FUEL PROPERTIES

 


      FUEL PROPERTIES

Every engine is run by FUEL. The word "FUEL",in a dictionary would mean "Material such as coal, gas, or oil that is burned to produce heat or power" but in the real world i would define FUEL as "any solid, liquid or gas which has made a superabundant impact on the world's economy or in other words it is the driver of world's economy and is the prime driving source for every mode of transportation, production, communications, etc". In simple terms FUEL runs the world we live in or we can't live without fuel.

Now as Marine professionals what impact has fuel made on us?
Well the answer is simple.

Fuels run our ship i.e. fuel is the prime source for our ship's propulsion and electricity. Without fuel on board, ship is a Dead ship.

So what's all the fuss with fuel?

Unlike water as discussed in section "BOILER WATER", fuel too is full of impurities.

Before we proceed further it is important for us to know where we get our ship's fuel. To understand this let's go Back to School.

BACK TO SCHOOL

Formation of fuel is like a fairy tale which started millions and millions of years ago, where there lived tiny marine organisms who died and decomposed in the bottom of the ocean to form oil and natural gas, which drives our world today. A simple pictorial representation below will explain the entire process of formation of oil and natural and natural gas.

Now that we know from where this oil or crude oil is coming from, let's find out how and where our Marine Fuels come from.
Once this Crude oil is extracted from the under the earth, it is transported to a Refinery where it is subjected to the "REFINING PROCESS".

 Lets now again have a look at a simple Refining process picture which explains it all.

                                                                                                                                        Source: eetindia    

A brief description of how Refining Process works.

Refining process is a compilation of following methods:

• CRACKING - Breaking large hydrocarbons into smaller pieces which in turn is of 2 types i.e THERMAL and CATALYTIC 
• UNIFICATION (Done by a process called CATALYTIC REFORMING) - Combining smaller pieces to larger ones 
• ALTERATION (Done by a process called ALKYLATION) - Rearranging various pieces to form desired hydrocarbons 

Now that we have understood the Refining Process, lets now take a look at the types of Marine Fuel used on board ships.


CLASSIFICATION OF FUEL FOR MARITIME USE

• HFO or MFO (HEAVY FUEL OIL) or (MARINE FUEL OIL) - This type of fuel is purely Residual oil. 
• IFO (INTERMEDIATE FUEL OIL) - A blend of Heavy fuel oil and gas oil, with more of marine diesel oil and less of gas oil. IFO380 or IFO180 fuels are generally used on board ships. 380 and 180 represents the fuel viscosity. Another type of fuel which now recently has gained a lot of importance in the maritime industry is LS380 or LS180 which is Low Sulphur fuel. 
• MDO (MARINE DIESEL OIL) - A blend of heavy Diesel oil which may contain small amounts of black refinery products, but has a low viscosity and as a result does not require heating and can be used in internal combustion engines 
• MGO (MARINE GAS OIL) - is pure gas oil made from distillate only 

Fuel properties define engine's performance, reliability, efficiency, life, and TBOs (Time between Overhauls) and fuel properties at the same time illustrate the engines impact on environment.

Lets try to understand the concept this way,

To run an engine we need to burn fuel. When we burn fuel,certain ash formation and smoke is inevitable. Now that we are burning residual fuels, formation of deposits, harmful smoke is quite natural. At this point, am sure your mind will pop with this question "Why is maritime industry not making use of fuels which are formed higher in the refinery process?" 

The answer is simple. Residual fuels are very cheap compared to other distillate fuels. Also if Residual fuels were to be used in our cars, we could expect them to be very heavy and quite huge as to burn and use HFO, as fuel requires a heating plant.





So who defines the minimum standards for fuel which can be used on board?

As residual fuels are quite full of impurities, refining them to a certain level is very important, so that they give a good combustion effect and at the same time minimise effects on the enviornment. Now all this is pre- defined and regulated. Fuel on board needs to have a certain minimum specification and ISO governs these standards.

What is ISO?

ISO is International organization for standardization and is the world's largest developer of voluntary International standards. These standards help give state of the art specifications for products, services and good practice, and this helps make any industry more efficient and effective.

For any further information about ISO you can visit their website ISO .


Now that we know what ISO lets take a look at which particular section of ISO deals with marine fuels.

