Difference between VIT & Super VIT

Modern engines MAN B & W low speed 2-stroke have large Super VIT mechanism to lower the fuel injection and increases the maximum pressure loads, which optimize fuel consumption. But this lead to a another question " Was there a normal vit before super vit ?", is there any major significance of super in VIT ?

Back in the days of conventional VIT, A time-variable injection is obtained through unique profiles in the piston of fuel pumps . Therefore, it was a solid link in-between the injection time and fuel index. Therefore, it was impossible adjusting fuel indexes of each pumps without any much change in injection timing. Therefore, the super VIT was developed to make it possible to regulate the fuel flow & injection time and can be set independently of each other.

VIT = Variable beginning of injection + Variable ending of injection = Adjustable timing of injection.

Therefore, the maximum combustion pressure reaches a load range of 75-100%.

Super VIT

Super VIT = Adjustable Timings + Adjustable Break Point

breaking point

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It is the load point at which the maximum cylinder pressure is reached and the injection time is advanced. Above the breakpoint, the injection time is gradually decelerated until a load of 100% MCR reaches the initial setting. Usually 85% MCR load.

What is the variable injection time and how does it work?

In modern engines, both the start and the end of the injection are varied due to economic considerations. With the start of constant injection, the maximum firing pressure of the engine decreases as the engine power is reduced, which reduces the thermal efficiency. To improve the thermal efficiency and the specific fuel consumption, it is necessary to maintain the maximum pressure , even at low load. This is achieved by advancing the fuel injection time at low load, called VIT.

Recommended Book For Marine Engineers (AD)

The cylinder bottom has a thread cut in. The cylinder is moved by a rack and pinion. The cylinder can move freely upwards and downwards in the pump housing. This has changed the effect of the discharge opening position relative to the piston stroke. Therefore, changes are made at the start of the injection. The end of the injection is changed in the normal manner, the rotation of the piston.

The reason for using VIT is to achieve more economical fuel consumption. No change in the injection time at low load (up to 40% MCR). As the load increases, the fuel timing is advanced to about 85% of the MCR, with the engine Pmax being reached. The synchronization is then delayed to maintain a combustion at constant pressure between 85% and 100% of the MCR.

The VIT also allows small adjustments in time for the fuel pump for different fuel ignition qualities. The wear of the fuel pump and changes the timing of the camshaft due to the extension of the chain can be compensated to some extent by VIT.

The vit can be achieved by:

1. Mechanical - Pneumatic: Low-pressure air is fed to a pressure control valve, the output of which is supplied with the VIT Pump Servos. At the output of the link controller, a pivot rod moves, the position of which determines the output of the control valve.

The position of the control valve which can be used to allow various degrees of ignition of the fuel is adjustable and the time of the camshaft changes due to the extension of the chain. A link from governor output moves a pivoted bar and the position of the contol valve is adjusted.

2. Electro pneumatic: The air signal at the fuel pump Actuators VIT Operating VIT racks are realized in the electronic control as an electrical signal of 4 to 20 mA. The signal is sent to an IP converter to generate the pneumatic control signal between 0.5 bar (minimum setting VIT) and 5 bar (maximum value VIT).

The difference between the two above is the use of the breakpoint. Mechanical break point is fixed, with the electrical switching point is variable depending on the pressure recovery. While operation adjustments are easier since the correction values ​​are entered directly into the controller. The change in the fuel quality or wear of the pump may require adjustment of the VIT.

Super Vit:

In VIT, adjustable times are obtained by a special profile in the fuel pump pistons. Therefore, there is a fixed relationship between the timing of the fuel injection and the fuel index. Therefore, you must adjust the injection time without changing the fuel index. But this is possible with the new Super Vit. For this reason, the super-VIT, where it is possible to introduce the fuel flow and the injection time, is set independently of each other.

Super-VIT is available in both mechanical and electronic versions. In the electronic version an I / P air control of the converter supply pressure relative to the servo single cylinders instead of the pilot valve actuated by the fuel frame in the mechanical version. The I / P converter receives the pilot signal control.

The advantage of the electronic version is that the break point is calculated from the real conditions, the engine load is calculated by the engine speed and the speed of the fuel, while the compression pressure is calculated by pressurized purge air. Based on these calculations, the controller calculates output to I / P.

Has the Sulzer engine uses Super VIT? If so, what is the difference between VIT and VIT Super in Sulzer?

At Sulzer, VIT was able to set the start and end of the fuel injection independently, that is the same characteristics as the MAN B & W Super Vit. Therefore it is from a "Super-VIT" in Sulzer engine conversation. The only change they made was to replace the mechanical control Sulzer VIT to electronic control for the following engines.

#Cover Image credit: http://www.marinesite.info 

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Author: Amit                                                                                      Article Requested by: suzi karan

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Refrigeration, Air conditioning system and ventilation

Refrigeration: is a process in which the temperature of a space or its contents is reduced to below that of surroundings.

Uses of Refrigeration:
1. Domestic fridge rooms on ships for preserving foodstuffs for the crew.
2. Accommodation air-conditioning system.
3. Reefer control air dryers in engine room.
4. Refrigeration is used in the carriage of some liquefied gases like LPG and LNG.
5. Reefer containers for carrying food stuffs.
6. Reefer ships where the entire cargo space is refrigerated for carriage of perishable fruits and meat products.
7. To cool bulk CO2 for firefighting systems.

Live & Dead Cargoes: The perishable foodstuffs carried as refrigerated cargo or as stores on ships can be categorized as dead produce such as meat and fish or as live produce such as fruit and vegetables.

Fruit and vegetables are regarded as live cargoes until consumed, because they continue to ripen though slowly under refrigerated conditions. Fruit and vegetables continue a separate existence during which oxygen is absorbed and CO2 is given off, with the generation of heat.

The purpose of refrigeration in the carriage of perishable foodstuffs is to prevent or check spoilage, the causes of spoilage are:

1 excessive growth of micro-organisms, bacterial and fungal;
2 changes due to oxidation, giving poor appearance and flavours;
3 enzymatic or fermentive processes, causing rancidity;( it refers to the spoilage of a food in such a way that it becomes undesirable,and usually unsafe for consumption).
4 drying out (dessication);
5 The metabolism and ripening processes of fruit and vegetables.

Principle of Refrigeration.

When a liquid evaporates a cooling effect is produced. For example, a few drops of volatile liquid i.e. after shave,poured on to the hand gives a cold sensation, as it evaporates rapidly taking heat out of the skin.

Evaporation of leaked water from porous earthen pot surface keeps the water inside the pot surface cool. Similarly if liquid CO2 is made to vaporize at a coil as shown, the heat to vaporize the liquid CO2 will be taken from the surrounding i.e. in this case a bottle of water.
The draw back in the example shown is that the cylinder will soon become empty of liquid CO2 and the cooling effect will stop unless cylinder is recharged with further liquid CO2.

Vapor Compression System

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Vapour compression cycle is the most commonly used system of refrigeration. In this system a gas called as refrigerant is used as a medium of heat transfer and is alternately condensed and evaporated to remove heat from the spaces being cooled.

(The temperature at which fluid boils or condenses, is known as saturation temperature and varies with pressure).

The system consists of 4 processes namely:

1. Compression: Compression of the gas is carried out in the compressor, which delivers the gas at high pressure and temperature, raising the saturation temperature, so that it is higher than the sea water temperature or air cooling the condenser.

2. Condensation: The compressed high pressure gas is now condensed to a high pressure liquid in a condenser to below saturation temperature relating to compressor delivery pressure by circulating sea water (or air in case of domestic refrigerators).

