Boiler water chemistry

1. Boiler Water Chemistry
It ismandatorythat we needto feedagoodqualitywaterintoboiler.If we feedimpuritiesorpoorquality
waterintoboiler,the followingproblemsmayarise
1. Formationof scales& depositsonthe boilertubes
2. Carry overof wateralongwiththe Steam
3. Internal corrosiononthe Boilertubes
1. Formation of scales& depositson the boilertubes:
 Initiallythe boilerfedwithpurifiedgoodqualityof water,thoughthe waterispurified,some minor
impuritiesdogetthroughthe boiler(as100% purificationimpossible).
 Andthe concentrationof impuritiesgetsincreasesas the more andmore waterevaporatesand
becomessteam.
 The major impuritiesare:
i. Magnesium(Mg) and Coliseum(Ca) - causesHardnesstowaterand deposition
ii. Silica– causesscalesformationonthe LP Turbine blades
iii. Iron & Copper– Causescorrosion
Higher concentrationof impurities:
 Afterincrementof certainlevel concentrationof impurities,athighertemperaturestheydoget
participatedanddepositedon inside of the boilertubesandbecome hardscales.
 These depositsactsas partial heatinsulatorand preventthe adequate cooling boilertubes (from
inside) onthe locations of deposits.
Andthe resultisthe temperature of tube wall raisessignificantlyandcausestube failureincertain
locations.Hence highconcentrationof impuritiesinfeedwatercausesfrequenttube failures.

2. Methodsof reductionof impurities:
1. Continuousblowdownformthe Boilerdrum(forsubcritical sets)
2. Chemical Treatment
3. Condensate polishingunit
Silica:
At highertemperaturesandpressures,the silicavaporizesintogas,passesthroughthe Superheaterand
thenTurbine.
At lowertemperaturessilicabecomesbacktosolidformanddepositonthe Turbine bladesonthe LP
blades.
These depositsonthe LPblades,affectsthe performanceof Turbine andfailure of LPblades.
Thiscan be removedbyblasting.
The Silicalimits:
For Subcritical Boilers:
For Supercritical Boilers:<=2ppm

3. Methodsof reductionof Silica:
Continuousblowdownformthe Boilerdrum(forsubcritical sets)
2. Carry overof water along with the Steam
The higherconcentrationsof solidsinboiler wateriscausesthe carryoverof wateralongwithsteam.
The effectof highconcentrateddissolvedandsuspendedsolids iscausestoincrease foaminginboiler
drum andwhichin turncausescarryover of water.
The carryover mayalso happenwithfollowing:
Higherwaterlevelsindrum
Highalkalinity
Major issueswithcarryoverof water
1. Possible depositioninthe SHtubes
2. The water dropletsmaydamage the turbine bladesconsiderable,whentheyhitthe blades.

4. 3. Internal corrosionon the Boilertubes
The internal corrosioncausesthe removingof metal fromtube wall,if thisactioncontinuousthe thickness
of tubesmayreduce andthisthicknessreducedtube wall cannotwithstandthe operatingpressure and
fail incertainlocationdue torupture.Andthisremovedmetal causesincrementof total solidsinboiler
waterand turnsto deposition.
Removal of metal fromtube wall Failure of boilertube
The major reasonsof corrosions
1. Acidattack
2. Causticattack
3. Dissolvedgasses(O2andCO2)