OPERATION OF INERT GAS SYSTEM
The term purging is used in this manual to mean the introduction of I.G. into a cargo tank with the object of reducing the hydrocarbon content and/or further reducing the oxygen content.
Purging is carried out by dilution method or displacement method.
o Dilution method
Dilution method means forced mixing with high velocity injection of I.G.
On a vessel which is not fitted with purge pipe in cargo tanks, this method is applies. In this case, the tank atmosphere is diluted by inert gas. Diluted gases are emitted to atmosphere.
o Displacement method
Displacement method means changing of tank atmosphere by utilization of density difference between inert gas, hydrocarbon gas and fresh air. On a vessel is fitted with purge pipe in cargo tanks, this method is applies. In this case, I.G. being supplied should be kept at proper velocity to prevent minimize forced mixing in cargo tanks.
Gases are emitted to atmosphere through the purge pipe.
The tank atmosphere is therefore displaced by inert gas.
The following objective are achieved by purging.
o The primary inerting of cargo tanks after delivery of the vessel or dry docking
o The restoration of a satisfactory inert atmosphere, if for any reason, unsatisfactory Inert Gas was present
o The reduction of the content of hydrocarbon gases
Precautions
o In the dilution method, opening for gas release to atmosphere should be far away as possible from the inlet in order to ensure the required gas circulation within a cargo tank
o The O2 content of supplied I.G. must normally be kept less than 5%
o When the O2 content of the cargo tanks is reduced below 11.5%, the tank atmosphere will reach a non flammable condition but it is necessary to further reduce the O2 content below 8% as measured at. outlet position gas/air to increase the safety factor
o The velocity injection of I.G. is important for both Displacement and Dilution methods. Therefore it is necessary to consider how many tanks are purged simultaneously.
Cargo Loading, Ballasting of cargo tanks (Heavy weather ballast)
Inert gas is not supplied to tanks when loading or ballasting. For these, operations, gas from the cargo tanks is vented through the Inert Gas Mainline in proportion to the increase in the liquid levels.
Precautions
Before loading cargo or ballast, the following items must be confirmed.
o I.G. supply valves at each cargo tanks are open
o Vent riser main valve is open
(above two items are very important to avoid structural damage.)
o Deck isolating valve is closed
o Seal water level of deck seal unit shows correct reading
o Seal liquid of the P/V breaker is at the proper level.
Duty officer must check that the mesh of the flame arrester is not blocked (check at least 24 hours prior use).
Duty officer must ensure that the cargo oil or ballast water does not overflow into the I.G. supply line. This is because the diameter of the I.G. supply line is less than that of the cargo lines and structural damage could occur due to over pressurizing.
The vent riser main valve must be shut after completing loading cargo or ballasting.
Discharging Cargo, Deballasting (Heavy weather ballast)
The I.G.S. operation is same procedure when discharging cargo at loading port or deballasting on passage.
In this case, the I.G. is supplied to the cargo tanks at a rate at least equal to the discharge rate.
At the discharging port, the I.G.S. must be operated BEFORE commencing discharge of cargo.
Precautions
o Tank pressure should always be kept more than 200 mm.Aq of positive pressure
o In the case of taking soundings etc., then to release pressure by opening of the mastriser valve, and then measure as soon as possible in order to avoid fresh air entry to the tanks.
o After completion of the above work, close mastriser valve, and top off with IG as/ if required.
o Insufficient supply capacity of I.G. comparing with the discharging rate causes a vacuum in the tank and can result in structural damage.
Therefore I.G. supply capacity must exceed the discharging rate (volume) in order to have positive pressure.
o In case it is impossible to maintain positive pressure with one blower out of order, then slow down the discharging rate
o The I.G. that is being supplied should normally have an O2 content of below 5% but there is a possibility it may increase due to alteration of the load on the boiler. I.G. with an O2 content up to 8% is acceptable for short period of time.
o In case of a light load on the boiler and insufficient supply of good quality O2 content I.G., increase boiler load
o After completion of discharging cargo, tanks are to be pressurized
o In the case of ballasting after completion of discharge, the tanks may be pressurized.
