boilers - water treating 0

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is not alreadyin the public domain may not be copied, reproduced, sold, given, ordisclosed to third parties, or otherwise used in whole, or in part, withoutthe written permission of the Vice President, Engineering Services, SaudiAramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX10504 John Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Boilers - Water Treating

Engineering Encyclopedia Vessels


Saudi Aramco DeskTop Standards

CONTENTS PAGE

IMPURITIES IN WATER AND POTENTIAL PROBLEMS................................. 1

Sources of Water .......................................................................................... 1

Uses of Water ............................................................................................... 1

Boiler Feedwater .......................................................................................... 2

Hydrostatic Testing Water............................................................................ 2

Impurities in Water....................................................................................... 2

Quality of Water ........................................................................................... 4

Scale.................................................................................................. 4

Corrosion .......................................................................................... 4

Solids ................................................................................................ 8

Caustic .............................................................................................. 8

Boiler Water Quality Limits ......................................................................... 9

BOILER WATER TREATMENT ......................................................................... 10

Deaeration .................................................................................................. 12

Internal Chemical Treatment - Deaerator ................................................... 13

Internal Chemical Treatment - Boilers ....................................................... 13

CALCULATING BOILER BLOWDOWN RATE ................................................ 14

Priming and Foaming ................................................................................. 15

Turbine and Superheater Fouling ............................................................... 15

Solids, Sludge, and Silica ........................................................................... 15

Continuous Blowdown Rate....................................................................... 16

Blowdown Facilities ................................................................................... 17

WORK AID 1 - CALCULATE BOILER BLOWDOWN RATE .......................... 19

GLOSSARY .......................................................................................................... 20



Saudi Aramco DeskTop Standards 1

IMPURITIES IN WATER AND POTENTIAL PROBLEMS

Sources of Water

The usual sources of water are:

Purchased or municipal water. Surface fresh water. Subsurface groundwater. Sea water.

The major concerns with purchased water are the cost, the reliability of supply, and thepotential quality variations. Water for Juaymah and Yanbu Gas Plants is purchased.

Surface fresh water can come from rivers, streams, lakes, or ponds. These waters usuallycontain suspended matter, organic matter, dissolved solids, dissolved gases, and other man-made and natural pollutants. Surface fresh water is rare in Saudi Arabia.

Subsurface groundwater can originate from springs and shallow or deep wells. These watersare usually relatively free of suspended matter. They can have wide quality variations. Evennormally fresh water wells can have salt water intrusion or limited availability during dryperiods. Wells are a common source of water throughout Saudi Aramco.

Seawater is often used offshore or in arid regions such as Saudi Arabia. This water has a highdissolved solids content, frequently over 45,000 ppm. Waste heat or low-level heat is used inmany cases to evaporate seawater as a first step in water treatment. Desalination plants areused to produce high-quality water.

Uses of Water

Water has many uses both in municipalities and in plants. The main uses of water in SaudiAramco plants are:

Once-through cooling water. Recirculating cooling makeup water. Domestic (sanitary) water. Boiler feedwater. Firefighting water. Crude desalting.

In addition to these main uses, water is used for engine cooling, chemical mixing, hydrostatictesting, and other minor uses.




Boiler Feedwater

Boiler feedwater is one of the main uses of water by Saudi Aramco. Boiler feedwater consistsof returned condensate and makeup water.

Makeup water is frequently a low percentage of total feedwater, often less than 10%, becausemost of the condensate is returned. Makeup water must be treated in nearly all cases.Returned condensate can also require treatment, particularly to remove oil and control pH.The water quality required depends on the use of the water. Higher pressure boilers require abetter quality of water.

Hydrostatic Testing Water

Fresh water is preferred for hydrostatic testing because it is less corrosive than brackish or saltwater. Almost any source of fresh water is acceptable. Protection from corrosion must beconsidered.

If chemical additives are used for corrosion protection, disposal of the water must be plannedand environmental requirements considered.

Impurities in Water

Water supplies contain dissolved ions shown below. A water analysis must be performed inorder to determine if these impurities are within acceptable limits for the intended use. Theseimpurities consist of cations, which are positively charged ions in water, and anions, whichare negatively charged ions. Impurities are conventionally expressed in parts per million byweight (ppmw), which is equivalent to milligrams per liter (mg/l).

The total hardness is equal to the sum of calcium plus magnesium. The total alkalinity isequal to the sum of bicarbonate plus carbonate plus hydroxide.

