Gas tankers advance course 2003

Advanced Course Gas Tankers Advance Course INTRODUCTION Gas Tankers Advance Course INTRODUCTION 1.1 ABOUT STCW International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), 1978, as amended, sets qualification standards for masters, officers, and watch going personnel on seagoing merchant ships STCW was adopted in 1978 by conference at the International Maritime Organization (IMO) in London, and entered into force in 1984 The Convention was significantly amended in 1995 The 133 current stateparties to the Convention represent approximately 98 percent of the world’s merchant vessel tonnage 1.1.1 Limitations discovered Between 1984 and 1992, significant limitations to the 1978 Convention became apparent Many people felt that the Convention included vague requirements that were left to the discretion of parties to the Convention Others felt that there were growing problems with: (a) a lack of clear standards of competence, (b) no IMO oversight of compliance, (c) limited port state control, and (d) inadequacies that did not address modern shipboard functions Meanwhile, the U.S deferred ratification efforts and worked for almost a decade to effect necessary changes to our licensing regulations 1.1.1 Amendments adopted in 1995 On July 7, 1995, a conference of parties to the Convention, meeting at IMO headquarters in London, adopted the package of amendments to STCW The amendments entered force on February 1, 1997 1.1.2 Effective dates The provisions of the Convention not tied to individual mariner certification became effective when the IFR (Interim Final Rule) was published However, provision was made for certain new requirements to be introduced over a longer period Full implementation is required by February 1, 2002 For issuance of licenses and documents, the effective dates of the new requirements will be according to transitional guidance published by the STW Subcommittee Mariners already holding licenses have the option to renew those licenses in accordance with the old rules of the 1978 Convention during the period ending on February 1, 2002 Mariners entering training programs after August 1, 1998 are required to meet the competency standards of the new 1995 Amendments For persons seeking original licenses, the Coast Guard anticipates that most new training requirements will be incorporated into courses approved by the Coast Guard, or by equivalent courses To ensure that the competency objectives of the 1995 amendments are met, parties must implement quality assurance programs, with IMO reviewing each parties’ national program Again, this represents a fundamental change in thinking for the international community It will be mandatory that the "pulse" of the new system be checked on a recurring basis to ensure its "good health." Gas Tankers Advance Course 1.1.3 Familiarization training: Both the STCW Convention and the U.S implementing regulations use the term familiarization training or similar terminology five different ways: a Companies are required to ensure that seafarers who are newly assigned to a ship are familiarized with their specific duties and with all ship arrangements, installations, equipment, procedures and ship characteristics that are relevant to their routine or emergency duties Written instructions are to be issued by the company to each ship to ensure this ship-specific familiarization takes place b All persons who are employed or engaged on a seagoing ship other than passengers are required to receive approved familiarization training in personal survival techniques or receive sufficient information and instruction to be able to take care of themselves and take proper action when an emergency condition develops This includes locating and donning a lifejacket, knowing what to if a person falls overboard, and closing watertight doors c Officers and ratings who are assigned specific duties and responsibilities related to cargo or cargo equipment on tankers must complete an approved tanker familiarization course if they have not had a minimum period of seagoing service on tankers d Masters, officers and other personnel who are assigned specific duties and responsibilities on board ro-ro passenger ships must complete familiarization training which covers subjects such as operational limitations of ro-ro ships, procedures for opening and closing hull openings, stability, and emergency procedures e Masters, officers and other personnel who are assigned specific duties and responsibilities on board passenger ships other than ro-ro passenger ships must complete familiarization training which covers operational limitations of passenger ships 1.2 THE COURSE The International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW 78/95), which contains mandatory minimum requirements for training and qualifications of masters, officers and ratings of chemical tankers This training is divided into two parts: Level 1: Chemical tanker familiarization - a basic safety training course for officers and ratings who are to have specific duties and responsibilities relating to cargo and cargo equipment Gas Tankers Advance Course Level training can also be covered through an appropriate period of supervised shipboard service where an approved shipboard training programme is conducted by qualified personnel Level 2: Advanced training programme on liquefied gas tanker operations An