Water Treatment (Steam Boilers)
Introduction
Untreated water contains dissolved minerals, gases and particulates. The removal or otherwise ‘treatment’ of each of these is critical to efficient boiler operation for different reasons. Minerals lead to scaling that acts as in insulator reducing boiler efficiency; gases can be corrosive and particulates can contribute to both problems. Water treatment is dynamic and varies from boiler to boiler and can vary month to month with the same boiler.
Water quality is primarily an issue with steam boilers that use a lot of make-up water. Closed-system hot water boilers are the least effected by water quality because they use the least amount of make-up water and operate at lower temperatures.
The common minerals in water that lead to scaling problems are iron, calcium, magnesium and silica. When water containing these dissolved minerals are heated, it looses its ability to hold the minerals in solution. When they come in contact with metal boiler parts, scale forms. In addition to reduced efficiency, scale can lead to boiler tube failure if the tubes are over-heated.
Oxygen and certain other gases in water are corrosive. Deaerators and chemicals that remove oxygen can reduce the corrosiveness of the water.
Primary indicators of boiler water treatment are pH, TDS (Total Dissolved Solids), TSS (Total Suspended Solids) and hardness.
Water pH
Water pH is a measure of its relative acidic or alkalinity. A neutral level is pH = 7. A number lower than 7 is acidic and higher than 7 is alkalinic (caustic). Both extremes are corrosive to boiler metal.
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Methods to Remove Water Impurities
The best way to remove impurities is before they enter the boiler. Small amounts of impurities can be effectively treated inside the boiler to keep them in solution or allow them to be discharged via blowdown.
External Treatment
External treatment refers to the chemical and mechanical treatment of the water source. The goal is to improve the quality of this source prior to its use as boiler feed water, external to the operating boiler itself. Such external treatment may include:
Internal Treatment
Even after the best and most appropriate external treatment of the water source, boiler feed water (including return condensate) still contains impurities that could adversely affect boiler operation. Internal boiler water treatment is then applied to minimize the potential problems and to avoid any catastrophic failure, regardless of external treatment malfunction.
Monitoring Water Quality
Water quality monitoring varies from weekly litmus test strips to continuous electronic instrumentation and automated chemical treatment. The size of the boiler, the importance of water quality and the skills of the boiler operators are all factors in deciding how best to monitor boiler water quality.
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