WATER CHEMISTRY CONTROL IN HRSG

WATER SOURCES TO HRSGBFW MAKE UP WATERDIFFERENCE

TYPES OF IMPURITES

ImpurityResulting inGot rid bySoluble GasesH2SCorrosion of boiler tubesAeration, deaeration and chemical treatmentO2COSuspended solidsSediment and turbiditySludge and scale carryoverClarification, filtration and chemical treatmentCarryover, foaming and corrosionOrganic matterDissolved colloidal solidsOil and greaseFoaming, depositionCoagulation, filtrationHardness Ca & MgScaling, inhibits HT, boiler tube burn thruSoftening and internal treatmentNa, Alkalinity,Na2CO3,Foaming, corrosion, embrittlementIon exchange, deionizationSulphatesHard scales if Ca presentDeionizationChloridesPriming, foamingDeionizationFe, MnRusting, resistence to HTAeration, filtration, ion exchangeSilicaScalingDeionization, lime-soda process

Best HRSG Water Treatment Control External Treatment

Internal Treatment

External methods of conditioningClarificationFiltrationIon exchangeMembrane separation

ClarificationRemoves all types of solids & large particles sediments, oil, natural org. matter, colouretc.Consists of 4 steps screening, coagulation-flocculation, sedimentation, fine filtration.Screening protects downstream units from large, easily separable objects.Three types:Fine screening (spacing < 10 mm)Medium screening (spacing 10 40 mm)Coarse screening (spacing > 40 mm)Coagulation-flocculation removes suspended solids & colloidal particles.

Clarification (contd.)Important factors velocity gradient, time, pHFlotation to separate particles having density lesser than water.Three types:NaturalAidedInducedInduced flotation facilitated through bubbling of air; 2 types Dissolved air flotation (DAF) (bubbles of 40 70 mm)Mechanical flotation (bubbles of 0.2 2 mm)

FiltrationSeparates undissolved solids from water by means of a filter - porous substance, membrane or permeable fabric.Three types of filtration:Micro filtration (pore size 0.1 10 m)Ultra filtration (pore size 1-100nm)Nano filtration (pore size < 1 nm)Micro filtration removes bacteria; used for biological wastewater treatment, effluent treatment, separation of oil-water emulsions.Ultra filtration - separation of suspended solids, colloids, bacteria, virus.

Filtration (contd.)2 ultra filtration module configurations:Pressurized system or pressure-vessel configurationImmersed systemNano filtration water softening, decolouring, micro-pollutant removal (org. matter, heavy metals, pesticides).Ultra & nano filtration pressure driven processes.Pre-treatment protects filtration membranes; microfiltration pre-filter for ultra filtration and so on.

Ion Exchange Resins - acidic/basic radicals with ions fixed on them; exchanged with ions present in water.Theoretically removes 100 % of salts; does not remove organics, virus or bacteria.2 types of resins gel type (microporous) and macroporous or loosely cross-linked type.3 systems of resin beds:Strong acid cation + Strong base anionStrong acid cation + weak base anion + Strong base anionMixed-bed DeionizationIon exchange plant softens water, removes heavy metals, produces demineralized water.

Reverse Osmosis (RO)By applying pressure greater than osmotic pressure, water flows from the higher concentration solution to lower one.Mostly used for desalination; also for waste water treatment.Applied pressure depends on the type and salinity of water.Working pressure: < 15 bar for tap water (< 1500 ppm)15 25 bar for brackish water (< 8000 ppm)50 75 bar for sea water (35000 45000 ppm)RO plant preceded by pretreatment to avoid membrane fouling by sediments, bacteria, metal oxides & chlorine. RO permeate water more acidic than the feed water due to dissolved CO2. Common post-treatment are pH neutralization and remineralization.

ElectrodionizationCombines membrane separation and ion-exchange to provide high efficiency demineralization process. Electric potential transports & segregates charged aqueous species.Electric current continuously regenerates resin; no need for periodical regeneration.Deionization chamber - ion exchange resin, packed between cationic & anionic exchange membranes.

ElectrodionizationAdvantagescontinuous operationeliminates use of chemicals for regenerationlow power consumptionDisadvantagesNot used for water with hardness > 1requires purification pretreatmentPre-removal of CO2

INTERNAL TREATMENT OF HRSG

Internal Treatment methodsInternal treatment can constitute the unique treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when good quality raw water is available.

PURPOSE OF INTERNAL TREATMENT1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale;2) condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal;3) provide anti-foam protection to allow a reasonable concentration of dissolved and suspended solids in the boiler water without foam carry-over;4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion.