advanced training programme for masters, officers and others who are to have immediate responsibilities for cargo handling and cargo equipment In addition to level training, such personnel must have completed level and have relevant experience on liquefied gas tankers before signing on to these positions on board This course covers the requirements for level and level training required by STCW 95 Chapter V Regulation V/1 - 1.2, 2.2 and Section A-V/1 regulations 15 - 21 Gas Tankers Advance Course 02 Actual Gas Cargoes Gas Tankers Advance Course ACTUAL GAS CARGOES In the late 1920th transportation of liquefied gases in bulk started In the very beginning it was transportation of propane and butane in fully pressurised tanks When the steel quality became better and the knowledge about propane and butane was better they started to carry those liquefied gases under temperature control From the mid-1960th we have carried fully refrigerated liquefied gases and now the biggest gas carriers are more than 125 000 m3 Liquefied gas is divided into different groups based on boiling point, chemical bindings, toxicity and flammability The different groups of gases have led to different types of gas carriers and cargo containment system for gas carriers • • • • • • • IMO divides liquefied gases into the following groups: LPG - Liquefied Petroleum Gas LNG - Liquefied Natural Gas LEG - Liquefied Ethylene Gas NH3 - Ammonia Cl2 - Chlorine Chemical gases The IMO gas carrier code define liquefied gases as gases with vapour pressure higher than 2,8 bar with temperature of 37,8oC IMO gas code chapter 19 defines which products that are liquefied gases and have to be transported with gas carriers Some products have vapour pressure less than 2,8 bar at 37,8oC, but are defined as liquefied gases and have to be transported according to chapter 19 in IMO gas code Propylene oxide and ethylene oxides are defined as liquefied gases Ethylene oxide has a vapour pressure at 37,8oC on 2,7 bar To control temperature on ethylene oxide we must utilise indirect cargo cooling plants Products not calculated as condensed gas, but still must be transported on gas carriers, are specified in IMO’s gas code and IMO’s chemical code The reason for transportation of non-condensed gases on gas carriers is that the products must have temperature control during transport because reactions from too high temperature can occur Condensed gases are transported on gas carriers either by atmospheric pressure (fully cooled) less than 0,7 bars, intermediate pressure (temperature controlled) 0,5 bars to 11 bars, or by full pressure (surrounding temperature) larger than 11 bars It is the strength and construction of the cargo tank that is conclusive to what over pressure the gas can be transported Gas Tankers Advance Course Examples of some gas pressure at 37,8oC and boiling point at atmospheric pressure: Condensed gas Methane CH4 Propane C3H8 n - Butane C4H10 Ammonia NH3 Vinyl Chloride C2H3Cl Butadiene C4H6 Ethylene oxide C2H4O 2.1 Gas Boiling point pressure at atmospheric 37,8oC pressure bars in oC absolute Gas - 161 12,9 - 43 3,6 - 0,5 14,7 - 33 5,7 - 14 4,0 -5 2,7 10,7 at LPG LPG - Liquefied Petroleum Gas is a definition of gases produced by wet gas or raw oil The LPG gases are taken out of the raw oil during refining, or from natural gas separation LPG gases are defined as propane, butane and a mixture of these Large atmospheric pressure gas carriers carry most of the LPG transported at sea However, some LPG is transported with intermediate pressure gas carriers Fully pressurised gas carriers mainly handle coastal trade LPG can be cooled with water, and most LPG carriers have direct cargo cooling plants that condenses the gas against water The sea transport of LPG is mainly from The Persian Gulf to Japan and Korea It is also from the north- west Europe to USA, and from the western Mediterranean to USA and Northwest Europe LPG is utilised for energy purposes and in the petro-chemical industry 2.2 LNG LNG - Liquefied Natural Gas is a gas that is naturally in the earth Mainly LNG contains Methane, but also contains Ethane, Propane, Butane etc About 95% of all LNG are transported in pipelines from the gas fields to shore, for example, gas pipes from the oil fields in the North Sea and down to Italy and Spain Gas carriers transport the remaining 5% When LNG is transported on gas carriers, the ROB and boil off from the cargo is utilised as fuel for propulsion of the vessel Cargo cooling plants for large LNG carriers are very large and expensive, and they will use a lot of energy Small LNG carriers have cargo-cooling plants, and can also be utilised for LPG transportation Gas Tankers Advance Course The sea transport of LNG is from the Persian Gulf and Indonesia to Japan, Korea and from the Mediterranean to Northwest Europe and the East Coast of USA and from Alaska to the Far East LNG is used for energy purposes and in the petro-chemical industry 2.3 NGL NGL - Natural Gas Liquid or wet gas is dissolved gas that exists in raw oil The gas separates by refining raw oil The composition of wet gas varies from oil field to oil filed The wet gas consists of Ethane, LPG, Pentane and heavier fractions of hydrocarbons or a mixture of these Atmospheric pressure gas carriers and semipressurised gas carriers carry the most of the wet gas Ethane can only be transported by semi-pressurised gas