HAZARDS OF OXYGEN IN A CARGO TANK
[NOTE: Simple diagram given in Cargo Work by Capt.Erol Fernandes]
v CORROSION
v COMBUSTION
v EXPLOSION OF PETROLEUM GASES
THE CHANCES OF ALL THESE THREE OCCURING CAN BE REDUCED BY INTRODUCING INERT GAS INTO THE TANK.
PURPOSEOF THE INERT GAS SYSTEM
OXYGEN IS THE MAIN CAUSE OF EXPLOSION OF PETROLEUM
GASES.ATMOSPHERE CONTAINS ABOUT 21% OF O2,WHICH IS ENOUGH FOR SUPPORTING COMBUSTION OR EXPLOSION OF ANY GAS.
IF THE O2 CONTENT IN A TANK CONTAINING A LIQUID WHICH GIVES OUT FLAMMABLE VAPOURS IS DECREASED ,THEN THE CHANCES OF EXPLOSION ARE LESS.
EXPERIMENTS HAVE SHOWN THAT WHEN THE O2 CONTENT IN THE AIR IS LESS THAN 4% THEN THE CHANCES OF COMBUSTION ARE ALMOST ZERO.
NOW THE BEST REMEDY FOR PREVENTING EXPLOSIONS ON CRUDE OIL TANKERS IS BY INTRODUCING A GAS WHICH CONTAINS LESS THAN 4% O2 INTO THE CARGO TANKS.
GENERATION OF INERT GAS
THE BOILER EXHAUST WHICH CONTAINS VERY LESS O2 BY VOLUME IS COOLED AND INTRODUCED INTO THE CARGO TANKS.
THIS GAS GOES THROUGH VARIOUS PARTS OF THE INERT GAS SYSTEM BEFORE ENTERING INTO THE TANK.
THE COMPONENTS OF INERT GAS ARE AS FOLLOWS.
O2 VOL % 4.2%(this can be adjusted depending upon the %age of air supplied to the boiler)
CO2 VOL % 13%
SO2 VOL % 0.009%
N2 VOL % 76.8
INERT GAS TEMPERATURE= THE SEA WATER TEMP.+ 5DEG C.
GENERAL ARRANGEMENT OF TH INERT GAS SYSTEM.
THE GENERAL ARRANGEMENT OF THE INERT GAS SYSTEM IS AS FOLLOWS:
1.THE INERT GAS GENERATOR-
EXHAUST GAS FROM THE MAIN BOILER OR THE AUXILARY BOILER IS USED.
2. SCRUBBER
THIS IS INSTALLED TO CLEAN AND COOL THE EXHAUST GAS.
3. DEMISTER
THIS IS INSTALLED TO SEPARATE WATER AND SOLID CONTAINED IN THE INERT GAS COMING FROM THE SCRUBBER SUCKED BY THJE FAN.
4. IG FAN
THIS IS A SINGLE STAGE CETRIFUGAL BLOWER DRIVEN BY THE MOTOR,WHICH DRAWS OUT THE GAS COMING THROUGH THE UPTAKE OF THE BOILER AND SENDS IT OUT TO THE DECK SEAL UNIT THROUGH THE SCRUBBER AND DEMISTER.
5. CONTROL VALVE
THIS VALVE ADJUSTS THE PRESSURE IN THE INERT GAS LINE IN ACCORDANCE WITH THE PRE SET PRESSURES.
6. DECK SEAL UNIT.
THE INERT GAS COMING FROM THE IG BLOWERS IS SENT TO THE INDIVIDUAL TANKS TROUGH THE DECK SEAL UNIT.THE MAIN PURPOSE OF THE DECK SEAL IS TO PREVENT THE BACK FLOW OF HC GAS INTO THE IG SYSTEM AND THEN INTO THE BOILER.
THE DECK SEAL IS AN ADDITIONAL PRECAUTION IN CASE OF FAILURE OF THE NON RETURN VALVE SITUATED AFTER THE DECK SEAL AND BEFORE THE IG ISOLSTING VALVE.
7. DECK ISOLATING VALVE.
THE INERT GAS COMING OUT OF THE DECK SEAL UNIT IS DISTRIBUTED TO EACH CARGO TANK THROUGH THIS VALVE.
THE INERT GAS SYSTEM ALSO CONSISTS OF VARIOUS PRESSURE VACCUM DEVICES WHICH WILL BE DFISCUSSED IN BRIEF LATER.
For example,PRESSUREVACCUM VALVES,PRESSURE VACCUM BREAKER,MAST RISERS AND PRESSURE VACCUM BREATHERS.
