BOILER WATER

Boilers for steam generation have varying operating pressures and capacities. The use of a high pressure boiler allows a greater generation capacity but to minimise some of the problems discussed below requires a much higher water quality than a low pressure system. Temporary hardness in boiler feed water gives an accumulation of chalk scum in the boiler and scale on the walls and heating tubes. Deposition on the latter greatly reduces the rate of heat transfer and the boiler becomes increasingly less efficient. At high temperatures and pressures, both calcium and magnesium carbonates and magnesium hydroxide are much less soluble than under ambient conditions and contribute to scaling. Simple phosphates such as Na2HPO4, added to boiler feed water, will precipitate insoluble calcium and magnesium phosphate in a form that does not form a crust on the boiler walls and pipes. Polyphosphate sequestrants are less helpful since they tend to hydrolyse rapidly to simple phosphates in boiling water. Treatment with simple phosphates also ensures absorption of colloidal silica. This is significant, since silica scale on pipes and walls is very difficult to remove. The precipitation of calcium and magnesium in a non-crusting form in the boiler produces suspended material, the rate of accumulation being greater the higher the pressure and the capacity of the boiler. It is quite common to add dispersants such as polyacrylates to the feed water as these keep the precipitates well dispersed to prevent scaling. Discharge of the sludge is necessary from time to time to avoid excessive accumulation and to keep the salinity of the water within reasonable limits. In addition, dispersants and anti-foam chemicals prevent carry- over of sediment and foam with the generated steam. It is vital that there is minimum corrosion of the boiler and piping. This can be caused by acids, and by dissolved carbon dioxide and oxygen. Iron fittings corrode rapidly if boiler feed water is too acidic, so it is usual to condition the water to about pH 8 – 9 by addition of NaOH. This also ensures that all carbon dioxide is converted into bicarbonate. Too high a pH causes caustic embrittlement of non-ferrous metal fittings such as rivets. On the other hand, at pH values below 6, carbon dioxide can attack iron to form ferrous bicarbonate. This can occur in the boiler and in the piping for steam distribution and condensate return. Sometimes, volatile amines, such as ammonia or cyclohexylamine, are added to the water to neutralise any acidity and prevent this. Dissolved oxygen is a major source of corrosion. Preliminary heating removes most oxygen, since this gas is much less soluble in hot water. Alternatively, it reacts with reducing agents such as sodium sulphite or hydrazine added to the feed water. Sulphite is oxidised to sulphate but hydrazine has the advantage that it does not produce any ionic products.

Following matters are important in dye-house- Steam making with boiler, steam produce with boiler,The quality of steam produced from boiler, The capacity of boiler, pressure in the boiler, Mechanism of steam producing in the boiler, dye-house steam produce with boiler, End use of the boiler steam, The quality of water required to produce boiler water.