Total Failure
The term "TOTAL FAILURE" is used to mean that when it is impossible to supply inert gas into the tanks due to equipment failure.
When total failure occurs while discharging cargo, the following procedures should be followed.
o Discharging of cargo should be STOPPED
o If there is insufficient pressure in the tanks, only then open the ventriser main valve to avoid causing a vacuum in the tanks
o Notify terminal. Advise terminal and charterer estimated time to repair inert gas system. Investigate cause and if possible carry out repairs.
IMPORTANT
If the inert gas plant breaks down during discharge and air enters the tank, no dipping, sampling or other equipment should be introduced into the tank for 30 minutes after the cessation of the inert gas.
After 30 minutes equipment may be introduced provided that all metallic components are securely earthed.
During the re-inerting of a tank following a breakdown and repair of the inert gas system, no dipping, ullaging, sampling or other equipment should be inserted until it has been established that the tank is fully inerted.
This should be done by monitoring the efflux gas from the tank being inerted, when it is known that the efflux gas is fully representative of the gas condition throughout the tank. However, if it is necessary to introduce a gas sampling system into the tank for this purpose, there should be a wait of 30 minutes following the cessation of inert gas injection before insertion of the sampling system.
Metallic components of the sampling system should be securely earthed.
During the initial inerting of a non-gas free tank, the sample precautions should be taken as when re-inerting after breakdown and repair of the inert gas system.
The earthing of introduced metallic conducting equipment as recommended above must be maintained until 5 hours have elapsed from the cessation of inert gas injection.
Crude Oil Washing (C.O.W.)
o Prior to commencing C.O.W., confirm O2 content of tanks is below 8%
o During C.O.W., the O2 content of I.G. being supplied should be below 5%
o When due to insufficient load on the boiler, the O2 content of the delivered IG goes over 8%, C.O.W. must be stopped.
Boiler Load
Diesel Vessels
In port
In diesel vessels where the boiler output is based on the demand cargo pumps, flue gas of good quality O2 content should be available.
When final stripping or slow discharging is required and good quality inert gas can not be obtained, boiler load should be increased.
This can be done as follows;-
o Operate clean ballast pump sea to sea
o Idling deck machinery. (Winch, Windlass, etc.)
On passage
To obtain I.G. on passage through operation of the auxiliary boiler, the load on the boiler must be created by the following: -
Operate clean ballast pump sea to sea
Idling deck machinery. (Winch, Windlass, etc.)
If possible and lines provided to dump steam then using tank cleaning heater.
It is possible to decrease the air necessary for boiler combustion to smoke limit by changing over ACC installation from auto to manual after increasing the boiler load as described above.
Loaded Passage
Air is likely to enter the cargo tanks due to variations in the air/sea temperatures. However, this can be prevented by keeping a positive pressure on the cargo tanks whenever the vessel is under-way at sea. To give the required protection, pressurization has to be done at the start of the voyage. (In heavy weather, when rolling, pressure changes may cause opening of the vent riser breather valve for release of excess pressure.)
Pressure should be recorded on a daily basis. This will help to determine when repressurization will be required to remain above the alarm setting.
PRECAUTIONS as follows
Ballast Passage
Pressurization at discharge port.
To prevent air entering the cargo tanks on the BALLAST passage, the cargo tanks should be pressurized before leaving the discharge port.
After sailing from the discharging port, if the cargo tanks contains unsatisfactory pressure and it is necessary to improve the tank condition by pressurizing with good quality IG.
O2 content in tanks must always be maintained below 8%.
Tanks must always be maintained at positive pressure of over 200 mm.Aq.
Tank pressure brought up to approximately 750 mm.Aq initially will be subject to range of pressure variation due to temperature and lapse of time. However, O2 content will not appreciably change.
If daily record of tank pressure indicates pressure decreasing to less than 200 mm.Aq, the tank should be pressurized with good quality I.G.
