Reduced scale tank fire extinguishing tests

One of the goals of the ETANKFIRE project was to provide guidelines for tank firefighting operations. Two series of fire extinguishing tests in reduced scale have been conducted within WP1 and WP2 to understand the influence of foam characteristics, application techniques and rates. Tank fire conditions were simulated by using a larger amount of fuel and longer preburn times than those specified in various foam approval standard tests. Prior to the tests a literature review was made, primarily focused on real tank fire experiences involving ethanol or other water-miscible fuels and an evaluation of the test conditions for various standards for foam concentrates on water-miscible liquids and foam system design standards for water-miscible liquids.

In total 29 extinguishing tests were conducted in the first “small scale” test series using a 0.41 m2 fire tray and 14 tests were conducted in the second, “laboratory scale” test series using a 3.14 m2 fire tray. A fuel depth of 450 mm was used in most of the tests to simulate a tank situation where control and extinguishment is not primarily a result of water diluting the fuel. This corresponds to the use of 185 l of ethanol in the small scale tests and 1413 l of ethanol in the laboratory scale tests. The ethanol fuel used in the tests was a blend of 97 % pure ethanol denaturized with 3 % gasoline (designated E97). Alcohol resistant foam (AFFF-AR 3x3) was the main extinguishing media used but some tests were also conducted with other media, such as 3F-AR 3x3 (fluorine free foam), cellular glass, liquid nitrogen and aqueous vermiculite dispersion (AVD). The foam application method used in most tests was to apply the foam towards the back wall of the test tray, i.e. the method used in most foam standard test methods for water-miscible fuels.

Small scale tests identified the most important parameters for successful extinguishment

The intention of the first test series in small scale (0.41 m2) was to provide a better understanding of the various parameters that might influence the extinguishing process, such as the amount of fuel, preburn time, type of application, application rate,  foam type and foam characteristics. The smaller scale test results showed that the simulated tank fire conditions using an increased depth of fuel, prolonged preburn time and a slightly more severe foam application (e.g. increasing the impact position on the tank wall from 0.35 m to 1.05 m above the fuel surface) had a serious negative influence on foam extinguishing performance. In several tests the fire could not be controlled at all during the test (normally 15 minutes of foam application). Even when the application rate was doubled, from 6.1 l/m2 min to 12.2 l/m2 min, no extinguishing effect was obtained until the fire was significantly influenced by dilution of the fuel.
The characteristics of the foam (higher expansion ratio and longer drainage time) were the most important factors for suppression performance. These enhanced characteristics were obtained by generating the foam as CAF (Compressed Air Foam) but the most important improvement was obtained by doubling the foam concentration, from the nominal value of 3 % up to 6 %, which resulted in extinguishment in less than 2 minutes. The small scale tests also showed that using a combination of a layer of cellular glass covered by a layer of foam improved the performance. The cellular glass floated on the fuel and formed a barrier between the fuel and foam, resulting in an effective extinguishment. 

Two tests were also performed to evaluate the use of a “foam pourer” application (Type II discharge outlet according to NFPA 11) mounted on top of the wall of the test tray as this is expected to result in a more gentle application. However, the results indicated that gentle application of the foam is not guaranteed by the use of foam pourers. During cold conditions the foam flowed gently along the wall down to the fuel surface, but after the 15 min preburn time the steel tank wall temperature was in the range of 550 °C. This caused an immediate evaporation of the foam at the wall surface and formed a steam layer that pushed the foam stream away from the wall. The consequence was a free fall of the foam down to the fuel surface resulting in severe foam destruction. Some foam was also blown outside the test tray due to the thermal updraft from the fire.  There was no control of the fire in the test in which aspirated foam was used, although the foam application continued for more than 18 minutes. In the second test, the foam was generated as CAF, and although this also resulted in a free fall to the fuel surface, the CAF was not damaged as much and established a foam layer that extinguished the fire in about 5 minutes.

Laboratory scale tests verified that enhanced foam characteristics are crucial for tank fire extinguishment

The laboratory scale tests (3.14 m2) were focused on verifying the extinguishing performance of the most promising small scale test results. Firefighting foam was used in these tests because it is the most commonly used suppressant agent for this type of fire at the present time. However, one test involved the use of cellular glass with a subsequent application of foam.

In the test series, the importance of the foam characteristics was evaluated further. Using the nominal foam concentration (3 %), an application rate of 7.26 l/m2 min, and an impact position of 1.05 m above the fuel surface resulted in no control of the fire during 15 minutes of foam application. These results agree with the small scale test results. Doubling the foam concentration to 6 % resulted in extinguishment in about 4 minutes. Following the positive result, further tests were made with lower application rates, both at 4.77 l/m2 min and at 3.63 l/m2 min, the latter rate still provided complete extinguishment within 5.5 minutes. Further proof of the improved performance was obtained in a test in which the impact position was changed to 2.05 m above the fuel surface. Using an application rate of 4.77 l/m2 min, extinguishment was obtained in less than 3.5 minutes. Also in this test series, further improvement of the foam characteristics was obtained by generating the foam as CAF.  Using the reduced application rate (3.63 l/m2 min, impact position 1.05 m) and 6 % foam concentration resulted in complete extinguishment within 2 minutes. This indicates that, although using twice the nominal foam concentration, improved performance is possible without using a larger overall quantity of foam concentrate.
The last test in the test series used a combination of cellular glass with a subsequent foam application with the aim to make the extinguishing operation even more robust. To evaluate the benefit of this combination, aspirated foam was applied using direct (Type III) application, with a nominal foam concentration of 3 % and the lowest (3.63 l/m2 min) application rate. After the 15 minute preburn time a 50 mm layer of cellular glass was applied that reduced the fire intensity by about 25 % within about 2 minutes.  Following a waiting period of 10 minutes, the foam application described above was initiated on top of the cellular glass. The glass layer protected the foam from direct contact with the fuel and a foam layer was established immediately, although the foam layer spread across the surface somewhat slower compared to the free fuel surface. The fire was completely extinguished in less than 5 minutes.

A full report presenting full details of all the small scale and laboratory scale tests can be downloaded here.

Foam application using a “foam pourer” in the 0.41 m2 tray, showing how the foam is pushed away from the tank wall due to steam formation.
 
Foam application towards the back wall of the 3.14 m2 “tank fire” test tray. Using a nominal foam concentration of 3 %, the fire could not be controlled while at double the concentration (6 %) the fire was extinguished even at half of the initial application rate.

 

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Contact Persons

Henry Persson

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Francine Amon

Phone: +46 10 516 51 66

RISE Research Institutes of Sweden, Phone 010-516 50 00, E-mail info@ri.se

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