Petroleum Calculation Series: Gas Tanker Terms

Absolute Pressure
The absolute pressure is the total of the gauge pressure plus the pressure of the surrounding atmosphere.

Absolute Temperature
The fundamental temperature scale with its zero at absolute zero and expressed in degrees Kelvin. One degree Kelvin is equal to one degree Celsius or one degree Centigrade. For the purpose of practical calculations in order to convert Celsius to Kelvin add 273. It is normal for the degree Kelvin to be abbreviated in mathematical formulae to ‘K’ with the degree symbol being omitted.

Absolute Zero
The temperature at which the volume of a gas theoretically becomes zero and all thermal motion ceases. It is generally accepted as being -273.16°C.

Activated Alumina
A desiccant (or drying) medium which operates by adsorption of water molecules.

Describes an ideal process undergone by a gas in which no gain or loss of heat occurs.

Aerating means the introduction of fresh air into a tank with the object of removing toxic, flammable and inert gases and increasing the oxygen content to 21 per cent by volume.

A separation area used to maintain adjacent areas at a pressure differential. For example, the airlock to an electric motor room on a gas carrier is used to maintain pressure segregation between a gas-dangerous zone on the open deck and the gas-safe motor room which is pressurised.

Approved Equipment
Equipment of a design that has been type-tested and approved by an appropriate authority such as a governmental agency or classification society. Such an authority will have certified the particular equipment as safe for use in a specified hazardous atmosphere.

Auto-ignition Temperature
The lowest temperature to which a liquid or gas requires to be raised to cause self-sustained spontaneous combustion without ignition by a spark or flame.

Avogadro’s Law
Avogadro’s Hypothesis states that equal volumes of all gases contain equal numbers of molecules under the same conditions of temperature and pressure.

This is the abbreviation for a Boiling Liquid Expanding Vapour Explosion. It is associated with the rupture, under fire conditions, of a pressure vessel containing liquefied gas.

Boil-off is the vapour produced above the surface of a boiling cargo due to evaporation. It is caused by heat ingress or a drop in pressure.

Boiling Point
The temperature at which the vapour pressure of a liquid is equal to the pressure on its surface (the boiling point varies with pressure).

Booster Pump
A pump used to increase the discharge pressure from another pump (such as a cargo pump).

Bulk Cargo
Cargo carried as a liquid in cargo tanks and not shipped in drums, containers or packages.

Canister Filter Respirator
A respirator consisting of mask and replaceable canister filter through which air mixed with toxic vapour is drawn by the breathing of the wearer and in which the toxic elements are absorbed by activated charcoal or other material. A filter dedicated to the specific toxic gas must be used. Sometimes this equipment may be referred to as cartridge respirator. It should be noted that a canister filter respirator is not suitable for use in an oxygen deficient atmosphere.

A white powdery substance produced by the reaction of ammonia with carbon dioxide.

A substance capable of causing cancer.

Cargo Area
That part of the ship which contains the cargo containment system, cargo pumps and compressor rooms, and includes the deck area above the cargo containment system. Where fitted, cofferdams, ballast tanks and void spaces at the after end of the aftermost hold space or the forward end of the forward most hold space are excluded from the cargo area. (Refer to the Gas Codes for a more detailed definition).

Cargo Containment Systems
The arrangement for containment of cargo including, where fitted, primary and secondary barriers, associated insulations, interbarrier spaces and the structure required for the support of these elements. (Refer to the Gas Codes for a more detailed definition).

Cascade Reliquefaction Cycle
A process in which vapour boil-off from cargo tanks is condensed in a cargo condenser in which the coolant is a refrigerant gas such as R22 or equivalent. The refrigerant gas is then compressed and passed through a conventional sea water-cooled condenser.

A process occurring within the impeller of a centrifugal pump when pressure at the inlet to the impeller falls below that of the vapour pressure of the liquid being pumped. The bubbles of vapour which are formed collapse with impulsive force in the higher pressure regions of the impeller. This effect can cause significant damage to the impeller surfaces and, furthermore, pumps may loose suction.
Certificate of Fitness
A certificate issued by a flag administration confirming that the structure, equipment, fittings, arrangements and materials used in the construction of a gas carrier are in compliance with the relevant Gas Code. Such certification may be issued on behalf of the administration by an approved classification society

Certified Gas Free
A tank or compartment is certified to be gas-free when its atmosphere has been tested with an approved instrument and found in a suitable condition by an independent chemist. This means it is not deficient in oxygen and sufficiently free of toxic or flammable gas for a specified purpose.

The isolating space on a ship between two adjacent steel bulkheads or decks. This space may be a void space or ballast space.

Reliquefied gases which collect in the condenser and which are then returned to the cargo tanks.

Compression Ratio
The ratio of the absolute pressure at the discharge from a compressor divided by the absolute pressure at the suction.

Critical Pressure
The pressure at which a substance exists in the liquid state at its critical temperature. (In other words it is the saturation pressure at the critical temperature).

Critical Temperature
The temperature above which a gas cannot be liquefied by pressure alone.

The study of the behaviour of matter at very low temperatures.

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.

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.

The mass per unit volume of a substance at specified conditions of temperature and pressure.
The temperature at which condensation will take place within a gas if further cooling occurs.

A process which is accompanied by the absorption of heat.

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.

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 entropy 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.

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.

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.

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

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.

Gas-Dangerous Space or Zone
A space or zone (defined by the Gas Codes) within a ship’s cargo area which is designated as likely to contain flammable vapour and which is not equipped with approved arrangements to ensure that its atmosphere is maintained in a safe condition at all times. (Refer to the Gas Codes for a more detailed definition).

