Working Principle of Fresh Water Generator on Board Ship

Fresh water generator(FWG) is one of the essential machineries on board ship, After-all a large amount of fresh water is utilized on ship every day. On board ship Fresh water is used for the most part by huge boilers having steam turbines as main propulsion or cargo pumps driven by turbines. They all in all record for 30+ Tons of fresh water every day separated from the day by day necessities of 10+ ton for crew consumption as drinking, cooking, washing,Tank cleaning and different utilities. Adequate consumable water might be taken up against the cargo load capacity in the port for satisfying crew and machinery necessity yet isn't prescribed to utilize shore water for boilers because of their low quality, so Fresh water generator is unavoidable on board ship.

But why do we need the FWG on board in first place?

In the middle of good old days people used to bring across lots of water in their ship. It was good however it expands the load on the ship and furthermore water lack issues were always there. Be that as it may, the nature of shore water will be extremely (too much) bad to be used in water tube boiler and filling (expansion) tanks. The storage space that could have been used for fresh water can as a result be used for fuel or added/more space for payload and products (that are bought and sold) when fresh water generator is installed on a ship. As you may know the first (or most important) concern for running a ship or any transportation business is to make money. Further more Boiler needs demineralized water to reduce overall scale development and corrosion issues. It is also normally used to build the general efficiency of the running machinery, mostly it uses the engine warmth/heat by Cooling The Hot Jacket Cooling Water At Around 85 Deg Celsius To 65 Deg Celsius.

Working Principal Of FWG?

Extremely important rule of all low weight freshwater generator is that, boiling point of water can be decreased by lessening the pressure of the air surrounding it. By keeping up a low pressure, water can be bubbled at low temperatures say 55 degree Celsius. The source of warmth for the fresh water generator could be waste heat rejected by generators and main engine jacket cooling water. This decrease In Saturation Temp lead to early vaporisation of water due to vacuum created within the chamber. It Is Created By The Help Of An Air Ejector, Which Extracts The Air From That compartment/chamber. An Air Ejector Works On Bernoulli's Rule and being used side by side with a brine ejector used to suck salt water from the chamber. The vacuum help disappear the water leaving behind the salt or salt water to be sucked by salt water ejector. It is then passed through the dimmister (A sort of punctured plate with very minute gaps) which traps any water or salt particle traveling through it. The evaporated water is then condensed together with the help of ocean water and after that tested for sality with the help of salino-meter.

Regulations Regarding Production of sea water on board ship:

1. Ship must be 12 nautical miles away from the nearest coastline to start the FWG.
2. Engine must be running at full ahead sea speed during start of FWG.
3. Ensure main engine parameters are normal.
4. Ship is not in congested water.
5. Ship is not maneuvering.
6. There is no oil/chemical spill reported in the visinity of the ship.

Safety Equipments on A Typical FWG:

1. Vacuum Breaker For Releasing Vacuum when we shut down the FWG.

2. Relief Valve For Releasing The Excess Pressure.

3. High Salinity Alarm: It Is Fitted To The Salinometer As It Measures Higher Salt Content In The Water Produced.

4. Temperature Gauge.

Starting Procedures for Fresh water generator:

• First we need to ensure that suction, discharge and overboard valves for ejector pumps are under open condition. Now once been checked, start the ejector pump.
• Close the vacuum breaker valve.
• Make Sure the S.W pressure at ejector (air) is not less than 3 bar. Now wait for 10-15 minutes to build vacuum in the generation chamber of fresh water generator. Wait until it shows more than 90% vacuum.
• Now open the sea water feed to the evaporator. Check for the sea water level in the shell through the sight glass and ensure it is at optimum level controlling feed through feed valve. Once the heating coils are fully submerged by the sea water open the J.C.W valves to the evaporator slowly.
• Purge out any air in the system by opening air vent at the top of the evaporator.
• Check for evaporation from the sight glass and then start the salinometer.
• Divert the outlet back to the feed water until the salinity is under the limit.
• When the salinity level is as per the desired or set value, open the discharge valve of distillate pump which sends the water to the fresh water holding tank through flow meter.
• Increase the evaporation rate and start the discharge pump. Check for the sytem to be stable.

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Author Amit                                                          Article requested by: NIÑO GONZALES

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Ship side valve Functions, Definitions and Specification

The valves on the side of the ship / Ship-side valves on the top refer to valves that are connected to the hull, without having anything to medium with the valve on the side of the ship. During the dry dock, the valves are checked by the dry dock personnel, while the work of the ship's engineers is to check the internal valves. For total seaworthiness of the ship, the ship side valves must always be in proper condition, They must have local as well as remote control operation which must be permanently connected. Pipes of the inlet and the outlet to the sea must be provided with valves or taps attached directly to the outer coating or to the plating’s of fabricated water boxes attached to the shell plating. These accessories must be fixed with threaded bolts equipped with hazelnuts.

Heads or bolts screwed into heavy steel pads built into the plating. Screw holes must not penetrate into the plating. Valves for the Ship side Side Applications must be installed so that the pipe section can be located directly inside the valve without removing the sealed integrity of the hull. Taps and accessories of the ship, when made of steel or other approved material with low resistance to corrosion, must be adequately protected against wastage/rusting. In the visual inspection, the valves should not show any sign of defects on their body, their internals and the spindle. The Thread and other parts must be in good condition, which ensures freedom of movement in the operating conditions. Ship side valves are made and tested in accordance with the rules and regulations of the classification society in the presence of an expert.

Markings on the Ship Side valve of the Ship: The following data are marked on each side of the valve.

a) Nominal size of the drill (eg NB 80);

(b)Nominal pressure according to section 1.4.

c) Test pressure

d) Brand / seal of the manufacturer.

e) Marker / Stamp of Classification Surveyor.

f) Flow direction

g) The weight of the valve to be marked.

h) Metallic tag numbers provided with the table.

Why are they so important?

In a Ship, all that has been installed has a purpose, besides the valves on the side of the ship is controlled and supervised by the classification society. Maritime organizations (classification society and flag state) know a series of incidents of local flooding machinery space installations and averaging two reports of floods received annually. The side flaps ensure the sealed integrity of the shell and the internal working sea tube from the sea. Operation of the valves allows maintenance of pipeline equipment. They are used to prevent the ingress of water into the compartments after a failure in a pipeline or equipment. Therefore, the valves must provide a watertight barrier when closed. In normal commercial vessels, the ship is docked dry and the ship's valves are inspected to meet classification society standards. This inspection confirms the condition of the valve and closes it when closed. For installations, such as mobile drilling units of fleets or ships of intervention, the inspection may be performed by dry dock times or repair in the port, however, permanently anchored facilities these inspections are carried out. They become more complicated as the right holder must be able to demonstrate that the valve is sufficiently leak-proof and in good condition. This can be combined by a close visual inspection and on-site testing. After repairing or installing a new valve, the dry dock repair shop usually provides a 5-year warranty on these valves. However, if the valves are damaged during the warranty period, the workshop will bear the cost of the diver's time and expenses.

