Lesson 11. Scale and sludge formation in boilers

In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reach saturation point, they are thrown out of water in the form of precipitates on the inner walls of the boiler. If the precipitation takes place in the form of loose and slimy precipitate, it is called sludge. On the other hand, if the precipitated matter forms a hard, adhering crust/coating on the inner walls of the boiler, it is called scale.

Sludge is a soft, loose and slimy precipitate formed within the comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. Sludge’s are formed by substances which have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc.

11.1 DISADVANTAGES OF SLUDGE FORMATION :

  1. Sludges are poor conductor of heat, so they tend to waste a portion of heat generated.
  2. If sludges are formed along with scales, then former gets entrapped in the latter and both get deposited as scales.
  3. Excessive sludge formation disturbs the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge-glass connection, thereby causing even choking of the pipes.

11.2 PREVENTION OF SLUDGE FORMATION :

(1) By using well softened water, (2) By frequently ‘blow-down operation’, i.e., drawing off a portion of the concentrated water. Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler. Scales are difficult to remove, even with the help of hammer and chisel. Scales are the main source of troubles. Formation of scales may be due to;

(1) Decomposition of calcium bicarbonate

Ca(HCO3)2 → CaCO3  ↓ + H2O + CO2 ↑

                         Scale

However, scale composed chiefly of calcium carbonate is soft and is the main cause of scale formation in low-pressure boilers. But in high-pressure boilers, CaCO3 is soluble.

CaCO3  +  H2O   →  Ca(OH2)2  (soluble) + CO2 ↑

 

11.3 DEPOSITION OF CALCIUM SULPHATE :

The solubility of calcium sulphate in water decreases with rise of temperature. Thus, solubility of CaSO4 is 3,200 ppm at 15oC and it reduces to 55 ppm at 230oC and 27 ppm at 320oC. In other words, CaSO4 is soluble in cold water, but almost completely insoluble in super-heated water. Consequently, CaSO4 gets precipitated as hard scale on the heated portions of the boiler. This is the main cause of scales in high-pressure boilers. Calcium sulphate scale is quite adherent and difficult to remove even with the help of hammer and chisel.

(1) Hydrolysis of magnesium salts : Dissolved magnesium salts undergo hydrolysis (at prevailing high temperature inside the boilers) footing magnesium hydroxide precipitate, which forms  a  soft type of scale  e.g.,

 MgCl2  +  2 H2O →  Mg(OH)2 ↓ + 2HCl ↑

(2)  Presence of silica (SiO2), even present in small quantities, deposits as calcium silicate (CaSiO3) and/ or magnesium silicate (MgSiO3). These deposits stick very firmly on the inner side of the boiler surface and are very difficult to remove. One important source of silica in water is  the sand filter

Disadvantages of scale formation :

(1) Wastage of fuel : Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a steady supply of heat to water, excessive or over heating is carried out  and this causes increase in fuel consumption. The wastage depends upon the thickness and the nature of scale :

thickness of scale (mm)

0.325

0.625

1.25

2.5

12

Wastage of fuel

10%

15%

50%

80%

150%

 

(2) Lowering of boiler safety : Due to scale formation, over-heating of boiler is to be done in order to maintain a constant supply of steam. The over-heating of the boiler tube makes the boiler material softer and weaker and this causes distortion of boiler tube and makes the boiler unsafe to bear the pressure of the steam, especially in high-pressure boilers.

(3) Decrease in efficiency : Scales may sometimes deposit in the valves and condensers of the boiler and choke them partially. Tills results in decrease in efficiency of  boiler.

(4) Danger of explosion :  When thick scales crack, due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes  formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler.

Removal of scales : (i) With the help of scraper or piece of wood or wire brush, if they are loosely adhering. (ii) By giving thermal shocks (i.e., heating the boiler and then suddenly cooling with cold water), if they are brittle. (iii) By dissolving them by adding them chemicals, if they are adherent and hard. Thus, calcium carbonate scales can be dissolved by using 5-10% HCl. Calcium sulphate scales can be dissolved by adding EDTA (ethylene diamine tetra acetic acid), with which they form soluble complexes. (iv) By frequent blow-down operation, if the scales are loosely adhering.

 

11.4 PREVENTION OF SCALES FORMATION :

(1)  External treatment includes efficient 'softening of water’ (i.e.  removing  hardness producing constituents of water).

