IX. WATER TREATMENT, CHEMICALCLEANING AND STANDBY PROTECTION OF

HIGH PRESSURE BOILERS

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Contents1. INTRODUCTION

2. FEED WATER TREATMENT

3. BOILER WATER TREATMENTS

4. SILICA CONTROL

5. CRITERIA FOR OPERATION DURINGCONDENSER LEAK

6. MEASUREMENT AND CONTROL

FREQUENCY OF MEASUREMENTS STEAM-WATER CYCLE

7. CARRY OVER IN STEAM

8. CHEMICAL CLEANING

9. STANDBY PROTECTION

10. CONCLUSION

FIG. IX-1 RECOMMENDED CO-ORDINATED PHOSPHATE CURVE

FIG. IX-2 RECOMMENDED MAXIMUM SILICA CONCENTRATIONSIN BOILER WATER

FIG. IX-3 STREAM DRUM INTERNALS (TYPICAL ARRANGEMENT )UTILITY BOILERS

FIG. IX-4 TYPICAL ARRANGEMENT OF DRUM INTERNALS FORINDUSTRIAL BOILERS

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IX. WATER TREATMENT, CHEMICALCLEANING AND STANDBY PROTECTION OF

HIGH PRESSURE BOILERS

1. INTRODUCTION

It is important to maintain proper quality of feed water, boiler water and steam for trouble-freeoperation of boilers and turbines. Proven treatment methods should be employed to ensurethe required quality of water and steam. The quality requirements become more stringent forhigh-pressure boilers, as they are generally designed to closer tolerances. The make-up waterfor High Pressure (HP) units should be demineralised to meet the stringent quality requirements.Improper quality of feed or boiler water results in deposits and corrosion in the boiler watertubes. Without proper boiler water quality, it is not also possible to achieve desirable steampurity for trouble-free operation of superheaters, reheaters and turbines. In spite of good watermanagement, the internal surfaces of tubes in a boiler become dirty over a period of operation.It is essential to periodically clean the boiler. The frequency of cleaning should be establishedbased on the operating history and cleanliness of the boiler. Proper selection of solvent andprocedure for cleaning is important. The cleaning should be done in such a way that theintegrity of tubes is not impaired. Equally important is proper lay-up of boiler when it isunder shut-down. Any wet surface exposed to atmosphere is corroded by oxygen. The surfacesshould be kept filled with treated water, with nitrogen blanketing. Adequate attention byoperating personnel for proper water management and cleanliness of a boiler will increaseavailability of the unit.

2. FEED WATER TREATMENT

The feed water is made up of condensate and make-up water. The quality of condensatedepends upon that of quality of steam from tubines, contamination in the process and the typeof chemicals used for treatment purposes whereas the make-up water quality depends uponthe treatment plant employed for the production of make-up. The make-up water for HP unitsshould be either dernineralised or evaporated. Demineralisers can produce water quality withnearly zero hardness and with very lowsilica of less than 0.01 ppm. All ionised salts areremoved in this process which greatly minimises the potential for boiler deposits, corrosionand turbine fouling.The following are the recommended feed water limits for HP drum type units:

Total solids - 50 ppb (max.)Total iron - 10 ppb (max.)Total copper - 10 ppb (max.)Total silica - 20 ppb (max.)Oxygen - 5 ppb (max.)pH - 8.8 - 9.2 (Copper alloy system)

9.2 - 9.4 (Copper free system)Cation conductivity - 0.3 micro mho/cm (max.)

