Boilers - Water Treating 0

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Boilers - Water Treating

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Vessels File Reference: MEX10504

For additional information on this subject, contact John Thomas on 875-2230

Engineering Encyclopedia

Vessels Boilers - Water Treating

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



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 the potential quality variations. Water for Ju’aymah and Yanbu Gas Plants is purchased. Surface fresh water can come from rivers, streams, lakes, or ponds. These waters usually contain suspended matter, organic matter, dissolved solids, dissolved gases, and other manmade and natural pollutants. Surface fresh water is rare in Saudi Arabia. Subsurface groundwater can originate from springs and shallow or deep wells. These waters are usually relatively free of suspended matter. They can have wide quality variations. Even normally fresh water wells can have salt water intrusion or limited availability during dry periods. 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 high dissolved solids content, frequently over 45,000 ppm. Waste heat or low-level heat is used in many cases to evaporate seawater as a first step in water treatment. Desalination plants are used to produce high-quality water. Uses of Water Water has many uses both in municipalities and in plants. The main uses of water in Saudi Aramco 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, hydrostatic testing, and other minor uses.

Saudi Aramco DeskTop Standards

1



Boiler Feedwater Boiler feedwater is one of the main uses of water by Saudi Aramco. Boiler feedwater consists of returned condensate and makeup water. Makeup water is frequently a low percentage of total feedwater, often less than 10%, because most 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 a better quality of water. Hydrostatic Testing Water Fresh water is preferred for hydrostatic testing because it is less corrosive than brackish or salt water. Almost any source of fresh water is acceptable. Protection from corrosion must be considered. If chemical additives are used for corrosion protection, disposal of the water must be planned and environmental requirements considered. Impurities in Water Water supplies contain dissolved ions shown below. A water analysis must be performed in order to determine if these impurities are within acceptable limits for the intended use. These impurities consist of cations, which are positively charged ions in water, and anions, which are negatively charged ions. Impurities are conventionally expressed in parts per million by weight (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 is equal 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.


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COMMON IMPURITIES IN A WATER SUPPLY

Cation Impurities Calcium  MagnesiumHardness in water Sodium

Chemical Symbol

Factor to Convert to CaCO3 Equivalent

Ca+2 Mg+2 Na

2.5 4.1

HCO3-1 CO3-2

0.8 1.67 2.9

2.18

Anion Impurities Bicarbonate Carbonate Alkalinity Hydroxide  Chlorides Sulfates

OH-1 Cl-1 SO4-2

Nitrates

NO3-1

0.8

CO2 SiO2

1.14 0.83

1.4 1.0

Other Impurities Carbon Dioxide Silica

FIGURE 1


3



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. These deposits reduce heat transfer, increase tube metal temperatures, and cause possible equipment failure. 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 chemical treatment.

Corrosion Corrosion affects distribution piping, feedwater piping and heaters, boiler internals, and condensate piping. The main causes are oxygen, carbon dioxide, chlorine, and excess alkalinity. Corrosives act in different ways. Oxygen causes pitting or formation of small pits in distribution piping, feedwater systems, and boilers. It also aggravates corrosion in condensate systems. Oxygen can be removed externally in a deaerator, or it can be scavenged internally by adding sulfite or hydrazine.


4



Page 1 of 3 COMMON CHARACTERISTICS AND IMPURITIES IN WATER CONSTITUENT Turbidity

Color

Hardness

Alkalinity

Free Mineral Acid

Carbon Dioxide

pH

Sulfate

Chloride

CHEMICAL FORMULA DIFFICULTIES CAUSED None. Usually expressed Imparts unsightly appearance to in Jackson Turbidity water; deposits in water lines, Units process equipment, boilers, and so on; interferes with most process uses. None Decaying organic material and metallic ions causing color may cause foaming in boilers; hinders precipitation methods such as iron removal, hot phosphate softening; can stain product in process use. Calcium, magnesium, Chief source of scale in heat barium, and strontium exchange equipment, boilers, salts expressed as pipe lines, and so on; forms . curds with soap; interferes with CaCO3 dyeing and so on. Foaming and carryover of Bicarbonate (CHO3-1) solids with steam; -2 carbonate, (CO3 ), and embrittlement of boiler steel; hydroxyl (OH-1), bicarbonate and carbonate expressed as CaCO3 produce CO3 in steam, a source of corrosion. H2SO4, HCl, etc., expressed as CaCO3 titrated to methyl orange end-point. CO2

Corrosion

Hydrogen ion concentration defined as: pH = log 1 (H+1) (SO4)-2

pH varies according to acidic or alkaline solids in water; most natural waters have a pH of 6.0 - 8.0

Cl-1

Corrosion in water lines and particularly steam and condensate lines.