carriers, which have direct cascade cooling plants and are allowed to carry cargo down to –104oC This is because Ethane has a boiling point at atmospheric pressure of –89oC This will create too high condense pressure if using water as cooling medium The cargo is condensed against Freon R22 or another cooling medium with boiling point at atmospheric pressure lower than –20oC Wet gas is transported from the Persian Gulf to the East, Europe to USA and some within Europe There is also some transport of wet gas in the Caribbean to South America NGL is utilised for energy purposes and in the petro-chemical industry COMPOSITION OF NATURAL GAS 2.4 2.5 LEG LEG - Liquefied Ethylene Gas This gas is not a natural product, but is produced by cracked wet gas, such as, Ethane, Propane, and Butane or from Naphtha Ethylene has a boiling point at atmospheric pressure of -103,8oC, and therefore has be transported in gas carriers equipped with cargo compartment that can bear such a Gas Tankers Advance Course low temperature Cascade plants are used to condense Ethylene As critical temperature of Ethylene is 9,7oC one can not utilise water to condense Ethylene The definition of Ethylene tankers is LPG/LEG carrier Ethylene is very flammable and has a flammable limit from 2,5% to 34% by volume mixed with air There are stringent demands regarding the oxygen content in Ethylene The volume of ethylene must be less than 2% in the gas mixture to keep the mixture below the LEL “lower explosion limit” Normally, there are demands for less than 0,2% oxygen in the gas mixture in order to prevent pollution of the cargo Ethylene is utilised as raw material for plastic and synthetic fibres Ethylene is transported from the Persian Gulf to the East, the Mediterranean to the East and Europe, the Caribbean to South America There is also transport of Ethylene between the countries Malaysia, Indonesia and Korea 2.6 AMMONIA NH3 The next gas we will focus on is Ammonia, which is produced by combustion of hydrogen and nitrogen under large pressure Ammonia is a poisonous and irritating gas, it has TLV of 25 ppm and the odour threshold is on 20 ppm It responds to water and there are special rules for vessels that transport Ammonia We can locate the rules in the IMO Gas Code, chapters 14, 17 and 19 When ammonia gas is mixed with water, a decreased pressure is formed by volume part water absorbing 200 volume parts ammonia vapour A decreased tank pressure will occur if there is water in the tank when commence loading ammonia and the tank hatch is closed With an open hatch, we can replace the volume, originally taken up by the ammonia gas, with air One must not mix ammonia with alloys: copper, aluminium, zinc, nor galvanised surfaces Inert gas that contains carbon dioxide must not be used to purge ammonia, as these results in an carbamate formation with the ammonia Ammonium carbamate is a powder and can blockage lines, valves and other equipment The boiling point for ammonia at atmospheric pressure is –33oC, and must be transported at a temperature colder than –20oC One can cool ammonia with all types of cargo cooling plants Ammonia is transported with atmospheric pressure gas carriers or semi-pressurised gas carriers Gas carriers carrying Ammonia must be constructed and certified in accordance with IMO’s IGC code for transportation of liquefied gases The definition for ammonia tanker is LPG/NH, carrier Ammonia is utilised as raw material for the fertiliser industry, plastic, explosives, colours and detergents There is a lot of transportation from the Black Sea to USA, from USA to South Africa and from Venezuela to Chile 2.7 CHLORINE CI2 Chlorine is a very toxic gas that can be produced by the dissolution of sodium chloride in electrolysis Because of the toxicity of Chlorine it is therefore transported in small quantities, and must not be transported in a larger quantity than 1200m3 The gas carrier carrying chlorine must be type 1G with independent type C tanks That means the cargo tank must, at the least, lie B/5 “Breadth/5” up to 11,5 meter 10 Gas Tankers Advance Course The cooling process of the cargo side is plotted into a Mollier diagram for ethylene and is, as follows: The cooling process on the freon side of the cascade plants is plotted equal The condensation pressure of the loading side depends on the temperature of the freon liquid circulating through the loading condenser The pressure in the liquid separator again depends on the suction pressure of the freon compressor and determines the freon temperature (We assume that the freon side has the right filling) Notice that the condensation pressure for ethylene lies at 17,3 bars, which corresponds to a temperature of –27oC The pressure in the liquid separator is 0,2 bars, which corresponds to a temperature of –37oC This gives a temperature difference of 10oC in the cargo condenser, usually too high for this type of heat exchanger If the freon pump don’t deliver sufficient liquid to the cargo condenser, the cause is probably one or a combination of the following: • · • · • · reduced heat transmission caused by incrustation in the loading condenser reduced heat transmission caused by too large share of oil in the Freon liquid too high condenser pressure caused by unknown gas on the loading side Cascade plants are used both for LPG and ethylene The plant exists both on atmosphere gas carrier LPG and for semi pressured