practical knowledge for marine engineers

8/12/2019 Practical Knowledge for Marine Engineers

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How to Know if the Piston is at Top Dead CenterWhen checking tappet clearance on marine engines, we have to ascertain that

the piston is at TDC. Though markings are provided on the flywheel, the marine

engineer must know the other methods for this like inspection of the camshaft

and the fuel pump window.

Why Know the TDC Position of the Piston?During the maintenance of a four stroke marine diesel engine there are times

when we must know whether the particular units piston is at the top dead centerof not. For example when checking the tappet clearances of the engine it is

important to know which unit is at TDC.

Referring to the flywheel would indicate two units, but only one can be at

injection TDC. So which one is it?In this article the various methods to find out the position of the piston would bediscussed. Some are very simple using conventional methods. Other methods

are a little bit complicated, but nevertheless important whenever you require an

independent method to find TDC.

Flywheel MethodThe flywheel is the simplest method to know which unit is at TDC. If the fly

wheel shows two units, simply open the bonnet covers and check visually. The

unit at TDC will have both the inlet and the exhaust valve closed and hencerelaxed springs; the other unit would have both the arms of the rocker arm at

different levels. In addition the push rods of the unit at TDC would be loose andcan be turned by hand because of the release of the clearances. There is a word

of caution however: this method is only useful in a working generator which

you have just stopped to check the tappet clearances. In case you have removed

the rocker arms for any reason the spring height and the push rod freeness check

would lead you nowhere and misguide you.

Flywheel Marking


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Fuel Pump MethodThe most accurate method to know the position of the piston without opening

up the piston is the fuel pump window. The fuel pumps have a window and as

the plunger goes up and down, so does the mark on the bottom spring holder.On the body of the fuel pump there are cut marks which show the start of

injection. In a diesel engine the start of injection is the injection TDC where

both the inlet and the exhaust valves are closed. In this injection TDC we can

check the tappet clearance. It must be noted however that the injection TDC is

not the absoluteTDC as the piston is still some way down depending on thedesign of the engine. An injection TDC may be around 5 degrees before TDC.

Dial Gauge MethodIn this method the fuel injector is taken out and from the opening a dial gauge is

put inside. Then the turning gear is engaged and the engine turned over. The

pointer of the dial gauge will move in one direction and then stop and start in

opposite direction. The moment the pointer of the dial gauge stops and changes

its direction of movement is the TDC of the unit. This method is not normally

used in day-to-day practice, but may be used in the calibration of the flywheel if

it is not calibrated, or after some repairs.

Dial Gauge

Camshaft MethodThe camshaft window of the engine can be opened up and the camshaft

inspected. The cam of the engine has a base circle, and acceleration and dwellperiods. If the roller of the follower is at the base circle, then the particular
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valve is closed by spring action. When both the exhaust valve and the inlet

valve follower are on the base circle, then the unit is also at TDC. It must be

remembered that as a four stroke engine has two rotations of the crankshaft

there is one injection TDC where the injection and the combustion take place.

The second time the piston is at TDC is when the exhausting of the flue gasestake place. It is very important to identify the combustion TDC as tappets haveto be adjusted at that point.

Cam Profile

Crankcase MethodIn this method the crankcase doors are opened up and the piston is visually

checked whether is going up or down. This is the surest method, but a bitcumbersome. It should be used when you have a strong doubt about the other

methods.

Valve Spring MethodThis is not an independent method but is used in conjunction with the flywheel

method. In this method if the flywheel is indicating two units, you can check the

springs of both the units. The unit in which the springs are loose is the one at

TDC. The caution is that this method is useful for an engine in use. If you have

removed the rocker arms during the overhaul and thereafter you want to use thismethod then it can cause errors.

Push Rod MethodThis method is like the spring method and you check that the push rods are free

to turn. The unit at TDC will have loose springs. The care that must be taken isthat it should be used along with the flywheel method and should be used in a

working engine. By a working engine, I mean the engine that was running and

has been stopped for tappets adjustment.

Spill Timing Method
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This is a very accurate (and tedious) method generally used to check the start of

injection in the fuel pump. It will also give you the injection TDC. It is used not

in the tappet checking process, but instead to find the start of injection when

you have power or thermal balance problems.

Basically in this method the delivery valve of the fuel pump along with thedelivery valve spring are removed. There after a special "U" shaped pipe is put

in place of the high pressure pipe. After this operation the engine is turned by

the turning gear and slowly brought near the expected TDC.

Soon oil will start spilling out of the pipe because the oil is entering from the

inlet port, which is uncovered by the plunger. Keep turning the engine slowlyand the oil quantity will reduce. The point where the oil flow just stops is the

start of the injection. At this moment the plunger of the fuel pump has closed

the inlet port and if you observe the fuel cam, you will find the follower is no

longer at the base circle.

As this method involves the spilling of fuel, it is called the spill timing method.

How Does An Air Ejector Work?An air ejector or steam ejector is a device which uses the motion of moving

fluid (Motive Fluid) to transport another fluid (Suction fluid). It is has a wide

range of application in steam ejector in boiler condenser, fresh water generatorand in priming the centrifugal pump.

Air Ejector TheoryIt works on the principle of convergent /divergent nozzle as it provides the

venturi effect at the point of diffusion as the tube gets narrows at the throat the

velocity of the fluid increases and because of the venturi affect it pressure

decreases, vacuum will occur in the diffuser throat where the suction line will

be provided.

An air ejector which uses the high pressure motive fluid such as air or steam toflow through the convergent nozzle the function of the convergent nozzle is to

convert the pressure energy of the motive fluid into the velocity energy.

As in convergent nozzle the following effect takes place,

P1-pressure of the fluid entering the nozzle.

V1- velocity of the fluid entering the nozzle.

P2- pressure of the fluid leaving the nozzle.
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V2- velocity of the fluid leaving the nozzle.

y Bernoullis theorem:P1 V1 = P2 V2.

As the pressure energy before entering the convergent nozzle is greater and thevelocity is less for the fluid. At the point of discharge the pressure energy is

converted into the velocity so the velocity will be greater and the pressure will

be less during the discharge.

Divergent nozzle the opposite effect takes place velocity energy is convert into

pressure energy, at the point of diffusion there is a low pressure or vacuum iscreated which is used to suck the other fluid for the motion.

In the fig: C- Convergent Nozzle.

D- Divergent Nozzle.

V- venturi Point or Diffuser Point.

Air Ejector

Appliction of Air Ejectors:1.Steam Jet Air Ejector:It is one of the types of air ejector which is used in the steam like near the

condenser to remove the non condensable gases and some vapour entering into

main condenser by an air ejector and it is cooled by the main condensate and

released in the ejector condenser.

A steam is used as the motive fluid to withdraw air and dissolved gases from the

condenser by the ejector action. In each stage of the steam jet ejector, high

pressure steam is expanded in a convergent /divergent nozzle. The steam leavesthe nozzle at a very high velocity in the order of 1220 m/s and a proportion of
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the kinetic energy in the steam jet transferred by interchange of momentum to

the body of air which entrained and passes along with the operating steam

through a diffuser in which the kinetic energy of combined steam is re-

converted to pressure energy.

The maximum pressure ratio that can be obtained with a single stage is roughly5:1 and consequently it is necessary to use two or even three stages in series to

establish a vacuum in the order of 724mm of Hg with reasonable steam

consumption.

Design Feature:There are a variety of ejectors designed in service which work on the same

principle. Older unit have heavy cast steel which serves as a vapour condenser

and also contains diffusers. These are arranged vertically the steam entering at

the top. More recent design has the diffuser arrangement externally and vapor

condenser shell is some what lighter in construction.

Horizontal singe element two stage air ejectors this unit comprises a stack u-

tubes contained in a fabricated mild steel condenser shell on which is mounted asingle element two stage air ejector.