Loaded Passage (Full Cargo)
On completion of loading, the vent riser main valve should be closed, and pressure adjustment in the tanks is controlled by the breather valve.
The ullage space in the tanks are usually then in an over rich condition.
If necessary, pressurize with I.G.
Tank pressure may decrease due to small gas leakage from tank cleaning holes and hatches.
In cold weather, the temperature will tend to reduce the pressure in the ullage space.
If the tank pressure indicates vacuum, pressurize with I.G.
In rough sea the pressure in the ullage space is influenced by the ship's movement. Therefore, pay strict attention to the deck main pressure gauge.
Tank Cleaning (Water washing)
During tank cleaning, there are conditions which could produce ignition and explosion. e.g.
o Sparks caused by equipment/fittings falling down inside the tank
o Static electricity produced by water jets from cleaning machines.
It is therefore essential to ensure that the tanks are fully inerted to less than 8% O2 content prior to commencing and during tank cleaning.
FIXED TANK CLEANING MACHINES (Crude Oil washing/ water washing)
Before commencement of tank cleaning, measure the O2 content of the tank atmosphere. While tank cleaning is in progress, maintain positive pressure throughout by operating the I.G.S.
Any pressure built-tip while tank cleaning can be released through the vent riser main valve.
PORTABLE TANK CLEANING MACHINES (Water Washing)
If necessary, these machines are used for additional cleaning, especially before dry docking or tank inspection.
During this operation, tank cleaning holes are open, and air may enter the tanks.
Therefore, the I.G.S. must be operated and the tanks being cleaned supplied with I.G. to continuously maintain a positive pressure and O2 content less than 8%.
Precautions
o Measure the O2 content in the tanks which are scheduled for washing prior to commencing tank cleaning
o If the measurementof O2 content indicates over 8%, purging of the tank must be started
o During tank cleaning, continuously maintain positive pressure in excess of 200 mm.Aq.
o O2 content of the I.G. being supplied is normally less than 5%, but must not be permitted to exceed 8%
o If the O2 content of the I.G. supplied exceed 8%, stop supplying I.G. Also stop the washing.
o If there is insufficient time for purging all tanks which are scheduled for washing before tank cleaning, simultaneous operation of cleaning one tank and purging another tank can be carried out.
Example 1
Measure tank pressure and O2 content and confirm over 200 mm.Aq and below 8% respectively. After that operate I.G.S. and commence tank cleaning with fixed machine
o If no problems are encountered during operation, tank cleaning can be continued until completion
o When O2 Content of I.G. being supplied can not be maintained below 8%, stop supply of I.G. and then if O2 content in tank below 8% and tank pressure maintained over 200 mm.Aq, tank cleaning can be carried out on continuously
o When tank pressure is below 200 mm.Aq and/ or O2 content in tank is over 8%, tank cleaning with fixed machines must be stopped.
Example 2
When tank cleaning is carried out without the I.G.S, in gas free condition, portable machines only should be used
Gas Freeing
Before gas freeing is carried out either in preparation for dry docking or repairs, it is important to recognize the procedure necessary in order to avoid flammable mixture in the cargo tanks when air is introduced.
To ensure the tank atmosphere does not pass through the explosive range when gas freeing, the tank has to be purged with I.G. until the hydrocarbon gas content is below the Critical Dilution Line.
Gas freeing there are five methods as follows.
o Gas freeing by portable blower
o Gas freeing by inert gas blower
o Gas freeing by fixed blower (External High Capacity Blower)
o Gas freeing by combined portable and inert gas blowers
o Gas Freeing by ballasting.
Gas Freeing By Portable Blower
This gas freeing is usually accomplished by the use of several blowers.