CHEMICALS USED FOR INTERNAL TREATMENTPHOSPHATE (TSP, NALCO 7208) CHELATES POLYMERSAll above chemicals condition the calcium and magnesium.Chelates and polymers forms water soluble complexes.Phosphates precipitates the hardnessSludge conditioners are also used to precipitate hardness

MECHANISM OF PHOSPHATE TREATMENT FOR HARDNESSAt boiler operating temperatures calcium bicarbonate in the feed water breaks down to form calcium carbonate. Since it is relatively insoluble, it precipitates. Internal treatment with phosphates transforms calcium bicarbonate to calcium phosphate and sodium carbonateSodium carbonate in the water partially breaks down to sodium hydroxide and carbon dioxide.. In the presence of hydroxide alkalinity, magnesium bicarbonate precipitates as magnesium hydroxide or reacts with silica to form magnesium silicate.These minerals are precipitated from solution in form of sludge, which must be conditioned to prevent its sticking to the boiler metal. The conditioned sludge is then removed from the boiler by blowdown.

MECHANISM OF CHELATES AND POLYMERS FOR HARDNESSWhen chelate is used for internal treatment, it reacts with calcium and magnesium salts to form soluble complexes. These complexes are in the form of dissolved solids and are removed by blow down. Dispersant polymers used in conjunction with chelate produces reaction products, which are better conditioned. They do not precipitate and are more easily removed by blow down.

INTERNAL TREAMENTS FOR SULFATESThe boiler temperature makes the calcium and magnesium sulfates in the feed water insoluble. With phosphates used as internal treatment, calcium reacts with the phosphate producing hydroxyapatite, which is much easier to condition than calcium sulfate. With chelates or polymer used as internal treatment, calcium and magnesium react with these materials producing soluble complexes that are easily removed by blowdown.

INTERNAL TREATMENT FOR SILICAIf silica is present in the feedwater, it tends to precipitate directly as scale at hot spots on the boiler metal and or combines with calcium forming a hard calcium silicate scale. In the internal treatment for silica, the boiler water alkalinity has to be kept high enough to hold the silica in solution. Magnesium, present in most waters, precipitates some of the silica as sludge. Special organic materials or synthetic polymers are used to condition magnesium silicate from adhering to the boiler metal.

INTERNAL TREATMENT FOR SLUDGE CONDITIONINGStarches effective on high silica feedwater and where oil contamination is a problem Lignins effective on phosphate type sludge Tannins fairly effective on high hardness feedwater Synthetic polymers Highly effective sludge conditioners for all types of sludges Note: For low pressure boilers Only.

INTERNAL TREATMENT CHEMCIAL DOSAGETSP or NALCO 7208LP DRUMS: 5-15 PPMHP DRUMS: 1- 5 PPMNa: Po4

TESTS FOR TREATMENT CONTROLpHConductivityPhosphateSilicaIronNa:Po4ChloridesSulfates

ROLE OF BLOW DOWNBlowdown is the discharge of boiler water containing concentrated suspended and dissolved feed water solids. As the blowdown water is replaced with lower solids feed water, the boiler water is diluted. With proper regulation of blowdown, the amount of solids in the boiler water can be controlled. The amount of blowdown needed depends on how much feed water impurities a given boiler can tolerate.

BLOW DOWN EXAMPLEIf a particular boiler can tolerate 500 ppm maximum dissolved solids, and the feed water contains 50 ppm, it can be concentrated only about 10 times. This means that for every 100 pounds of water fed to the boiler about 10 pounds of boiler water must be blown down to keep the dissolved solids from exceeding 500 ppm. Total dissolved solids is not the only limiting factor in determining blowdown, other considerations include suspended solids, alkalinity, silica and iron.

CORROSION CONTROL IN CONDENSATE SYSTEMCorrosion in steam condensate system is caused by carbon dioxide and oxygen carried into the system by steam. Dissolved carbon dioxide in condensed steam forms corrosive carbonic acid. If oxygen is present with carbon dioxide, the corrosion rate is much higher, and is likely to produce localized pitting. Ammonia, in combination with oxygen, attacks copper alloys.

PREVENTION OF CONDENSATE CORROSIONCondensate treatment is done by use of volatile amines, ammonia to neutralize carbon dioxide or volatile filming inhibitors to form a barrier between the metal and the corrosive condensateNalco 72310, Nalco 352, Ammonia 25%

REMOVAL OF NON CONDENSIBLE GASES BY DEAERATION

WHAT IS IT

REMOVING OXYGEN

REMOVING OXYGEN

REMOVING OXYGEN

REMOVING OXYGEN

CORROSION PROCESS

PITTING

GENERAL SKECTH

DA PROBLEMS

MONITORING

OXYGEN SCAVANGING BY CHEMICALS

HYDRAZINEELIMINOX

Corrosion Control contd.Control: downtime and storageOxygen in-leakage and pH loweringDry storage: long downtime, month or moreCompletely driedApplied dessicants like quicklime, silica gel, activated aluminaWet storage: short downtimeCleaning, inspection and filling with deaerated feedwaterAddition of scavenger, heat addition

DepositsScaling/deposition from carryoverCarbonate/Phosphate controlAddition of certain amounts of carbonate/phosphate for ensuring precipitate in the form of salts. Prevention of SulphatesFor removal of hardness, Ca and MgPrecipitation in bulk instead of at walls, non-adherentOrganic supplements: fluid sludge formation (polymer addition)Bottom blowdown removes sludgeChelant controlCombination of additivesBlowdown

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