HomeEngineering Knowledge GeneralMarine Boilers (introduction and Operational guide)

Marine Boilers (introduction and Operational guide)

Type of boilers

  1. Cochran Exhaust Gas Boiler
  2. Composite Boiler
  3. Alternative Boiler
  4. Economizer as an Exhaust Gas Boiler

Cochran Exhaust Gas boiler

  • A double pass, Vertical type, in which Exhaust gasses from the marine engine pass through two banks of the tube.
  • Served as an efficient silencer, when the boiler is in use.
  • A separate Silencer always fitted along with an exhaust gas boiler to be used when the boiler is generating more steam than required.
  • All or part of exhaust gasses can be directed to the silencer and atmosphere, without going through the boiler.
  • Working pressure is around 7n bars

Composite Boiler(composite type Cochran boiler)

  • If exhaust Gasses and oil fire can be used at the same time, it is termed composite boiler.
  • In the double pass, composite type Cochran Boiler, it provides a separate tube nest for exhaust gas passage, situated immediately above the return tube nest from the oil-fired furnace.
  • Exhaust gasses from the oil-fired furnace and Main engine, pass through the tubes, which are surrounded by boiler water.
  • Separate Uptake provided for Exhaust Gasses and oil-fired smoke.
  • Heavy Changeover valves are fitted, to divert the gasses straight to the funnel. when desired.

Alternative Boiler(Alternative type Cochran boiler)

  • If the Exhaust Gasses and oil fire can be used only one at the time, it is termed Alternative Boiler
  • Double pass, Alternative Cochran Boiler can be Oil Fired and Heated by exhaust Gasses alternatively.
  • Since both systems use the same combustion chamber, one system being blank, while the other one is in operation.
  • Only one uptake Required.

Economizer as Exhaust Gas Boiler

  • In this System, a separate exhaust Gas Economizer EGB is connected to an oil-fired Auxiliary Boiler9or an Accumulator)By means of piping and a set of Circulating pumps.
  • Heat Absorbed from Exhaust Gas in EGE is transmitted by working Fluid, to auxiliary Boiler or Accumulator, from which steam is drawn for use.
  • The Economizer unit cannot deliver steam, directly to the steam range.
  • Straight gas lead from the main engine exhaust manifold, pass through EGE under the funnel.
  • This arrangement permits the auxiliary Boiler or Accumulator,to be placed in any convenient position in the Engine room.
  • Inlets and outlets of piping coils are connected to external headers (Distributing and Collecting), that are simply inserted in the exhaust trunk way.
  • Water from Auxiliary boiler or Accumulator is fed by circulating pump through a non-return valve into distribution chest or header and from it, water passes into coils.
  • Water and steam from outlets of these coils pass into the collecting header and then to the steam space of the auxiliary Boiler or Accumulator.
  • EGE safety Valve is Adjusted at a slightly higher pressure than safety valves of auxiliary Boiler, in order to ensure that economizer operates in flooded conditions at all times.

Boiler safety Devices as for UMS status

  • Flame failure – Photocell shutdown the combustion system and gives an alarm.
  • Fuel temperature-Deviation from the set temperature range, causes the burner to be shut off, and alarms are given for both low and high temperatures.
  • Fuel pressure-Low fuel pressure causes the automatic controller to shut down the burner and alarms given.
  • low and high water level-Level is maintained by feed pump, controlled by float operated on/off switch.
  • Very low water level-Burner stopped and raised the alarm.
  • Very high water level-Burner stopped or the Main engine slows down and alarms given to avoid foaming and carryover.
  • Low and High steam pressure-if steam demand drops, high steam pressure will shut down the burner and/or Main Engine speed reduced. A low steam pressure alarm will be given if there is a fault in combustion conditions.
  • Air/Fuel Ratio- The air register damper controller keeps the correct ratio, and shuts down the system and alarm given on deviation.
  • Draught fan failure-Air supply fan failure operates audible and visual alarms.
  • High flue gas temperature- Burner Stopped and the alarm was given.
  • Smoke density-Emitted smoke through uptake, being monitored and if deviate from normal limit, shut down system and alarm given.