Precautions
o I.G. supply valve of the tanks should be closed and blank flanges fitted
o Some of the blowers should be fitted with flexible ducts
o Combination use of blowers with and without ducts is considered preferable
o Exhaust openings should be as far away as possible from the sources of fresh air supply
Gas Freeing By I.G. Blower
Precautions
o Exhaust openings should be as far away as possible from the source of fresh air supply
o All I.G. supply valves other than the tanks to be gas freed must be closed
o If the time of gas freeing extends over a long period, attention must be paid to the tank pressure on other tanks which may be affected by temperature alteration
o In the case of small tanks, it is sometimes impossible to gas free them due to the fresh air from the blower not reaching the bottom half of the tank. In this case, portable blowers with a flexible duct should always be used.
Gas Freeing By Fixed Blower (Golar Vent)
Precautions
o The cargo pipe line should be washed by sea water and then drained completely
o Tank bottom should also be drained as dry as possible
o I.G., supply valve at the tanks should be closed and blank flanges fitted
o A suitable number of cleaning holes or I.G. supply hatch should be opened as necessary
o When cleaning work on other tanks is being carried out at the same time, keep the pressure around 750 mm.Aq in the tanks being cleaned to prevent the possibility of fresh air leaking back through the cargo pipe lines from the tanks being gas freed.
Gas Freeing Combining The Use Of Portable Blowers And I.G. Blower
Fig. 2-7D Next page
Refer to items A and B.
CAUTION!
When entering tanks gas freed by any the above mentioned methods, it is essential to pay strict attention to the possibility of the existence of pockets of hydrocarbon gas still being present even after the hydrocarbon and O2 content has been checked and found to be satisfactory. (O2 21 %) (H/C <1%LEL)
MEASUREMENT OF OXYGEN and HYDROCARBON GAS CONTENT and PRESSURE
In order to keep the atmosphere within non-flammable limits it is necessary to check and understand the oxygen and hydrocarbon gas content in the tanks. For this reason, the measurement of the oxygen and hydrocarbon gas content and the checking of the tank pressure is Important and should be done correctly by the appropriate means.
Measurement should be done:-
o By proper instrument - calibration checked
o At required measuring locations and depths
o At required time intervals
Precautions During Measurement
When the measurement of the oxygen and hydrocarbon gas content and pressure is made, the following, points should be noted for a safe and correct operation.
The Chief officer is responsible for taking all measurement. If the Chief officer cannot personally take measurement for whatever reason, another officer must be delegated to take his place. In such case, he must inform the Chief officer of his finding and obtain his advice before next work
The measuring instruments must be maintained regularly and also be checked and adjustment before using
Be careful during measurement not to inhale gas
Be careful that nothing can fall into the measuring opening
Before entering any enclosed space, take measurement of gas and oxygen
When measurement is taken, at least two persons should be engaged in the work. i.e. One to carry out measurement and other to maintain safe distance.
Measuring Instruments and Their Range.
Oxygen content analyzer RIKEN OA-225R
o This instrument is "Intrinsically safe"
o Before using, carry out "range check" with fresh air
o The Sensor's life is about one year, but its life becomes limited according to the frequency of use
o Even if not used in a year, renew it annually
o When not inuse the instrument sensor to be capped and sealed
o Dipping the sensor into water or keeping it in a wet atmosphere is to be avoided
o If used in Flue Gas directly from boiler this gas is to be cooled and then it can be measured properly.
Hydrocarbon gas content analyzer
RIKEN type 18 & type 21
o Instrument for gas content low range measurement (measuring range 0-2%)
o Before using, carry out "range check" with fresh air
o Before working inside tanks or whenever checking the gas free condition for entering drydock, only this type of instrument must always be used. Do not use the M.S.A. TANKSCOPE for testing to decide if it is safe to work inside tanks
o In an atomosphere containing Inert Gas and Hydrocarbon gas it is impossible to measure the Hydrocarbon gas content correctly because this instrument also collects CO2 gas together with Hydrocarbon gas.
In this case the M.S.A. TANKSCOPE must be used.
