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.
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.
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.
ACCUMULATION OF SCALE/SEDIMENT
Water used in boilers contains minerals that can cause scale or sediment deposits on water-side surfaces. The most common minerals found in boiler water are calcium, magnesium, iron, and silica. As water temperature increases, these minerals precipitate from the water and adhere to the boiler's metal, forming hard-scale deposits. Three problems are created by this accumulation. First, scale buildup can result in physical damage to a boiler. Second, and more commonly, boiler efficiency is reduced as a result of scale's insulating properties, requiring more fuel to maintain required temperatures and pressures. In extreme cases, this can cause overheating of a boiler tube, which can lead to failure. Third, when deposit buildup occurs regularly, frequent shutdowns are required for removal of that buildup.
Other contaminants in boiler water that do not attach to metal will settle in the boiler as sediment. Because sediment deposits can affect heat transfer, sediment can accumulate in areas in which it can affect the function of controls or protective devices, such as in the float chamber of a low-water fuel-cutoff device. This sediment can interfere with a device's operation, making it unable to shut down a boiler in the event of a low-water condition. Sediment also can block the openings of boiler blowdown lines so sediment or loose scale cannot be drained from a boiler.
Another potential problem occurs when sediment accumulates in piping connections to a water-level sight glass. If these connections are blocked, the sight glass will not provide an accurate indication of boiler water level, which could prevent an operator from seeing a low-water condition.
PREVENTIVE MAINTENANCE: LOW WATER
Because low water occurs when a boiler needs water and does not call for it or calls for water and does not get it, the first step in preventing low water is to ensure that feedwater is provided to a boiler reliably. Most steam and hot-water boilers rely on an external-float low-water fuel-cutoff device to control feedwater supply. The device contains two switches: one to start and stop feedwater pumps and one to shut down a boiler, protecting against low water. Because the device provides feedwater control and acts as a protective device, its proper operation is essential. Often, this device also contains the water column to which the water-level sight glass is attached.
An external-float low-water fuel-cutoff device in a steam boiler must be flushed regularly to remove sediment deposits and test the electrical circuits that shut down the burner. To prevent possible damage to the float, the drain valve should be opened slowly while the device is flushed. At least twice a year, a boiler should be shut down for routine maintenance. This includes disassembly and examination of the float chamber, as well as examination of the float assembly. The piping connecting the device to the boiler also should be opened in a check for scale and sediment accumulation.
Every year, low-water fuel-cutoff devices should be tested by conducting a slow-drain test of a boiler to determine that the devices will shut the burner off before the water drops below a safe level. Extreme caution must be exercised when conducting this test to prevent damage caused by allowing the water level to drop too far without the burner shutting off. The manufacturer should be consulted for additional information on maintaining its specific low-water fuel-cutoff device. Installations with two low-water fuel-cutoff devices may require that one be disabled by attaching a jumper to bypass the device during testing. This should be done only by a qualified person following a written procedure designed to verify that the device has been returned to operation.
Electronic-conductivity-probe low-water fuel cutoffs should be removed and their probes examined for scale accumulation, corrosion, and deterioration that will prevent their proper operation. Sediment can cause a false water-level indication. Because these devices cannot be flushed, most are equipped with a test button that can test the circuits to determine if they will interrupt burner operation. This test should be done daily — no less than weekly — while a steam boiler is in operation.
Age can impact the reliability of electronic-conductivity-probe low-water fuel cutoffs. Most manufacturers recommend periodic replacement of components. The electrical components should be examined for signs of corrosion, loose connections, and wire-insulation deterioration. Mechanical switches should be checked for pitting and damage to contact points. With mercury switches, observe to see if the mercury has separated into pieces that can hang on the contacts and prevent circuit interruption.
Maintaining feedwater controls and low-water protective devices is just half of the maintenance necessary to prevent damage from a low-water condition. Feedwater-supply equipment, such as pumps and pressure reducers, must receive proper maintenance to ensure operating reliability. Routine maintenance, as well as any additional maintenance recommended by the manufacturer, should be performed regularly.
