Unit 6 Water Treatment Flashcards
Impurities in a boiler can;
Result in lose of efficiency, and can create a dangerous situation that may result in catastrophic failure
Sludge
Produced elsewhere but carried along by flowing water, will deposit on the boiler heat transfer surfaces
Scale
Formation from salts, which have solubility in source water, but are totally insoluble under the conditions inside a boiler
Corrosion
Happens in the hottest areas of the boiler, includes water wall, screen tubes, and superheater tubes
Carryover
From impurities downstream of the boiler. Negatively affects the purity of the steam that contacts parts of the energy plant
Two methods of treating boiler water
External and internal water treatment
External water treatment
Takes place before the water enters the boiler
Internal water treatment
Adding specific chemicals to boiler water that target problematic impurities
Direct amine measurements can be made using a;
Gas chromatograph
Shock feeding only one type of biocide will result in;
Organisms becoming immune to it
Biological attack on cooling tower wood occurs when;
The cellulose deteriorates
Suspended solids
Substances that have not been dissolved, but are held or suspended in the water.
Examples of suspended solids;
Clay, sand, silica, organic materials
Particles smaller than ________ are considered to be dissolved solids
2um
Larger materials in water such as tree and branches are considered;
Not to be suspended solids, since they are easily dealt with using screens
If suspended solids are not removed;
They can form deposits within the boiler, which can reduce heat exchange, which in turn can lead to metal failure. Can also lead to erosion of metal surfaces or plug up equipment.
A higher concentration of suspended solids can lead to;
Foaming, which results in carryover or priming from the steam drum
Most common methods of reducing suspended solids externally;
Through gravity settling like clarifiers and pressure filtration
How to remove suspended solids internally;
Through boiler blowdown
Dissolved solids are usually:
Ionic compounds dissolved in water
Ionic compounds with calcium and magnesium cations are called
Calcium and magnesium salts
Brackish water
Water containing high dissolve salt concentrations
When conducting water hardness tests, calcium and magnesium appear as
Dissolved solids
Ppm is equivalent to
Mg/L for liquids
Bicarbonate of calcium and magnesium
Cause temporary hardness because they will precipitate out of the water at temperatures below the boiling point. these can be removed before they enter the boiler
Sulfates of calcium and magnesium cause;
Cause permanent hardness since they do not precipitate until the water boils at which point they form hard, dense scale on boiler surfaces
Technologies that remove silica
Lime softening, reverse osmosis and ion Exchange
Two ways sludge and scale reduce heat transfer
Scale insulates boiler surfaces, impeding heat transfer
Scale reduces the flow of boiler water
Sludge and scale build-up can lead to
Boiler overheating
3 ways foaming can occur in a boiler
Dissolved solids or by the presence of oils or suspended solids
Dissolved solids are most commonly removed externally using
Some sort of ion exchange such as lime softeners cation exchangers using zeolites, and anion exchangers and demineralizers
Dissolved gases
Gases that are in solution
Examples of dissolved gases
Nitrogen, ammonia, oxygen, and carbon dioxide
As the temperature increases, the solubility of gases in water tends to
Decrease, which results in the release of dissolved gases
2 dissolved gases of primary concern to power engineers
Oxygen and carbon dioxide, because of their ability to produce boiler corrosion
Dissolved oxygen in boiler water
Produces a form of ferrous metal corrosion due to improperly treated feed water.