Hardness and alkalinity are usually expressed in ppmw of calcium carbonate equivalent(CaCO3). Factors to convert impurities to CaCO3 equivalent are listed in Figure 1.




COMMON IMPURITIES IN A WATER SUPPLY

Cation Impurities Chemical SymbolFactor to Convert

to CaCO3 Equivalent

CalciumMagnesium

Hardness in water Ca+2

Mg+22.54.1

Sodium Na 2.18

Anion Impurities

BicarbonateCarbonateHydroxide

Alkalinity

HCO3-1

CO3-2

OH-1

0.81.672.9

Chlorides Cl-1 1.4Sulfates SO4-2 1.0

Nitrates NO3-1 0.8

Other Impurities

Carbon Dioxide CO2 1.14Silica SiO2 0.83

FIGURE 1




Quality of Water

The following types of impurities found in water are a concern in steam generating systems.For a summary of water impurities, difficulties, and treatment methods, see Figure 2.

Scale-forming and deposit-forming insoluble solids.

Soluble salts and dissolved gases that can enhance or cause corrosion.

Dissolved solids, oil, and silica that can carry over into the steam from a boiler.

Caustic (sodium hydroxide - NaOH), which can cause embrittlement.

Scale

Scale and deposits result when insoluble salts deposit on heat transfer surfaces. Thesedeposits reduce heat transfer, increase tube metal temperatures, and cause possible equipmentfailure.

Among the significant scale- and deposit-forming impurities are:

Calcium. Magnesium. Silica. Phosphates.* Oil. Iron, copper. Other suspended solids and turbidity.

* NOTE: This impurity can be added unintentionally during internal chemicaltreatment.

Corrosion

Corrosion affects distribution piping, feedwater piping and heaters, boiler internals, andcondensate piping. The main causes are oxygen, carbon dioxide, chlorine, and excessalkalinity.

Corrosives act in different ways. Oxygen causes pitting or formation of small pits indistribution piping, feedwater systems, and boilers. It also aggravates corrosion in condensatesystems. Oxygen can be removed externally in a deaerator, or it can be scavenged internallyby adding sulfite or hydrazine.




Page 1 of 3

COMMON CHARACTERISTICS AND IMPURITIES IN WATER

CONSTITUENT CHEMICAL FORMULA DIFFICULTIES CAUSEDMEANS OF

TREATMENTTurbidity None. Usually expressed

in Jackson TurbidityUnits

Imparts unsightly appearance towater; deposits in water lines,process equipment, boilers, andso on; interferes with mostprocess uses.

Coagulation, settling, andfiltration.

Color None Decaying organic material andmetallic ions causing color maycause foaming in boilers;hinders precipitation methodssuch as iron removal, hotphosphate softening; can stainproduct in process use.

Coagulation, filtration,chlorination, adsorptionby activated carbon.

Hardness Calcium, magnesium,barium, and strontiumsalts expressed asCaCO3.

Chief source of scale in heatexchange equipment, boilers,pipe lines, and so on; formscurds with soap; interferes withdyeing and so on.

Softening, distillation,internal boiler watertreatment, surface activeagents, reverse osmosis,electrolytes.

Alkalinity Bicarbonate (CHO3-1)

carbonate, (CO3-2), and

hydroxyl (OH-1),expressed as CaCO3

Foaming and carryover ofsolids with steam;embrittlement of boiler steel;bicarbonate and carbonateproduce CO3 in steam, a sourceof corrosion.

Lime and lime-sodasoftening, acid treatment,hydrogen zeolitesoftening,demineralization,dealkalization by anionexchange, distillation,degasifying.

Free Mineral Acid H2SO4, HCl, etc.,expressed as CaCO3titrated to methyl orangeend-point.

Corrosion Neutralization withalkalies.

Carbon Dioxide CO2 Corrosion in water lines andparticularly steam andcondensate lines.

Aeration, deaeration,neutralization withalkalines, liming, andneutralizing amines.

pH Hydrogen ionconcentration defined as:pH = log 1

(H+1)

pH varies according to acidic oralkaline solids in water; mostnatural waters have a pH of 6.0- 8.0

pH can be increased byalkalies and decreased byacids.

Sulfate (SO4)-2 Adds to solids content andincreases corrosive character ofwater.

Demineralization,distillation, reverseosmosis, electrodialysis.