For instance, when purging a tank by Inert Gas withO2 content 5%, even if measurement for hydrocarbon gas was taken by "RIKEN- 1 8 " or "RIKEN-21 ", the correct hydrocarbon gas content cannot be obtained because these instruments collect both hydrocarbon gas and CO2 gas.
o When the battery is changed, it must only be done inside the accommodation. It is prohibited to change in a place where gas may exist or can enter.
M.S.A. Tankscope
o This is the instrument for measurement of hydrocarbon gas in the 0-20% range
o Before measurement, a battery check and zero check must be done
o This instrument is "Intrinsically safe"
o When the gas content is less than 2%, the "RIKEN-18" or "RIKEN-21" should be used
o As the filament-units life (2 spares set in the instrument) depends on the measuring frequency or gas content, its life may be limited and must be checked
More measurement of high content gas, the shorter the life.
o When the battery is changed, it should only be done inside the accommodation. It is prohibited to change in a place where gas may exist or can enter.
Measuring Range Of Instruments.
The following diagram shows the measuring range of the above-mentioned three types of instrument.
Important Points in Taking Measurements in Tanks.
Selection of measuring locations for oxygen and hydrocarbon content will depend on the operation, tank size and tank construction. For example, measurement of the atmosphere in tanks where the gas is moving such as during purging or gas freeing is different for that required in a tank which is static during the voyage.
Gas distribution is not always uniform, therefore it is necessary to measure hydrocarbon and oxygen at different locations and depths. Measurement at one location and depth may not always be sufficient and certainly additional measurements will be necessary where pockets of gas are likely to be found i.e. near to tank bottom and in corners in way of tank structure.
In general, when measuring oxygen, the upper part of the tank is the most important, and for hydrocarbon gas, the lower part of the tank is the most important.
When purging, gas inlet speed and the location and number of tank openings are very important to ensure the required atmosphere changes are taking place within the tank.
Measuring Points While Purging.
Gas distribution within cargo tanks during purging depends on many factors e.g. type of crude, velocity of inlet gas and tank structure. Just after commencing purging, it is therefore very important to measure several locations in the tanks to establish the gas distribution so that as the operation proceeds it is possible to confirm whether the gas concentrations are changing as required.
When purging, gas release to atmosphere through a deck opening(s) should be as far away as possible from the Inert Gas supply inlet.
Thefollowing points will assist in maintaining the gas inlet speed:-
o the area of the opening should be sufficient to ensure gas release is not restricted. If the area is too small, the tank pressure will increase and gas inlet speed will be reduced
o the deck main pressure should be kept as high as possible to maintain the gas inlet speed to the tank(s)
o the number of tanks OPEN for purging at any one time.
REMARK : When the tank atmosphere lies in the Explosive Range before purging ortank atmosphere will pass through Explosive Range during purging, measurement should be done according to ISGOT recommendations which must be fully complied with.
SHORT TANK
Purging is permitted in two small tanks at the same time. But the following points are to be noted.
If after taking the above measurement twice, oxygen content is changing in the same ratio then measurement need only be made at the gas outlet. If not, the decrease of oxygcn content at, the bottom point is slower than at the other points, it indicates that Inert Gas is not reaching the bottom. In this case, only one tank should be purged.
LONG TANK
In early stage, measure at points A, B, C, and A’, B’, C’ continuously. If the decreasing ratio of O2 content at point A is slower than at B, then measure the point A continuously until operation is completed.
The effect of purging causes pockets as above, but these will disappear as the purging continues.
SHORT TANK
If purging too fast in a short tank or in slop tanks, the atmosphere in the lower part of the tank can be compressed by gas pressure and therefore not mix.
In this case, it is better to reduce the inlet velocity by reducing the Deck Main Pressure,
Measure the point A.B.C.
If the decrease of O2 content at points A, B, are slow, the above mentioned compression may have occurred.
Measurement Point While Gas Freeing.
While gas freeing, the gas distribution in tanks depends especially on the way the gas freeing operation is being carried out.
If the method of gas freeing operation is not correct, it is time wasting and pockets of gas are likely to remain.
