By Maklub Al Mostofa
Scale formation and corrosion are the main two factors that determine the efficiency of the internal parts and as well as lifetime of the boiler. Badly corroded and scaled boiler can fail within very short time.
SCALE FORMATION IN BOILER
Depending upon the sources boiler water contains various types of salts and impurities. Under operating conditions all the salts comes out of the water. These salts cause formation of scale inside the boiler. The more the water contains solids and salts the more the boiler is prone to scale formation.
Scale formation can be prevented in two ways.
1. External treatment:
– Use as pure water as possible.
– Proper feed water treatment.
– Keep the cascade tank clean. Maintain the cascade tank filter and feed pump filters cleaned.
2. Internal treatment:
– Carry out partial blow down of the boiler regularly and effectively.
– Regularly test the boiler water to determine the dissolved solids and salts.
– Chemical dosing should be done as per the test result to keep the boiler scale free.
Significance of regular partial blow down:
1. As the temperature rises the chloride comes out of solution that raises the boiler chloride level. Regular partial blow down from the bottom helps to maintain the chloride level.
2. Due to operating conditions the water may become acidic. Partial blow down of the water may reduce chemical dosing.
3. Remove sludge or mud from the boiler that precipitates at the bottom of the boiler.
4. Reduce the dissolved solid.
5. Reduce floating particles by scum blow down.
6. Reduce the water level in the boiler to prevent carry over or priming.
What is coagulant?
BOILER COAGULANT is a liquid sludge conditioner designed to prevent the formation of solid and sticky deposits in boilers.
How does it work:
boiler coagulant is a physical dispersant product that prevents the formation of large particles in liquid. It functions by keeping solids as small particles and prevents agglomerations from forming.
Boiler coagulant is primarily used in conjunction with hardness / phosphate control. The solids can be removed by the blow down as usual.
In addition boiler coagulant can help to remove small amounts of oil contamination if it arises, by blow down. Oil contamination must of course be stopped if it has arisen.
Boiler coagulant prevents the formation of adherent deposits and sludge in boilers and thereby reduces clean-downs.
Normal dosage is 20ml daily/tone of boiler water capacity. Typically this equates to 0.1-0.3 liters/day. This is the recommended initial dose.
Boiler coagulant should be dosed directly to the boiler via the bypass pot feeder installed in the boiler feed water line.
Why the chloride level is always higher in the boiler than the condensate water in the cascade tank?
Answer: The water enters the boiler from the cascade tank will evaporate .Produced steam is very fine and will contain so chloride as it is heavier .So the chloride will remain in the water drum. As more feed water will enter the boiler the chlorides will be added .This process is continuous and if the chlorine id not blown down regularly it will go beyond limit though using distilled water.
The feed system is also prone to atmospheric contamination (sea environment is salty) at various points of the feed system.
HARD AND SOFT WATER:
Boiler feed water quality is most important factor that contribute the boiler efficiency. Poor control or treatment of the feed water could lead the boiler to be damaged within very short time.
Depending upon the purity boiler water be divided into two types
Hard water contains scale-forming impurities while soft water contains little or none.
Hardness is caused by the presence of the mineral salts of calcium and magnesium and it is these same minerals that encourage the formation of scale.
There are two common classifications of hardness:
• Alkaline hardness: it is also known as temporary hardness because the hardness is removed by boiling. Calcium and magnesium bicarbonates are responsible for alkaline hardness. The salts dissolve in water to form an alkaline solution. When heated up the salts decompose to release carbon dioxide and form soft scale or sludge.
Non-Alkaline Hardness Salts also known as permanent hardness salts are due to the presence of sulphates, chlorides, nitrates and silicates of calcium and magnesium. With the exception of silicates and the calcium sulphate, the permanent hardness salts are all very soluble in water and do not normally produce scale, but they are electrolytes and their presence, therefore, favours corrosion by galvanic action.
These salts precipitate out of solution as the temperature rises, and form hard scale, which is difficult to remove.
