BOILER WATER TREATMENT

The purpose of boiler water treatment is fourfold:

1.To react with any remaining feed water hardness and prevent its precipitation as scale on boiler metal;

2.To condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non adherent to boiler metal;

3.To provide anti-foam protection to permit a reasonable concentrations of dissolved solids in boiler water without introducing carry-over;

4.To eliminate dissolved gases from water and to provide sufficient alkalinity to prevent boiler corrosion.

        i.  Scale Formation and Prevention

Typical constituents of scales and deposits in boiler are:

Calcium carbonate, Calcium sulphate, Complex silicates of Mg, Fe, Ca and Al, Calcium phosphate, Magnesium hydroxide, Magnesium phosphate, Iron and copper oxides.

A build-up of scale leads to reduced boiler efficiency and in water tube boilers can eventually lead to tube failures as overheating.

The soluble calcium (Ca) and magnesium (Mg) salts are cause of hardness in water, and most of the scales and deposits formed when fresh water or shore water is used in boiler plants are largely compounds of calcium and magnesium.

      The calcium and magnesium salts may be divided into two groups:

                     i.            The bicarbonates of calcium and magnesium, which are easily decomposed by heat, cause alkaline hardness sometimes known as temporary hardness. The decomposition of these bicarbonates by heat may be represented by the following equations:

Ca (HCO₃)₂ → CaCO₃ + CO₂ + H₂O

The CO₂ is liberated in the boiler and will produce an acid steam condensate. Trace quantities of magnesium and calcium bicarbonate may be present in evaporated sea water and will decompose in the boiler to produce insoluble carbonate which, if not removed by boiler blow down, will form a deposit on boiler tubes.

                   ii.            The chlorides, sulphates and nitrates, CaCl₂, MgCl_2, CaSO₄, Ca (NO₃) and MgNO₃, are not decomposed by boiling. They cause non-alkaline hardness or permanent hardness.

Treatment: In low pressure, auxiliary marine boilers operating at pressures up to 15 bar, internal treatment with sodium phosphate and sodium carbonate or sodium hydroxide is used to prevent hard adherent scales of calcium sulphate by precipitation of all calcium as calcium phosphate and magnesium as magnesium hydroxide. The precipitate can then be removed by boiler blow down provided the sludge is maintained in a mobile state and prevented from baking into boiler metal.

Again trace amounts of Ca and Mg, sulphate, chloride and nitrates will be present in evaporated sea water and treatment with sodium phosphate and sodium hydroxide is necessary to remove these potentially scale forming salts.

      Provided a slight excess of phosphate is present in boiler water at all times the calcium salts entering with feed will be precipitated as calcium phosphate. e.g:-

3CaSO₄ + 2Na₃PO₄ → Ca₃(PO₄)₂ + 3Na₂SO₄

3CaCl₂ + 2Na₃PO₄ → Ca₃(PO)₄ + NaCl

Recommended phosphate reserve to be maintained in boiler water for 0 to 15 bar water tube boiler is 30 to 70 ppm PO₄.

v Boiler Corrosion and Prevention

The corrosion process is essentially an oxidation/reduction reaction. It may occur in various forms including caustic cracking. The most usual forms however, are pitting, general wastage and corrosion fatigue.

      When metal dissolves in water, whether the water is alkaline or acidic, the metal is oxidized and oxidizing agent is usually hydrogen ion. Thus, iron will dissolve more readily in acid solutions which have high hydrogen ion concentration. While corrosion is an electrochemical process it may be represented simply as:

Fe → Fe⁺⁺ + 2 electrons

The reaction continues in presence of oxygen as follows:

4electrons + O₂ + 2H₂O → 4 OH^-

Fe⁺⁺ + 2 OH^- → Fe (OH)₂   (ferrous hydroxide).

      The corrosion reactions are caused directly or indirectly by oxygen, carbon dioxide or certain salts, e.g. magnesium chloride in boiler water.

      Corrosion can proceed undetected under a layer of scale. Corrosion fatigue occurs when the metal is in contact with a corrosive medium and the metal is subjected to fluctuating stress.

