Marine Knowledge since 10: TANKERS ABOVE 30000DWT IS A MUST!!!!!

TANKERS ABOVE 30000DWT IS A MUST!!!!!

INERT GAS SYSTEM/GENERATOR

Inert gas system(IGS) or inert gas generator(IGG) are only used in tanker ships. This is the most important in tanker ships. The major function of this system is for safety. Safety for cargo tanks. For a more clear picture eloborating this will be more specific. In atmosphere there is 21% of oxygen. Oxygen has its presence in all the place even in an enclosed cargo tank. So when there is loading or discharging operation is done in tanker ships with oxygen in it most probably the ship may explode because of pressure, temperature and air so called the mixture for combustion. So to overcome this problem IGG & IGS are introduced. The working principle are quite simple and easy. IGG & IGS both have different working procedures but the production is the same. The end product is inert gas. You may think why specificalyy inert gas is used. What is an inert gas?

DEFINITION OF INERT GAS

Inert gas is a type of gas whichcontains low volume of oxygen and have an excessive amount of nitrogen and carbon dioxide and also contains sulphur oxide.

So from here we can conclude that inert gas would help in reducing oxygen content. So to produce this inert gas there are 2 ways. One is IGG and the other one is IGS. First and foremost i would like to explain how an IGS works.

HOW AN IGS WORKS?

Inert gas system(IGS) depends on the boiler. You might think what is the connection betwen IGS and a boiler. There is an obvious huge connection between them. IGS is operated by the flue gas of the boiler. Yes. The flue gas produced from the boiler is more less have the properties of an inert gas. So flue gas is used in IGS. This flue gas from boiler is not wasted by discharging through funnel but is used as inert gas for cargo tanks.

This gases are cooled directly by seawater then is sucked via the uptake valve to the venturi scrubber by a blower. This cooling process have 2 stages. The first stage, flue gas is cooled directly by sea water in the venturi scrubber. The second stage, the gas is cooled by sprayers in the combustion chamber. In the venturi scrubber the sulphur oxides and particles are washed out, while the content of nitrogen, carbon dioxide and oxygen remain unchanged. At the end of the cooling section the water droplets/mist is seperated from the flue gas by a help of demister. This is to prevent the flue gas that will be supplied to the cargo tanks is moist. The cooled and clean flue gas is then supplied to blowers or downstream the generator. The cooling and scrubbing water is dscharged through a drain line beneath the combustion chamber.

After that the clean flue gas is discharged through the IG line to deck seal water. The main purpose of the deck seal is to provide constant pressure and temperature to the gas. The deck seal also helps to clean any remaining soot or carbon from the flue gas. Then from there the flue gas is inerted in the cargo tank as an inert gas.

HOW IGG WORKS?

The inert gas generator(IGG) should be reliable for a good combustion process. The process of an IGG is more less similar to the IGS after the combustion process. The IGG is fitted with a burner. The burner system has a specialized design in accordance with the Aalborg industries’ Inert System Ultramizing Combustion System(ISUCS).

The ISUCS is meant by combustion air is supplied under pressure to the burner where the fuel is atomized in such a way where a perfec fuel/air mixture is achieved, so that the behaviour/properties of the flame will be identical to the gas/air mixture.

The product is a light blue flame which barely or will not produce any soot. Actually this soot cannot be tolerated in the IGG neither in the cargo tanks nor piping system. So to prevent the soot especially during long run of the generator a well balanced and extremly stable combustion process must occur. We will look into the problem if there is soot in the system later. The hot inert gas that is produced after the combustion process contains nitrogen, carbon dioxide, sulphur oxide and a very low volume of oxygen.

Then this hot combustion gases are cooled indirectly first by a jacket water cooling in the combustion chamber. This is called as pre cooling. The main cooling process is taken place during the cooling/washing section in the combustion chamber where the gases are cooled directly with seawater and at the same time the sulphur oxides are washed out. Sulphur is present in the fuel and is oxidized during combustion and produce as sulphur oxide.

At the end of the cooling process, the water droplets/mist are seperated from the gas by a demister which then allows only the flow of gas through it. The required final pressure of the IGG is achieved by a blower upstream the combustion chamber and a pressure regulating valve in the IG line. A water loopseal under the cooling section discharges the water and at the same time maintaining the required pressure and also prevent the inert gas from escaping.

FUEL TO AIR RATIO/OXYGEN CONTENT

The fuel to air ratio has been set over the whole capacity range by means of the adjustable screw in the fuel oil regulatiing valve. In case there is drastic changes in air humidity, temerature or quality, the oxygen content becomes too high or too low. In this event, the fuel oil pressure before the fuel oil regulating valve must be incresed or decresed by means of the pressure control valve in the fuel oil return line.