Silica can lead to hard scale and react with calcium and magnesium salts to form silicates which can severely resist heat transfer across the fire tubes and cause them to overheat.
What is dissolved oxygen?
Why it exists in boiler water?
Dissolved oxygen (DO) is the amount of oxygen (O2) dissolved in the water.
Oxygen enters the water at the surface of the water where exchange between the atmosphere and the water can take place.
The amount of dissolved oxygen that the water can hold depends on
– The temperature. DO reduce as the salinity increases.
– Salinity of the water. DO reduces as the temperature rises
How dissolved oxygen is removed where no mechanical de-aerators are used ?
The condenser is a heat exchanger which removes the latent heat from exhaust steam so that it condenses and can be pumped back into the boiler. This condensing should be achieved with the minimum of under-cooling, i.e. reduction of condensate temperature below the steam temperature. A condenser is also arranged so that gases and vapors from the condensing steam are removed
It is also done by chemical dosing.
Cascade tank also assists in removing oxygen as the temperature is maintained within 80-90 degree celciuous.
Why it dissolved oxygen is dangerous?
Boiler water ionizes into H+ and OH- ions. Dissimilarity in the ferrous metal forming the boiler surface will cause the surface to become partly positive and partly negative. The positive H+ ion attracted by negative cathodic area and by taking one electron it becomes H atom. While the negative OH- ion attracts positive anodic area and will produce ferrous hydroxide. If the oxygen is present it will react with ferrous hydroxide and corrode the boiler.
If the boiler water is properly alkalized and does not contain dissolves oxygen the H atoms will form a stable layer on the metal surface. In still water the ferrous hydroxide will remain insoluble and form a protective layer of magnetite in contact with the iron and no further corrosion will take place.
The main purpose of boiler and feed water treatment is to have a scale free and corrosion free boiler internals.
To prevent corrosion the boiler tubes needed to be passivated. Corrosion occurs when such passivated layer of magnetite becomes unstable or when chemicals are allowed to come between it and the base metal.
What is corrosion?
Oxidation: it is the chemical process by which metals gives up electrons.
Reduction: it is the gain of electrons in a chemical reaction. It is also known as redox. It is the opposite of oxidation.
Cathode: The cathode of a device is the terminal where current flows out..
Anode: An anode is the electrode in a polarized electrical device through which current flows in from an outside circuit.
IONS are the charged particles of substances. Ions are of two types
– Positive Ions
– Negative Ions
CHEMICAL COMPOSITION OF WATER:
pH elaboration is power of hydrogen. It is defined as the reciprocal logarithm of hydrogen ion concentration in water.
We know, water is the composition of hydrogen and oxygen. When it ionises splits into OH- & H+.
1liter water contains 10-14gm ions at 25 degree C. At neutral condition it contains 10-7gm H+& 10-7 gm OH- ions.
If OH- ion concentration increases it becomes alkaline.
If H+ ion concentration increases it becomes acidic :
The pH value is influenced by temperature.
pH value can be changed by chemical dosing.
Corrosion of boiler metal:
- It is an electrochemical reaction of iron where corrosion occurs at cathode as the boiler metal oxidizes and dissolves.
- This metal is extracted from the natural ore by reduction process in the refiner. This metal has natural tendency to go back to its natural state through oxidation and interaction with suitable environment. This natural tendency is the driving force of corrosion.
- It is the general tendency of metals to be oxidized. Corrosion is a reduction/redox reaction.
- Corrosion occurs at the anode, where metal oxidizes and dissolves.
- At cathode, reduction takes place
- Reaction at anode: At anode Iron is oxidized to Fe++ , ferrous ion , Fe = Fe2+ + 2e¯
- Reaction at cathode: O2 gets reduced to OH- ions at cathode, Fe2++ ions are combining with OH- ions to form Fe(OH)2, ferrous hydroxide, Fe2+ + 2OH¯ = Fe(OH)2.