Adds to solids content and increases corrosive character of water. Adds to solids content and increases corrosive character of water.

MEANS OF TREATMENT Coagulation, settling, and filtration.

Coagulation, filtration, chlorination, adsorption by activated carbon.

Softening, distillation, internal boiler water treatment, surface active agents, reverse osmosis, electrolytes. Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying. Neutralization with alkalies.

Aeration, deaeration, neutralization with alkalines, liming, and neutralizing amines. pH can be increased by alkalies and decreased by acids. Demineralization, distillation, reverse osmosis, electrodialysis. Demineralization, distillation, reverse osmosis, electrodialysis.

FIGURE 2 Saudi Aramco DeskTop Standards

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Page 2 of 3 COMMON CHARACTERISTICS AND IMPURITIES IN WATER (CONT’D) CONSTITUENT Nitrate

Fluoride

Silica

CHEMICAL FORMULA (NO3)-1

F-1

SiO2

Iron

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

Manganese Oil

Mn+2 Expressed as oil or chloroform extractable matter, ppmw. O2

Oxygen

Hydrogen Sulfide

H2S

Ammonia

NH2

Conductivity

DIFFICULTIES CAUSED Adds to solids content, but is not usually significant industrially; useful for control of boiler metal embrittlement. Not usually significant industrially.

Scale in boilers and cooling water systems: insoluble turbine blade deposits due to silica vaporization.

Discolors water on precipitation; source of deposits in water lines, boilers, and so on; interferes with dyeing, tanning, paper mfr., and so on. Same as Iron. Scale, sludge, and foaming in boilers; impedes heat exchange; undesirable in most processes. Corrosion of water lines, heat exchange equipment, boilers, return lines, etc. Cause of “rotten egg” odor; corrosion. Corrosion of copper and zinc alloys by formation of complex soluble ion.

Expressed as micromhos, specific conductance.

Conductivity is the result of ionizable solids in solution; high conductivity can increase the corrosive characteristics of a water.

MEANS OF TREATMENT Demineralization, distillation, reverse osmosis, electrodialysis. Adsorption with magnesium hydroxide, calcium phosphate, or bone black; Alum coagulation; reverse osmosis, electrolytes. Hot process removal with magnesium salts; adsorption by highly basic anion exchange resins, in conjunction with demineralization; distillation. Aeration, coagulation, and filtration, lime softening, cation exchange, contact filtration, surface active agents for iron retention. Same as Iron. Baffle separators, strainers, coagulation, and filtration, diatomaceous earth filtration. Deaeration, sodium sulfite, corrosion inhibitors, hydrazine or suitable substitutes. Aeration, chlorination, highly basic anion exchange. Carbon exchange with hydrogen zeolite, chlorination, deaeration, mixed bed demineralization. Any process which decreases dissolved soils content will decrease conductivity; examples are demineralization, lime softening.

FIGURE 2 (CONT'D)


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Page 3 of 3 COMMON CHARACTERISTICS AND IMPURITIES IN WATER (CONT’D) CONSTITUENT Dissolved Solids

CHEMICAL FORMULA None

Suspended Solids

None

Total Solids

None

DIFFICULTIES CAUSED

MEANS OF TREATMENT

“Dissolved Solids” is a measure of total amount of dissolved matter, determined by evaporation; high concentrations of dissolved solids are objectionable because of process interference and as a cause of foaming in boilers. “Suspended Solids” is the measure of undissolved matter, determined gravimetrically; suspended solids plug lines, cause deposits in heat exchange equipment, boilers, etc. “Total Solids” is the sum of dissolved and suspended solids, determined gravimetrically.

Various softening processes, such as lime softening and cation exchange by hydrogen zeolites, will reduce dissolved solids; demineralization; distillation; reverse osmosis, electrolytes.

