Fire Protection Systems PDF

Fire Protection Systems • Chapter 15 f£ OBJECTIVE 8 Explain the purpose of and describe a typical foam system for process buildings and tanks. FOAM SYSTEMS Where flammable liquid fire protection is required for permanently installed hazards, such as fuel storage tanks or dip tanks containing flam...

Fire Protection Systems • Chapter 15 f£ OBJECTIVE 8 Explain the purpose of and describe a typical foam system for process buildings and tanks. FOAM SYSTEMS Where flammable liquid fire protection is required for permanently installed hazards, such as fuel storage tanks or dip tanks containing flammable or combustible liquids, air-foam generating and distributing devices are installed internally in the tank. These fixed devices, which are piped to a source of foam solution, may be arranged for manual control or automatic activation by fire detectors in the event of a fire. Foam Chambers for Large Fuel Storage Tanks Fire protection of large outdoor fuel tanks requires that several foam chambers with foam-makers be installed at equally spaced positions slightly below the top rim of the tanks, as shown in Figure 25. These chambers are connected to lines on the ground that supply the foam solution to each foam-maker simultaneously if the flammable contents of the tank ignite. Frangible seals at the discharge outlet of the foam chamber prevent vapour from entering the foam piping. These seals are designed to burst when foam pressure is applied. A screen for the air inlet to the aspirating foam-maker prevents clogging from foreign matter, such as bird nesting material. A universal or swing pipe joint is installed at ground level in the foam solution inlet pipe to prevent fracturing of the supply piping if an explosion precedes a tank fire. Figure 25 - Air-Foam Chamber at Top of Storage Tanks -Top rim of tank Tank side Foam maker barrel Air inlet Swing joint Tank base Foam solution supply pipe (Courtesy of National Fire Protection Association) 3rd Class Edition 3 • Part A2 809 Chapter 15 • Fire Protection Systems Internal Tank Foam Distributing Devices A prime requirement for efficient fuel tank extinguishment by topside foam devices is that the foam must be applied to the burning surface without undue plunging into the fuel or allowing the foam to become coated with burning fuel. This gentle application of foam must be accomplished at any level of the contents of the tank. Many devices have been developed to gently apply foam from one point regardless of burning fuel level. These devices are listed as type I foam-discharge outlets for tanks and are required for some alcohol-type foams. When foam discharge into a tank is deflected to run down the inside tank shell to the burning fuel surface, it is called a type II outlet for foam application. Central Foam Distributing Systems Central foam distributing systems are systems that consist of an enclosure that houses a foam concentrate supply tank and a proportioning device, as shown in Figure 26. The foam solution is supplied under pressure from this foam house to the piping system. The foam solution is controlled by appropriate valves so that the foam chambers with foam-makers on the burning tank receive the foam solution Figure 26 - Schematic Arrangement ofAir-Foam Protection for Storage Tanks Foam made here at chambers Foam chamber ^^^^ r——]- 1 I - T-T-I Concentrate storage Foam solution hydrant Drain' Water supply Pump (Courtesy of the National Fire Protection Association) Semi-fixed systems of similar design are more frequently used with mobile foam concentrate supply from foam trucks. The truck proportions and pumps foam solution to the pipe laterals, feeding the foam-makers from a safe location outside the dike. Fbced systems that consist of automatically operated combinations of foam spray systems and foam monitors are often installed to protect chemical processing plants. Alcohol-resistant foams are usuaUy required in these locations. In these designs, where there may be a high fire risk, process vessels, pumps, and piping are often all included within the foam distribution pattern for overall protection. The sensing of heat by fire detectors can automatically activate the system. 3rd Class Edition 3 ' Part A2 Fire Protection Systems • Chapter 15 Foam-Water Sprinkler Systems In areas where flammable and combustible liquids are processed, stored, or handled, a water discharge may be ineffective for controlling or extinguishing fires. The foam-making sprinklers (aspirating-type) and deluge or spray nozzles that use AFFF foams have successfully replaced water sprinkler nozzles for such systems, so that fires in these occupancies are controlled and property is safeguarded. When supplied with a foam solution, sprinkler system piping grids that are equipped with foam-water nozzles generate air-foam in essentially the same sprinkler pattern as when water is discharged from the same nozzle. This dual capability affords the system with class A and B extinguishment ability. Fixed sprinkler systems that use these nozzles require foam concentrate tanks, proportioners, and suitable pumps in order to supply the system with foam solution and water. Detection devices may also be used to activate the system, or the system may be activated manually. Closed-head sprinklers may also be used and are now recognized in the NFPA 30 Flammable and Combustible Liquids Code. 3rd C/ass Edition 3 - Part A2 811