While dissolved in boiler water, oxygen causes
A form of corrosion called pitting, which is localized and hard to spot
Oxygen removal is accomplished by a combination of
External and internal methods
Carbon dioxide, when dissolved in water forms
Carbonic acid
When condensate forms, the carbon dioxide dissolves, forming
Carbonic acid
Carbonic acid causes
Grooving on metal surfaces of condensate return lines
As more dissolved carbon dioxide is present,
The pH drops
Carbonic acid formation in condensate lines can be inhibited by adding
Neutralizing agents internally to the boiler or directly to the steam lines
The lower the pH value,
The more acidic the water is
A high pH value indicates
A basic or alkaline solution
Hard and softened water contains
Carbonate and bicarbonate salts of calcium, magnesium and sodium
A safe range of pH is
Between 8.5 and 12.7. Outside this range, corrosion can be significant
Magnetite
Prevents further oxidation of the boiler heat transfer surface. Therefore considered highly beneficial
Caustic embrittlement
May result from high pH levels. Very difficult to detect, due to its fine cracking. Caustic embrittlement commonly results in boiler metal failure
High pH boiler water may cause
Foaming in the boiler drum
Most boilers operate with a pH of about
9.5 to 10.5, balances corrosion and foaming
Sodium hydroxide, also known as caustic soda is commonly added to
Raise pH
To lower pH
The boiler is blown off or it’s continuous blowdown rate is increased
Three main steps Municipal potable water systems use to purify their water
Remove solids, remove pathogens, disinfect
When the temperature is low, water holds
Slightly more oxygen
Turbidity
Is a measure of water cloudiness
Municipal Water Treatment process
Clarification, filtration, disinfection
When boilers use Municipal Water as feedstock, it requires further processing. This will;
Reduce hardness, remove oxygen and other dissolved gases, adjust the pH
External treatment for dissolved oxygen will only lower the level to the range of
10 ppb
The first stage of water treatment is usually
The removal of suspended solids
Settling ponds
Slow or delay the velocity of water, this allows suspended solids to settle, by gravity, to the bottom
Clarifier vs settling pond
Clarifiers can finish in an hour or two, while settling ponds may take days. With clarifiers, the accumulations of solids are drained from the bottom
How a clarifier works
Water is introduced in the centre of the clarifier, and flows to the bottom middle section. As the water flows, the particles settle out, and are removed by a rake that moves the sludge to a central blow off point
Backwashing
Involves reversing the flow of water through the filter, thus freeing trapped solids. Backwash flow rate must be correct, otherwise it will be not be effective
Once the initial removal of suspended solids is done, the water usually
Passes through a filter to remove the last of the suspended particles
Four common types of filters
Pressure filters, filter Aid tubular filters, cartridge filters, activated carbon filters
Overtime, filters become plugged with suspended particles
And must be backwashed
Filter Aid tubular filter
Closed cylindrical housing that contains several screen type cylindrical tubes. Can be quite large and similar in concept to an air cleaning baghouse
Cartridge filters
Similar to an oil filter in an automotive engine. Outer shell can be either steel or plastic. Inner cylinder made of woven fibres
Activated carbon filter
Often used to remove taste or odours from water. Also used to remove chlorine, especially from Municipal Source water. Not very effective against dissolved solids
Chlorine must be removed to protect;
Reverse osmosis membranes and ion exchange material
Water is considered hard when it contains over;
Over 120 PPM (120mg/L) of calcium or magnesium ions
Scale could possibly lead to;
Deposit corrosion, and even cause metal failure
Calcium and magnesium ions are unaffected by;
Clarifiers or filters
Four methods to soften water
Lime soda systems, sodium zeolites, demineralizers, reverse osmosis
Lime soda softeners
Lime reacts with impurities that cause carbonate hardness. Soda ash reacts with impurities that cause non carbonate hardness. After these reactions, they become suspended solids, and are no longer dissolved solids. Now they can be removed using clarification, flocculation, or filtration.
Two categories of lime soda softeners
Hot ( 95-100°C )
Cold ( 25-30°C )
Lime soda hot process versus cold process
Hot process is more common because it is quicker, and the solids are easier to filter out. The hot process can remove silica, whereas the cold process cannot
Sodium zeolite system
Rely on the principles of ion exchange to soften water. They cost less than lime soda softeners, are easier to control, and the chemicals are safer for operators. Magnesium and calcium ions are replaced with ones that do not cause scale and are highly soluble. Can bring the hardness down to zero, while the sodium increases dramatically
When a sodium zeolite softener is removed from service the following procedure is followed;
4
- Backwashing
- Brining (regeneration)
- Slow rinse (displacement)
- Fast rinse
Sodium zeolite softener backwashing stage
Consists of flowing water backwards through the bed. This expands the volume of the bed by about 50%, and loosens dirt.
Sodium zeolite softener brining stage
Salt brine is added to the vessel, while the resulting calcium and magnesium chloride salts are discharged to waste.
Sodium zeolite softener slow rinse stage
Water flows downwards, and forces the remaining brine into areas it may not have penetrated earlier
Sodium zeolite softener fast rinse stage
Water flow is almost doubled, and completely removes the brine from the zeolite bed. Used or spent brine is often flushed to the Sewer
Demineralizer
Rely on ION Exchange. 2 exchange units are installed in series, cation exchanger and anion exchanger. Can remove most ions except for silica. Instead of using a brine solution, the Cation exchanger uses a diluted solution of sulphuric acid through the bed. The anion exchanger uses a dilute solution of caustic soda through the bed.
