BOILER WATER

Typical Marine Boiler system Layout

Introduction

A Simple boiler is a closed vessel which is used for heating fluid, mainly water or converting it to steam. Steam thus produced can be used as a heating medium or as a working fluid in a prime mover, where it turns thermal energy to mechanical energy, which in turn may be converted to Electrical Energy.

                                                                           BACK TO SCHOOL

                                                       Q. What is a Prime Mover?

 A. A Prime Mover is an Engine which coverts fuel to useful work.

For Example, as we are dealing with steam, a Turbo Alternator is the right choice.  A Turbo Alternator is a machine which has an engine which is rotated by steam (A Turbine) i.e. Heat Energy Converted to Mechanical Energy and this Engine is coupled to an Alternator where this Mechanical Energy is Converted to Electrical Energy.

Now that we know what is a Boiler, so a Marine Boiler would be: A Boiler used for Marine Purpose.

In majority of the Boilers the primary source of fluid being circulated within the boiler walls is "Water".

As we know that Water is Universal solvent therefore it carrying impurities is inevitable.

So in this topic we are dealing with water as a primary constituent in Boilers working and its effects.

TERMINOLOGY USED WHEN DEALING WITH BOILER WATER SYSTEMS

1.  MAKE-UP WATER (MW)- The raw water, softened water or demineralized water required for steam generation i.e. water from our very own "DISTILL WATER TANK".

2.  CONDENSATE WATER (CW)- After the Heat transfer process of steam in heat exchangers or heating coils, the steam reverts back to its liquid phase which is termed as Condensate Water.

3.  BLOWDOWN WATER (BW)- The part of the water which is purely drained in order to remove impurities to an acceptable level.  Make up water commensurates for Blow down loss.

4.   FEED WATER (FW) - Feed water is the total of condensate water and make up water which is fed to the boiler for production of steam.

                                                      FW=MW+CW

5.  BOILER WATER (BW) - Water which is present in the steam generating section of the boiler i.e. the water drum is Boiler Water.

                                                         

Now that we have learnt the basic terminology about Boiler water, it is very important that we now revise the meaning of two very important terms "HARD WATER" and "SOFT WATER" before we proceed further.            

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HARD WATER

Hard Water contains an appreciable quantity of dissolved minerals.  Hardness is primarily composed of Calcium (Ca++) and Magnesium (Mg++) minerals.

Water Hardness is measured in grains per gallon or milligrams of calcium per litre or it can be expressed in ppm (parts per million).

Various water test kits are available to measure hardness.

ppm (parts per million) - ppm is usually defined as 1mg/litre.

SOFT WATER

Soft water is treated water in which the only cation (positively charged ion) is sodium.  Measuring unit is the same as that of Hardness of water.

  

Now that we have learnt the meaning of hard and soft water it is very important to for us to know what is  Alkalinity and Acidity.  

ALKALINITY

Alkalinity of water is a measure of how much acid it can neutralize to the equivalence point of carbonates or bicarbonates or it can also be defined as the measure of Bicarbonates (HCO3), Carbonates (CO3) and Hydroxyl Ions (OH) in water. 

Alkalinity is usually measured in mEq/l (milliequivalent per litre) or ppm.

Alkalinity is measured by a process known as TRITATION.

So what makes Alkalinity?

• Carbonate in presence of CO2
• Borate, Hydroxide, Phosphate, Silicate, Nitrate, Dissolved Ammonia, Bicarbonate 

Alkalinity is determined in 2 ways i.e.

• M Alkalinity or Total Alkalinity - This is defined as the total sum of carbonates, bicarbonates and hydroxides
• P Alkalinity or Phenopthelin -  This is defined as half of carbonates and hydroxides i.e. half of carbonate alkalinity and all of hydroxide alkalinity.

Now that we have learnt what is Alkalinity, lets take a look at Acidity.

But before we proceed into acidity it is necessary that we understand what is pH.

