Chapter 5 - The degree of explosion danger for NEC Class II locations

Abstract

Hazardous areas for Class II locations, as defined by the NEC, are areas that produce combustible dust in which sources of hazard exist that may produce combustible dust from products grouped under E, F, and G. This chapter focuses on the explosion hazard of coal and coke dust. . The NEC divided the explosion hazard for coal and coke dust into: Division-1— the dangerous condition, and Division-2 — the remotely dangerous condition. A location must be considered dangerous, when coal or coke dust exists in the atmosphere in sufficient quantities because of normal operations. The degree of explosion danger in fossil power plants is high when combustible dust from coal is exposed to the atmosphere, this is exemplified in the chapter with: (1) Coal Fuel Unloading Systems. (2) Coal Pulverizers. (3) Crusher House. The main feature of a dust ignition proof enclosure is that it excludes ignitable amounts of dust. The most effective dust suppression system is a water spray that keeps the fuel and dust wet during the dumping process in fossil power plants. Even pulverizer units are considered dust tight. The coal pulverizing system in a fossil power plant is the final stage of the fuel supply. Pulverizing equipment reduces the coal to the required fineness before it enters the boiler furnace. Different types of pulverizing units: (1) The “Impact Mill” (2) The “Ball Mill.” The most commonly used pulverizer fuel system is the direct fire unit in which the fuel is pulverized near the point of use. Proper maintenance and regular housekeeping for reducing the amount of coal dust to below the explosive limit may not be the answer for keeping the coal dust below the lower explosive limit. However, during the first year of operation the pulverizer equipment may maintain its dust tightness, but in the years following, the equipment may start leaking because of wear and handling. Coal dust is also produced in reclaim hoppers, storage silos and crusher houses, and may cause dust explosions. Therefore, the use of enclosed machines must be encouraged; they give better protection against the entry of coal dust.