gas carriers LPG/ ethylene This 385 Gas Tankers Advance Course plant is an example of a usual cascade plant that can also be used to re-condense ethylene Normally screw compressors are used both on the loading side and on the freon side in the cooling plant onboard big atmosphere pressure gas carrier LPG There are many different configurations of cascade plants The freon side is frequently equipped with super-feed or intermediate pressure container with subcooling Some plants have freon pumps, which pump freon through the cargo condenser, where others use thermal expansion valves Freon compressors can be piston compressors or screw compressors Screw compressors can be built with one or two stages Plants with and without MT-containers, with and without flash cooling, with and without de-super-heaters, 2-stage compression with and without sub-cooling, exist on the loading side The compressors on the cargo side have to be oil free piston or screw compressors The configuration possibilities are many and the variation in plants from vessel to vessel is what one faces onboard The understanding of the cooling process and knowledge to your vessel’s “special” plant is a basic assumption for safe and economic operation of the plant 386 Gas Tankers Advance Course 14- Insulation and Heat Transfer 387 Gas Tankers Advance Course 14.1 INSULATION AND INSULATION MATERIAL There are three different methods in transporting heat: thermal conductance, convection and radiation The insulation material’s’ main task is to reduce heat transmission from thermal conductance Most insulation material’s insulating qualities emerge from stationary gases, bad thermal conductance capability and thereby good insulation capability The thermal conductance capability is expressed by a material’s thermal conductance number (thermal conductivity) and states the heat quantity measured by Watt, which is transported through to surface of meter thickness when the temperature difference is 1K Thermal conductivity Natural pure metals From to 400 W/m K 80 W/m K 2,4 W/m K From 0,10 to 0,60 W/m K Wood From 0,1 to 0,3 W/m K Glass W/m K Polypropylene 0,12 W/m K Sand 0,35 W/m K Natural gases From 0,008 to 0,048 W/m K Stationary air 0,024 W/m K Freon 22 0,012 W/m K Iron Ice at -20 oC Natural liquids One can see that the best heat conductor or worst insulation materials are pure metals The worst heat conductors or best insulation materials are stationary gases It is of most importance that the gases are kept as stationary as possible, because the total thermal transmission is higher if convection also arises This is in practice solved by trying to catch the gas inside the smallest possible cavity, or by keeping the gas inside a net of thin fibres The thermal conductance figure will mainly increase at higher temperature because of larger convection The best insulation material regarding thermal transmission is a composition of a firm material and a gas with the separately lowest thermal conductance figures First of all, it is the thermal conductance figure that is of interest when looking at the important qualities for an insulation material Lower thermal conductance figures render possible thinner insulation and thereby place saved 388 Gas Tankers Advance Course The upper temperature limit for the material’s relevance normally has no importance for an insulation material that should be used for cool insulation, but one prefers that the material tolerate highest possible temperature considering fire The insulation material’s lower temperature is of major importance In the plastic insulation condensation can occur at a lower temperature and thereby increase the thermal inductance figure for the material Thermal expansion and elasticity are two qualities of great importance for plastic insulation materials, for example cargo tanks on gas tankers Changing temperature for tank shells and insulation can lead to periodical expansions and compressions As the thermal expansion co-efficiency for the insulation material can be to times larger than the steel, cracks may easily arise if the insulation material don’t have good elasticity A plastic insulation material like polyurethane has good adhesion firmness to the steel priming and good elasticity and is thereby resistant to cracks An insulating problem area is around the loose tank cradle of cylinder cargo tanks All of the expansion and compression movement of longitudinal direction of the tank takes place here Large demands are made both to material and for the insulation to be good and long performance A special developed insulation with especially good elasticity is suitable for such areas All insulation materials that are used onboard must be fire resistant Insulation materials made of plastic are added or built up in such a way that in case of fire, fireextinguishing vapour are released, and they are self-quenching The insulation material will only burn as long as a foreign fire source is present One must not underestimate the risk of fire in the plastic insulation and the consequences by such a fire Fire can easily arise in such materials in connection with weld work Ignition of the insulation on the cargo tanks has occurred with fatal consequences The large amount of thick, black and poisonous smoke that is formed by such a fire has prevented escape from hold space and serious poisoned injures Where the material is exposed