3. Expansion: The high pressure liquid is then passed through an expansion valve to reduce its pressure, after passing thru the expansion valve the refrigerant consists of low temperature liquid and a small quantity of vapour both at low pressures.
The pressure drop through the expansion valve causes saturation temperature of the refrigerant to fall so that it will boil at the low temperature of the evaporator.

4. Evaporation: The liquid refrigerant containing small quantity of vapour is now passed thru an evaporator which is located in the space required to be cooled. Here the refrigerant absorbs heat from the surrounding secondary coolant (air or brine) receives latent heat and evaporates, cooling the surrounding space. The evaporated liquid (gas) is passed to the compressor suction for the entire process to repeat itself.

Vapor absorption cycle

In the early years of the twentieth century, the vapour cycle of absorption using ammonia water systems was popular and widespread. After the development of vapour compression cycle, the vapour absorption cycle has lost much of its importance because of its low performance (about one fifth of the vapour compression cycle). Today, the steam absorption cycle is mainly used when fuel oil is present, but no electricity.

Absorption Type Refrigeration Unit :
1. Hydrogen vapor which is insoluble in water, leaves the absorber and rises until it meets ammonia liquid falling into entry of evaporator. Due to hydrogen pressure causing lowering of ammonia pressure, this results in vaporization of ammonia.
2. Ammonia and hydrogen vapor are carried down to the absorber where water absorbs and dissolves ammonia and hydrogen vapor re cycles.
3. Ammonia vapor which is highly soluble in water, rises with the water vapor from the generator to the seperator where the water vapor and some ammonia vapor condenses.
4. Ammonia vapor then rises, is liquefied in the condenser, reduced in pressure and vaporized in the evaporator and falls to be absorbed in absorber. Ammonia, dissolved in water, falls down into lower pipe to the generator.
5. Water vapor leaves the generator, is condensed in the separator, falls through the absorber dissolving the ammonia vapor and returning to generator.
The unit requires no compressors or pumps and is silent and vibration less. Condenser, evaporator and vapor liquid separator are air cooled, with fins welded or brazed on to the piping to give extended surface heat transfer.  

AIR-CONDITIONING

The basic principles of air conditioning: Air conditioning is the process of treating air so as to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space. 

Action involved:
Temperature control
Humidity control
Air filtering, cleaning and purification
Air movement and circulation
Winter conditioning relates to increasing temperature and humidity of air whilst summer conditioning relates to decreasing temperature and humidity of air.
What are the objectives of air conditioning on ships ?
1. To extract excess heat
2. To raise air temperature when required
3. To add moisture as required
4. To reduce moisture content as required
5. To maintain sufficient air flow
6. To remove dust


When moisture evaporates from a surface, the latent heat required, is drawn from the surface causing it to be cooled. If a thermometer bulb is covered by a wetted fabric and exposed to the air, the rate of evaporation will depend upon the humidity of the surrounding air. As the heat required must come from the bulb, this results in a lower temperature reading than if the bulb was dry.

Important definitions in air-conditioning:

Hygrometer or Psychrometer: Hygrometer is an instrument to measure the humidity of air. This consists of an ordinary thermometer which gives the dry bulb temperature and a wet bulb thermometer (wetted with gauze cover).The wet bulb reading will be less than the dry bulb reading, the difference is quoted as the wet bulb depression.

The drier the air, the more rapid the moisture evaporation from the gauze giving a cooling effect. Thus greater the difference between the dry and wet bulb readings, drier the air and lesser the relative humidity.

Relative humidity (r.h.): The relative humidity is a measure of the amount of water vapor in the air (at a specific temperature) compared to the maximum amount of water vapor air could hold at that temperature, and is given as a percentage value.

Relative humidity depends on the temperature of the air, as warm air can hold more moisture than cold air. A relative humidity of 100 percent indicates that the air is holding all the water it can at the current temperature and any additional moisture at that point will result in condensation.

A relative humidity of 50 percent means the air is holding half the amount of moisture that it could. As the temperature decreases, the amount of moisture in the air doesn't change, but the relative humidity goes up (since the maximum amount of moisture that cooler air can hold is smaller).

Dewpoint (d,p): is the temperature to which unsaturated air must be cooled to bring it to saturation point and to cause moisture to precipitate. (If an unsaturated mixture of air and water vapour is cooled at constant pressure, the temperature at which condensation of water vapour begins is known as the Dew point.)
Or
The atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.
The dew point is the temperature to which the air must be cooled at constant pressure in order for it become saturated, i.e., the relative humidity becomes 100%.
A higher dew point indicates more moisture present in the air.

Physcometry: It is the study of properties of mixture of air and water vapour. This subject is important to air-conditioning because the systems handle air-water vapor mixtures, not dry air.

Some air-conditioning processes involve the removal of water from the air-water vapor mixture (dehumidification) while some involve the addition of water (humidification).

A convenient way to represent the properties of air-water vapor mixtures is the psychrometric chart. On the chart, such properties as dry bulb temperature, wet bulb temperature, dew point, relative humidity, humidity ratio, specific volume, and enthalpy are presented in graphical form.

 
Comfort Zone: The condition of the air in a space depends on its temperature, humidity and movement. The effect of the air on people in a space varies considerably between one person and another, so it is only possible to stipulate a fairly wide zone.

Under summer conditions relative humidity between 30% and 70%, average about 50% and thermometer readings 19 deg to 25deg, average 22deg gives the best degree of summer comfort.


Ventilation is defined as the circulation of air around a space to clean and refresh it, but not changing the temperature.

Air Velocity: The early air conditioning systems were rather bulky because designs were based on low air velocities in the distribution ducts, with velocities in the order of 10 m/s or less. In later years with very substantial increases in air velocities, reaching a maximum of about 22.5 m/s in the ducts and producing a large reduction in the space occupied by the equipment. Higher velocity systems have increased operating costs but lower installation costs.

What is Low velocity system in ventilation?

A low velocity System is one in which the velocity of air at the beginning of main duct is 5 to 10 m/s and successively lower there after which results in low frictional resistance. Thus in this system we require a fan which is having low power rating, but the only problem with this type of arrangement is that it will require large sized and expensive ducts and installation will be difficult.

What is High velocity system in ventilation?

In high velocity system, velocity of air at the beginning of the main duct is 15 to 30 m/s. Since it includes a high power fan that will produce a high air pressure, it requires small sized ducts and would result in
economy of material,
low manufacturing and installation cost
easy installation on board ship,
considerable space saving in the ship.

This system will have high recurring cost and will also result in high noise levels. Also since the machinery will be running at high speed, the frictional loses in this system will be more.

Typical Air-conditioning System

The main components of the system, such as the oil separator filter, condenser, expansion valve and evaporator, are explained in the refrigeration system; The components, which are generally unique to the air conditioners, are described below:

Compressor: It can be stroke based or rotatory. In almost all cases, a method for changing the amount of the feed is taken. The piston compressors therefor is also a rotary feed unit for the speed.

Compressor protection: Compressors are under similar protection systems as of refrigiration plant, low pressure cooling zone, high pressure part (manual reset) and this cuts the differential oil pressure. In addition, it has a lock for the compressor installed, can not be started when the air handlink unit fan is not started. When the fan stops, the compressor is turned off.

An alternative is to mount the solenoid valve upstream of the compressor as shown in the diagram above, which only open when the fan is running. The compressor is triggered with a low suction pressure. The purpose is to prevent liquid back to the compressor.

