by Grant Lobdell
According to section 12.6.2 of the current, 2016 edition of National Fire Protection Association (NFPA) 11 Standard for Low-, Medium-, and High-Expansion Foam, “Samples of [foam] concentrates shall be sent to the manufacturer or qualified laboratory for quality condition testing.”
Expansion and Drain Time
The 2016 edition of NFPA 11 does not contain a formal definition for foam quality. However, according to the seventh edition of Underwriters Laboratories Inc. (UL) 162 Foam Equipment and Liquid Concentrates, a standard used in the listing of foam concentrates, low expansion foam quality is defined as “a measure of a foam’s physical characteristics, expressed as the foam’s: a) 25 percent drain time, and b) Expansion value”. Based on this definition, at minimum, the quality condition testing required in section 12.6.2 of the 2016 edition of NFPA 11 must evaluate a foam’s expansion ratio and drainage time. The foam concentrate is passed through a test nozzle and the resulting expanded foam solution is collected and measured to determine the expansion ratio. The drainage of the resulting expanded solution is also monitored and the time at which 25% (low expansion foam concentrate)/50% (high expansion foam concentrates) of the expanded foam has returned to an unexpanded solution is recorded as the drainage time.
When the foam type and brand of foam are known, the expansion ratio and drainage time results are compared to manufacturer’s specifications where available as required by 12.6.3 in the 2016 edition of NFPA 11. In the absence of manufacturer specifications, Dyne requires the foam quality meet or exceed the expansion ratio and drainage time requirements as specified in relevant industry standards such as MIL PRF 24385 (2017) Performance Specification: Fire Extinguishing Agent, Aqueous Film-Forming Foam (AFFF) Liquid Concentrate, for Fresh and Sea water. Where applicable, a 10% degradation of the foam quality from the time of initial certification to the time of the periodical foam quality follow up testing is allowed. Failure to meet the expansion and/or drainage time requirements indicates the foam concentrate is no longer performing as designed.
In addition to the expansion and drain time, Dyne Fire Protection Labs also reports a spreading coefficient for all film forming foams such as aqueous film forming (AFFF) and film-forming fluoroproteins (FFFP) foams. According to the definition of film formation in the 2016 edition of NFPA 11 (3.3.7), such foam concentrates must have a spreading coefficient greater than 0 dynes/cm when measured according to ASTM D 1331 using cyclohexane as the substrate. The spreading coefficient is determined by subtracting the surface tension of the foam solution mixed at the nominal concentration and the interfacial tension between the foam solution and the fuel the foam is protecting against (cyclohexane by definition) from the surface tension of the fuel by itself. Surface and interfacial tension measurements are obtained using a tensiometer which measures the force exhibited on a piece of metal as it passes through the surface of a liquid. Failure to achieve a positive spreading coefficient suggests the foam solution may not form a film on a flammable liquid as designed.
Dyne Fire Protection Labs additionally reports several physical properties of a foam concentrate: refractive index, density, viscosity (if applicable), and pH. These parameters, while not a direct measure of performance, are indicators of the current state of the foam concentrate chemical composition. Each property is measured with an appropriate device and compared to manufacturer’s specifications where available. A change in a physical property over time, while indicating a change in chemical composition to some degree, may not be detrimental provided the foam’s performance is still acceptable. Such a change, however, can indicate potentially harmful storage conditions which can be addressed before becoming significant. Where the performance of a foam concentrate is not acceptable, changes in the physical properties of a foam are often clues as to why. For example, a foam that fails the performance tests due to dilution will typically also have a refractive index and density very similar to water.
The International Maritime Organization (IMO) requires all foam concentrate samples also be analyzed for sediment. The foam concentrate is centrifuged to force all the sediment to settle out of solution which can then be quantified. The sample is also passed through a 180 µm sieve to assess if any of the sediment collected is larger than 180 µm in size which could pose a threat to any small orifices contained in the proportioning equipment and/or discharge devices. IMO MSC.1/Circ. 1312 (dated 10 June 2009) Revised Guidelines for the Performance and Testing Criteria, and Surveys of Foam Concentrates for Fixed Fire-Extinguishing Systems requires the sediment collected be less than 0.25% of the sample.
In addition to the sediment test, the IMO requires all alcohol resistant (AR) protein-based foam concentrates to also undergo a small-scale fire test as well as a chemical stability test to prove AR performance. Dyne Fire Protection Labs offers a small-scale fire test that involves placing the foam sample on isopropanol (IPA), a polar solvent, and monitoring how long before 50% of the foam blanket has dissolved after a section of IPA is exposed and ignited. This is commonly referred to as a burn-back test and is a small-scale representation of the burn back test conducted as part of the large-scale fire tests described in UL 162. The chemical stability test offered by Dyne Fire Protection Labs, and as described by IMO 1312, involves applying the foam blanket produced by the sample onto acetone and monitoring if the solution mixes with the polar solvent or if it sits on top as desired.
All of the additional tests required by the IMO can be requested for any sample even where not required.
©Dyne Fire Protection Labs 2020