Subsidence, filtration, usually preceded by coagulation and settling.

See “Dissolved Solids” and “Suspended Solids.”

Source: GPSA Engineering Data Book

FIGURE 2 (CONT'D)


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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 or condensate. It can also be formed by hydrazine decomposition. A deaerator will remove ammonia. Abnormal alkalinity produces film corrosion and turbine fouling. Excessive chelates or dispersants can cause corrosion in steam piping and throughout the steam 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 chemical treatment.

Solids Carryover of solids from boiler water into the steam is caused by inadequate separation in a boiler drum, by volatilizing of silica, and by foaming resulting from oil contamination of boiler 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 dissolved solids (TDS), alkalinity, oil, and silica in the boiler drum. Caustic Caustic embrittlement is the cracking of metal along grain boundaries. It can result from too much caustic in boiler water, particularly in poorly controlled caustic-pH programs where caustic is added for pH control.


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Boiler Water Quality Limits Saudi Aramco has established boiler water quality limits for gas plant boilers operating in the range of 400-650 psig. This covers most of Saudi Aramco's boilers. Quality limits for boiler feedwater, steam drum water, condensate return, and steam are listed in Work Aid 1. These boiler feedwater and steam drum water qualities are based on the use of demineralized or desalinated water. All limits are the same for the two sources, except that limits on the chloride content are added when desalinated water is used. Silica limits are well established, based on the maximum level to prevent vaporization and carryover 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 (for example, 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 monitored to confirm that alkalinity levels, particularly hydroxyl alkalinity (B alkalinity), do not exceed 40 mg/l. Excessive free hydroxyl alkalinity introduces the potential for caustic attack. High levels of total alkalinity may cause carryover of boiler water salts into the steam system, leading to possible fouling of superheater tubes or turbine blades.


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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 hardness ions of calcium and magnesium, silica, chlorides, oil, organics, suspended solids, and other impurities.

•

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

•

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

•

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

Boiler water treatment is illustrated in Figure 3, which is a simplified flow plan of the water treatment facilities at Uthmaniyah. This shows the many treatment steps that can be required in 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. + +

•

Preheating.

Other external water treatment processes are used in other plants. These include the following: -

•

Cation units. Anion units.

Reverse osmosis. Sodium zeolite softening. Multistage flash evaporation.

Deaeration.


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SIMPLIFIED FLOW PLAN - WATER TREATING FACILITIES AT UTHMANIYAH

FIGURE 3 Saudi Aramco DeskTop Standards

11



•

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. Both carbon 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 the cooler portions of boilers, because the oxygen is released there first. Carbon dioxide causes condensate line corrosion, especially in combination with oxygen. Deaerators are required to remove oxygen in the boiler feedwater before the water is fed to the boilers. Boilers operating above 600 psig require deaerators capable of reducing oxygen to less than 0.007 ppm. In pressure-type deaerating heaters and deaerators, the oxygen removal (deaeration) level achieved is a function of the temperature, pressure, and degree of stripping. Deaeration is based on the fact that oxygen has an inverse solubility curve in water. A pressure deaerator uses steam to heat the water to the saturation temperature, where the oxygen solubility is very low. 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 the maximum amount of oxygen. If the deaerator is working properly, the temperature of the storage section of the deaerator will be within 2 to 3°F of the steam saturation temperature at the operating pressure of the deaerator. A positive steam plume is required at the vent on pressure units to assure effective venting of the stripped gases. Typical steam pressures used in 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 is distributed over trays, and steam is injected to strip the dissolved gases from the water as it cascades down from tray to tray. The spray type uses spray nozzles to atomize the water into droplets. Some deaerators combine both trays and sprays.