Listed below are few Ship-side valves that is seen on vessels.

              Name of the Valve                                Side                          Type                        Specification

1  Sea water suction for low sea chest                   Starboard      Butterfly valve           10K 550A

2  Sea water suction for high sea chest                  Port               Butterfly valve           10K 550A         

3  Sea water suction F.W.G sea chest                     Starboard      Butterfly valve           10K 125A         

4  Main cooling S.W discharge overboard             Port               Butterfly valve           10K 300A               

5  O.W.S discharge overboard                                Port              SDNR                         10K 40A          

6  Fire & G.S pump discharge overboard               Starboard     SDNR                         10K 200A             

   (fire and ballast pump)                                                             (Screw Down Non-Return valve) 

7  Steam blow valve to high sea chest                    Port              SDNR                         16K 40A            

7a Air vent for high sea chest                                 Port              Ordinary Globe valve(SDGV)   10K 40A                         

8  Steam blow valve to low sea chest                     Starboard     SDNR                         16K 40A           

8a Air vent for low sea chest                                  Starboard     SDGV                         10K 40A              

9  Steam blow valve F.W.G sea chest                     Starboard     SDNR                         16K 40A      

9a Air vent for F.W.G sea chest                              Starboard     SDGV                         10K 40A                     

10 F.W.G discharge overboard                                Port             Butterfly valve            10K 125A                         

11 Boiler blow down direct overboard                    Port            SDNR                          16K 40A               

12 Soil/waste water discharge overboard                Starboard   Storm valve                  10K 100A                         

13 Stop valve for waste water overboard                Port            Storm valve                  10K 100A                  

14 Gally/provision refer drain overboard                Starboard   Storm valve                  10K 100A                       

15 I.G system scrubber cooling tower                     Starboard   Butterfly valve             10K 200A                             

   discharge overboard 

16 Draught gauge overboard valve                          Starboard   SDGV                         10K 50A                      

17 Weather deck scupper overboard                        Port                                                125A  

18 Weather deck scupper overboard                        Starboard                                       125A  

19 Main water ballast pump discharge overboard   Port            Butterfly valve            10K 550A             

20 ODME discharge overboard                               Starboard   Butterfly valve            10K 350A    

What does K and  A stand for in specifications?

 K : is the pressure.

 A : is the Nominal diameter of the pipeline.  

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How to do crude oil washing on ships in 10 easy steps

COW or Crude Oil Washing cleans cargo tanks with jets of high pressure oil when the ship is unloaded. The crude oil is pumped through the washing machines to the jets and serves as a cleaning agent which is then pumped to land with the load/Product Discharge. It has been found that crude oil is more reliable washing medium than water. The washing / dissolving effect results in heavy, waxy and asphaltic deposits on the sides and bottom of the Holding tank to convert back into the liquid so that they are easily discharge To the shore. The Crude Oil Washing (COW) techniques has several advantages over washing with water, using Cow as the washing medium reduces the Sludge deposition, thereby reduces the discharge (Stripping) time and increasing the load/cargo flow. There low deposition of mud at the bottom of the tank which reduces Ship constant and so increase its dead weight. On the other hand washing with water takes more time due to more deposition of mud at the bottom. They not only cause delay in washing but also release hydrocarbon gases which becomes a challenge to gas free the tank. There are three essential prerequisites for the washing of petroleum product:

A fully functional inert gas system for maintaining the tank atmosphere in an inert state during the whole wash cycle.
Fixed Piping Lines for Tank washing system
Means to ensure that the bottom of the tank is clean and dry at the end of the operation.

The vessel must be equipped with an inert gas system which can function properly and produce Inert Gas with a maximum oxygen content of 5%. This should be maintained during the COW to ensure that the oxygen content does not exceed this value. No COW operation to be performed if the Inert Gas installation of the ship is not working properly and the oxygen content is not within the desired limit. The oxygen content must not exceed 8% by volume. The IG pressure in the tank must not be less than 200 mm. wg. The vessel to be washed with oil must be equipped with a fixed tank wash system connected by permanent lines to the main load venting system or by separate laundry lines of the loading system. The crude oil washing of discharging tanks takes place in a single step or in several stages during the discharge of the charge.

One Step Washing:

As soon as the tank is almost emptied , Washing starts and the tank is dried under pressure during the last washing step using stripping. The machine adjusts to the vertical angle from 0 to 140 degrees. Since single-stage washing is performed only during the last tank discharge stage, it is necessary to use the ship's instructor to remove the pump or reduce the speed of the charge pump so that the vacuum pump is effective. This method is generally useful between discharges at two terminals or between ignition operations.

Multi-stage washing

This involves washing the interior areas of the tank in stages as the load is pumped out of the tank. The revolutions of the charge pump are not reduced. Depending on the back pressure in the distributor of the vessel, this process results in a very small time loss during the bulk discharge of the tank. The wax and sediment collected in the spacers and other structural elements above the bottom of the tank are removed and pumped with the load to the ground. Any wax or sediment on the underside of the tank remains as long as the soil is not washed and stripped as a separate step. This increases the total discharge time, but is required for sludge control.


The machines are pre-set in the first stage at a vertical angle of 120 ° to 60 °. In the second step, when the loading level is about two thirds of the tank depth, it is set at a vertical angle of 70 ° to 30 ° and during the third stage when it is set to about 1 meter of the residual charge in the tank to 40 ° to 0 ° is. Each step is superimposed by approximately 10 ° in the preceding step, and the cycle is normally 11/2, ie, in the first step, for example, the nozzle moves in the horizontal plane by slowly changing its angle from top to bottom or from 120 * to 60 * from top to bottom, or from 60 * to 120 *, and from top to bottom, or from 120 * to 60 *

The regulation requires that the cow jets cover at least 90% of the horizontal surface area of the floors, rods and structural elements and at least 85% of the vertical direct zone. Shaded areas should be covered with splashes or deformations.

List of oil washing equipment controls
In the case of unloading of raw materials, the master must notify the competent authority and the terminal (or any other ship in transit to the ship), at least 24 hours in advance or at the time of compliance with the regulations. The washing of crude oil should only be approved once each step of the COW operation must be specified in the oil wash operation plan. There must be an operational checklist for the use of the crew at each port of discharge. This includes the control and calibration of all instruments to be used during the operation of the cow.

