Common Boiler Problems, Part 3: Poor Water Treatment
Boiler feedwater is treated to protect it from two basic problems: the buildup of solid deposits on the water side of the tubes, and corrosion.
Prevention of Scaling or Buildup
The need for proper feedwater treatment is obvious for prevention of scaling or buildup if you will consider the comparison of an industrial or commercial boiler and a pot of boiling water on the stove. The boiler is actually an oversized distillery in that the water entering the boiler is vaporized to steam, leaving the solids behind. Depending on the amount of solids in the water, or hardness, the residue is sometimes visible when a pot containing water is boiled until all the water is vaporized.
The same thing occurs inside the boiler and, if left unchecked, can destroy it. Boilers rely on the water to protect the steel boiler tubes from furnace temperatures which greatly exceed the melting point of the tube material. A buildup of deposits on the internal boiler tubes will produce an insulating layer which inhibits the ability of the water to remove the heat from the tube. If this continues long enough, the result is localized overheating of the tube and an eventual blowout.
In order to prevent this buildup on the tubes, the level of solids in the boiler feedwater must be reduced to acceptable limits. The higher the operating pressure and temperature of the boiler, the more stringent the requirements are for proper feedwater treatment.
Prevention of Corrosion
The most effective method of controlling corrosion within the boiler is proper deaeration of the water. The removal of oxygen from the water drastically reduces the potential for corrosion. This is most often accomplished through the use of deaerators. These units typically utilize steam to both preheat the feedwater and remove the oxygen, carbon dioxide, and other gases from the make-up water. Oxygen scavenging chemicals are also commonly injected into to the deaerator to provide an additional measure of protection. Additionally boiler steam drum, or feedwater, has generally supplied chemicals at a controlled rate for even further protection.
In order to prevent problems with poor water treatment, the following are recommended:
- Verify that your boiler feedwater is of sufficient high quality for the temperatures and pressures involved. Water quality standards based on operating pressures and temperatures as recommended by ABMA should be followed.
- Verify that the water leaving the deaerator is free of oxygen, that the deaerator is operated at the proper pressure, and that the water is at saturation temperature for the pressure.
- Verify proper operation of the water treatment systems on a regular basis. Loss of resin from a softener or demineralizer can create problems if the resin escapes into the feedwater. Such resins can melt on the tube surfaces, resulting overheated tubes, etc.
- Never use untreated water in a boiler.
- Adjust continuous blowdown to maintain the conductivity of the boiler water within acceptable limits and blow down the intermittent blowdown on a regular basis.
- It is also important to blow the sludge out of all dead legs of the low-water trips, water column, etc., on a regular basis to prevent sludge buildup in these areas. The buildup of sludge can disable the low water trips.
- The water side of the boiler should be inspected on a regular basis. Should any signs of scaling or build up of solids on the tubes be noted, adjustments to the water treatment should be made.
- The water side of the deaerator should also be inspected on a regular basis for corrosion. This is an important safety issue because a deaerator can rupture from corrosion damage. All the water in the deaerator would immediately flash to steam in the event of a rupture.
Proper treatment of the boiler feedwater is absolutely critical to enable a normal life expectancy of the unit. This is one of the most serious boiler “destroyers”. A qualified water treatment specialist is invaluable in determining the best method for your plant and your site specific water requirements to ensure longevity of boiler equipment and efficient operation.
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