The condensate from the main or auxiliary condenser is used as the cooling

medium. High velocity operating steam emerging from the 1 st stage ejector

nozzle entrains the non-condensable and vapour from the main condenser andthe mix discharge into the inter condenser.

Most of the steam and vapour is condensed when it comes into contact with the

cool surface of the tubes, falls to the bottom of the shell and drains to the main

or auxiliary condenser.

The remaining air and water vapour are drawn into the second stage ejector anddischarged to the steam drain tank and non-condensable gases are at last

discharged to the atmosphere through vacuum retaining valve.

2.Fresh Water Generator:The next main application of the air ejector in marine field is in fresh water

generator as it is used to remove the air and non condensable in the evaporatorchamber so as to maintain the vacuum inside the chamber. Thus the efficiency

of the generation increases at low temperature of the sea water.

3. Self Priming of Centifual Pumps:It is also employed in priming of the centrifugal pumps by the air ejector, which

removes the air inside the casing of the pump by the suction effect created bythe air ejector thus by flooding casing with the liquid so that it helps in stating

of the pump.

Test and Overhaul of Fuel Injection Valves


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This article discusses the testing and the overhaul of fuel injectors of marine

engines,the testing of the needle and guide condition of the fuel valve, and the

procedure to overhaul and inspect the injectors taken out of the marine diesel

engines.

Fuel Valve Checks

The fuel valves taken out from the engine must be checked for function and

performance. Even in engines which are stopped on heavy fuel oil in ports the

fuel injector taken out must be immediately tested with diesel oil before they get

cold as this will flush and clean the components. It must be noted that if the fuel

valves taken out are tested after they have cooled, will show bad performanceeven if they were performing satisfactorily in service.

In the majority of cases the fuel injectors have a good spray profile but they

open up at a less pressure. The pressure adjustment can be done without

opening up the valve and should be done so. The engine manufacturers also

instruct that unless the fuel injector valve has a major problem like holes choked

or valve dripping, they should not be opened up. The valve should be cleaned

from the outside, pressure checked, pressure adjusted and tagged.

Inspection and RepairsIn the case where the fuel injector valve is not performing as required and has

some defect, then it needs to be opened up and overhauled. The assembly and

the disassembly have to be done as per the instructions given by the engine

manufacturer. However, below is a general guide about what you will most

likely have to do.

After the fuel valve has been disassembled then the following checks have to be

done:
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1. The needle guide should be immersed in clean diesel oil and the needle taken

out and checked for free movement. In the case of any resistance which may be

due to the presence of carbon or fuel sludge the needle may be put in and pulled

out in succession many times while keeping it submerged in diesel oil. It is

important to do this in a container full of clean diesel oil so the contaminantscan be flushed away.

2.After the needle guide has been cleaned, the needle should be taken almost

out and then let it fall in with its own weight. A free and smooth movement with

small jerks as the clearance is making way for the oil to come out is an

indication that the clearances are all right and the needle guide is in good

condition. It must be noted that the needle should fall fully into the seat.

3. On the other hand if the needle falls fully in one go, then the clearances haveincreased and the fuel will leak past the spindle and less fuel will go in the

cylinder. The needle must be inspected for any wear marks if this happens. The

needle guide can be used but must be changed soon.

4. If the needle does not go down and gets struck then it must be thoroughly

cleaned again. If still there is no improvement then the needle might have

become bent. Check the needle for any signs of overheating.

5. The push rod end should be checked for any abnormal wear.

6. The seating between the nozzle body and the valve body if damaged can be

repaired by lapping with fine lapping paste. It must be noted that the lapping

paste should be thoroughly flushed away with clean diesel oil and thereafterblown dry with compressed air.

7. Check the nozzle spring for breakage, poor seating and other defects. Change

if required.

8. Check the leak off pipes, shims, packing etc for the condition. If the fuel

valve is water cooled, the cooling pockets should be cleaned with compressed

air.

Tests and Adjustments1. After the parts are cleaned and inspected the fuel valve is assembled as per

the manufacturers instructions and thereafter tested for function and

performance.

2. The assembled fuel valve is installed on the test stand and after purging thepipe line the manual handle is operated in quick succession. The nozzle should

start discharging with a sharp crackling noise at the set pressure. The pressure at

which the injector is supposed to fire depends upon the manufacturers engine

design but normally is between 250 to 350 kg/cm2 with an allowance of plus or


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minus 10 kg/cm2.

3. In case the lifting pressure is not correct, it can be adjusted by the adjusting

screw.

4. The spray characteristics should be satisfactory and as per the manufacturersadvice.

5. All the holes of the injector should be firing and can be checked by a torch

light or a filter paper can be folded as a cone and then the injector tested. The

holes on the filter paper will show the number of holes firing. In this procedureyou must be careful as the high pressure spray can enter the skin and is toxic for

us.

6. The spray angle should be as stated by the manufacturer. The atomization of

the fuel should take place and solid spray should not come out.

7. Clean diesel oil should be used for the testing purpose.

8. In the case that the fuel valve is dripping the needle guide should be taken out

and repaired.

CautionThe needle and the guide is always a pair and should not be interchanged withanother one. Cleanliness is the most important factor in making fuel valves. A

clean fuel valve lasts a longer time. The fuel under pressure can enter the skinand the blood stream and is toxic for humans. Take care that you stay away

from the spray. The fine mist can catch fire and in inflammable. Do not smoke

or use naked lights where the fuel injectors are being tested.

A Guide to Grinding and Lapping Paste

Grinding pastes are used for controlled removal of metal, polishing, and fitting

applications. They are used in various applications like fuel injector and exhaust

valve overhaul, globe valve overhaul, and threading and tapping applications,

among others. Grinding Paste

Lapping paste is a mixture of hard abrasive particles in a suitable base. The base

can be oil-based like grease or water soluble lubricant. The hard particles used

are carborandum, aluminum oxide, silica or silicon carbide, glass, boron

carbide, etc.

Lapping pastes are used for controlled removal and are used for the close

mating of surfaces and for removing rust and the brightening of a metal surface.

They are used for lapping of the exhaust and the inlet valves of engines, seatrepairs of globe valves, the overhaul of reciprocating compressor valves, etc.


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The lapping paste are graded from extra coarse to extra fine. In extra coarse

there are few grits of larger size abrasives, and it is used for rough lapping. In

extra fine grinding paste, there are a very large number of grits of very fine

particles and it is used for fine lapping. The larger the grit rating of the lapping

paste, the finer is the paste. Correct Methods of Manual Lapping

When using hand lapping there are two methods of lapping used, the first is the

figure of eight lapping and the other is circular. In case a guide is available, like

the nozzle nut in a fuel injector the circular method can be used. The figure of

eight lapping needs a mature hand and best not left to novices. It should beremembered that if you can stop that leakage with a minimum amount of

lapping then over lapping should be avoided. Under lapping is always preferred

to over lapping and lapping must not be done just for lapping sake but the

advice of the manufacturers must be sought.

In a figure of eight method a numeric shape of eight is made on thesurface plate by the operator. In a circular method the job piece is rotatedcircularly in place.

The exposed metal should give a uniform polished look and the quality of the

lapping can be made out by the patterns. Any stray scratches on the lapped

surface should be avoided as under high pressure they can become channels forleakage.

Role of Lubricants in Lapping PasteOil and grinding paste have contrary functions, while abrasive increases the

rubbing and cutting the oil seeks to reduce it. However oil is used to control the

abrasive and cutting action. It also is the base in which the abrasive particlesfloat and move under the action of the mating surfaces. Oil or grease is

However used in a certain measured amount.

Some people like to use additional lubricant during lapping process. However it

must be remembered that the addition of more oil is reducing the effort and the

cutting action of the abrasive particles. The abrasive particles are also washedaway if extra lubricant is added.

