Boiler feed water chemistry

Introduction

  Steam is convert by the water those water is used in boiler is called boiler feed water. The water is available from different source like river, sea, pond, ground water but this water is not use directly in the boiler of the purpose of steam generation .

    Before using this water in a boiler, these impurities are remove by different method.

Water impurities remove method -water impurities are responsible to decrease boiler efficiency .Water impurities are solidify in tube so the heat is not transfer properly and then the boiler efficiency decrease. Before transfer the water in boiler tube this impurities are remove. There are mainly two type of impurities available in the water first is undissolved impurities and second is dissolved impurities. We are discuss about how to remove this impurities 

*Undissolved impurities- There are three process to remove undissolved impurities. 

:: Sedimentation- In this method the raw water is collected in the tank with which the sand ,mud and other heavy suspended material are settle down by the gravity of force and the clean water is on the tank . Then the is decant a other tank, this water is clear but it is not useful for drank. 

 

:: Filtration-In filtration process water is pass through a bed, this bed  is made by  porous material ( sand ,gravel ,charcoal). when water is passed through this bed then suspended material are collected in the bed and clean water is collected in the bottom of the filter bed , after some quantity of water are pass through this bed it is wash with water, water are normally  flow in reverse direction and the sediments are remove in the filter bed ,this process is called backwash .  after remove this sediments filter is used again.

               In water treatment plant , pressure sand filter and active carbon filter are also used to make the water completely free form suspended solids. 

Pressure sand filter- The pressure sand filter  is consists a multiple layer of sand with a verity of  size  and special gravel . This filter is designed to remove the turbidity and suspended particles present in feed water with a minimum pressure drop.

 

                                           

Active carbon filter- Active carbon and charcoal is used at active carbon filter. Active carbon is a black solid substance resembling granular or powdered charcoal . active carbon reduces organic chemicals , chlorine, lead and tastes and odours

                                            

                                                                                   

                                                            

  A   Multi Grade Filters (MGF) is also used in water treatment but it is used only low turbidity water , this filter is consists by a multiple layer of gravel , sand and active carbon , it a combination for PSF and ACF filter .

 

 

        :: Coagulation- In coagulation , colloidal impurities are remove in water by the conglomeration of small colloidal particles to the bigger particles having more mass to settle by the gravity of force.                                                                                                             colloidal particles have some electric charge developed on the surface , so repel each other and cannot conglomerate into large particles ,  we add some chemical such as alum which are produces positive charge to neutralization  the negative charge of particles then the particles can be stick together, forming a bigger particles, then these particles can be settle down by gravity of force and the colloidal impurities are remove in boiler water .

                          

                                   

        After this process water is used to drinking but this water is not transfer in boiler for the purpose of steam generation (chlorination is done to kill bacteria) . Only undissolved impurities are remove by above process . The water still contains dissolved salt and minerals. Present this salt and minerals are responsible  for hardness of water. Their are two type of hardness in water.                                                                               

       First temporary hardness and second is permanent hardness. Temporary hardness are remove by boiling the water bicarbonates and magnesium are responsible for temporary hardness. permanent hardness remove by many method.

         *Dissolved impurities- after remove undissolved impurities in water ,its it use drinking but their don't sent in boiler for steam generation because some salt of calcium , iron,  aluminum,  magnesium  and other gas etc are present in water then as it form a sludge.                       

                 Natural source of water contain silica. Present of silica is not desirable in feed water, as its form hard scale. Sometime silica is evaporated and carried out in steam which form an insoluble deposit on the turbine blades  , then the efficiency of turbine is decrease.                                         So some treatment are apply to remove the salt in boiler feed water . 

 Those treatments are apply inside the boiler it is called internal treatment and those treatment are apply outside the boiler it is called external treatment.

:: Internal  treatment- In internal treatment ,chemical are added in boiler water. these chemical are react with water salts and convert a harmful sludge after some time this sludge settle down by gravity for force .later then this sludge remove by blow down .

Some internal treatment are basically use in boiler.

