water is very important engineering material which is mainly used for the production of steam in boilers .it is essential requirement for industries and power houses. The water thus is called boiler feed water. The boiler feed water should be free from dissolved calcium and magnesium salts. If the boiler feed water contains impurities beyond the prescribed limits they lead to the following problems.
1. priming and foaming ( carry over)
2. Boiler corrosion
3. Scale sludge and formation
4. Caustic embitterment
1) Priming and Foaming:
Due to rapid boiling, the steam may carry some water droplets along with it. This is called wet steam .The process of wet steam production is called Priming. It can reduce the heat of the steam and cause corrosion in the pipelines.
Priming is due to:
a) Improper design of boiler
b) High water level
c) High velocity of steam
d) sudden boiling
e) due to presence of large quantity of suspended organic mater , oil matter and
Priming can be controlled by
i) Proper boiler design
ii) Maintaining proper water level
iii) Proper boiling
If oils and greases are present, they produce bubbles on the water surface. This will increase the wet steam production. This is known as “Foaming”.
Foaming is prevented by adding
i) Anti foaming agents (e.g.) synthetic poly amides , castor oil
ii) Coagulants (e.g.) Aluminium hydroxide
Foaming and priming are collectively known as ‘ Carry over”.
Disadvantges of priming and foaming:
Priming and foaming may cause the following boiler troubles.
the actual height of the woter in boiler is not judged.
Wastage of heat with the result that it becomes difficult to keep up steam pressure and efficiency of the boiler is lowered.
Water concentrated with dissoled salts may deposit on the parts of the machinery , which cause corrosion due to the formation of concentration cell in that part resulting the decay of metal on that spot.
Corrosion in boilers can almost always be traced to one or both of two problems. The most common cause is dissolved oxygen entering the system via the feed-water. The oxygen causes very localized corrosion to occur in the form of pitting. The pits are small but deep pinpoint holes which eventually can penetrate tube walls and cause their failure. Another common cause of corrosion in boiler systems is low pH within the boiler. This reduced pH may result from carbon dioxide infiltration or form contamination by other chemicals.
Oxygen corrosion is normally controlled by driving the oxygen from the feed-water in a deaerating heater or by chemically removing it with an oxygen scavenger such as sodium sulfite.
There are many contaminates which can infiltrate a boiler system and cause low pH levels to develop. Manufacturing wastes such as sugar or acids from plating operations which can be returned to the boiler with condensate can be a source of problems because they concentrate in the boiler. Oxygen can infiltrate the boiler system at virtually any point. When dissolved, oxygen is present in boiler feed water attach on feed lines, pumps and economizers can be expected. The severity of the attach depends upon the concentration of the oxygen and the temperature of the water.
“ The decay of boiler material by chemical or electrochemical attack of its environment is called boiler corrosion.”
It may be due to three major reasons:
i) Dissolved Oxygen
ii) Dissolved CO2
iii) Dissolved salts like MgCl2
i)Corrosion Due to dissolved oxygen:
Dissolved oxygen in presence of water, causes corrosion.
4Fe + 6 H2O + 3O2 4 Fe (OH)3 (Rust)
Prevention from oxygen:
a) Chemical method -
Adding Sodium Sulphite:
2 Na2SO3 + O2 2 Na2SO4
This method results in other precipitates which can have some side effects. So this method is less preferred.
N2H4 + O2 N2 + 2 H2O
This method results in inert gas and pure water, it is preferred.
b) Mechanical deaeration method:
This is based on the principle that at high temperature and low pressure and high
exposed area, the solubility of gases in water is decreased. So, the water is fed into the
mechanical deaerator which is provided with vaccum pump, heaters and perforated
plates. The out coming water will be free from dissolved gases
ii)Corrosion due to Carbon – di – oxide:
Salts like Calcium bicarbonate on heating produces CO2 . CO2 dissolves in water to form carbonic acid which corrodes the boiler metal.
Ca (HCO3)2 ∆ CaCO3 + H2O + CO2
Mg(HCO3)2 Mg(OH)2 +2CO2
H2O + CO2 H2CO3
Prevention from CO2 :
By adding calculated amount of ammonium hydroxide
2NH4OH + CO2 (NH4)2CO3 + H2O
2. Mechanical deaeration method ( similar to oxygen method)
iii)Corrosion due to Dissolved salts like MgCl2
Dissolved salts like MgCl2 cause acid formation. This will be prevented by alkali neutralization.
MgCl2 + 2 H2O Mg(OH)2 + 2 HCl (Corrosive acid)
HCl + NaOH NaCl + H2O
3)Scal and sludges :
Boiler are used for the generation of steam in power plant, where water is continuosly heated to produce steam. As more and more H2O is removed from the water in the form of steam ,the boiler water gets concentrated with dissolved salts prograssivly reaches the saturation point. At this point the dissolved salts are precipitated out and slowy settle on the inner walls of the plate. The precipitation takes place in two ways
a) the precipitation in the form of soft loose and slimmy deposits formed comparatively in the colder portion of boiler is called sludge.
b) the precipitation in the form of hard,stick ery firmly on the inner walls of the boiler is called scale
scales and sludges formation in boiler:
When hard water is used for boilers on continuous evaporation salts present in the water gets saturated and finally deposits in the areas where the flow is slow. When these precipitates are loose and slimy in nature, it is called s ludge
whereas when these precipitates are hard and they adhere strongly to the inner surface of the boilers, they are calledscale.
can be easily removed by scrapping with a brush. Sludge is formed by the presence of MgCO3 , MgSO4 , MgCL2, CaCl2 etc. These salts are more soluble in hot water.