DETAILED PROCESS DESCRIPTION OF IGS

FLUE GAS/AIR SUPPLY

· Have 2 blowers each with 100% capacity

· Casing of blowers have internal lining and inspection openings

· There is a rinsing and drain connection for the cleaning of blowers.

· Rinsing should be done manually during run down of the blowers at normal stop of the unit

BOILER UPTAKE VALVE

· This valve situated in between the boiler and the venturi scrubber.

· Normally in closed postion

· The valve operated automatically

· This valve is controlled by the limit switch

· The uptake valve will have some leakage when in closed position, so in preventing the leakage a high performance valve is installed behind this valve

· This high performance valve do not have leakage because there is a sealing air connection after the uptake valve.

· In this case, when theuptakevalve is closed th high performance valve also will be in close position

VENTURI SCRUBBER

· The flue gases(400^oC) coming fromthe boiler, enters the venturi scrubber via a flexible joint.

· Designed to ensure high efficiency in removal of sulphur and soot particles from the flue gas

· Also precools the flue gas at the same time

· The velocity inside is over 50m/s

· Water supply fixed by the orifice

· The gas speed atomizes the injected water and the fine droplets catches the soot and also other small particles/impurities that is found in the flue gas.

DETAILED DESCRIPTION OF IGG

COMBUSTION AIR SUPPLY

· Combustion air is supplied to the main burner with required pressure by the blower

· The oil pressure must be adjusted in such a way, that the oxygen content of the inert gas is produced, will be between its limit.

PILOT BURNER

· Certain interval of ignition sequence, pilot burner is supplied with fuel oil with 16bar pressure and instrument air for atomization and combustion; adjusted by a pressure regulator(approx.1.4bar)

· Ignition occurs with help of an ignition transformer and a spark plug

· The burning of the flame is monitored by an UV-flame detecter, in conjunction with the flame safeguard relay in the electric system.

MAIN BURNER

· The main burner is ignited by the pilot burner

· Fuel oil is atomized in 2 steps:

1. The fuel oil is dispersed by a spray nozzle

2. It is subjected to a tangential impulse flow of combustion air which, added to the mainly axially orientated impulse flow of the liquid itself, results in an ultrafine dispersion of the liquid.

· The tangential impulse flow is created by supplying the air through slots in a ring fitted around the spray nozzle.

· An optimal atomization of the fuel oil is achieved if the differetial pressure over the slots of the atomizing ring approximately 0.1bar

· At a lower atomizing pressure, the quality of combustion will grow worse and the possibilty of soot being form will be greater

COMBUSTION CHAMBER AND SCRUBBING TOWER

· Main and pilot burners are mounted on the combustion chamber.

· The combustion chamber is surrounded by a cooling water jacket to discharge the generated heat as much as possible.

· The cooling water jacket is spiral, so the cooling water flows around the combustion chamber wall and an optimum cooling effect is achieved

· Hot inert gas flows through the cooling tower, where they are cooled directly with seawater and at the same time sulphur oxides are washed out from the inert gas

· Seawater is supplied by the orifice to the sprayer and the combustion chamber jacket

· Seawater leaving sprayers flows out via the waterseal

· Sufficient cooling is needed for the combustion chamber

These are the working principle and detailed description of each parts of the IGS and IGG. There might be some questions regarding the oxygen content in the inertgas produced. Some might ask if an inert gas contain oxygen how it can it be considered as safety.

In a cargo tank if the oxygen content is more than 8% itis considered dangerous and can cause explosion if correct method is not used. IGS or IGG that produces inert gas can have an oxygen content of 3-5%. If the oxygen content is more than that an alarm will be rang and a safety interlocking device will open the purge valve and closes the main delivery valve. This is to reduce the oxygen content in the inert gas.

The inert gas will be blown off at the begining stage of combustion for 5 minutes. This is to ensure that the oxygen level inside the combustion chamber reduces. After 5 minutes if there is still footage of oxygen can be detected the fuel to air ratio should be checked and set back to required ratio.

This IGS and IGG can only be found in ship more than 30000DWT. Some ships which sail to America or similar countries only should have IGG. IGS is considered less safety in such countries so they banned the usage of IGS in ships that sail to this countries.

IGG


Blower: Deliver IG to Cargo Tank

I will be posting the working principles of each component of IGG and IGS next week.... for more detail description of IGG and IGS stay tuned in www.mksince10.blogspot.com