KGiSL Institute of Technology(Autonomous)
(Approved by AICTE, New Delhi; Affiliated to Anna University, Chennai)
Recognized by UGC, Accredited by NBA (IT,CSE&ECE)
365, KGiSL Campus, Thudiyalur Road, Saravanampatti, Coimbatore – 641035.
DEPARTMENT OF SCIENCE AND HUMANITIES
Name of the Faculty : Department of Chemistry
Subject Name & Code : 24UCH181/ ENGINEERING CHEMISTRY,
24UCH182/ APPLIED CHEMISTRY &
24UCH183/MATERIAL CHEMISTRY
Branch & Department : I Year UG- B.E.& B.Tech (Common to All)
Year & Semester :I/I
Academic Year :2024-25
29-09-2024 Department of Chemistry 1
SYLLABUS
TOPIC- 2 BOILER TROUBLES (SCALES & SLUDGES, PRIMING & FOAMING,
CAUSTIC EMBRITTLEMENT, BOILER CORROSION)
OUTCOME
At the end of the topic, students will be able to understand and identified the various troubles using
hard water in industry
29-09-2024 Department of Chemistry 2/25
BOILER FEED WATER
In industry one of chief uses of water is generation of steam by boilers.
Boiler feed water – the water fed into the boiler for the production of steam.
Specifications of boiler feed water :
• zero hardness.
• Free from dissolved gases like O2, CO2, etc.
• Free from suspended impurities.
• Free from dissolved salts and alkalinity.
• Free from turbidity and oil.
• Free from hardness causing and scale forming constituents like Ca and Mg
salts.
29-09-2024 Department of Chemistry 3/25
REQUIREMENTS OF BOILER FEED WATER
Type Amount
Hardness < 0.2 ppm
Soda alkalinity 0.15 - 1.0 ppm
Caustic alkalinity 0.15 - 0.45 ppm
Excess soda ash 0.3 - 0.55 ppm
Dissolved gases like 0 ppm
CO2,O2
29-09-2024 Department of Chemistry 4/25
BOILER TROUBLES (OR) DISADVANTAGES OF
USING HARD WATER IN BOILERS
• Formation of deposits (scales and sludges).
• Priming and foaming (carry over).
• Caustic embrittlement.
• Boiler corrosion.
29-09-2024 Department of Chemistry 5/25
Formation of scales & sludge's in boilers
• When water is continuously converted into steam in boilers or heat exchangers, the
concentration of dissolved salts in water increases.
• When the salt concentration reaches their saturation point, they are thrown out in the form of
precipitates in the inner walls of the boilers.
• The least soluble one gets precipitated first.
29-09-2024 Department of Chemistry 6/25
SLUDGE AND SCALE
Sludge
• The precipitate is loose and slimy.
• Formed by the substances like MgCl2, MgCO3, MgSO4 and
CaCl2.
• Have greater solubility in hot water than cold water.
Scale
• Precipitate forms a hard and adherent coating on the inner
walls.
• Formed by the substances like Ca(HCO3)2, CaSO4 and Mg(OH)2.
29-09-2024 Department of Chemistry 7/25
COMPARISON OF SCALES AND SLUDGES
29-09-2024 Department of Chemistry 8/25
DISADVANTAGES OF SCALES & SLUDGES
Wastage of fuels
Decrease in efficiency
Boiler explosion.
1. Wastage of fuels
The heat transfer from boiler to inside water in not efficient because of the low thermal
conductivity of scales.
Overheating is done to provide steady supply of heat to water which causes wastage of fuel.
The wastage of fuel depends on the thickness and nature of the scale, which is shown in the
table.
29-09-2024 Department of Chemistry 9/25
DISADVANTAGES OF SCALES & SLUDGES
2. Decrease in efficiency
• Due to the deposition of scales in the valves and condensers
of the boiler and choke.
3. Boiler explosion
• Sometimes due to over heating the thick scales may crack and
causes sudden contact of high heated boiler material with water.
• This causes formation of a large amount of steam and high
pressure is developed which may lead to explosion.
29-09-2024 Department of Chemistry 10/25
PREVENTION OF SCALES FORMATION
• Scales can be removed using scraper, wire brush etc. at the initial stage.
• If scales are brittle, they can be removed by thermal shocks.
• By using suitable chemicals like dilute Acids and EDTA (for CaSO4 scale) with which they
form suitable complexes.
• If the scales are loosely adhering, they can be removed by frequent blow down operation
29-09-2024 Department of Chemistry 11/25
PRIMING AND FOAMING (CARRY OVER)
The phenomenon in which the water liquid droplets carry with them some of the
dissolved salts and suspended impurities called carry over.
• Priming – The small droplets of liquid water associated with steam (wet steam).
• Foaming – The formation of stable bubbles above the water surface. It is carried over by
steam leads to excessive priming.
Causes of priming Causes of foaming
Improper boiler design Presence of oil, grease, finely
divided
Very high water level in the boiler particles
Sudden increase in boiling rate
High steam velocity
29-09-2024 Department of Chemistry 12/36
PREVENTION OF PRIMING AND FOAMING
• PRIMING • FOAMING
the addition of coagulants such as
Controlling the velocity of steam.
ferrous sulphate and sodium
aluminate.
Keeping the water level lower.
Using good boiler design. adding antifoaming agents such as
castor oil or polyamides
Using soft water.