Air handling unit

In the diagram above a single unit contains an evaporator fed through a gas compressor. A belt driven fan supplies air to the evaporators through an air filter with fine mesh. This filter is removed and washed regularly in a soapy solution containing a disinfectant. The air flows over the evaporator, where it is cooled, and gives water vapor. The water condenses and is transported in a collecting basin and pipelines. The previous draft a collector was installed to remove water droplets entrained in the air, they are not always equipped. A perforated tube is installed after the evaporator allows the low steam quality to be introduced into the air to improve the moisture when it is too dry.

The fresh air is taken from the outside atmosphere and the recirculated air is the return air housing. The air is distributed to the ships during their stay in the port or during navigation normally, Air is recirculated normally on tankers during port stay or during sailing when any cargo or IG related operations are on to prevent cargo vapours from entering the accommodation spaces. For the mass stay at loading port air is taken back for ship carring raw materials such as coal, iron ore etc., To avoid them from entering the air housing.

Thermostat in AC system

A thermostat is the component of an Air-con system which regulates the temperature of the space to be cooled so the temperature is maintained near a desired set point temperature. The thermostat does this by indirectly switching the compressor on or off, to maintain the correct temperature.

As long as the desired temperature in the accommodation space is not reached the Air con compressor keeps working and thereby cooling the accommodation, when the desired temperature is reached the thermostat actuates and closes the liquid line magnet valve(solenoid valve) located on liquid line after the condenser, the compressor then eventually stops on low suction pressure cut off. However the air handling unit blower keeps running all the time.

Now as the accommodation temperature starts going up above the desired set point the thermostat energizes the liquid line magnet valve, the suction pressure now goes up as the gas starts flowing to the compressor, the compressor then immediately starts on L.P cut in thereby the cooling now again commences, this cycle is repeated to maintain the desired temperature in the accommodation spaces.

Thermostats are normally located in the air handling unit; they sense the temperature of Return air. Alternately they are also located in one of the cabins on the top deck.

Recently, digital thermostats have no moving parts to measure the temperature, and instead rely on thermistors or other semiconductor devices, such as a resistance thermometer (resistance temperature sensor). Each has an LCD screen that displays the measured temperature and the set temperature.

What is Capacity Control?
Capacity control of an air-conditioning plant can be defined as a system which controls the output of the plant as per the load in demand. Refrigerating capacity control with reciprocating compressors running at constant speed consists of controlling the quantity of gas delivered to match the fluctuating load

Holding the valves open: This is the most common method used in unloading in multi cylinder V & W type compressors. It is accomplished by lifting of suction valves, usually of 2 cylinders together by means of push pins. When the suction valve is lifted the gas drawn during suction stroke is pushed back into the suction line during the upward stroke of the piston. No work is done except frictional work during such idling. The push pins are operated by oil pressure. More and more cylinders are unloaded as the suction pressure or evaporator temperature continues to drop.
Normally in warm weather area the AC plant is always running at full load, ideally the air conditioning plant will start unloading when the ship goes into colder weather. The thermostat control will come into action only when further drop in temperature takes place, and this will stop the air-con compressor.

#Cover Image credit: Chitre Sir (Marine Faculty) / If you have any problem regarding post, please contact us!

# 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                                                                Article Requested by: Pranesh Devadiga

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STCW & SHIP FIRE PREVENTION

FIRE TRIANGLE: IF ANY ONE SIDE OF “FIRE TRIANGLE” IS REMOVED, FIRE WILL BE EXTINGUISHED.

METHODS OF FIRE FIGHTING: THERE ARE FOUR METHODS OF FIGHTING FIRE

• SMOTHERING – REMOVING THE AIR.
• STARVING - REMOVING THE BURNING MATERIAL FROM THE SURROUNDING.
• COOLING – REMOVING THE HEAT.
• INHIBITING – RETARDATION OF THE COMBUSTION REACTION.

Types of fire

In the European Standard "Classification of fires" (EN 2:1992, incorporatiing amendment A1:2004), the fires are classified as:

Class A fire: Ordinary combustibles such as wood, paper, carton, textile, and PVC;

Class B fire: Flammable liquids and solids which can take a liquid form, such as benzene, gasoline, oil;

Class C fire: Flammable gases, such as butane, propane, and natural gas;

Class D fire: Combustible metals, such as iron, aluminum, sodium, and magnesium;

Class F fire: Cooking media, such as oils and fats, in cooking appliances;

A fire involving energized electrical equipment is not classified by its electrical property.

SMOTHERING

IF THE OXYGEN CONTENT OF THE ATMOSPHERE IN THE IMMEDIATE NEIGHBORHOOD OF BURNING MATERIAL CAN BE SUFFICIENTLY REDUCED COMBUSTION WILL CEASE. THIS PRINCIPLE IS INEFFECTIVE IN THE CASE OF CELLULOID AND SIMILAR SUBSTANCES, WHERE BURNING OF THE MATERIAL CONTAINS WITHIN ITSELF, IN A CHEMICALLY COMBINED FORM.

PRACTICAL ASPECT: ON A SMALL SCALE IT IS EMPLOYED IN A SNUFFING  CANDLE. PERSONS CLOTHING CAN BE SMOTHERED WITH A RUG / BLANKET

ON A LARGE SCALE, IN CAPPING A BURNING WELL. BATTERING DOWN OF A SHIP’S HOLD WHEN A FIRE BREAKS OUT BELOW DECKS WILL OFTEN HOLD THE FLAMES IN CHECK UNTIL PORT IS REACHED.

PRACTICAL APPLICATION OF SMOTHERING METHOD

• USE OF FOAM: THIS FORMS A VISCOUS COATING OVER THE BURNING MATERIAL AND LIMITS THE SUPPLY OF AIR.
• USE OF DRY POWDER: SODIUM BICARBONATE FROM A PRESSURIZED EXTINGUISHER. CARBONATE WHEN FINELY DIVIDED ABSORB HEAT AND THUS HAS COOLING AND FLAME INHIBITING EFFECT.
• USE OF INERT GAS: THE VIGOROUS DISCHARGE OF INERT GAS, SUCH AS CARBON DIOXIDE & NITROGEN IN THE IMMEDIATE VICINITY OF THE FIRE REDUCES THE OXYGEN CONTENT OF ATMOSPHERE BY PROVIDING A TEMPORARY BLANKET EFFECT.

STARVATION

THE EXTINCTION OF FIRE BY STARVATION IS APPLIED IN THREE WAYS. BY REMOVING COMBUSTIBLE MATERIAL FROM THE NEIGHBORHOOD OF THE FIRE. 

Ex: DRAINING OF FUEL FROM BURNING OIL TANKS, DISCHARGING OF CARGO AT A SHIP FIRE.

BY REMOVING THE FIRE FROM THE NEIGHBOURHOOD OF COMBUSTIBLE MATERIAL.

Ex: PULLING APART A BURNING HAYSTACK.

BY SUB-DEVIDING THE BURNING MATERIAL. 

Ex: SMALLER FIRES PRODUCED MAY BE EXTINGUISHED MORE EASILY, SUCH AS EMULSIFICATION OF THE SURFACE OF BURNING OIL.

COOLING

THE PROCESS OF REMOVING THE HAET FROM FIRE IS CALLED COOLING. THIS IS NORMALLY DONE BY WATER, AS WATER HAS MORE COOLING EFFECT AND VERY HIGH LATENT HEAT. APPLICATION OF JET OR SPRAY OF WATER TO A FIRE, EMULSIFICATION OF THE SURFACE OF OIL BY MEANS OF THE EMULSIFYING TYPE OF SPRINKLER HEAD.

