Boilers

1. 1
SCALE & SLUDGE
FORMATION
PRIMING AND
FOAMING
BOILER CORROSION
CAUSTIC
EMBRITTLEMENT
BOILER PROBLEMS

2. Causes
Highsteamvelocity,+nceoflargequantitiesofdissolvedsalts,Very
highwaterlevelinboiler,Suddenboilingandimproperdesignof
boiler
Prevention:
byusingsoftwater,fittingmechanicalsteampurifiers,controlling
steamvelocity,improvingdesignofboiler,removingscale&sludge
timetotime
2
PRIMING
Processofwetsteamformationiscalledpriming
(Whenaboilerisproducingsteamrapidly,somedropletsofliquidwaterarecarried
alongwiththesteam)

3. FOAMING:
Formation of persistent foam or bubbles, at the surface of water
in boiler which do not dissolve or break easily
normal buble carry over buble
Causes
+nceofdissolvedsubstanceslikeoil,grease,andalkalineimpurities
Oilandalkalisreacttoformsoap,lowerstheS.T.&increases
foaming
Prevention:
byusinganti-foamingchemicalslikecastoroil
Byadditionofcoagulantssuchassod.aluminate
3

4. DISADVANTAGES OF PRIMING & FOAMING
Wetsteamcarriesoverthedissolvedsaltsofboilerwaterto
turbineblades
B.P.ofwaterisincreasedduetofoaming-resultsinwastage
offuel
Reducesefficiencyofboiler
Dueto+nceoffoam,actualwaterlevelcannotbeassessed
inboiler
4

5. BOILER CORROSION
Decay of boiler material due to chemical or electrochemical
reactions with its environment
1.Dissolved oxygen
2.Dissolved CO
2
3.Mineral acids
5
CAUSES

6. 1.Dissolvedoxygen
Watercontains8ppmofdissolvedoxygen.Onheatingwaterin
boilers,dissolvedoxygenissetfree.
In+nceofwater,thisoxygenreactswithironwallsofboilerat
hightemperaturetoformrust
2Fe+2H
2O+O
2→2Fe(OH)
2
4Fe(OH)
2+O
2→2[Fe
2O
3.2H
2O]
rust
Hencetopreventthiscorrosion,itisessentialtoremove
dissolvedoxygeninboilerfeedwater.Itcanbedonebyadding
hydrazine(N
2H
4),Sod.Sulphite(Na
2SO
3)orsodsulphideNa
2S
6

7. 2.DissolvedCO
2
CO
2formscarbonicacidwhichhasacorrosiveeffect
onboilermaterial
CO
2+H
2O→H
2CO
3
•CO
2is+ntinboilerwateroritresultsfrom
decompositionofbicarbonates+ntinwater
Ca(HCO
3)
2---->CaCO
3+H
2O+CO
2↑
7
*CO
2can be removed by adding NH
3
2NH
3+ CO
2+H
20 (NH
4)
2CO
3

8. 8
Boiler Tube: Oxygen Pitting Pipe: Low pH Corrosion

9. 9
3.Mineralacids
HydrolysisofdissolvedsaltslikeMgCl
2presentinboilerwater
resultsinformationofHCl
MgCl
2+2H
2OMg(OH)
2+2HCl
HClreactswithironwalls&formrust
Fe+2HClFeCl
2+H
2
FeCl
2+2H
2OFe(OH)
2+2HCl
4Fe(OH)
2+O
22[Fe
2O
3.2H
2O]
rust
SmallamountofHClmaycausecorrosiontoalargeextentdue
tochainreaction

10. CAUSTIC EMBRITTLEMENT
“The phenomenon in which the material of
aboilerbecomesbrittledue to the accumulation of caustic
substances”
•Aswaterevaporatesintheboiler,conc.ofsod.carbonateincreases
•Sod.Carbonateisalsousedinwatersofteningbylime-sodaprocess,
duetothissomesodiumcarbonatemaybeleftbehindinthewater
•Astheconcentrationofsod.carbonateincreases,it
undergoeshydrolysistoformsod.hydroxide.
Na
2CO
3+H
2O→2NaOH+CO
2
10