Chloride Cl-1 Adds to solids content andincreases corrosive character ofwater.

Demineralization,distillation, reverseosmosis, electrodialysis.

FIGURE 2




Page 2 of 3

COMMON CHARACTERISTICS AND IMPURITIES IN WATER (CONTD)

CONSTITUENTCHEMICALFORMULA DIFFICULTIES CAUSED MEANS OF TREATMENT

Nitrate (NO3)-1 Adds to solids content, but isnot usually significantindustrially; useful for controlof boiler metal embrittlement.

Demineralization, distillation,reverse osmosis,electrodialysis.

Fluoride F-1 Not usually significantindustrially.

Adsorption with magnesiumhydroxide, calciumphosphate, or bone black;Alum coagulation; reverseosmosis, electrolytes.

Silica SiO2 Scale in boilers and coolingwater systems: insolubleturbine blade deposits due tosilica vaporization.

Hot process removal withmagnesium salts; adsorptionby highly basic anionexchange resins, inconjunction withdemineralization; distillation.

Iron Fe-2 (ferrous)Fe-3 (ferric)

Discolors water onprecipitation; source ofdeposits in water lines, boilers,and so on; interferes withdyeing, tanning, paper mfr.,and so on.

Aeration, coagulation, andfiltration, lime softening,cation exchange, contactfiltration, surface activeagents for iron retention.

Manganese Mn+2 Same as Iron. Same as Iron.Oil Expressed as oil or

chloroformextractable matter,ppmw.

Scale, sludge, and foaming inboilers; impedes heatexchange; undesirable in mostprocesses.

Baffle separators, strainers,coagulation, and filtration,diatomaceous earth filtration.

Oxygen O2 Corrosion of water lines, heatexchange equipment, boilers,return lines, etc.

Deaeration, sodium sulfite,corrosion inhibitors,hydrazine or suitablesubstitutes.

Hydrogen Sulfide H2S Cause of rotten egg odor;corrosion.

Aeration, chlorination, highlybasic anion exchange.

Ammonia NH2 Corrosion of copper and zincalloys by formation ofcomplex soluble ion.

Carbon exchange withhydrogen zeolite,chlorination, deaeration,mixed bed demineralization.

Conductivity Expressed asmicromhos, specificconductance.

Conductivity is the result ofionizable solids in solution;high conductivity can increasethe corrosive characteristics ofa water.

Any process which decreasesdissolved soils content willdecrease conductivity;examples aredemineralization, limesoftening.

FIGURE 2 (CONT'D)




Page 3 of 3

COMMON CHARACTERISTICS AND IMPURITIES IN WATER (CONTD)

CONSTITUENTCHEMICALFORMULA

DIFFICULTIES CAUSED MEANS OF TREATMENT

Dissolved Solids None Dissolved Solids is ameasure of total amount ofdissolved matter,determined by evaporation;high concentrations ofdissolved solids areobjectionable because ofprocess interference and asa cause of foaming inboilers.

Various softening processes,such as lime softening andcation exchange by hydrogenzeolites, will reduce dissolvedsolids; demineralization;distillation; reverse osmosis,electrolytes.

Suspended Solids None Suspended Solids is themeasure of undissolvedmatter, determinedgravimetrically; suspendedsolids plug lines, causedeposits in heat exchangeequipment, boilers, etc.

Subsidence, filtration, usuallypreceded by coagulation andsettling.

Total Solids None Total Solids is the sum ofdissolved and suspendedsolids, determinedgravimetrically.

See Dissolved Solids andSuspended Solids.

Source: GPSA Engineering Data Book

FIGURE 2 (CONT'D)




Carbon dioxide also causes condensate system corrosion. It can be removed in a deaerator,degasifier, or decarbonator.

Ammonia attacks copper alloys. Ammonia is sometimes added for pH control in feedwater orcondensate. It can also be formed by hydrazine decomposition. A deaerator will removeammonia.

Abnormal alkalinity produces film corrosion and turbine fouling.

Excessive chelates or dispersants can cause corrosion in steam piping and throughout thesteam system.

Impurities that enhance corrosion include:

Oxygen. Carbon dioxide. Ammonia.* Alkalinity.* Chlorides. Sulfites.* Hydrazine.* Chelates.* Organics.

* NOTE: These impurities can be added unintentionally during internal chemicaltreatment.

