After evacuation, the next step consists of filling the insulation spaces with nitrogen. The cycle is repeated until the oxygen content in the spaces is less than 2%.
Procedure for Initial Filling With Nitrogen
Liquid N2 is supplied from shore to the liquid manifold where it passes to the stripping/spray header via the appropriate ESDS liquid valve. It is then fed to the LNG vaporizer and N2 gas produced is passed at +20°C to the each insulation spaces.
At this stage, ship’s N2 generators are not used due to required capacity for initial filling.
It is assumed, though unlikely, that all valves are closed prior to use.
a) Install the spool piece on LNG vaporizer outlet to insulation spaces.
b) Prepare the LNG vaporizer for use.
c) Adjust set point of the temperature control valve to +20°C.
d) Open valves related from liquid manifold to be use to LNG vaporizer through stripping/spray header and carry out line cooldown operation. For safety reasons, ensure that the hull water curtain on the connected side is in operation.
e) Open the isolation valves CN278, 276, 285, 283 for the insulation exhaust control system.
f) Crack open the primary space supply valves CN475, 375, 275, 175 on each tank.
g) Adjust the opening of the primary space supply valves for balancing the pressure rise in all the spaces. During filling, always maintain the pressure in the primary space 100 mbar above the secondary space.
h) When the pressure in the primary spaces reaches 300 mbarA (100 mbar above the pressure in the secondary spaces), crack open the secondary space supply valves CN474, 374, 274, 174 on each tank.
Adjust the opening of these valves for balancing the pressure rise in all the spaces.
i) Set the opening of the control valves CN277, 284 at 4 mbar.
Operating Procedure for the Completion of the Nitrogen Filling a) The final filling of the insulation spaces, up to 2 mbar is carried out
at reduced flow rate. Three cycles are usually necessary.
b) After the final filling, check the oxygen content in all the spaces. If it is higher than 2%, repeat inerting operation. There is also the possibility to check the O2 content at the vacuum pump discharge.
c) Do not shut down the LNG vaporizer until it has been warmed through to the ambient temperature.
! Caution
Changes in temperature or barometric pressure can produce differentials far in excess of 30 mbar in the insulation spaces which are shut in. With the cargo system out of service and during inerting, always maintain the primary insulation space pressure at, or below, tank pressure and always maintain the secondary insulation space pressure at or below the primary insulation space pressure.
Operating Procedure for Normal Inerting
The primary and secondary insulation spaces are filled with dry nitrogen gas which is automatically maintained by alternate relief and make-up as the atmospheric pressure or the temperature rises and falls, under a pressure of between 2 mbar and 4 mbar above atmospheric.
The nitrogen provides a dry and inert medium for the following purposes:
To prevent formation of a flammable mixture in the event of an LNG leak To permit easy detection of an LNG leak through a barrier
To prevent corrosion
Nitrogen, produced by the two N2 generators and stored in a pressurised buffer tank of 26 m3, is supplied to the pressurisation headers through make-up regulating valves located in the cargo compressor room. From the headers, branches are led to the primary and secondary insulation spaces of each tank.
Excess nitrogen from the insulation spaces is vented to mast No.2, through the exhaust regulating valves.
Both primary and secondary insulation spaces of each tank are provided with a pair of pressure relief valves which open at a pressure, sensed in each space, of 10 mbar above atmospheric. A manual bypass with a cut out valve and a ball valve is provided from the primary space to the N2 vent mast for local venting and sweeping of a space if required.
The nitrogen production plant is maintained in an automatic mode. One nitrogen generator is able to maintain normal demands on the system. When a high nitrogen demand is detected, the second unit will start automatically (See section 1.9).
a) Adjust the set point of the nitrogen supply regulating valve CN579 to the secondary header at 2 mbar, and the regulating valve CN576 to the primary header at 2 mbar.
b) At the forward part of the trunk deck, ensure that the valves CN276, 278, 283, 285 are open.
c) Adjust the set point of the nitrogen exhaust regulating valve CN277 (primary) at 4 mbar and regulating valve CN284 (secondary) at 4 mbar.
There is a standby exhaust regulating valve CN280, which can be connected to either the primary or secondary system in the event of failure of one of the master regulating valves.
The nitrogen supply to the insulation spaces has a standby regulating valve CN572, which can be connected to either the primary or secondary system in the event of failure of one of the master regulating valves.
In the event of cargo gas leakage into insulation spaces, this can be swept with a continuous feed of nitrogen by opening the exhaust from the space, allowing a controlled purge. Close monitoring of the gas analyser on this space will be necessary during purging.
In cases where other consumers reduce the availability of nitrogen for the insulation spaces, the pressure may temporarily fall below the atmospheric pressure.