ISO has developed over 19000 international standards and all these standards are included in the ISO Standards Catalogue. There are three ways to find our standard i.e Standard on Marine fuels

1. Browsing by International Classification for standards (ICS)
2. Browsing by Technical committees (TC)
3. Search the standards catalogue using the number of standard i.e. all ISO standards are numbered 

Finally, our standard for Marine Fuel is found. It's ISO 8217, where the number 8217 simply states the standard number in the standards catalogue.

Apart from fuel specifications, there are many ISO standards which outline minimum standards which must be adopted at the time of fuel testing.

Below table illustrates the fuel properties & test methods as outlined in the various ISO standards. (This table is what describes Fuel Analysis report)

Table below is for Fuel Oil.


                                                                                                                                        Source - Chevron


Table below is for Diesel Oil




                                                                                                                                        Source - Chevron


All other parameters seem quite familier but what's with these abbrevations RMA, RMD, RME, RMB, DMX, DMA, DMZ, DMB etc?

Like MAN B&W engine designations we have Fuel designations which tell us a lot about the fuel.

Category of fuel consists of these three letters

1. The first letter of this category is always the family letter which is "D" or "R".  "D" is for Distillate and "R" is for Residual
2. The second letter, "M" is for the application which is Marine
3. The third letter X,A,B,C,....K, indicates a particular property as specified in the product specification of ISO8217

Let's now take a look at fuel properties one by as mentioned in the fuel report and its impact on engine:

• VISCOSITY -  The recommended viscosity range at engine inlet is 13-17 cSt (mm2/s).  The preheating temperature can be estimated from the approximate viscosity vs. temperature chart which can be found in Instructions manual.  For a standard 380 cSt fuel (at 50 deg c), the fuel must be preheated to about 130 deg c.  Viscosity cannot be considered a quality criterion and is stated only for handling purposes i.e. how would pumps, preheaters and centrifuges behave.
• Density at 15 deg c - Density is related to fuel quality.  As stated above, we have seen that fuels are derived from extensive refinery process which imparts large amounts of carbon content, fuels become more aromatic and thus heavier.  This means that fuels with higher density are high in carbon residue and asphaltenes.  Density is normally measured at higher temperatures, and the density at 15 deg c is calculated on the basis of tables which, depending on their origin, date of issue, and the data on which they are based.   

The next question which might crop in your mind now is what's with this 15 deg c?  Answer is simple, value stated for density is at 15 deg c.  Now you may wonder why 15 deg c, why not 20 deg c?  Answer to this would be it was AMERICAN PETROLEUM INSTITUTE(API) long time back, which had set a standard for measuring density at 60 degree fahrenheit or 15.6 degree celsius.  So next time if surveyor asks you why its 15 degrees you know the answer.

Density also governs the water separation ability of the fuel i.e. we adjust gravity disks on purifiers as per density. 

• CCAI (CALCULATED CARBON CONTENT AROMATICITY INDEX) and CETANE INDEX - Gives a value on Ignition quality of residual fuels.  CCAI gives us an idea about how much is the ignition delayed during a combustion process i.e. greater the CCAI index, longer will be the ignition delay.  During the combustion process there is a delay i.e. once the fuel is injected in the cylinder it takes a while to get ignited . (This topic will be dealt with in details under section INDICATOR DIAGRAMS later only on nalinbaijal.blogspot.com)  This delay however is controlled, but if the delay prolongs then a large amount of fuel is injected before the combustion starts, producing a quick and a violent raise of pressure.  This produces what is called "DIESEL KNOCK".  CCAI is calculated from density and viscosity of the fuel.       

CETANE INDEX - Cetane Index acts as a substitute to Cetane number for diesel fuel.  Cetane number is a measure of diesel fuel ignition delay which can also be defined as the period between the start of injection of fuel in the cylinder and the first identifiable pressure increase during combustion of the fuel.  Higher the Cetane number, lesser is the ignition delay and lower the Cetane number higher is the ignition delay.

It is important that engine is run within specified CCAI or CI limits or other wise stresses on engine components might increase considerably and special attention needs to be paid to the following engine components:

1. Connecting rod big-end bearing shells
2. Piston, piston rings and liners
3. Main bearing shells
4. Cylinder head with studs and gaskets
5. Tie Bolts
6. Intake and Exhaust valves

To mitigate the effects of ignition delay or out of specification CCAI and CI, following steps are to be taken

1. Keep the engine load within 50-85%
2. Maintain inlet air temperature as high as possible
3. Lubricating oil must be in excellent working condition as there is a possibility of compromising oil's property due to blow by (higher combustion pressures)

• SULPHUR - Sulphur content of fuel oil or diesel oil has zero impact on combustion process, but still it is one of the most important parameters in the above table. 