Pre-Arrival Checks at Discharge Ports

1. Notification to the terminal of the operation of the cow.
2. Is the oxygen analyzer successfully tested and operated?
3. Is the water heater and the flushing system of the motor compartment insulated?
4. All Valves Marked washing is isolated from that of engine room pipe line.
5. Are all V/vs closed for stationary tank washing machines?
6. Are the tanks cleaning lines under pressure and tested for leakage?
7. Do you make portable drives for the M/C quick tank sink that will be tested?
8. Have gauges on the top line relief manifold has to be verified.
9. Should the trimming system control equipment cheked?
10. Has the communication system been reviewed and tested?
11. Has the organizational plan been set up and defined the duties and responsibilities of crew?

 Checklist before cow Operation

1. All checks and conditions are in order prior to arrival.
2. Unloading/operating the raw oil wash has been discussed with ship and shore staff and is readily available the agreed plan for a simple reference?
3. Was the communication link between the control and control stations and the monitoring/shore station repaired and functioning properly?
5. Has the fixed and portable oxygen analyzer been verified and calibrated?
6. Is the inert gas generator functioning properly and the oxygen content of the inert gas is delivered below 5% of the volume?
7. Is the oxygen of the tank washed about below 8% in volume?
8. Is the gas pressure is positive for the entire cargo tank?
9. Keep person responsible for checking all the lines for leaks as soon as the operation starts?
10. Are stationary machines for the necessary washing and portable motor units, if installed, mounted and repaired?
11. Have valves and lines both in pump-room and deck been checked?

Checklist-During the operation of the cow

1. Is the quality of the inert gases supplied often controlled and recorded?
2. Are all deck and machine lines often tested for leaks?
3. Is the current tank only available for COW?
4. Is the pressure on the appropriate tank wash line indicated?
5. Is the processing time of the tank disk indicated?
6. Is the washing machine, which works with the propeller groups, often properly controlled and working?
7. is a responsible person constantly stationed on the bridge?
8. Will trim be satisfactory when bottom washing is in progress as specified in the COW manual?
9. Is the level in the reservoir for tank washing often checked to avoid overflow?
The cow should be abandoned immediately if:There is an error in the inert gas system or oxygen exceeds the permissible limit, or The pressure in the tank falls below the air pressure or a minimum required (usually 200 mm water gauge).

D Checklist-After cow operation

1. Are all the valves between the discharge and the washing line of the tank closed?
2. Has the laundry been drained of oil from the crude tank?
3. Are all valves closed to the washing machine?
4. Are the cargo pumps, tanks and pipes properly drained as indicated?

Sludge and sediment control

Under certain circumstances, substantial sludge may also form in the containers, even in the case of crude oils which are not normally associated with such accumulations. Studies of incidents of large sludge accumulation show that there are specific critical temperatures at which crude oil begins to precipitate hydrocarbon species that form mud. This temperature, cloud point, is the temperature at which the crude waxes change from their liquid phase to suspended, nearly solid particles with associated oil components. These particles separated by phases are installed at the bottom of the reservoir and form mud. Sludge from paraffinic crude oil, once formed, is extremely difficult to return to a liquid phase by heating alone. Even an effective COVER program, which eliminates them from boats, has only managed to solve the problem in shore-based containers.

Electrostatic risks
The oil used for COWing must be free of suspended water to minimize the formation of electrostatic charges by high pressure jets. To ensure that the oil supply to the VAE is dry, the cargo tank to be used must be discharged to the ground before the start of the COW. At least one meter of load must be discharged. This removes all bottoms from the tank before the mold starts. VACA procedures that remove the flushing fluid from the discharge current or use a sliding container that is then filled with clean oil will avoid this problem.

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How inert gas is produced on board ship?

The Inert gas is produced in the IGG by the combustion of fuel with air. Marine diesel oil (MDO) or MGO is drawn from the storage tank and pumped through a filter to the main burners. A blower delivers the air necessary for combustion.
Oil and air is mixed in the correct proportion in an air-atomizing burner, which is ignited by the pilot burner. The fuel-air ratio and the oxygen content of the IG can be regulated by a valve in the fuel supply line to the main burner. Purple-blue to rose-purple flame indicates good combustion whereas yellow flame means too much fuel oil or too low delivery air pressure causing low atomizing pressure and risk of soot formation.

The inert gas thus generated consists of mainly Nitrogen and carbon dioxide. It is first cooled in the combustion chamber by means of a sea water jacket, which surrounds the combustion chamber. The gas then comes in direct contact with a water spray. The resultant IG comprises of:

Oxygen  :            2% by volume
Carbon dioxide: 13% by volume
Nitrogen   :        85% by volume

Due to perfect combustion, there is a complete absence of soot. The temperature of the gas leaving the generator is about 5-10* above the cooling water temperature.
During the operation, should there be a reduction in demand for the IG, the excess gas is vented to the atmosphere. The operation is continuously monitored for flame, water or control air failure and excessive cooling water level. Should any emergency condition arise, the IGG automatically shuts off and sounds an audible alarm.
This is a costly installation, which is usually found in product or chemical tanker. With some clean cargo tainting could be a problem, if flue gas from boiler is used.

Chemical tankers & Product tankers are normally FRAMO tankers hence IGG is fitted on these ships, secondly the quality of IG plays a very important role in maintaining the quality of chemicals being transported.
Conventional tankers with turbine driven COP will always have a large water tube boiler and thus Inert gas on these ships will be produced from boiler flue gas. Boiler will be used continuously during cargo discharge for running the cargo oil pump turbines (COPT).
On Framo tankers, the cargo discharge system consists of each cargo tank having a fixed submersible cargo pump, this pump is coupled to a hydraulic motor. The hydraulic motor is driven by high pressure hydraulic oil (220 bar), this high pressure hydraulic oil is delivered by hydraulic power-packs located in the engine room, these power packs are normally driven by diesel engines. Thus these ships use an IGG for production of inert gas. They do not have large boilers as no turbines are present, hence IG from boiler uptake is not possible.
Carbon Molecular Sieve type Nitrogen Generator

The molecular sieve nitrogen generating plant is usually installed on ships such as chemical carriers where oxygen critical cargoes may be carried and purity of the inert gas is of utmost importance.
The system uses the pressure swing absorption method using a carbon molecular sieve which has the unique property of absorbing oxygen in preference to and at a faster rate than nitrogen. When air passes through the molecular sieve, oxygen is absorbed. Nitrogen gets across and is collected. However the sieve gets saturated with oxygen after sometime and needs to be changed over.
The system uses two vessels packed with the carbon molecular sieve. The vessels are alternatively on line or in the process of regeneration. As one reaches saturation, the system switches to the second. Change over time is appox. 1 minute.