If the boiler water is properly alkalized and does not contain dissolves oxygen the H atoms will form a stable layer on the metal surface. In still water the ferrous hydroxide will create a protective layer of magnetite and no further corrosion will take place.
What is magnetite layer in boiler?
Magnetite is an iron oxide, [Fe3O4]. It deposits in the form of thin layer on boiler steel surface and passivate the surface and thus it resists the influence of water and contaminants to further react with the steel material.
Magnetite is formed on clean, pickled steel by two reactions:
1. Electro chemical reaction that takes place as follows:
3Fe (OH)2 = Fe3 O4 + H2 + 2H2O
The iron hydroxide is initially produced by reaction between iron and water. The reactions start around 100C and increase as the temperature increases.
2. Hot oxidizing reaction when magnetite is formed directly at temperatures 300 C or approximately at 30 bar boiler pressure. The reaction follows this route:
3Fe + 4H2O ( 300C+) = Fe3 O4 + 4H2
Types of corrosion:
a. Galvanic corrosion
b. Acidic corrosion
c. Caustic corrosion
d. Hydrogen corrosion
e. Stress Corrosion
f. Corrosion fatigue
g. Pitting corrosion
We all know it needs two dissimilar materials to create a galvanic cell. Boiler tube material is steel. Then how the galvanic cell forms?
Boiler condenser tubes are made of copper and boiler tube material is made of steel. Copper may react with oxygen and may be carried as copper oxides inside the boiler. This two dissimilar materials are mainly responsible for galvanic corrosion.
Galvanic cells also form due to temperature difference, scales, salts, bacteria, oil contamination, conductivity, scratches in the material etc.
Acidic water has an excess of hydrogen ions which leads to hydrogen evolution. The Protective film of hydrogen gas on the cathodic surface breaks down as the hydrogen combines and bubbles off as diatomic hydrogen gas.
Acid corrosion may also occur due to heavy salt water contamination or by acids leaching into the system from demineralisation regeneration.
Oxygen: We know at cathode, Fe2++ ions are combining with OH- ions to form Fe(OH)2, ferrous hydroxide, Fe2+ + 2OH¯ = Fe(OH)2
If oxygen is present it will react with ferrous metal surface to form red iron oxide F2O3 result in pitting corrosion
The ferrous hydroxide then combines with oxygen and water to produce ferric hydroxide, Fe (OH) 3, 4 Fe (OH) 2 + O2 + 2 H2O –> 4 Fe (OH)3.
Ferric hydroxide dehydrates to rust,
F2O3, Fe (OH)3 ⇌ FeO(OH) + H2O . FeO (OH) ⇌ F2O3 + H2O.
Rust consists of hydrated iron (III) oxide, F2O3·nH2O and iron ( III) oxide – hydroxide ,(FeO(OH), Fe(OH)3).
CO2: React with water to form carbonic acid, which reduce the pH of the water and accelerate corrosion
Ammonia: Attack the copper base alloy in the present of oxygen
Hydrogen irons are generated by concentration of acid under a hard dense deposit. It can penetrate the grain boundary of tube metal and react with carbon and produces methane gas. This carbon loss weakens the tube metal and methane gas exerts a pressure which separates the grains of tube.
Hydrogen attack can also occur when hydrogen is released by caustic corrosion.
Caustic: Caustic is the other form of solid alkali added to the boiler
Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe3O4) layer.
This form of attack can take place at high pressures due to excessive concentrations of sodium hydroxide. The sodium hydroxide forms local concentrations nearly coming out of solution and forming thin film close to the heating surface. This breaks down the magnetite layer and then reacts with the steel to produce a soluble compound which then deposits on the surface in the form of a layer of loose porous oxide.
While high temperature surface suffers from poor circulation of water and the surface is under stress it may form a series of f line cracks in the wall. Corrosive conditions aggravate the condition.
It needs two factors to act together which leads to stress corrosion.