This condition is not critical insofar as the differential pressure (Ps - Pp) between the secondary spaces pressure (Ps) and the primary space pressure (Pp) does not exceed 30 mbar.
(Ps - Pp) < 30 mbar Warning
When the depression in the primary insulation spaces, relative to the secondary insulation spaces, reaches 30 mbar, the two insulation spaces shall be immediately inter-connected - which will involve a manual operation.
When put in communication, and therefore subjected to the same nitrogen pressure, the primary and secondary insulation spaces can withstand a large depressurisation without any damage.
It should be noted that, even with the tanks fully loaded, a pressure lower than atmospheric pressure in the primary insulation spaces is not harmful to the primary membrane. In this respect, it should be noted that this membrane is subjected to a –800 mbar vacuum pressure - both during global testing at the construction stage and also for the insulation spaces’ cycles purging.
Illustration 3.2.2a Drying Cargo Tanks
IG021 IG020
CG405 TO E/R FROM E/R
IG022 CG527
CG525
CG524 CG523
CG522
CG516
CG512
CG508 CG507
CG503 CG504
CG515
CG511 CG514
CG513 CG002
CG509
CG505
CG501
CN683
CG528
CS004
CS003
CS002 CS001
CG510
CG506
CG502 No.2 HIGH DUTY COMP.
No.1 HIGH DUTY COMP.
No.2 LOW DUTY COMP.
No.1 LOW DUTY COMP.
TO INS.PRESS.
DEMISTER CG519
CG518 CG517
CG532
CG530 No.2 B.O/W.UP
HEATER
CG521
CS506
CS505 CS504
CS501
CF101
CF102 CF201
CF202 CF302
CF301
CF402
No.4 CARGO TANK No.3 CARGO TANK No.2 CARGO TANK No.1 CARGO TANK
CF401 CS502
LNG VAPORIZER FORCING VAP.
CS503 CG526
CG520
GAS MAIN
DRY AIR LINE
HUMID AIR LINE
VAPOUR MAIN STRIPPING/SPRAY MAIN LIQUID MAIN No.1 B.O/W.UP
HEATER
3.2.2 Drying Cargo Tanks
During a dry docking or inspection, cargo tanks which have been opened and contain humid air must be dried to avoid primarily the formation of ice when they are cooled down and secondly, the formation of corrosive agents if the humidity combines with the sulphur and nitrogen oxides which might be contained in excess in the inert gas. The tanks are inerted in order to prevent the possibility of any flammable air/LNG mixture. Normal humid air is displaced by dry air. Dry air is displaced by inert gas produced from the IGG.
Dry air is introduced at the bottom of the tanks through the filling piping. The air is displaced from the top of each tank through the dome and the vapour header, and is discharged from the vent mast No.1.
The operation, carried out from shore or at sea, and will take approximately 20 hours to reduce the dew point to less than -20°C.
During the time that the inert gas plant is in operation for drying and inerting the tanks, the inert gas is also used to dry (below -40°C ) and to inert all other LNG and vapour pipework. Before the introduction of LNG or vapour, any pipework not purged with inert gas must be purged with nitrogen.
Operating Procedure for Drying Tanks (See Illustration 3.2.2a)
Dry air, with a dew point of -45°C, is produced by the IGG at a flow rate of 14,000 Nm3/h.
a) Prepare the dry air/inert gas plant for use in the dry air mode.
b) Open blind flange valve to connect the inert gas/dry air feeder line to the liquid header.
c) Open valves CG527, CL602, CL410, CL310, CL210 and CL110 to supply dry air to the liquid header.
d) Open tank filling valves CL400, CL300, Cl200 and CL100.
e) Open tank vapour valves CG401, CG301, CG201 and CG101.
f) Open CG107, CG402, CG302, CG202 and CG102 to vent through the No.1 vent mast. Eventually, tank pressure is controlled via the regulating valve CG106, set at 100 mbar by the inching control, manually set on the IAS.
g) Start the dry air production. When dew point is -45°C, open the valve IG022 upstream of the two non-return valves on the dry air/inert gas discharge line.
h) Monitor the dew point of each tank by taking a sample at the vapour domes. When the dew point is -25°C or less, close the filling and vapour valves of the tank.
Humid air which may be contained in the discharge lines from the cargo pumps, float level piping and any associated pipe work in the cargo compressor room must be purged with dry air.
i) When all the tanks are dried, stop the plant. Close the supply valve CG527, CL602 to the LNG header and close valve CG107 to the venting system at the mast riser No.1.
Note !
It is necessary to lower the tank’s dew point by dry air to at least -20°C, before feeding tanks with inert gas in order to avoid formation of corrosive agents.
