Liquefied Gas Carrier

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Handling VINYL CHLORIDE MONOMER (VCM) & BUTADIENE

Vinyl chloride monomer (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.



Special care for VCM and butadiene

Because of their specific characteristics, VCM and butadiene require special attention during cargo operations. This section highlights characteristics and procedures of the two cargoes which differ from the procedures included in the other sections of these operating instructions.


Risk of carrying VINYL CHLORIDE MONOMER (VCM)

Vinyl Chloride Monomer (V.C.M.) is carried as a liquid in certain LPG ships, which have been converted or built for the purpose.

V.C.M. is a chlorinated hydrocarbon and has flammability limit of 4 - 33% by volume and toxicity characteristics similar to many cargoes of hydrocarbon origin. In addition, however, long-term exposure to high concentrations has been linked with a rare form of cancer, the TLV has therefore been set at 10 ppm - a level well below the bottom of the flammable range. V.C.M. has an odour threshold of 2000 ppm and therefore smell cannot be relied upon as a means of detection.

Fully refrigerated gas carrier underway
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:-
  1. Cargo vapours are returned to the cargo tank or shore installation for reclamation or destruction during cargo transfer;


  2. 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.


  3. 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.


  4. 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.


  5. 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


  6. A restricted gauge is neither to be used as a check on the required closed gauge nor as a means of sampling.


  7. 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.


  8. 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.


  9. 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


  10. and protective garments, provided clean and dry for each use, to prevent skin contact with liquid VCM.


Carrying Butadiene & associated risks

Butadiene is an unsaturated hydrocarbon. It is a colourless liquid with a faint, sweetish characteristic odour.

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 0C 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

Loading guideline

(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 ships 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 used.

As a precaution against cargo pumps becoming blocked with ice, they must be rotated periodically.


Cooling

(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 90C; butadiene 60C; 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 +40C. Ensure lubricating oil is compatible.

NOTE 2 : When compressing VCM and butadiene, the compressor discharge temperatures must not exceed 90C and 60C respectively. The compressor suction pressure is not to be allowed to exceed 1.5 barg in either case.


Discharge VCM

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.







Related Information:

  1. Tackling fire onboard LNG & LPG ships


  2. Detail guideline for Ballast operation at sea by LPG carrier


  3. Handling cargo related documents for LPG carrier


  4. Cargo sampling procedure for liquefied gas cargo


  5. Cargo measurement and calculation guideline for LPG carriers


  6. Handling Propylene oxide, Ethylene oxide mixtures






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