Reverse osmosis
Uses a membrane technology, and not strictly a filtering process. Can remove up to 99% of dissolved solids, organic matter, and colloidal matter. Commonly used to desalinate seawater. Creates pure water, while concentrating the impurities.
Daeration
The Daerator applies a pressure to the water, and heats it just below the saturation temperature. This frees the dissolved gases from the water, which are then vented from the daerator.
Four important operating parameters to consider for efficient daerator operation
Temperature
Turbulence
Time
Surface area
2 basic daerator configurations
Tray type: In what water is directed into a steam chamber, as the water heats up, it then Cascades down several trays. This increases the surface area of the water, which promotes greater water to steam contact.
Spray type: Inlet water is directed into a similar steam chamber, however, it is broken up into small particles with spray nozzles to promote water to steam contact.
External vent condenser
When the vent condenser is mounted Outside The Daerator vessel.
Sodium hydroxide solution is often used to
Raise the pH level within the boiler. The solution is fed either directly to the boiler drum, or to the feed water before it enters the drum
Caustic embrittlement, also known as caustic;
Occurs when metal under stress is attacked by a concentrated caustic solution. Creates a corrosion pattern that runs around the circumference of the tube.
How to prevent caustic embrittlement
Limit the boiler water sodium hydroxide concentration to the minimum required to achieve proper pH and alkalinity
Caustic gouging
Another form of caustic embrittlement, also a result of concentrations of sodium hydroxide
How to prevent caustic gouging
Maintain clean boiler metal surfaces, and monitor for the presence of excess sodium hydroxide.
Foaming
The result of high concentrations of dissolved and suspended solids in boiler water. Foaming makes it difficult to determine water level, and also allows liquid water to lift over in the steam line, also known as carry over.
Foaming is effectively controlled by
The use of continuous surface below down.
Oxygen corrosion
Dissolved oxygen causes pitting in the boiler and condensate lines. It is highly localized, and can be difficult to spot.
Preventing oxygen corrosion
By using oxygen scavengers, which combine with oxygen so that is it is no longer available to react with boiler metal.
Two chemicals used as oxygen scavengers
Sodium sulfite and hydrazine. One disadvantage of sulfite is that it increases dissolved solids, which can lead to foaming.
Hydrazine can cause
Cancer, so it must be handled very carefully according to its SDS.
The most common used chemical to prevent corrosion caused by oxygen and carbon dioxide;
Amines
Two main types of amines
Neutralizing and filming
Neutralizing amines
They are volatile, the flash off and travel with the steam to the rest of the system. They are alkaline, they will neutralize any acids formed in the condensate.
Filming amines
Not as soluble, and they’re usually injected into the steam header. When steam condenses, these chemicals will create a thin coat over the piping surfaces, protecting the surfaces from acidity and corrosion.
Molybdate
Corrosion inhibitor which acts to protect the boiler against General corrosion and oxygen pitting. Common in hot water and low pressure steam boilers, and in closed-loop cooling systems.
Concentration of molybdate is generally maintained at;
75 to 125 ppm, controlled by blowdowns
Chemicals added so soft scale will form, rather than hard scale
Sodium phosphate
Chelants
Commonly used in boiler water treatment programs to trap metallic ions, and make scales more soluble. The Trap and hold ions. However, they are more expensive and complicated to use. Chelants must be used carefully because they can cause large clumps of scale to be released.
Common chelant used;
EDTA
Organic compounds used to prevent sludge deposits
Starch, tannin, lignin, alginates. These coat the scale precipitates and prevent them from adhering to boiler internals
The method used to supply the necessary chemicals to the boiler water is usually a
Small positive displacement motor driven metering pump
Valve’s needed for the chemical feed line to the boiler
A shut off valve next to the boiler, a check valve between the chemical feed pump discharge isolation valve and the shut-off valve at the boiler
Chemical feed pumps are;
High pressure, low capacity types with adjustable flow rates.
Chemical feeders
Supply chemicals to the return line of a hot water circulating system, or directly into a boiler.
Bypass chemical feeder
Feeds chemicals, supplied in liquid, briquette, or ball form into the feed water or circulating line
The most important part of any treatment program
Make the dosing adjustments necessary for maintaining correct boiler water chemistry.