INHIBITING

IN THIS HEAT PRODUCING CHEMICAL REACTION OF SUBSTANCES IS CHECKED OR STOPPED. THIS APPLIES TO ONLY FLAMING MODE AND NOT TO THE GLOWING MODE. IN THIS FLAME INHIBITING EXTINGUISHING MEDIUM INTERFERES WITH THE ACTIVE SPECIES IN FLAME CHAIN REACTION WHICH WOULD OTHERWISE FUNCTION AS “CHAIN CARRIERS”. THIS METHOD EXTINGUISHES THE FLAME VERY RAPIDLY.

    Ex: DCP OR HALON GAS – THESE ACT BY CHEMICAL INTERFERENCE WITH THE CHAIN REACTION OF FLAME PROPAGATION.

PROPERTIES OF FLAMMABLE MATERIAL

FLAMMABILITY: THIS IS THE ABILITY OF SUBSTANCE TO BURN. IN THIS VAPOR GIVEN OFF BY FLAMMABLE MATERIAL CAN BURN WHEN MIXED WITH AIR IN THE RIGHT PROPORTION IN THE PRESENCE OF AN  IGNITION SOURCE.

IGNITION POINT: IT IS THE LOWEST TEMP., TO WHICH A FLAMMABLE SUBSTANCE MUST BE HEATED TO IGNITE.

FLASH POINT: IT IS THE LOWEST TEMP., AT WHICH THE VAPOUR OF THE SUBSTANCE ARE AVAILABLE IN SUFFICIENT QUANTITY TO PRODUCE A MOMENTARY FLASH WHEN A FLAME IS APPLIED.

FIRE POINT: IT IS THE TEMP., AT WHICH THE HEAT FROM THE COMBUSTION OF BURNING VAPOR IS CAPABLE OF PRODUCING SUFFICIENT VAPOUR TO ENABLE COMBUSTION TO CONTINUE.

SPONTANEOUS / AUTO / SELF IGNITION TEMP: IT IS THE LOWEST TEMP., AT WHICH THE SUBSTANCE WILL IGNITE SPONTANEOUSLY i.e., THE SUBSTANCE WILL BURN WITHOUT THE INTRODUCTION OF A FLAME OR OTHER IGNITION SOURCE.

UPPER FLAMMABLE LIMIT: IT IS THE CONCENTRATION OF FLAMMABLE VAPOUR IN AIR ABOVE WHICH THE MIXTURE BECOMES TOO RICH TO IGNITE AND PROPEGATE COMBUSTION.

LOWER FLAMMABLE LIMIT: IT IS THE CONCENTRATION OF FLAMMABLE VAPOUR IN AIR BELOW WHICH THERE IS INSUFFICIENT FLAMMABLE VAPOUR TO SUPPORT AND PROPAGATE COMBUSTION.

STATIC ELECTRICITY: IT IS THE ELECTRICITY PRODUCED IN DISSIMILAR MATERIALS THROUGH PHYSICAL CONTACT AND SEPARATION i.e., A SAMPLING APPARATUS LOWERED IN TO A TANK CONTAINING CHARGED PETROLEUM LIQUID.

FLAMMABLE RANGE: IT IS THE RANGE OF CONCENTRATION OF A FLAMABLE VAPOUR IN AIR WITH WHICH THE VAPOUR AND AIR MIUXTURE IS FLAMABLE.

FLAMMABILITY: IT IS THE ABILITY OF SUBSTANCE TO BURN VAPOUR GIVEN OFF BY A FLAMMABLE MATERIAL, WHEN MIXED WITH AIR IN RIGHT PROPORTION IN THE PRESENCE OF THE IGNITION SOURCE. NORMAL AIR CONTAIN -  21%  OF O2  AND 78% OF N2 AND SMALL TRACES OF CO2 , WATER VAPOUR etc. IN CARGO TANKS AS THE PERCENTAGE OF HYDROCARBON INCREASES, OXYGEN CONTENT DECREASES.

LOWER FLAMABLE LIMIT (LFL): FOR A MIXTURE OF AIR AND THE FUEL TO BE COMBUSTIBLE, MINIMUM QUANTITY OF THE FUEL VAPOUR SHOULD BE PRESENT IN THE MIXTURE. THIS IS EXPRESSED AS PERCENTAGE OF FUEL VAPOUR IN THE MIXTURE OF AIR AND VAPOUR. THIS IS CALLED LOWER FLAMABLE LIMIT. THE MIXTURE WILL NOT BURN UNLESS FUEL VAPOUR CONTENT IN THE COMBUSTIBLE MIXTURE IS ABOVE LFL.

EVERY TYPE OF FUEL HAS DIFFERENT LFL.

POINT “F” – MINIMUM OF O2 REQUIRED FOR COMBUSTION  ( 11% BY VOLUME).

    LFL – LOWER FLAMABLE LIMIT

          – 1% OF C2H2 AT POINT “A”

UPPER FLAMABLE LIMIT (UFL):

WHEN THE PERCENTAGE OF FUEL VAPOUR IS INCREASED IN THE MIXTURE, THE PERCENTAGE OF AIR AND HENCE THE PERCENTAGE OF OXYGEN REDUCES.

SINCE THE OXYGENN CONTENT BELOW 11% IN THE MIXTURE DOES NOT SUPORT COMBUSTION, A STAGE MAY BE REACHED WHEN THE PERCENTAGHE OF VAPOUR IN THE MIXTURE IS SO HIGH THAT RESULTING PERCENTAGE OF OXYGEN IS TOO LOW TO SUPORT THE COMBUSTION AND MIXTURE WOULD NOT BURN EVEN WHEN IGNITED. THIS IS CALLED UPPER FLAMABLE LIMIT.

MIXTURE WILL NOT BURN IF THE FUEL VAPOUR CONTENT MORE THAN THE UFL.

UFL – UPPER FLAMABLE LIMIT

              - 11% OF C2H2 AT POINT “B”

EVERY FUEL HAS A LFL and UFL AND THE PERCENTAGE OF FUEL VAPOUR RANGE BETWEEN BETWWEN THESE TWO LIMITS IS CALLED “FLAMABLE RANGE”

POINT “A” – AT THE END OF BURNING IF THE PRODUCT OF COMBUSTION ARE ANALYSED, IT IS FOUND THAT ALL OF C2H2 VAPOURS ARE CONSUMED, WHILE SOME “O2” IS UNUSED AND STILL AVAILABLE.

POINT “B” – AT THE END OF BURNING, ON ANALYSIS, IT IS FOUND THAT ALL OF “O2” IS CONSUMED AND SOME UNBURNT C2H2 IS STILL AVAILABLE.

SOURCE OF IGNITION  (HEAT ENERGY)

1. CHEMICAL HEAT ENERGY
2. ELECTRICAL HEAT ENERGY
3. MECHANICAL HEAT ENERGY
4. NUCLEAR HEAT ENERGY

CHEMICAL:

• HEAT OF COMBUSTION
• HEAT OF PARTIAL OXIDATION
• SPONTANEOUS HEATING
• HEAT OF DECOMPOSITION (ONION, POTATO etc., IN CARGO HOLD)
• HEAT OF SOLUTION (MIXTURE OF TWO CHEMICALS)
• ELECTRICAL:
• RESISTANCE HEATING
• DI-ELECTRIC HEATING
• INDUCTION HEATING
• MECHANICAL:
• FRICTIONAL HEATING
• FRICTIONAL SPARKS
• HEAT OF COMPRESSION

CLASSIFICATION OF FIRE

FIRES ARE CLASSIFIED ACCORDING TO THE TYPES OF MATERIAL WHICH ARE ACTING AS FUEL. FIRES BURNING WOOD, GLASSFIBRE, UPHOLSTERY AND FURNISHINGS. FIRES BURNING LIQUID SUCH AS LUBRICATING OIL AND FUELS. FIRES BURNING GAS FUELS SUCH AS LIQUIFIED PETROLEUM GAS. FIRES BURNING COMBUSTIBLE METALS SUCH AS MAGNESIUM AND ALUMINIUM. FIRES BURNING ANY OF THE ABOVE MATERIALS TOGETHER WITH HIGH VOLTAGE ELECTRICITY.