The important consideration while gas freeing:
o number of deck openings
o velocity of inlet gas
o location of inlet/outlet relative to each other.
Gas measurement is not only for the gas condition in the tanks but is also the best means of checking the success of the gas freeing operation.
Hereunder, we show an example of gas freeing and explain the measurement points.
LONG TANK (Using portable blower)
SWASH BHD has enough opening. Using 2 sets of turbine blowers
1 set with duct 1 set without duct, also fit a duct to gas outlet side
If tank capacity is more than 20,000m3 , 4 sets of blowers should be utilized.
SHORT TANK (Using portable blower)
If gas freeing from tank top especially in SHORT TANKS or slop tank, atmosphere in tank bottom is compressed and air does not mix with the gas.
In this case the duct should always be used.
Bottom part is compressed and would not mix with air.
LONG TANK (Using portable blower)
SWASH BHD has not enough opening.
It is considered impossible to gas freeing by Inert Gas Blower. Therefore use four(4) portable blowers, i.e. two with duct and two without duct.
Point A and A' hydrocarbon are most likely to remain.
SHORT TANK (Using inert gas blower)
Gas freeing by means of using the Inert Gas blower should be done one or two tanks at a same time, and the deck main pressure should be kept sufficiently.
As there is no HORIZONTAL GIRDER under the SUPPLY HATCH, fresh air reached the tank bottom and tank gas freed quickly.
SHORT TANK (Using inert gas blower)
There is no SWASH BHD but there is a HORIZONTAL GIRDER directly under the INERT GAS SUPPLY HATCH.
In the top figure, six(6) times atmosphere changes have taken place but the O2 content in the bottom under the IG supply hatch (point) A' is almost unchanged.
After that, a portable duct was inserted into the opening (gas outlet) and gas freeing continued. After about two hours the O2 content reached over 20%.
The point where it takes the longest time for oxygen content to reach 20%.
Measure the points A.B.C. and A'B'C' twice. After that measure the points A and A' at frequent
intervals until gas freeing is completed. Especially at point A' hydrocarbon gas is most likely to remain.
SHORT TANK (Using inert gas blower)
Deck main pressure should be kept sufficiently. Gas freeing should be done tank by tank.
Gas freeing is difficult in places where hull structure is complex such as the Fore and Aft tanks. In this case, special attention should be given to arranging the DUCT to the place where gas is most likely to remain.
Measurement Points in Tanks.
BEFORE AND DURING TANK CLEANING
Before tank cleaning
Oxygen content should be measured at three different depths, that is at upper, middle and bottom points in the cargo tank through the ullage hole.
During tank cleaning
Measurement of oxygen content is not necessary while positive pressure is maintained in the tanks.
In case measurement is required during tank cleaning, stop the cleaning machines to protect the sensor of the instruments from splash water.
CARGO DISCHARGING
Generally, it is not necessary to measure oxygen and hydrocarbon gas content.
Should measurement be required, should be done at three different depths, that is upper, middle and bottom points in the cargo tanks through the ullage hole.
Measurement of Tank Pressure
Tank pressure should be checked every day taking the following precautions.
Tank pressure can be measured by compound gauge fitted on the deck main line.
This pressure gauge reading is transmitted to the CCR and Navigation Bridge through the pressure transmitter.
Check the compound gauge by comparison with the manometer or direct reading gauge and allow for any difference.
Drain out the sensing tubes (if sensor type fitted) to ensure correct indication.
Relations Between Injection Velocity of Inert Gas/Fresh Air and Its Effective Attainment Distance
For Dilution Method
It is very important to ensure high injection velocity of inert gas/fresh air for effective gas distribution in the tanks. If the injection velocity is not sufficient, gas pockets may remain in the bottom part of the tanks, especially during gas freeing. Fresh air injected with insufficient velocity will make a "short circuit" and only pass through upper part of the tanks, because of its lighter density when compared with inert gas and/or hydrocarbon gas. When purging with the Inert Gas, the density is heavier than oxygen (fresh air), so that, even if injection velocity is not sufficient, purging will probably be successful but at the expense of time.