Types of boiler burner used in marine boilers

  1. Simple pressure jets
  2. Spill type pressure jets
  3. Variable Orifice-type pressure jets.
  4. Steam assisted pressure jets.(Steam Blast jet burner)
  5. Spining cup atomizers.(Rotary cap Burners)

Pressure jet

atomisation of the fuel is achived by forcing the fuel under pressure through an orifice at the end of the burner,and swiller plate,the pressure energy in the fuel is converted to velosity.

Spill type pressure jet

Atomization is the same as simple pressure jet type. the burner differs in that a proportion of the supplied fuel may be spilled off. which allows for an increased turndown ratio.

Variable orifice pressure jet

A rotating cap oil burner atomizes the oil by throwing it off the edge of a tapered cup. The cup rotated at high speeds between 2000-7000 rpm by either an air turbine driven by primary combustion air, or by an electric motor driving.

The fuel oil is supplied to the inner surface of the cup through the hollow end of the spindle. centrifugal force causes it to spread out evenly into a thin film and moves out along the taper until it reaches the lip of the cup, where the velocity of oil causes it to break up into fine particles as it passes into the surrounding air stream.

High oil supply pressure is unnecessary as this pressure plays no direct part in the atomization process. thus this type of burner can be used with a gravity type oil fuel supply system.

the oil throughput is controlled by a regulating valve placed in the fuel supply line and thus can easily be adapted automatic control.

Here the wide turndown ratio available with this type of burner is a great advantage. values of over 10:1 are possible.

Steam Assisted

These atomize the oil by spraying it into the path of a high-velocity jet of steam. compressed air is therefore seldom used, except when lighting up from cold. sketch shows in diagrammatic form the general arrangement of a steam blast jet burner of the widely used Y-jet type.

in this steam flows along the central passage, and is then expanded through a convergent-divergent nozzle, where its pressure energy is converted to kinetic energy resulting in a high-velocity jet of steam.

Oil sprayed into this jet is entrained by it being torn to shreds and atomized in the process. the exit ports are arranged tangentially, thus giving the necessary swirl to the oil droplets in order to form the hello rotating cone of fine particles of oil needed for the efficient combustion of residual fuel oil in the boiler furnace.

There is no need to fit the usual swril vanes for the secondary air stream only a ventury shaped throat and tip plate are required.T

The throughput of oil is controlled by varying the oil supply pressure.it has large turndown ratios of up to 10:1 without having unduly high pressures. the oil supply pressure ranges from about 140-2000kN/m2, with a corresponding steam pressure of 140-1500kN/m2.

Boiler burner turn down Ratio      

trn down ratio of a boilermarine progress

The turndown ratio (TDR) of a burner indicates the flexibility of the burner to cope with boiler variations without losing the efficiency of atomization. The ratio of the maximum to minimum oil throughput of the burner is known as the turn-down ratio of the Burner.

In the case of pressure jet burners this can be stated in terms of the squre root of the ratio of maximum to minimum oil supply pressure.

How to Know Boiler tube is leaking?       

  1. Excessive feed water consumption from the feed water tank.
  2. Continuous running of feedwater pump.
  3. If the Leakage is large, low boiler water level and drop steam pressure under continuous firing of the boiler.
  4. Some water comes out from the furnace cover
  5. White smoke from boiler uptake.

Possible causes of Leaking

  1. External wastage due to water side corrotion & Pitting,Being using the bad feed water always forming heavy scales and corrosion.
  2. Wastage of ligaments may be caused by using wet steam for soot blowing.
  3. Unequal thermal expansion between tubes and tube plates due to overheat or design fault.
  4. Deformatio or panting tube plates under pressure.

Possible source of water leaking

  1. Leakage from tubes
  2. Distored furnance crown plate.
  3. Furnance shell plate,opposite to burner opening due to flame impingment.
  4. Lower section plate of furnance,due to damage bricks-works.

Leakage Check

Water tube boiler

Stop the Firing and open up the combustion chamber,leakage can see easily & individually boiler water tubes.

Smoke tube boiler

By opening the smoke side drain valve,if the water come out,we can know that the boiler tube is leaking.After open up the fireside cover and fill up the boiler water level untill all smoke tubes are flooded,we can easily cheack which one is leaking.