    BACK TO SCHOOL

pH is also referred as "the power of hydrogen".  Water,(H20) which is two hydrogen atoms bonded covalently to an oxygen atom.  In a water solution, water molecules would disassociate into the component ions the H+ ion and the OH-  ion.

When there are equal number of products and reactants, this situation is often referred to as an equilibrium reaction in chemistry and water is the best example of equilibrium reaction as water (H20) will contain H+ and OH- at all times.  It is because of this balance, pH is determined.

So when there are more H+ ions than OH-, the solution is referred to as Acidic and when there are more OH- ions than H+ the solution is referred to as Base.

pH scale ranges from  0 to 14 with 0 being most Acidic and 14 being most basic of alkaline.  Therefore 7 being the mid value simply illustrates value of neutral.  

So now it can be concluded that value less than 7 is acidic and value greater than 7 is more alkaline.

A change of 1 pH in pH scale represents a change of 10 times of relative alkalinity or acidity i.e. a pH of 4 is 10 times more acidic than a pH of 5.

Having learnt the basics of water lets now take a look at the problems and impurities associated with water.

• Total Suspended Solids (TSS) - The measure of particulate matter which is suspended in a sample of water or waste water.  Method for Measuring TSS - A known volume of water is filtered and dried and weighed to determine the residue.  TSS is measure in mg/L.
• Total Dissolved Solids (TDS) - This represents dissolved constituents, e.g. calcium, Chlorides, Sodium, etc.  It is measured in mg/L.
• Total Solids - It represents the sum of TSS and TDS.
• Turbidity - Finely suspended matter which does not settle and impart a muddy or cloudy appearance to water.
• Conductivity - is a measure of water's ability to conduct electricity in cooling water.  It indicates the amount of dissolved minerals in water.  It is measured in Micro ohms.
• Dissolved Gases - O2 and CO2 can be readily dissolved by H2O.
• Total Cations - Cations are positively charged metallic parts such as Mg++, Ca++, Na+, K+ etc and are attracted to cathode.
• Total Anions - Anions are negatively charge non-metallic parts such as alkalinity i.e bicarbonates, carbonates, chlorides, sulphates, etc and are attracted towards anode.
• Iron and Manganese - can exist in water as a dissolved cation
• Oil and Grease - can exist in water as an emulsion
• Silica - Normally exists in water as an anion or as a colloidal suspension.  Measured in mg/L or Sio2.

Now that we have learnt the problems/impurities associated with water lets take a look at how can we relate these problems with each other and what complications can happen because of their very existence in boiler.

• Hardness and Cations 

We have learnt that Cations Mg++ and Ca++ are the main constituents of Hardness.  Hardness is primarily responsible for scale formation.  Sum of Ca++ and Mg++ is called Total Hardness.

Total Hardness can be broken down into 2 categories:

1. Carbonate or Temporary Hardness - Calcium and Magnesium bicarbonates are responsible for Alkaline Hardness.  These salts when dissolved in water, form an alkaline solution.  When heat is supplied to this solution it decomposes to release CO2 and form a soft scale or sludge which is commonly termed as Calcium Carbonate Scale.  CO2 thus released, combines with the water to form Carbonic Acid which causes corrosion of the boiler internals.
2. Non Carbonate or permanent Hardness - This is also due to Ca and Mg salts, but in the form of sulphates and chlorides.  As the water temperature increases, solubility of these salts decreases and as a result they precipitate out of the solution and form hard scale which is hard to remove.

• Hardness and Silica

Silica in Boiler water can react with Calcium and Magnesium salts to form silicates which results in the formation Hard Scales.  These scales can inhibit proper heat transfer across the boiler tubes and can result in localized overheating.

• Conductivity and TDS

Greater the amount of minerals present in water more conductive it is.  Dissolved minerals can be Ca, Cl, Na, etc.  So greater the amount of TDS in water, greater will be the conductivity of water.