During lapping if you want fast removal of the metal you must lap till the effort

to move the mating parts reduces. The reduction of the force indicates that the

abrasives have broken down to harmless paste and are now acting like a

lubricant. Therefore you must remove the old lapping paste and put fresh paste.

If sufficient care is not taken during this time then metal to metal contact would

take place and the effect of spot welding and scuffing would spoil the finish

desired. Taking Care of lapping Paste


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Lapping is an art and a master machinist would work down from a coarse to a

very fine grade in a proper sequence to get the desired finish and precision as

required. Each grade has carefully controlled similar sizes of abrasive particles

suspended in the base. Any alien particle of a coarse abrasive in an extra fine tin

can create frustration especially when you are on your final finish.It is a good practice to keep all the lapping and the grinding paste cans closed

when not in use. Also when a fresh charge of lapping paste is taken it should be

rubbed thoroughly between the fingers to make out for any contaminants. This

is very beneficial when you are doing fine lapping.

Cleanliness in Precision LappingCleanliness is one of the most important factors in precision lapping as is the

quality of the lapping paste and the skill of the operator. It is a good practice

that the work table is cleaned with clean rags and blown dry with compressedair before the lapping. If a surface plate is being used then it should be washed

with clean kerosene and then blown dry with compressed air.

Also when removing used lapping paste from items being lapped, they should

be

How to Take Accurate Readings Using Micrometer Screw Gauge?

Micrometer screw gauge is used for measuring small dimensions with acuteprecision. Screw gauge is used for dimensions smaller than those measured by

vernier calipers. Learn how to take accurate measurements using a screw gaugein the article inside.

IntroductionA micrometer screw gauge is used for measuring dimensions smaller than those

measured by the vernier calipers. A micrometer screw gauge is a small

measuring device which works on the screw principle. To more about the

different parts and the working principle of the micrometer screw gauge

readhere.

Having a U shaped metallic frame, a micrometer screw gauge measures eventhe minutest length with acute precision. In the following article we will learn

how to read a micrometer with utmost accuracy.
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Using Micrometer Screw GaugeJust like a vernier calipers, a micrometer screw gauge also carries two scales - a

main scale and an auxiliary scale. The main scale is a millimeter scale graduated

to 0.5 mm, whereas the auxiliary scale is divided into 50 equal divisions. Theauxiliary scale is on the thimble of the screw gauge and measures hundredth of

the measurement. The jaws of the gauge are moved rotating the thimble. The

auxiliary scale on the thimble is also known as the vernier rotating scale.

Moreover, the thimble is so adjusted that 2 revolutions of the thimble will allow

the jaws to move by 1 mm. This means that a single rotating will move the jawsonly by 0.50 mm. The main scale lies on the part of the screw gauge known as

the sleeve.

wiped with soft tissue paper and cleaned with compressed air.

How is the Reading Taken?

In order to take the reading using a screw gauge, the object is placed between

the jaws which are moved by the thimble. The ratchet knob is used to adjust the

object firmly between the jaws. For accurate reading, the thimble should be

moved until three clicks are heard from the ratchet. The ratchet ensuresaccuracy and also prevents the object from getting damaged. The main scale

reading is taken by considering that marking on the sleeve which is visible just

to the left of the thimble. It is also to note that the 0.5 mm divisions that areprovided below the main scale should also be considered while taking the

reading. The auxiliary scale reading is taken by observing the marking on thethimble that coincides with the main scale on the sleeve. The auxiliary reading

figures will follow the main scale reading figures in the final reading. Lets take

an example to understand this.

Example 1Imagine that the scales have come to the positions as shown in the figure below,after the jaws are kept around the object. The lock can be used to assure that

readings dont change due to the movement of the thimble. In the figure, it can


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be seen that the marking on the main scale which is just to the left of the

thimble is 7 mm. However, the half scale division that is visible below the main

scale shouldnt be neglected and thus the reading on the main scale will be

7.5mm. For the auxiliary scale reading, it is noted that the 22nd

division on the

thimble scale matches with the main scale. Thus the thimble scale readingwould be 0.22 mm. The final reading will be the addition of these two readingsi.e. 7.5 + 0.22 = 7.72 mm.

Example 2Lets take one more example to understand it properly. Suppose the scales came

to the positions shown in the figure. The main scale reading would be the

marking that is fully visible immediately to the left of the thimble, i.e. 5.5 mm.

For the auxiliary reading, the 30thdivision of the thimble matches with the main

scale and thus its reading will be 0.30 mm. The final reading will be theaddition of the readings of both the scale i.e. 5.5 + 0.30 = 5.80 mm.


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Calculating the Stoichiometric Air-Fuel Ratio

Achieving fuel-efficiency in Internal Combustion Engines (ICE) is crucial to

operation. In order to efficiently burn fuel in an ICE, you need the correct air-

fuel ratio.

Internal combustion engines burn fuel to createkinetic energy. The burning of fuel is basically the reaction of fuel with

oxygen in the air. The amount of oxygen present in the cylinder is the

limiting factor for the amount of fuel that can be burnt. If theres toomuch fuel present, not all fuel will be burnt and un-burnt fuel will be

pushed out through the exhaust valve.

When building an engine, its very important to know the air-fuel ratio at which

exactly all the available oxygen is used to burn the fuel completely or atleast tothe best possible value. This ratio is called the stoichiometric air-fuel ratio.

Theoretically, this ratio will exist only for a stoichiometric mixture, which is an

ideal mixture and in practice this mixture has never been formed for any

machine so far. As every combustion cycle in an ICE is short lived, so itbecomes almost impossible to achieve the ideal ratio. However, air-fuel ratios

close to it can be achieved by modifying engine design and making use of

proper admixtures and catalysts to keep a check on the pressure and temperature

of the fuel.

The fuel combustion process takes place under very hot and pressurized

conditions and to avoid any unsafe consequences, excess air operations arecarried out. Excess air level keeps a check on the various factors like fuel

composition variation, oxygen availability and pressure, that can lead to an

explosion.
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Air fuel meters or air fuel gauges are used to measure the air to fuel ratio of

engines. Wideband oxygen sensors are used to measure the air-fuel ratio of

ICEs. Mainly there are two types of band sensors, depending upon the purpose

and type of engine. For heavy-duty and multi-purpose engines, using a wide

sensor is advisable. For general diagnostic purposes, a narrow band sensor ismost suitable. Advanced fuel ratio analyzers and gauges help a great way inmonitoring the performance of an engine and tune it up according to the

requirements.

Calculating the RatioThe air to fuel ratio is the property of fuel and chemical composition of the fuelthat defines the value for this ratio. Most of the fuels we use in internal

combustion engines are hydrocarbons, and their burning will obviously result in

the release of hydrogen and carbon as residuals, along with heat and pressure.

Below is an example of the oxidation reactionof methane (natural gas)as a

fuel.

CH4 + 2(O2) CO2 + 2(H20)

If we look up the atomic weightsof the atoms that make up octane and oxygen,

we get the following numbers:

Carbon (C) = 12.01

Oxygen (O) = 16

Hydrogen (H) = 1.008

So 1 molecule of methane has a molecular weight of: 1 * 12.01 + 4* 1.008 = 16.042

One oxygen molecule weighs: 2 * 16 = 32 The oxygen-fuel mass ratio is then: 2 * 32 / 1 * 16.042 = 64 /

16.042

So we need 3.99 kg of oxygen for every 1 kg of fuel Since 23.2 mass-percent of air is actually oxygen, we need : 3.99 *

100/23.2 = 17.2 kg air for every 1 kg of methane.

So the stoichiometric air-fuel ratio of methane is 17.2.

