Boilers have a long-standing and notable place in American history as seen from the versatility of applications that range from steam engines to power generation. In a nutshell, boilers provide a means to transfer one form of energy to another. Under controlled conditions, the transfer of energy takes place within a pressurized vessel. Regardless of the type, almost all boilers have common elements: a burner, combustion chamber and heat exchanger.
Although boilers are manufactured in a variety of sizes and configurations, their design and maintenance is governed by industry standards such as those published by the American Society of Mechanical Engineers (ASME – Boiler & Pressure Vessel Code). The good news is that, after more than a century of usage, boilers continue to provide an economical means in which to control our environment. The not-as-good news is that boilers use combustion to produce heat. Any process that involves combustion of a fuel-air mixture in a confined space is not always forgiving, especially when the operation of a boiler goes awry.
We will discuss how boiler heating systems operate, common causes of boiler malfunctions and, when combustion is not controlled, resulting damage.
Boiler Heating Systems
Hydronic and Steam Systems
Residential and commercial boilers frequently utilize hydronic or steam systems. Inside the combustion chamber, a mixture of fuel and air is combined and ignited. Depending upon the type of system, hot gases associated with the combustion of natural gas or propane heat water to an elevated temperature or convert water to steam. Energy associated with the heat of combustion is transferred to water (liquid or vapor) and circulated within a closed system.
While both hydronic and steam systems use the heat of combustion to elevate the temperature of water, the main distinction between a hydronic system and steam system is that the hydronic system circulates heated water, not steam. Heat exchangers, such as radiators, transfer heat from hot water or steam to the surrounding environment. Water in a hydronic system is returned to the boiler for reheating and then circulated back through the system. In steam systems, steam is condensed before reheating.
Inspection and Maintenance
All boiler manufacturers require routine inspection and maintenance to ensure safe and efficient operation. Boilers located in commercial and public building are required by state agencies to undergo periodic maintenance. In Colorado, boiler maintenance is regulated by the Department of Labor and Employment – Division of Oil and Public Safety.
Fire-Related Structural Damage
Combustion chambers for hydronic and steam heating systems must be insulated from the surrounding boiler components. As mentioned, periodic boiler maintenance is a critical requirement. Inadequate boiler maintenance, such as the failure to inspect the integrity of the combustion chamber and exhaust flue, can lead to an uncontrolled fire with subsequent structural damage.
For wood-framed structures, fire-related damage includes pyrolysis and charring with a commensurate loss in member strength and stiffness. Although concrete is more resistant than wood to fire-related damage, concrete elements can undergo spalling, scaling and cracking. Likewise, and at elevated temperatures, the strength and stiffness of structural steel can be diminished. Sagging of steel members such as bar joists, purlins and girts is a primary indication that the structural integrity of the structural element has been compromised.
For any structure, an important consideration is damage segregation. The extent of damage attributable to fire (and fire-suppression efforts) versus damage associated with long-term decay or corrosion, cyclic wetting and drying or a condition of defect in original construction is often a question that needs to be answered.