Lets make our life simpler

Lets learn the above mentioned relations this way:-

Ca(++)  +  Mg(++) = TOTAL HARDNESS

Ca bicarbonate + Mg bicarbonate = Alkaline Hardness

Alkaline Hardness + Water = Alkaline Solution

Alkaline Solution + Heat = CO2 + Calcium Carbonate Scale

CO2 + H2O = Carbonic Acid

Ca Sulphate or Mg Sulphate + Heat = Hard Scale

Having learnt problems, the next question that would certainly pop in our minds is "What are the consequences of these problems and impact on Boiler Systems".

There are three main problems which water can incur in Boiler systems.  These problems are:

1. SCALE formation
2. CORROSION
3. CARRY OVER

Lets take a look at these problems one by one

SCLAE formation 

As stated earlier Calcium, Magnesium and silica when heated, precipitate out to form a dense coating of minerals on the water side of the boiler.  This layer of coating is technically called Scale.

Scales can be very dense or very porous, can be loosely held or tenaciously bonded to the surface.

Consequences of Scale formation

• Scales have a thermal conductivity of an order of magnitude less than the corresponding value of bare metal.  As a result even a thin layer of scale acts as an insulator and restricts heat transfer
• Scale deposits progressively narrows pipe internal diameter, roughen tube surface and restricts water flow
• Scale deposits are responsible for the formation of hot spots, thus causing the metal temperature to rise, which eventually causes the flue gas temperature to rise.  If this condition were to be allowed to prolong for a considerable period of time, then this would cause the tubes to eventually rupture due to overheating.
• Scales not only cause the tubes to fail but also reduces the boiler efficiency, causing an increased fuel consumption

Lets now take a look at the principle factors which contribute to the formation of scales 

• As mentioned earlier Ca, Mg and silica are the primary factors
• High alkalinity of water 
• High operating temperatures
• High concentrations of TDS
• In sufficient Blowdowns
• Low condensate recirculation
• Other impurities such as iron. copper, oil, grease, etc

So now at this point we can easily conclude that governing factor for scale formation is hardness of water.  Higher the hardness, greater the amount of minerals present, more is the probability of scale formation or in simpler terms higher the level of scale forming salts more are the chances of scale formation.

  

Lets find out the symptoms that prove the presence of Scale in Boiler

• Flue gas Temperature - A trend showing an increase in flue gas temperature over a period of time depicts the presence of scale
• Visual inspection - Scales can be visually found whenever boiler is inspected for maintenance (Boiler inspections are to comply safety precautions at all times)
• Fuel Consumption - If boiler is consuming more fuel than before (taking into consideration steam consumption being the same), then it would depict the presence of scale

We now know what is scale, factors that lead to the formation of scales and symptoms of scale.  What remains is how to prevent scale formation?

Steps to Control Scale formation

• External Pretreatment - of boiler make up water to remove hardness/scale forming salts or minerals.  This is achieved by the use of water softeners, dealkalizers, demineralizers, reverse osmosis, flitration and clarification.
• Internal treatment -  Carbonate/Phosphate treatment of boiler water helps keep the scale forming minerals/salts in precipitated form, which is then attached to a polymer molecule, which accumulates to the bottom of boiler as sludge and can be removed by blowdown.
• Blow Down practises - Blowing down boiler water helps limit the concentration of scale forming minerals/salts
• Prevention of Condensate Loss - Condensate water is very pure and free from hardness causing minerals.  Make up water on the other hand has impurities.  As learnt before Boiler Feed Water is the sum of make up water and condensate water.  So lesser the amount of make up water we add lesser the quantity of impurities being introduced to the system.  To get maximum condensate back in the hotwell it is very important that we keep the steam system free from leakages and keep dump condensor in good working condition.

Getting into the Chemistry of chemical Treatment

As we have learnt that internal chemical treatment is done in order to remove hardness of water.
There are two methods of Internal Chemical Treatment.