When the composition of a fuel is known, this method can be used toderive the stoichiometric air-fuel ratio. For the most common fuels, this,however, is not necessary because the ratios are known:

Natural gas: 17.2 Gasoline: 14.7 Propane: 15.5
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Ethanol: 9 Methanol: 6.4 Hydrogen: 34 Diesel: 14.6

You may find it interesting that methanol and ethanol both have a very low air-

fuel ratio, while the carbon chain length is comparable to methane and ethane.

The reason for this is that alcohols like methanol and ethanol already carry

oxygen themselves, which reduces the need for oxygen from the air.

The Bottom LineIn order to be able to judge if an air-fuel mixture has the correct ratio of air to

fuel, the stoichiometric air fuel ratio has to be known. If the composition of a

fuel is known, this ratio can be calculated rather easily.

How to Take Accurate Readings Using Vernier Calipers?

Vernier calipers are small mechanical devices used for taking precision

measurement. Though a bit complex to use it in the first place, a vernier caliper

is supposed to be known as the most widely used instrument in the engineering

field. Find out how to take measurements using vernier calipers

IntroductionVernier caliper is used for measuring length of objects with acute precision. An

important measuring device for engineers, vernier calipers consists of two

scalesa main scale and a sliding or vernier scale. It is important to know the

different parts of the vernier calipers before using it. Know more about Verniercalipers and its various parts inhere.

The main scale of the Vernier calipers shows reading in millimeters, whereas

the sliding vernier scale is divided into ten equal parts and has a least count of

0.1 mm. Readings of both the scales are important for reaching the final

reading.

Reading Vernier Calipers
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Using vernier calipers in the first place might seem a bit difficult; however with

practice one can easily gain the expertise. It is to note that reading vernier

calipers requires a bit of calibration to be done by the user. Reading of each

scale is taken with respect to the other scale. This means that the reading of the

main scale is taken using the markings on the vernier scale and vise-versa. Letus now understand how the readings of objects are taken using Vernier calipers

Order directly at the specialist. A huge range of measuring tools.

How the Readings are Taken?A Vernier Calipers allows readings up to a precision of 0.02 mm. The object

whose outer diameter is to be measured is kept between the main jaws, whereas

the object whose inner diameter is to be measured is placed around the smaller

jaws. The jaws are so kept that they just touch the surface of the object softly.This is done by moving the vernier scale.

The reading of the main scale is taken where the patch just on the left of the

vernier scale coincides with the marking on the main scale. Whereas the

measurement of the vernier scale is taken by observing the division on the

vernier scale that lines up exactly with that of the main scale. We will see how

this is done taking with the help of examples.

Example 1Imagine that the scales, after placing the object in between the jaws, came to the

position as shown in the figure below. Make sure that the screw clamp is

tightened to prevent the scales from moving. If you observe the vernier scale

carefully, you will notice that the small patch immediately to the left of 0coincides with the 4thmarking after 3 of the main scale. This means that the

main scale reading is 34 mm. This main scale reading is the first significant

figure of the measurement. The vernier readings will be kept after 34 mm to

form the final reading.

Again observing the figure carefully, it is observed that the 60thdivision exactly

coincides with one of the divisions on the main scale. Thus the vernier reading

is 60 and it will be placed right after the main scale reading, i.e. 34.60 mm.

Thus, the final reading of the vernier is 34.60mm.


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Example 2Lets take one more example. In the figure below, the main scale reading, justleft to the zero on the vernier scale is 37 mm. Thus the main scale reading is

37mm. For the vernier reading, it is observed that the 46th

division exactlycoincides with one of the main scale readings. Thus the vernier scale reading is

46 and thus the final reading comes to 37.46mm.

While taking the main scale reading, in case the small division on the left of 0

on the vernier scale, doesnt match any of the markings on the main scale andlies exactly in the center of two divisions than the smaller reading is taken as the

main scale reading. Also, many people complain that there are instances when

more than two divisions of the vernier scale coincide with those of the main

scale. However, in reality such situation is not possible, and if observed

carefully, only one division would be found coinciding exactly with one on themain scale.


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Impressed Current Cathodic Protection System

Corrosion:

The ships hull is corroding in sea water. Generally this is electro-chemical

reaction in which the metal combines with an oxygen, to form a metal oxide or

other compound. This depends upon the nature of the environment. Different

metals have different tendencies to corrode, activity or potential.

Some metals and alloys have two positions in the series, marked Active and

Passive.

The active position is when the corrosion is occurring and approaches the

electro-chemical series position for the material. The passive position relates to

a non-corroding situation where the material is protected by a self forming

surface film.

If two metals are placed in an electrolyte (e.g. sea water or damp soil) and are in

direct electrical contact, a current will pass through the electrolyte from themore active metal onto the least active metal.

The least active metal does not corrode and is termed the cathode. The more

active metal, the anode, passes into solution and the flow of electrical current

increases. This is a metal ion and electron transfer process i.e., it corrodes

Cathodic Protections:

The anodic and cathodic areas in a corrosion cell may be due to the electrical
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contact of two dissimilar metals, galvanic corrosion. Anodic and cathodic areas

may be formed on a single metal surface as micro-cells for instance by rain

drops on uncoated steel. Alternatively, they may be close but discrete cells

found when accelerated corrosion occurs at uncoated anodic areas on a

generally coated cathodic structure.Large currents can occur at small anodic areas and lead to rapid corrosion ofmarine structures such as ship's internal tanks, external hull plates, sheet steel

piling in harbours and tubular structures common in jetties and petrochemical

drilling and production platforms.

Cathodic Protection is a system of preventing corrosion by forcing all surfaces

of a structure to be cathodes by providing external anodes.

Sacrificial anode cathodic protection achieves corrosion prevention on a

particular structure or component by forming galvanic cell where an additional

anode of zinc, magnesium or aluminium corrodes in preference to the structure.

The galvanic corrosion current (see simple cell before) available from this

anode / electrolyte / structure combination should be sufficient to overcome thelocal surface corrosion currents on the structure until no current flows from

anodic areas of the structure i.e the structure is entirely cathodic or under

complete cathodic protection As indicated previously, a metal can be madecathodic by electrically connecting it to a more anodic metal within the

electrolyte. The most commonly used anodic metals are alloys of aluminium,zinc and magnesium. Anodes of these metals corrode preferentialy, the

corrosion current of the anode achieving cathodic protection of the structure to

which they are connected.

The anodes deteriorate as an essential part of their function and they are

therefore termed sacrificial

Introduction of ICCPA metal also can be made cathodic by electrically connecting it to another
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metallic component in the same electrolyte through a source of direct electric

current. The current flow from this metallic component must be sufficient to

overcome the natural corrosion current. Thus we will direct the current flow to

occur off the surface of added metallic component (anode), into the electrolyte

and onto the metal (cathode).All we need is to measure what the natural corrosion current is. So we add onemore electrodereference cellcompletely passive metal. The potential

difference between the hull and reference cell will form the natural corrosion

current. So another electrodeanode - with a power source is introduced so that

the current flow from this electrode is sufficient to overcome the natural

corrosion current.Because an external current source is employed, this type of protection is

termed 'IMPRESSED CURRENT CATHODIC PROTECTION'.

Cathodic Protection

A source of direct current is required, this is generally obtained from mains

power units that contain a transformer and rectifier.

The magnitude of this current may be automatically controlled in response to a

continuous monitor of the cathode / electrolyte potential or may be manually

controlled after intermittent measurement.

The impressed current anode material is ideally non-consumed by the passage

of current from it into the electrolyte, in practice the materials used are acompromise between this ideal and the cost and physical properties of available

materials. Impressed current anodes are made from graphite, silicon iron, lead

alloys some with platinum dielectrodes, platinised titanium or more exotic

combinations such as platinum clad niobium. The selection of the correct anode

material is critical in the formulation of an effective and economic cathodic

protection schemeGenerally, for a given current demand, less impressed current anodes than

sacrificial anodes are required for protection, as high anode currents are

feasible.