29-09-2024 Department of Chemistry 13/25
CAUSTIC EMBRITTLEMENT (INTERCRYSTALLINE CRACKING)
• A form of corrosion caused by high concentration of NaOH in the
boiler feed water.
Na2CO3 + H2O 2NaOH + CO2
Fe + 2NaOH Na2FeO2 + H2
Sodium
ferroate
• It is a type of electrochemical corrosion occurs when the
concentration of NaOH is above 100 ppm.
• It occurs at the stressed parts like bends, joints, rivets, etc.
29-09-2024 Department of Chemistry 14/25
PREVENTION OF CAUSTIC EMBRITTLEMENT
Caustic embrittlement can be prevented by,
using sodium phosphate as the softening agent instead
of Na2CO3.
adding chemicals such as tannin, lignin to the boiler
water blocks the hairline cracks.
adjusting the pH of the feed water carefully between 8
and 9.
29-09-2024 Department of Chemistry 15/25
BOILER CORROSION
Corrosion in boilers is due to the presence of
• Dissolved oxygen
• Dissolved carbon dioxide
• Dissolved salts like MgCl2
1. Dissolved oxygen
• Water containing dissolved oxygen when heated in a boiler, free oxygen is
evolved, which corrodes the boiler material.
4Fe + 6H2O + 3O2 4Fe(OH)3
Rust
29-09-2024 Department of Chemistry 16/25
BOILER CORROSION
2. Dissolved carbon dioxide
• When water containing bicarbonates is heated, CO2 is evolved which makes the water acidic (by
forming carbonic acid). This leads to intense local corrosion called pitting corrosion.
Ca(HCO3)2 CaCO3 + H2O + CO2
CO2 + H2O H2CO3
carbonic
acid
3. Dissolved MgCl2
• Hydrochloric acid, produced from MgCl2, attacks the boiler in a chain-like reaction producing
HCl again and again which corrodes boiler severely.
MgCl2 + 2H2O 2HCl + Mg(OH)2
Fe + 2HCl FeCl2 + H2
FeCl2 + 2H2O Fe(OH)2 + 2HCl
Corrosion by HCl can be avoided by the addition of alkali to the boiler water
29-09-2024 17/25
Department of Chemistry
PREVENTION OF BOILER CORROSION
Prevention of boiler corrosion by
• Removal of Dissolved Oxygen Chemical and mechanical
method
• Removal of CO2 Chemical and mechanical
method
• Removal of acids Neutralization method
29-09-2024 Department of Chemistry 18/25
PREVENTION OF BOILER CORROSION
1.Removal of Dissolved Oxygen (DO)
• Chemical method
• Mechanical method
Chemical method :Removal of Dissolved Oxygen
Sodium sulphite, Hydrazine are the chemicals used to remove DO.
Na2SO3 + O2 2Na2SO4
Sodium Sodium
sulphite sulphate
N2H4 + O2 N2 + 2H2O
Hydrazine
• Hydrazine is an ideal compound to remove DO in water, since the products are
water and inert nitrogen gas.
29-09-2024 Department of Chemistry 19/25
• Mechanical method – Removal of DO by de-aeration method.
29-09-2024 Department of Chemistry 20/25
2. Removal of dissolved CO2 – NH4OH (Chemical method )
2NH4OH + CO2 (NH4)2CO3 + H2O
Ammonium Ammonium
hydroxide carbonate
• CO2 along with water can also be removed by de-aeration method
(mechanical method)
3. Removal of acids – Neutralization method
Corrosion of acids can be avoided by addition of alkali to the boiled water.
Department of Chemistry 21/25
29-09-2024
ASSESSMENT
1. If the precipitate is loose and slimy is called
a. Sludge b. scale c. prime d. Foam
2.If the precipitate is hard and adherent is called
a. Sludge b. scale c. prime d. Foam
3. The process of production of wet steam is
a. Sludge b. scale c. priming d. Foaming
4. The process of formation of stable bubbles above the surface is
a. Sludge b. scale c. priming d. Foaming
5. The water fed into the boiler for the production of steam is called is
a. Boiler feed water b. boiler corrosion
29-09-2024
c. caustic embrittlement d. Scale
Department of Chemistry 22/25
ASSESSMENT
1. ____________is caused by using highly alkaline water.
2. Foaming is caused by the formation of ___________.
3. The pH value of feed water should be maintained greater than ___________to reduced corrosion.
4. To avoid scale formation _________is added to boiler drum when alkalinity of boiler water is low.
5. In boilers, the feed water treatment is done mainly for removing_________.
Answers
1. Embrittlement.
2. Oils & alkalis
3. > 9
4. Trisodium phosphaste
5. Corrosion, scale & sludge formation, priming
29-09-2024
& foaming and Embrittlement.
Department of Chemistry 23/25
SUMMARY
29-09-2024 Department of Chemistry 24/25
REFERENCES
1. S.S Dara, “A Text book of Engineering Chemistry ”S.Chand & Company LTD, New
Delhi.2021
2. Wiley, “Engineering Chemistry”, John Wiley & Sons. InC, USA.
3. P.C.Jain & Monicka Jain, “Engineering Chemistry” , Dhanapat Rai Publising Company Pvt.
Ltd. 2017.
THANK
YOU
29-09-2024 Department of Chemistry 25/25