Before we proceed further at this point it becomes important for us to understand the meaning of the term SOX.  

SOX is used to indicate the general oxides of Sulphur (SO2, SO3, etc). 

In diesel engines, fuel is injected in cylinder in which air is at very high pressure due to compression by moving piston. This compression raises the temperature of the air sufficiently to cause the fuel to ignite. Combustion proceeds around the periphery of the fuel spray at temperatures around 2000°C.

Oxides of sulphur are formed during the combustion process, by combination of the sulphur in the fuel with oxygen. The prime constituent of SOx is SO2. The amount of SOx formed in an engine depends primarily on the concentration of sulphur in the fuel. SOx emissions from ship engines are relatively high because they burn high sulphur content fuels. This Sulphur dioxide then rises into the atmosphere and is oxidized once again in the presence of atmospheric hydroxyl radicals to form sulphur trioxide (SO3). Sulphur trioxide reacts with atmospheric water droplets or vapors to form sulphuric acid (H2SO4) and eventually results in Acid Rain.

Recently many engine modifications and numerous law modifications were made and are still being formulated to help prevent our environment and protect human lives from impact of emissions.

Sulphuric acid can also be formed during combustion and its effects are counteracted by adequate lube oils and temperature control of the combustion chamber walls.

What does IMO say about sulphur?

In 2008 the revised Annex VI to Marpol 73/78 was adopted and required the sulphur content of any fuel used on board ships not to exceed :

• 4.50% m/m prior to 1 January 2012

• 3.50% m/m on and after 1 January 2012

• 0.50% m/m on and after 1 January 2020 or 2025,

depending the outcome of a review to be completed by 2018 to determine availability of fuel oil to comply

with the fuel oil standard. Additionally, the revised Annex VI to Marpol 73/78 restricts the sulphur content of fuel oil used on board ships operating within an Emission Control Area (ECA) to :

• 1.00% m/m on and after 1 July 2010

• 0.10% m/m on and after 1 January 2015.

Annex VI to Marpol 73/78 allows for alternative technologies/methods which are at least as effective in terms of emissions reductions. Currently adopted ECA areas are the Baltic Sea, North Sea and English Channel, the U.S. Caribbean ECA (including designated waters adjacent to Puerto Rico and the US Virgin Islands) and the North American ECA (including waters adjacent to the Pacific Coast, the Atlantic/Gulf Coast and the eight main Hawaiian Islands, extending up to 200 nautical miles from coasts of the United States, Canada and the French territories). In addition, the EU directive 2005/33/EC extended the           1.5 m/m % S limit to ferries operating to and from any EU port.

The EU directive also has set a maximum limit of 0.1 m/m % on the sulphur content of marine fuels used by ships when at berth for more than 2 hours. The process to review the EU Directive 2005/33/EC started in 2011. Therefore, the limits and requirements stated above are subject to change. In California, the Ocean Going Vessels (OGV) Clean Fuel regulation applies to OGV main diesel engines, auxiliary diesel engines and auxiliary boilers, and requires:

1) the use of marine diesel DMB:

— at or below 0.5 m/m % sulphur

— at or below 0.1 m/m % sulphur as of January 1, 2014; or

2) the use of marine gasoil (DMA/DMZ):

— at or below 1.5 m/m% sulphur prior August 1, 2012

— at or below 1.0 m/m% sulphur on and after August 1, 2012

— at or below 0.1 m/m% sulphur on and after January 1, 2014.