Disadvantages of sludge
i) poor heat conduction due to the presence of sludge on the surface;
ii) difficulty in the operation of the boiler;
iii) if sludge is formed along with the scale and is trapped within the scale formed and so it is difficult to remove and
iv) it clogs the pipe lines and other connections to the vessel near the places where water circulations are is slow.
Removal of sludges.
Sludge formation can be prevented by
i) using soft water for boiler operation and
ii) removing the concentrated salty water from time to time so that deposition of sludge is prevented.
are the hard deposits on the inner surface of the boilers which are difficult to remove. This scale formation takes place due to the following reasons:
a) In low pressure boilers scale formation occurs due to the formation of CaCO3
Ca(HCO3)2 CaCO3 + H2O +CO2
gets converted to soluble Ca(OH)2. But here CaSO4 forms the hard scale. Since the solubility of CaSO4 decreases with increase in temperature, and at high temperature the precipitated CaSO4 forms hard scale. Similar hard scales are formed when SiO2 is present in the hard water. It deposits as CaSiO3 or MgSiO3. These calcium or magnesium silicate scales are very difficult to remove . Dissolved magnesium salts also precipitate as Mg(OH)2 forming soft type of scale.
Disadvantages of scale formation
are similar to sludge formation but the severity is more, since its removal is more difficult.
i) Poor heat transfer from boiler to water leading to increase in fuel consumption. The increase in thickness of the scale from 1.25 mm to 12 mm leads to increase in fuel consumption from over 50% to 150%.
ii) Due to the overheating of the boiler, different parts of the boiler become weak and distorted and so the operation of the boiler becomes un safe, particularly the high pressure boilers. The thick scales may sometimes lead to
explosion due to sudden development of high pressure.
iii) Valves and condensers of the boilers are chocked due to scale formation and
boiler efficiency decreases.
Removal of scales can be done by:
i) Wooden scraper or wire brush, suitable for removing loose scales.
ii) Blow down operations for loose scales. The operation actually involves the re-moval of very hard water from a tap at the bottom of the boiler and replenishing the water with softened water called ‘make up’ water.
iii) Giving thermal shocks, which involve alternate heating and cooling to make the scales brittle.
iv)Chemical treatment with 5-10% HCl for carbonates and EDTA treatment for Ca/Mg salts forming complexes.
These are methods for the removal of the scales, when they are formed. There are also some methods for the prevention of scale formation by internal or external treatment.
Internal treatment :
Internal treatment involves addition of chemical to the boiler water either to
i) precipitate the scale forming impurities in the form of sludges , which can be easily removed or
ii) convertthe impurities to soluble compounds, so that scale formation can be avoided. Important internal treatments involve.
a ) Colloidal Conditioning:
Organic substances like kerosene, tannin, agar-agar are addedto form gels and form loose non-sticky deposits with scale-forming precipitates, whichcan be easily removed by blow-down operations in low pressure boilers.
b) phosphate condition:
Different sodium phosphates like NaH2PO4 , Na2 , HPO4 , Na3 and Po4 are added to highpressure boilers to react with the hardness forming impurities to form soft sludge of calcium and magnesium phosphates and finally this can be removed by blow downoperation.
3CaCl2 + Na3PO4 → Ca3(PO4)2 + 6NaC
c ) Carbonate conditioning:
Sodium carbonate is added to the water of low pressureboiler whereby the scale forming CaSO4 gets converted to loose sludge of CaCO3 , which can be easily removed by blow-down operation.CaSO4
CaSO 4 + Na2CO3 = CaCO3 + Na2SO4
d ) Calgon conditioning:
Calgon i.e., sodium hexa meta phosphate when added to boiler water , reacts with scale forming CaSO4 and forms soluble complex compound
CaSO4+ [ Na4P6O8]2 → [Ca2P6O18]2 + 2Na2SO4
e) Sodium aluminate conditioning:
Sodium aluminate is hydrolysed yielding NaOH and gelatinous Al(OH)3. The NaOH formed reacts with magnesium salts to precipi-tate Mg(OH)2. This Mg(OH)2 and Al(OH)3 are flocculent and entraps the colloidal as well as the finely divided impurities like silica in the boiler water and the loose precipitate is finally removed by blow down operation.
NaAlO2 + 2H2O → NaOH + Al(OH)3↓
MgCl2 + 2NaOH → Mg(OH)2↓ + 2NaCl
f) Electrical conditioning:
Rotating mercury bulbs on heating by the boiling water emitelectrical discharges that prevent scale formation by the particles.
Tablets of radioactive salts placed inside a boiler emitradiations , which prevent scale formation
It is the intercrystalline cracking of boiler due to NaOH. NaOH content is increased due to the dissolved salts like Na2CO3 which is added during internal treatment.
It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.
The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.
Na2CO3 + H2O 2 NaOH + CO2
This NaOH occupies the hair line cracks of boiler metal and converts the insoluble Fe into soluble Sodium Ferroate. Thus it makes the cracks bigger in bents, joints and crevices.
The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate (Na2FeO2). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows
Fe + 2 NaOH Na2FeO2 + H2 ↑
Prevention of caustic embrittlement:
1. As softening agent, we can use sodium phosphate instead of sodium carbonate.
2. The hair line cracks can be sealed by waxy materials like Tannin and Lignin.
Disadvantages of caustic embrittlement :
The cracking or weakening of the boiler matel causes failure of the boiler.