Boiler Water

A boiler is a closed pressure vessel in which a fluid is heated for use external to itself by direct application of heat resulting from the combustion of fuel (solid, liquid or gaseous) or by the use of electricity or nuclear energy. In general there are two types of boilers to consider - steam and hot water. Boilers are further categorized into two groups:

• fire tube boilers
• water tube boilers

The fire tube boilers have a capacity restriction up to 20 kg/cm² due to design limitations. Water tube boilers have no such design limitations.

A hot water boiler is a closed loop system in which the heated water is circulated throughout the system by means of a pump. This type of boiler uses very little chemical treatment since the make-up water entering the system is small.

Steam boilers can be categorized into two groups:

• heating boilers
• process boilers

The heating boilers have little make-up water entering the system. In case of process boilers whenever there is an excess of 10 % loss of water from the boiler, it is suggested to pre soften the make-up water. This would reduce the cost of chemical treatment and frequency of washout periods.

Most sources of water contain impurities in the three states:

• soluble salts in dissolved state
• suspended solids
• dissolved gases

Some soluble impurities on heating decompose to insoluble salts, such as, bicarbonates of calcium and magnesium. While some dissolved impurities forms scale (soft/hard) and others cause corrosion.

There are three methods of treatment:

• External Treatment
• Internal Treatment
• Blowdown

External treatment adjusts raw water analysis before water enters the boiler. Internal treatment adjusts the boiler water analysis by feeding chemicals directly to the feed water or the boiler. Blowdown helps in regulating parameters within desired limits, such as, TDS, insoluble suspended precipitates.

Scaling in boilers occurs due to decomposition of soluble salts or solutes reaching their solubility limits. These salts form scale on the heat transfer surface and may be due to carbonates of calcium or magnesium, calcium sulphate, magnesium silicate and silica, The treatment recommended consists of chemicals such as phosphate/organic phosphonate, chelating/sequestering agents.

Corrosion in boilers occurs due to dissolved gases such as oxygen and carbon dioxide. Oxygen is responsible for corrosion and can be eliminated by using oxygen scavengers, such as, sodium sulphite or hydrazine. Sodium sulphite is used in low-pressure boilers while hydrazine is recommended for high-pressure boilers. Carbon dioxide passes out with the stern and causes corrosion in steam/condensate pipelines. This is due to acidic nature of carbon dioxide. In order to control pipeline corrosion volatile neutralizing amines are used.

Blowdown in simple terms refers to purposeful withdrawal of water from the boiler. The higher the amount of blowdown, the greater is the loss in fuel efficiency. Blowdown can be intermittent or continuous. Transferring heat in blowdown to incoming make up water often pays for the investment.