BOILER WATER CHEMISTRY

The main objective of feed and boiler water treatment in a power plant is to maintain the plant in a condition where it can operate reliably and efficiently.It aims:-

• To maintain the integrity of the feed, boiler and steam system by minimizing corrosion to an acceptable level.
• To maintain the effectiveness of heat transfer surface by minimizing the deposition of corrosion products & scale.
• To maintain a high level of steam purity and thus prevent problem in the super heaters,   re heaters, & turbine.

The water has to be treated to obtain required quality before it is allowed to go into the boiler.The ultra-pure water so obtained is further conditioned in the feed system and then within boiler so that water is not aggressive to any of the variety of construction materials used in boiler, feed and steam cycles.Corrosion by oxygen and carbon dioxide in the feed lines and economizers causes small amounts of corrosions debris carried into the boiler and deposited into the tube surfaces. These deposited acts as a potential areas for further corrosion.In the control of such kind of corrosion certain chemicals are added to the feed water to remove or neutralize gases that have entered into the system and escaped during mechanical deaeration.HYDRAZINE TREATMENT OF FEED WATER

• Dissolve oxygen is most objectionable species in water. It brings about rapid corrosion by conversion Iron (in steel) into Iron Oxides.
• The mechanism of corrosion of metals by oxygen is by the formation of differential aeration cells in the system.
• To remove oxygen (Dissolve oxygen) from feed water HYDRAZINE is dosed. It reacts according to the reaction:-

N₂H₄   + O2   → N₂ + 2H₂O

From the reaction we see that no solids are produced in the system. In fact the product N2 & H2O are completely harmless. Any surplus hydrazine that remains after removing the oxygen is decomposed at high temperature to give ammonia and nitrogen.

3N₂H₄   → 4 NH₃ + N₂

Hydrazine also forms protective film on the metal surfaces-

6Fe₂O₃ + N₂H₄ → 4Fe₃O₄ + N₂ + 2H₂O

DOSING POINT OF HYDRAZINEHydrazine is usually dosed at Deaerator Outlet after mechanical scavenging of dissolved oxygen in deaerator. Hydrazine dosing is a continuous process. AMMONIA TREATMENT OF FEED WATERCarbonates or bicarbonates salt enters into the boiler due to condenser tube leakage/ seepage, dissociates to produce carbon dioxide under boiler condition. CO₂ In the absence of dissolved oxygen in water causes general metal thinning i.e, uniform attack on metal.Feed water is ammoniated in order to protect steel against corrosion by carbonic acid according to the reaction

 NH₄OH + CO₂  → NH₄HCO₃

NH₄HCO₃ + CO₂ → (NH₄)₂CO3 + H₂O

In the boiler, ammonium bicarbonate and carbonate disintegrate into ammonia, carbonic acid and water. The carbonic acid and ammonia thus formed turns into vapour and are removed from the boiler.When the steam has been cooled and condensed in the condenser, ammonia dissolved in the condensate, and carbonic acid is removed together with permanent gases. Therefore ammonia should constantly be introduced in the feed water to insure a desired  pH value. So the purpose of ammonia dosing is

1. to insure a desired pH value (to protect corrosion) and
2. to neutralize the presence of  carbon dioxide.

Amines which decompose to give ammonia, can also be used to neutralize carbon dioxide. They are called neutralizing amines like Cyclohexileamine and Morpholine.DOSING POINT OF AMMONIANH₃ is dosed at  Deaerator Outlet or at  BFP (BOILER FEED PUMP) SUCTION.PHOSPHATE TREATMENT OF BOILER WATERIt is well known that carbon steel has minimum corrosion in the pH range of around  9 to 10 .To achieve this pH  TSP (Tri Sodium Phosphate) is added into the boiler. Sometime sodium hydroxide may also be added but it provides an extra ionic load.TSP reacts with scale forming salts (which may enter by poor quality of make-up or condenser leakage) as follows:-   

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

2Na₃PO₄  + 3MgSO₄  → 3Na₂SO₄ + Mg₃( PO4)₂  ↓

So phosphate if present in boiler water will react with scale forming salts and for corresponding phosphate of Ca / Mg which are insoluble and formed a sludge. When  CBD ( Continuous Blow Down ) is operated, this sludge is bled-off from the boiler drum and this one gets rid off the scale forming salts.DOSING POINT OF  PO₄ TSP is directly dosed into the boiler drum at pressure of 150 Kg/cm².WATER AND STEAM   QUALITY STANDARDS  :-CONDENSATE WATER:-

• pH                        8 – 9.0
• ACC ( μs/cm)       ≤0.30
• SiO₂ (ppm)          ≤0.02
• NH₃ (ppm)          5 (max)
• DO (ppb)             ≤ 50

ECO (IN-LET)

• pH                         8-9.0
• ACC (μs/cm)         ≤0.30
• SiO₂(ppm)            ≤0.02
• N₂H₄                    10-30 ppb
• DO 0 ppb max
• Cu 0 ppb max

BOILER WATER (DRUM) :-

• pH                      9.2-9.5
• K( μs/cm)           ≤50.0
• SiO₂                   0.5 ppm max
• PO₄                    0-4.0 ppm max 
• Cl                       0 ppm max

 MAIN STEAM

• pH                      8-9.0
• K(μs/cm)             0-4.5
• ACC(μs/cm)       ≤0.30
• SiO₂(ppm)         ≤0.02
• NH₃                   5 ppm max
• Na                     0 ppb max   

    PRIMARY STATOR WATER

• pH                      6.0-7.0
• K(μs/cm)             0 max
• DO(ppm)            0-5.0
• Fe                       0 ppb max
• Cu                       0 ppb max         

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