– Corrosive environment
Due to corrosive action protective layer may break down and form a local weak point. If this area is subjected to heavy alternating stress fatigue crack may result. The bare metal will then be subjected to further corrosive action causing the process to continue. Mechanical stress of boiler parts may be due to mal-operation of the boiler, raising steam too rapidly from cold, missing or poorly connected internal feed pipes, fluctuating feed temperature and steaming conditions.
Corrosion is mainly a local corrosion. It needs relatively a large cathodic area and a small anodic area. Hence the intensity of attack at the anode is high. Large area differences could be caused by mill scale, oxide films, acid pockets of water, scale from salts, pores or crevices, oils, gases and ingress of metals into the boiler. Corrosion rate increases with temperature, hence where metal surfaces are hottest failure may take place earlier.
What is caustic hide out?
Caustic is the other form of alkali added to the boiler
As the name implies the caustic present in the boiler water remains untraced during water test. Hence it is termed as caustic hide out.
Due to the rapid evaporation the sodium hydroxide forms local concentrations that come out of solution and forming thin film close to the heating surface.
However if the evaporation rate is reduced the hydroxide is released back into normal circulation and the alkalinity is apparently restored. This phenomenon is referred to a caustic hideout.
This breaks down the magnetite layer and then reacts with the steel to produce a soluble compound which then deposits on the surface in the form of a layer of loose porous oxide.
Q. When a steam boiler water tube is started leaking,
(a) How do you know?
(b) How will you check the leaking source?
(c) What are the remedies available?
(d) How will you repair to proceeds the rest of journey?
How to know:
1. As steam consumption is more feed pump will be continuous running.
2. Excessive feed water consumption from cascade tank or feed water tank.
3. If the large amount of leakage, boiler water level will be low, steam pressure will be drop and continuous firing of boiler.
4. Some water comes out from furnace cover.
5. White smoke emitting from boiler uptake.
The possible sources of the entry of water may be considered at
1. The leakage tubes.
2. Distorted furnace crown plate.
3. Furnace shell plate opposite of the burner opening due flame impingement.
4. The lower section plate of furnace due damage brick-works.
The possible causes of leakage:
1. External wastage: due to waterside corrosion and pitting. Corrosion and pitting may occur due to
– Poor quality feed water
– Ineffective treatment
2. Uneven thermal expansion: this can happen between tube and tube plate due to local overheating. Overheating could be due to low water condition, heavy scale, oily deposits or forcing of the boiler.
3. Deformation of tube plate: Under pressure would have the overheating effect that tube fail at the tube end of the tube plate.
For water tube boiler (Z boiler):
1. Stop the firing
– open up the combustion chamber,
– Fill up the boiler water level to full.
– leakage can seen easily be seen & identify the individually boiler water tube
For smoke tube boiler:
– Stop the firing
– open the smoke side drain valve,
– If there is any leakage water will come out.
Once tube leakage is confirmed steps will be taken to identify the leaking tube.
– open up the fire side cover
– fill up the boiler water level until all smoke tubes are flooded,
– We can easily to check which one is leaking ligaments.
Repairing could be carried out by inserting tube stopper or new tube renewal.
The defective expanded tube is found originally expanded and bell-mouthed at the tube ends. The first step is cropping at the ends about 50mm from the tube plate and chisel off. The remaining pieces are removed by chiseling and knocking out after heating and cooling to achieve shrinkage.
Then the tube holes to be cleaned and polished before dye penetrant test for any cracks. Minor blemishes at the tube hole are made good with light rolling by an expander.
The usual diametrical clearance between the tube and tube hole being about 1.5mm, this must be taken into account while renewal of this tube. The tube ends of the new tube are cleaned thoroughly and carefully roller expanded into the hole in the tube plate.
When the new tube is placed in the tube hole, they must project through the tube plate by at least 6mm. The bell mouthing is to be 1mm for every 25mm of outside diameter plus 1.5mm.