THESE CLASSIFICATIONS ARE ALSO USED FOR EXTINGUISHERS AND IT IS ESSENTIAL TO USE THE CORRECT CLASSIFICATION OF EXTINGUISHER FOR A FIRE, TO AVOID SPREADING THE FIRE OR CREATING ADDITIONAL HAZARDS. MANY FIRE EXTINGUISHER WILL HAVE MULTIPLE CLASSIFICATIONS SUCH AS A, B and D

FIRE EXTINGUISHING AGENTS

CARBON DIOXIDE: AT NORMAL TEMP., CO2 IS 1.5 TIMES HEAVIER THAN AIR. IT IS EASILY LIQUEFIED AND BOTTLED. IT IS STORED UNDER PRESSURE OF 50bar IN STEEL CYLINDERS. WHEN CO2 IS APPLIED TO FIRE, THE LIQUID CO2 BOILS OFF RAPIDLY AS A GAS, EXTRACTING HEAT FROM THE SURROUNDING ATMOSPHERE. IT EXTINGUISHES FIRE BY SMOTHERING i.e., BY DISPLACING OXYGEN. ABOUT 20-30 % OF THE ATMOSPHERE SHOULD CONTAIN CO2 TO EXTINGUISH THE FIRE. CO2 IS QUICK AND CLEAN, NON-TOXIC, DOES NOT HARM MOST FAFRIC AND DOES NOT CONDUCT ELECTRICITY. SO IT CAN BE USED ON ELECTRICAL IT DOES NOT DAMAGE EXPENSIVE CARGO OR MACHINERY. IT LEAVES NO UNDESIRABLE RESIDUE TO BE CLEARED OFF AFTER USE. SO IT CAN BE USED TO EXTINGUISH FIRE IN CARGO AND MACHINERY SPACES BY PROVIDING FIXED CO2 INSTALLATIONS.

ADVANTAGES OF CARBON DIOXIDE:

1. NON-CORROSIVE.
2. DOES NOT CONDUCT ELECTRICITY.
3. LEAVES NO RESIDUE.
4. EASILY AVAILABLE.
5. NOT SUBJECTED TO DETERIORATION IN QUALITY WITH AGE.

DISADVANTAGES OF CARBON DIOXIDE:

1. TOXIC AND HAZARDOUS TO HUMAN BEING.
2. LITTLE COOLING EFFECT & DANGER OF RE-IGNITION IF AIR IS READMITTED IN ROOM.
3. SOLID CO2 PARTICLES GENERATE SPARK DUE TO STATIC ELECTRICITY, SO UNSUITABLE IN OIL TANKERS

WATER: IT IS MOST EFFICIENT , CHEAPEST AND READILY AVAILABLE MEDIUM. IT HAS A COOLING EFFECT AND NO-TOXIC. IT ABSORBS HEAT AND EXPANDS TO 1700 TIMES OF IT’S VOLUME TO PRODUCE STEAM WHICH HAS SMOTHERING EFFECT.
IT IS SAFE TO USE ON MOST OF THE FIRES, IT CAN BE EASILY DIRECTED OVER A CONSIDERABLE DISTANCES.

DISADVANTAGES:

1. IT CONDUCTS ELECTRICITY.
2. IT CAN CAUSE DAMAGE TO CARGO & MACHINERY.
3. LOSS IN STABILITY OF SHIP WHEN USED IN LARGE QUANTITY.

FOAM:
IT IS MOST SUITABLE FOR EXTINGUISHING FIRES INVOLVING FLAMMABLE LIQUIDS. IT EXTINGUISHES BY FORMING A LAYER OF SMALL BUBBLES ON THE SURFACE OF THE LIQUID PREVENTING FUEL FROM VAPOURISING AND RESTRICTING THE OXYGEN SUPPLY TO IT.
IT IS INSOLUBLE IN MOST OF THE LIQUIDS AND LIGHT IN WEIGHT, SO IT FORMS A BLANKET TO COVER THE SURFACE OF BURNING LIQUID AND THUS EXTINGUISHES THE FIRE.

FOAM CAN BE GERATED BY MECHANICAL OR  CHEMICAL MEANS.

MECHANICAL FOAM IS GENERATED BY AGITATION OF A DILUTED FOAM COMPOUND SOLUTION IN PRESENCE OF AIR. THESE COMPOUND INCLUDE SOAPS, GLUE AND WETTING AGENT MIXTURES. CHEMICAL FOAM IS GENERATED BY MIXING AN ALKALI (SODIUM BI-CARBONATE) WITH A SOLUTION OF ALUMINIUM SULPHATE, IN PRESENCE OF A STABILIZER (i.e., SOAP) THIS PRODUCES SKIMMED BUBBLES CONTAINING CO2 “HIGH EXPANSION FOAM” HAS A VERY HIGH EXPANSION RATION OF AROUND 1000 TO 1 INSTEAD OF 8 TO 1 FOR STANDARD FOAM. THIS TYPE FOAM IS USED FOR DEALING WITH CARBONACEOUS FIRES IN COMPARTMENTS WHICH ARE INACCESSIBLE AND WHICH LEND THEMSELVES TO COMPLETE FLOODING  OF THE COMPARTMENT.

DRY CHEMICAL POWDER:

WATER CANNOT BE USED ON MOST FIRES AS IT CAN BE EXPLOSIVELY DISASTROUS. IN VIEW OF THIS OTHER MEDIUMS ARE USED. CHIEF AMONG THEM IS DRY CHEMICAL POWDER. IN THIS SODIUM BICARBONATE IS USED AS BASIS ALONG WITH ADDITION OF METALLIC STEARATE AS WATER PROOFING AGENT. DRY CHEMICAL POWDER IS EXPELLED FROM CONTAINER BY GAS PRESSURE.

IT IS ESPECIALLY EFFECTIVE ON BURNING LIQUIDS SUCH AS LIQUIFIED GAS, OIL ESCAPING FROM LEAKING LINES AND JOINTS. ON ACCONT OF IT’S UNIQUE ABILITY TO QUICKLY STOP COMBUSTION OF GASES, CHEMICAL PRODUCTS etc., IT HAS BECOME MOST POPULAR MEDIUM IN GAS CACCIERS AND TANKERS. DRY CHEMICAL POWDER IS ELECTRICAL NON-CONDUCTOR , THUS IT CAN BE UTILISED ON FIRES INVOLVING LIVE ELECTRICAL EQUIPMENTS. IT INHIBITS THE COMBUSTION REACTION.

Solas convention

The International Convention for the Safety of Life at Sea (SOLAS) is an international maritime safety treaty. It ensures that ships flagged by signatory States comply with minimum safety standards in construction, equipment and operation.

1974 version

The intention had been to keep the Convention up to date by periodic amendments, but the procedure to incorporate the amendments proved to be very slow,As a result, a complete new convention was adopted in 1974 which includes all the agreements and acceptable procedures. Even though the Convention was updated and amended numerous times, the Convention in force today is sometimes referred to as SOLAS, 1974.