11. 11
•PresenceofNaOHmakesthewateralkaline.
•NaOHattacksthesurroundingmaterialanddissolvestheiron
oftheboilerassodiumferrate(Na
2FeO
2).
•Itdecomposesleadingtorustingofboiler
3Na
2FeO
2+4H
2O→6NaOH+Fe
3O
4+H
2
•Thiscausesembrittlementofboilerpartslikerivets,bendsand
joints
Na
2CO
3+H
2O→2NaOH+CO
2

12. Prevention
•by using sodium phosphate instead of sodium carbonate
as softening reagents
•by adding tannin or lignin to boiler water-blocks the hair-
line cracksand prevents infiltration of caustic salts into
these areas
•Adding Na
2SO
4to boiler water also blocks hair-line cracks
12
Tannic acidC
76H
52O
46 from wood
Ligninorlignenis a complexchemical compoundmost commonly derived fromwood, and an integral part of
the secondarycell wallsofplantsand somealgae. C
9H
10O
2, C
10H
12O
3, C
11H
14O
4

13. BOILER
Adevicetogeneratesteamforheatingaswellasto
generatepower
BOILERFEEDWATER
Waterusedinboilerinthisregardistermedasboiler
feedwater
Boilerfeedwater=Returnwater+Makeupwater
13

14. SPECIFICATION FOR BOILER FEED WATER
It should be free from:
Hardness (sludge & scale formation)
Organic matter, oil, grease (foaming)
Dissolved gases like CO
2, H
2S, O
2(to avoid corrosion)
Alkalinity (caustic embrittlement)
Turbidity and sediments
14

15. 15
Following methods
(a)External Treatment Methods-(water treatment before being fed
into boilers)
•Lime Soda Process,
•ZeoliteProcess
•Ion Exchange Process
(b) Internal Treatment Methods-(water treatment inside the boiler)
Can be achieved by adding certain chemicals to water in the
boiler
•Colloidal conditioning
•Carbonate conditioning
•Phosphate conditioning
•Calgonconditioning
Treatment of boiler feed water

16. Internal Treatment Methods of Boiler Feed Water
1.Colloidalconditioning
•Scaleformationcanbereducedbyaddingcolloidal
substanceslikekerosene,agar-agar,starchetctotheboiler
water
•Thesesubstancessurroundtheminuteparticlesofscale
formingsalts&preventtheircoagulation
•Hencesaltsremainlooseintheformofsludgeandcanbe
removedbyblow-downoperation.
•Thisprocessissuitableforlowpressureboilers.
CnH2*n+2.Keroseneisamixtureofhydrocarbonsofthisformula,agar(C
12H
18O
9)
n,
starch(C
6H
10O
5)n
16

17. 17
2.CarbonateConditioning
•FormationofCaSO
4scalescanbeavoidedbyadditionof
Na
2CO
3
•CaSO
4getsconvertedintoCaCO
3
CaSO
4+Na
2CO
3→ CaCO
3+ Na
2SO
4
Loosesludge
(Canberemovedbyblowdownoperation)

18. (3) Phosphate Conditioning
Addition of sod. Phosphate
2 MgCl
2+ 2 Na
3PO
4→ Mg
3(PO
4)
2+ 6 NaCl
Soft sludge
2 CaCl
2+ 2 Na
3PO
4→ Ca
3(PO
4)
2+ 6 NaCl
Soft sludge
18

19. 19
(4) CalgonConditioning-
Addition of Sod. Hexa-meta-phosphate (calgon)
Calgonconverts scale forming salts into soluble complexes
Na
2[Na
4PO
3)
6] 2Na
+
+ [Na
4(PO
3)
6]
2-
Calgon
CaSO
4 + [Na
4PO
3)
6]
2-
[Ca
2(PO
3)
6]
2-
+ 2Na
2SO
4
Soluble complex
(do not cause any hardness)