Repair to proceed jorney

  • Shut down the fire and cool down gradually.
  • When pressure is reached to around 4 bar,blow down the boiler water properly.
  • When noise level fall,shut the blow down properly.
  • When boiler pressure reached near to atmosphere pressure, open vent valve to avoid vaccume in the boiler
  • Open the furnance and throughly ventilate.
  • Upon cleaning finish,find and mark the leaking tube carefully.
  • After complete drain out of boiler water,open steam aad water drums carefully and ventilated.
  • Clean the leaking tube hole and drive in the plug from both ends.
  • Box up both water and steam drums,fill water and pressure test.
  • If ok,box up the boiler and flash up with correct procedures.
  • Voyage could be contineud.
  • During voyage,avoid using excess steam pressure and keep close watch to boiler.
  • Permanant repair(tube renewal)should be carried out in arrival port.

Decition to repair in port

If the problem is happend in port and having enough time and spares,it should be repaired permanantly by reneawal of leaking tubes by ship crew or available labour.After cooling down,open up and inspect the leakage for the reneawal of tubes.

  1. Crop both ends of leaking tube about 50mm from the tube plate and chisel out.
  2. Remove the remaining pieces by chiseling and knocking after heating and cooling to achive shrinkage.
  3. Clean and polish the tube holes for dye check for any cracks.then are made good with light rolling by an expander.
  4. The usual diametrical clearance between the tube and hole must be about 1.5mm.
  5. The ends of new tube should be cleaned throughly and carefully expanded by rolling into the hole and in the tube plate.new tube must be projected from the tube plate by 6mm at least.
  6. The bell mouthing should be 1 mm for every 25 mm of outside diameter plus 1.5mm

Boiler overheating

Overheating is always caused by the presence of some insulating medium between the furnance and water chamber.

Causes:

  • High degree formation of scale,sludge,mud and oil
  • Poor water circulation
  • Feed water shortage
  • Local overheating
  • Fault combustion
  • Incorrect raising of steam
  • Direct flame impingment,resulting deformation andlocal buiging.

To prevent boiler overheating

  • Test boiler water and give proper chemicsl treatement.
  • Properly blowdown boiler water whenever neccesary.
  • Regular cleaning of furnance and chemical cleaning to water side.
  • Maintain proper boiler water level
  • Maintain efficient feed water pump and circulating pump.
  • Maintain proper fuel burning procecc including burner cleaning
  • Maintain proper function of boiler safety devices.

Furnace Distortion

  1. High degrees of scale,sludge,oil and mud formation.
  2. Poor water circulation.
  3. Water storage.
  4. Incorrect flashing up of boiler or raising of steam pressure
  5. Local overheating
  6. Faulti combustion
  7. Directly flame impingment,resulting deformation or a local bulging opposite the burner opening.

Correction

  1. Apply heating on bulging area and push back to original shape by pressure jack if the matieriel is good and the internal surface are not severely corroted.then crackes test must be followed.
  2. By crop and renew the defective portion and pressure test.no doubler plating is recommended.
  3. Temporary repair of weakness furnance by pushing back the deformation and by welding plate.
  4. Stiffeners to be provided circumferentially around the furnance on waterside.
  5. When the deformation is acute the only remedy is renewal.

Boiler mounting

  • Feed water check(two number)

These are fitted to give final control over the entry of feed water into the boiler.they are non return valves.one is main valve and the other is auxiliary.

  • Air vent cock

It is fitted to release air when filling water or initiallly rising steam and to prevent vaccum condition when completely emptying the boiler.it is fitted at the highest point of the boiler.air vent is closed when the boiler pressure is 0.5 bar,while rising steam in the boiler.

  • Water level gauge

Every boiler is to be fitted with at least two indipendent means of indicating the water level.one of which is to be direct reading gauge glass.the other means is to be either an addition glass gauge or an approved equelant device.the have steam,water and drain cock to get satisfactory operation of the boiler at all time.

  • Pressure gauge

It is fitted to show the boiler steam pressure.there are two red lines on the dial.lower line is normal working pressure and upper line is maximum permissible pressure.