to strains the material’s compressive-strength is important The strain points will, for example, be large in the support points for an insulated pipeline To evade lasting compression or crumbling of the insulation material, special compressive-strengthen materials are used in these areas Of the chemical qualities for insulation materials, it is the corrosive qualities that often are underestimated When the insulation material of glass or mineral wool got a high humidity, it will have a strong corroding effect on metals It is difficult over time to avoid this onboard Even stainless steel pipes corrode in such an environment and if this is allowed to proceed, an expensive replacement is soon the result The capability to resist humidity or diffusion resistance is an important quality that must be evaluated in choosing insulation material, as well as, in planning maintenance When insulating between a warm and cold side, for example, a cargo tank, the cold side will be tight (tank shell) and the warm side will be surrounded by air with high humidity Because of a higher saturation pressure on the warm side in proportion to the cold side, moist air will be forced through and will condense against the cold side The thermal conductance figure will, because of the humidity, increase and the water will freeze and destroy the insulation The damage extent will accelerate if such a process is continuously unchanged If the diffusion resistance for stationary air is set to 1, the proportional diffusion resistance for mineral wool will be about 1,5 and for polyurethane it will be about 60 389 Gas Tankers Advance Course This indicates that insulation of polyurethane is about 40 times more resistant against moist air to leak through than an insulation of mineral wool Regardless of which insulation material that is used, moist air will penetrate into the material and destroy it A diffusion-tight damp-latch on the warm side is, at all times, imperative on a chill insulation The most commonly used materials and methods are: • • • • • • • • • Thin aluminium foil glued on the insulation This method is suitable for insulation non-exposed for mechanical wear and tear, for example, some loading tank constructions Galvanised and stainless steel sheet that is fastened by pop cones and the joint seals with jointing compound This method is expensive, but strong against mechanical wear and tear and necessary for “foaming” Glass-fibre armed polyester This method is more moderate, but gets easily fragile Most suitable for repair One or more layers with asphalt emulsion armoured with multiple layers of glass fibre fabric Moderate method and easy to maintain, but weak for mechanical wear and tear Sprayed mastic with or without armouring Moderate method, but week against mechanical war and tear and requires more maintenance on exposed places Most suitable for repair and sprinkling on insulation not exposed to rough weather The insulation material on gas ships may be divided into the following three different groups based on structure and material: Cellular plastic, which is expanded plastic raw material, built up in a cellular structure Wallboard, which is built up of a net with thin fibres Expanded volcanic perlite, which is built up on a cellular structure 14.1.1 Polyurethane There are a number of insulation materials, which are built up of raw plastic materials The most used is polyurethane Mixing isocynate and polyole, normally in the proportion 1:1 makes polyurethane Isocyanate has a resemblance to thick oil and polyole has a resemblance to clear liquid A chemical reaction that is exothermic is actuated when mixing the liquids If one adds some water, carbon dioxide is formed, which because of the reaction-heat evaporates and “blows” up the material Foam with about 90% closed cells and a very low thermal conductance number appear Polyurethane-foam based on CO2 has a relatively long time of expansion 390 Gas Tankers Advance Course The qualities of the polyurethane-foam can be improved by using different freon materials as a blowing agent Because of the freon material’s lower boiling point, the expansion is quicker R11 was used earlier as a blowing agent The thermal conductance capability for R11 is only half of CO2 If applying polyurethane foam with a sprayer, or frothing, freon is used as the blowing agent Freon evaporates with speed in normal surrounding temperature and one obtains pre-expansion, when the mixture leave the spray One can mix polyurethane foam for smaller repairs in a bucket before pouring into a mould As there are different suppliers on the market, it is recommended to check with the supplier about the composition and if water has to be used when mixing There are also a number of machines for spraying on the market, also disposable spray equipment with smaller containers for iciyanat and polyol Disposal-spray equipment of this type or a simple modifying of the paint sprayer onboard, is suitable for smaller repairs to the insulation Polyurethane is also available as half-cups for insulation of pipes It is important that the dimension of the cup fit to the outer diameter of the pipe so that air leak is avoided The polyurethane cups can easily be cut for matching the pipe bend, bend and valves It is recommended to lay two layers with half cups on larger pipes, so that the connections are displaced in proportion to each other The half