Illustration 3.2.3a Inerting Cargo Tanks
IG021 IG020
CG405 TO E/R FROM E/R
IG022 CG527
CG525
CG524 CG523
CG522
CG516
CG512
CG508 CG507
CG503 CG504
CG515
CG511 CG514
CG513 CG002
CG509
CG505
CG503 CG501
CN683
CG528
CS004
CS003
CS002 CS001
CG510
CG506
CG502 No.2 HIGH DUTY COMP.
No.1 HIGH DUTY COMP.
No.2 LOW DUTY COMP.
No.1 LOW DUTY COMP.
TO INS.PRESS.
DEMISTER CG519
CG518 CG517
CG532
CG530 No.2 B.O/W.UP
HEATER
CG521
CS506
CS505 CS504
CS501
CF101
CF102 CF201
CF202 CF302
CF301
CF402
No.4 CARGO TANK No.3 CARGO TANK No.2 CARGO TANK No.1 CARGO TANK
CF401 CS502
LNG VAPORIZER FORCING VAP.
CS503 CG526
CG520
GAS MAIN
DRY AIR LINE
HUMID AIR LINE
VAPOUR MAIN STRIPPING/SPRAY MAIN LIQUID MAIN No.1 B.O/W.UP
HEATER
3.2.3 Inerting Cargo Tanks (See Illustration 3.2.3a)
Inert gas, with an oxygen content of less than 1% and a dew point of -45°C, is produced by the IGG with a flow rate of 14,000 Nm3/h. The inert gas is primarily nitrogen and carbon dioxide, containing less than 1% oxygen with a dew point of -40°C or below.
Emergency pump wells have to be inerted with nitrogen before inerting the cargo tanks.
Warning
Inert gas from this generator and pure nitrogen will not sustain life. Great care must be exercised to ensure the safety of all personnel involved with any operation using inert gas of any description in order to avoid asphyxiation due to oxygen depletion.
a) Prepare the IGG for use in the inert gas mode.
b) Open the blind flange valve to connect the inert gas/dry air feeder line to the liquid header.
c) Open the valves CG527, CL602, CL410, CL310, CL210 and CL110 to supply inert gas air to the liquid header.
d) Open tank filling valves CL400, CL300, CL200 and CL100.
e) Open tank vapour valves CG401, CG301, CG201 and CG101.
f) Open CG107, CG402, CG302, CG202 and CG102 to vent through the No.1 vent mast. Eventually, tank pressure is controlled via the regulating valve CG106 set at 100 mbar by inching control, manually set on the IAS.
g) Start the inert gas production. When oxygen content is less than 1% and dew point is -45°C, open valve IG022 upstream of the two non-return valves on the inert gas discharge line.
h) By sampling at the vapour dome, check the atmosphere of each tank by means of the portable oxygen analyser. O2 content is to be less than 2% and the dew point less than -40°C.
i) During tank inerting, purge for about 5 minutes the air contained in the lines and equipment by using valves and purge sample points.
j) When the inerting of the tanks, lines and equipment is completed, set the regulating valve CG106 to 150 mbar in order to pressurise all the tanks to this pressure.
k) When the operation is completed, stop the supply of inert gas and close valves IG022 and CG527, CL602, CL410, CL310, CL210, CL110, CL400, CL300, CL200 and CL100 and close the blind flange valve.
Note !
Until the ship is ready to load LNG for gas filling, the tanks may be maintained under inert gas as long as necessary. If required, pressurise the tanks to 20 mbar above atmospheric pressure and, to reduce leakage, isolate all the valves at the forward venting system.
The inert gas is purged and replaced by natural gas(NG) produced by the LNG vaporizer(+20℃ outlet temperature) fed with LNG supplied by the loading terminal one point eight(1.8) complete volume changes are required for displacing the inert gas and attaining a co2 content ≤ 1% by volume this operation is complete in 20 hours.
Illustration 3.2.4a Gassing-up Cargo Tanks (Stage-1)
IG021 IG020
CG405 TO E/R FROM E/R
IG022 CG527
CG525
CG524 CG523
CG522
CG516
CG512
CG508 CG507
CG503 CG504
CG515
CG511 CG514
CG513 CG002
CG509
CG505
CG501
CN683
CG528
CS004
CS003
CS002 CS001
CG510
CG506
CG502 No.2 HIGH DUTY COMP.
No.1 HIGH DUTY COMP.
No.2 LOW DUTY COMP.
No.1 LOW DUTY COMP.
TO INS.PRESS.