• FLASH POINT - Flashpoint refers to the lowest temperature at which a fuel can vaporise to form an ignitable mixture in air. Again Flash point has nothing to do with Combustion of fuel.  It is a legal requirement with regard to storage of fuel and acts as a safeguard against fire only.  
• HYDROGEN SULPHIDE - Fuel can also contain H2S (Hydrogen Sulphide), but in varying concentrations depending on how it is manufactured. H2S gas is pungent, colourless, highly toxic and flammable. Exposure to high levels of H2S gas can be fatal and inhalation can result in loss of life. Although the toxicity of H2S gas remains the primary hazard, lesser risks in longer term may include corrosion within bunker tanks and pipelines, and may cause damage to other system components.
• ACID NUMBER - Acid Number (AN) has been included in the standards to take into account any potential damage to marine diesel engines (primarily fuel injection equipment) due to acidic nature of fuel. Testing for AN can give indication of presence of acidic compounds.  It should always be kept in mind in the event of AN exceeding limits, it may still be fit for purpose depending on nature of acid
• TOTAL SEDIMENT AGED and TOTAL SEDIMENT HOT FILTRATION - Inorganic material naturally occurring in crude oil is removed in the refinery's distillation. Some minor contamination (for example, iron oxides) of a finished heavy fuel can not be excluded. The biggest risk for sediment formation in heavy fuel is due to potential coagulation of organic material inherent to the fuel itself.  The total sediment aged is the total amount of sediment that can be formed under normal storage conditions, excluding external influences. If the total sediment aged of the heavy fuel oil markedly exceeds the specification value (0.10% m/m maximum) for all grades of IFOs and HFOs), problems with the fuel cleaning system can occur, fuel filters can get plugged and combustion can become erratic. The total sediment by hot filtration is measured on all DMB category products that fail the visual inspection which requires the sample to be bright and clear. Organic type sediment can occur in DMB marine diesel and in intermediate fuel oils. The cause of the formation of organic sediment is due to the thermal cracking of the heaviest molecules of crude. Asphaltenes, the heaviest molecules of crude, can be made unstable by thermal cracking, and therefore must be carefully monitored by the refineries. The asphaltene sediment formation is a function of time and temperature (excluding external influences), and an unstable fuel will only reach its final sediment formation after a certain storage time. The sediment present in a sample of heavy fuel at a particular moment if is given by the total sediment by hot filtration test, than there is no certainty that this figure corresponds to the condition of the bulk of the fuel at that same time. The total sediment aged test shows the total amount of sediment that can be formed under normal storage conditions, excluding external influences. 
• CARBON RESIDUE, MICRO -  Carbon residue is determined by a laboratory test which is performed under specified reduced air supply. It has nothing to do with combustion conditions in an engine. It gives an indication of the amount of hydrocarbons present in the fuel which have difficult combustion characteristics. In Micro carbon residue method  a weighed quantity of sample is placed in a glass vial and heated to 500℃ under an inert (nitrogen) atmosphere in a controlled manner for a specific time. The sample undergoes coking reactions and volatiles formed are swept away and reported as a percent of the original sample as “carbon residue (micro).” Micro Method offers advantages of better control of test conditions, smaller samples, and less operator attention.  Micro Carbon Residue method gives a measure of the tendency of the fuel to form carbon deposits.  This test is more advantageous than Conradson Carbon Residue Test and as a result Micro Carbon Residue method is mainly adopted.
• POUR POINT and CLOUD POINT- Pour point is the lowest temperature at which a fuel will continue to flow when it is cooled under specified standard conditions. This property is used in determining at what minimum temperature fuel should be stored and pumped.  Temperatures below Pour Point can result in wax formations.  Cloud Point is for distillate fuels(diesel) and is the measure of temperature at which clear distillate fuel becomes cloudy due to the formation of wax crystals.  Compliance with this parameter ensures that the fuel is suitable for use in ambient temperatures down to -15 deg c without heating the fuel.
• WATER - Water in fuel is a contaminant and is not a good sign. The percentage of water in the fuel can be translated into a corresponding energy loss . Water is removed onboard the vessel by centrifugal purification. If after purification, the water content remains too high, water vapor lock can occur and pumps can cut out. If water-contaminated fuel reaches the injectors, combustion can be erratic. Water in fuel that remains standing in lines for a longer period can cause corrosion.  If fuel is contaminated by sea water than salt in fuel amy cause sodium deposits on valves and turbochargers.  If water cannot be removed by centrifuging than homogenising is recommended.
• ASH - The ash content is a measure of the metals present in the fuel, either as inherent to the fuel or as contamination.  Part of ASH could be catalytic fines. Heavy cycle oil is used worldwide in complex refining as a blending component for heavy fuel. Mechanically damaged catalyst particles (aluminum silicate) cannot be removed completely in a cost-effective way, and are found in blended heavy fuel. Fuel treatment onboard ships has a removal efficiency of approximately 80% for catalytic fines which can cause abrasive wear of fuel pumps, injectors and cylinder liners.  Also placing a fine filter after the centrifuge can prove quite effective.
• VANADIUM and SODIUM - Vanadium is bound in chemical complexes in the fuel and as a result it cannot be removed.  Vanadium deposits are very hard and may cause extensive damage to turbocharger nozzle ring and turbine wheel.  The only way to remove vanadium deposits is to disassemble the components and erase the deposits mechanically.  Sodium as mentioned above is present in fuel as salt or sea water contamination and can be removed by centrifuging.  Vanadium and Sodium in combination can lead to exhaust valve corrosion and turbocharger deposits.  This can occur if the weight ratio of sodium to vanadium exceed 1:3, and especially when there is high vanadium content in fuel.  Magnesium, either present in the fuel due to salt water contamination or added intentionally via additives can, to some extent increase the melting point of vanadium and thus preventing the formation of deposits.
• ALUMINIUM AND SILICON (AL+Si) - Limit of AL and Si has been introduced in order to restrict the content of catalytic fines mainly AL2O3 and SiO2, in oil.  Catalytic fines as mentioned earlier give rise to abrasive wear, and their content can can be reduces by centrifuging or by placing a fine filter after the purifier.  
• USED LUBRICATING OIL (ULO) - The use of used lubricants (predominantly used motor vehicle crankcase oils) in marine fuels first surfaced as a potential problem in the mid-1980s. Calcium, zinc and phosphorous are considered main elements of Used Lubricating Oils. A fuel oil is considered to contain ULO when either calcium and zinc or calcium and phosphorus exceed the limits as stated in the tale above. This, however, does not necessarily imply that the fuel oil is not suitable for use. Generally, 10 mg/kg Zn corresponds to approximately 1% used oil in the fuel.