Solids

Carryover of solids from boiler water into the steam is caused by inadequate separation in aboiler drum, by volatilizing of silica, and by foaming resulting from oil contamination ofboiler water. Solids carryover can result in superheater failure, steam turbine blade fouling,and process catalyst fouling. The main causes of such problems are high total dissolvedsolids (TDS), alkalinity, oil, and silica in the boiler drum.

Caustic

Caustic embrittlement is the cracking of metal along grain boundaries. It can result from toomuch caustic in boiler water, particularly in poorly controlled caustic-pH programs wherecaustic is added for pH control.




Boiler Water Quality Limits

Saudi Aramco has established boiler water quality limits for gas plant boilers operating in therange of 400-650 psig. This covers most of Saudi Aramco's boilers. Quality limits for boilerfeedwater, steam drum water, condensate return, and steam are listed inWork Aid 1.

These boiler feedwater and steam drum water qualities are based on the use of demineralizedor desalinated water. All limits are the same for the two sources, except that limits on thechloride content are added when desalinated water is used.

Silica limits are well established, based on the maximum level to prevent vaporization andcarryover of silica, which can foul turbine blades.

The chloride limit is specified for desalinated makeup water to prevent internal corrosion.

The conductivity levels specified are typical operating levels, rather than absolute limits.Conductivity is correlated to the maximum level of the limiting constituent in boiler water (forexample, silica or chloride). Because of the ease and reliability of measuring conductivity,conductivity is the primary parameter for controlling boiler water blowdown.

Alkalinity should not be a limiting or controlling parameter. However, it should be monitoredto confirm that alkalinity levels, particularly hydroxyl alkalinity (B alkalinity), do not exceed40 mg/l. Excessive free hydroxyl alkalinity introduces the potential for caustic attack. Highlevels of total alkalinity may cause carryover of boiler water salts into the steam system,leading to possible fouling of superheater tubes or turbine blades.




BOILER WATER TREATMENT

The four main steps for treating boiler water and reducing impurities are the following:

External treatment, upstream of the boiler and deaerator. This can reduce the hardnessions of calcium and magnesium, silica, chlorides, oil, organics, suspended solids, andother impurities.

Deaeration. This can reduce the amount of oxygen, carbon dioxide, and ammonia in thewater.

Internal chemical treatment in the boiler or deaerator. This can control scale andcorrosion that result from impurities not removed in external treatment.

Blowdown. This can remove solids that accumulate and concentrate in the boilerbecause of evaporation.

Boiler water treatment is illustrated in Figure 3, which is a simplified flow plan of the watertreatment facilities at Uthmaniyah. This shows the many treatment steps that can be requiredin a single plant. The flow sequence includes the following steps:

Wells as the water source.

External water treatment.

- Sulfuric acid injection.- Aeration in a tower.- Iron removal filters.- Cartridge filters.- Electrodialysis.- Demineralization.

+ Cation units.+ Anion units.

- Preheating.

Other external water treatment processes are used in other plants. These include thefollowing:

- Reverse osmosis.- Sodium zeolite softening.- Multistage flash evaporation.

Deaeration.




SIMPLIFIED FLOW PLAN - WATER TREATING FACILITIES ATUTHMANIYAH

FIGURE 3




Internal chemical treatment.

- Nalco 356 injection.- Sulfite injection.- Nalco 7200 injection.

Blowdown.

Deaeration

Gases dissolved in water, such as oxygen and carbon dioxide, must be minimized. Bothcarbon dioxide and oxygen can cause corrosion of carbon steel in steam generation facilities.Dissolved oxygen is a major contributor to the pitting corrosion experienced in boilers,especially in economizers and downcomer tubes. Corrosion frequently is more severe in thecooler portions of boilers, because the oxygen is released there first. Carbon dioxide causescondensate line corrosion, especially in combination with oxygen.

Deaerators are required to remove oxygen in the boiler feedwater before the water is fed tothe boilers. Boilers operating above 600 psig require deaerators capable of reducing oxygen toless than 0.007 ppm.