PREVENTIVE MAINTENANCE: CORROSION
External corrosion can be prevented by ensuring that the operating environment is kept dry and protected from the weather. Correcting leaks in valve stems and pipe connections will help control external corrosion. Be alert to signs of corrosion, take action to determine and eliminate the source of moisture, and repair the corrosion. Whenever there is evidence of external corrosion to a boiler, outer insulation should be removed and the boiler examined to determine if thinning of the boiler has occurred. Any welded repairs must be performed by an authorized boiler-repair firm.
Internal corrosion is addressed in two ways. The first is to prevent oxygen from entering a boiler. This is done through the use of a deaerator, which uses steam to scavenge oxygen from water. The deaerated water then is supplied to the boiler. Deaerators are pressure vessels and must be maintained and inspected according to the manufacturer's recommendations. Of particular concern in larger deaerators is corrosion at welded joints, which can lead to a vessel's failure. Routine non-destructive testing generally is recommended for these larger deaerator vessels.
Another way to control corrosion is to chemically treat boiler water to scavenge oxygen. This normally is done in conjunction with chemical treatment to prevent scale. Whenever a boiler is shut down for maintenance, the water-side surfaces should be examined to determine if corrosion is present. If evidence of corrosion is found, the boiler should be examined further to determine if thinning has occurred.
When a boiler is shut down for an extended period of time, such as when a heating boiler is shut down for the summer, it should be placed in a wet- or dry-layup condition to prevent internal corrosion. In a wet layup, chemicals are added to control oxygen levels, and the boiler is filled completely with water. In a dry layup, the boiler is drained and moisture-scavenging material is placed in the boiler furnace to control moisture levels in the boiler. In both cases, a qualified boiler-water-treatment firm should be consulted to determine the proper method of maintaining a dry or wet layup.
PREVENTIVE MAINTENANCE: SCALE/SEDIMENT ACCUMULATION
Preventing scale or sediment accumulation differs depending on the boiler and the temperature and pressure at which it is operated. Low-pressure hot-water heating boilers in a closed-loop system generally require little makeup water to maintain the system. As a result, the chance for introducing contaminants is low. Most hot-water boilers do not require treatment of their water to prevent scale formation. However, when makeup water contains high levels of scale-forming minerals, even small amounts of makeup water can result in scale formation. In such situations, it may be necessary to pre-treat the makeup water. This usually is done by installing a water softener in the boiler makeup-water line. A qualified water-treatment firm should be consulted to determine if this approach is necessary.
Steam boilers may require more makeup water, which increases the chances of scale- or sediment-forming contaminants being introduced. The most common method of preventing scale or sediment accumulation in steam boilers is chemical treatment of boiler water. It also is common to pre-treat feedwater with a water softener to remove minerals. Depending on the size of a boiler, how it is used, and the nature of the contaminants, there are several methods of chemically treating boiler water. Along with the use of chemicals, the routine draining, or blowdown, of a boiler is a way to remove loose scale or sediment. Chemical treatment of a boiler requires specific knowledge of boiler chemistry and should be done only by a qualified water-treatment firm. Regular inspection of water-side surfaces is necessary to determine the treatment program's effectiveness.
Modern steam and hot-water boilers are among the most reliable, efficient, and safe fixtures in our industrial, commercial, and public infrastructure. However, when boilers are neglected and their required routine maintenance is inadequate, like any machine, they can fail. These failures are preventable. The key to years of efficient and trouble-free operation is regular maintenance. With fuel and equipment-replacement costs continuing to rise, deferring maintenance no longer is affordable. Remember these three things: maintenance, maintenance, maintenance.
For past HPAC Engineering feature articles, visit www.hpac.com.
Jerry Theodorou is a director of underwriting for The Hartford Steam Boiler Inspection and Insurance Co., managing the company's warranty programs. With degrees from Cornell University and the Massachusetts Institute of Technology, he has held positions in underwriting, operations, management, and new-product development on three continents. He can be reached at firstname.lastname@example.org.