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All measurements must be made at H.P. heater outlet or at economiser inlet. Oxygen can bemeasured at deaerator outlet if cycle is equipped with deaerating heater. For H.P. boilers,hydrazine is used for oxygen scavenging. 10 to 20 ppb of residual hydrazine should bemaintained at economiser inlet for proper oxygen control. Maximum oxygen removal shouldbe effected by good deaerators. Hydrazine is used only to reduce residual oxygen to withinlimits. Good deaerators are capable of producing oxygen to less than 10 ppb. It is not desirableto add sodium sulphite for residual oxygen removal for H.P. boilers, as

it increases the total solids in feed water and it can decompose at higher temperatures and theproducts of its decomposition are corrosive to pre-boiler system. In the case of L.P. boilerswhere sodium sulphite is used, it should be added after the tapping point for desuperheaterspray so that the spray water is not contaminated to result in deposits in superheaters andturbines. The limits on iron and copper are important as any corrosion in the pre-boiler systemresults in iron and copper oxides which are transported and deposited inside the boiler tubes.These form porous deposits which provide for concentration of any small amounts of corrodentto cause corrosion.

The pH is maintained as indicated above for feed water is found to produce least corrosion inpre-boiler system. The required pH is obtained by the dozing of Ammonia or any neutralisingamine such as cyclohexylamine. Ammonia is more popular for the purpose in the case of H.P.boilers. Ammonia and hydrazine should preferably be injected at the condensate pump outlet.

3. BOILER WATER TREATMENTS

There are two popular methods for boiler water treatment. They are co-ordinated phosphate-pHmethod and volatile method. The former is the most suitable method for H.P. drum typeboilers. This method envisages the use of sodium phosphates to maintain required pH (asshown in Figure IX-11). It eliminates the presence of free caustic in boiler water, if phosphate- pH relationship is maintained below the curve. Any free caustic present in boiler waterconcentrates under porous deposits formed inside water wall tubes cause gouging type ofattack which is termed as caustic gouging . When this corrosion occurs, the material thicknessis reduced without loss of ductility and the tube fails. The recommended boiler water limits inthis method are as follows for drum pressure above 100 kg/sq.cm.g.

Phosphate - 5-10 ppm

pH - 9.4-9.7

There are three different kinds of phosphates. By judicial combination of these phosphates,the phosphate residual and pH should be maintained as above. Trisodium phosphates tend toincrease the pH. Disodium phosphate is neutral salt and monosodium phosphate is acidic,tending to lower the pH.

The other treatment method, namely volatile treatment is desirable only for oncethrough units.In this method a boiler water pH of 8.5 to 9.0 is maintained by the addition of ammonia or anyvolatile amine in feed water. This does not have any buffer to combat the situation arising outof any sudden contamination. Any condenser leak producing acidic environment in boilerwater lowers the pH and acidic corrosion will occur. Caustic gouging can occur due to causticconcentration. Under volatile treatment, it is essential to have sensitive monitoring instrumentto detect any sudden change in pH and corrective action should be immediately initiated.

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It is necessary to switch over to co-ordinated phosphate treatment under such conditions. Thevolatile treatment requires a condensate polisher to be installed to take care of suddencontamination of condensate.

Sometimes, apprehensions are raised against co-ordinated phosphate treatment from the pointof view of phosphate hideout. There is no danger of corrosion in phosphate hide-out. Ithas been established that phosphate concentrated even under hide-out condition is notaggressive to the tube material. It is nothing more than a nuisance to the operators for thecontrol of boiler water chemistry. Any phosphate salts, hidden out, are available to take careof any hardness in-leakage. The intent of phosphate treatment is to provide conditionsconducive to the precipitation of calcium as calcium hydroxyapalite 3Ca3(PO4)2 Ca (OH)2and magnesium as serpenline, 3Mgo 2SiO2.2H2O.It has been found that phosphate hide-outis the least in a comparatively clean boiler as there are very little deposits available for phosphateto hide-out in a clean boiler. It is never advisable to resort to any boiler water treatment withfree caustic, even in small amounts.

4. SILICA CONTROL

Demineralisers are capable of reducing silica to very low levels. Silica of less than 0-02 ppmis recommended for feed water during normal operation. The silica level in boiler water willbe much higher depending upon the blow down percentage. Silica limits are fixed for boilerwater as shown in Figure IX-2 based on pressures as silica volatility is related to pressure.These silica limits are fixed to limit the silica carry over into steam within 0.02 ppm Highersilica carry over will result in silica deposits in turbine blades which are difficult to be removed.Silica in boiler water can form hard scales, reacting with any calcium or aluminiumcontamination salts. These hard scales will result in overheating of tubes and are difficult toremove.