In pressure-type deaerating heaters and deaerators, the oxygen removal (deaeration) levelachieved is a function of the temperature, pressure, and degree of stripping. Deaeration isbased on the fact that oxygen has an inverse solubility curve in water. A pressure deaeratoruses steam to heat the water to the saturation temperature, where the oxygen solubility is verylow. Steam stripping is provided to reduce the oxygen partial pressure in the vapor phase.Since the solubility of oxygen is a function of the partial pressure, these two steps remove themaximum amount of oxygen. If the deaerator is working properly, the temperature of thestorage section of the deaerator will be within 2 to 3F of the steam saturation temperature atthe operating pressure of the deaerator. A positive steam plume is required at the vent onpressure units to assure effective venting of the stripped gases. Typical steam pressures usedin pressure-type deaerators vary from 2 to 60 psig.

There are two basic types of deaerators, tray and spray. In the tray type, the water isdistributed over trays, and steam is injected to strip the dissolved gases from the water as itcascades down from tray to tray. The spray type uses spray nozzles to atomize the water intodroplets. Some deaerators combine both trays and sprays.




Internal Chemical Treatment - Deaerator

After deaeration, an oxygen scavenger is added to the boiler feedwater to destroy the residualfree oxygen in the water. Hydrazine and sodium sulfite are the most cost effective oxygenscavengers.

Catalyzed sodium sulfite is used in most Saudi Aramco plants. This removes the free,dissolved oxygen, but adds dissolved salts to the boiler water. Sulfite reacts with oxygen toform sodium sulfite salt. Sodium sulfite is added to the boiler feedwater to maintain a residualconcentration of 20 to 30 ppm SO3 in the boiler blowdown. Some plants have replacedsodium sulfite injection with diethyl hydroxylamine (DEHA).

Internal Chemical Treatment - Boilers

Many types of chemical treatments are available for use with boiler water to protect the boilerfrom scale and corrosion. These chemicals generally react with the impurities in the water toform compounds that will not deposit on the boiler tube surfaces and can be removed with theblowdown. These compounds can be completely water soluble or can be free-flowingsludges.

The choice of chemicals used depends upon the type and amount of impurities in the water,which are largely the result of the type of water treating system used. Improper use of boilerchemicals can cause additional problems in the boiler, including corrosion or other types ofdeposits on the boiler tubes.

Polymers are used in most Saudi Aramco boilers to control scale deposition on the boiler tubesurfaces. Nalco 7200 Transport-Plus is injected into the boiler feedwater at a rate of 2.5ppm. This chemical solubilizes the hardness ions in the feedwater. Particulate iron is alsodispersed by the action of the polymer. This chemical also helps prevent carryover bycontrolling foaming in the steam drum.

The dosage rate is determined by the total hardness and total iron in the feedwater. Controlincludes monitoring the total reacted and residual product in boiler feedwater. Testing forresidual product in the boiler water is also required. Determination of product effectiveness isby measurement of % transport of Ca, Mg, Fe, and SiO2. Aim for 100% transport. Thisindicates that the system is in balance.

% transport =

boiler water concentration(feedwater concentration x cycles)

x 100

where: cycles = ratio of feedwater rate to blowdown rate.




CALCULATING BOILER BLOWDOWN RATE

All dissolved and suspended solids entering a boiler with the feedwater remain in the drumsand tubes as steam is generated. The continual addition of feedwater produces higher andhigher concentration of solids in the boiler water. A point can be reached beyond whichoperation is completely unsatisfactory. This situation may be caused by dissolved solids,silica content, or alkalinity. Every boiler has a limit above which foaming and carryoveroccur. To keep boiler water concentrations below this limit, remove some of the concentratedboiler water from the unit as blowdown.

The intermittent or manual blowdown is taken from the bottom of the mud drum. Thisblowdown is mainly intended to remove any sludge formed in the boiler water. With polymertreatment, both suspended and dissolved solids are present in the water. These must beremoved to prevent solids from settling and caking on the heat transfer surfaces. The manualblowdown should be used approximately once per day for a few seconds to removesuspended solids which may have settled in the mud drum.

A continuous blowdown system helps to keep the boiler water within the concentration limitson a relatively constant basis. Removing a small stream of water continuously saves water,chemicals, and heat. The heat in the continuous blowdown water can be recovered in a heatexchange system installed in the blowdown system. The continuous blowdown is usuallylocated below the normal water level in the steel drum.

Proper regulation of boiler blowdown is very important in boiler operation. Too littleblowdown allows the concentration of suspended and dissolved solids to become too great,resulting in scale formation and carryover of impurities in the steam. Too much blowdownwastes fuel and feedwater. Globe valves with position indicators allow for accurate control.