1. Carbonate Cycle - Recommended for systems which are operating at a pressure of 9.6 Bar or less.  This cycle is based on Precipitating  the hardness to form carbonates.  Once hardness precipitate is formed, it is conditioned by the use of synthetic or natural polymers. In this cycle a physical reaction occurs between the calcium carbonate and the polymer, where calcium carbonate attaches to the polymer molecule and drops to the bottom of the boiler as fluid sludge.  These molecules can be removed by blowdown.
2. Phosphate Cycle - This is based on precipitating the calcium and magnesium hardness with phosphate. Once Hardness/phosphate precipitate is formed, it is conditioned by the use of synthetic or natural polymer.  A physical reaction occurs between the calcium phosphate and the polymer, where calcium phosphate attaches to the polymer molecule and drops to the bottom of the boiler as fluid sludge, from where it can be removed by blowdown.  It is important that a pH of above 9.5 is maintained as this will ensure a proper reaction between Ca and Mg ions in the phosphate cycle.  Phosphate cycle however is disadvantageous at the same time as Magnesium phosphates formed are very sticky sludge and frequently this stick sludge attaches it self to the boiler surfaces and as a result hinders heat transfer.

Now we know Carbonate and Phosphate Cycle but what's with these Conditioners?

As stated above once the precipitate is formed a conditioner/softner is to be used.  As the precipitates so formed are quite sticky, they may result in the formation of deposits especially with phosphate cycle.  However these deposits are not restricted to phosphate cycles. Many a times such sludge/deposits are found in carbonate cycles also. So in order to transform these deposits into a bulky transportable sludge, instead of deposits, these scale conditioners are used.

These scale conditioners physically bond with calcium carbonate/phosphate precipitate.  Most common Conditioners are sodium polycrylate, lignin, sodium polymethacrylate, and suffonated copolymers.

As metioned many times in this blog, an effective blow down schedule is critical for proper working of boiler.  

Corrosion

Corrosion is deterioration of materials by chemical interaction with their environment.  The most common source of corrosion in boiler is dissolved gases i.e. oxygen, carbon dioxide, etc.  Of all these gases oxygen is most aggressive.

Consequences of Corrosion

• Corrosion in boilers if not controlled can lead to devastating results.  
• Corrosion can cause complete failure of equipment which can have a catastrophic result, and include loss of life and property
• It reduces the strength of materials and as a result reduces their working life which leads to premature failures.
• Unexpected shut down of plants which further influences the operation and economic drive

Now that we can see corrosion can have devastating results and as a result the study and control of same is prudent to achieve proper and efficient operation of our marine boiler.

Lets take a look at the principle factors which contribute to the formation of corrosion

• Oxygen and other dissolved gases - The most common source of corrosion in boiler is dissolved gases i.e. oxygen, carbon dioxide, etc.  Of all these gases oxygen is most aggressive.  The main sources for introduction of oxygen in boiler are :

1. Make up water
2. Condensate return system
3. Direct air leakage on suction side of pumps
4. For systems under vacuum, the breathing action of condensate receiving tanks
5. Open condensate receiving tanks
6. Leakage of non-deaerated water used for condensate pump seal cooling

We have earlier studied how carbon dioxide is introduced in boiler due to the decomposition of alkaline solution upon heating.  (Recalling - Alkaline solution is calcium and Magnesium bicarbonates which are responsible for alkaline hardness when dissolved in water).  CO2 so released combines with water to form carbonic acid, which causes corrosion of boiler internals.

• Dissolved or Suspended Solids - (Recalling) - " Greater the amount of minerals present in water more conductive it is" and as a result this would accelerate the chemical reaction pertaining to corrosion.
• Acidity and Alkalinity i.e. very high pH - A boiler may be subjected to an Acidic Attack or a Caustic Attack depending on the pH which is the determining factor of this case.

So what is Acidic Attack and Caustic Attack?

1. Acidic Attack -  If the pH has significantly dropped below 8.5 then a phenomenon called "water side thinning" may occur.  Water side thinning means thinning of water side walls due to acidic attack.  This attack is mainly in the stress regions of boiler i.e maximum thinning occurs along the side of the tube towards flame.
2. Caustic Attack - Before we get into Caustic attack it is important for us to understand what is "magnetite".

