirely reliant upon F3 installations to provide levels of expected life safety that are now potentially unlikely to be achieved.
Over-reliance on small-scale fire test standards without larger scale verification, without conducting realistic parallel testing on specific fuels in use (like gasoline or adequate substitutes as NRL and Briggs propose), could be placing both life safety and critical infrastructure at unexpectedly and unacceptably increased danger, right now – are your staff and facilities safe?
Test fuel change could provide reassurance
NRL highlighted that: ‘A switch to heptane as the reference [fuel] may result in some F3 formulations being qualified and then later lacking anticipated performance when being used by firefighters on gasoline fires.’ Unacceptable danger to life safety and critical infrastructure could result. Quick revision of approval test standards requires rectifying this fundamental weakness with F3s before it causes unpredicted and unacceptable loss of life and/or major critical infrastructure damage in major incidents. Meanwhile foam users should perhaps be checking their foam fire protection arrangements. Where F3 is being used, undertaking additional realistic fire testing using gasoline (and/or other volatile hydrocarbons used on site), could verify continued acceptability of their existing, and perhaps intended, use of F3 agents, particularly where existing delivery nozzles, realistic site application rates and larger scale fires are used.
Why gasoline’s divergent effects on F3s?
NRL investigated gasoline’s divergent effects on F3s, finding the aromatics were hardest to extinguish, in the order trimethylbenzene (TMB) > xylenes > toluene > benzene. Mechanisms involved extraction of potential F3 surfactant components into the fuel. Varying responses show some F3s are more vulnerable than others, but all still seem to suffer significant adverse effects.
It seems Jet A1 also contains these four aromatics, but in smaller quantities than gasoline, which may help explain why F3s generally find aviation fuel more challenging.
Could Heptane +25% TMB be effective future test fuel?
Results showed F3 being destabilized by aromatics, diminishing F3’s ability to extinguish pool fires. The team designed and tested a two-component alternative test fuel for unleaded gasoline. NRL found that: ‘Increasing the TMB content to 25% results in a [F3] foam degradation time coincident with that for gasoline.’ Also, when fire tested this 25% TMB in heptane delivered similar results to F3s on gasoline and fuel vapour transport measurements. Results for C6AFFF were also close to gasoline and heptane. Might this be an appropriate future approval test substitute for heptane?
Unique fluorosurfactant benefits may prove impossible to mimic
Extensive NRL testing observed: ‘If sufficient fluorosurfactant properties important for fire suppression can be maintained while incorporating a sufficient susceptibility to biodegradation and a non-toxicity, this approach of retaining some fluorine in the surfactant structure may have significant merit.’ Doesn’t C6AFFF meet this challenge with swift control and least foam and firewater runoff created?
Graphs of commercial 3% MilSpec C6AFFF (AFFF-3) as ‘control’ (Fig. 1) compared with four commercial 3% F3 results (Fig.2), shed more light on further implications. We clearly see close relationships between gasoline and heptane extinguishment results on 19cm pool fires with the C6AFFF-3, which also has 35% less Fluorine content than
C6AFFF-4 tested. Lower gasoline foam volumes at 50, 30 and 20sec extinguishment times were recorded for C6AFFF-3 compared to C6AFFF-4, although C6AFFF-4 required slightly lower foam volumes on heptane extinctions of 30 and 20sec. Both C6AFFFs extinguished the MilSpec28ft2 (2.6m2) gasoline pool fire in the required <30 sec, which all F3s seem unable to achieve.
Four commercial Fluorine Free Foam (F3) alternatives were similarly tested on 19cm gasoline and heptane pool fires. Their extinction profiles displayed considerable variation in effectiveness on both gasoline and heptane fires. Results were substantially divergent compared to C6AFFF-3, with the best F3 (NF Universal Green) requiring over three times more foam to extinguish gasoline at 50sec and 2.5 times more foam to extinguish heptane at 50sec. At 30sec extinguishment this best F3 required over five times more foam to extinguish gasoline, over twice as much to extinguish heptane, than C6AFFF-3. The worst commercial F3 surprisingly required eight times more (2,000mL/min foam) to achieve its best extinction on gasoline at 35sec.
Only the best F3 extinguished gasoline under 30sec, requiring high flow rates (>2,000mL/min) – nearly six times more than C6AFFF-3. Heptane required double the foam flow to achieve its 30sec extinguishment, compared to C6AFFF-3. The worst F3 seemed unable to extinguish heptane below 50sec, requiring 12 times more foam to do so than C6AFFF-3. How can such potential excess volume be accommodated in real world fires?
NRL confirmed: ‘General assessments of the [F3] fire-extinguishing performance range from optimistic to pessimistic. In the <1,000mL/min flow region gasoline is the predominantly more difficult fuel fire to extinguish with the differences becoming quite large at the 500 mL/min flow rate.’ Such significant impacts on F3 volumes may not be adequately considered by regulators seeking to avoid essential Major Hazard Facility (MHF) uses for C6 foams, perhaps not fully appreciating the serious volume implications of F3s, plus the potential compromises to life safety, critical infrastructure and environment quality that may result.
NRL’s extensive testing found ‘two important premises in trying to develop new F3 formulations to replace AFFF formulations are:
- Mechanisms of foam activity for pool fire suppression are fundamentally different for AFFF and F3 based foam formulations; and
- Effectiveness of fuel fire suppression by F3 formulations is dependent on the identity of the fuel, with the gasoline fire being the more difficult to extinguish while fuel fire suppression by AFFF formulations is relatively insensitive to the identity of the fuel.’
Yet C6AFFFs work quickly, reliably, effectively and flexibly on all hydrocarbon fuels similarly, providing exceptional re-ignition resistance. NRL recognizes: ‘It is likely to involve significant out-of-the-box thinking and chemistry, particularly if one tries imitating some of fluorocarbon surfactant’s more important properties.’
Speed of control minimizes potentially carcinogenic smoke and breakdown products affecting firefighters and nearby communities, reducing amounts of foam used, reducing containment overflows potentially causing major polluting events, even where F3s are used. As occurred at West Footscray’s chemical factory fire (Melbourne, August 2018). EPA Victoria confirmed >2,000 fish died and 16 times accepted PFAS recreational water levels were detected immediately downstream of this fire site for two weeks. Only non-fluorinated foams were used, taking a slow 17 hours to control and five long days to extinguish. Aren’t there better answers?
Best way forward
Capturing, containing, treating and safely disposing of all foams should be a requirement for any incident irrespective of the foam type used in order to prevent run-off, whether derived from firefighting foams or other breakdown products of the fire, from reaching our environment. Effective, affordable containment methods and treatment options for both fluorinated and non-fluorinated foams are now available to ensure this result.
In spite of improvements made in F3s, C6 foams still provide the fastest, most effective, reliable agents providing life safety and critical infrastructure protection for MHFs, without compromise. In the words of the UK Environment Agency: ‘In summary … foam buyers’ primary concern should be which foam is the most effective at putting out the fire. All firewater and all foams present a pollution hazard. … The key to preventing [the] worst pollution is to have a response plan to clear potential fire hazards. … All fire water runoff will be detrimental to the environment if allowed to enter water courses. … [the] best technique is to prevent pollution from entering in the first place.’
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