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.
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