 

References

MAN B&W, CHEVRON, WIKIPEDIA.

A word from nalinbaijal.blogspot.com

After writing this post, I can only say that your comments and likes are the fuel that drives me to write more and help more mariners appearing for examinations and broaden your horizon of knowledge. So please contribute and lets try to help each other.

It has been rightly said

Coming together is a beginning
Keeping together is progress
Working together is success

So come on my fellow engineers let's contribute!!!!!

ENGINE DESIGNATIONS AND MEANING

ENGINE DESIGNATIONS AND MEANING

We always hear this statement or tell others that:
"oh i have workd on 6S50MCC engine on my last ship"  or "my last ship was propelled by SULZER RTA68-7-flex engine"

Now do we really understand what the above statement means or what are we talking about?

We have worked on different types of MAN and Sulzer engines in the past and will continue to do so.
We have come across so many engine designations and types in our past, but how many of us know exactly what these designations mean?  This post mainly deals with Engine Designations and what story they tell us about that particular Engine.

P.S. - this post is dealing with only MAN and Sulzer make engines.

MAN B&W Engines

Below picture illustrates MAN B&W Engine Designations.
P.S.  We are not dealing with the old designations of EE, FF, GF, GFCA, GB as these types of engines are now getting obsolete.
Picture explains it all so no further description is required.

Now that we have Learnt about MAN B&W Engines, lets now take a look at Sulzer Engines.

SULZER ENGINES

Unlike MAN B&W engines, Sulzer Designations do not have any technical meaning but simply kept as an easily recognised identifier for the Sulzer low-speed engines.
The letter "R" in the RD, RND, RND..M, RLA, RLB, RTA and RT-flex engine types, goes back to the Sulzer RSD two-stroke, low-speed engine types introduced in the 1950's. The letter "R" stood for "REVIDIERTER" which is a german word for "REVISED".  During this period SD engines were "REVISED", and so were denoted with letter "R" and called RSD.  Then in 1956 RSAD engines were introduced which were the turbocharged versions.

However as of today letter R has lost any connotation with "REVISED" and now is simply kept as an easily recognised identifier for Sulzer low-speed engines.
When electronically-controlled common rail systems were introduced in 1998, the designation RTA was adopted to RT-flex to emphasise the key feature of "FLEXIBILITY" given to the new type of technology.
Also RTA engines represent uni-flow scavenged engines (with exhaust valve). So now if somebody talks about SULZER RTA68-7 then it simply means -
 SULZER RTA -  uni-flow scavenged engine (with exhaust valve)
68 - Diameter of Piston (in cm)
7 - Number of Cylinders

So now if some body asks you or tells you "oh i have worked on 6s50mcc engine on my last ship"  or "my last ship was propelled by SULZER RTA68-7-flex engine"  you now know the answer.

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.            

BACK TO SCHOOL

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".

     

 Back to School

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?

BACK TO SCHOOL

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.

                                                 

This Blog is for my fellow Marine Engineers who are preparing for their Class Examinations.

First Post will be on

"Boiler water"

Coming Soon!!!!