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.

CONCLUSION

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.

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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 jerry_theodorou@hsb.com.

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