Basic requirements for a good blaze like this are three the fuel some means of
ignition and plenty of oxygen to keep it all going three sides of the fire
triangle if any one of them is missing there can't be a fire in fire prevention
one effective way of removing the oxygen side is to replace it with a long
combustible inert gas
clearly seagoing tankers full of combustible cargo and flammable cargo
gas our considerable fire risks so for some time now it's been a widely
accepted practice to inert their tanks conveniently onboard ship there's a
plentiful supply of inert gas IG from the boiler exhaust with careful boiler
combustion the 21% oxygen in air can be reduced to as little as two in the flue
gas and typically to five so is nothing lower than 10% supports of fire the
boiler is already made IG plant if we can divert some inert
flue gas and get it along to the tanks we're in business
although we've still got volatile cargo and probably the means of ignition
starved of oxygen there'll be no fire so this is the basis
of any IG system at sea and as it's our surest protection against explosion and
fire let's take a close look at how it all works
as we've seen we're off to a good start with the boiler if we fit valves in the
uptakes we can tap gas off as we need it but fresh from the boiler flue gas isn't
much use it's scorching hot dirty and highly corrosive so the next step is to
cool it down and clean it to do that we have a scrubbing time scrubbers vary
from ship to ship but essentially they all accept the hot flue gas through a
water seal cooling it and passing it through baffles and seawater sprays till
the solids have washed out and the corrosive sulfur is dissolved finally
there's a Dee misting stage to remove or water vapor the efficiency of these
systems is such that hot flue gas arriving at as high as 300 degrees
centigrade leaves within one or two degrees of sea water temperature and
with better than 95% of its solids washed out
conditions inside the tower are pretty ferocious so periodic checks are made to
ensure that anti-corrosion linings are intact before inspection it's essential
to isolate and thoroughly ventilate the enclosed space the atmosphere is tested
and oxygen must reach a respirable 21 percent before anyone goes inside during
operation of the IG system the quality of flue gas in and out of the scrubbing
tower needs to be constantly checked so here in the engine room
recorders monitor each uptake and sound alarms if the oxygen content rises above
a preset prescribed safe level normally five or six percent among other safety
measures at the stage are a thermal probe to warn of dangerously high IG
temperatures and a sight glass in the engine room against flooding in the
tower and the possibility of water getting into the boiler upticks in
automated ships this device like most of the others is backed up by automatic
alarms in the control room
after the scrubbing tire the processed inert gas is delivered to the deck by
fans there are two to each system these are typical heavy duty models on a
modern VLC see the fans distribute gas to the cargo tanks via a water seal and
a branching deck main the deck water seal is a non mechanical non-return
valve with a continuous supply of sealing water and an alarm against water
failure IG passes freely as long as pressure from the fan is greater than
any bat pressure from the tanks but sure back pressure exceed the fan sealing
water is driven up the inlet to form a plug preventing flammable cargo vapor
getting back to the engine room beyond the water seal the deck layout is
completed by a block valve purge pipes and mast venting with a bypass pressure
or vacuum relief valve set to operate at two and a quarter and minus 1/2 PSI
short high pressure or vacuum still develop despite the PV valve as a final
safeguard we have an oil seal this is a branch from the gas main submerged in a
bath of oil the seal is filled to a set level when the cargo tanks are at
atmospheric pressure then under high pressure oil is blown out to relieve the
system and for excessive vacuum air is drawn in clearly pressures are critical
so they are permanently displayed by repeaters in the engine room on the
bridge and in the cargo control room again mirror automatic alarms against
any failure well that's the basic layout boiler scrubbing tar and fans water seal
and oil seal purge pipes riser and relief valve essentially this is the IG
system for any tanker
the details do vary between crude and product carriers and between early and
later ships but whatever the vessel the principle is the same
now let's look at the operational side we'll begin with the startup procedure
and the tanks follow there
first open discharge valves from the scrubber and Dec water seal open water
supply valves to both units start salt water pump to the scrubber and open the
discharge open the boiler up take the suction and discharge valves on the IG
fan and finally start the fan test all alarms and check the flue gas quality
meanwhile deck personnel must top up the oil seal close all masked vents check
the bypass and tank lid relief valves and open the appropriate purge pipes for
manifold valves check discharge from the deck water seal
and when the engine room reports ready opened the main block valve at the poop
front I G will now drive the air out of the tanks through the purge pipes or in
later ships through the direct load lines to the manifolds issue from the
pipes is tested with a portable analyzer till it matches the oxygen content of
the IG supply
when it does when all the tanks are full of inert gas the purge pipes can be
closed pressure through the system will now rise to equal the fan at this stage
we can close the block valve and turn off the IG
on passage in ballast unloaded IG will be slowly leaking away all the time the
so to maintain the correct positive pressure routines must be established
for checking and topping up
the next major operations for the system are loading and discharging loading
procedures for a crude carrier are the same for cargo for balance
with the IG shut down all risers are open till the IG pressure drops to zero
now the loading begins and in flowing cargo displaces the remaining IG through
the vapor lines
when loading is complete all vents are closed gas pressure is checked and if
necessary the IG started to bring the system back to pressure at the discharge
port the procedure is reversed with the IG running and all apertures and vents
closed cargo in the tanks is pumped out and the ige takes its place in fact
towards the end better than just taking its place the gas pressure on top
actually helps drive the liquid out usefully speeding up the last stages of
a discharge on product carriers loading discharging and loaded passage may be
complicated by cargo parcels that need careful segregation serious losses can
be incurred if valuable products are put off spec by vapor contamination through
a common line against this possibility product ships are fitted with duplicate
gas mains and supplied with portable relief valves for fitting in specific
vapor lines
between the discharge and loading ports tanks will normally be washed before
washing it's essential to purge all tanks with IG to clear any traces of
cargo gas that would be dangerous if mixed with air purging can be done on a
time basis two changes of atmosphere generally being sufficient
although thoroughly purged it's essential to keep the IG running
throughout tank cleaning pressure will be kept low but nevertheless before
lifting any covers it's important to check that each tank is depressurized by
opening the appropriate sighting port or purge pipe
when washing is complete the tanks are Reaper all vents re closed
and the system is brought back to pressure
a lot of maintenance of course is carried out on inert it ships but for
repair work inside tanks obviously they must be thoroughly ventilated first gas
freeing can be done in two ways with portable fans or with the IG system
blowing air for the first method the individual tank is locked open isolated
from the IG main and portable fans are mounted in the tank washing apertures
either to suck out the IG or to blow in fresh air during this exercise the tank
atmosphere must be carefully monitored when the oxygen gets back to a normal
21% and the hydrocarbon reading on the explosive meter is zero the tank can be
entered but the first man in must take breathing apparatus and an analyzer to
check the remote parts of the tank the portable fan should be kept running all
the time to disperse any hydrocarbon vapor but might be released from
disturbed scale or sludge the second but slower way to gas free the tanks is with
the IG blower with the boiler uptakes closed and a blank removed on its
suction side the fan delivers fresh air to the deck main and ventilates the
whole system
gasps freeing them completes our cycle we've seen all the major operations
primary inerting loading and discharging purging tank washing and gas freeing for
entry we've seen the layout most of the nuts and bolts and something of how the
whole thing works but there's still plenty to learn transferring cargo
maintenance record sheets and much more so read the manuals see the film again
and learn the drills the IG system properly operated and maintained is
essential to your safety the ships and the safety of everyone on board