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Internal Chemical Treatment - Deaerator After deaeration, an oxygen scavenger is added to the boiler feedwater to destroy the residual free oxygen in the water. Hydrazine and sodium sulfite are the most cost effective oxygen scavengers. 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 to form sodium sulfite salt. Sodium sulfite is added to the boiler feedwater to maintain a residual concentration of 20 to 30 ppm SO3 in the boiler blowdown. Some plants have replaced sodium 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 boiler from scale and corrosion. These chemicals generally react with the impurities in the water to form compounds that will not deposit on the boiler tube surfaces and can be removed with the blowdown. These compounds can be completely water soluble or can be free-flowing sludges. 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 boiler chemicals can cause additional problems in the boiler, including corrosion or other types of deposits on the boiler tubes. Polymers are used in most Saudi Aramco boilers to control scale deposition on the boiler tube surfaces. Nalco 7200 “Transport-Plus” is injected into the boiler feedwater at a rate of 2.5 ppm. This chemical solubilizes the hardness ions in the feedwater. Particulate iron is also dispersed by the action of the polymer. This chemical also helps prevent carryover by controlling foaming in the steam drum. The dosage rate is determined by the total hardness and total iron in the feedwater. Control includes monitoring the total reacted and residual product in boiler feedwater. Testing for residual product in the boiler water is also required. Determination of product effectiveness is by measurement of % transport of Ca, Mg, Fe, and SiO2. Aim for 100% transport. This indicates that the system is in balance. % transport =

boiler water concentration x 100 (feedwater concentration x cycles)

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


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CALCULATING BOILER BLOWDOWN RATE All dissolved and suspended solids entering a boiler with the feedwater remain in the drums and tubes as steam is generated. The continual addition of feedwater produces higher and higher concentration of solids in the boiler water. A point can be reached beyond which operation is completely unsatisfactory. This situation may be caused by dissolved solids, silica content, or alkalinity. Every boiler has a limit above which foaming and carryover occur. To keep boiler water concentrations below this limit, remove some of the concentrated boiler water from the unit as blowdown. The intermittent or manual blowdown is taken from the bottom of the mud drum. This blowdown is mainly intended to remove any sludge formed in the boiler water. With polymer treatment, both suspended and dissolved solids are present in the water. These must be removed to prevent solids from settling and caking on the heat transfer surfaces. The manual blowdown should be used approximately once per day for a few seconds to remove suspended solids which may have settled in the mud drum. A continuous blowdown system helps to keep the boiler water within the concentration limits on 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 heat exchange system installed in the blowdown system. The continuous blowdown is usually located below the normal water level in the steel drum. Proper regulation of boiler blowdown is very important in boiler operation. Too little blowdown 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 blowdown wastes 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 a function of the operating pressure. In some cases, the blowdown from a high pressure application 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 occur inside the boiler to produce clean and dry steam. Most boilers have effective mechanical separators in the boiler drum when the water boils smoothly. When boiler water primes or foams, however, impurities are carried over in the steam. While water priming and foaming are partly controlled through careful operation of the boiler drum level and chemical injections, respectively, they are highly dependent on maintaining proper boiler blowdown.


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Priming and Foaming Priming is caused by too area for steam release, or increases, erratic hot gas operator must maintain instructions.

high a water level in the boiler drum, which decreases the surface by upsets in boiler water circulation because of sudden steam load flow, or sudden increases in heat input. To prevent priming, the the boiler water level in accordance with the manufacturer's

Foaming is caused by chemical conditions in the boiler water that result in excessive dissolved and suspended solids. Some boiler waters will foam when a sudden change in their chemical 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 and can 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. Superheater fouling results in increased pressure drop and ultimately tube rupture because of high tube wall temperatures. Turbine fouling can be measured by frequent monitoring of turbine steam flow and corresponding steam chamber pressure and comparing the information to the clean condition. The method can indicate a fouling condition over a period of three to four days. It is not useful, however, for isolating a steam carryover problem where a number of boilers supply a common steam header that then supplies a turbine. Solids, Sludge, and Silica Boiler blowdown is adjusted to maintain steam purity. Procedures for determining total dissolved solids, sludge, and silica are as follows: •

Total dissolved solids (conductivity): The amount of dissolved solids can be calculated from the sodium salts naturally present in the feedwater, soluble silica, and any soluble chemicals added for treatment. The amount of dissolved solids in the boiler drum is measured with a conductivity meter, which indicates the amount of dissolved salts by the electrical conductivity of the water. Excessive amounts of dissolved salts cause foaming and carryover of impurities in the steam. A continuous conductivity monitor/recorder on the boiler blowdown is recommended.


