Gassing Up
Why to Gas up?
Tank will be cooled to well below dew point temperature
Dry Ice deposition from CO2 (IG contains 15% by volume) occurs at -78°C
(opposite is sublimation, dry ice turns to vapour without passing through the liquid phase)
Gassing-up
• Process of replacing inert gas with Methane
• Inert gas contains uncondensable gases
• Nitrogen and CO2 can not be condensed by ship’s reliquefaction plant
• Venting to atmosphere is subject to MARPOL Annex VI
• Return to shore until tank is full with next cargo vapour
Gassing-up
• Gassing-up at sea only available to fully or semi-pressurised vessel (often with deck tanks)
• Gassing-up using cargo from shore
• Before commencing gasing-up O2 contents must of tanks should be less then 5% by Volume.
• Compressors for re-liquefaction can be started after gas concentration into tank reach at least 90%
Time
% O
2
1.5%
20.8%
5%
Gassing Up
Time
Gassing Up
Inert Gas LNG
Steam
VaporiserVent (or ashore)
Inert Gas
LNG
Steam
LNG Vaporiser
Gassing Up
Liquid at -160°VaporTemp?
Max temp +80°C
Min temp +10°C ?
Methane entersthe tank throughthe vapour line atthe top of the tankand exits throughthe liquid fillinglines to the ventriser or ashore.
Compressor & Motor Room Arrangements
•Gas Tight Seal through bulkheads (10.2.4.2, 10.2.4.3)
•Positive and negative ventilation (12.1.4, 12.1.5)
•Entry via airlock (3.6)
•Trip of non approved electrical power on loss of positive pressure (3.6.4)
Airlock
Suction from top and / or bottom
MAIB Tanker Awareness by WMA
High Duty Compressor
760 kw3570 rpm
9552 rpm
Nitrogen
Air
Compressor Inlet Guide Vanes
Inlet Guide Vanes
Minimum +80° Maximum -30°In line with shaft 0°
HD Compressor Alarms and Trips
Lube Oil Lo Lo 90kPa
Nitrogen seal pressure Lo 10kPa
Lube oil inlet temp Hi 65°C
Excess vibration 80µm
Discharge gas temp Hi 100°C
Bearing Lube oil temp Hi 105°C
Rotor axis excess position +/-0.2mm
Bulkhead seal temp Hi 120°C
Bulkhead seal press Lo 3kPa
Differential pressure
vapour header / atmospheric = 0.3kPa
Vapour header / IBS pressure header = 0 kPa
Any tank very high liquid level
Oil temperature
Oil pressure
High gas discharge temperature
Low seal gas pressure
Electric power failure
Motor room ventilation failure
Cofferdam Heating NO96 System
Cofferdam Heating MKIII System
CONDENSATE RETURN LINE
VENT HEADER
INERT GAS HEADER
LIQUID HEADER
Reliquefaction
VapourCompressor
L.P.G.Vaporizer
Inert Gas Generator
L.P.G.Heater
Air Blower
VAPOUR HEADER
liquid vapour
Gassing-up Liquid Supply from Shore Vapour Return Shore