During initial commissioning of the unit, it is likely to get higher silica levels in boiler water.Proper silica limits should be maintained by adequate blow down. The alkaliboil out of boilerand pre-boiler system precommissioning cleaning should remove considerably the silicaaccumulated on the inside surfaces during construction of the unit. If silica accumulation isexcessive any amount of cleaning cannot remove completely the silica deposits. The remainingsilica can only be slowly removed during initial days of operation. It is advisable to cut in H.P.heaters at as low a load as possible to get rid of silica at the earliest. At lower pressure, moresilica is tolerated. Large surfaces such as feed heaters are major sources to contribute to highsilica levels during initial operation of the unit. Pressure should not be raised until silica iswithin limits.

If the unit has been effectively cleaned by boil-out operation, silica level will be brought intocontrol within a few days of initial operation. During normal operation, the sources of silicacontamination are only either condenser leakage or demineralisers. Demineralisers are oftenexhausted to have silica break earlier than break of other anions. Demineralisers should beregenerated before silica break occurs.

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5. CRITERIA FOR OPERATION DURING CONDENSERLEAK

In most of the cases, condenser leaks produce acidic environment and the pH is reduced.Conductivity measurements should

be used to detect condenser leaks. Condenser leaks should be detected immediately. Leakageinstrumentation should be installed for each compartment in the case of condensers withsegmented compartments. When the hot well total solids exceed 0.5 ppm the condenser leakshould be considered abnormal and it is excessive it it exceeds 2 ppm and the load should beimmediately reduced as necessary to permit isolation of damaged condenser section. If thehot well concentration cannot be quickly reduced below 2 ppm orderly shut-down should bearranged. As stated earlier, when condenser leak is detected, if the boiler water is on volatiletreatment it should be changed to co-ordinated phosphate treatment to maintain excessphosphate of 5-20 ppm and pH of 9.5 - 10.5. The blow down should be increased to limit totaldissolved solids concentration of boiler water within 100 ppm. Unnecessary use ofdesuperheating spray water should be avoided either by permitting reheat temperature to fallor by reducing load. The operation of the unit should not be continued if pH cannot bemaintained above 8.0 or TDS in boiler water below 100 ppm During condenser repair, theboiler unit and auxiliary equipment should be properly laid-up as described later. These arethe limits prescribed for drum pressure above 100 kg/sq.cm. Similar limits are given forlower pressure also.

6. MEASUREMENT AND CONTROL

It is well known to everyone that proper measurement and control is very essential but it isoften neglected. In the modern world, where instrumentation has made such an advance,obsolete methods and instruments are still used. To-day, very reliable instruments are availablewhich can accurately measure very low levels of contamination. Similarly reliable on-lineanalysers are available which can be installed for continuously monitoring critical parameters.It is not enough if the instruments are installed. It is equally important to maintain themproperly in good condition. The instruments should be checked for its accuracy periodicallyand recalibrated whenever necessary

It is desirable to have on-line instruments for measuring and recording the critical parameterslike:

i) conductivity in hot well and at D.M.plant outlet

ii) conductivity, pH, and silica in feed water and boiler water

iii) sodium and silica in saturated and superheated steam.

Whenever the on-line instruments are not working these parameters should be monitoredfrequently in laboratory. Even where they are working, the values should be cross checked

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with laboratory measurements. For coastal power stations, where condensers are cooled bysea water, condenser leakage is a potential danger. To guard against this, an on-line conductivityrecorder at hot well outlet provided with a cation exchange column in H+ form should beinstalled. Any spurt in conductivity values will immediately indicate the condenser leak. Theon-line instrument coupled with an alarm will alert the operator. It is important to rememberthat the cation exchange column should be properly regenerated periodically. On-Line cationconductivity measurement of steam is also a reliable monitoring of steam purity.

