The physical properties of a liquefied gas depend on its molecular structure. Some compounds have the same molecular formula but a different arrangement within the structure. These different compounds of the same basic substance are called ISOMERS for example N-BUTANE x ISO-BUTANE.

The single most important physical property of a liquefied gas is its saturated vapour pressure/temperature relationship. This property governs the design of the containment system suitable for each cargo.

LPG tanker – A gas carrier built for the transportation of petroleum gases in bulk. LPG tankers often can carry some other gases such as ammonia, propylene and vinyl chloride. The cargo list for a LPG carrier specifies what type of cargoes the vessel shall be designed for. Practically all LPG carriers can handle LPG, ammonia, propylene and VCM. For larger gas carriers there is a tendency that some of the cargoes are removed from the cargo list. Wärtsilä Oil & Gas System has a significant market share in the delivery of cargo handling system for both LPG and LEG vessels.

Considering the temperature and the pressure needed, the LPG carriers can be divided into three categories: full pressure type, semi-refrigerated type and fully-refrigerated type. Small size ships (less than 4000m3 of cargo capacity) are usually the full pressure type. The semi-refrigerated technique is used for the cargo spaces around 7500m3, and fully-refrigerated technique is intended for the cargo spaces between 10,000m3 and 100,000m3.

Escape of LPG on deck

Whether a spillage of liquid or a leak of vapour, the priorities of the Emergency Party are to:
a) Stop the flow of gas
b) Prevent ignition
c) Disperse the vapour cloud
The flow of gas from a source which is between two valves on the Emergency Shut Down system will be limited. However, personnel advancing to close a valve manually close to the source of a spillage or leak must:

a) Wear breathing apparatus and protective clothing.
b) Be protected by a massive water wall
Care must be taken to avoid cargo coming into contact with the skin.
Any leak of LPG will produce a rapidly expanding cloud of explosive vapour, which must be prevented from coming into contact with a source of ignition. It is imperative therefore to isolate any source of ignition and ensure that no vapours enter the accommodation. Early consideration must be given to altering course and/or speed, and if necessary to stopping ventilation fans. Personnel who have to leave the accommodation to close vent intakes must wear breathing apparatus and protective clothing. If a vapour cloud approaches a known source of ignition, consideration must be given to attempting to "bend" its path by putting up a solid water wall.

The rate of dispersal of a vapour cloud will depend on climatic conditions. However, the use of liberal quantities of water in spray form will increase the rate of vaporisation, and, in the case of liquid spillage, reduce the risk of cold fractures of steel. Solid water jets must not be used on liquid spills, as they will result in splashing of cold liquid. Fixed water sprays may also assist in vapour dispersal if near enough to the cloud.




Sources of ignition

Smoking
There are frequently local regulations about smoking which must be rigidly observed. Smoking may be permitted but only under controlled conditions at times and in places specified by the Master. Personnel when working aboard the ship must not carry matches or more particularly lighters, and the risk of doing so is to be impressed on all.


Portable Electrical Equipment.
Only approved Safety torches or hand lamps are to be used. Portable Electric Equipment self contained or on extension cables are not to be used in a gas dangerous place or zone unless the equipment is intrinsically safe.
Portable domestic radios, electronic calculators, tape recorders and other non-approved battery equipment are not to be used in a gas dangerous place or zone.

Communication Equipment.
When berthed the ships normal communication equipment is not to be used unless certified safe. Main radio transmitters are not to be used during cargo operations. This does not apply to permanently and correctly installed VHF equipment or Satellite communication systems


Hot work, hammering, chipping and power tools
Before any hot work, hammering, chipping, or power tools including shot blasting are used the responsible officer is to examine the area to be worked and satisfy himself that such work can be safely undertaken and a hot work permit certificate issued. Non - sparking tools are not to be used as they do not significantly reduce the risk of igniting a flammable vapour.
Aluminium paint is not to be smeared across steel. A heavy smear of aluminium on rusty, steel if struck can cause an incandescent spark.


Ship to Shore Bonding
On some jetties this may be required but it is considered to be of minimal benefit. Cargo hoses and loading arms are to be fitted with an insulating flange to ensure discontinuity between ship and shore.

Auto - Ignition
The vapours from flammable liquids including fuel and lubricating oil may ignite if the liquid comes into contact with any surface heated above the auto - ignition temperature e.g. steam lines, exhaust manifolds, over heated equipment. Immediate steps are to be taken to rectify any leakage and to remove any soaked rags on other material including lagging.


Static Electricity
Static Electricity can cause sparks capable of igniting a flammable gas. The cargo system of a gas carrier is electrically bonded to the ships hull to prevent any build up of charges, bonding connections must be maintained in good order.


Steam
A jet of steam may become charged in passing through a nozzle. Steam is not to be injected into a tank compartment or piping system which may contain a flammable mixture.

Carbon Dioxide
Liquid Carbon Dioxide when released under pressure may form particles of solid carbon dioxide these may become electrostatically charged. For this reason it must not be released into tanks or spaces which contain flammable mixture.




Principles of refrigeration
Cold liquid refrigerant (Liquid Gas Cargo) is evaporated in an evaporator coil (Cargo tank) which being cooler than its surroundings draws heat to provide the latent heat of vaporisation. The cool vapour is drawn off by a compressor which raises both the pressure and the temperature of the vapour and passes it to the condenser.

The pressure of the vapour having been increased the vapour now has a temperature of condensation greater than the temperature of the condenser cooling fluid (sea water). The vapour is condensed to a high pressure liquid and the sensible heat of desuperheating the vapour together with the latent heat of condensation is removed via the condenser coolant which is warmed up in the process.