Impressed current systems of cathodic protection are more sophisticated in
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design than sacrificial systems.

The tipycal elements of ICCP

Control Panel Anodes Reference cell

The interconnection is similar to given picture below:

Although modern hull coatings can provide some protection against corrosion
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they seldom offer a complete solution. For this reason, most operators choose to

protect their vessels with a purpose designed impressed current cathodic

protection system.

Using an arrangement of hull mounted anodes and reference cells connected to

a control panel(s), the system produces a more powerful external current tosuppress the natural electro-chemical activity on the wetted surface of the hull.This eliminates the formation of aggressive corrosion cells on the surface of

plates and avoids the problems which can exist where dissimilar metals are

introduced through welding or brought into proximity by other components

such as propellers.

An essential feature of ICCP system is that they constantly monitor theelectrical potential at the seawater/hull interface and carefully adjust the output

to the anodes in relation to this.

Therefore, the system is much more effective and reliable.

Blow-Down Procedure for Marine Boilers

Boiler blow down is done to remove carbon deposits and other impurities from

the boiler. Blow down of the boiler is done to remove two types of impurities
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scum and bottom deposits. This means that blow down is done either forscum

or for bottom blow down.Moreover, the reasons for boiler blow down are:

1. To remove the precipitates formed as a result of chemical addition to the

boiler water.

2. To remove solid particles, dirt, foam or oil molecules from the boiler

water. This is mainly done by scum valve and the procedure is known as

scumming.

3. To reduce the density of water by reducing the water level.

4. To remove excess water in case of emergency.

Procedure for Scumming and

Bottom Blow Down

Below is the procedure for

boiler blow down using the

blow down valve located at

the bottom of the boiler. Inorder to do scumming, instead

of bottom blow down, the

scum valve is to be opened.

**WHY TO OPEN V/V 1 FIRST??

TO AVOID THE FULL LINE COMONIG

INTO PR OF BOILER WATER AND

HENCE CHANCE OF FAILURE BCOZ THIS

LINE WITH TIME CORRODES DUE TO

THE NATURE OF SERVICEAND ALSO WE CAN USE A LOW SCHEDULE NO. PIPE LEADING TO

ECONOMICAL SAVING

**2.N/R V/V,,,,3.RETURN TYPE V/V

WHY 2 V/VS 2AND3..2SHOULD BE OPENED FULL TO PREVENT WIRE DRAWING OF ITS SEAT BCOZ

OF HIGH VELOCITY OF WATER(THE PR. HEAD ON WATER CONVERTING INTO VELOCITY DUE TO

THROTTLING OR EXPANSION).HENCE WE ARE NOW LEFT WITH NO OPTION TO FIT A OTHER V/V TO

CONTROL THE QUANTITY OF BLOW DOWN OR RATE OF BLOW DOWN.
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Steps for blow down procedure are as follows:

Kindly refer the diagram to understand the blow down procedure properly.

1. Open the overboard or ship side valve(1) first.

2. Open the blow down valve (2), this valve is a non-return valve.

3. The blow down valve adjacent to the boiler (2) should be opened fully so

as to prevent cutting of the valve seat.

4. The rate of blow down is controlled by the valve (3).

5. After blow down close the valve in reverse order.

6. A hot drain pipe even when all valves are closed indicates a leaking blow

down valve.

BOILER NOTES

Economizers are heat exchangers which are fitted in a boiler to increase the

efficiency of the boiler. This is done by extracting the heat from the exiting gas

and using it to heat the feed water entering the boiler.

INTRODUCTION

To obtain an acceptable degree of efficiency and reduce fuel consumption as

much as possible by introducing further heat recovery surface so that the gas

temperature at the funnel may be as low as practicable, the gas temperature

leaving a boiler cannot be reduced much below 30oC above the saturation

temperature. In radiant types a much higher exit gas temperature is usually

found. To carry out this further heat exchange, surfaces such as economizers

and air heaters are commonly used.

In many radiant boiler types, economizers are also found arranged integrally

within the boiler unit. In this location they consist of a number of multi-loop

elements of plain tubes connected at their ends to inlet and outlet headers.

Since are situated in a hot gas temperature zone and are required to perform a

considerable heat exchange duty, a portion of the water pumped through

them may be converted into steam. These steaming economizers are arranged


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so that water enters the lower header and the steam and water mixture leaves

from the top header to the steam drum where the steam and water separate.

Economizers are used externally to boilers for further heat recovery.

Economizers are found in the cooler gas zone and are fed with watertemperatures around 116

oC or 185

oC depending upon whether the feed cycle

includes high pressure feed heaters after the de-aerator.

BOILER WITH THE ECONOMIZER

INSPECTION ON GAS SIDE:

Before going into economizer inspection, first inspect the gas side of the boiler.

It gives you a clear picture of boiler working condition and the efficiency ofheat transfer surfaces.

1. Check exterior of drums for sign of tube roll, leakage, corrosion, sooterosion and overheating.

2. Condition of outside drum insulation.3. Drum seals for signs of air leakage.4. Inspect drum support for cracks and expansion clearance.5. Check all the blow-down connection for expansion and flexibility of

support.

6. Inspect all piping and valves for leaks.7. Visually check water wall tubes and fins for cracks.8. Check exterior of all tubes for corrosion, carbon-build up, erosion,

blisters and sagging.

9. Inspect tubes at soot blower for sign of steam impingement.10.Check header seals for signs of air leakage.11.Examine exterior of headers for corrosion, erosion, thermal cracking and

condition of insulation.

12.Condition of refractory.


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13.Around the burner assembly check refractory, tube condition andaccumulation of soot or carbon.

14.Check soot blowers for distortion, worn bearings, rubbing of tubes,condition of nozzle cracks, freedom of movement and effectivelubrication.

INSPECTION ON ECONOMIZER:

1. The major problem at the economizer section is low temperaturecorrosion and problems from gas side deposits.

2. Sliding and leaky expansion joints at the casing may allow accumulationof soot with severe acid attack.

3. Inspection of tubes bends by opening the inspection covers needs to becarried out to check these.

4. Uptake area may show cracked expansion bellows sign of acid corrosion.General cleanliness of these areas indicates the combustion performance in

boiler.

Design and Setting of the Marine Boiler Safety Valve

Safety valves are fitted to protect the boiler from the effect of over pressure.

At least two safety valves are fitted to each boiler steam drum, but if there is a

super heater, another safety valve should be fitted on it.

Introduction: Marine Boiler Safety Valves

The pressure setting of the superheater safety valve should be less that the

designed pressure of the boiler, i.e. less than that of the steam drum safety

valve, to ensure flow of steam through the superheater under blow off

conditions. The pressure setting of one steam drum safety valve should be

same as the design pressure of the boiler. The pressure setting of another

safety valve should be 2-3 % more than the designed pressure of the boiler.

Classification of Boiler Safety Valves

There are three types of safety valves used in marine boilers:
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1. Improved high lift safety valve2. Full lift safety valve3. Full bore safety valve

Boiler Safety Valve

Improved High Lift Safety Valve:

1. Wingless valve improves steam flow and reduces risk of seizure.2. Waste steam pressure acting on the piston gives increasing valve lift.3. Special shaped seat deflects steam towards lip on valve and increases

valve lift.

4. The valve lifts, the force to compress the spring increases, so the highervalve lifts the greater the increasing in boiler pressure.

5. Waste steam pressure keeps cylinder in place while piston moves, alsoby having a floating cylinder, seizure risk is reduced.

6. A lip is placed around the valve seat so that when the valve lid lifts,escaping steam is trapped in the annular space around the valve face,

the resultant buildup of pressure acting upon the greater valve lid area

causes the valve to lift sharply. This arrangement gives another

advantage to close the valve cleanly and sharply with very little blow

down effect.