Boiler concentration limits applied to control corrosion and fouling in the boiler vary as afunction of the operating pressure. In some cases, the blowdown from a high pressureapplication is suitable for makeup to a lower pressure steam generator.

Steam that is dirty and wet can cause deposits in superheaters, turbines, and control valves,and process contamination can result. A good separation of water and steam must occurinside the boiler to produce clean and dry steam. Most boilers have effective mechanicalseparators in the boiler drum when the water boils smoothly. When boiler water primes orfoams, however, impurities are carried over in the steam. While water priming and foamingare partly controlled through careful operation of the boiler drum level and chemicalinjections, respectively, they are highly dependent on maintaining proper boiler blowdown.




Priming and Foaming

Priming is caused by too high a water level in the boiler drum, which decreases the surfacearea for steam release, or by upsets in boiler water circulation because of sudden steam loadincreases, erratic hot gas flow, or sudden increases in heat input. To prevent priming, theoperator must maintain the boiler water level in accordance with the manufacturer'sinstructions.

Foaming is caused by chemical conditions in the boiler water that result in excessivedissolved and suspended solids. Some boiler waters will foam when a sudden change in theirchemical composition occurs. High amounts of dissolved and suspended solids, alkalinity,oil, and organic contaminants that can act as surfactants in the boiler water promote foaming.Commercial antifoams, blended into water treating formulations, have been successful andcan be verified for effectiveness by monitoring steam purity with a sodium analyzer.

Turbine and Superheater Fouling

The measure of steam carryover is the rate of turbine and superheater fouling. Superheaterfouling results in increased pressure drop and ultimately tube rupture because of high tubewall temperatures. Turbine fouling can be measured by frequent monitoring of turbine steamflow and corresponding steam chamber pressure and comparing the information to the cleancondition. The method can indicate a fouling condition over a period of three to four days. Itis not useful, however, for isolating a steam carryover problem where a number of boilerssupply a common steam header that then supplies a turbine.

Solids, Sludge, and Silica

Boiler blowdown is adjusted to maintain steam purity. Procedures for determining totaldissolved solids, sludge, and silica are as follows:

Total dissolved solids (conductivity): The amount of dissolved solids can be calculatedfrom the sodium salts naturally present in the feedwater, soluble silica, and any solublechemicals added for treatment. The amount of dissolved solids in the boiler drum ismeasured with a conductivity meter, which indicates the amount of dissolved salts bythe electrical conductivity of the water. Excessive amounts of dissolved salts causefoaming and carryover of impurities in the steam. A continuous conductivitymonitor/recorder on the boiler blowdown is recommended.




Continuous Blowdown Rate

The blowdown rate depends on the boiler feedwater quality. Blowdown is usually calculatedas a percent of steam production. Because of the high quality of demineralized water,blowdown may be about 1% with a system designed for a 5 to 10% blowdown rate. Withzeolite-softened water, blowdown may be 5 to 10%, with a system designed for 10% or more.

X =

AB - A

100( ) = 100C - 1

(Eqn. 1)

where: X = Blowdown rate, % of steam flow.

A = Concentration of impurity in boiler feedwater.

B = Target concentration of impurity in steam drum. Refer to Work Aid 2 for target concentration limits.

C = Cycle of concentration.= Ratio of feedwater rate to blowdown rate.

Y =

100C

(Eqn. 2)

where: Y = Blowdown rate, % of feedwater rate.

For example, assume that for a 600 psig boiler, the feedwater has 0.05 ppmw silica, and thetarget concentration in the boiler water is 10 ppmw. The blowdown rate required to controlsilica would be:

X =

0.0510 - 0.05

100( ) = 0.502% = 0.5%

The cycle of concentration would be:

C =

100X

+ 1 = 200




Blowdown Facilities

Blowdown facilities limit solids buildup in the boiler water caused by evaporation. Thesystem must safely dispose of the flashing steam and hot liquid.

Blowdown from high-pressure boilers may be flashed at several levels. For example,600-psig blowdown may be flashed at 125 psig, at 15 psig, and at atmospheric pressure.Drums are used as the flash vessels. Flashed steam is recovered except for that steam flashedat atmospheric pressure. The liquid is flashed at a lower pressure or sent to the sewer or wastedisposal. Flashing in the sewer should be avoided because of the personnel hazard. Heatexchange between the blowdown waste liquid and cold makeup water is common whenenergy costs are high.