* Soda ash (sodium carbonate Na₂) treatment - soda ash treatment is also use in low pressure boiler . In this process soda ash is added in boiler water , it react with chlorides and sulphates of magnesium and calcium to convert into insoluble sludge .  

Cacl₂ + Na₂Co₃ → CaCo₃ + 2NaCl  

MgSo₄ + Na₂Co₃   →MgCo₃  + Na₂So₄

* Phosphate treatment or Hp dosing ( high pressure dosing) – In high pressure boiler soda ash treatment is not used ,  it  is form caustic soda and increase  with the increase in temperature. Hence it is very difficult to maintain the ph valve of boiler water .Also , at higher PH solubility of calcium carbonate increase . so ,in high pressure boiler phosphate treatment is preferred as internal treatment.  

                HP dosing is only use in drum type boiler , chemicals are added in boiler in  boiler drum because phosphate work on absolute temperature and create a harmful sludge , this sludge are remove by CBD(continuous blow down).

Trisodium phosphate (Na₃PO₄), Disodium phosphate(Na₂HPO₄), Monosodium phosphate(NaH₂PO₄) are use in hp dosing. TSP is highly alkaline , DSP is less alkaline and MSP is slightly alkaline . It is use to different purpose. This chemical is also called orthophosphates.

 :: External treatment – we are discussed internal treatment method for remove the dissolve impurity in boiler feed water but there are not sufficient in a high pressure boiler.

Feed water treated externally before sending in boiler. By using external treated water, the quality of the boiler water can be maintain with little blowdown. Particularly   , in case of once though boiler the feed water is treated externally , as no further internal treatment is possible.

We are used some process to remove the dissolved salts and minerals in boiler feed water

*Softener process

*RO ( reverse osmosis )

* Demineralization process/ Cation and anion exchanger

Some process are used to remove the dissolve gases in feed water

* LP dosing

* Degasser

* Deaerator

We are discussed in following process in details.

SOFTNER- In softener is make hard water into soft water. softener is used for low pressure boiler. It is easily remove scale forming calcium and magnesium iron from water by iron exchange process using resin. Hard water contains salts of magnesium and calcium. Water hardness is measured in grains per gallon or milligrams per liter(mg/liter) or equivalent part per million(ppm).

          In this process, softener vessel is filled with negatively charge resin these resin is react with positively charge calcium and magnesium iron and it is exchange, sodium into water because resin is in a sodium form. The water is come out is soft form. After some time resin is discharge so first resin bed is backwashed to remove any filtered particles and recharge with sodium chloride or NaCl.

Chemical reaction

RCa + NaCL→ RNa + CaCl₂

RMg + NaCL→ RNa + MgCl₂

 

 

                                         

Softener working process

Demineralization process– In industrial water treated by demineralization process, in this process dissolved impurities are remove form feed water and process to steam generation. Demineralised water complete (almost) free of dissolved minerals like calcium, magnesium, iron, copper etc.  

* Principle of demineralization process –Water free from undissolved  suspend  solids(from  pressures filter and active carbon filter) passes through a cation  exchange containing a bed of cross-linked polymers called cation resin. This resin containing hydrogen ions (H⁺) .When water passes through the bed , earlier mentioned cation are substituted by these H⁺ ions of the resin.

Water coming out form cation exchange is acidic in nature which passes through another exchange, i.e anion exchanger ,anion exchanger contains another type of resin called anion resin containing hydroxide ions OH^- . When water from the cation passed through anion resin, earlier discussed anion are substituted by OH^- ions of resin.

In between the cation and anion exchanger, there is a degasifier unit .Here, carbonate and bicarbonates are removed. 

 

                             

DM water treatment process

Degasser- As state in earlier section, when carbonates and bicarbonates are passed though the cation and anion exchanger, carbonic acid (H₂CO₃) is formed . carbonic is a weak acid. Soft drinks available in market contain carbonic acid. This can easily be broken into water and carbon dioxide gas.

                               H₂CO₃  ^→ H₂O +CO₃