THE SCRUBBER UNIT
THE PURPOSE OF THE SCRUBBER IS TO PURIFY THE EXHAUST GAS AND TO REMOVE SOLID SUBSTANCES AND TO COOL THE EXHAUST GAS.
WORKING
THE EXHAUST GAS COMING OUT OF THE BOILER ENTERS THERS THE SCRUBBER AND PASSES THROUGH A WATER SEAL IN AWATER TANK AT THE BOTTOM OF THE UNIT.THE GAS THEN MOVES UPWARDS.SEA WATER IS INTRODUCED AT THE TOP OF THE TOWER FOR COOLING AND PURIFICATION.THE SEA WATER IS THEN DISTRIBUTED INTO THE SPACE BY MEANS OF A SPRINKLER SYSTEM.
THE WATER SEAL AT THE START IS TO PREVENT T6HE INERT GAS COMING OUT OF THE CARGO TANK FROM ENTERING INTO THE BOILER.ITS SOUGHT OF ANU\OTHER NON RETURN DEVICE.ITS OTHER FUNCTION IS TO COOL THE HIGH TEMPERATURE EXHAUST GAS.
A HIGH LEVEL SWITCH FUNCTIONS WHEN THE WATER LEVEL IN THE UNIT INCREASES TO A CERTAIN LEVEL.
THE DEMISTER
THE DEMISTER IS PROVIDED TO ELIMINATE WATER AND SOLID SUBSTANCE CONTAINED IN THE INERT GAS THAT HAS PASSED THROUGH TH SCRUBBER.
WORKING
INERT GAS CONTAINING WATER AND DUST IS FLOWN INTO THE DEMISTERT AND IT IS GIVEN CENTRIFUGAL FORCE BY A VANE AT THE ENTRANCE .THE VANE ALSO PROVIDES IN SEPARATING THE WATER AND DUST.THE WATER AND SOLID SEPARATED IS EXHAUSTED FROM THE DRAIN OUTLET.THE CLEAN INERT GAS IS NOW SUCKED BY THE FAN.
INERT GAS BLOWERS
BLOWERS ARE USED TO DELIVER THE SCRUBBED GAS TO THE CARGO TANKS.SHIPS REQUIRE AT LEAST TWO BLOWERS WHICH TOGETHER SHALL BE CAPABLE OF DELEVERING INERT GAS TO THE CARGO TANKS AT A RATE OF ATLEAST OF 125% OF THE MAXIMUM RATE OF DISCHARGE CAPACITY OF THE EXPRESSED AS A VOLUME.IN SOME SHIPS THERE IS AN ADDITIONAL SMALL CAPACITY BLOWER WHICH IS USED DURING TOPPING UP OF CARGO TANKS.
DECK SEAL UNIT.
THE MAIN PURPOSE OF THE DECK SEAL IS TO PREVENT HC GAS IN THE TANK FROM CONTER FLOWING IN THE BOILER.
TYPES OF DECK SEAL UNITS
v SEMI DRY TYPE
v WET TYPE
v DRY TYPE
WORKING OF ASEMI DRY TYPE DECK SEAL UNIT.
AT FIRSTTHE WATER LEVEL AT OPENING OF INLET IS THE SAMEAS THAT OUTSIDE OF THE PARTITION PIPE AND THE GAS FROM THE BLOWER ENTERS THE DECK SEAL AND BUBBLES UP THROUGH THE WATER.
WHEN THE FLOW IS FULL THE WATER LEVEL IS PUSHED DOWN WITH THE PRESSURE AND THEN THE INERT GAS FLOWS WITHOUT COMING IN CONTACT WITH THE WATER.THE HONEY COMB SCREEN PROVIDES REVULUTION FLOW TO THE GAS AND CAN ELIMINATE THE WATER DROPLETS WHICH ARE CARRIED OVER.
WORKING WITH COUNTER PRESSURE
WHEN HC FROM THE CARGO TANK FLOWS INTO THE DECK SEAL,THE GAS EXERTS A PRESSURE ON THE WATER INSIDE THE SEAL AND THE WATER IS PUSHED INTO THE INLET DUCT AND HENCE SEALS THE INLET DUCT BY SEA WATER.
OTHER FEATURES
v INORDER TO KEEP THE NORMAL WATER LEVEL IN THE DECK SEAL UNIT,SEA WATER IS SUPPLIED AND SIMULTANEOUSLY THE WATER IS DRAINED FROM THE OVERFLOW PIPE.
v A STEAM HEATING COIL IS PROVIDED IN THE TANK OF THE UNIT INORDER TO PREVENT FREEZING DURING NAVIGATION IN COLD AREAS.