Safety gear when testing boiler water
Heat resistant gloves, eye protection and protective coveralls
Never take a sample from
A sight glass
Sample cooler
Reduces the temperature of the sample to a standard temperature usually 24° Celsius. It then flows to a sample header.
Testing area
An open area, so fumes do not accumulate. Should be equipped with a fume Hood, preferably. There should also be a supply of hot and cold water. Distilled water must also be available for final rinsing of any equipment.
Titration
Allows precise determination of a reaction endpoint, and thus the amount of reactant in the sample. The main piece of equipment used is called a burette
To conduct a hardness water test
Take a water sample, and then titrate to determine total hardness
Low phosphate in boiler water May indicate
Hard water is entering the boiler
Electronic pH meter
Automatically measures the temperature of the sample, and adjusts the reading accordingly. Probes need to be recalibrated from time to time, using calibration Solutions.
Ph tests are commonly performed on
Condensate and boiler water
Excessively High condensate pH indicates
An overfeeding of neutralizing amines
Excessively low condensate pH indicates
An underfeed of neutralizing amines
Excessively High boiler water PH
Increases the potential for caustic embrittlement , cracking and Gouging. Reduce the feed of caustic soda, and if necessary increase the rate of continuous blowdown
Excessively low boiler water pH
Increases the likelihood of acidic corrosion. Increase the feed of sodium hydroxide to the boiler. If the total alkalinity is low, reduce continuous blowdown
For boiler water, it is more important to know the _______ than to determine the pH
Hydroxyl alkalinity
Total dissolved solids (TDS)
Measures all the salts in solution
The dissolved solids can be determined using a;
Conductivity meter. The concentration of dissolved solids can be determined by multiplying the liquid water conductivity by a constant Factor. As the concentration of dissolved solids increase, so does the conductivity
Manual conductivity measurement
Pour the water into the sample cup at the top of the metre, or submerge the top part of the metre directly into the water.
Excessively High total dissolved solids
Results in forming, priming and carry over. Increase the rate of continuous blowdown.
Excessively low total dissolved solids
Indicates excessive blow down or blow off. Reduce the amount of blow down and blow off
Molybdate salts are often used as
Corrosion Inhibitors in closed-loop heating and cooling water systems.
Alkalinity is a measure of a solution’s ability
To resist changes in PH
Alkalinity test
Performed on both Source water and boiler water. Source water is tested to determine the efficacy of the feed water treatment program. Boiler water test allows the operator to make the necessary adjustments and total alkalinity are in the prescribed range
Three types of alkalinity
Carbonate, bicarbonate, and hydroxyl
Two types of alkalinity tests
Phenolphthalein alkalinity; tests for hydroxides and carbonates
Methyl alkalinity (total alkalinity); tests for hydroxide, carbonate and bicarbonate
Excessively High total alkalinity
May result in foaming or carry over. Increase the rate of continuous blowdown and blowoff
Excessively low total alkalinity
Indicates excessive continuous blowdown or bottom blow off. Decrease the rate of continuous blowdown and blow off
Sodium sulfite
Eliminates the presence of dissolved oxygen. Usually added to the feed water system to protect the decorator, the feed water lines and pump, and the boiler
Excessively high sodium sulfite
May cause sludge and scale formation. Reduce the rate of sulfite addition to the feed water
Excessively low sodium sulfite
Will result in dissolved oxygen, pitting and corrosion. Increase the amount of sulfite fed to the feed water
Phosphate residual range
20 to 60 ppm
Excessively High phosphate
Will cause foaming and carry over. Reduce amount of phosphate fed to boiler or increase continuous blowdown and bottom blow off
Excessively low phosphate
Will result in scale formation. Phosphate feed rate should be increased
A small amount of carbon dioxide in contact with condensate can produce
Corrosive Solutions
Oxygen enters the boiler make up water or condensate through
Contact with air
When both carbon dioxide and oxygen corrosion occur in a system
The total combined corrosion rate is significantly higher
4 main ways of Preventing condensate line corrosion
Mechanical inspection/maintenance
Installation of dealkalization equipment
Neutralizing amines
Filming amines
Dealkalization Equipment
Used to treat the raw make up water, by reducing water alkalinity to zero. This will reduce the cost and complexity of condensate corrosion control.
