Before answering the question, let us examine the following scenarios: On Dec 29, 2009, a boiler exploded at a palm oil mill in Sabah, Malaysia, killing one boilerman and injured many others. On September 24, 2010, nine people were killed and four others injured when a boiler exploded in an aluminum plant in Xiaoyi City, China, and on Feb 22, 2011 in Talkatora Industrial Estate in Uttar Pradesh, India, three persons were killed and six received serious burn injuries following a huge boiler explosion in Kiran Plywood Factory.
Based on those incidents, despite having various protections and inspections by local jurisdictions, we can conclude that explosion is actually not very rare in modern boilers. It happened almost every year although today, the casualty did not exceed 20 people per year. The good news about boiler explosion is that it can be prevented. The bad news however, some personnel are not aware of the mechanism of boiler explosion and tend to stick to the routine they have been practicing for years, and thus accident happens. So the answer is, yes and no.
Investigations after investigations were made and human errors seem to be the main cause for boiler explosion. Other causes are inadequate boiler operating procedure, improper boiler maintenance, or inoperative boiler controls and safety devices. Therefore, special considerations must be made on standard operator training.
The topic of boiler explosion is too broad to be covered in a short article; therefore, I shall only discuss the causes of fireside explosion, since furnace explosion is more common than waterside explosion. First of all, what is fireside explosion? Well, in a fireside explosion, an accumulated combustible mixture ignites almost simultaneously, creating a force which exceeds the yield strength of the boiler furnace, casing or uptake, causing catastrophic structural damage. The sudden load explosion in the boiler furnace can be heard miles away.
The principal cause of furnace explosion for oil-firing boiler is the accumulation of unburned fuel in the furnace due to incomplete or non-ignition. The accumulated oil on the hot furnace floor begins to volatize and releases its combustible gases when the operator initiates another trial for ignition. When the mixture of unburned fuel with air is in explosive proportion, explosion will occur. Explosive proportion is where the lower explosive limit (LEL) of diesel fuel marine (DFM) is 0.6% by volume vapor to oxygen. Once the LEL is reached, explosion may occur. Maximum explosion normally occurs at 2.0% by volume DFM vapor to oxygen.
There are many ways the oil may enter the furnace in an unburned state. Poor atomization can cause discharge of unburned oil into the furnace. There are three ways of atomizing fuel, which are forcing under pressure of 5 to 15 bars, steam atomizing, ranging from 5 bars to 10 bars of steam, and mechanical atomization (rotary cup atomizers) with a rotary cup rotates at 8,000 rpm. Obviously, if the atomizing pressure is too low, oil cannot atomize properly and much oil will drop on the furnace floor in an unburned state. Sometimes atomizing steam can be wet due to high condensate which is often due to poorly insulated steam line or malfunctioned steam traps, which can result in loss of atomization.
Most importantly, atomization share this site is affected by the viscosity of the oil which in turn affected by the temperature. To prevent this, the oil tips must be clean, the oil temperature must be correct to keep the oil at oil firing viscosity of 200 to 220 SSU (Saybolt Seconds Universal), and the atomizing steam (or air) pressure and fuel oil pressure must be properly adjusted. This means that the oil must be heated up to 60oC for light oil and as high as 95oC for heavy oil.
Fuel inlet valve can also be a problem. Fuel can also enter share our website the furnace through leaky fuel inlet valves on idle burners. Sometimes, the fuel inlet valve cannot secure fuel supply to atomizers promptly when fires are share this site extinguished or there is a slight delay in fuel shut off if the flame extinguished unexpectedly.
During starting up, sometimes difficulty is experienced in establishing ignition due to failure of photocell or flame scanner (or other mechanical problems) that can prompt the boilermen to attempt starting up the boiler for several times, which resulted in pool of unburned fuel to be accumulated after each trial.
Failure to purge furnace properly, including furnace, boiler and uptake areas can also cause boiler explosion. During boiler startup, purging sequence is required to drive out all combustible and explosive gas from the furnace through the uptake before ignition. Purging sequence must be sufficient. As a rule of thumb, an ideal purging should give five changes of volume. For a large boiler, normal set time would be 20 minutes while for a medium or smaller, a shorter time may be required, and the air flow must be low, which is only 25% of the normal full-load air. Purge air requirement is normally 4 SCFM for light fuel or 15 SFCM for heavy fuel. A word of caution, never bypass the purging sequence by all means.
Learn how flame impingement and soot blowing can contribute to fireside explosion. Besides overpressure, learn how low water and other causes can cause waterside explosion and implosion. And most importantly, know the ways to prevent boiler explosion from me in detail.