The oxygen from the saturated sieve is removed by applying a reduced pressure to the vessel and then venting to the atmosphere. The nitrogen gas thus generated is fed to a buffer vessel from where it is distributed to the inert gas line. The purity of the nitrogen depends on the flow rate and can be as high as 99.5%. The lower the flow rate, the higher the purity and vice versa.

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Properties And Working System Of Marine Fuel Oil

Properties Of Marine Fuel

Density:

• This Is Described As The Weight Of A Unit Volume Of A Liquid.
• The Unit Is Kg/ M3.
• The Density Is Measured At A Standard Temp., Of 150c As It Varies With The Temp.
• If Density Is Taken At Different Temp., A Correction Factor Is Applied To Ascertain Correct Density At That Temp.
• Reciprocal Of Density Is Specific Volume.
• This Is Important Property For Estimation Of Bunker Capacity.

Viscosity:

• This Is Defined As The Resistance Of Fluids To Change Shape, Which Is Due To The Friction Between The Molecules Of A Fluid Producing A Frictional Drag.
• Absolute Dynamic Viscosity: This Is Defined As The Force Required To Shear A Plane Fluid Surface From Other Plane Surface, Over An Area Of 1sq. M. At The Rate Of 1m/Sec., When The Distance Between The Two Surfaces Is 1m. Absolute Viscosity Is Difficult To Ascertain And Therefore , Kinematic Viscosity Is Generally Used To Measure.
• Kinematic Viscosity: This Is The Ratio Of Absolute Viscosity To Density Of The Fluid At A Particular Tyemp. This Is Measured By Flow Of A Set Of Volume Of A Fluid At A Particular Temp., And Through A Specifically Calibrated Instrument. It Is Measured On The Basis I.E., Number Of Seconds For A Calibrated Instrument Like Redwood No.1, At 380c.
• Centistoke: This Is The Unit Used By International Standard And Measured At 500c. Higher Temperature At The Time Measurement Can Be Used For Viscous Fluids And Appropriate Corrections Can Be Made Later.
• Temperature Affects The Viscosity , Which Decreases With The Temperature Of The Liquid.
• Viscosity And Temp., Play A Vital Role While Selecting Oil For A Particular Service.
• For Best Results During Automisation Of Fuels Of High Viscosity, It Is Necessary To Heat The Fuel To Bring Down The Viscosity To About 30 Centistokes Or Still Lower At The Injector Point.
• Viscosity Of Diesel Oil Is 7centistoke At 380c.
• Viscosity Is Always Quoted At A Certain Temp., Without Which, It Has No Meaning.

Flash Point:

• This Is The Minimum Temperature At Which The Oil Begins To Give Out Flamable Vapours, Which Would Cause Momentary Ignition On Application Of Heat / Flame In A Specified Apparatus.
• The Test May Be Specified As ‘OPEN’ Or ‘CLOSED’ Depending On The Type Of Apparatus.

Fire Point:

• It Is The Minimum Temperature At Which The Vapours Given Out By The Heated Oil Are Sufficient To Ignite And Give Out More Vapours By The Heat So Produced, So That They Burn Continuosly.
• This Temp., Is Different From Flash Point And Is Higher By Anything Up To 400c.

Acidity (ALKALINITY):

• It Is Indicated By Neutralisation Number.
• The Number Is The Mass In Miligram Of An Alkali  Required For Neutralising The Acid Present In 1gm Of The Sample.Neutralsation Number Can Also Be Expressed As Parts Per Million Per Ml Of Sample Of Oil.
• Total Base Number (TBN) Is Often Used For Alkalinity Indication Of Lubricating Oils.

Ash:

• This Is Expressed As Percentage By Mass Of The Original Sample Of Oil, Which Is Evaporated And Ignited Until All The Traces Of Carbon Have Disappeared
• The Ash Left After Above Process Contains Hard And Abrasive Minerals Such As Quartz, Silicates, Iron, Aluminium Oxides, Sand Etc.

Preparation Of Heavy Fuel Oil:

• Present Days Modern Marine Diesel Engines Are Run Continuosly On Heavy Fuel Oil Only
• The Main Purpose / Object Of Preparing The Heavy Fuel Oil For Use In Two Stroke Diesel And Four Stroke  Engine Is To Remove The Impurities Like Water, Sludge, Catfines. Also To Heat The Fuel To  Get The Correct Injection Viscosity As Recommended By Engine Manufacturer.

Preparation:

• After Receipt On Board Fuel Oil Is Stored In Bunker Tanks. In These Storage Tanks Fuel Is Heated To Just The Required temp To Keep It Pumpable.
• The Fuel Is Transferred From The Storage Tanks To The Settling Tank By Using A Transfer Pump. The Settling Tanks Are Normally Two In Number Having Capacity Of  24 Hours Use.
• In The Settling Tank Fuel Is Further Heated And Retained For As Long As Possible. In The Settling Tank Some Of The Heavier Solid Impurities And Water Get Separated By Gravity And Collect At The Bottom Of Tank. These Have To Drained Out Regularly.
• Heavy Fuel Oil Is Then Purified Further By Centrifuges. Centrifuges I.E., Purifiers And Clarifier Are Connected In Series. In Centrifuges The Impurities Such As Solid, Liquid And Sludge Are Removed Completely.
• In case Of Alfa Laval Purifiers Then By Using Alcap System  The Oil Is Clarified And At Same Time Separated Water Is Removed From Bowl.
• Purified Oil Is Then Pumped To The Daily Service Tanks. From Daily Service Tank Oil Flows Through A Three Way Valve To A Mixing Tank. 
• A Flow Meter Is Fitted In The System To Indicate The Fuel Consumption.
• A Mixing Tank Installed In Fuel Oil System Of Engine, Is Designed To Operate On Heavy Fuel Oil. The Purpose Is To Produce A Gradual Variation Of Fuel Quantity During Transition Period From Diesel Oil To Heavy Oil Or Vice Versa. The Supply Oil Changes In Viscosity And Temp., Progressively With This System. Mixing Tanks Has Proved To Be Useful For De gasification And Entrainment Of Air From The System. Mixing Tank Can Also Be Used As A Metering Tank. This Is Used To Collect Recirculated Oil And Also Acts As A Buffer Tank As It Will Supply Fuel Oil When Daily Service Tank Is Empty.
• Booster Pump Draw Fuel From Mixing Tank, Raises It’s Pressure And Passes Through The Heater. Oil Is Heated Up With Curresponding Reduction In Viscosity In Accordance With The Temperature – Viscosity Relationship.
• Controled Temperature By Viscosity Regulator Ensures Fuel Have A Viscosity Of Combustion Quality.
• A Pressure Control Valve Provides Constant Supply To Engine Driven Fuel Pump Ensuring High-Pressure Injection Supply Pressure.
• Pre-Warming Bypass Valve / Line Is Used To Heat Up The Fuel Before Starting The Engine.
• Service Tank Diesel Oil Are Connected To System Through A Three-Way Valve. The Engine Can Be Started And Manevier On Diesel Oil Or Even A Mixture Of Diesel Oil And Heavy Oil.
• Further The Sestem Include Various Safety Devices Viz., Low Level Alarms And Remotely Operated Tank Outlet Valves Viz., Quick Closing Valves, Which Can Be Closed From Outside The Engine Room In The Event Of Fire.