THERE IS ALSO ANOTHER TYPE OF SEMI DRY TYPE DECKSEAL UNITS IN WHICH INSTEAD OF BUBBLING THROUGH THE WATER TRAP IN THE BEGINNING,THE WATER
IS STORED IN A SEPARATE TANK.WHEN THE IG FLOWS THROUGH THE DECK SEAL IT DRAWS THE SEALING WATER INTO SEPARATE U SHAPED HOLDING CHAMBER BY VENTURI ACTION.HENCE A LITTLE AMOUNT OF WATER PARTICLES ARE CARRIED AWAY,HENCE THIS IS ALSO A SEMI DRY TYPE OF DECK SEAL UNIT.
WORKING OF A WET TYPE DECK SEAL UNIT
THIS IS THE SIMPLEST TYPE OF WATER SEAL.ITS WORKING IS THE SAME AS THE FIRST TYPE OF SEMI DRY TYPE WATER SEAL,THE ONLY DIFFERENCE IS THAT THE IG KEEPS BUBBLING THROUGH THE WATER THROUGHOUT THE INERTING PROCESS.IT DOES NOT PUSH THE WATER DOWNWARDS WHEN THE PRESSURE INCREASES.
WHEN THE INERT GAS PLANT IS OPERATING,THE GAS BUBBLES THROUGH THE WATER FREOM THE SUBMERGED UNERT GAS INLET PIPE.WHEN THE TANK PRESSURE EXCEEDS THE PRESSURE IN THE INERT GAS INLET LINE THE WATER IS PRESSED UP INTO THIS INLET PIPE AND THUS PREVENTS BACKFLOW.
DRAWBACKS OF WET TYPE DECK SEAL UNIT.
THE DRAWBACKS OF THIS TYPE OF WATER SEAL IS THAT WATER DROPLETS MAY BE CARRIED OVER WITH THE INERT GAS WHICH,ALTHOUGH IT DOES NOT IMPAIR THE QUALITY OF THE INERT GAS,COULD INCREASE CORROSION.
HOWEVER A DEMISTER IS FITTED IN THE GAS OUTLET FROM THE WATER SEAL TO REDUCE ANY CARRY OVER.
WORKING OF THE DRY TYPE DECK SEAL UNIT.
IN THIS TYPE OF WATER SEAL THE WATER IN THE SEAL TANK IS DRAINED WHEN THE INERT GAS PLANT IS IN OPERATION.THE CHAMBER IS AGAIN FILLED WITH WATER WHEN THE WHEN THE INERT GAS PLANT IS EITHER SHUT DOWN OR THE TANK PRESSUIRE EXCEEDS THE INERT GAS BLOWER DISCHARGE PRESSURE.FILLING AND DRAINAGE ARE PERFORMED BY AUTOMATICALLY OPERATED VALVES CONTROLLED BY THE LEVELS IN THE WATER SEAL AND DROP TANKS AN BY THE OPERATING STATE OF THE BLOWERS.
ADVANTAGES AND DRAWBACKS
v THE ADVANTAGE OF THIS TYPE IS THAT THE WATER CARRIED OVER IS PREVENTED
v THERE IS A RISK OF FAILURE OF THE AUTOMATICALLY CONTROLLED VALVES WHICH MAY MAKE THE WATER SEAL INEFFECTIVE.
PRESSURE VACCUM SITUATIONS IN THE INERT GAS SYSTEM.
THE IG IN THE TANK IS MAINTAINED AT A CERTAIN PRESSURE.PRESSURE IS PROPORTIONAL TO TEMPERATURE.
AS THE TEMPERATURE OUT SIDE INCREASES THE IG IN THE TANK AND THE MAIN IG LINE EXPANDS AND THIS CAUSES ARISE IN THE PRESSURE OF THE INERT GAS IN THE IG LINE AND THE TANK.
WHEN THE PRESSURE IN THE TANK EXCEEDS A CERTAIN VALUE,THE EXCESSIVE PRESSURE CAUSES THE TANK TO EXPLODE.
SIMILARLY,WHEN THE PRESSURE IN THE TANK DECREASES AND REDUCES BELOW ZERO,A VACCUM IS CREATED IN THE TANK.WHEN THE VACCUM IS BELOW A CERTAIN VALUE,THE VACCUM CAUSES THE TANK TO IMPLODE.
BOTH THESE SITUATIONS ARE CATASTROPHIC.
INORDER TO AVOID SUCH SITUATIONS ON BOARD ALL VESSELS WITH IG SYSTEM ARE PROVIDED WITH PRESSURE VACCUM DEVICES.
