Wednesday, October 25, 2017

Liquefied Gas Handling Principles On Ships and in Terminals Definition



Definition
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).

AAvogadros Law, Avogadros  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.

Daltons 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 ships 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 systems  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 ships 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 ships 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 ships 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.

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

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).

Liquefied Gas, 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 (see 1.1).

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 ships cargo tank as stated on the ships 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 Daltons 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 ships 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).

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.
Source : SIGTTO Personal Handbook.

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