The following minimum frequencies are recommended for the various parameters to monitorand control the water and steam regime. There should be no hesitation on the part of thepower station authorities to employ the required personnel. The personnel should be wellqualified and motivated. They should also be interested to have a well equipped laboratorywith necessary instruments. Any investment on this count will soon be repaid by betteravailability of the unit.

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FREQUENCY OF MEASUREMENTS STEAM-WATER CYCLE

Continuous Once in Once in Once in Twice Once a

on-line 2 Hrs 4 Hrs 8 Hrs weekly month

Feed water Conductivity before - PH silica Hardness Iron Copper Permanganate

and after resin Ammonia OilOxygen

Hydrazine

Boiler water Conductivity - PH, Sodiumphosphate

Silica

Saturated steam Conductivity after resin- - Sodium Silica - -

Superheated steam Conductivity after Sodiumresin - -

Make-up Conductivity Silica pH Hardness Permanganate(Once 30

minutes-at end)

NOTE : Should on-line conductivity instruments be out of service manual conductivity readings should be taken every 2 hours and More frequently during upset conditions (start-up, trip, load swings, condenser leak etc.)

The laboratory should be equipped with the following instruments to perform efficiently thewater analysis :

i) A precision pH meter.

ii) A conductivity meter with selector switch to cover the range 0-10, 0-100, 0-1000ms/cm.

iii) A spectro photometer capable of measuring ppb level.

iv) A flame photometer capable of measuring low concentrations of sodium (upto 0.001ppm).

v) A Nesseler disc for Incligo-caramine test for measuring dissolved oxygen.

The spectro photometer is a versatile tool, since almost all constituents important for boilerwater chemistry such as iron, copper phosphate, ammonia, chloride, silica and hydrazine canbe accurately measured. In addition, the laboratory should have routine equipments likesemi-micro balance, water baths, air oven, dessicator etc. and sufficient glass wares like

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pipettes, burettes, flasks, beakers etc. It is necessary to use standard solutions and chemicalsof high purity for measurement and analysis.

Without proper sampling, no measurement is meaningful. It is the best if samples could becollected at the rate of about 50 litres/hr and at a temperature of about 400C. For high pressureboilers where the samples are having low concentration of constituents, any slightcontamination will seriously affect the results. The samples should be protected from theatmospheric gases like CO2 and 02, the silica from atmosphere, sodium contamination fromperspiration etc. In coal fired units, coal dust and atmospheric emission can contaminate thesamples. All the more important is that collecting vessel should be clean and free fromcontamination. It is desirable to arrange for the sample collection in a covered enclosure. It isa sound practice, if the samples can be led into the laboratory itself which will not only avoidthe contamination but also will reduce the physical

strain in sample collection. At many places, the sample collection points and the laboratoryare located wide apart. In such cases it is desirable to have an express laboratory close to theboiler duplicated with a pH meter, conductivity meter, a Nesseler disc and other routinelaboratory wares. There can be a central laboratory where more sophisticated instruments canbe housed and the laboratory should be airconditioned. it is very important to keep the samplinglines and cooling water lines free from blockage. it is desirable to keep the sampling linescontinuously flowing so that the lines are always kept clean.

Continuous monitoring, systematic analysis and proper control go a long way in maintainingtrouble-free operation of the power stations. Any problem should be immediately located andremedial action should be taken. It is beneficial it blow down and dozing systems are controlledby the plant chemists. It is needless to emphasise on proper logging of data. Only based on thecorrect history of operation, preventive action can be taken in time to avoid costly downtimeof the units. A well maintained record also helps to schedule the operational cleaning in righttime. Prevention is better than cure , and, proper measurement and control help to achievethis noble end.

