Define scale and sludge. Give the causes, disadvantages of scales in boiler.

Define Scale

Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler. Scales are difficult to remove, even with the help of a hammer and chisel. Scales are the main source of trouble. The formation of scales may be due to the Decomposition of calcium bicarbonate.

Define Sludge

Sludge is a semi-solid slurry that can be produced from a range of industrial processes, from water treatment, wastewater treatment, or on-site sanitation systems.


The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Sewage sludge is the solid, semisolid, or slurry residual material that is produced as a by-product of wastewater treatment processes. This residue is commonly classified as primary and secondary sludge.

Sludge is a substance that is in solid and liquid form. An example of sludge is a mass of mud formed in a river bed after a flood. An example of sludge is the treated material from a sewage plant.

Also Read:- Difference Between Sludge and Scale in Boiler

Scale & Sludge Formation in Boilers

Boilers are used for steam generation. When hard water is evaporated, progressively the concentration of dissolved salt is increased. When their saturation points are reached, the dissolved salts of calcium and magnesium along with other soluble impurities are precipitated on the inner walls of boilers and in due course of time adhere to the metal surface in the form of scales and sludges.

When the precipitate formed is soft, slimy, and loose, known as sludge, and is hard and firmly adhering to the inner walls of boilers known as scale. The sludges are generally formed by the substance, which has a greater solubility in hot water than in cold water. So, sludges are formed in colder parts of boilers.

Causes of  Scales in Boiler

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots.

The precipitation of impurities from the feed water on the heat transfer or metal surface on the boiler causes scale formation. During evaporation, the deposits become hard and concentrated, which hinders heat transfer and causes hot spots, eventually leading to local overheating. Feedwater contaminants such as calcium, magnesium, iron, silica, and aluminum are the major causes of deposit formation. Scale formation occurs due to salts that are not completely insoluble in the boiler water. Hence the salts reach the surface in a soluble form and precipitate.

The presence of calcium and magnesium salts is the main cause of scaling in boilers, along with the high concentration of silica to the water alkalinity in the boiler. Scale salts generally consist of carbonates, bicarbonates, and sulfates that form slowly in a concentrated pattern, reducing heat transfer. If overlooked, scales can be extremely difficult to remove even by using chemical treatment.

The carbonate deposits are generally granular or porous. The calcium carbonate crystals are large and matted, which leads to dense scale formation in boilers. Carbonate deposits can be identified by suspending them in an acid solution as the carbon dioxide bubbles will effervesce from the scale.

The sulfate deposit is denser compared to the carbonate deposit as the crystals are smaller and bond together. The sulfate deposit is brittle that does neither easily pulverize nor effervesce when introduced to acid.

Deposits with high silica are hard with extremely small crystals that lead to a dense scale formation in boilers. The scale is light in color and brittle which causes difficulty in pulverizing. It is not soluble in hydrochloric acid.

Causes of Scale Formation in Boiler:

Let us look at the factors that contribute to the scaling in steam boilers:

Feedwater quality:

One of the most impactful factors causing scales in the boiler is the low quality of feed water. Boiler manufacturers suggest the pretreatment of feedwater that involves analyzing the water source, its quality, and the removal of suspended or dissolved solids through the process of filtration, softening, or demineralization. Feedwater of low quality can result in scaling and deposition because of hardness and iron.

Contaminated condensate:

When the steam vapor is returned to its liquid state, it forms a condensate taking place during utilizing steam in process operations. It retains about 13% of the steam energy that is used in boilers to reduce make-up water and fuel costs. Contaminated condensate, if not treated properly, causes scaling in boilers.

Boiler Pressure:

The amount of scale formation in the boiler depends on the pressure at which the steam boiler operates. Depending on the industry and system, boiler manufacturers suggest the ideal chemical properties and pressure to use for the optimal functioning of the boiler.

Boiler Design:

Some boiler designs are prone to scaling on a greater scale than others. Few steam boilers have varying heat flow rates at the tube surface with the same pressure rating. Boiler design and efficiency can be major factors causing scaling in boilers.

Irregular Boiler Maintenance:

Irregular and poor maintenance of the boiler system or negligence with feed water treatment causes scaling that may cause harm to the steam boiler and the process plant.

Prevention of Scale Formation in Boiler:

If scaling is neglected, it may cause a reduction in heat efficiency by acting as an insulator that would slow down the heat transfer. Scaling, if left unchecked, may lead to overheating and rupture of the tube in the steam boiler. Some of the ways to prevent scale formation in boilers are:

Boiler Water Treatment: Scaling is caused by the presence of insoluble salts, calcium, and magnesium in the feed water. It is crucial to ensure proper boiler water treatment before it is utilized for operations.

Water Softeners: Installing water softeners can help in preventing limescale in the steam boiler.

Removal of Scales: Scales that are formed loosely on a boiler surface can be removed with a wire brush or a scraper.

Cooling Water Inhibitors: Cooling water inhibitors are suitable for extending the solubility of the salts and preventing scaling in boilers.

Thermal Shocks: Thermal shocks are conducted for a scale that is brittle in nature.

Chemical Cleaning: If the scale formation is extensive in the boiler, then boiler manufacturers can perform chemical cleaning for the removal of scale and corrosion in boilers.

Blowdown Operations: Frequent blowdowns can help in removing the scale that is formed loosely on the surface of the boiler.

Regular Maintenance: Regular monitoring and maintenance of the steam boiler can help in preventing scale formation before it gets denser.

Disadvantages of Scale & Sludge Formation

1. Decrease in Efficiency of Boiler: The excessive scale formation may cause clogging of boiler tubes which decreases the efficiency of the boiler.

2. Wastage of Fuel: Scale is a poor conductor of heat. This results in a reduced rate of heat transfer, and thus the evaporative capacity of the boiler will be reduced. Thus scale formation also decreases the efficiency of the boiler and causes a waste of fuel.

3. Danger of Explosion: The scale formation also leads to the uneven expansion of the boiler metal. Due to uneven expansion, thick scales crack and water comes suddenly in contact with overheated iron plates. This results in the formation of a large amount of steam suddenly which in turn develops a sudden high pressure. The sudden high pressure thus developed may lead to the explosion of the boiler.

4. Boiler Becomes Unsafe: The overheating of boilers occurs due to scale formation and metal becomes soft and weak. This makes the use of the boiler unsafe, particularly at high pressures.

5. Bagging: The distortion of boiler material is known as bagging. The superheating of the boiler leads to the distortion of the boiler material.

6. Corrosion: There are certain salts formed from scale, that corrode the tubes, and at the higher temperature so maintained, there is still greater corrosion of the boiler tubes

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