All natural water contains dissolved mineral salts such as calcium, magnesium and iron bicarbonates. As the temperature of the water increases, these bicarbonates decompose into calcium, magnesium and iron carbonate salts. These carbonate salts are much less soluble than the bicarbonates. 

The result is a precipitation of the excess carbonates which become hard, rigid deposits. Mineral deposition occurs on heat transfer surfaces in a boiler. This deposition acts as insulation, lowering the heat transfer coefficient, which reduces the heat transfer capability, significantly increasing fuel consumption and fuel costs. 

The insulating properties of these deposits subject metal surfaces to excessive temperatures causing them to crack, blister, or rupture. In extreme cases, complete system failure occurs. Mineral deposits result in increased maintenance and fuel costs and reduced equipment life.

When water evaporates in a boiler, the hard components that were in the water, such as calcium salts, magnesium salts, and other insoluble materials, form deposits on the tubes and other internal surfaces. These deposits are known as scale. 

Actually, the temperature of the water determines how well the different salts dissolve and how long they remain dissolved. Some salts are such that the hotter the water, the better they stay dissolved. Other salts stay dissolved while the water is at a relatively low temperature but form solid crystals (scales) that come out in increasing amounts as the water gets closer to becoming steam.

The scale-forming salts stay dissolved in the water and in the cooler parts of the boiler, but when the water reaches the hot tubes, these salts start forming solid particles that come out of the water and stick to the hot metal parts as scale deposits. These deposits are highly objectionable because they are poor conductors of heat, actually reduce efficiency, and are frequently responsible for tube failures. Some of the principal scaleforming salts to be considered in most cases are listed as follows:

Scale is made up of three main parts: 

• calcium sulfate, 
• calcium carbonate, 
• and silicates of calcium and magnesium. 

Scales that are principally calcium sulfate or chiefly of the aforementioned silicates are very hard; those scales that are principally calcium carbonate with little silicate are somewhat softer. A scale consisting chiefly of calcium carbonate may appear only as a thin, porous, soft scale that does not build up in thickness.

Deposits act as insulators and slow heat transfer.  The insulating effect of deposits cause the boiler metal temperature to rise and may lead to tube-failure by overheating.  Large amounts of deposits throughout the boiler could reduce the heat transfer enough to reduce the boiler efficiency.  The graph demonstrates that different types of deposits will effect boiler efficiency differently.  This is why it is important to have an analysis of deposit characteristics.

When feedwater enters the boiler, the elevated temperatures and pressures cause the components of water to take on dramatic changes.  Most of the components in the feedwater are soluble; they are dissolved in the water.  However, under heat and pressure most of the soluble components come-out of solution as particulate solids, sometimes in crystallized forms and other times as amorphous particles.  The coming-out of solution is referred to as retrograde solubility, and means that as temperature increases, ability to stay in solution decreases.  When solubility of a specific component in water is exceeded, scale or deposits develop.

Figure illustrates such a situation in a boiler tube. In the illustration there is a 56C (100F) drop across the tube from the outside surface (called “fireside”) to the inside surface or waterside. 

As the deposit restric ts heat transfer, the surface of the tube under the deposit increases in temperature (to 427C or 800F in this case). 

Eventually, as the deposit thickens, temperature will increase, the metal will soften, and the tube will blister as indicated by the dotted line. 

Because of very high furnace temperatures, modern naval boilers can tolerate no more than a few thousandths of an inch of scale on tube surfaces without suffering tube ruptures.

Scale can be prevented by the intelligent use of proper water treatment, and that is one of the objectives of the boiler water test and treatment program.

The alkaline pH in the boiler serves to minimize boiler metal corrosion and also provides hydroxide ions needed to react with the magnesium ions that would otherwise turn neutral water acidic. The reaction is:

The magnesium hydroxide forms a sludge as long as the water remains alkaline.

Both the sodium hydroxide and disodium phosphate in the water react with calcium and magnesium to form various phosphate sludges as follows:

Sludge, if allowed to accumulate in the lower boiler sections, will reach so large an amount that particles will circulate with the boiler water. As sludge circulates, it begins to adhere to the tube surfaces. The adhering sludge is, at first, soft and is removable by mechanical cleaning.

If allowed to remain on tube surfaces, the soft sludge is converted by heat to hard, baked-on sludge. The baked-on sludge is different from scale (scale forms in place, sludge is carried to high heat transfer areas) but it acts just like scale in that it restricts heat transfer with resulting blistering of the tube and eventual rupture. 

Mechanical cleaning of the watersides will not remove baked-on sludge nor scale. Scale is prevented by proper chemical treatment and sludge is kept low in concentration by maintaining feedwater  purity and by effective blowdown, primarily by expelling the boiler water contained in the lower sections of the boiler. 

This is called a bottom-blow and must only be done on a secured boiler due to the high pressure.

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