Fig:Fully refrigerated LPG carrier at sea
It must be noted however, that to date no cases have been recorded relating to exposure to the
monomer. The only cases have been linked with production processes, and in particular to cleaning of
the autoclaves used in the production of PVC.
Nevertheless, it is policy that exposure levels should not exceed 10 ppm unless suitable protection such
as CABA and possibly chemical suits are used.
Ships, which are in the VCM trade for all or part of the time, have specialised detection and protective
equipment. In addition, modified emergency procedures have been developed to deal with the
problems, which may arise from the presence of the gas.
(a) VCM is a colourless liquid with a characteristic sweet odour. It is highly reactive, though not
with water, and may polymerise in the presence of oxygen, heat and light. Its vapours are both toxic
and flammable. Aluminium alloys, copper, silver, mercury and magnesium are unsuitable for vinyl
chloride service. Steels are, however, chemically compatible.
(b) Extract from I.M.O. Regulations / USCG
With VCM - the person in charge of cargo transfer operations shall ensure that:-
Carrying Butadiene & associated risks
- Cargo vapours are returned to the cargo tank or shore installation for reclamation or
destruction during cargo transfer;
- Continuous monitoring for vapour leaks takes place during all cargo transfer operations. Fixed
or portable instruments may be utilised to ensure that personnel are not exposed to VCM vapour
concentrations in excess of 1 ppm averaged over any 8 hours period or 5 ppm averaged over any
period not exceeding 15 minutes. The method of monitoring and measurement must have an accuracy
(with confidence level of 95%) of not less than plus or minus 50% from 0.25 through 0.5 ppm, plus or
minus 35% from over 0.5 ppm through 1.0 ppm, and plus or minus 25% over 1.0 ppm.
- Cargo transfer operation is discontinued or corrective action is initiated by the person in charge
to minimise exposure to personnel whenever a VCM vapour concentration exceeds 5 ppm for over 15
minutes, action to reduce the leak can be continued only if the respiratory protection requirements of
29 CFR 1910.93q (g) are met by all personnel in the area of the leak.
- Those portions of cargo lines which will be open to the atmosphere after piping is disconnected
are free of VCM liquid and that the VCM vapour concentration in the area of the cargo piping
disconnect points is not greater than 5 ppm.
- Any restricted gauge fitted on a tank containing VCM is effectively out of service by locking or
sealing that device so that it cannot be used; and
- A restricted gauge is neither to be used as a “check” on the required closed gauge nor as a
means of sampling.
- Sign bearing the legend:
"DANGER - SUSPECT AGENT IN THIS AREA"
"PROTECTIVE EQUIPMENT REQUIRED"
"AUTHORISED PERSONNEL ONLY"
must be posted whenever hazardous operations, such as tank cleaning, are in operation.
- A tank ship undergoing cargo transfer operations must be designated a "regulated area"
having access limited to authorised persons and requiring a daily roster of authorised persons who may
board the ship.
- Employees engaged in hazardous operations, such as tank cleaning, must be provided and
required to wear and use respiratory protection in accordance with the provisions of 29 CFR 1910.93q
- and protective garments, provided clean and dry for each use, to prevent skin contact with liquid
Butadiene is an unsaturated hydrocarbon. It is a colourless liquid with a faint, sweetish characteristic
Water is soluble in butadiene, particularly at elevated temperatures. On cooling water-saturated
butadiene the solubility of the water decreases and the water will separate out as droplets which will
settle as a layer in the bottom of the tank. For instance, on this basis, for a 1,000 cu metres tank, 100
cc of free water would require to be drained from the bottom of the tank. On further cooling to below
0°C this layer of water would increase in depth and freeze.
Butadiene is chemically more reactive than saturated hydrocarbons and may act dangerously with
chlorine. It can be chemically reactive with materials of construction. It is chemically incompatible with
copper, silver, mercury, magnesium, aluminium and monal. Butadiene streams often contain traces of
acetylene which can react to form explosive acetylides with brass and copper.
Cargo operation guideline for VCM and Butadiene
(a) All inhibited liquid must be removed before a ballast passage between consecutive cargoes. If
a second cargo is to be carried between such consecutive cargoes the reliquefaction plant must be
thoroughly drained and purged before loading the second cargo, if compatible. Practical steps are to
be taken to ensure that polymers do not accumulate in the ship’s system.
(b) Before loading VCM / butadiene all air is to be excluded from the tanks by purging with
nitrogen. The quality of the nitrogen is to be sampled and demonstrated to have an oxygen content
not exceeding 0.1% by volume.
(c) All connections are to be purged with nitrogen.
(d) When loading butadiene it is important to have a dry atmosphere in the tanks to avoid the
possibility of ice forming in the cargo. If water is allowed into the tank methanol injection is to be
As a precaution against cargo pumps becoming blocked with ice, they must be rotated
(a) When carrying VCM and butadiene cargo temperatures are to be controlled so as to maintain a
positive pressure in the tanks.
(b) VCM and butadiene cargoes are carried with inhibitors to prevent polymerisation.
(c) During cargo loading, the reliquefaction plant can normally be operated without intercooling,
but if the gas temperatures are approaching their upper limit, VCM 90°C; butadiene 60°C; intercooling
will be necessary. If incondensable cause an unacceptable increase in condenser pressure, these are
to be returned with the condensate into the tank, or discharged ashore.
(d) During reliquefaction the inhibitor is present only in the liquid phase, i.e. it will not be present
in the condenser during a normal open reliquefaction cycle, and the condensate will not contain it
either, and as polymerisation is accelerated by high temperatures, polymerisation will occur
downstream from the compressor discharge, which includes the condenser, piping and cargo tanks.
Prevention is to supply stabilised liquid, i.e. one containing inhibitor into the condensers using the
following procedure: After cooling down, inhibited cargo liquid can be circulated with a deep well pump
via the liquid line to the condenser, returning to the tank via the condensate line - the condenser is to
be kept approximately half full during this operation.
At the same time, gas is swept from another tank by a compressor into the condenser to pressurise the
inhibited liquid back to the tank. During this operation a careful watch must be maintained on the
distribution of condensates back to the tanks to avoid overfilling.
Normally it is possible to reliquefy VCM and butadiene without the need for intercooling.
On completion of this operation the lines must be emptied into the tanks and blown through with gas.
NOTE 1 : The solubility of VCM in lubricating oil increases as temperature rises and pressure
drops. Pressure in the compressor crankcase must not exceed 1 bar - achieved by controlling the first
stage compressor suction pressure to a maximum of 0.5 bar, and the glycol heating system must be in
operation to prevent condensation, even if the compressors are shut down. Temperatures should be
maintained at approximately +40°C.
Ensure lubricating oil is compatible.
NOTE 2 : When compressing VCM and butadiene, the compressor discharge temperatures must
not exceed 90°C and 60°C respectively. The compressor suction pressure is not to be allowed to
exceed 1.5 barg in either case.
Prior to discharging, all connections must be purged with nitrogen. Due to its high density VCM must
not be discharged with deepwell pumps running in series with booster pumps.
If, during stripping, it becomes necessary to increase pressure above the liquid residues by introducing
gas from another tank, the pressure difference between the two tanks is to be approximately 2.5 bar.
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- Cargo measurement and calculation guideline for LPG carriers
- Handling Propylene oxide, Ethylene oxide mixtures
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