7. CARRY OVER IN STEAM

Carry over of salts in steam occur either due to mechanical or vapour carry over. Efficientdrum internals can only reduce mechanical carry over. in order to limit silica carry over,concentration of silica in the drum water must be limited to a specified value for a givenoperating pressure. Fig. IX-2 shows silica limit in boiler water for various pressures in orderto limit it to 0.02 ppm at drum outlet to reduce problem of turbine deposits. Silica is alwayscarried over in vapourous form. The vapourous carry over of remaining salts, mainly sodiumsalts, is significant only at very high pressures, above 200 kg/sq.cm. Continuous monitoringof cation conductivity is essential and it is also desirable to monitor sodium and silica.

7.1 Drum Internals

The typical drum internals of high pressure boilers are shown in figures IX-3 and 4. FigureIX-3 is the one normally used for high pressure power boilers and the other for industrial andpower boilers of lower pressures, having evaporating bank tubes.

High pressure drum internals employ turbo separators as main separating device. The spinnerblades of turbos impart the centrifugal motion to the up-coming mixture of water and steam.

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The water particles being heavier are thrown apart and the steam gets separated, rises andflows into further stages of drum internals namely secondary separators and final drier screen.These perform the final purification of steam.The other type of drum internals employs deflection baffle. The change of direction of flowthrough the baffle separates bulk of water from steam. Further there are two stages of screensto perform final purification of steam.

7.2 Water Level:

The maintenance of proper water level is a prime factor for the proper functioning of internals.Fluctuations in levels, sudden surges, or prolonged maintenance of high levels may lead toexcessive carry over and such frequent occurrences will lead to turbine deposits, superheatertube failures etc.

There are chances of water level indicators showing false levels due to sub-cooling as thegauge glass is exposed to the cooler atmosphere outside the boiler. Because of the greaterdensity of this cooler water, it will support a proportionately greater height of water in thedrum. In high pressure boilers where the saturation temperature of the water is high, thedegree of sub-cooling may be appreciable and thus introduce a significant error in the waterlevel indication. Compensated level controllers and remote indicators are not directly affectedby this phenomenon; however, they are sometimes calibrated against the gage glass zeroreading and thus reflect the same error. To avoid the effect of subcooling it is also recommendedto ensure that the pipe connecting the drum and water level gauge on the water side should beperfectly insulated.

In all boilers, there is a natural gradient in water level which is required to move the circulatingboiler water through the drum. For this reason, the water close to a downcomer would beexpected to be lower than at a location midway between downcomers. Normal difference inlevel are not a problem, however, wide variations may result in localised carry over. Anexcessive water level gradient may be an indication of restricted flow through the drum or anunusual firing condition in the boiler.Directly over a dlowncomer, a vortex may exist which causes turbulence in the region. Waterlevel measurements in the affected zone are erratic and meaningless. Level measurementsshould be restricted to locations sufficiently far from a dlowncomer to avoid this phenomenon.

7.3 Boiler Water Concentration

Excessive levels of boiler water concentration will increase the carry over. According to thequality of feed water, blow down rates are to be monitored either continuously or intermittentlyso that the recommended limits of concentrations are not exceeded. The level of concentrationto be maintained decreases with the increase of operating pressure as the separating efficiencyof drum internals decreases with increase of pressure.

8. CHEMICAL CLEANING

Cleanliness of the boiler and pre-boiler system is an important aspect for the efficient andtrouble-free operation of the boilers. Mill scale is formed during various stages of fabrication.Rusting takes place to some degree during erection. It can be excessive if proper care is nottaken during storage and construction. Mill scale on boiler tubing is normally very thin withthe exception of the areas near welds and bends. Even though mill scale is initially uniform,

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its brittleness upon cooling can produce flaking. The resulting non-uniform surface withdiscontinuities is undesirable from the stand point of corrosion susceptibilities. Duringoperation, mill scale can be readily removed from the steam generating surfaces andsubsequently redeposited in critical areas. In H.P. units, these surfaces should be chemicallycleaned prior to operation.