The latest Convention in 1974 included the "tacit acceptance" procedure whereby amendments enter into force by default unless nations file objections that meet a certain number or tonnage.

Types of Bulkheads.

'A' Class Divisions

Class differences "A" are the bulkheads and decks of steel or any other equivalent material which is capable of preventing the passage of smoke and flames to the end of the test standard of the hour of fire. They are well reinforced. They are insulated with unauthorized combustible materials so that the average temperature of the un-exposed side does not rise above the initial temperature or temperature at any point above 140 ° C, including any junction, rise above 180 ° C above the original temperature, Within the time, specified below:

"A-60" 60 minutes

"A-30" 30 minutes

"A-15" 15 minutes

"A-0" 0 minutes.

A prototype or platform partition should be tested according to the FireCode test procedure to ensure that it meets the above requirements for integrity and temperature rise.

Class A fire protection doors

The construction of the doors in the class A walls and the fastening means in the closed position provides a fire resistance, such as the passage of smoke and flames, as far as possible, similar to the bulkheads in which they are located.

Class A fire-resistant doors, made of steel or an equivalent material.

In These doors should be capable of being opened and closed from each side of the bulkheadby one person only.

Fire doors in main vertical zone bulkheads, galley boundaries etc., other than power operated water tight doors should be (1) self closing and and be capable of safe operation with an angle of inclination of upto 3.5 degrees opposing closure, and in no more than 40 sec. and no less than 10 sec. with the ship in upright position. (3) the doors except for those for emergemcy escape trunks Shall be capable of of remote release from the central control stations and should also be capable of release individually from a position at both sides of the door.  Release switches shall have an on-off function to prevent automatic resetting of the system.  (4) hold back hooks not subject central control station release are prohibited, (5) a door closed remotely shall be capable of being reopened locally from both sides of the door and after such opening, the door shall automatically close again.(6) there should be fire door position indicator on the indicator .

Indicator panel in the central control room and the release mechanism shall be so designed that the door will automatically close in the event of power supply failure. (7)  local power supply accumulators for power operated doors shall be provided in the immediate vicinity of the doors to enable the doors to be operated at least 10 times (open and close)  after  disruption of the control system or the central power supply using local controls. (8) the components of the local control system shall be accessible for maintenance and adjusting.

(9) In the event of a fire, the control system shall operate the door at the temp. of at least 200 deg. For at least 60 min. served by the power supply and the power supply for all other doors not subject to fire shall not be impaired, and (10) at temperatures  exceeding 200 deg. C the control system shall automatically be isolated from the power supply and shall be capable of keeping the door closed upto at least 945 deg.C.

'B' Class Divisions

Class differences "B" are the bulkheads, decks, blankets and coatings

Approved non-combustible materials to prevent sufficient flame to pass through the end of the first half hour of the standard fire test. They have an insulation value so that the average temperature of the un-exposed side does not exceed 140 ° C above the initial temperature or the temperature at any point, including any junction, rises more than 225oC to the initial temperature, the following phases:

Class "B-15" 15 minutes

Class "B-0" 0 minutes.

A prototype or platform partition should be tested according to the FireCode test procedure to ensure that it meets the above requirements for integrity and temperature rise.

 

Class B fire protection doors

The construction of all doors in the partition type "B" and the means in the closed position represents the equivalent of fire as far as possible bulkheads in which the doors of the exception that can lie vent openings in the lower part of the doors, with a total area of ​​the lower opening Or equal to 0.05 m2. (1) These doors must be closed. Retaining hooks are not permitted. The doors of class B fire must be built with non-flammable materials approved. Class B fire protection doors built with approved non-combustible materials in accordance with the test method Code for fire.

Class differences, C '

Class differences "C" are the non-flammable materials, the bulkheads, decks, ceilings and linings, which limit requirements for the passage of smoke and flames or temperature rise. Door Class "C" is the type of doors installed in the class partitions "C". The approved noncombustible materials must be constructed.

Fire dampers

Valves are penetrating into the ventilation ducts installed bulkheads and bridging the class "A" to maintain the division of fire resistance capability and prevent the spread of smoke and flames in adjacent compartments through the ventilation system.

fire integrity

Resistance to fire is to keep the grounding element's resistance with a bulkhead or deck intact for a period of time, 60 minutes for class "A" and 30 minutes for class "B".

Fire Detection Systems

The PURPOSE of this regulation is to detect a fire in the space of origin and to provide for alarm for safe escape and fire fighting activity.  For this purpose :

(1) Fixed Fire Detection  and fire alarm system installations shall be suitable for the nature of the space,fire growth potential  and potential generation of smoke and gases, (2)  manually operated call points shall be placed effectively to ensure a readily accessible means of notification and (3) fire petrols shall provide an effective means of detecting and locating fires and alerting the navigation bridge and fire teams.  Fire Detection Systems. – General Requirements.

A fixed fire detection and alarms system shall be provided as per this regulation.

 A fixed fire detection and alarm system and a sample extraction smoke detection system required as per regulations mentioned in Chapter II-2, Part C, of SOLAS shall be of an approved type and comply with Fire  Safety Systems Code.

The function of such systems shall be tested under varying conditions of ventilation after installation.

Such installed systems shall be periodically tested for proper functioning under simulated fire conditions to the approval of Administration.

Detection in Machinery Spaces:

A fixed fire detection and fire alarm system shall be installed in (i) periodically unattended machinery spaces and (ii)m/c spaces containing all automatic and remote control systems and equipment approved in lieu of continuous manning of the space and also  in continuously manned spaces having a central control room.

This system shall be so designed and installed that all the fire detectors  of  suitable type will be capable of detecting the smoke or fire rapidly in any part of the spaces under working varying condition of temperature and ventilation.

Detection in Accommodation and Service Spaces and Control Stations:

(1)  Smoke detectors shall be fitted in all stairways, corridors, and escape routes and if found necessary, special purpose smoke detectors shall be fitted within ventilation ducting.

Fire Extinction Systems. – General Requirements

This system is brodadly divided in three parts:

Regulation 8 of Part C of Chapter II-2 of SOLAS.  Control of Smoke spread :  For this purpose, means of controlling smoke in atriums, control  m/c spaces and concealed spaces shall be provided.

For example, in most cargo ships of any type of all the supply air fans (usually 4) for machinery spaces, at least one fan is reversible and can be used as an exhaust fan in the event of fire and generation of smoke in the m/c spaces.

In accommodation, gallies, Navigation Bridge, Steering Flat  a provision for exhaust fans is made to expel smoke from the concerned spaces.

(2) Regulation 9 of Part C of Chapter II-2 of SOLAS.

Containment of Fire:  For this purpose (i) the ship shall be subdivided by thermal and structural boundaries,

 (ii) thermal insulation of boundaries shall have due regard to the fire risk of the space and adjacent spaces,

(iii)  The fire integrity of the divisions shall be maintained at openings and penetrations by using approved materials and methods by adhering to the relevant regulations of the SOLAS.

(3) Regulation 10 of Part C of Chapter II-2 of SOLAS.

Fire fighting:  For this purpose, following functional requirements will be met : (A) fixed fire extinguishing systems shall be installed, having due regard to the fire growth potential of the protected spaces, and (B) adequate number of approved type of fire extinguishing appliances shall be readily available.

 (C) Water supply systems  with approved type and sufficient number of fire pumps, fire mains, hydrants, hoses and nozzles complying with the regulations of the SOLAS requirements shall be provided.

(i)  Fire mains and hydrants materials readily rendered ineffective by heat shall not be used.

(ii) Their arrangement shall be such as to avoid the possibility of freezing, with adequate provisions for the drainage of the mains.

