The cargo tanks on fully refrigerated LPG carriers are normally built of low-temperature carbon-manganese steel. The cargo tanks are designed for LPG, Ammonia and some chemical gases with a minimum temperature of -48 deg C. The cargo tanks are normally insulated either with Polyurethane or Polystyrene. Some of the older fully refrigerated gas carriers have Perlite as tank insulation.

Fully refrigerated gas carriers are normally equipped with independent type A or B prismatic cargo tank or membrane tanks. Fully refrigerated carriers with independent tank type A must have a full secondary barrier. This is achieved by using low-temperature steel in the hull structure around the cargo tank. If independent tank type B is utilized either prismatic or spherical tanks, only a partly secondary barrier is demanded. This is achieved by utilizing low-temperature steel in the hull under the cargo tank.

Independent prismatic cargo tanks are normally divided into two in the longitudinal direction with a center bulkhead that runs to the top of the tank dome. The center bulkhead is built to improve the stability on the carriers by reducing the effect of the free liquid surface when the tanks are loaded.

There are normally one or more valves in the center bulkhead that is called intermediate valves. These intermediate valves are installed down in the pump sump for the liquid to flow from one side to the other. It is important that the intermediate valves are closed when there is no loading or discharging of cargo. Normally there are two pumps in each cargo tank.

With the intermediate valves open, one can discharge the entire cargo tank with one pump.

Fully refrigerated carriers with membrane tanks are without a center bulkhead. Such gas carriers are built with a trunk on deck that the membrane tank is formed out of, and thereby reduces the effect of the free liquid surface.

Fully refrigerated carriers are generally equipped with the same cargo handling equipment as Semi-pressurised carriers. Some carriers also have coils in the pump sump that is used for liquid free the tank, hot gas is blown through the coils. Some carriers are also equipped with strip lines in the tank that either are connected to ejectors or transportable membrane pumps, this is utilized when loading naphtha etc. Some atmospheric pressure tankers do not have booster pumps or heat exchangers (cargo heaters).

Actual cargo for this type of gas carrier is LPG, Ammonia, Naphtha, and some chemical gases, such as, Propylene, Butadiene and VCM. Information of the type of cargo the tanker transports is located in IMO Certificate of Fitness.

When atmospheric pressure gas carrier are carrying flammable products, the hold space or the inter-barrier space must have a content of neutral atmosphere with either dry inert gas or dry nitrogen. When carrying non-flammable products, one utilizes dry air or dry nitrogen on the hold space.

This gas carrier type carries a lot of LPG from the Persian Gulf to the Far East and USA. Ammonia is transported from The Black Sea to USA and the Far East.

Advantages:

  • Transports large weight in proportion to volume because the cargo is at all times loaded and transported at atmospheric pressure.
  • Easier cargo tank construction than Semi pressurized tanker
  • Tanks and lines are insulated.
  • Have large cargo cooling plant.
  • Large tankers are more efficient (cargo weight).

Disadvantages:

  • Not so flexible for cargo change as Semi pressurized tankers.
  • Pressure limitation, not possible to heat up cargo on route.
  • Carrier without heat exchanger (cargo heater) can only unload at atmospheric pressure (fully cooled).
  • Limited access on terminals and ports with limitations to draught.

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Related Terms

CARGO PUMP

Centrifugal cargo pumps used, differ according to the type of cargo e.g. product tankers (crude oil, etc) would have a separate pump room with conventional centrifugal pumps, probably vertical overhung impeller, sometimes called barrel-type cargo pump installed. This double eye inlet pump with either a straight-through or 90° suction-discharge angle with pipe connections in the bottom half of casing has two external bearings above the impeller, the upper one takes all the hydraulic thrust and the lower acts as radial load-bearing. This pump has certain advantages over its counterparts, namely:

  1. Impeller can be sited lower in the pump room thus improving suction conditions and reducing stripping time.
  2. Removal of impeller without disturbing pipe joints.
  3. Easier access to bearings and shaft seal without removal of rotating elements.

Chemical, LPG or multi-product tankers: Here a separate pump is sited in each tank. Pumps driven through line shafting coupled to hydraulic motors on deck will be deep well, single or multi-stage with radial or mixed flow impellers respectively. Or, submerged pumps electrically or hydraulically driven with, usually, single elements. The line-shaft pump, despite some bearing problems is proving the more popular, especially for LPG carriers. Submersible pumps eliminate line shaft bearings and gland problems but expensive problems could occur due to hydraulic fluid leakage into the cargo and vice-versa.

LPG

Liquefied Petroleum Gas

HEEL

The amount of liquid cargo retained in a cargo tank at the end of discharge. It is used to maintain the cargo tanks cooled down during ballast voyages by re-circulating through the sprayers. On LPG ships such cooling down is carried out through the reliquefaction plant and on LNG ships by using the spray pumps.

L.P.G

Liquid petroleum gas (LPG) is an alternative fuel to diesel or petrol.

PETROLEUM GAS

A gas derived from petroleum. May be called LPG for Liquefied Petroleum Gas

LPG

Liquefied Petroleum Gas - this group of products includes propane and butane which can be shipped separately or as a mixture.
The LPG gases are taken out of the raw oil during refining, or from natural gas separation. LPG gases are defined as propane, butane and a mixture of these. Large atmospheric pressure gas carriers carry most of the LPG transported at sea. However, some LPG is transported with intermediate pressure gas carriers. Fully pressurised gas carriers mainly handle coastal trade. LPG can be cooled with water, and most LPG carriers have direct cargo cooling plants that condense the gas against water.
The sea transport of LPG is mainly from The Persian Gulf to Japan and Korea. It is also from the north- west Europe to USA, and from the western Mediterranean to USA and Northwest Europe.
LPG is utilised for energy purposes and in the petro-chemical industry

RELIQUEFACTION PLANT

A plant fitted on many liquefied gas carriers. It is used to change the cargo vapor back to liquid keep the cargo cooled.

GASSING-UP

A procedure of introducing a cargo vapor into the cargo tanks on a gas carrier to remove inert gas or nitrogen from the cargo tank atmosphere.

FULLY PRESSURIZED GAS CARRIER

These ships are the simplest of all gas carriers in terms of containment systems and cargo-handling equipment and carry their cargoes at ambient temperature. Type C tanks - pressure vessels fabricated in carbon steel with a typical design pressure of 17.5 bars, corresponding to the vapour pressure of propane at 45°C, must be used. Ships with higher design pressures are in service: 18 bar is quite common - a few ships can accept up to 20 bar. No thermal insulation or reliquefaction plant is necessary and cargo can be discharged using either pumps or compressors.
Because of their design pressure tanks are extremely heavy. As a result, fully pressurized ships tend to be small with maximum cargo capabilities of about 4,000m3 and they are used to carry primarily LPG and ammonia. Ballast is carried in double bottoms and in top wing tanks. Because these ships utilize Type C containment systems, no secondary barrier is required and the hold space may be ventilated with air, Appendix 1, shows a section through a typical fully pressurized ship.

CDI

Chemical Distribution Institute, (London). Establishes standards for carrying Hazardous Chemicals. An independent organization that inspects chemical carriers and gas carriers for compliance with existing international rules, codes, industrial and manufacturing standards applicable to specialized vessels. Inspection CDI orders or charterer, or the shipowner. This kind of inspection is not mandatory but has recently become widespread.

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