     

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Magnetite is a natural protective film formed on boiler surfaces as it resists the influence of water and contaminants to further react with steel material.  Magnetite has an excellent thermal conductivity i.e. heat transfer efficiency is promoted by Magnetite.  Please note that and always remember Magnetite film formation is a natural process and normal treatment chemicals have absolutely nothing to do with the formation, improvement or retardation of pure magnetite under normal operating conditions.

So, Caustic attack is stripping of this Magnetite film which normally occurs at a very high pH of 12.9.  Caustic Corrosion is mainly found in Phosphate treated boilers. 

• Velocity - High fluid velocities increase corrosion by transporting oxygen to the metal at a faster rate and carrying products of corrosion again at a faster rate.  However on the contrary, if the water velocity is low, then deposition of suspended solids may occur, establishing localized corrosion cells, thereby increasing corrosive rates.
• Temperature - Oxygen is the dominant factor in corrosion.  Now the primary source for introduction of O2 in boiler is from make up water.  It is always important to keep the make up water (water in feed water tank) at at least 80 Deg c. as oxygen content of water reduces with rising temperature.  Thus greater the temperature less oxygen will be introduced in boiler and hence less corrosion.  Also a very high temperature of make up water (feed water tank) would cause frequent failure of boiler feed water pump mechanical seal damage and demand frequent overhauling, also in many cases pump tends to lose suction.  Also elevated temperature in boilers is not good as high temperature itself does not cause corrosion but accelerates the rate of corrosion.  This means high temperature provides that driving force which accelerates the reaction which causes corrosion and as a result even small quantities of dissolved oxygen can cause serious corrosion.

Lets find out the symptoms that prove the presence of Scale in Boiler

• Most common type of boiler corrosion is Pitting attack. This type of corrosion causes small but deep pinpoint holes that eventually penetrate boiler tube walls and cause their failure
• Another type is general attack where corrosion is uniformly distributed over the metal surface throughout the boiler system network.

All this is inspected by visual inspection of boiler and other boiler water tests help to determine the condition of boiler water which will be explained later in this topic.

Materials which are susceptible to corrosion

• Carbon Steel - Carbon Steel is the primary metal which is used in boiler construction and is highly susceptible to corrosion.
• Iron - Iron is carried into the boiler in various forms of chemical composition and physical state.  Most of the iron found in the boiler mainly enters as iron oxide or hydroxide.  Any soluble iron in feed water is converted to the insoluble hydroxide when exposed to high alkalinity and temperature in boiler.

These iron compounds are of 2 types i.e.

1. Hematite or Red Oxide (Fe2O3) - Hematite exists in the condensate system or when the boiler is out of service
2. Magnetite or Black Magnetic Oxide (Fe3O4) - Magnetite as stated earlier are formed by natural process and exist in an operating boiler

Steps to Control Corrosion

Now that we have learnt O2 is the determining factor in corrosion together with CO2, it is important that entry of Oxygen and Carbon dioxide in boiler is prevented or reduced to a minimum.  This is achieved by adopting the following methods:-

• As stated earlier "oxygen content of water reduces with rising temperature" therefore lower the feed water temperature, larger the volume of dissolved oxygen present.  This makes it important to heat the feed water to reduce the O2 content
• Oxygen can also be reduced/removed by mechanical means i.e use of Deareators and vacuum degasifiers.  Deaerators not only help remove oxygen, carbon dioxide and other non-condensable gases from feed and make up water but also return condensate to an optimum temperature for minimizing solubility of the undesirable gases and providing the best and optimum water temperature for injection into the boiler
• Any remaining oxygen can be dealt with the use of chemical oxygen scavenger such as catalyzed sodium sulphite
• CO2   is normally controlled by an aerator or decarbonator.  Also chemical inhibitors in the form of neutralizing amines are generally used.