The number and position of hydrants should be such that at least two jets of water from two different hydrants may reach any part of the ship.

There will be an  isolating valve and a relief valve on the fire main line as approved by the regulations.

Pressure at the hydrants: For cargo ships above 6000 grt   it should be 0.27N/mm2.

(vi) Fire pumps : For cargo ships of 1000 grt  and above, there will be at least two independently driven fire pumps, which will  have the provision for being started and stopped by local and remote  switches (normally situated on the Bridge).

Emergency fire pump :  The space containing the emergency fire pump shall not be contiguous to the boundaries of the m/c spaces  or those spaces containing main fire pumps.  There will be no direct access between the m/c space and the emergency fire pump spaces.

The emergency fire pump shall have two sources of electrical power supply, one from the power mains and the other from the emergency generator bus bars.

Ventilation arrangements to the emergency fire pump spaces shall be such as to preclude the possibility of smoke from m/c space fire entering or being drawn into the space.

(viii) Fire hoses shall be of non perishable material  approved by the Administration and should have a length of at least 10 mtrs, but not more than 15 mtrs in m/c spaces and not more than 25 mtrs for open  decks on ships with breadth in excess of 30 mtrs.

(ix) Standard nozzle sizes shall be 12mm, 16mm and 19 mm.  For accom. Spaces a nozzle size greater than 12 mm and for cargo spaces a nozzle size greater than 19 mm need not be used.  The size should be adequate to obtain the max. discharge possible from two jets at the approved pressure.  Nozzles shall be of approved dual purpose type( i.e. spray/jet) incorporating a shut off.

(x) Portable fire extinguishers:  Their type and design shall comply with the requirements of the Fire Safety systems Code.

Ships of 1000 grt annd above will have at least 5 portable fire extinguishers in accom. Spaces, service spaces and control stations.  CO2  fire extinguishers shall not be placed in accom. Spaces.  In control stations and other spaces containing electrical or  Or electronic equipment, only those fire extinguishers whose extinguishing media are not electrically conductive or harmful  shall be used.

Spare charges shall provided for 100% of the first ten extinguishers and 50% for the remaining fire extinguishers capable of being recharged on board. Instructions for recharging shall be carried on board.

(xi) Fixed fire extinguishing systems: These systems for m/c spaces may be any of the following types: Fixed gas fire extinguishing system, a fixed high expasion foam fire extinguishing system or a fixed pressure water spraying fire extinguishing systems, all complying with the provisions of the Fire Safety Systems Code.

Here fire extinguishing systems using Halon 1211, 1301, and  2402 and perfluorocarbons shall be  prohiibited.

(xii) There shall be at least one portable foam applicator unit complying with the provisions of the Fire Safety Systems Code.  In addition, in each space, approved foam type fire extinguishers each of at least 45 ltr capacity shall be provided.  Additionally, there shall be provided a sufficient number of portable foam extinguishers so located that no point in the space more than 10 m walking distance from an extinguisher and that there are at least two such extinguishers in each space.

(xiii) Paint Lockers shall be protected by  (i) Co2 system designed to give a minimum volume of free gas equal to 40% of the gross volume of the paint locker, (ii) a dry powder system giving at least 0.5 kg powder/m3 of the space, (iii)  a water spraying or sprinkler system connected to ship’s fire mains designed for 5 ltr/m2 /min.  All above systems shall be operable from outside the protected space.

(ivx)  Fire Extinguishing systems for cargo spaces:  Fixed Co2 or inert gas system complying with the provisions of the Fire Safety System Code.  For cargo tank protection on Tankers a fixed deck foam fire extinguishing systems complying with provisions of the Fire Safety System Code shall be used.

(xv) Approved Fire Fighter’s Outfits (atleast two in  nos.) shall be carried by all ships.  Additionally, on tankers two extra sets will be carried.

 # The article was written by two author and then merged before publishing. Various books are used as reference before writing this article but no part is being reproduced but stated in a different easy way.

# Cover image credit: Pintrest  (If you have any problem regarding the post contact us!)
 

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Author Arpit Singh & Amit                        

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Reefer (refrigeration) ship system, Working and cargo Refrigeration

A reefer ship is a cargo vessel that specializes in carriage of cargo that requires to be maintained at a temperature other than ambient temperature. Reefer ships can carry any frozen or cooled cargo including meat, fish, fruits, vegetables. The temperature range is -30 to +12 deg C depending on the type of the cargo.

A reefer ship design incorporates more pipelines than those on a tanker; these lines are refrigerant lines which lead to each cargo hold. Usually this refrigerant is secondary cooling element and is brine. These brine lines lead to a cooling battery pair located in each deck. Each such brine line feeds a bank of cooling coils per battery, which cools the forced air flow generated by cooling fans over each such coil.

Secondary Refrigerants

The primary refrigerants can be very searching, that is they can escape through minute clearances, so it is essential to keep the number of possible leakage points to a minimum. Secondary refrigerants are usually liquids, and are used to transfer heat from the substance being cooled to a heat exchanger, where the heat is absorbed by a primary refrigerant. Secondary Refrigerants like brine are used where the installation is large and complex (reefer ships), to avoid circulation of expensive primary refrigerants in large quantities. In this case, the primary refrigerant is circulated to the evaporator in the secondary refrigerant, which is then transferred to the space to be cooled.

What are the advantages of secondary refrigerant ?

1. Low initial cost

2. Low maintenance cost

3. Suitable for large refrigeration plant

4. Easily produced on board by mixing CaCl2 and distilled water.

5. Easily store as a salt on board

BRINE: Brine is made by mixing 250 grams of calcium chloride in 1 liter of fresh water. As the percentage of calcium chloride is increased the freezing point of the resultant brine will decrease. Sodium dichromate or lime may be added to maintain brine in alkaline condition in order to prevent corrosion.

Common salt brine may be used under emergencies as a replacement. Sea water can not be used as brine as it is highly corrosive and has scale forming properties. Also sea water does not have sufficiently low freezing point.

ETHYLENE GLYCOL:Systems have been designed in which brine is replaced by one of the Glycols, for example ethylene glycol. The glycols have an advantage of being non-corrosive and may be used at much lower temperature than brine.

TRICHOLROETHYLENE: Trichloroethylene has also being used as a secondary refrigerant, but has a disadvantage of being toxic and a solvent of many synthetic rubbers and other materials normally used as jointing.

Refrigeration system of a Reefer ship using  Brine as Secondary refrigerant

This system of refrigeration using brine as secondary refrigerant is used on reefer ships for cooling cargo holds. Vapour compression system using Primary Refrigerants cools the brine at the evaporator coil. Brine gets cooled to about -150C and is circulated to the refer holds. By controlling the flow rate of brine through each hold at the return valves, we can control the temperature of any hold.

A circulating fan maintains the air circulation over cooling coils in the cargo holds.

Cargo Refrigeration

Central Refrigeration system

Refrigerated cargo vessel typically requires a system that provides cooling at various temperature. The arrangements installed could be in three parts i.e the installation of the cooling unit, the main system, brine circuit system with the flow of air to cool the charge in the charging space.

In the figure, a cooling system is shown. The coolant flow through the cooler which is divided into four units, with their own expansion valves for each of them. The four units are used for controlling the amount of the evaporator surfaces, depending on the degree of charge at a time of charging is required and the capacitor, which gives greater flexibility in the system. The large oil separator is a feature of the screw compressor systems and illustrates the return oil circuit.

Each circuit has its own primary refrigerant evaporator in the brine cooler (as shown in the figure) resulting in a completely independent gas system. Probably there will be three of these systems in a freight ship or a container.