PRESSURE VACCUM VALVES
PV VALVES ARE SPECIAL DEVICES THAT ARE FITTED AT THE IG INLET OF EACH INDIVIDUAL TANK.WHEN THE TANK IS ISOLATED THE PV VALVES WORK FOR THE INDIVIDUAL TANKS.WHEN NOT ISOLATED THE PV VALVES CAN BE USED TO RELASE PRESSURE OR TO RELEASE VACCUM IN THE ENTIRE IG LINE.
WORKING OF PV VALVES.
THE PV VALVE FROM INSIDE CONSISTS OF TWO CYLINDRICAL BLOCKS.
THE LOWER ONE WHICH IS HEAVIER THAN THE OTHER LIFTS UP WHRN THE PRESSUREW IN THE IG LINE EXCEEDS A CERTAIN VALUE.THE UPPER BLOCK LIFTS UP WHEN THE VACCUM IN THE IG LINE DECREASES BELOW THE SET VALUE.
THE PV VALVE IS SET AT A POSITIVE PRESSURE OF 1600 MMAQ AND AT A NEGATIVE PRESSURE OF 700 MMAQ,ie THE LOWRE CYLINDER AUTO MATICALLY LIFTS UP WHEN THE PRESSURE IN THE IG LINE EXCEEDS
+1600mmAQ AND THE UPPER BLOCK GETS LIFTED UP TO SUCK IN AIR WHEN THE PRESSURE IN THE TANK GETS BELOW -700mmAQ.
THE LATEST FORM OF PV VALVES ARE THE HIGH VELOCITY VENTS.
THESE VENTS WORK AS PER THE SAME PRINCIPLE AS THAT OF THE PV VALVES,THESE VENTS VENT OUT THE IG AT A VERY HIGH VELOCITY SO AS RELEASE THE IG HIGH ABOVE THE DECK LEVEL THUS REDUCING HEALTH HAZARDS CAUSED DUE TO INHALING IG.THE OPENING OF THESE VENTS ARE CONE SHAPED SO AS TO ENHANCE HIGH VELOCITY VENTING.
THE PV VALVES CAN ALSO BE MANUALLY OPERATED WHEN REQUIRED TO RELEASE PRESSURE OR TO COUNTER VACCUM IN THE IG LINE AND THE CARGO TANKS.
LIQUID FILLED PRESSURE VACCUM BREAKERS
ONE OR MORE PV BREAKER SHOULD BE FITTED ONBOARD UNLESS THE PV VALVES HAVE THE CAPACITY TO PREVENT EXCESSIVE PRESSURE OR VACCUM.THIS DEVICE REQUIRES LITTLE MAINTAINANCE,BUT IT WILL ONLY OPERATE AT THE REQUIRED PRESSURE IF THEY ARE FILLED TO THE CORRECT LEVEL WITH WATER.A SUITABLE OIL OR A FRESHWATERT/GLYCOL MIXTURE SHOULD BE USED TO PREVENT FREEZING IN COLD WEATHER.
WORKING OF PRESSURE VACCUM BREAKERS.
THE PV BREAKER IS A CYLINDER LIKE STRUCTURE IN WHICH THE BRANCH IG LINE ENTERS.AT NORMAL PRESSURE THE WATER RISING IN THE DUCT AND THE WATER IN THE CYLINDER ARE AT THE SAME LEVEL.
WHEN THE PRESSURE IN THE LINE INCREASES TO A CERTAIN SET LIMIT THE GAS IN THE LINE PUSHES THE THE LIQUID IN THE BREAKER OUT AND HENCE MAKES WAY TO GET OUT OF THE IG LINE AND HENCE PRESSURE IS RELEASED.SIMILARLY WHEN THE VACCUM CROSSES A CERTAIN SET LIMIT,THE WATER IN THE BREAKER IS PULLED INSIDE THE LINE AND HENCE MAKES WAY FOR AIR TO GET INTO THE IG LINE AND HENCE THE VACCUM IS COUNTERED.
PV BREAKERS ARE SET TO A POSITIVE PRESSURE OF 1800mmAQ AND ANEGATIVE PRESSURE OF -500mm AQ.
IN ADDITION TO THE PV VALVES AND THE PV BREAKER THERE IS ALSO THE MAST RISER WHICH IS USED AS A MANUAL MODE OF RELEASING PRESSURE OR TAKING IN AIR.IT IS ALSO USED TO VENT OUT THE IG DURING LOADING OPERATIONS.
[NOTE: Simple diagram given in Cargo Work by Capt.Erol Fernandes]
