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Cargo tank inerting prior to gassing up - LPG tanker procedure
LPG cargo tanks - why inerting ?
Inerting cargo tanks and pipework systems is undertaken primarily to ensure a non-flammable
condition in the subsequent gassing up with the vapour of the cargo to be loaded. For this purpose a
reduction in the oxygen concentration to 5% by volume is generally judged to be adequate, although
lower values are usually obtainable and preferred.
For some of the more active chemical gases, VCM or butadiene, oxygen levels as low as 0.1% may be
required to avoid chemical reaction with the incoming gassing-up vapour. This level will be difficult to
achieve using shipboard inert gas plant.
Inerting by displacement
This is generally considered to be the most efficient method of inerting tanks. It relies on stratification
in the tank as a result of the difference in vapour densities between the gas entering the tanks and the
gas already in the tank. The heavier gas is introduced beneath the lighter gas, and at a low velocity to
minimise turbulence. If perfect stratification could be achieved with no mixing at the interface, then
one tank volume of the inert gas would completely displace the gas already in the tank.
some mixing does occur and it will be necessary to use more than one tank volume of inert gas. This
may vary from 1.25 to 4 times the tank volume, depending upon the relative densities and tank and
piping configurations. There is little density difference between air and inert gas; inert gas from a
combustion generator is slightly heavier than air while nitrogen is slightly lighter. These small density
differences make inerting by displacement alone very difficult to achieve, and usually the process
becomes partly displacement, partly dilution.
Before introducing nitrogen a purge with dry air is usual. The relative densities of nitrogen and air are
similar which makes the change over from air to nitrogen, and the clearance of all air from the tank
difficult. In order to improve the separation during this process the dry air in the tank should be as
cold as possible, and the nitrogen as warm as possible.
The nitrogen must be introduced through the upper distribution line with displacement taking place in
the direction top to bottom.
This process must not be hurried. Nitrogen gas is expensive, and the need to re-inert following an
unsuccessful first attempt is costly and time consuming.
Tanks can be inerted in parallel or series, and with flow within the tanks or either top to bottom or
bottom to top, depending on the relative densities of the two gases.
Inerting by dilution
In the dilution method the incoming gas mixes with the vapour already in the tank. This can be done
in several ways depending on the type of the vessel:
Repeated pressurisation (not on fully refrigerated vessels)
Dilution can be achieved by a process of repeated pressurisation of the tank with inert gas using a
compressor, followed by a release of the compressed contents to atmosphere. Each repetition will
bring the tank contents nearer to the oxygen concentration level of the injected inert gas. Thus to
bring the tank contents to a level of 5% oxygen within a reasonable number of repetitions, an inert gas
quality better than 5% oxygen content is required.
Quicker results will be achieved by more numerous repetitions each at a lower pressurisation level, than
by fewer repetitions using the higher pressurisation levels, of which the tank and compressor may be
Repeated vacuum (not on fully refrigerated vessels)
Inerting by successive dilutions may be carried out by repeatedly drawing a vacuum on the tank by the
compressors, and then breaking the vacuum using inert gas. If, for instance a 50% vacuum can be
drawn then on each vacuum cycle half the oxygen content of the tank will be withdrawn.
Some of the
withdrawn oxygen will, of course, be replaced by the oxygen content of the subsequent vacuum
breaking inert gas but, if the quality of inert gas is good, this method is probably the most economical
in the use of minimum inert gas quantity in order to achieve the desired inerting level in the tank. The
overall time taken may be longer than with pressurisation because of the reduction in capacity of the
compressor on vacuum, and the limitation of the rate of vacuum breaking output capacity of the inert
Inerting by dilution can be a continuous process. An increased flow of inert gas and hence better
mixing and a reduction in overall time may be achieved by maintaining the tank under vacuum by
passing the diluted efflux through the compressor. Care must be taken to ensure continued good
quality inert gas under the increased outflow conditions of the inert gas generator.
It is immaterial where the inert gas inlet or the tank outlet is located provided that good mixing is
achieved. It is generally found more satisfactory to introduce the inert gas at high speed through the
vapour line and exit through the liquid loading line. Where several tanks are to be inerted it may be
possible to achieve a reduction in the total quantity of inert gas used, and in the overall time, but
inerting two or more tanks in series. This procedure also provides a ready way of inerting pipework
and equipment at the same time.
Inert gas from a combustion type generator must never be used in preparation for carrying
ammonia because of the reaction of ammonia vapour with the carbon dioxide content of such inert
gas to form carbamates.
Normally, however, inerting prior to loading ammonia is not required because
it is recognised that ammonia vapour, though flammable, is not readily ignited. Liquid ammonia must
never be sprayed into a tank containing air as there is a risk of creating a static charge which would
cause ignition, and the conditions for ammonia stress corrosion cracking. If the ship's Flag
Administration or the loading terminal require inerting prior to loading ammonia then nitrogen should
Compressors and reliquefaction
If the compressors are used to create vacuum in the tanks, they are to be connected on the suction
side to the tank gas suction lines on deck, and on the discharge side to the gas discharge lines on deck.
If condensable gas is drawn from the tanks this may be reliquefied and discharged ashore via the
condensate and liquid cross over lines, or to the deck tank or another cargo tank.
When using the compressors care must be taken to avoid raising the level of non-condensable gases
(nitrogen/inert gas) thus causing an increase in the temperature and pressure in the condenser, and
overheating at the compressor outlet as this will stop the reliquefaction process.
Preparation for gassing up and cooling down
If possible a quantity of the next, or compatible, cargo should be taken into a deck tank for the voyage
to the loading port. This will enable the gassing up and cooling down process to be started during the
ballast passage. The limited quantity of cargo may mean that only one or two tanks can be prepared,
but this would be a useful contribution to minimising port time.
- LPG cargo loading special guideline
- Tackling fire onboard LNG & LPG ships
- Detail guideline for Ballast operation at sea by LPG carrier
- Handling cargo related documents for LPG carrier
- Cargo sampling procedure for liquefied gas cargo
- Cargo measurement and calculation guideline for LPG carriers
- Handling Propylene oxide, Ethylene oxide mixtures
- Special characteristics of Vinyl Chloride Monomer & Butadiene
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