Steam boilers are responsible for a third of the energy that powers industry in the United States. From food production to paper making to chemical processing to electric power generation, steam continues to be a major energy source. Of the three principal sources of energy — electricity, fuel, and steam — steam accounts for 34 percent. In commercial and institutional settings, such as schools, hospitals, churches, prisons, shops, and malls, boilers are the workhorses that provide heat and hot water. What is more, boilers are efficient in producing heat energy when compared with other available sources — an important consideration with today's high fuel costs.

Current boilers incorporate knowledge gained from more than 200 years of engineering advancements in design, metallurgy, fabrication, controls, and supervisory systems. Boiler failures, however, still occur, causing businesses to suffer losses of production, income, and, worst of all, life. Because it is known how and why they occur, these failures are preventable.

Three major causes of boiler failures are low water, corrosion, and scale/sediment accumulation. Of those, low water is the most significant. Low water results when a boiler's water level drops below the safe point, exposing heating surfaces. Should the boiler continue to fire with insufficient water to absorb heat, metallurgical change can weaken its metal, resulting in ruptures of boiler tubes, drums, or shells. Low water may cause significant damage to a boiler or catastrophic failure.

Corrosion can occur internally or externally. Internal corrosion occurs when oxygen dissolves in boiler water or a boiler is exposed to moisture and oxygen while idle. In either case, as its metal corrodes and thins, a boiler weakens until it no longer can hold the desired pressure. External corrosion generally results from a boiler being exposed to the elements without adequate protection. Moisture from leaks in valve packing or piping flanges can seep into a boiler's insulation, trapping moisture against the metal. Just as with internal corrosion, the metal weakens as it corrodes.

Accumulation of scale or sediment also is a major cause of boiler breakdowns. Boilers essentially are heat-transfer machines. Heat from combustion is transferred to boiler tubes or sections and absorbed by boiler water. Scale or sediment accumulations on water-side surfaces disrupt heat transfer. A boiler then overheats, which causes a decrease in efficiency and damages the boiler.

Let's take a closer look at these three causes of boiler failures and look at ways to prevent them.

LOW WATER

A low-water condition most commonly results when a boiler requires water but fails to call for it or calls for water but fails to receive it. These conditions usually are the result of a malfunction in a control device or mechanical component. Low water itself usually is not a problem because a boiler is equipped with protective devices designed to detect low-water conditions and shut down the unit before damage results. However, if protective devices fail or are not installed properly, severe damage can occur.

Advances in technology have resulted in highly sophisticated devices for the control and supervision of boilers. Yet it is remarkable that the key to preventing damage from a low-water condition is as high-tech as the float in a typical toilet tank. The device most commonly employed to control a boiler's water level and provide protection against damage from low water is an external float-style low-water fuel-cutoff device, also referred to as a low-water cutoff. This device consists of a ball float inside of a chamber that is attached to an arm that connects to a mercury or mechanical switch. The switches can control feedwater supply as well as shut off a burner should boiler water drop below a safe level.

Although float-style low-water fuel-cutoff devices have stood the test of time as reliable devices, they can and sometimes do fail. The float arm can get caught in the arm guide. Scale or sediment in the float chamber can prevent the float from moving. Damage or deterioration of electrical components can prevent the device from working. Regular maintenance is the key to ensuring the reliability of these devices.

Another type of low-water fuel cutoff is a device using a conductivity probe. The device is inserted into a boiler, the probe immersed in the boiler's water. Should the water level drop below that point, the circuit will be broken and the burner shut off. While there are no moving parts to maintain, scale accumulation on or corrosive deterioration of a probe can lead to its failure.

Among the most dangerous events that can happen to a boiler is the sudden addition of water after the boiler has been fired dry. Without the cooling action of water, contact with a hot metal surface causes water to flash immediately to steam, expanding up to 1,600 times the volume of the water. This can result in an explosion that rips apart the vessel or, in extreme cases, turns the boiler into a projectile.

CORROSION

As mentioned previously, boiler failures can occur as a result of external or internal corrosion. Corrosion causes a deterioration and thinning of metal that diminishes a boiler's ability to contain pressure. Once metal thins to a point at which a boiler no longer can contain pressure, failure occurs.

External corrosion most commonly is the result of exposure to excessive moisture. This can happen when a boiler is exposed to the weather without proper protection of its components. Water can accumulate under a boiler's outer insulation, where corrosion can begin. The same condition can happen when a boiler room experiences excessive leakage from failed roofing or broken windows. Corrosion more commonly is caused by leaks within a boiler, such as between sections in a cast-iron boiler, as well as leaks from valve packing and piping flanges. Because excessive moisture can corrode the metal under a boiler's outer insulation, the extent of the corrosion often is unseen.

Internal corrosion is considered far more serious, as it progresses more rapidly than external corrosion. When oxygen dissolves in boiler feedwater/makeup water, it is seen as pitting on the surface of boiler drums, tubes, and shells. As corrosion continues, the pitting deepens until the component is too thin to contain pressure and fails. More widespread corrosion can occur when air is allowed to enter an idle boiler. The mix of oxygen and water results in surface corrosion.

Whether external or internal, corrosion is a serious condition that must be prevented and addressed.

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