It is preferable however to maintain the require injection velocity.
Increase of injection velocity (V1 means increase of pressure (P2 and Pi)
Therefore;
Maintain high pressure of supplying inert gas/fresh air in order to ensure high injection velocity.
In addition, exhaust opening must be of sufficient area, i.e., more than three(3) times the inlet area. If not, insufficient exhaust area increases pressure inside of tanks (P3 ). Consequently, pressure difference between inside of the tank (P3) and injection (P2) is reduced, and then injection velocity (V1) reduced.
Remark:
To increase pressure at the (P1) close the I.G. Supply valve on other tanks.
Don't adjust the deck Main Pressure (P1) by tlrottling the I.G. supply valve on the tank being supplied with Inert Gas/Fresh air.
We show Actual ship's data of MSTs follows.
o Fig 2-9 Actual ship's data -of MSTS. (90,000 DWT)
These data show Relations between Deck Main Pressure and Gas velocity in the cargo tanks.
TestResult
Deck Main Press. V1 V2 V3
mm.Aq m/sec. m/sec. m/sec.
380 1.2 0.55 0.25
480 1.5 0.68 0.20
600 1.8 0.88 0.15
700 2.0 1.00 0.24
820 2.3 1.20 0.27
1910 2.5 1.5 0.15
Remarks.
o Used two blowers (5,500 m/h x 2 sets)
o Adjusted Deck Main Press. by throttling the I.G. supply valve on No. I Port and Starboard Tank
o Gas inlet diameter is 250 mm shown as follows.
Fig.2-10 shows Velocity curves at the points of 8.9 m, 19.5 m, 25 m and 30 m in depth . Both 8.9 m and 19.5 m are actual data, and both 25 m and 30 m are presumption curves based on above two actual data.
Concerning the above four curves, theoretical gas velocity is more 3 to 4.3 times higher than these curves.
Minimum mixing gas velocity is 0.2 m/sec. theoretically, but it is better to keep the velocity more over 0.5 m/see. to ensure effective dilution.
For example, if the tank depth is 20 m, keep Deck Main Pressure more over 350 mm.Aq.
By Displacement Method
Injection velocity must be kept slow in order to minimize gas mixing in the tank atmosphere. Therefore, purging or gas freeing for this method should be carried out in more than one tank at a time.
I.G.S. OPERATION RECORD BOOK
It is very important from an I.G.S. operation point of view, to record I.G.S. operation data in the I.G.S. Record Book. This is also useful for improvement of operation in future.
According to the following entry procedure the person on duty should record all I.G.S. data and keep the Book in good order.
ENTRY PROCEDURE
Notwithstanding operation or non-operation, I.G.S. data should be recorded as required below. Example is attached.
Under Operation.
Data to be recorded every hour after start of I.G.S. Refer to (Fig. 2-1 1) in connection with recording items.
Oxygen content should be confirmed and recorded in the Remarks column before tank cleaning, or crude oil washing.
Non-Operation
Measurement should be carried out once a day before noon. Refer to Fig. 2-1 I in connection with recording items.
Measurement of Oxygen Content
o Measuremet of O2 % should be carried out once as soon as possible after sailing
o On passage, measurement should be carried out when there is some possibility of fresh air entry to tank, for example, loss of system pressure or after heavy weather (rolling)
o Measuring tanks are as follows
· The nearest tank to vent riser
· One tank forward (wing tank)
· One tank aft (wing tank).
Notes
o Water supply and P/V breaker should be recorded Yes (O) or No (X) after confirmation of adequate water/oil supply
o Items re tank condition should be recorded using following words:
a(gas free), b(clean/dirty ballast), c(laden full/half), d(discharging cargo), e(crude oil washing), f(tank cleaning).
o In the remarks column the following items should be recorded.
· I.G.S. trouble and the cause(s)
· Comments for future operations
· Others.
o Scrubber water supply under non-operation means scrubber seal water.
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