Hard water

Water which does not form a permanent lather with soap due to the presence of dissolved salts in the water. Hardness is of two types: temporary hardness, due to the presence of bicarbonates; permanent hardness, due to the presence of sulphates. Both types of hardness can be removed using ion exchange methods.

Head

The height of the surface of a liquid above a specific point, especially when used as a measure of the pressure at that point.

Heading

The direction in which the longitudinal axis of a craft is pointed, usually expressed in degrees from north (true, magnetic or compass).

Heat exchanger

Equipment in which heat is transferred between liquids or liquids and gases through a tubular wall on a continuous basis. The transfer of heat is achieved by conduction through the tube walls from a hot to relatively cool material by the countercurrent, or concurrent, flow of hot and cold materials.

Heave

The up-and-down movement of a semi-submersible drilling platform or drill ship.

Heave compensation

The removal of the effect of wave motion from the recorded sea-bed profile.

Heavy crude

Crude oil with a high specific gravity and a low API gravity due to the presence of a high proportion of heavy hydrocarbon fractions.

Heavy fractions (heavy ends)

The high-molecular-weight, high-boiling-point fractions that emerge from the lower part of a fractionating column during the oil refining process.

Height

The vertical distance of a level, a point or an object considered as a point, measured from a specified datum. The vertical dimension of an object.

Height of tide

The vertical distance from the chart datum to the level of the water at any time.

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