7. The improved high lift safety valve makes use of waste steam pressureto increase the valve lift; this is done by allowing the pressure to act

upon the lower spring carrier which fits within a floating ring so forming

in effect a piston. The pressure acts upon this piston causing it to move

up, helping to compress the spring and so increasing the valve lift.

8. Loose fitting key or pad lock is provided to ensure proper closing ofvalve.

9. Loose pin is provided to secure valve lid and allow thermal expansion.


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10.Adjustment of the valve is carried out by means of a compression nutscrewing down on to the top spring plate.

11.A compression ring is fitted after the final adjustment to ensure nofurther movement takes place.

12.A cap is then fitted over the compression nut and the top of the valvespindle, a cotter is passed through and padlocked to prevent tampering

by unauthorized person.

13.Clearance between this cap, the valve spindle and cotter are such as toprevent the valve being held down externally.

14.Easing gear is fitted so that in the event of an emergency the valve canbe opened by hand to a full lift D to release the boiler pressure.

Valve Area: As = A (1 + Ts / 555)

As- Aggregate area through the seating of valve (mm2) for superheatedsteam.

A-Aggregate area through the seating of valve (mm2) for saturatedsteam.

Ts- Degree of superheated steam in oC.15. Valve Area (As) greater than (A) due to specific volume of steam increases

with increases of temperature at constant pressure and more escape area is

required to avoid accumulation of pressure.

16. The area of valve chest must be at least (1/2) A.

17. The waste steam pipe and steam passage must be at least 1.1 A.

Manual Hand Trying of Boiler Safety Relief Valve:

To check the proper working condition of the boiler safety valve we carry out

the Hand trying out the Boiler Safety valve at regular intervals. The safety

valve is provided with the easing gear which manually lifts the safety valve and

releases the excess pressure in the boiler. When the easing gear is pulled, the

valve will be opened by hand to a full lift of D to release the boiler pressure.Before carrying out the process the boiler safety valve has to be drained.


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Boiler Safety Valve Drain:

Draining of the boiler safety valve is necessary as to prevent any build-up of

water in the pipe line causing head of water to form over the valve lid so

increasing the blow off pressure. So at regular intervals the boiler safety valve

should be drained.

1. Drain pipe must be fitted to the lowest part of the valve chest on thedischarge side of the valve.

2. The pipe should be led clear of the boiler.3. The pipe must have no valve or cock fitted through its length.4. The open drain of the pipe should be regularly checked.5. If the pipe becomes chocked, there is possibility of overloading the valve

due to hydraulic head, or damage due to water hammer.

6. The waste steam pipe of the boiler safety valve should be well securedso that no load of the pipe is on the safety valve, which can be the cause

of additional stress on the valve.

Pressure Setting of the Boiler Safety Valve:

If it is found that the boiler safety relief valve is not lifting at the designed

lifting pressure, manual pressure setting of the boiler safety valve has to be

done for the proper and safe operation of the boiler. The adjustment can be

carried out on this type of valve to give the desired discharge and blow down

characteristic.

1. Safety valve pressure setting can be done from high to low pressure orvice versa.

2. Take necessary personal safety precaution and arrange tools i.e. gaggingtool and master gauges.

3. Slowly raise the boiler pressure and blow off the safety valves manuallyfew times for thermal expansion and to reduce the thermal stress on thevalves.


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4. Then screw down all the safety valves higher than the setting pressure atwhich you are going to set.

5. Raise the boiler steam pressure 2-3 % more than the designed pressureof the boiler, then stop firing and unscrew the first valve slowly, when itblows off at 2-3 % more than the designed pressure then note this

opening and closing pressure of the valve and finally gag it.

6. Raise the boiler pressure at the designed pressure of the boiler andunscrew the 2nd valve, when it blows off at designed pressure then note

this opening pressure and check the closing pressure also. Recheck the

setting pressure and gag the valve.

7. Then set the superheater safety valve lower than the designed pressureof the boiler in same procedure.

8. Finally take out the gagging tools. Pressure setting should be done inpresence of surveyor

Boiler Inspection or Survey Carried Out at Regular Intervals

The boiler is vital equipment on ships. It is used as main propulsion (in steam

ships) and for auxiliary heating in other ships. It is very sensitive and dangerous

equipment, where there should be regular inspections and surveys carried out

to avoid accidents and outages.

BOILER INSPECTION

Introduction

Normally boiler inspection will be carried out onboard the ship by a port state

control and during the dry dock. They are used to carry out the inspection and

see the working condition of the boiler. During the inspection they will conduct

an in-depth analysis of the boiler condition considering various factors to find

the working condition of the boiler. If necessary they will replace damaged

parts of the boiler needed for continued safe operation.

NEED FOR BOILER SURVEY OR INSPECTION

1. Boilers are inspected to maintain the Class requirement.
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2. Regular internal inspection and external examination during such surveyconstitute the preventive maintenance schedule the boiler goes through

to have a safe working condition.

FREQUENCY OF BOILER SURVEY

1. Water tube high pressure boilers are surveyed at two year intervals.2. All other boilers, including exhaust gas boilers, are surveyed at two

yearly intervals until they are eight years old and then surveyed

annually.

PLANNING FOR BOILER SURVEY

1. Confirm time available, manpower, and time required.2. Check before shutting down boiler.3. Check for spares e.g. manhole door joints, gauge glass, packing and

steam joints.

4. Check the tools required e.g. gagging tool, torque spanner, rope, chainblock etc.

5. Check manual for special instruction and past records.6. Steam requirement for the next port should be considered e.g. Tankers

require steam in discharged Port.

7. Briefing to other engineers of work involved.SHUTTING DOWN THE BOILER FOR INSPECTION

Before inspection is to be carried out, the boiler which is firing should be shut

down. These are the steps to be followed before shutting down the boiler for

inspection.

1. Inform the chief engineer and inform the duty officer in the bridge.2. Change over M/E, A/E, and Boiler to diesel oil.3. Top up diesel oil service tank, stop heavy oil and lube oil purifiers.


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4. Stop all tank and tracing steam heating and carry out soot blowing.5. Change over from automation to manual firing of boiler.6. Stop the firing of the boiler and purge boiler for three to five minutes.7. Switch off power and off the circuit breaker for forced draught fan, FO

pump, feed pump, and combustion control panel. Hang necessary

notices.

8. Shut main steam-stop valve and shut all fuel valves to boiler.9. Let the boiler cool down, do not blow down now.10.When the boiler pressure is about 4 bars, carry out blow down.11.When boiler pressure is slightly higher than atmospheric pressure, open

the vent cock to prevent formation of vacuum.

12.Let the boiler cool down.13.Once sufficient cooled, open top manhole door first with all safety

precaution.

14.Mark the nut on the top manhole, slacken the dog-nut, and secure itwith a rope.

15.Knock the manhole door gently, but do not open it as it may containsteam or hot water.

16.Conform nothing coming out; open the door fully with the help ofsecuring rope.

17.Do not open immediately open the bottom door, since the boiler is stillhot and if opened relatively cool current of air will pass through the

boiler causing a thermal shock.

18.Allow further cool down before opening bottom manhole door.19.Open the bottom manhole door with the same precautions and open

the furnace side door also.

20.Ventilate foe period of 12 to 24 hours.


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21.Then check for oxygen, flammable vapour, and toxic gasses.22.If it is safe, prepare for entry.

PREPARATION FOR ENTRY

These are the steps to be carried out before entering the boiler for inspection.

1. Prepare a long rope, wooden plank oxygen analyzer, safety hand lamp,and safety torch attached with rope.

2. Get a pouch to carry tools and keep track of the number of tools to bebrought into boiler.

3. Personnel safety protection wear, e.g. helmet, safety shoes, hand gloves,etc.

4. No extra instruments to be brought in and clear pocket contents as itmay fall into boiler.

5. Keep an emergency breathing apparatus ready.6. Remain in communication and ensure proper lighting.7. Check boiler internals before making an entry, e.g. foothold and

handhold.