Importance Of Viscocity In Fuel Oil:

• Steady Values Of Viscosity Is To Be Maintained For Proper Atomization. Atomization Is Breaking Down Of Fuel Oil To Fine Droplets So That A Large Surface Of Oil Is Exposed To Heat And Oxidation.
• Low Viscosity Cause Too Fine Atomization And Thus Droplets Will Not Penetrate Deep Enough. This Will Cause Burning Close To Nozzle Tip. Because Of This Poor Combustion Nozzle Operation May Be Troublesome.
• On The Other Hand High Viscosity Cause Too Large Droplets Of Oil And Have Enough Energy To Strike Metallic Surface. This Will Lead To Overheating. This Also Create Problems In Functioning Of Centrifuges

Settling Tank

• The Fuel Is Transferred From Bunker Storage Tanks To The Settling Tanks Prior To Centrifuging, For Use In Diesel Engines And Also For Burning In Boilers.
• The Fuel Is Heated In Settling Tank And Retained There For As Long As Possible. The Settling Period Should Not Be Less Than 24 Hours For Optimum Separation Of Impurities.
• While The Fuel Is In Settling Tank Some Of The Solids, Sludge And Water Separates Out And Settle Down At The Bottom Of The Tank From Where They Can Be Drained Off Through The Drain Connections Which Must Be Of Self Closing Type.
• For Use In Diesel Engines, Only Purification By Settling Is Not Enough As Subsequent Centrifuging Is Carried Out To Separate The Impurities. Still Draining Of The Settling Tank Regularly Once In A Watch Is Recommended.
• High And Low Level Alarms Are Fitted On The Tanks.
• Fuel Oil Is Heated In The Settling Tank For Faster Separation Of Impurities, For This Reason Settling Tanks Are Provided With Heating Arrangement.
• The Rate Of Separation Of Impurities In Settling Tank Depends On Following Factors.
• Higher The Density Difference Between Oil And Other Impurities, Greater Will Be The Rate Of Separation.
• Bigger The Size Of The Impurity Particle Better Will Be The Rate Of Separation.
• Lower The Viscosity Of Oil, Better Will Be The Rate Of Separation.
• As Heating Results In Better Separation, The Fuel Is Generally Heated To A Minimum Temp., Of 500c Or To A Temp., About 700c.
• One Of The Solid Contaminats In The Fuel Oil Is A Cat (CATALYTIC) Fine, Having Density Of 2600 To 2800 Kg/M3 And Having Particle Size Of 10 To 50 Microns. These Compound Take Long Time To Settle Even When The Fuel Temp., Is Raised To 700c.
• An Ideal Settling Tank Should Have Inclined Bottom To Facilitate The Separation And Draining.

Safety Devices Fitted  On Settling Tank;

Fuel Outlet Valve: This Is Remotely Operated Quick Closing Valve Fitted On The Settling Tank.  It Is Arranged To Operate From Outside The Engine Room In Case Of Emergency.
Air Pipe: This Pipe Is Led To Above The Upper Deck Level And External To The Deck House. The Outlet Of This Pipe Is Fitted With Metallic Wire Gauge Screen Called Flame Trap To Prevent Fire Hazards.
Thermometer:This Is Used For Measuring The Temp., Of Oil
Sludge Valve / Cock: This Is Used For Draining Water And Sludge  Collected At The Bottom Of The Tank. This Valve Must Be Self-Closing Type.
Over Flow Pipe: It Is Fitted At The Top Of The Tank And Led To An Over Tank In The Double Bottom. An Alarm Activated By An Over Flow Condition Is Sometime Fitted To The Tank.
Alarms: These Are Fitted To Warn High Fuel Temp., And Low Fuel Level.
Dumping Valve: This Valve Is Used In The Event Of Fire, To Dump The Oil To The Double Bottom Tank.
Quick Closing Valve: In Case Of Emergency To Stop The Engine, This Valve Cuts The Supply Of Oil To The Engine. The Arrangement Provided Outside The Engine Room  To Close This Valve.
Sounding Valve: This Pipe Is Provided To Take Sounding Of The Tank For Checking Fuel Level.

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Author Amit                                                                 

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Basic concept on Ship stability and free surface area

Stability Of Ships

Two Principal Forces Which Act On A Ship Floating Freely Are – Weight  &  Buoyancy.

For The Ship To Float, It Must Displace It’s Own Weight Of Water.

Then To Be In Equilibrium Condition, The Centre Of Weight & The Centre Of Buoyancy Must Be Vertically In Line.

External Or Internal Forces Can Move The Ship In Either Transverse Direction Or Longitudinal Direction. But It’s Ability To Return To It’s Original Stable Position I.E., Equilibrium Condition Is Related To The Stability.

Upright Position Of Ship :  A Vessel Is Said To Upright If It Is Rolling Slightly About The Upright Position.
Statical Stability : This Is The Measure Of The Tendency Of A Ship To Return To The Upright Position If Inclined By An External Force.
Upright Position Of Ship : In The Upright Position, Weight Of The Ship Acts Vertically Down Through The Centre Of Gravity ‘G’. While The Upthrust Acts Through The Centre Of Buoyancy ‘B’.
Equilibrium Condition Of Ship : When Weight Of The Ship Is Equal To The Upthrust And The Centre Of Gravity And The Centre Of Buoyancy Are In The Same Vertical Line, The Ship Is Said To Be In Equilibrium Condition.