The high pressure liquid then passes through an expansion valve to the low pressure side of the cycle, in doing so flash evaporates to a mixture of cold liquid and vapour. The mixture passes to the evaporator (cargo tank) to continue the cycle.



Related Information:

1. Various type LPG tanker - Design characteristics and usability

Gas carriers range in capacity from the small pressurised tankers of between 500 and 6,000 m3 for shipment of propane, butane and the chemical gases at ambient temperature up to the fully insulated or refrigerated seagoing tankers of over 100,000 m3 capacity for the transport of LNG and LPG. Between those two distinct types is a third tanker type – semipressurised gas carrier. .....


2. LPG tanker cargo work equipments & product line system

The cargo handling equipment can vary in range from a simple manual system with no reliquefaction plant as fitted to a small fully pressurised ship, or a complicated fully automatic cascade type reliquefaction plant. For individual ships please refer to the Shipbuilders Cargo Operations Manual. ....


3. LPG tanker cargo pipe line inspection and testing guideline

Cargo tanks are to be inspected internally during every dry-docking and in circumstances where a particular tank is required to be gas freed for repairs. All other tanks, including ballast and void spaces must be internally inspected on a 12-month cycle to establish the condition of steelwork, pipework, fittings and coatings of anodes, as applicable. .....


4. Carriage of LPG cargo at sea & safety guideline

During the loaded passage, the cargo is warmed by heat input from sea water and atmosphere, causing the temperature and saturation pressure (cargo tank) to rise. It is therefore necessary to maintain strict control of the cargo temperature and pressure at all times during the loaded passage. On vessels other than pressurised LPG carriers this is achieved by reliquefying the boil-off and returning it to the tanks. ....


5. LPG reliquefaction plant safety guideline

The reliquefaction plant is designed to maintain a constant tank pressure at a maximum ambient air and sea water temperatures. .....


6. Preparations for LPG cargo discharging, pumping & stripping guideline

When the vessel arrives at the discharge terminal, cargo tank pressures and temperatures must be at values appropriate to the terminal requirements to allow maximum discharge rates to be achieved. Before the discharge operation begins, the pre-operational ship/shore procedures must be carried out, i.e. ship/shore information exchange, ship/shore safety check list. .....


7. Preparations for loading compatible cargo onboard LPG tanker

Compatible cargoes are those substances which can be loaded consecutively without prior need to gas free the tanks. However, care must be taken to fully comply with Charter Party, shippers or other stated requirements for the cargo changeover, as these may require more stringent procedures....


8. Preparation for changing different grade cargo or drydocking -LPG tanker guideline

Changing non compatible grades Gas freeing is necessary when changing between incompatible cargoes, and when preparing the ship for dry-dock or repairs. Liquid residues can be removed by the total tank heating procedure or by boiling off procedure. ....


9. Cargo tank inerting prior to gassing up - LPG tanker procedure

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. .....


10. LPG cargo tank purging & safety guideline

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.....


11. LPG cargo tank cooling safety procedure

Before loading a refrigerated cargo, the tanks must be adequately cooled down in order to minimise thermal stresses and excessive tank pressure during loading. Cool down consists of introducing cargo liquid into a tank at a low and carefully controlled rate. The lower the cargo carriage temperature, the more important the cool down procedure becomes. .....


12. LPG cargo loading special guideline

Before the loading operation begins, the pre-operational ship / shore procedures must be thoroughly discussed and subsequently carried out. The appropriate information exchange is required and the relevant ship / shore safety check list must be completed. .....


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

When the ship is not carrying cargo or is lightly loaded, sufficient ballast must be carried to ensure that the ship’s stress, stability, draft, trim and propeller immersion is within permissible limits to guarantee the safe handling of the vessel in the prevailing or expected conditions. .....


14. Handling cargo related documents for LPG carrier

The transportation of liquefied gases is subject to the same commercial documentation as applies to oil cargoes. The master should request the correct technical name of the cargo as soon as possible and before loading. The master must only load a cargo which is listed on his certificate of fitness. Data sheets for these cargoes should be on board. ....


15. Cargo sampling procedure for liquefied gas cargo

The sample container must be completely clean and compatible with the cargo to be sampled and is to be able to withstand the extremes of temperature and pressure anticipated. ....


16. Cargo measurement and calculation guideline for LPG carriers

Liquefied gas cargoes are measured and calculated in a similar manner to that of other bulk liquid cargoes such as crude oils and petroleum products. However, as liquefied gases are carried as boiling liquids in a closed containment system the quantity of vapour has also to be measured when calculating the total quantity onboard. .....


17. Handling Propylene oxide, Ethylene oxide mixtures

Propylene oxide (PO) is an eserine with wide flammable limits of 2.8 - 37% by volume. The TLV of this product is 100 ppm whilst its odour threshold is 10 ppm. Exposure to the liquid or high concentrations of the vapour can lead to eye burns, skin irritation and blistering, vomiting, lack of co-ordination and depression ....


18. Special characteristics of Vinyl Chloride Monomer & 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. .....


19. Gas measuring equipments calibration procedure

All vessels are supplied with portable gas measuring equipment, according to the specific requirements of the vessel owners. Personnel must fully understand the purpose and limitations of vapour detection equipment, whether fixed or portable. .....


20. Hazards with carrying propylene oxide mixtures

Propylene oxide (PO) is an eserine with wide flammable limits of 2.8 - 37% by volume. The TLV of this product is 100 ppm whilst its odour threshold is 10 ppm. Exposure to the liquid or high concentrations of the vapour can lead to eye burns, skin irritation and blistering, vomiting, lack of co-ordination and depression .....

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