DEMISTER CG519
CG518 CG517
CG532
CG530 No.2 B.O/W.UP
HEATER
CG521
CS506
CS505 CS504
CS501
CF101
CF102 CF201
CF202 CF302
CF301
CF402
No.4 CARGO TANK No.3 CARGO TANK No.2 CARGO TANK No.1 CARGO TANK
CF401 CS502
LNG VAPORIZER FORCING VAP.
CS503 CG526
CG520
GAS MAIN
LNG LIQUID LINE
LNG VAPOUR LINE
VAPOUR MAIN STRIPPING/SPRAY MAIN LIQUID MAIN No.1 B.O/W.UP
HEATER
LNG MIXTURE AND INERT GAS LINE
3.2.4 Gassing-up Cargo Tanks
Introduction
After lay up or dry dock, the cargo tanks are filled with inert gas or nitrogen. If the purging has been done with inert gas, the cargo tanks have to be purged and cooled down when the vessel arrives at the loading terminal. This is because, unlike nitrogen, inert gas contains 15% carbon dioxide (CO2), which will freeze at around -60°C and produces a white powder which can block valves, filters and nozzles.
During purging, the inert gas in the cargo tanks is replaced with warm LNG vapour. This is done to remove any freezable gases such as carbon dioxide, and to complete the drying of the tanks.
Description
LNG liquid is supplied from the terminal to the liquid manifold where it passes to the stripping/spray header via the appropriate ESDS liquid valve. It is then fed to the LNG vaporizer and the LNG vapour produced is passed at +20°C to the vapour header and into each tank via the vapour domes.
At the start of the operation to fill the cargo tanks, the piping system and LNG vaporizer are vapour locked. The stripping/spray header can be purged into the cargo tanks via the vapour dome through the arrangement of spray valves containing the control valve until liquid reaches the LNG vaporizer. The LNG vapour is lighter than the inert gas, which allows the inert gases in the cargo tanks to be exhausted up the tank filling line to the liquid header. The inert gas then vents to the atmosphere via the No.1 vent mast.
When 5% methane (the percentage figure will be specified by the particular port authority) is detected at No.1 vent mast riser, the exhaust gas is directed ashore via the HD compressors by pass line, or to the boilers through the gas burning line.
This operation can be done without the compressors, subject to existing back pressure, or with one or both HD compressors in service. If possible, it is better not to use compressors to avoid creating turbulence inside the tanks.
The operation is considered complete when the CH content, as measured at the top of the cargo filling pipe, exceeds 98% by volume.
The target values for N2 gas and inert gas CO2 is equal or less than 1%. These values should be matched with the LNG terminal requirements.
This normally entails approximately 2.0 changes of the volume of the atmosphere in the cargo tank.
On completion of purging, the cargo tanks will normally be cooled down.
There are exceptional cases where it may be necessary to undertake the purging of one or more tanks at sea using LNG liquid already on board. In this case the liquid will be supplied to the LNG vaporizer via the stripping/spray header using the stripping/spray pump of a cargo tank containing LNG liquid. Due to local regulations on venting methane gas to the atmosphere, some port authorities may require the entire operation to be carried out with the exhaust gases being returned to shore facilities.
Operating Procedures to Purge the Cargo Tanks with LNG Vapour 1. Stage One
(See Illustration 3.2.4a)
It is assumed, though unlikely, that all valves are closed prior to use.
a) Install the following spool pieces:
Liquid header to compressors (only if compressors are required) liquid header to No.1 vent mast.
b) Prepare the LNG vaporizer for use.
c) Adjust the set point of the temperature control valve to +20°C.
d) Using the IAS, adjust the set point of the pressure control valve CG106 to 60 mbar (or required value) by using the inching control (remote manual).
e) At No.1 vent mast, open valve CL107.
f) Open valve CS003, the stripping/spray header crossover valve to the manifold.
g) Open valves CS004 and CS601 on the stripping/spray header to enable supply to reach the LNG vaporizer.
h) Open valve CS501, the inlet valve to the LNG vaporizer.
i) In the cargo machinery room open the outlet from the LNG vaporizer CG530.
j) Open valves CG528 and CG601 to allow supply to the vapour header.
k) Open the header valves to the vapour domes.
No.1 Tank CG102 CG101 No.2 Tank CG202 CG201 No.3 Tank CG302 CG301 No.4 Tank CG402 CG401
For safety reasons, ensure that the hull water curtain on the connected side is in operation.
l) Open CS808 (if using the after liquid manifold on the port side), the isolating valve to the stripping/spray line.
m) Using the IAS, open the individual tank loading valves.
No.1 Tank CL100 CL110 No.2 Tank CL200 CL210 No.3 Tank CL300 CL310 No.4 Tank CL400 CL410
n) Using the IAS, open CL803, the liquid manifold valve on the port side,
n) Using the IAS, open CL803, the liquid manifold valve on the port side,