Consider A Ship Inclined By External Force To An Angle ‘Ǿ’.
               In This Case Centre Of Gravity Remains In The Same Position, But The Centre Of Buoyancy Moves From ‘B’ To ‘B1’.
    Therefore Buoyancy Acts Through ‘B1’  And The Weight Still Acts Through ‘G’, This Creates
    The Righting Moment = Δg X Gz -------------------(1)
    This Moment Tends To Turn The Ship To It’s Upright Position.
    Righting Lever = Gz = Gm Sinǿ
    Now

The Vertical Through New Centre Of Buoyancy -‘B1’ Intersects The Centreline At ‘M’ Which Is Called The Metacentre. The Height  Of ‘M’ From The Centre Of Gravity Is Called The Metacentric Height- ‘GM’

Stable Ship : ‘GM’ Is Said To Be Positive When ‘G’ Lies Below ‘M’. In Such Case Ship Will Roll Back To Original Upright Position When Disturbed By External Or Internal Force.

Tender Ship : A Stable Ship With Small Metacentric Height Will Have Small Righting Lever At Any Angle And Roll Easily. In Such Case Ship Is Said To Be Tender Ship.

Stiff Ship : A Stable Ship With Large Metacentric Height Will Have A Large Righting Lever At Any Angle And Will Have Considerable Resistance To Rolling. In Such Case It Is Called The Stiff Ship.

Unstable Ship : When Metacentre ‘M’ Lies Below The Centre Of Gravity ‘G’, Then ‘GM’ Is Said To Negetive, Which Increases The Angle Of Heel. In Such Case The Vessel Is Said To Be Unstable And Will Not Return To Upright Positon.

Neutral Equilibrium : When Centre Of Gravity And The Transverdse Metacentre ‘M’ Coinside, The Righting Lever Is Zero And There Is No Righting Moment Acting On The Ship. In Such Case Ship Will Remain In Inclined Position With An Angle ‘Ǿ’, And The Ship Is Said To Be In Neutral Equilibrium Condition.

Stability At Small Angle Of Heel
Transverse Metacentre :
The Height Of Transverse Metacentre Above The Keel ‘KM’ May Be Found Out, By Considering The Small Inclination Of The Ship About It’s Centre Line.
For Small Angle Of Heel
Upright And Inclined Water Line Intersects.
Volumes Of The Emerged And The Immersed Wedges Are Equal For Constant Displacement.
    Now
    The Distance Of The Transverse Metacentre Above The Keel Is Given By
    Km = Kb + Bm -------------------------------------------(1)
    Note : ‘KB’ Is The Distance Of The Centre Of Buoyancy Above The Keel. This Can Be Found Out From The Hydrostatic Curve.

‘BM’ Can Be Found Out As Follows.
        Let Us Consider A Ship As Shown In Figure, Whose Volume Of Displacement Is ‘▼’, Lying Upright At Water Line ‘WL’. The Centre Of Buoyancy ‘B’ Being On The Centre Line Of The Ship.
        If The Ship Is Now Inclined By An Angle ‘Ǿ’, It Will Lie At The Water Line W1l1, Which Intersects The Original Water Line ‘WL’ At ‘S’. Since ‘Ǿ’ Is Small It May Be Assumed That ‘S’ Is Lying On The Centre Line.
        A Triangular Wedge Of Buoyancy ‘W1SW’ Has Been Moved Across The Ship To ‘L1SL’ Causing The Centre Of Buoyanmcy To Move From ‘B’ To ‘B1’
    Now
    Moment Of Shift Of Ship’s Buoyany From B To B1
    = Moment Due To Shift Of Buoyancy Wedge
Therefore
    ▼ X Bb1  =  V X Gg1
                                         V X Gg1                                        
                Bb1  =   ---------
                              ▼
                                   V X Gg1
             Bm Tanǿ = ---------- -----------------( As Bb1 = Bm Tanǿ )
                                      ▼
                     V X Gg1
        Bm = --------------  ----------------------------------(2)
                        ▼ X Tan∆

    Now To Determine The Value Of (v X Gg1) , Ship’s Length Is Devided In To Thin Strips Of Length
    Δx. The Half Width Of Original Water Line Is ‘Y’
    Therefore
    Immersed C/S Area Of Wedge = ½ Y X Y Tanǿ X Δx
                                                                   = ½ Y2tanǿ X Δx
    Volume Of Immersed Wedge  = Σ ½ Y2tanǿ X Δx

The Volume Of This Wedge Is Effectively Moved From One Side To The Other By A Distance Of (4Y/3)
    Therefore
    Total  Moment Of Shift Of Wedge
                        = Σ ½ Y2tanǿ X Δx  X (4Y/3)
                                                        = Tanǿ (2/3) Σ Y3 Δx
    We Know That,  (2/3) Σ Y3 Δx = Ixx
    (IXX = Second Moment Of Area Of Water Plane         About The Centre Line Of The Ship )
    Therefore
    Total  Moment Of Shift Of Wedge = Ixx X Tanǿ --(3)
    From Equation (2) And (3)
                                                                                     Ixx
    Height Of Metacentre From ‘B’  =  ------ ------------(4)
                                                                                      ▼
Metacentric Diagram
In This Diagram Kb (HEIGHT Of Centre Of Buoyancyfrom Keel ) & Bm ( Height Of Metacentre Above The Centre Of Buoyancy )  Are Plotted Against The Ship’s Draught As Shown In Figure.
As The Position Of Buoyancy-B & The Position Of Metacentre-M Depends Only Upon The Geometry Of The Ship & Draught Of The Ship At Which It Is Floating, Position Of ‘B’ & ‘M’ (HEIGHT Of Metacentre) Can Be Found Out Without The Knowledge Of Loading Of The Ship At Any Intermediate  Draught.

Stability At Large Angle Of Heel
Stability Discussed So Far Is For Small Angle Of Heel With Certain Assumptions ( Two Water Planes Intersect At The Centreline,Wedges Formed Are Right Angled Tringles) The Metacentric Height ‘GM’ Was Taken As The Measure Of Stabilty.
When Ship Heels To An Angle Greater Than 100, Above Assumptions Cannot Be Made & The Principle On Which Initial Stability Were Based Are No Longer True. Insteadthe Righting Lever ‘GZ’, Which Is The Peprendicular Distance Between Vertical Lines Through The Centre Of Gravity & The Inclined Centre Of Buoyancy, Is Used As Measure Of Stability.

