Foam concentrate rheology and foam system design

Abstract

NFPA and EN design standards for systems of firefighting foams devolve the problem of flow calculations for each foam concentrate to the product manufacturers. The data provided by manufacturers do not represent the engineering specifications needed for foam system design. This contribution examines the rheology of foam concentrates, comprising three FfreeF (fluorine-free foams) and one AR-AFFF (alcohol-resistant aqueous film-forming foam). These foam concentrates comprise solutions of polymers (predominantly polysaccharides) combined with surfactants and solvents. We reveal that all products exhibit significant shear thinning and that the formulations display substantial yield stress and slow response to changes in stress. These foam concentrates are sensitive to shearing and require many hours to recover from any deformation during handling. These non-Newtonian properties of the concentrates arise mainly from the presence of polysaccharides in their chemical make-up. We provide guidance for the types of measurement needed to provide engineering specifications for fire system designers and the limits of use for these measurements. The contribution highlights the design implications of the slow-yielding foam concentrates for design of foam-based fire-suppression systems.