Inspection Carried Out In Boiler Superheater and In Steam Drum

SUPER HEATERS

The superheater is a device which converts saturated steam or wet steam to

dry steam, and it is used in driving the lager turbines in the marine propulsion

system. In the superheating process the temperature of the steam is only

raised, keeping the pressure at a constant level.

Superheating process can be done by three methods:

1. Radiant superheating: In this type, the superheating tubes are placeddirectly in the combustion chamber.
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2. Convention superheating: In this type of super heaters the superheatingtubes are placed outside the combustion chamber on the path of the hot

gases.

3. Separately fired: In this type the superheater tubes are placed in theseparate combustion chamber outside the boiler. This is separately fired

to maintain the required temperature of the superheated steam outlet.

In the superheater zone the products of combustion were still at a high

temperature and deposits from impurities in the fuel condensed out on the

tubes, reducing heat transfer and steam temperature. Eventually gas passages

between the tubes would become so badly blocked that the forced draught

fans would be unable to supply sufficient air to the burners, combustionbecome impaired and the fouling condition accelerated. Sodium and vanadium

compounds present in the deposits proved very corrosive to superheater tube

causing frequent repeated failure. Due to the fouled conditions there was a

loss of efficiency and expensive time consuming cleaning routines were

required.

Inspection on Superheater

1. Internal and external examination of heaters.2. Thermal crack at the headers due to high stresses set up across the thick

welded section is possible.

3. Super heater safety valve and stop valve.4. Super heater drains and vents valves and manhole openings to check.5. Efficiency of the screen plates to ascertain these protect headers

from direct heat of furnace.

6. Superheater tubes are also prone to high temperature creep failures andthermal fatigue cracking sudden quenching can cause fatigue failure.

7. Check for deposit accumulation in header.8. Drain valve from headers to examine.

Super Heater Walk-In Spaces:


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1. Supports of horizontal super heater tubes to check for burning away andleave the unit unsupported and cause drainage problems.

2. Super heater support tubes may also crack due to effect of bendingfatigue stresses due to misalignment of tubes in the tube holes.

3. Build-up of deposit is most troublesome defect in super heater. Thesemay result in high furnace pressure, loss of super heater and poor

combustion.

4. Special attention and suspicion to be reserved for tubes through whichthere still exist gas paths as they operate under excessive metal

temperature.

5. Oxide scaling inside or outside may cause tube failure and worst casehydrogen fire when iron burns in steam at above 700*C in exothermic

reaction, and destroys all boiler, economizer and air heater.

Now you have a clear picture on the various inspections carried out on the

marine boiler parts for the safe and efficient working of the boiler.

Inspection carried out in Boiler Superheater and in Steam Drum

Learn how inspection is carried out in boiler steam drum, headers and super-

heater tubes.

INTRODUCTION

The steam drum is one of the important parts of the boiler which acts as the

reservoir for the steam generated and for water required for the boiler. Mainly

all the boiler mountings are mounted on the steam drum and it should possess

sufficient strength to withstand the high temperature and pressure of the

steam generated.

As before, in inspecting the generating tubes, headers, and superheater tubes

of the boiler, the inspection has to be carried out in the boiler steam drum.

Check the steam drum for corrosion, scaling, and pitting:

1. Manhole seats and surface condition.
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2. Condition of all feed, chemical feed, blow down lines and inside pressureparts or chocking, security, and leaks.

3. Check for freedom of expansion of drums and headers.4. Inspect tubes for corrosion, excessive deposits, flare-cracking, and

pitting.

5. Inspect hand-hole plates and stud threads.6. Make a complete waterside examination and check for scale build up as

necessary.

7. Measure thickness of scales by using commercially available gauges.OUTSIDE STEAM DRUM:

1. All internal (removed from drum) checked and tested.2. Feed regulator, feed check valve, water gauge fittings, and drum safety

valves examined. Attention to securing arrangement of seats in valves

covers to valve chest to drum nozzles.

3. Welded connection of drum to casing to check for any possible damagecreating gas leakages.

4. Areas of drum not protected by tubes from heat radiation and shieldedrefractory. Thermal cracking of the refractory material to be checked.

STEAM DRUM

HEADERS

Boiler headers are the water feeders to the generating tubes in boiler. The

headers are connected in between the steam drum and the water drum.

Normally the water from the water drum enters the main headers from there

and many generating tubes are connected where the steam is generated.

Rear and Side Wall Headers:

1. Sufficient doors or handhole plugs to remove for assessment of internalcondition of headers and tubes.


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2. Check for pitting and corrosion of headers, rear walls, floors, roofs, andside wall tubes.

3. Check for casing defects for possible gas or air leakage.Bottom Header:

This contains the furnace tubes and the down comer tubes. A number of

handhole doors is provided for internal inspection and repair to the tubes.

Inspection for deposits of sludge must be carried out during the survey. Regular blowing down from this header will be necessary to keep it clear

of sludge deposits.

Repairs in Marine Boilers

This article discusses the general repairs needed in the marine boiler and how

to repair them directly on board. Some common repairs are leaking of tubes in

both smoke tube and water tube boilers, busting of tubes, and leakages in the

manhole joints.

Introduction

Some of the common repair work carried out on the marine boiler while on

board the ship is plugging of the tubes and replacing the leaky manhole joints.

Other major repairs like the renewal of the damaged tubes and furnace

rebuilding must be carried out in the dry dock. The plugging of the boiler leaky

tube is a temporary repair which must be carried out in order to fire the boiler.

Whatever the situation, and in any condition the boiler must run to supply the

working steam.

When the gasket becomes damaged or gets old, smoke starts to come out of

the boiler in the case of the water tube boiler. In the smoke tube boiler, the

water starts to leak outside the boiler. This must be repaired on board by

replacing the leaky manhole joints.

Replacement of a Leaky Manhole Joint

1. Maintain proper spigot clearance- 1.5 mm to position the door centrallyfor evenly loading the gasket.
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2. Never use an old gasket.3. Do not over strain the door studs, which may stretch.4. Pull-up studs by re-tightening the nut after steam rising or warming up.5. Avoid causing damage to door by holding it by a rope and gently

lowering it inside or taking it out.

6. Mark the dogs and nuts to fit back correctly in the same door.7. Check for wear and tear on the studs and nuts.8. Carefully check the matting/ landing surface for corrosion and erosion

on the door and boiler before reassembling.

Repairs in Smoke Tube Boilers

Procedure for Plugging of a Damaged / Busted Smoke Tube:

1. Hydrostatic testing to mark the leaky tubes.2. Cut the tubes on one end and clear of the tube plate. At the other end

the tube is collapsed inside the tube plate.

3. Pull out the tube from the collapsed end.4. Insert a short tube into the tube plate and weld it in place.5. Lap the spare tapered plugs on both stud ends in the tube plates.6. Insert the tube plugs and tack weld it.7. Alternatively, the plugs can be held in place by a long steel bar threaded

and bolted at both ends.

8. Hydrostatic pressure test to confirm no leaks.9. Flush up the boiler and re-inspect the plugs for leaks under full steam

pressure.

Temporary Repairing Procedureto Rectify the Leakage in Smoke Tube:

1. Stop the burner, allow the boiler to cool and remove the soot.


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2. Allow boiler to depressurize, and open the blow down valve to drain theboiler.

3. Enter the boiler flue box and cut a hole in the side of the relevant smoketube.

4. Clean the rim of the smoke tube with a wire brush.5. Cut a circular plate (15 mm thick) of the same diameter as the smoke

tube and chamfer the top edge to 30 degrees by grinding.