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Continuous Blowdown Rate The blowdown rate depends on the boiler feedwater quality. Blowdown is usually calculated as 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. With zeolite-softened water, blowdown may be 5 to 10%, with a system designed for 10% or more. X =

A 100 (100) = B - A C - 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 =

100 C

(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 the target concentration in the boiler water is 10 ppmw. The blowdown rate required to control silica would be: X =

0.05 (100) = 0.502% = 0.5% 10 - 0.05

The cycle of concentration would be: C =

100 + 1 = 200 X


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Blowdown Facilities Blowdown facilities limit solids buildup in the boiler water caused by evaporation. The system 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 flashed at atmospheric pressure. The liquid is flashed at a lower pressure or sent to the sewer or waste disposal. Flashing in the sewer should be avoided because of the personnel hazard. Heat exchange between the blowdown waste liquid and cold makeup water is common when energy costs are high. Figure 4 shows a typical blowdown system arrangement. It includes a medium-pressure and low-pressure flash drum for continuous blowdown from a boiler steam drum and atmospheric flash drum for intermittent blowdown from a boiler mud drum. Condensate from the continuous blowdown low-pressure flash drum is routed through a heat exchanger to site drainage. Condensate from the intermittent blowdown drum is sent directly to the sewer after an atmospheric flash. The intermittent blowdown drum is piped so that it can spare the continuous blowdown system for maintenance.


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TYPICAL BOILER BLOWDOWN FACILITIES

FIGURE 4


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WORK AID 1 - CALCULATE BOILER BLOWDOWN RATE SAUDI ARAMCO WATER QUALITY CONTROL LIMITS The following quality limits apply to demineralized or desalinated water. Boiler Feedwater

Steam Drum Water

Condensate Return

Steam

<10

1000-1500

10

10

pH

8.2-9.5

10.0-11.5

8.2-9.2

8.2-9.2

Iron, mg/l

<0.025

Conductivity at 25 °C, µS/cm

<0.05

Copper, mg/1 Silica, mg/l

< 0.025 <0.05

0-15

Sodium, mg/l

<0.02 <0.05

Chloride, mg/l

<0.05*

Dissolved Oxygen, mg/l

<0.007

<50*

Residual Sulfite, mg/l

20-30

Total Alkalinity, mg/l

50-300

Hydroxyl Alkalinity (B Alkalinity), mg/l

<40

Suspended Solids, mg/l

<50

N-7200, mg/l

2.5 x Cycles

* Monitored when desalinated water is used. mg/l = ppm Source:

Saudi Aramco Report PD-4L/324/84


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GLOSSARY alkalinity

The total carbonate, bicarbonate, and hydroxide ion concentration in the water expressed as ppm calcium carbonate equivalent. These ions react with acid.

anion exchanger

A vessel containing insoluble resin that is capable of exchanging one anion, usually hydroxide ions, for other undesirable 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 the temperature of the steam.

biochemical oxygen demand (BOD)

A measure of the oxygen consumed in the oxidation of organic and oxidizable inorganic materials in wastewater (expressed in ppm).

biocide

A poisonous chemical substance that can kill living organisms.

blowdown

The removal of a portion of water from a system or boiler drum to control the concentration of dissolved and suspended impurities in the system or boiler water.

calcium and magnesium 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 caustic soda

Sodium hydroxide (NaOH).

caustic embrittlement

A type of boiler corrosion characterized by cracking of the metal along the grain boundaries. It may occur when highly stressed metal is exposed to concentrated boiler water. It is usually associated with high concentrations of sodium hydroxide.


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coagulation


A process whereby suspended and colloidal particles, which cause turbidity and color in water, are combined by physical means into masses large enough to settle.

21



degasification/ decarbonation

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

demineralization

The removal of dissolved ionic matter from water.

desuperheating

Spraying water on steam in the system to reduce the temperature 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 matter through a porous material to remove the suspended matter. The suspended matter is normally reduced to less than one NTU of turbidity.

hardness

Same as calcium and magnesium hardness.

ion

Electrically charged particle formed when a molecule dissociates 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.

nephelometric turbidity 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 decaying plants and other forms of pollution.

pH

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

potable water

Drinkable water.

raw water

Water that has not yet been processed by a water treating plant.


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23



silica

An impurity present in water sources, expressed as reactive SiO2.

suspended solids

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

total dissolved solids (TDS)

Impurities, normally salts, that are present in the water in solution, 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 and dissolved impurities.

turbidity

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


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Boilers - Water Treating 0

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