Sl (Systeme International) Units
An internationally accepted system of units modelled on the metric system consisting of units of length (metre), mass (kilogram), time (second), electric current (ampere), temperature (degrees Kelvin), and amount of substance (mole).
SIGTTO
Society of International Gas Tanker and Terminal Operators Limited.
Slip Tube
A device used to determine the liquid-vapour interface during the ullaging of semi and fully pressurised tanks. See also Restricted Gauging (see 4.9.1).
SOLAS
International Convention for the Safety of Life at Sea, 1974; as amended.
Span Gas
A vapour sample of known composition and concentration used to calibrate gas detection equipment (see 9.7.3).
Specific Gravity
The ratio of the density of a liquid at a given temperature to the density of fresh water at a standard temperature (see 8.3.2).
Temperature will affect volume and the comparison temperature must therefore be stated; e.g. specific gravity 60/60°F — substance and water at 60°F; specific gravity 15/4°C — substance at 15°C, water at 4°C. (The use of this term is being superseded — see Relative Liquid Density.)
Specific Heat
This is the quantity of energy in kiloJoules required to change the temperature of 1 kg mass of the substance by 1°C. For a gas the specific heat at constant pressure is greater than that at constant volume.
Spontaneous Combustion
The ignition of material brought about by a heat-producing chemical reaction within the material itself without exposure to an external source of ignition (see 2.20).
Static Electricity
Static electricity is the electrical charge produced on disimilar materials caused by relative motion between each when in contact (see 2.22).
Submerged Pump
A type of centrifugal cargo pump commonly installed on gas carriers and in terminals in the bottom of a cargo tank. It comprises a drive motor, impeller and bearings totally submerged by the cargo when the tank contains bulk liquid (see 4.2).
Superheated Vapour
Vapour removed from contact with its liquid and heated beyond its boiling temperature.
Surge Pressure
A phenomenon generated in a pipeline system when there is a change in the rate of flow of liquid in the line. Surge pressures can be dangerously high if the change of flow rate is too rapid and the resultant shock waves can damage pumping equipment and cause rupture of pipelines and associated equipment (see 4.1.3).
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Toxicity Detector
An instrument used for the detection of gases or vapours. It works on the principle of a reaction occurring between the gas being sampled and a chemical agent in the apparatus (see 9.7.4).
TLV
This is the abbreviation for Threshold Limit Value. It is the concentration of gases in air to which personnel may be exposed 8 hours per day or 40 hours per week throughout their working life without adverse effects. The basic TLV is a Time-Weighted Average (TWA). This may be supplemented by a TLV-STEL (Short-Term Exposure Limit) or TLV-C (Ceiling exposure limit) which should not be exceeded even instantaneously (see 9.3.1).
Upper Flammable Limit (UFL)
The concentration of a hydrocarbon gas in air above which there is insufficient air to support combustion (see 2.20).
Vapour Density
The density of a gas or vapour under specified conditions of temperature and pressure (see 2.16).
Void Space
An enclosed space in the cargo area external to a cargo containment system, other than a hold space, ballast space, fuel oil tank, cargo pump or compressor room or any space in normal use by personnel.
Explanation of symbols
Chapter 1
Introduction
This chapter provides an overview of the liquefied gases carried by sea and it concludes with some advice on the safety issues involving the ship, the terminal and the ship/shore interface. The latter point is of the utmost importance as this is where ship and shore personnel meet to plan safe operations. Subsequent chapters provide much greater detail about gas carrier cargoes and the equipment utilised on the ship and at the terminal jetty. They also cover operational and emergency procedures. Questions of health and safety are also covered and Chapter Six is devoted exclusively to ship/shore interface matters.
A thorough understanding of the basic principles outlined in this book is recommended as such knowledge will help ensure safer operations, better cargo planning and the efficient use of equipment found on gas carriers and on jetties.
LIQUEFIED GASES
A liquefied gas is the liquid form of a substance which, at ambient temperature and at atmospheric pressure, would be a gas.
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Most liquefied gases are hydrocarbons and the key property that makes hydrocarbons the world's primary energy source — combustibility — also makes them inherently hazardous. Because these gases are handled in large quantities it is imperative that all practical steps are taken to minimise leakage and to limit all sources of ignition.
The most important property of a liquefied gas, in relation to pumping and storage, is its saturated vapour pressure. This is the absolute pressure (see 2.15) exerted when the liquid is in equilibrium with its own vapour at a given temperature. The International Maritime Organization (IMO), for the purposes of its Gas Carrier Codes (see Chapter Three), relates saturated vapour pressure to temperature and has adopted the following definition for the liquefied gases carried by sea:
Liquids with a vapour pressure exceeding 2.8 bar absolute at a temperature of 37.8°C
An alternative way of describing a liquefied gas is to give the temperature at which the saturated vapour pressure is equal to atmospheric pressure — in other words the liquid's atmospheric boiling point.
In Table 1.1 some liquefied gases carried at sea are compared in terms of their vapour pressure at 37.8°C — the IMO definition — and in terms of their atmospheric boiling points.
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