Steam pressure gauge shoud be connected to a boiler through a length of coiled pipe or siphon to avoid steam coming in to contact with the bourdon tube of the pressure gauge.

  • Main steam stop valve

Steam stop valve is the steam outlet to the process and is usually of an ordinary right angled screw- down type.the stop valve is not designed as a throtlling valve and should be fully open or closed.it should always be opend slowly to prevent sudden rise in downstream pressure and associated water hammer.

  • Safety valve

The safety valves are normally in paires,on a single chest,to protect the boiler against overpressure.they are arranged to open automatically at the pre-set blow off pressure and close when the steam pressure in boiler has returned to a safe level.

  • Salinometer cock

It is water outlet valve or cock,and cooling arrangement is provided for the sampling and analysis of feed water.

  • Scum valve

It is connected to shallow dish positioned at the normal water leveling the boiler.this valve enables the blowing down or removal of scum and impurities from the water surface.

  • Blow down valve

It is fitted at the bottom of the boiler and enables sediment to be discharged from the bottom of the boiler.it may be used when partially or completely emptying the boiler.

Boiler gauge glass regulation

  • Every boiler is fitted with at least two independent means of indicating the water level
  • One of which is to be direct reading gauge glass and the other one is to be either an additional glass  or an approved equelant device.
  • Water tube boilers are to fitted with two systems of water level detection,which are to be independent of any other mounting on the boiler.
  • Glass water gauges are to be so located that the lowest visible level in the glass is either not lower than 51mm(2inch) above the lowest permissible water level.
  • The lowest permissible water level is just above usually 25mm(1inch)above the top row of tubes when cold.

Boiler gauge glass blowdown

  • It is possible for the small water or steam passage to block with scale or dirt and the gauge will give an incorrect reading.
  • To check that passages are clear a “blowing through” procedure should be followed.
  • Close the water cock and open drain cock C.
  • The boiler pressure should produce astronge jet of steam from the drain.
  • Steam cock is now closed and cock “B” opend
  • A jet of water should now pass through the drain.
  • The absence of a flow through the drain will indicate that the passage to open cock is blocked
  • Then close drain cock and open water cock B,water level rise up.Finally open steam cock A.

Boiler Blow Down

Steps for blow down procedure are as follows

  1. Open the overboard or ship side overboard valve first.
  2. Open the bottom blow-down valve, this valve is a non-return valve.
  3. The blow-down valve adjacent to the boiler should be open fully so as to prevent the cutting of the valve seat.
  4. The rate of blowdown is controlled by the intermediate valve.
  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.

Purpose of Boiler blow down

Boiler blowdown is done to remove carbon deposits and other impurities from the boiler. Blowdown of the boiler is done to remove two types of impurities.

scum and bottom deposits. This means that blow down is done either scum or bottom blowdown. moreover, the reasons for boiler blowdown 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.

Boiler water level indicating methods.

  • Turbulence gauge glass
  • Reflex plate gauge glass
  • Double plate gauge glass
  • Conductivity/capacitive electrode level detectors.
  • Magnetic float switch (Mobray Float switch controller and alarm)

Boiler remote level indicator

Boiler remote gauge glass level indicator-image credit Reeds General engineering knowledge

The sketch shows igema boiler water level indicator to read from remote station.it consists of a U tube,filled with coloured indicating liquid whose density is greater than that of water and it does not mix with water,having connection to thesteam and water spaces of boiler.

the gauge would have some form of lighting ,to make it easier to read.the upper limb is connected to steam space through a condenser and weir.this provides a constant head of water on one side.the other side is subjected to a variable head of water depending on the actual water level in the boiler.

For equlibrium,the forces acting on either side must be equal.

If water level stable

H=h+px

P= Density of indicating fluid.

As the water level in the boiler rise, the head of the right-hand side(h) increases and forces some of Igema fluid from the reservoir, into the left-hand side(x and H will be reduced)increasing the height of fluid in the gauge.

this is turn display some of the water from the weir, which overflows back to the boiler. when the boiler water level falls, “h” will be reduced, ‘x’ and ‘H’ will be increased. The level of water in the condenser reservoir is maintained by condensing steam.

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