      Caustic cracking results from the contact of concentrated caustic soda solutions with steel which has not been stress relieved, e.g. in riveted seams.

      The main cause of boiler, condensate and feed system corrosion results from the dissolved gases, oxygen and CO₂ in feed water.

Treatment:

Mechanical removal of Gases: Air can enter through any openings such as make up, drain, or cascade tanks and especially system under vacuum such as turbine seals and condensers. To deal with this problem, marine steam systems are equipped with air ejectors, hot wells and sometimes de-aerating heaters. The following are the main points to check in plant operation to reduce entry of O₂ and CO₂:

1.      Check all points of possible air leakage in the condensing and vacuum sections of the plant. (i.e. defective flanges, gaskets, valve packing, cracked valve bonnets, open return line drain valves, insufficient steam pressure on gland seals, malfunctioning of steam traps etc.)

2.      Check the temperature of water in tanks operating at atmospheric pressure. Sine O₂ and CO₂ gases readily dissolve in cool water, the water in all atmospheric water tanks should be heated to highest temperature possible without causing vapour lock in pump suction.

3.      Check for inefficient operation for de-aerating heater. One thermometer should be installed in steam and other in water space of de-aerator. When unit operating correctly, the temperature difference of two should be within 1⁰ or 2⁰ C.

Chemical De-aeration: The chemicals used for removal of residual oxygen from the feed water are Hydrazine (N₂H₄) or Sodium sulphate.

1.      Hydrazine must be continuously dosed into the feed water to react with all the oxygen present and to produce a small reserve of hydrazine in feed water (0.05 to 0.3 ppm N₂H₄). The chemical reaction with oxygen can be expressed as:

N₂H₄ + O₂ → 2 H₂O + N₂

As the products are nitrogen gas and water, hydrazine treatment does not increase the dissolved solids content of boiler water unlike sodium sulphite which forms sodium sulphate.

The excess hydrazine in the boiler breaks down to give ammonia which provides suitable alkaline conditions in steam condensate system.

3 N₂H₄  → 4 NH₃ + N₂

The treatment advised for prevention of corrosion can be summarized as follows:

a)      Efficient mechanical de-aeration of the feed water for all high pressure boilers;

b)      Maintenance of correct level of alkalinity in the boiler.

c)      Hydrazine or sulphite addition to maintain reserve of one these chemicals in boiler;

d)      Maintenance of correct pH level in condensate system;

e)      Feed filtration to reduce ingress of copper and iron oxides to the boilers.

v Carry-over and Prevention

Carry-over is the name used to describe contamination of steam by boiler water and the solids which it contains. There is always a little entrainment of water with steam, but gross contamination can cause erratic superheat temperature and can interfere turbine efficiency. Boiler water solids entrained in steam can form deposits in super heaters, on turbine blading and in valves.

      Carry-over can be caused by purely mechanical factors among which are high boiler water levels, steaming in excess of boiler rating, sudden increase in steam demand and operation at steam pressure below that for which boiler is designed.

      Carry-over may be caused by slugs of water surging into the steam off take and this is usually called as priming. High water levels fluctuations in steam load or overloading of boiler encourage priming.

Foaming: Is due to small stable bubbles of steam collecting on boiler water surface. The water film around each bubble is made more stable by an increase in suspended or dissolved solids in boiler water; foaming may also be caused by oil or organic matter. Operating a boiler at less than the design pressure increases the volume of steam produced although the weight of steam is same. This increases the tendency of foaming. Anti-foams available are complex organic compounds, characterized by the presence of poly amide or poly oxide groups.

v Recommended Boiler Water Characteristics (Water tube, up to 15 bar)

Phosphate ppm PO4	P.Alkalinity ppm CaCO3	T.Alkalinity	Hydrazine ppm N2H4	Sulphite ppm Na2SO3	Chlorides Max.ppm CaCO3	Conductivity Max
30 - 70	50- 300	Less than 2×P.Alkalinity	0.05-0.3	20-30	300	700 µ mhos

Hardness test of boiler water is not necessary when phosphate is above lower limit of control range.