Dalton's Law of Partial Pressures

This states that the pressure exerted by a mixture of gases is equal to the sum of the separate pressures which each gas would exert if it alone occupied the whole volume (see 2.17).

Dangerous Cargo Endorsement

Endorsement issued by a flag state administration to a certificate of competency of a ship's officer allowing service on dangerous cargo carriers such as oil tankers, chemical carriers, or gas carriers.

Deepwell Pump

A type of centrifugal cargo pump commonly found on gas carriers. The prime mover is usually an electric or hydraulic motor. The motor is usually mounted on top of the cargo tank and drives, via a long transmission shaft, through a double seal arrangement, the pump assembly located in the bottom of the tank. The cargo discharge pipeline surrounds the drive shaft and the shaft bearings are cooled and lubricated by the liquid being pumped (see 4.2).

Density

The mass per unit volume of a substance at specified conditions of temperature and pressure (see 2.16).

Dewpoint

The temperature at which condensation will take place within a gas if further cooling occurs (see 2.18).

Endothermic

A process which is accompanied by the absorption of heat.

Enthalpy

Enthalpy is a thermodynamic measure of the total heat content of a liquid or vapour at a given temperature and is expressed in energy per unit mass (k Joules per 1 kg) from absolute zero. Therefore, for a liquid/vapour mixture, it will be seen that it is the sum of the enthalpy of the liquid plus the latent heat of vaporisation (see 2.19.1).

Entropy

Entropy of a liquid/gas system remains constant if no heat enters or leaves while it alters its volume or does work but increases or decreases should a small amount of heat enter or leave. Its value is determined by dividing the intrinsic energy of the material by its absolute temperature. The intrinsic energy is the product of specific heat at constant volume multiplied by a change in temperature. Entropy is expressed in heat content per mass per unit of temperature. In the Sl system its units are therefore Joule/kg/K.

It should be noted that in a reversible process in which there is no heat rejection or absorption, the change of entropy is zero.

Entropy is the measure of a system's thermal energy which is not available for conversion into mechanical work.

Many calculations using enthalpy or entrophy require only a knowledge of the difference in enthalpy or entropy at normal operating temperatures. Accordingly, to simplify calculations, many different enthalpy or entropy tables have been produced which have different baselines. Care should be taken when using such tables as they do not provide absolute values (see 2.19.2).

Explosion-Proof/Flameproof Enclosure

An enclosure which will withstand an internal ignition of a flammable gas and which will prevent the transmission of any flame able to ignite a flammable gas which may be present in the surrounding atmosphere (see 4.8).

Flame Arrestor

A device fitted in gas vent pipelines to arrest the passage of flame into enclosed spaces.

Flame Screen

A device incorporating corrosion-resistant wire meshes. It is used for preventing the inward passage of sparks (or, for a short period of time, the passage of flame), yet permitting the outward passage of gas.

Flammable

Capable of being ignited.

Flammable Range

The range of gas concentrations in air between which the mixture is flammable. This describes the range of concentrations between the LFL (Lower Flammable Limit) and the UFL (Upper Flammable Limit). Mixtures within this range are capable of being ignited (see 2.20).

Flash Point

The lowest temperature at which a liquid gives off sufficient vapour to form a flammable mixture with air near the surface of the liquid. The flash point temperature is determined by laboratory testing in a prescribed apparatus (see 2.20).

Frost Heave

The pressure exerted by the earth when expanding as a result of ice formations. It is a situation which can arise as a result of the low temperature effects from a storage tank being transmitted to the ground beneath.

Gas Codes

The Gas Codes are the Codes of construction and equipment of ships carrying liquefied gases in bulk. These standards are published by IMO (see Appendix 1 — References 1.1, 1.2 and 1.3).

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