Let Us Consider
A Ship Which Is Inclined To Some Angle ‘Ǿ’ From The Vertical.
‘WL’ Is The Initial Water Line And W1li New Water Line When Inclined.
The Volume Of Displacement In Each Case Is Same.
If The Side Of The Ship Were Vertical Along It’s Length, Then The Two Water Line Would Intersect At The Centreline At Point ‘P’
    Further,
Volume Wpw1 Which Has Emerged Will Be Equal To The Volume Which Has Been Immersed. Let This Volume Be ‘v’
The Centroid Of These Two Wedges ‘g’ And “g1’ Be Located At A Distance Of ‘d’

Now
    Moment Of Shift Of Ship’s Buoyancy
    = Moment Due To Shift Of Buoyancy Wedge
    Bc X ▼ = D X V
                       D X V
            Bc = -------  ------------------------------------------------(1)
                         ▼
    &   Gz = Bc – Bg Sinǿ
                       D X V
                 =  -------- - Bg Sinǿ          
                        ▼
                                                   D X V
     Righting Lever ‘GZ’ = -------- - Bg Sinǿ -------------(2)
                                                      ▼
    Note ;  This Is Called Atwood’s Formula. From This Formula If ‘v’ And ‘d’ Are Evaluated For A Range Of Angles Of Inclination ‘Ǿ’ , The Graph Of Righting Lever ‘GZ’ Verses The Angle Of Inclination ‘Ǿ’ Can Be Drawn. The Curve So Drawn Is Called ‘CURVE Of Statical Stability’

Curve Of Statical Stability

In This Curve Righting Lever Can Be Seen, Rising To A Maximum Value And Then Slowly Fall To Zero.
A Ship Inclined Beyond The Point Of Zero ‘GZ’ Will Be Unstable.
Angle Up To Zero ‘GZ’ Point Is The Range Of Ship Stability At That Particular Load Condition.
Ship Operator Should Know This For Safe Operation Of The Ship.

Free Surface Effect

If The Tank On A Ship Containing Liquid Is Not Fully Filled, The Liquid Moved Through The Tank In The Direction Same As The Heel. For This Reason, The Focus Of The Vessels Central Gravity Shifts Away From The Centre Reducing The Righting Lever "GZ" And Height Metacenter Which Results In Increase In Heel Angle. This Effect Is Known As The Effect Of The Free Surface.
We Consider A Full Tank Water In Ship Of Displacement "Δ" With Inclination To Any Angle Ǿ.
The Focus Of Vessel Moves From "G" To "G1", As Wedge Of Liquid Moves Through The Tank

Tank Divisions Use In Tankers:

Largest Ship With Free Surface Effect Must Be Left Space Of Oil Tank For Expansion.
Tanker Was Built Initially With Centeline Bulkhead And Expansion Tanks. Twin Length, Bulkhead Were Introduced Without Expansion Tanks And Successfully Proved Because It Deals With The Loss Of Metacentric Height Of Design Due To The Free Surface Effect.
It Is Not Possible To Design A Dry Cargo Ship In Same Way As C.G Position Varry With The Nature Of Deposition Of Cargo. Effect Of The Free Surface Is Dangerous For A Ship With Small Metacentric Height And Can Make The Ship Unstable.
In These Ship Tanks Required To Be Pressed Up. If The Ship Is Initially Unstable And Helling To Port, Then Any Attempt To Fill The Water Ballast Results In Reduction Of Stability.

# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced  from any of the source but explained same thing in better detailed way.

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Author Arpit Singh and Amit                                                                 Article requested by: Deepak Kumar

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Overhaul and Repair of a Marine Turbocharger

Introduction
The turbocharger is a very sensitive device; It must be treated with caution. It is very important to know the detailed step by step, to disassemble and in this article we will discuss some safety measures that must be taken before and during the dismantling process.
A turbocharger has a turbine on one side and a compressor on the other. The disassembly should always keep the compressor side beginning to measure the critical clearance between the mounting of the cover on the compressor side and the compressor side tree. This is a very important area that needs to be met and must be supported as he left.

THE MAKER’S MANUAL MUST BE READ AND UNDERSTOOD BEFORE ANY WORK IS UNDERTAKEN ON ANY MACHINERY.

Safety while Dismantling the Turbocharger

Inform the operating personnel accordingly before starting any maintenance work on turbocharger.
As a precaution, place a receptacle for leaking oil under the turbocharger.
Before starting work, secure the rotor against turning.
Ensure that absorbent material is available to soak up any spilled oil.
Ensure that operation and process materials are drained, collected, and disposed of in a safe manner.
Ensure that all spares and tools are available for dismantling and assembling.
Dismantled safety devices must be reassembled and subjected to a functional test immediately after conclusion of maintenance and repair.

Turbocharger Overhauling

Tools Required for Dismantling

Open and ring spanner
Box spanner
Claw spanner
Tommy spanner
Bearing pushing tool
Bearing pulling tool
Pump disc locking plate
Pump removing tool set (provided by manufacturer)
Impeller removing tool set (provided by manufacturer )
Shaft pushing tool
Clearance measuring instruments
Screw driver

Preliminaries before Dismantling:

Before dismantling, exhaust gas from the turbine should be bypassed and a blanking plate should be fitted in turbine inlet casing.
Drain the lube oil from the built-in sump.
Remove the turbine side cooling water connection and drain all water

Turbocharger Sectional View

Image credit:www.auto-innovations.com

Turbine and Impeller

Image credit: www.romaga.com

Turbocharger Dismantling Procedure

Compressor Side Removal:
Dismantling should always be started from the compressor side.
1) First remove the filter silencer assembly or compressor inlet casing from position.
2) Remove the compressor end cover and drain plug on the compressor side.
3) Remove the suction cover and measure the critical clearance .It is the distance between the compressor end cover mounting face and shaft end .Mark it as K.
4) Pull the rotor shaft towards the compressor side until the impeller comes in contact with the insert and determine K2.
1. Impeller clearance L = K - K2
5) Thrust the rotor shaft towards the turbine side until the turbine disc and nozzle ring comes in contact with each other and measure K1
2. Disc clearance M = K1 - K
6) The above measured clearance is very important as this will determine the proper functioning of the labyrinth seal between the impeller and exhaust shield and also the alignment of the shaft.
7) Remove the lube oil pump assembly after removing the pump locking plate.
8) Remove the bearing nut and bearing nut washer.
9) Fix the bearing pulling tool in position and slowly tighten it. This will pull the ball bearing assembly out. Care should be taken while removing bearing to avoid any damage to the bearing and rotor shaft end threads.
10) Mark the position of the bearing in position to put it back as it is while assembling.
11) The ball bearing assembly should not be disturbed in any case. If it is damaged, the whole assembly should be replaced with the manufacturer's new part.
12) Now remove the compressor outlet casing with diffuser.
13) Remove the impeller nut and impeller washer.
14) Remove the impeller and inducer from position.