Figure 4 shows a typical blowdown system arrangement. It includes a medium-pressure andlow-pressure flash drum for continuous blowdown from a boiler steam drum and atmosphericflash drum for intermittent blowdown from a boiler mud drum. Condensate from thecontinuous blowdown low-pressure flash drum is routed through a heat exchanger to sitedrainage. Condensate from the intermittent blowdown drum is sent directly to the sewer afteran atmospheric flash. The intermittent blowdown drum is piped so that it can spare thecontinuous blowdown system for maintenance.




TYPICAL BOILER BLOWDOWN FACILITIES

FIGURE 4




WORK AID 1 - CALCULATE BOILER BLOWDOWN RATE

SAUDI ARAMCO WATER QUALITY CONTROL LIMITS

The following quality limits apply to demineralized or desalinated water.

BoilerFeedwater

Steam DrumWater

CondensateReturn Steam

Conductivity at25 C, mS/cm




GLOSSARY

alkalinity The total carbonate, bicarbonate, and hydroxide ionconcentration in the water expressed as ppm calcium carbonateequivalent. These ions react with acid.

anion exchanger A vessel containing insoluble resin that is capable ofexchanging one anion, usually hydroxide ions, for otherundesirable anions in the water, for example, sulfates, chlorides,and nitrates.

anions Negatively charged ions in the water, for example, sulfates,chlorides, nitrates, and bicarbonates.

attemperating Spraying water on steam coming out of a boiler to lower thetemperature of the steam.

biochemical oxygendemand (BOD)

A measure of the oxygen consumed in the oxidation of organicand oxidizable inorganic materials in wastewater (expressed inppm).

biocide A poisonous chemical substance that can kill living organisms.

blowdown The removal of a portion of water from a system or boiler drumto control the concentration of dissolved and suspendedimpurities in the system or boiler water.

calcium andmagnesium hardness

The concentration of calcium and magnesium ions in the water,expressed as ppm calcium carbonate equivalent.

cations Positively charged ions in the water, for example, calcium,magnesium, and sodium.

caustic or causticsoda

Sodium hydroxide (NaOH).

caustic embrittlement A type of boiler corrosion characterized by cracking of themetal along the grain boundaries. It may occur when highlystressed metal is exposed to concentrated boiler water. It isusually associated with high concentrations of sodiumhydroxide.




coagulation A process whereby suspended and colloidal particles, whichcause turbidity and color in water, are combined by physicalmeans into masses large enough to settle.




degasification/decarbonation

The process of removing dissolved gases other than air fromwater. Air from a blower comes into intimate, countercurrentcontact with the water droplets and scrubs out the gases.Degasification in boiler water treatment is usually used toremove carbon dioxide.

demineralization The removal of dissolved ionic matter from water.

desuperheating Spraying water on steam in the system to reduce thetemperature of the steam.

dissolved solids See Total Dissolved Solids.

dolomitic lime Lime containing 30-35wt% magnesium oxide.

filtration The process of passing water containing suspended matterthrough a porous material to remove the suspended matter.The suspended matter is normally reduced to less than oneNTU of turbidity.

hardness Same as calcium and magnesium hardness.

ion Electrically charged particle formed when a moleculedissociates into positive and negative particles, for example,salt into positive sodium and negative chloride ions.

makeup water Water added to a system to make up for losses or blowdown.

nephelometricturbidity unit (NTU)

A measurement of the turbidity of a sample of water,determined by light reflection.

organics As used in this course, complex acids resulting from decayingplants and other forms of pollution.

pH A measurement of the acidity or alkalinity of a system. Thereference temperature for pH is 25C (77F) and the pH scaleruns from 0 (highly acidic) to 14 (highly basic), with pH=7.0being neutral.

potable water Drinkable water.

raw water Water that has not yet been processed by a water treatingplant.




silica An impurity present in water sources, expressed as reactiveSiO2.

suspended solids Finely divided insoluble matter present in water. Thesuspended solids are normally inorganic material, such asclay, rock, silt, and so on.

total dissolved solids(TDS)

Impurities, normally salts, that are present in the water insolution, usually as ionic material.

total hardness Calcium plus magnesium hardness.

total solids The sum of the suspended and dissolved solids in water.

treated water Water that has been processed to reduce the suspended anddissolved impurities.

turbidity Lack of clarity due to the presence of suspended or colloidalmatter, expressed in Nephelometric Turbidity Units (NTU).

boilers - water treating 0

Documents