(2)  Internal treatment : In this process (also called sequestration), an ion is prohibited to exhibit its original character by 'complexing’ or converting it into other more soluble salt by adding appropriate reagent. An internal treatment is accomplished by adding a proper chemical to boiler water either : (a) to precipitate the scale forming impurities in the form of sludges, which can be removed by blow-down operation, or (b) to convert them into compounds, which will stay in dissolved form in water  and thus do not cause any harm.

Internal treatments methods are, generally, followed by 'blow-down operation', so that an accumulated sludge is removed. Important internal conditioning/treatment methods are;

(i) Colloidal conditioning : In low-pressure boilers, scale formation can be avoided by adding organic substances like kerosene, tannin, agar-agar (a gel), etc., which get coated over the forming precipitates, thereby yielding non-sticky and loose deposits, which can easily be removed by pre-determined blow-down operations.

(ii) Phosphate conditioning : In high-pressure boilers, scale formation can be avoided by adding sodium phosphate, which reacts with hardness of water forming non-adherent and easily removable, soft sludge of calcium and magnesium phosphates, which can be removed by blow - down operation, e.g.,

             3CaCl2  +  2Na3PO4    →   Ca3(PO4)2  +  6 NaCl

The main phosphates employed are : (a) NaH2PO4, sodium dihydrogen phosphate (acidic); (b) Na2HPO4, disodium hydrogen phosphate (weakly alkaline); (c) Na3PO4, trisodium  phosphate (alkaline).

 (iii) Carbonate conditioning : In low-pressure boilers, scale-formation can be avoided by adding sodium carbonate to boiler water, when CaSO4 is converted into calcium carbonate in equilibrium.

                           CaSO4 +Na2CO3 →  CaCO3 + Na2SO4

Consequently, deposition of CaSO4 as scale does not take place and calcium is precipitated as loose sludge of CaCO3, which can be removed by blow-down operation.

(iv) Calgon conditioning involves in adding calgon [sodium hexameta phosphate (NaPO3)6 to boiler water. It prevents the scale and sludge formation by forming soluble complex compound with CaSO4.

                  Na2[Na4(PO3)6]     →  2  Na+  +   [Na4P6O18]2-

Calgon

       

                2 CaSO4  +  [Na4P6O18]2 −   →  [Ca2P6O18]2 − +  2 Na2SO4

                       Soluble complex ion

  (v) Treatment with sodium aluminates (NaAlO2) : Sodium aluminates gets hydrolyzed yielding  NaOH and a gelatinous precipitate of  aluminium hydroxide.

             NaAlO2  +  2H2O     →  NaOH    +    Al(OH)3

                                                                                                                       Sodium meta-aluminate          Gelatinous precipitation

The sodium hydroxide, so-formed, precipitates some of the magnesium as Mg(OH)2 ,

                       MgCl2 +  2NaOH   →   Mg(OH)2  +  2 NaCI

The flocculent precipitate of Mg(OH)2 plus Al(OH)3, produced inside the boiler, entraps finely suspended and colloidal impurities, including oil drops and silica. The loose precipitate can be removed by pre-determined blow-down operation.

(vi) Electrical conditioning: Sealed glass bulbs, containing mercury connected to a battery, are set rotating in the boiler. When water boils, mercury bulbs emit electrical discharges, which prevents scale forming particles to adhere /stick together to form scale.

(vii) Radioactive conditioning: Tablets containing radioactive salts are placed inside the boiler water at a few points. The energy radiations emitted by these salts prevent scale formation.

(viii) Complex metric method involves addition of 1.5 % alkaline (pH = 8.5) solution of  EDTA to feed-water. The EDTA binds to the scale-forming cations to form stable and soluble complex. As a result, the sludge and scale formation in boiler is prevented. Moreover, this treatment : (1) prevents the deposition of iron oxides in the boiler, (2) reduces the carryover of oxides with steam, and (3) protects the boiler units from corrosion by wet steam (steam containing liquid water).

 

11.5 CAUSTIC EMBRITTLEMENT:

Caustic embrittlement is a type of boiler corrosion, caused by using highly alkaline water in the boiler. During softening process by lime-soda process, free Na2CO3 is usually present in small proportion in the softened water. In high pressure boilers, Na2CO3 decomposes to give sodium hydroxide and carbon dioxide,

             Na2CO3 + H2O → 2NaOH + CO2

and this makes the boiler water basic ["caustic"]. The NaOH containing water flows into the minute hair-cracks, always present in the inner side of boiler, by capillary action. Here, water evaporates and the dissolved caustic soda concentration increases progressively. This caustic soda attacks the surrounding area, thereby dissolving iron of boiler as sodium ferroate this causes embrittlement of boiler parts, particularly stressed parts (like bends, joints, rivets, etc.), causing even failure of the boiler. 