Turbocharger turbine side dismantling procedure for overhauling,for repairing damaged turbine blades, for cleaning cooling water spaces is detailed in the second page of the article "Overhaul and Repair of a Marine Turbocharger."
Turbine Side Removal
1) Remove the turbine end cover with sight glass on the turbine side.
2) Measure the clearance between the turbine end cover mounting face and shaft end.
3) Check the axial deflection of the pump disc cover. The permissible axial deflection of the pump cover is 0.05 mm.
4) Check the rotor shaft by turning by hand.
5) Remove the pump disc locking plate.
6) Loosen the lube oil disc cover and pump washer on the lube oil pump disc by removing the bolt.
7) Remove the outer shaft end nut and tab washer and then remove inner shaft end nut.
8) Remove the lube oil disc from position.
9) Loosen the bearing nut and bearing nut washer and remove from place.
10) Fix the bearing pulling tool on a resilient mounting and slowly tighten it, and this will pull the roller bearing on turbine side slowly out.
11) Care should be taken while removing the bearing to avoid damage to the shaft outer end threads and bearing.
12) Do not disturb the bearing assembly as improper bearing position may misalign the rotor shaft.
13) Before removing, put punch mark on the bearing in position so that it can be put back as it is.
14) Remove the turbine inlet casing from the turbine outlet casing.
15) Now the whole rotor shaft can be pulled out from the compressor side. While pulling out the shaft, care must be taken to avoid damage to the turbine blades and labyrinth sealing arrangements on the shaft.
16) Remove tab washer and remove seal plate to the turbine outlet casing.
17) Remove shroud ring and shaft seal from the turbine outlet casing.
18) Remove nozzle ring assembly from the turbine inlet casing.
Finally remove the air seal adjusting screw, anti-corrosion zinc assembly, sand cover, and other various accessories in position

# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced  from any of the source but explained same thing in better detailed way.

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Author Amit                                                                     

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What is marine boiler and Boiler system on oil tankers.

A boiler is a closed pressure vessel, in which water vapor is produced from distilled water / feed water. All boilers have a furnace or a combustion chamber in which fuel is combusted to release energy. Air is provided in the boiler furnace for assisting the combustion of the fuel. A large clearance between the combustion chamber and the water allows the transfer of combustion energy to the water, as heat. The boilers are equipped with a steam drum and a water drum, which ensure steam and water respectively, can be separated. It should also provide a variety of accessories and to ensure that the fuel, the supply of air and water is to meet the steam requirements.

VARIOUS USES OF STEAM ON SHIPS

For main engine propulsion/turbines (in case of steam ships)
For power generation (to run steam turbo generators)
For running auxiliaries (in case of steam ships)
For soot blowing and for the steam atomized burners.
For fresh water generation (Evaporators)
For fire major fighting (steam drenching)
For heating duties (ME fuel oil heater, Galley supply, Purifier, Calorifier, Galley, Accommodation heating, Sea chests tracer lines for pipeline heating)
For cargo heating
For fuel tank heating
For deck machineries
For running Cargo pump turbines
For operating bilge, stripping and other steam driven pumps.
For tank washing in tanker ships and general cleaning.
For using as a steam ejector media for ejector pumps and vacuum devices
For Driving steam driven deck machineries like winches etc.

Pressures used:
The working pressure used in marine boilers will vary from boiler to boiler as required.For Tanker Vessels Medium pressure 17-30 bar(normally 16bar).

Boiler system in oil tankers:

Due to the high demand in the oil-Tankers, high capacity composite boilers or Dual pressure boilers are used. The main reason for the introduction of double pressure boiler is the use of modern boiler water pipes of high yield, without fear of load damage or contamination of Fuel oils. The basic construction is made of a D-Type boiler design upon which is mounted a Steam/Steam generator drum. Steam heating by the main boiler warms the water in the steam generator which full fill all the requirements for a propulsion pump, the heating load and fuel tanks and all other tasks of turbines.

The drum is initially filled with the primary quality of feed water and is well balanced. Make-up is limited to small amounts due to leaks and the fuel pump can be simple. An example of this could be an alternative pump driven by steam or air. The chemical treatment/dozing is simple and requires only minimal for the addition or rinsing. The above design shows super heater in the system but this is usually installed where the generated steam is required for the turbine alternators.

Secondary drum.

The U-shaped heating elements welded through the door of the shaft and at the end covered with the drum head. The tubes are well supported. In the lower part of the housing a manhole can be installed to allow access to the heating elements. The secondary drum also acts as a receiver for the exhaust gases of the steam boiler. Typical pressures above 63 bar (278degC) for the primary and 23.5deg (219degC) for the secondary. Primary pressure of 35bar (242degC) and closer to 15bar high pressure (198degC) proved to be sufficient to drive turbines for cargo oil pumps on tankers.

On another hand Composite boilers are a combination of oil boilers and exhaust gas economiser. When the diesel engine is at full load, the fuel burner starts only when the steam demand is higher than the production of steam from the exhaust gas of diesel engines. Composite boilers are so arranged that these can generate steam on Main engine exhaust gases or by burning oil in the furnace. In most cases the gas flows are kept separate each having its own uptake this permits the oil firing to be used in conjunction with the engine exhaust gases. By this means the output of steam can be maintained independent of the engine power.

Most of the tank type auxiliary boilers can be modified for composite firing, the modification consists of an additional tube nest added to basic oil fired design, the engine exhaust gases are circulated thru this tube nest so providing heat for generation of steam. In port stays boiler pressure is controlled by start/stop of burner at required pressure, and during sailing the pressure control is by excess steam dump valve.

But on which principle D-type water tube boiler work?

This boiler has 2 drums, an integral furnace with wall mounted burners and is often referred to as ‘D’ type boiler because of its shape. The furnace is at the side of the 2 drums and is surrounded on all sides by water tube walls. These water wall tubes are connected either to upper and lower headers or a lower header and the steam drum. The larger steam drum is placed above a smaller water drum , The two drums are connected to a large number of pipes of small diameter, which carry feed water. These small diameter pipes are known as the generating tubes that provide the main heat transfer surfaces for the generation of steam. The water circulates between the two drums with the aid of large diameter downcomers. The superheater is located through several rows of screen tubes covered between the drums. The fireproof material is used in the manufacture of the furnace, the burner wall and behind the water walls. The fire-resistant material acts as an insulator, which prevents heat loss. The boiler is also provided with a jacket for the combustion air, around the air control registers, which are surrounded by the burner.

# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced  from any of the source but explained same thing in better detailed way.

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Author Arpit Singh                                                                             Article requested by: bilal ahmad

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