During operation, numerous solid constituents may enter the unit with feed water. Solublesalts, which remain in solution, can be removed by blow down. Insoluble salts may only bepartially removed by blow down. Some percentage of these insolubles deposit on boilersurfaces. If not removed, these deposits may accumulate over a period of time and they causetube failures. In utility system using evaporated or demineralised make-up, the bulk of thesedeposits will be iron oxide and copper. Control of pre-boiler corrosion can minimise thequantity of these materialsHowever, complete freedom from deposition is not possible. Periodicchemical cleaning of the boiler should be accepted as a routine maintenance practice. It is asound practice if the high pressure boiler is cleaned once in three to four years. The frequencyof cleaning should best be established taking into account the following factors:

i) the number of outages

ii) method of standby protection

iii) length of operation when feed water quality and treatment limits were not maintained

iv) length of operation when boiler water quality and treatment limits were not maintained

v) number of suspected and confirmed condenser leaks

vi) other contamination of the cycle such as demineraliser mis-operation.

A history card should be maintained on all the above aspects and the boiler should be morefrequently cleaned if the history card shows that the above aspects are had enough to makethe boiler dirty sooner.

Pre-operational cleaning must be preceded by an alkaline boil-out to remove silica, oil andgrease. Pre-boiler system of H.P. boilers should be thoroughly flushed with a hot alkalinesolution to remove oils, siliceous materials and particulate matter present following fabrication,storage and erection.

The solvent generally recommended and universally applied for removal of mill scale is 5%inhibited hydrochloric acid (HCI). Most routine operational scales are primarily iron oxideand inhibited HCI is the recommended solvent for their removal. A unit which oftenexperienced condenser leaks, is very likely to have hard salts present in operational scale ofthe unit. Higher the percentage of salts, the greater is the difficulty of their removal. Multistepcleaning of 3 to 5% solvent may be required to restore surfaces to a satisfactory clean condition.Corrosion of pre-boiler materials often leads to the deposition of copper in the furnace wallsof a unit. Significant amounts of copper can accumulate on internal surfaces in a large boilerin a 3 year period. Copper can be removed from steam generators employing either two stageor single stage solvent systems. The two stage system envisages ammonium bromate in thefirst stage and HCI in the next stage. Inhibited HCI containing an additive for copper

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complexing forms the single stage cleaning for copper bearing deposits.Unfortunately, acid cleaned surfaces are even more susceptible to corrosion than Uncleanedsteel. Passivation is beneficial only for a short period of time, but has little long-term effect.To produce optimum conditions, the period of time between the cleaning and start-up dateshould be held to a minimum.

Seldom it is found necessary to clean superheaters. Normal mill scale on tubing is thin, adherentand not readily eroded by steam. Steam blowing has proven effective in removing particulatematter, debris, loose oxide and atmospheric rust and is recommended for pre-operationalcleaning of superheaters regardless of other procedures, employed.

Operational deposits are more likely to form in water walls than in superheaters. There maybe deposits due to carry over, which can readily be removed by water lushing, since most ofthe carry over salts are soluble. The need to eliminate depositforming materials is relatedprimarily to their effect on the corrosion problems that are common in water walls. It is notdesirable to chemically clean superheaters during its life, unless it is found absolutely essential.Because of the geometry superheater cleaning requires special attention. Frequently, thesesections contain non-drainable, non-ventable surfaces that are hazardous to clean unless thereis clear under standing of the flow machanics. Any acid left over, is more dangerous. Whenchemical cleaning is found to be a must, HCI or any halogenic solvent is not desirable as thestainless steel superheater tubes are susceptible to the attack of stress corrosion cracking withthe use of these solvents. The use of organic solvents should only be considered for chemicalcleaning of such superheaters.