6. Fit the plate into the top of the smoke tube and weld it in position asshown.

7. Enter the boiler furnace and cut a similar hole in this end of the relevantsmoke tube.

8. Repeat steps 4 to 6 for lower plate.9. Refill boiler and check for leaks before start-up.10.Start-up boiler and check for leaks when pressurized.

Note: Any temporary repair to smoke tubes or boiler tubes should receivemore permanent attention as soon as conveniently possible.

Repairs in Water Tube Boiler

Instruction for Plugging / Repair of Water Tube Boiler & Economizer:

1. In case of tube failure, steam pressure has to be removed and the oilburner dismantled.

2. If the leakage is readily visible from the burner hole, the boiler can beemptied and repairs commence.

3. Otherwise, the boiler is given pressure by means of the feed pump. Theposition of the leakage will be indicated by the water flow.

4. This flow may not be visible from the burner hole. If it is not visible,remove the inspection door and enter the furnace. If the tube failure is

still not found, then enter the generating tube section. From here the


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bottom of the membrane walls and generating tubes can be inspected

for leakage.

5. If the leakage has resulted from the membrane walls or generating tube,the inspection door at the smoke connection pipe must be removed, andthe generating tube/ membrane tube in which the failure has occurred is

pointed out.

6. The leakage may also result from economizer.7. By removing the inspection door at the bottom of the economizer, it can

be determined which uptake has caused the leakage?

8. If necessary other inspection doors should be removed to point out thedamage register.

9. When a damaged tube or convection register has been removed, andthe remaining tube studs have been repaired/ plugged a new tube or

register should be mounted as soon as possible.

10.Operation for longer periods with one or more registers missing involvesthe risk of further damage to the boiler due to increasing heat leads on

the parts next to the ones removed.

Scope of Inspection of a ShipsBoiler

The boiler is one of the items of equipment on a ship which continuously keeps

on running during sailing and in port. As it is running continuously, it has to be

cleaned and inspected to check the condition of all internal working parts at

regular intervals.

SCOPE OF INSPECTION

The scope of inspection is to clean the boilers internal surfaces and to check

for corrosion and scale formation in the boiler. As the boiler normally runs

continuously, there are few chances to open the boiler. Thus, during the

inspection all the important checks will be carried out and it will be made sure

that the boiler will safely work without any problems until the next inspection.

Routine inspection is important because salt formation and scaling inside the
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boiler tubes will reduce the heat transfer rate and ultimately damage the tubes

due to overheating.

1. The inspection should include finding reasons for any abnormality foundand should also ensure that any repair carried out does not affect thatsafe working order of the boiler.

2. A complete inspection means full internal and external examination ofall parts of the boiler and accessories such as super-heaters, air heaters,

and all mountings.

3. The examination may lead the inspector to require hydraulic testing ofpressure parts or thickness gauging of plate or tubs that appear to be

checked for good working condition.

The Inspection is not completed until the boiler has been examined under

steam and the following items dealt with:

a) Pressure gauge checking against a test gauge.

b) Testing of water level indicators and protective devices.

c) Safety valves adjusted under steam to blow off at the required pressures.

d) The oil fuel burning system examined.

e) Testing of remote control gear for fuel shut off valves.

For a gas fired boiler, the chief engineer floats the safety valve at sea at the

first opportunity. Survey record is not assigned until a statement is received

from chief engineer about the pressure at which the safety valves were set.

Inspection Consists Of:

a) Examination of the items.

b) Statement whether a problem / defect exist.

c) Determining the cause of problem.

d) Define the repair and whether temporary / permanent.

The Main Benefits of Doing Inspection:


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By doing the inspection, we are manually cleaning the boiler scales and

chemical cleaning of the salt formation in the boiler parts and making the

boiler safe for operation. It also helps in checking the redundancy of the stand-

by boiler. During the inspection the newly signed in crew members and the

ships engineer will also have a chance to see the internal parts of boiler.

1. Boiler must be sufficiently cleaned and dried to make a thoroughexamination possible.

2. Boiler should be manually wire-brushed to clean the internal surfaces.3. In case of difficulty in manual cleaning, chemical cleaning with

hydrochloric acid plus inhibitor to prevent acid attacking the metal

without affecting removal of deposits is the best procedure.

4. For oil contamination, alkali boil-out using tri-sodium phosphate solutionis essential prior to acid cleaning. Through water flushing must be

carried out after acid cleaning to avoid acid concentration in crevices

and captive spaces.

5. All internals that may interfere with the inspection have to be removed.6. Wherever adequate visual examination is not possible, surveyor may

have to resort to drilling, ultrasonic, or hydraulic testing.

7. All manhole doors and other doors must be opened for reasonable timeprevious to survey for ventilation.

8. If another boiler is under steam arrangement of locking bar and othersecurity devices must be in position preventing the admission of steam

or hot water to the boiler under survey. The smoke trunking, exhaust gasshut-off etc., must be in position and in proper working condition.

9. Plants staff or repairers staff should stand by the manhole in case ofemergency and to take note for defects/ repairs required.

Before survey, the surveyor should acquaint himself with the boiler type in

question (drawings carried on board) and during the survey it is advisable to

follow a planned routine in order not to miss parts of the boiler or important

items.


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SAFETY VALVES

At least two safety valves have to be fitted to the boiler. They may be both

mounted on a common manifold with a single connection to the boiler. The

safety valve size must not be less than 38mm in diameter and the area of thevalve can be calculated from the following formula C x A x P = 9.81 x H x E

where

H= Total heating surface in m3

E = Evaporative rate in Kg steam per m2of heating surface per hour

P = Working pressure of safety valves in MN/m2absolute

A = Aggregate area through the seating of the valves in mm2

C = the discharge coefficient whose value depends upon the type of valve.

C=4.8 for ordinary spring loaded valves

C=7.2 for high lift spring loaded valves

C= 9.6 for improved high lift spring loaded valves

C= 19.2 for full lift safety valves

C= 30 for full bore relay operated safety valves

LIFT PRESSURE

The safety v/v must be set at a pressure not exceeding 3% of the approved

boiler working pressure. It is normal to set the suphtr safety below that of the

drum to ensure an adequate flow of stm for cooling purposes under fault

conditions. Similarly the superheater should be set to close last.

10% ACCUMULATION OF PRESSURE RULE.

With all the flames in full firing the stm stop is closed, the boiler pressure must

not increase by more than 10% in 7 minutes for water tube of 15 mins for tank

boilers with the safety lifted. this is normally waivered for superheater boilers.

Instead calculations and previous experience used.

BLOWDOWN
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The pressure drop below the lifting pressure for a safety v/v is set at 5% by

regulation although it is more normal to set v/v's at 3% to prevent excessive

loss of stm. For boilers with a superheater it is important that the superheater

v/v not only lifts first but closes last. Adjustement of the blowdown may be

necessary following adjustment of the popping setpoint (Increaseing set point

lengthens blowdown). Adjustment is achieved by altering the height of the

'adjusting guide ring' on the full lift safety valve design shown below. Over

raise adjustment of this ring can lead to mal-operation with the valve not fully

opening

SETTING

Must be set with the surveyor present except when on the waste heat unit. A

chief engineer with three years experience may then set the safety valve but

must submit information to surveyor for issue of certificate.

Superheated steam safety valves should be set as close to operating

temperature as possible as expansion can alter the relationships between

valve trim and guide/nozzle rings which can effect the correct operation of the

valve.

1. Two safety valves- each set independently2. Each safety valve must release entire steam flow in pressure

accumulation test

3. Surveyor uses specially checked gauge4. One valve gagged5. valve initially set to approximately the correct position then steam

pressure increased to set pressure

6. adjust valve to lift7. raise and lower pressure to check8. fit locks to both valves on completion
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