Alternative AFFF (Aqueous Film Forming Foam) Proportioning Systems for the SSN 21 Design - T&E.

The Aqueous Film Forming Foam AFFF fire suppression systems proposed for flammable liquid fuel hazards on the SSN 21 require proportioning systems to inject 3 AFFF concentrate into seawater systems to create AFFF solution. Alternative proportioning system each of which utilizes an air-pressurized concentrate storage tank are being evaluated by the Naval Research Laboratory NRL. One candidate system uses multiple single-user injection points e.g., orifice metering plates to proportion AFFF at each hose station or sprinkler system. The other system, which uses and in-line balanced pressure proportioner and ratio controller, is designed as a central system which can proportion over a range of demands. Proportioning using the orifice plate system is pressure sensitive and not flow sensitive. As pressure differential between the injected concentrate and seawater increases, flow rate increases. If the pressure differential can vary considerably during operation, AFFF flow and storage requirements must be overdesigned to meet the lowest design point, i.e., 3 at the smallest pressure differential. The size of the orifice in the orifice plated design may be so small e.g., 0.1 in. dia. that clogging of the orifice could be a problem. Proportioning using the balancing valve system is flow sensitive. A steady level of concentrate in maintained over a range of flows. The use of a central proportioning system creates in effect, a separate AFFF firemain system. The central proportioning system allows more effective use of a AFFF concentrate, but significant design and operational problems result from the requirements to have seawater only capability at AFFF hose stations.