Because they are completely independent, any method could be adapted controlling the output of brine at various temperature. Each temperature of the brine is identified by a color and has its own circulating pump. Cold brine is then fed to the cooling air space and controlled by the temperature of the air to leave the cooler.

The refrigerant in the loader space for the airflow over and through the front charge before returning. A fan arrangement and ducts direct the air to the cooling room and under load. The load is applied to the cooling air to the stand stacked by the load.

#Image source credit: www.nafsgreen.gr

# 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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Rudder angle indicator | Definition, Working and Circuit Diagram

A device to display the current position of the rudder installed in the control house of the bridge. In the angle indicator system Rudder is independent of the steering system and is only for the display. There is an IMO requirement which states regardless of the steering control system there must be installed steering indicator system installed on board. The specification of the rudder angle is required for each cockpit and steering position in the rudder emergency room.

The various regulatory authorities vary the requirements of the Rudder Level Indicator (RAI). DNV needs a second angle indicator independent bar on deck. Regulation of the Panama canal requires large instruments (min 192x192mm) Bridge, visible wing operator and IMO pull to train / MED (ISO 20673) requires that the accuracy of the system is over a degree.

Rudder angle indicator consists of a control unit, a transmitter and receiver. This includes indicators with different dimensions, different scales and rudder angle display.

A rudder angle indicator consists of a transmitter and a receiver direction of the rudder rudder. The emitter is connected to the steering head by means of a lever, etc. in the wheel housing. Communicates rudder direction and rotation angle for self-synchronization of the machine in the sender and causes the self-synchronization of the machine, in the receiver that I have in the wheelhouse or another that is for with their directions of synchronicity. Self-synchronous transmitters and display units used in the control angle display systems are manufactured by various companies refer to names such as Selsyns, Synchro, and Automatic Syns Telmotors.

Non Follow-up Steering System

Control equipment – conveys a signal of the desired rudder angle from the bridge to the steering flat where it is received to activate the power unit and transmission system until the desired rudder angle is reached. This equipment can be of 2 types (1)hydraulic telemotor systems (tele- means far away in Greek & motor means motion or movement) & (2) electrical electronic control equipment.

Hydraulic telemotor systems:  The telemotor employs master and slave principle. The transmitter is situated on bridge and the receiver at the steering gear unit. Mechanical movement is transduced hydraulically or electrically for distance telemetering and is then transduced back again.

Auto & Follow up Steering System

                            

Hydraulic Transmitter: As the bridge steering wheel is moved to starboard the rotating pinion causes the RH ram to move down, pushing oil out to the receiver unit along the RH pipe. The LH ram moves up, so allowing a space for oil to come from the receiver unit. The fluid being virtually incompressible, any down movement of the RH produces an identical movement at the receiver unit. This in turn displaces the same quantity of fluid which is taken up in the extra space created by the LH ram moving up. The fluid in the replenishing tank acts as reservoir. The casing is usually gunmetal with bronze rams, and copper pipes are led in by drilled leads in the casting. A device (called bypass valve) is required in the system to allow for variation in oil volume due to temperature changes and also to allow for equilibrium between both sides of the system. This bypass valve also has function of topping up the system in the case of leakages and acts as relief valve in case of pressure rise.

Bypass valve: Operation can only be carried out when the wheel is in the mid position. This is achieved by having the operating rod butting against a circular disc, in mid position of the wheel the slot in the driven revolving disc allows the operating rod to be depressed through it. With some types the operating rod is depressed by hand, whilst with some types the rod is automatically depressed by a cam each time the wheel passes mid position. In case of hand operated types the rod is operated at regular intervals and must be operated when either pressure gauge registers above 4.5 bar with wheel in mid position. When the rod is depressed both sides of system are connected thus giving pressure balance. The connection to the replenishing tank is also joined to both sides of the system, so that any expansion or contraction of the oil can be compensated.

Hydraulic Receiver: Consider the starboard (clockwise) movement of the bridge wheel. The depressed RH ram pressurizes the right hand side of the system. The pressure force acts on the central web of the moving cylinder until the movement caused corresponds to the movement of the ram in the steering telemotor. Oil is pushed back on the left hand side of the moving cylinder central web to the steering unit. After a small initial movement the LH sleeve butts against the nut and further movement by the moving cylinder to the left compresses the springs. When the steering wheel is returned to mid-ship the springs, which are under initial compression, return the moving cylinder to mid position. For port wheel rotation the LH ram of the steering unit moves down and the receiver moving cylinder goes in the opposite direction i.e. in this case left to right.

The moving cylinder is connected by a linkage to the control unit of the steering engine. Thus any movement of the bridge telemotor unit by wheel rotation is almost directly operating the control device which causes rotation of the steering engine and rudder movement.

electrical electronic control equipment: This system is based on the electrical and electronic circuits, the monitoring and control of the valves, which control the movement of the rudder. The system also includes a logic circuit that prevents the side rudder from reaching its physical limits. In the steering system can only work, under preset electronic limits. When the bar has reached a limit, the power of the solenoid valve closes automatically.

Electronic Steering Control

The system consists of the following parts: The control unit transmits to the angle of the desired direction from the bridge in the direction of the plane,Hand Steering without follow-up, Hand Steering with follow-up and Auto Steering using Gyro Compass.

The power supply provides the energy to move the rudder at the desired angle and the transfer unit to move the motions of the rudder.

The central steering of the movement of the rudder affect the ship's engines control. And rudder actuators single unit servo steering is the torque means is applied to the i-th rudder shaft. Lance or Quadrant

Control system of the device by means of which the commands from the computer to the power units of the steering mechanism and other necessary parts are transmitted to operate the steering. Including transmitters, receivers, hydraulic control pumps and motors associated hoses and cables.

The rudder actuator, the element that directly moves to a hydraulic pressure on the mechanical effect moves the rudder. Wheel drive means that the parts that transfer the force from the actuator to the aileron of the flow including the rudder. Power unit means:

• in the case of electrical steering gear ; an electric motor and its associated electrical equipment
• in case of electro-hydraulic steering gear ; an electric motor and its associated equipment and connected pump
• in case of other hydraulic steering gear ; a driving engine and connected pump

Steering Gear Regulations

1.Every ship is to be provided with two power unit (main and auxiliary) steering gear, each independent of the other. This is because if one fail another can take over the power. If however two identical power unit is available, the presence of stand-by(Auxiliary) is not required.  

2.The capacity of the unit must be such that, it may swing the rudder from 35º on one side to 35º to the other side of the ship with the maximum speed and at deepest draft.The time required to do so must not exceed 28 Seconds.

3.Steering gear must be power operated if rudder stock diameter is greater than 120mm. Mostly we use hydraulic power for operating the rudder post.

Power of the auxiliary steering shall be such that rudder can be swung from 15º on one side to 15º to the other in 60 seconds at deepest draft and of 7 knots.

5.Steering gear should be prevented from any abnormalities, such as overloading, short circuit, overload, and visual and audible indicator should be available on bridge, ECR and steering gear room, alarms and trips is also present to minimize the damage.

6.For Hydraulic Oil tank, Low level alarm is present.

7.A Tanker of 10000 GRT and more must be provided with two steering gear systems, So in case of failure the steering gear change automatically to the stand-by Steering gear system within 45 seconds, along-with alarm for indication. It is provided for a reason, that if in case failure occur the ship keep moving in the same direction.

#Cover image credit: http://marine-data.co.uk

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Author Arpit Singh & Amit                                                                Article Requested by: kesavan s

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