Gas-free Certificate
A gas-free certificate is most often issued by an independent chemist to show that a tank has been tested, using approved testing instruments, and is certified to contain 21 per cent oxygen by volume and sufficiently free from toxic, chemical and hydrocarbon gases for a specified purpose such as tank entry and hot work. (In particular circumstances, such a certificate may be issued when a tank has been suitably inerted and is considered safe for surrounding hot work.)

Gas-free Condition
Gas-free condition describes the full gas-freeing process carried out in order to achieve a safe atmosphere. It therefore includes two distinct operations: Inerting and Aeration.
(Note: — In some gas trades the expression ‘Gas-free’ is used to denote a tank which is just Inerted. Some gas carrier operations can stop at this stage; for example prior to special drydockings or cargo grade changes. However, in this book this condition is described as an ‘Inert condition’ and the expression Gasfree is reserved for the condition suited to tank entry or for hot work, as described on the Gas-free certificate).

The removal of toxic, and/or flammable gas from a tank or enclosed space with inert gas followed by the introduction of fresh air.

Gassing-up means replacing an inert atmosphere in a tank with the vapour from the next cargo to a suitable level to allow cooling down and loading.

Gas-Safe Space
A space on a ship not designated as a gas-dangerous space.

Hard Arm
An articulated metal arm used at terminal jetties to connect shore pipelines to the ship’s manifold.

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 recirculating 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.

Hold Space
The space enclosed by the ship’s structure in which a cargo containment system is situated.

The compounds formed by the interaction of water and hydrocarbons at certain pressures and temperatures. They are crystalline substances.

Hydrate Inhibitors
An additive to certain liquefied gases capable of reducing the temperature at which hydrates begin to form. Typical hydrate inhibitors are methanol, ethanol and isopropyl alcohol.

International Association of Classification Societies.

International Association of Ports and Harbors.

International Chamber of Shipping.

International Maritime Organization. This is the United Nations specialised agency dealing with maritime affairs.

Incendive Spark
A spark of sufficient temperature and energy to ignite a flammable gas mixed with the right proportion of air.

Inert Gas
A gas, such as nitrogen, or a mixture of non-flammable gases containing insufficient oxygen to support combustion (see 2.5).

Inerting means:
(i) the introduction of inert gas into an aerated tank with the object of attaining an inert condition suited to a safe gassing-up operation.
(ii) the introduction of inert gas into a tank after cargo discharge and warming-up with the object of: —
(a) reducing existing vapour content to a level below which combustion cannot be supported if aeration takes place
(b) reducing existing vapour content to a level suited to gassing-up prior to the next cargo
(c) reducing existing vapour content to a level stipulated by local authorities if a special gas-free certificate for hot work is required — see the note under gas-free condition.

Insulation Flange
An insulating device inserted between metalic flanges, bolts and washers to prevent electrical continuity between pipelines, sections of pipelines, hose strings and loading arms or other equipment.

Interbarrier Space
The space between a primary and a secondary barrier of a cargo containment system, whether or not completely or partially occupied by insulation or other material.

Intrinsically Safe
Equipment, instrumentation or wiring is deemed to be intrinsically safe if it is incapable of releasing sufficient electrical or thermal energy under normal conditions or specified fault conditions to cause ignition of a specific hazardous atmosphere in its most easily ignited concentration.

International Safety Guide for Oil Tankers and Terminals.

Descriptive of a process undergone by an ideal gas when it passes through pressure or volume variations without a change of temperature.

Latent Heat
The heat required to cause a change in state of a substance from solid to liquid (latent heat of fusion) or from liquid to vapour (latent heat of vaporisation). These phase changes occur without change of temperature at the melting point and boiling point, respectively.

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.

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.

This is the abbreviation for Liquefied Natural Gas, the principal constituent of which is methane.

Lower Flammable Limit (LFL)
The concentration of a hydrocarbon gas in air below which there is insufficient hydrocarbon to support combustion.

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.

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.

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

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.

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.

Mole Fraction
The number of moles of any component in a mixture divided by the total number of moles in the mixture.

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

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.

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.

Oil Companies International Marine Forum.

Oxygen Analyser
Instrument used to measure oxygen concentrations in percentage by volume.

Oxygen-Deficient Atmosphere
An atmosphere containing less than 21 per cent oxygen by volume.

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.

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.

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.

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).

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.
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.

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.

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.

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.

Saturated Vapour Pressure
The pressure at which a vapour is in equilibrium with its liquid at a specified temperature.

Secondary Barrier
The liquid-resisting outer element of a cargo containment system designed to provide temporary containment of a leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship’s structure to an unsafe level.

Sensible Heat
Heat energy given to or taken from a substance which raises or lowers its temperature.

Shell and Tube Condenser
A heat exchanger where one fluid circulates through tubes enclosed between two end-plates in a cylindrical shell and where the other fluid circulates inside the shell.

Silica Gel
A chemical used in driers to absorb moisture.

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).

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.

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.

Specific Gravity
The ratio of the density of a liquid at a given temperature to the density of fresh water at a standard temperature.
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.

Static Electricity
Static electricity is the electrical charge produced on disimilar materials caused by relative motion between each when in contact.

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.

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.

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.

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.

Upper Flammable Limit (UFL)
The concentration of a hydrocarbon gas in air above which there is insufficient air to support combustion.

Vapour Density
The density of a gas or vapour under specified conditions of temperature and pressure.

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.

Author: Faisal Yusuf

Hi, my name is Faisal Yusuf, but you can call me Surveyor. I live in Jakarta, Indonesia and work as a surveyor.

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