Caustic embrittlement can be avoided :

  1. by using  sodium  phosphate as softening  agent, instead of sodium carbonate ;
  2. by adding tannin or lignin to boiler water, since these blocks the hair-cracks, thereby preventing infiltration of caustic soda solution in these;
  3. by adding sodium sulphate to boiler water.  Na2SO4 also blocks hair-cracks, thereby preventing infiltration of caustic soda solutions. It has been observed that caustic cracking can be prevented, if Na2SO4 is added to boiler water so that the ratio :                              

is kept as 1:1:2:1 and 3:1 in boilers working respectively at pressures up to 10, 20 and above 20 atmospheres.

 

11.6 BOILER CORROSION

Boiler corrosion is decay of boiler material by a chemical or electro-chemical attack by its environment. Main reasons for boiler corrosion are:

(1) Dissolved oxygen : Water usually contains about 8 ml of dissolved oxygen per litre at room temperature. Dissolved oxygen in water, in presence of prevailing high temperature, attacks boiler material:

                                    2 Fe  +  2H2O  +  O2  →  2 Fe(OH)2

                                    4 Fe(OH)2  + O2   → 2 (Fe2O3.2H2O)

                                      Ferrous hydroxide             Rust

Removal of dissolved oxygen :

 (1) By adding calculated quantity of sodium sulphite or hydrazine or sodium sulphide. Thus;

                                 2 Na2SO3  +  O2 → 2 Na2SO4

                                                          N2H4  + O2 → N2 + 2 H2O                            

   Hydrazine

                                 Na2S  +   2 O2  →  Na2SO4

 (2) By mechanical de-aeration, i.e., water spraying in a perforated plate-fitted tower, heated from sides and connected to vacuum pump (see Fig. 2). High temperature, low pressure and large exposed surface (provided by perforated plates) reduces the dissolved oxygen in water

Flg. 2. Mechanical de-aeration of water

 (2) Dissolved carbon dioxide :  CO2 is carbonic acid,

                                    CO2   +  H2O  →   H2CO3

which has a slow corrosive effect on the boiler material? Carbon dioxide is also released inside the boiler, if water used for steam generation it contains bicarbonate, e.g.,

                  Mg(HCO3)2   → MgCO3    +    H2O    +   CO2

Removal of  CO2  : (1) By adding calculated quantity of ammonia. Thus,

                 2NH4OH  +  CO2 →  (NH4)2CO3 + H2O

(2) By mechanical-aeration process along with oxygen.

(3) Acids from dissolved salts:  Water containing dissolved magnesium salts liberate acids on hydrolysis, e.g.,

              MgCl2     +    2H2O  → Mg(OH)2 + 2HCl

The liberated acid reacts with iron (of the boiler) in chain like reactions producing HCI again and again. Thus

                      Fe + 2HCI → FeCl2 + H2

                     FeCl2 + 2H2O → Fe(OH)2 + 2HCl

Consequently, presence of even a small amount of MgCl2 will cause corrosion of iron to a large extent.

 

11.7 PRIMING AND FOAMING

When a boiler is steaming (i.e., producing steam) rapidly, some particles of the liquid water are carried along-with the steam. This process of 'wet steam' formation is called priming. Priming is caused by;

  1. the presence of  a large amount of dissolved solids;
  2. high steam velocities,
  3. sudden boiling ;
  4. improper boiler design
  5. sudden increase in steam-production rate.

Foaming is the production of persistent foam or bubbles in boilers, which do not break easily. Foaming is due to presence of substances like oils (which greatly reduce the surface tension of water).

Priming and foaming, usually, occur together. They are objectionable because; (i) dissolved salts in boiler water are carried by the wet steam to super-heater and turbine blades, where they get deposited as water evaporates. This deposit reduces their efficiency,  (ii) dissolved salts may enter the parts of other machinery, where steam is being used, thereby decreasing the life of the machinery; (iii) actual height of the water column cannot be judged properly, thereby making the maintenance of the boiler pressure becomes difficult.

Priming can be avoided by: (i) fitting mechanical steam purifiers; (ii) avoiding rapid changing steaming rate; (iii) maintaining low water levels in boilers, and (iv) efficient softening and filtration of the boiler-feed water.

Foaming can be avoided by: (i) adding anti-foaming chemicals like castor oil, or (ii) removing oil from boiler water by adding compounds like sodium aluminates.

 

 

Last modified: Friday, 11 April 2014, 5:39 AM