On the question of the required foam expansion ratio for sublayer extinguishing of oil and oil products fires in vertical steel tanks

Возможность эффективного тушения резервуаров подслойным способом во многом зависит от подъема пены на поверхность горючей жидкости. Критически рассмотрены существующие методики испытаний высоконапорных пеногенераторов для подслойного пожаротушения. Предложены расчетные формулы для определения величины минимальной кратности пены, способной к подъему, в зависимости от состава и плотности горючей жидкости, высоты столба горючей жидкости, типа используемого пенообразователя. Обоснованы условия применения существующих систем подслойного пожаротушения для резервуаров с различной высотой стенки. During the certification tests of high-pressure foam generators for sublayer oil and oil products fire extinguishing in the tanks there is created a conditional back pressure at the generator output, and the foam expansion ratio is measured when this back pressure is no longer present. In accordance with the standardized methodology the foam expansion ratio is measured in the measuring container by weight. The derived foam after passing throughout the pipeline is supplied to the measuring container where the foam expansion ratio is measured under the atmospheric pressure. In practice, during fire extinguishing the flammable liquid exerts a constant excess pressure on the formed foam. The expansion ratio of the foam decreases in proportion to the pressure value of the column. As a result, the foam expansion ratio can reduce to a value when the Archimedean buoyant force no longer occurs. In this case, the foam cannot be supplied to the surface of the flammable liquid and the fire cannot be extinguished. The back pressure created at the tank base depends on the height of the tank wall (level of the liquid fuel filling) and on its density. Foam rise is possible if the foam density is lower than the fuel density. Light oil products extinguishing requires greater expansion ratio foam than heavy oil products extinguishing. At the same time, heavy oil products cause greater back pressure and a decrease in the foam expansion ratio. Alcohols in the composition of modern high-octane gasolines produce an additional defoaming effect on the foam, which questions the feasibility of using a sublayer fire extinguishing system of modern high-octane gasolines. The article evaluates a set of parameters that affect the required foam expansion ratio for sublayer extinguishing of oil and oil products fires in tanks and justifies the minimum value of its expansion ratio.