Latent Heat of Vaporisation

Quantity of heat to change the state of a substance from liquid to vapour (or vice versa) without change of temperature (see 2.10.1).

Liquefied Gas

A liquid which has a saturated vapour pressure exceeding 2.8 bar absolute at 37.8°C and certain other substances specified in the Gas Codes (see 1.1).

LNG

This is the abbreviation for Liquefied Natural Gas, the principal constituent of which is methane (see 1.2.4).

Lower Flammable Limit (LFL)

The concentration of a hydrocarbon gas in air below which there is insufficient hydrocarbon to support combustion (see 2.20).

LPG

This is the abbreviation for Liquefied Petroleum Gas. This group of products includes propane and butane which can be shipped separately or as a mixture. LPGs may be refinery by-products or may be produced in conjunction with crude oil or natural gas (see 1.2.4).

MARVS

This is the abbreviation for the Maximum Allowable Relief Valve Setting on a ship's cargo tank — as stated on the ship's Certificate of Fitness (see 4.1.2).

Mlc

This is the abbreviation for metres liquid column and is a unit of pressure used in some cargo pumping operations (see 4.2).

Molar Volume

The volume occupied by one molecular mass in grams (g mole) under specific conditions. For an ideal gas at standard temperature and pressure it is 0.0224 m³/g mole (see 2.17).

Mole

The mass that is numerically equal to the molecular mass. It is most frequently expressed as the gram molecular mass (g mole) but may also be expressed in other mass units, such as the kg mole. At the same pressure and temperature the volume of one mole is the same for all ideal gases. It is practical to assume that petroleum gases are ideal gases (see 2.1).

Mole Fraction

The number of moles of any component in a mixture divided by the total number of moles in the mixture (see 2.17).

Mollier Diagram

A graphic method of representing the heat quantities contained in, and the conditions of, a liquefied gas (or refrigerant) at different temperatures (see 2.19).

NGLs

This is the abbreviation for Natural Gas Liquids. These are the liquid components found in association with natural gas. Ethane, propane, butane, pentane and pentanes-plus are typical NGLs. (See 1.2.4)

NPSH

This is the abbreviation for Net Positive Suction Head. This is an expression used in cargo pumping calculations. It is the pressure at the pump inlet and is the combination of the liquid head plus the pressure in the vapour space (see 4.2).

OCIMF

Oil Companies International Marine Forum.

Oxygen Analyser

Instrument used to measure oxygen concentrations in percentage by volume (see 9.7.2).

Oxygen-Deficient Atmosphere

An atmosphere containing less than 21 per cent oxygen by volume (see 9.3.2).

Partial Pressure

The individual pressure exerted by a gaseous constituent in a vapour mixture as if the other constituents were not present. This pressure cannot be measured directly but is obtained firstly by analysis of the vapour and then by calculation using Dalton's Law (see 2.17).

Peroxide

A compound formed by the chemical combination of cargo liquid or vapour with atmospheric oxygen or oxygen from another source. In some cases these compounds may be highly reactive or unstable and a potential hazard.

Polymerisation

The chemical union of two or more molecules of the same compound to form a larger molecule of a new compound called a polymer. By this mechanism the reaction can become self-propagating causing liquids to become more viscous and the end result may even be a solid substance. Such chemical reactions usually give off a great deal of heat (see 2.6).

Primary Barrier

This is the inner surface designed to contain the cargo when the cargo containment system includes a secondary barrier. (Refer to the Gas Codes for a more detailed definition) (see 3.2.1).

R22

R22 is a refrigerant gas whose full chemical name is monochlorodifluoromethane and whose chemical formula is CHCIF₂. It is colourless, odourless and non-flammable. It is virtually non-toxic with a TLV of 1,000 ppm. Its relatively low toxicity and flammability levels render it suitable for use on gas carriers and is approved for such use under the IGC Code (see 4.5).

Other refrigerant gases listed in the IGC Code are shown in Appendix 2 although many are now controlled with a view to being phased out under the Montreal Protocol (1987).

Relative Liquid Density

The mass of a liquid at a given temperature compared with the mass of an equal volume of fresh water at the same temperature or at a different given temperature (see 2.16 and 8.3.2).

Relative Vapour Density

The mass of a vapour compared with the mass of an equal volume of air, both at standard conditions of temperature and pressure (see 2.16).

Restricted Gauging

A system employing a device which penetrates the tank and which, when in use, permits a small quantity of cargo vapour or liquid to be expelled to the atmosphere. When not in use, the device is kept completely closed (see 4.9.1).

Rollover

The phenomenon where the stability of two stratified layers of liquid of differing relative density is disturbed resulting in a spontaneous rapid mixing of the layers accompanied in the case of liquefied gases, by violent vapour evolution (see 2.16.1).

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