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LPG tanker cargo tank purging (Gassing up) safety guideline
Requirement of tank purging (Gassing up)
Neither nitrogen nor C02, the main components of inert gas, can be condensed by the ship’s
liquefaction plant because at cargo temperatures they are above their critical temperatures. Purging
the inert gas out of the cargo tank with vapour of the cargo to be loaded is necessary so that the
reliquefaction plant can operate continuously and efficiently.
Similarly, on change of cargo without inerting, it may be necessary to purge out the vapour of the
previous cargo with vapour of the cargo to be loaded.
Purging at sea
Liquid can be taken directly from the deck tank (if fitted) through the tank sprays (with the exception of
ammonia) at a carefully controlled rate to avoid cold liquid impinging on warm tank surfaces. In this
case mixing tends to predominate and the mixed cargo/inert gas mixture can be taken into other tanks
or vented up the vent riser.
Alternatively, liquid from the deck tank can be vaporised in the cargo vaporiser and the vapour
introduced gradually into the top or bottom of the cargo tank, depending on the relative densities, to
displace the existing inert gas or vapour to other tanks or to the vent riser.
Only when the concentration of cargo vapour in the tanks has reached approximately 90% should the
reliquefaction plant be started and cool down of the system begin.
Due to the limited quantity available from the deck tank, it may not be possible to gas-up all the tanks,
but it should be possible for the ship to arrive at the loading terminal with at least one tank gassed-up
and partially cooled.
Purging alongside
If the ship arrives at the loading terminal fully or partially inerted, gassing-up will be completed using
cargo supplied from ashore. At certain terminals facilities exist to allow the operation to be carried out
alongside but these tend to be the exception as venting hydrocarbon vapours alongside may present a
hazard and is therefore prohibited by most terminals and port authorities.
Thus, before a vessel arrives alongside with tanks inerted, the following points must be considered:-
- Is venting allowed alongside? If so, what is permissible?
- Is a vapour return facility available?
- Is liquid or vapour provided for purging?
- Will only one tank be purged and cooled down initially from the shore? How much liquid must
be taken on board to purge and cool down the remaining tanks?
Before commencing purging operations alongside, the terminal will normally require to sample the tank
atmosphere to check that the oxygen is less than 5% for LPG gases (some terminals require as low as
2%) or the much lower concentration required for chemical gases such as VCM.Where no venting to atmosphere is permitted, a vapour return line must be provided and used
throughout the purging operation.
Either the ship’s cargo compressors or a jetty vapour blower can be
used to handle the efflux. Some terminals, while prohibiting the venting of cargo vapours, permit the
efflux to atmosphere of inert gas. Thus if a displacement method of purging is used, the need for the
vapour return flow to shore may be postponed until cargo vapours are detected in the mast vented
efflux. This point may be considerably postponed if tanks are purged in series.
Where a terminal supplies a cargo liquid for purging, it is to be taken on board at a carefully controlled
rate and passed through the ship’s vaporiser or allowed to vaporise in the tanks. If the supply is of
vapour, this can be introduced into the tanks at the top or bottom depending on the vapour density.
Where a vessel arrives alongside with its tanks containing a cargo vapour which requires to be replaced
with the vapour of a different cargo to be loaded, then the terminal will normally provide a vapour
return line. The vapours taken ashore will be flared until the desired vapour quality is achieved, at
which point cool down can begin.
If no facilities (return line) are available for the ship to purge alongside, it is common practice for the
ship to prepare one cargo tank and to take sufficient liquid on board so that the vessel can leave the
berth, purge and cool down the remaining cargo tanks using this liquid and then return ready for
loading.
Purging operations
If a liquid product is received it will have to be vaporised in the vaporisers, heated by sea water.
If two cargoes are to be carried and sufficient heat is obtained in the vaporiser from sea water, the two
purges can be done simultaneously.
For purging all tanks with a common gas the ship’s two gas handling systems can be integrated by
inserting spool pieces and removing line blinds. If two gases are to be used the systems must be
segregated by removal of the spool pieces and insertion of line blinds.
Purging may be done with the tanks connected in parallel or in series, with the flow of gas in the tanks
either from top to bottom or from bottom to top, using the tank upper and lower distribution lines and
the gas suction line, as appropriate.
The compressors may be required to boost the pressure of the purge gas to the cargo tanks. Single or
two stage compression may be used, but intercooling is seldom necessary.
If the source of the liquid is a deck tank and there is insufficient pressure differential between the deck
tank and the evaporator the deck tank must be pressurised by the compressors.
Before purging with the new gas all tank, plant and equipment should be drained, gases expanded and
liquids vaporised.
The Officer with overall responsibility for cargo operations should check with his opposite number
ashore if waste gas is allowed to be blown off at the mast.
Allow the cargo tanks to expend to atmospheric pressure before purging.
Blow off from the mast is strictly prohibited during a thunderstorm, or if a thunderstorm is imminent.
Butadiene and VCM must not be vented to atmosphere.
Liquid supplied by a deck tank
When the source of liquid is a deck tank the compressors are to be used to raise the pressure in the
deck tank. One of the following methods should be used:
- With high vaporisation pressures, i.e. high sea water temperature, liquid with low boiling point,
the main body of the purge gas from the vaporiser is led to the cargo tanks with the balance going to
the compressor to pressurise the deck tank.
- With low vaporisation pressure, i.e. low sea water temperatures, liquid with high boiling point
the compressors may be used to pressurise both the deck tank and cargo tanks simultaneously. The
main body of purge gas from the vaporiser is led to the cargo tanks with the balance going to
pressurise the deck tank.
Related Information:
- 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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