Chemical treatment for  internal problems in boilers 

 

In the boiler operation  it is very importance of protection of internal system must be smooth for smooth operation .During operation process there is happened various changes because of pressure  and temperature  ,heat ,air, dry air ,wet air ,oil and dissolved water flowing with high velocity ,having also presence of TDS and suspended matter and also other total dissolved solids .

During continuous operation metal erosion , corrosion ,scale formation by total dissolved solids takes place. These problems create abstraction in continues operation and can effect on boiler efficiency.

Internal treatment consists of adding chemicals directly to the water in boiler for removing dangerous scale forming salts which were not completely removed in the external treatment for water soften. This is mainly used as a corrective treatment to remove the slight residual hardness and also sometimes to remove the corrosive tendencies in water. This treatment is not usually applied to raw waters , except for small boiler, but it is usually practiced in large power station. In modern heavy-duty high pressure boilers, water of zero hardness is required, since even an egg shell thickness of scale may be extremely detrimental.

v  CARBONATE CONDITIONING:

 

For a salt to be precipitate sufficient amount of the ions forming the salt must be present so that the product of their concentration exceed a limiting value known as the solubility product. Thus, for a salt like CaCO3 to be precipitated, the product of the concentration of Ca+ and CO3 must exceed the solubility product of CaCO3.

When sodium carbonate solution added to the boiler water the co3 increases and when it becomes greater than KX (SO4). Only CaCO3 gets precipitate and CaSO4 remain in solution. Thus the deposition of scale forming CaSO4 is prevented.

Na2CO3+CaSO4 --------------CaCO3+Na2SO4

Carbonated conditioning is used only for low pressure boilers. In high pressure boiler the excess Na2CO3 might be converted into NaOH  due to hydrolysis as follows:

Na2CO3+H2O----------------------------2NaOH+H2CO3

H2CO3-------------H2O+CO2

NaOH Causes caustic embrittlement in high pressure boilers.

v  PHOSPHATE CONDITIONING

In this, an excess of a soluble phosphate is added to the boiler water to the precipitate the residual calcium ions in the form of a non-adherent precipitate of calcium phosphate and thus the prevent of scale formation.

The three sodium orthophosphates are using for phosphate conditioning are (Na3PO4, Na2HPO4and NaH2PO4 ) and also sodium pyrophosphate (Na4P2O2)  and sodium metaphosphate (NaPO3).the typical reaction of the various phosphates with the hardness represented as CaCO3 are given below

2Na3PO4+3CaCO3---------------------------Ca(PO4)2+3Na2CO3

2Na2HPO4+CaCO3----------------------------Ca3(PO4)2+2Na2CO3+CO2+H2o

2NaH2PO4+3caco3----------------------------------ca3(PO4)2+Na2CO3+2CO2+2H2O

2Napo4+3Caco3------------------------------------------Ca3(Po4)2+Na2CO3+2CO2

The quality of the feed water dictates the choices of the particular phosphate to be used. For instant, if the feed water tends to produce an acidic condition in the boilers, the alkaline trisodium phosphate   should be choosen. This treatment could be supplemented with NaOH if the required alkalinity could not be maintained with Na3PO4 alone.

If the feed water produce almost the right alkalinity desired in the boiler, it is preferable to use Na2HPO4 which is practically neutral.

If the boiler water becomes too alkaline, the acidic NaH2PO4 would be selected.

Both sodium pyrophosphate and metaphosphate are rapidly hydrolyzed under boiler water temperature to orthophosphates.

 

NaPO3+H2O---------------------------------NaH2PO4

Na4p2O7+H2O-------------------------- 2Na2HPO4

Thus, their behavior within the boiler is identical with that of orthophosphate mentioned above.

However NaPO3 solution are practically neutral, whereas NaH2PO4 solutions are acidic.

Hence the former could be preferred if the use of NaH2PO4 causes feedline corrosion.

The use of internal treatment combined with suitable blow-down to remove sludge has contributed largely to the operation of the modern high pressure steam boilers without the formation of hard scales.

However precautions should be taken to inspect them at least once in a six months and remove the scale and sludge accumulations.

 

v  COLLOIDAL CONDATIONING

Scale formation can be minimized by introducing into the boiler some colloidal conditioning agents such as glue, agar-agar, tannin , starches and sea-weed extract. This substances act as protective collides .They function by surrounding the minute particles of CaCo3 and CaSO4 and prevent their coalescence and coagulation thus the precipitate scale forming salts or maintained in lose suspended form which can easily be removed by blow-down operations . Thus the scale formation is prevented.

v Calogen conditioning :

Another approach for preventing scale formation is to convert the scale forming salts into highly soluble complexes which are not easily precipitated under the boiler conditions. In order to achieve this , sodium hexameta phosphate(NaPO3)6 or Na2 (Na4 P6 O18) (its trade name is calogen )is generally employed .This substance intract with the residue calcium ions forming highly soluble calcium hexameta phosphate. And thus prevents the precipitation of scale forming salts.

Na2(Na4P6O18 )     ---------------------------  2Na+ (Na4P6O18)

2Ca+(Na4P6O18 ) ------------------------------    4Na+(Ca2P6O18)

 

v Conditioning with EDTA:

Phosphate treatment fails to prevent the formation of iron oxide and cuprous deposition .sometimes, the phosphate chemicals  themselves become a sources of deposition formation. In distinction to phosphate treatment ,the conditioning of boiler water with complexing agents can secure scale free and sludge free operation of boiler units and sometimes ,also corrosion free operation under certain conditions. These reagents are known as complex ones or complexing agent such as ethylene diamine tetra acetic acid (EDTA )and its disodium salts (Na2EDTA) known as Trylon-B Or even its tetra sodium salts (Na4EDTA )

 

v  BOILER  COMPOUNDS :

Once ,during the early times of watt ‘s engine it is said that workman after cleaning the boiler and refilling with water hung a bag containing potatoes in the boiler to cook them .they forget about them and put the boiler into operation after closing it. then when the boiler was shut down again for manual removing of the scale it was found that the scale formation was much less, much of it  having come down in the form of loose sludge. The story seem to be true because for many years engineers used to through some potatoes in the boilers after every cleaning operation. Perhaps this is for runner of internal treatment of boiler salines to reduce the adherent scale formation by rendering it in the form of softer sludge which can be easily removed by blow down. slowly potatoes was replaced by starches and later by tannins and other organics and inorganic materials like soda ash and caustic soda, phosphates ,coagulant such as sodium aluminate and deoxidizer like sodium sulphate .Natural wood and plants extracts and several synthetic products have also been used.U.S.Navy was reported using a boiler compound consisting 47%  anhydrous disodium phosphate,44% soda ash and 9% starch. Castor oil compounds and various synthetics antifoam compound were also use in locomotive boilers to reduce foaming tendency. These boiler compounds do help in reducing scale formation when properly use under proper situation.