The definitions listed in this section relate to their usage within this book. Other publications may use similar terms with different interpretations
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 desiccant (or drying) medium which operates by adsorption of water molecules (see 4.7.1).
Adiabatic Describes an ideal process undergone by a gas in which no gain or loss of heat occurs (see 2.1).
Aerating, 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 (see 7.9.4).
Airlock 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 spon- taneous combustion
without ignition
by a spark or flame (see 2.20).
AAvogadro’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.
BLEVEhis 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 (see 2.20).
Boil-off, 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 (see 4.5).
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) (see 1.1).
A Booster Pump, pump used to increase the discharge pressure from another pump (such as a cargo pump) (see 4.2).
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 (see 9.9.1).
Carbamates white powdery substance produced by the reaction of ammonia with carbon dioxide (see 7.2).
Carcinogen 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 forwardmost
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) (see 3.2).
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
(see 4.5.2).
Cavitation, 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 (see 4.2).
Certificate of Fitness, 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 (see 3.7.1).
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.
Cofferdam , The isolating space on a ship between two adjacent
steel bulkheads or decks. This space may be a void space or ballast space.
Condensate, 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) (see 2.12).
Critical Temperature,The temperature above which a gas cannot be liquefied by pressure alone (see 2.12).
Cryogenics, 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 (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
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, En 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, flamable 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).
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) (see 3.5).
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 atmos- phere. 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 Gas-free is reserved for the condition
suited to tank entry or for hot work, as described on the Gas-free certificate).
Gas-Freeing, The removal of toxic, and/or flammable
gas from a tank or enclosed
space with inert gas followed by the introduction
of fresh air (see 7.9.3).
Gassing-up, 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 (see 7.3).
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
(see 5.1).
Heel, 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 (see 7.8).
Hold Space, The space enclosed by the ship’s structure in which a cargo containment
system is situated.
Hydrates, The compounds
formed by
the interaction
of water
and hydrocarbons
at certain
pressures and temperatures. They are crystalline
substances (see 2.7).
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 (see 2.7).
IACS, International Association of Classification Societies.
IAPH, International Association of Ports and Harbors.
ICS, International Chamber of Shipping.
IMO, International Maritime Organization.
This is
the United
Nations specialised
agency dealing
with maritime affairs.
Inert Gas, gas,
such as nitrogen,
or a mixture
of non-flammable
gases containing insufficient oxygen
to support combustion (see 2.5).
Inerting, mean :,
(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 (see 7.2.3/7.9.3).
Insulation Flangen, A 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 (see 5.1.4).
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 (see 4.8).
ISGOTT, International Safety Guide for Oil Tankers and Terminals (see Appendix 1 — Reference 2.4).
Isothermal, 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 (see 2.10.1).
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).
LNG, 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
(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 BarrierThis 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 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).
Saturated Vapour Pressure, The pressure at which a vapour is in equilibrium with its liquid at a specified
temperature (see 2.15).
Secondary Barrier, The liquid-resisting
outer element of a cargo containment system designed to provide temporary con- tainment 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 (see 3.2.2).
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 (see 4.7.1).
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
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).
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).
Source : SIGTTO Personal Handbook.
Tidak ada komentar:
Posting Komentar
Silahkan berkomentar yang sopan sesuai dengan tajuk postingan serta tidak mengandung SARA dan politik,
mohon maaf jika tidak ada tanggapan atau respon yang lambat.