Condensate polishers
Mixed bed ion exchangers, designed to purify condensate. They are used when condensate may become contaminated, they filter out particulate and suspended matter, and perform ion exchange to remove hardness and chlorides
Condensate polishers require
Regular back washing and regeneration
Condensate polishing is essential for
High pressure boilers, where feed water Purity is critical
For most purposes, most readings below ______ should be considered acceptable
0.5 ppm
Direct amine measurements can be made using a;
Chromatograph
Fluorometer
Device which measures and monitors the condensate amine concentration
Corrosion coupons
An inexpensive and simple way to evaluate corrosion rates. Only useful at estimating average corrosion rates, they do not provide instantaneous readings of system corrosiveness
Corrosion coupons are removed after
1 to 3 months
Sources of impurities in cooling water
3
Avian waste
Contact with the atmosphere
Raw makeup water
Water evaporates in cooling towers and leaves behind;
Dissolved solids
Since the operating temperatures are much lower in cooling towers, _______
Scaling is not as severe as boilers
Most cooling towers are open to atmosphere, so they are more susceptible to;
Airborne dust and debris, which can enter the water and eventually plug up the system
Biological fouling of recirculating cooling towers
Caused by algae, fungi, avian waste, and bacterial growth.
Biofilm or slime
Organic growths which can; reduce flow rates, release organic acids and waste products that can corrode metals, attack and Destroy wooden portions of a tower
Legionella bacteria
Bacteria that causes a serious lung infection, or a less serious flu-like infection. There is no vaccine for it, so prevention is the only option
Wood is composed of;
3
Cellulose, lignin, and natural extractives
Biological attack on cooling tower would occurs when
The cellulose deteriorates
The large white cloud seen above a power plant is often
Water vapour from cooling towers
Cooling tower Crystal modifiers
Promote the formation of soft sludge. When these are used, the cooling water should appear turbid, which indicates that they are taking effect
Cooling tower cycling control
Refers to the maintenance of the appropriate ratio of dissolved solids in the circulating water, to the dissolved solids in the makeup feed water
Cooling towers that operate at higher cycles of concentration
Require less blowdown
Two methods used for cycle control in cooling towers
Makeup proportional blowdown
Conductivity based blowdown
Sacrificial anodes
Used for corrosion control in cooling water systems. Directs corrosion to sites where it can be tolerated or accepted
Chemical inhibitors and three categories
The use of chemicals to form protective layers.
Cathodic, anodic, general
Chromates
While effective at corrosion control, they cause unacceptable environmental damage when released through blow down or system drainage. Therefore, it is no longer used in systems from which water can escape
Plugging or erosion of equipment
Screens and filters prevent the influx of solid matter and protect against plugging and erosion
Deposition occurs when the flow is
Too slow
Erosion occurs when the flow is
Too great
Two methods to protect against biological fouling
Mechanical methods, such as trash bars and strainers. Chemical methods that kill the microorganisms
Operators should be aware that biocides are
Poisonous
Differential microbial analysis (DMA)
Test to detect various microorganisms, conducted in an off-site Laboratory
Dip slides
Useful for determining Trends in bacterial activity. However, they do not provide precise measurements
Chillers
Heat exchangers that cool fluids. The process fluid is continuously circulated in a closed loop. The chiller may use refrigerant or some other cold process fluid to chill the water
Corrosion with Chillers
Corrosion rate is not as great, since they operate at much lower temperatures.
Galvanic corrosion
Occurs when dissimilar metals, are in physical contact with each other, and exposed to a fluid. This transfers electrons and metal ions between the two metals. The metal that gives off ions will waste away and fail.
Corrosion is likely to occur where ever
Deposits form
2 closed system corrosion inhibitors
Sodium nitrate, and Molybdate
Selective leaching, also called Dezincification
A form of corrosion in cooling systems. When this happens, one particular metal is preferentially attacked. It is usually zinc
Freeze stats
Used in Chillers, so that if refrigerant temperature gets too low, the chiller trips off
Sacrificial anodes can also be used to
Prevent galvanic corrosion
Borescopes
Inspect the inside of tubes and shells during shutdowns for signs of corrosion or deposition
Glycol
A colorless and odorless substance used mostly for manufacturing Plastics and antifreeze.
Two main types of glycol
Ethylene and propylene
Glycol reacts with oxygen to form
Organic acids that will increase corrosion, therefore they are equipped with Factory Blended corrosion inhibitors
Glycol concentrations should never be less than
20%
Glycol should not be used directly in a
Boiler or heater system
