(Reference: Power Engineering Third Class Edition 2.4; Part B1; Chapter 10: Internal Water Treatment for Boilers)
James R. Stringer
Water treatment is a critical component of building systems; be it for steam boilers, cooling towers, condensers, or closed loop systems. We first need to know what we are protecting against.
Scale is a hard layer of mineral deposit that forms on the boiler waterside. Scale forms usually in the hottest parts of the boiler, generally steam generating sections of steam boilers such as the risers and waterwall sections of watertube boilers.
The primary cause of boiler scale are undesirable minerals in the water such as calcium and magnesium as they exist in large quantities compared to other minerals. They deposit readily in water. Other deposits to watch out for are iron, copper, aluminum, and silica.
1) scale restricts heat transfer and causes metal to overheat.
2) scale traps water beneath it and encourages corrosion.
3) scale sloughs off and causes flow restrictions.
Sludge is a harmful product of suspended solid particles that are able to settle in a system. Sludge will accumulate on tube and boiler walls, increasing in size over time. This will affect the performance and efficiency of the system.
Corrosion occurs when there is a chemical reaction caused by impurities in the water that cause loss of metal (iron).
Corrosion weakens/thins metal and re-deposits iron into hotter areas that restrict heat transfer. The areas of concern are inside the boiler and the condensate system where scale can be deposited into the boiler. Boiler water is corrosive when it is outside the pH 8.5-12.7.
Corrosion also results in iron redepositing on the metal surfaces closer to the hotter areas of a boiler that can restrict heat transfer and cause overheating of the metal.
Natural corrosion can occur on freshly cleaned or exposed metal that produces a very thin layer of magnetite. This magnetite layer helps protect against further corrosion: once the magnetite layer is thick enough, the water can no longer react with the base metal. However, the magnetite layer is still susceptible to corrosion from other causes.
Acidic corrosion occurs when the pH is below 8.5 and it accelerates the lower the pH. Acidic water will first eliminate the magnetite layer, then cause general metal thinning and it can target areas of high stress.
Caustic corrosion occurs when the pH is above 12.7. Caustic corrosion causes localized gouges in the metal that appear deep and irregular.
Caustic embrittlement occurs when there is a leak and build-up of caustic and the crystalline structure of the metal changes, making the metal brittle. The metal must be under high amounts of stress. When the metal becomes brittle, it is weakened and can not withstand the pressure, leading to a sudden rupture or cracking.
Oxygen corrosion occurs when oxygen, generally introduced through feed water but can happen through leakage, is heated and becomes very acidic. It begins with small pits but can deepen in a localized or wide area. This pitting can progress rapidly and can cause metal failure expeditiously.
Carbon dioxide corrosion happens when CO2 reacts with steam condensate causing carbonic acid to be formed. Carbonic acid will then target the metal in a general pattern causing grooves in the metal.