9. STANDBY PROTECTION

Atmospheric corrosion of ferritic materials proceeds rapidly in the presence of oxygen andmoisture. The pitting that develops during down time seldom proceeds to failure. However,the resulting discontinuities on metal surface represent sites for future operational corrosion.Long outage without preservation can result in complete penetrations of thin walled tubes. Likewise, the oxides produced are objectionable and can be transported to critical heat transferareas in the boiler. In todays large boilers with their numerous, complex circuits and bends,it is difficult, if not impossible, to completely dry a boiler in preparation for storage. Drainingthe boiler while hot, may temporarily dry the surfaces. However, unless dry air can be circulatedto eliminate all the water vapour from the unit, recondensation will again result in moistconditions. For this reason, wet layup normally offers the most positive method of protectionfor large utility boilers. The components in the pre-boiler system also require careful protectionunder shut-down conditions.

The wet lay-up procedures require filling up of the boiler, economiser, superheaters, feedwater heaters (tube side) and associated piping with demineralised or condensate type watercontaining 10 ppm of ammonia and 200 ppm of hydrazine. The drum, superheaters and feedwater heaters (shell side) should be capped with nitrogen at a pressure of 5 psig. If the feedwater heaters are of copper alloy, it is recommended to fill the tube side with demineralisedwater containing only 0.5 ppm of ammonia and 50 ppm of hydrazine, since the surfaces aresusceptible to corrosion by ammonia if present in large quantities. Also it is important tomaintain the fluid temperature below 2000C before addition of hydrazine as it will decomposeat higher temperatures. For long outages, it is also required to isolate the reheaters and fill itwith treated water similarly. For short outages, (4 days or less) it is enough if the hydrazine

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and ammonia concentrations are maintained as those present during normal operation. Whenthe outage is long, it is necessary to periodically check for the presence of sufficient amountof ammonia and hydrazine and replenish, if required.

Though infrequent, oxygen corrosion or damage of boiler surfaces can occur if sound practicesare ignored. If proper storage and lay-up procedures are not followed, there is a definite riskof serious oxygen corrosion that could be costly in prolonged repairs and down time.

10. CONCLUSION

It is important that all the operating personnels are fully aware of all aspects of water treatment.Most of the ills on the water and steam side could be avoided by a good water management ina boiler. It is needless to say that adequate and sensitive controls and instrumentations areabsolutely necessary for maintaining proper water chemistry and steam purity in a boiler.Continuous logging of the measurements, review and correction are the important aspects ofday-to-day operation. Based on these, the operators should establish a feel of the working ofthe boiler. A proper history card of the working of the boiler alone can help to solve problemslike tube failures, corrosion, etc. The need for periodically cleaning the boiler and for properlay-up should never be ignored.

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FIG. IX-1 RECOMMENDED CO-ORDINATED PHOSPHATE CURVE

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FIG. IX-2 RECOMMENDED MAXIMUM SILICA

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FIG. IX-3 STREAM DRUM INTERNALS(TYPICAL ARRANGEMENT ) UTILITY BOILERS

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FIG. IX-4 TYPICAL ARRANGEMENT OF DRUM INTERNALSFOR INDUSTRIAL BOILERS

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Contents1. INTRODUCTION2. FEED WATER TREATMENT3. BOILER WATER TREATMENTS4. SILICA CONTROL5. CRITERIA FOR OPERATION DURING CONDENSER LEAK6. MEASUREMENT AND CONTROLFREQUENCY OF MEASUREMENTS STEAM-WATER CYCLE7. CARRY OVER IN STEAM8. CHEMICAL CLEANING9. STANDBY PROTECTION10. CONCLUSIONFIG. IX-1 RECOMMENDED CO-ORDINATED PHOSPHATE CURVEFIG. IX-2 RECOMMENDED MAXIMUM SILICA CONCENTRATIONS IN BOILER WATERFIG. IX-3 STREAM DRUM INTERNALS (TYPICAL ARRANGEMENT ) UTILITY BOILERSFIG. IX-4 TYPICAL ARRANGEMENT OF DRUM INTERNALS FOR INDUSTRIAL BOILERS