Getting into the chemistry of chemical treatment

Inhibitors used for O2-  Sodium sulfite is normally used for the treatment of dissolved oxygen.  Most of the oxygen scavengers contain a catalyist which speeds up the reaction of sulfite with O2.  In systems with de-aerator sulfites are fed to the storage tank of the de-aerator or to the suction or discharge side of feed water pump.  In systems without de-aerator sodium sulfites can be fed at almost any point in the feed water system, including the condensate tank or feed water tank.
Other common chemicals other than sulfites/bisulfites are Hydrazine, Carbohydrazine and Organic based Oxygen Scavenger.

Inhibitors used for CO2 - Amines are normally used for the treatment of CO2.  Amine refer to any number of chemicals derived from ammonia.  There are 2 types of Amines in practise which are

1. Neutralizing Amines - which neutralize the acid formed by Carbon Dioxide 
2. Filming Amines - which form a protective film in the metal

CARRYOVER

Carryover simply means any contaminant that leaves the boiler with steam.  Carryover can be 

• Solid
• Liquid
• Vapour

Consequences of Carryover

• Deposits in Non-return valves located in the boiler system
• Deposits in superheater (if provided)
• Deposits in control valves
• Deposits on turbines
• Deposits in heat exchangers

Lets take a look at the principle factors which contribute to Carryover

1. Mechanical factors

• Priming
• Sudden Load changes
• Boiler Design
• Soot Blowing
• HIgh water Level

     2.  Chemical Factors

• High Chlorides
• High TDS
• High Alkalinity
• Suspended Solids
• Oil
• Silica

Now that we have seen the various problems associated with boiler water lets take a look at the main purpose of BOILER WATER TREATMENT.

Boiler water treatment is done to:-

1. Eliminate the total hardness of the boiler water
2. Maintain correct pH and alkalinity values in feed water and boiler water
3. Prevent Corrosion, especially that caused by O2
4. Prevent formation of scale, by conditioning of sludge
5. Avoid Foaming

Now what's Foaming?

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There is a limit on maximum amount of salinity which a boiler water can sustain.  If this value exceeds than normal then there is a risk of formation of larger bubbles.  Larger the bubbles produced greater the turbulence on water surface and this causes foaming.  This foam may be carried with steam and can cause deposits.

Different types of tests are performed on board ships to determine the condition of boiler water and accordingly estimate the dosage which is to be done.  Lets take a look at a few of the parameters and their permissible limits

Tests for a Low Pressure Boiler 

P alkalinity - Recommended Limit - 100-300 ppm CaCo3

Chlorides - 200 ppm max.as cl.(For Boiler water)

Chlorides - 20 ppm max as cl (For Condensate water)

pH - 9.5-11.0 (For Boiler water)

pH - 8.3-9.0 (For Condensate water)

So what do if any of the values are above or lower than permissible limits 

If Chlorides are high - Make Blowdown

If Alkalinity if High - Make Blowdown

If Alkalinity is low - Add relevant Chemicals

pH is High - Make Blowdown

pH is low - Add relevant Chemicals

Test for Medium Pressure Boilers (31-60 Bar)

P alkalinity - Recommended Limit - 100-130 ppm CaCo3

M alkalinity - Recommended Limit - Below 2x P alkalinity

Phosphate - 20-40 ppm as PO4

Hydrazine - 0.03-0.15 ppm N2H4

Chlorides - less than 30 ppm

pH - 9.5-11.0 (For Boiler water)

pH - 8.3-9.0 (For Condensate water)

Whats New?

Hydrazine test.  Hydrazine scavenges and removes oxygen from condensate feed water and boiler water.

Hydrazine is a colourless liquid at ambient temperatures, completely misceble with water.
Hydrazine reacts with oxygen which results in the formation of Nitrogen and Water.  No solid is added to the boiler system.
If Hydrazine is over dozed, then at temperatures above 270 deg c it starts to breakdown, creating free ammonia.  Excessive free ammonia and oxygen can combine and form a corrosive condition on non-ferrous metals. 

Have a look at this video for clearer understanding
This Video shows Alkalinity test Method

This Video shows m Alkalinity Test method

Looking forward to your esteemed comments and feed back.