CONDITIONS OF A CRANKCASE EXPLOSION-
1. A source of heat or ignition which is required to vapourise the oil into a fine vapour.
2. The correct air to fuel ratio required for explosion.
3. Fine oil vapour with a high surface area to mass ratio.
HOW EXPLOSION TAKES PLACE-
• The source of ignition is mostly a hot spot due to a bearing running hot. This heat causes the lube oil in contact with the hot surface, to vapourise into a fine vapour. This oil vapour forms an oil mist in the presence of condensation in a relatively cooler section of the crankcase.
• When this fine oil mist ignites in the presence of a hot surface, a pressure rise occurs which depends on the weakness or richness of the oil particle to air ratio. An explosion occurs if this mixture is in the explosive range.
• A primary explosion is relatively slow as the crankcase atmosphere is too rich with oil vapour, but may cause a rupture of the crankcase allowing ingress of air. This ingress of air causes a very good air to oil-particle ratio and a secondary explosion occurs, which is more violent.
• A remedy for secondary explosions is the use of crankcase relief doors, which relieve the crankcase pressure if it exceeds 0.05 bar, thereby preventing any rupture to the crankcase and ingress of air. Constant monitoring of the crankcase oil mist is accomplished by an oil mist detector.
Self-closing crankcase relief valves are fitted at various points along the crankcase I to relieve pressure, irrespective of the origin in the crankcase.
• The valve should be self-closing to prevent ingress of air which leads to a secondary explosion. Self-closing function is achieved by the spring 3 which shuts the disc valve 2, if the crankcase pressure is less than 0.05 bar.
An oil-wetted metallic gauze 5 is fitted on the internal side for stopping a flame emerging from the relief valve.
• A rubber o-ring 6 provides oil tightness and sealing. A deflector 4 is fitted to deflect any flame or pressure wave in case of an explosion.