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Practical Guide
ASHRAM Journal
Boilers represent large capital expenditures. Their operational reliability and availability are often critical to facility profitability. Therefore, training and developing personnel responsible for this equipment is essential. Failure to follow a few well-established practices can result in catastrophe. The most common causes of catastrophic boiler failure include: fuel explosion, low water incident, poor water treatment, contaminated feedwater, improper blowdown techniques, improper warm-up, impact damage to tubes, severe overfiring, improper storage, and pulling a vacuum.
Fuel Explosion
Fuel explosion in the furnace is one of the most dangerous situations in the operation of a boiler. Most fuel explosions in boiler furnaces are caused either by a, "fuel-rich mixture" or improper purge techniques. A fuel-rich mixture occurs when high concentrations of unburned fuel exist in the furnace. Depending on the combustion control equipment, this can result from a variety of causes including instrument malfunction, fuel supply pressure variations, and equipment failure.
Many fuel explosions occur after a combustion problem that produces a burner trip. For example, when a fuel oil atomizing tip clogs and its spray pattern is disturbed, the resulting unstable flame can lead to flame failure. If fuel oil is sprayed into the furnace during successive attempts to relight the burner, a large inventory of vaporized fuel can accumulate. Unburned fuel can also accumulate if a burner operates for long periods of time with poor atomization.
A successful relight can ignite this potentially explosive inventory. Figure 1 shows the complete devastation of a utility boiler caused by a fuel explosion.
Unburned fuels in a furnace can ignite in a rapid or explosive manner. These events can be avoided by following a simple rule: Never add fuel or air to a dark, smoky furnace. Instead, "trip" the burners manually and purge the furnace thoroughly with air. Once this is done and the ignition problems are corrected, the burners can be relighted.
Low Water Incident
At temperatures above 800 deg F (427 deg C), carbon steel undergoes degradation that destroys its strength and integrity. Since typical furnace temperatures exceed 1,800 deg F (982 deg C), the cooling effect of water inside the boiler tubes is critical to prevent catastrophic damage. Continued firing of a boiler with low water will literally melt steel boiler...