flammable liquid pumps, and mixers may be interlocked with the dry chemical system and arranged to shut down automatically upon discharge of the system.
are arranged to discharge dry chemical directly on the hazard, without any enclosure (fig. 8-37). Typical local application systems are used for the protection of paint dip tanks and restaurant range hoods. Ventilating fans, conveyors, flammable liquid pumps, and mixers may be interlocked to shut down automatically upon discharge of the system.
discharge dry chemical through manually operated nozzles connected by hose or by piping and hose to a fixed supply (fig. 8-38).
Dry chemical used in approved systems is mostly sodium bicarbonate, very finely ground, to which has been added other ingredients to keep it free flowing and to resist caking. Other agents used in dry chemical extinguishing systems include potassium bicarbonate, potassium chloride, and monoammonium phosphatemultipurpose type.
The dangers dry chemicals used in fire extinguishing concentrations cause exposed personnel are temporary breathing difficulty and reduced visibility. In areas using total flooding systems, suitable means should be provided to permit evacuation of personnel. In areas using local application systems where the dry chemical is not confined, there is little hazard to personnel.
Dry chemical systems are used primarily for extinguishing fires in flammable liquids. Bicarbonate base dry chemical can be particularly effective for extinguishing fire in deep fat fryers caused by overheating. The saponification reaction between the dry chemical and fat or grease
Figure 8-37. - Local application dry chemical system installation.
prevents re-ignition by turning the fat to soap. Multipurpose dry chemical will not react with the fat or grease and can prevent the saponification reaction between the fat or grease and any bicarbonate base dry chemical subsequently used.
Dry chemical systems are not suitable for fires in materials (such as cellulose nitrate) that contain their own oxygen supply. They are not normally used for fires involving delicate electrical equipment such as telephone switchboards, computers, and certain other electronic equipment because the dry chemical will insulate the fine and delicate contacts. The contacts will then need complete cleaning.
Monoammonium phosphate and potassium chloride are slightly acidic, and in the presence of moisture can corrode metals such as steel, cast iron, aluminum bronze, and titanium. Corrosion can be minimized by prompt cleanup. Most dry chemical agents can be cleaned up by wiping, vacuuming, or washing the exposed materials or surfaces. Monoammonium phosphate will require some scraping and washing if exposed surfaces were hot when the agent was applied.
There are basically two types of dry chemical systems.
1. Gas cartridge systems that use a container of expellant gas that, when released by manual
Figure 8-38. - Stored pressure dry chemical cylinder with hose line.Continue Reading