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Introduction to Boiler Scales

Boiler Scales

As all of us are aware, in a boiler, steam formation takes place because of heat transfer from fire side to water side. So, performance of any boiler not solely but mainly depends on the efficiency of the heat transfer. Metals are good conductors of heat. Hence, all the contact surfaces between fire side and steam side are metals or metal alloys. In continuous operation of a boiler, it becomes quite difficult to maintain the metal tube surfaces clean, without any layer formation. Depending upon the chemistry of the feed water, different types of scales or deposits develop on the boiler tubes which affect the heat transfer taking place in boiler.

In general, the losses taking place on account of scales and deposit formation can be summarized as follows :

1.    Excess amount of fuel required to convert same amount of water into the steam (decreased heat transfer).

2.    Corrosion of copper alloy based heat exchanger tubes

3.    Excess wall temperatures which can eventually result in tube failures.

Observing all of the above losses which are resulted out of boiler scale formation, it is very important to avoid the scale formation.

On financial fronts, scales and deposits can aggravate following costs/ losses :

1.    Higher maintenance costs.

2.    High repair costs in case of tube failures.

3.    Down times which can affect the production line badly.

4.    Poor quality of steam which can worsen the quality of final product.

5.    Increased cost of steam because of blow down losses.

There are two types of boiler deposits, namely scales and sludge.

How scales are formed?

Water is called as universal solvent because of large number of compounds which can be dissolved in water. Though this is true, every substance can be dissolved only up to a certain limit in the water, called solubility of that substance. When the amount of a substance in the given water sample increases beyond this solubility limit, it starts to precipitate at the bottom. For producing steam, we supply boiler with feed water. Natural water contains many salts (calcium, magnesium etc.) in it. We cannot see these salts because they are dissolved in the water, as their quantity is below the solubility limit in the water. When this feed water goes into the boiler, it is evaporated, the salts being left behind. As a result, the concentration of salts goes on increasing and when it crosses the solubility limit of the water, the salts get deposited as scales. This type of scales is formed during the operation of the boiler.

Deposit constituents

As of now, we are clear that in every boiler, scale/deposit formation is going to take place. As we cannot bear the losses mentioned above, it is necessary that these scales and deposits are either prevented from being formed and also boilers are regularly monitored for their levels and cleaned when the layers are formed. There are different types of scales or deposits which get formed inside a boiler. To attain a good command over monitoring the deposits, it will be helpful if we understand the deposits in more details.

1.    Scales :

Scales are formed out of salts. These salts come inside the boiler through water in form of soluble solids and are precipitated when the concentration level increases because of evaporation. Scales are basically crystalline and more difficult to remove. The most common scale constituents are silica, aluminum, iron, magnesium etc. Scale salts are mostly carbonates, bicarbonates and sulphates. These scales are formed slowly, hence are formed in a dense pattern. Scales reduce heat transfer dramatically and many times are difficult to be removed even with the help of chemical and mechanical cleaning. Another different type of scale is Silica Scales. Silica is found commonly in most of the water supplies and is quite difficult to be removed. Silica can form a thin, glassy scale layer called amorphous silica which can be very hard to be removed.

2.    Sludge :

Along with the dissolved solids, there are some suspended solids in the boiler feed water. These suspended solids get precipitated as sludge. Sludge is normally soft in nature compared to scales, but many times sludge gets hardened because of heating inside the boiler. This heating many times makes the removal of sludge quite a difficult process.