cups and the connections are glued, and secure the density and strength and a water barrier is laid either with thin metal plates or mastic 14.1.2 Polystyrene Polystyrene is produced in two stages At the first stage polystyrene is pre-expanded with vapour where blisters from to mm are formed The pre-expanded material is sent up into large silos for de-aeration After to day in the silo, the pre-expanded material is filled in forms for further expansion and compression of the grains, to shape blocks Heat from steam or electrical elements are used in this last expansion The finished blocks are cut up into plates and pipe cups 14.1.3 Isolation of LNG ships with spherical tanks Cellular plastic is used as insulation material for Moss Rosenberg's spherical cargo tanks Dow Chemicals in USA and Technical Isolation in Norway developed a new method of mounting insulation, because the insulation materials contraction is 2,5 times more than aluminium and times more than steel at -163oC Poles of “Styrofoam” of about meters long are welded together and set continuously around the spherical tank, from equator against the bottom and on top of the tank An external aluminium foil of 0,25 mm is laid on also The room around the loading tank is filled with nitrogen with a dew point down to –40oC The purpose is partly to protect the tanks from corrosion and to reduce the “pressure” of humidity against the insulation 391 Gas Tankers Advance Course 14.1.4 Mineral wool Mineral wool is a collective term of different fibre rich insulation materials Rock wool and glass wool are two types of insulation material that are used The production method and user area is equal A mixture of several types of stones is used to produce rock wool The stones are melted in a temperature up to 1600oC and are dispatched over a wheel with very high rotation The melted mixture of stones is hurled out and chilled in long thin fibres Cementing agent based on plastic is added and hardened with hot air The amount of cementing agent varies and is determined by the material’s purpose of use Rock wool plates are elastic and a normal density of about 45 kg/m3 The plates are also delivered with larger density and firmness Glass wool is produced like rock wool by hurling and chilled melted glass to very fine fibres, only about 0,0025 mm diameter 14.1.5 Expanded Perlite Perlite is made of a volcanic rock species with perlite structure The main component is about 71% SiO2 and about 16% Al2O3 The raw material has some water content, which by heating to about 1200oC evaporates and “blows” up the material One obtains a 10 to 20 times expansion with numerous closed airtight cells Simultaneously the individual corns loosen form each other, the material “explodes” and forms sharp-edged corns with sizes from to mm with very large mechanical strength The density is about 60 kg/m3 One can easily fill the whole room around a loading tank with perlite It is first of all used on atmosphere pressure gas carriers Maintenance of insulation The cooling plant on a gas carrier is constructed and calculated for thermal leakage from cargo tanks and system when the ship is new Insulation is exposed for ageing, wear and tear and will in time be reduce if maintenance of the insulation is not kept If the insulation on cargo tanks and pipelines reduces, increased thermal leakage will occur Increased thermal leakage involves removal of more heat from the cargo Load time and time used to cool the cargo increase Preserving the insulation is good economy Regular control and systematic maintenance of this from day one will save large future expenses Protection of water and humidity is of high importance and that’s why we purge hold spaces with dry inert/ nitrogen With exception of mechanical wear and tear, there is nothing more destroying for the insulation then the humidity The only way to protect the insulation from humidity is to assure that the water barrier is intact Re-insulation of the insulation on cargo tanks is especially expensive The best and cheapest way to preserve this is to be sure that the atmosphere around the loading tanks is dry It is important to consider the different material’s capability to absorb the heat of radiation when working with external insulation materials A light water barrier absorbs less heat than dark In practice, this means that white pipe insulation absorbs less heat than one with red or orange colour on the pipe insulation One must also notice that pipe insulation faced with stainless steel plates radiates less 392 Gas Tankers Advance Course heat and thereby is warmer than a galvanised plate The thermal conductance is thereby larger Five good advice for maintaining the cargo tank insulation: Held the atmosphere in hold spaces dry by drying the atmosphere regularly, use dry inert gas or nitrogen, if possible Control the cargo tank insulation regularly Areas with ice or humidity indicate thermal loss Note these areas with spray paint to easier locate the areas when these need repairing Control external insulation regularly and repair wrecked water barrier plates immediately Areas with ice or humidity indicate thermal loss Note the areas with spray painting in order to easily locate these when need of repairing At all times, have necessary materials to repair wrecked insulation onboard, minimum materials to repair damage on water barrier As some of the insulation materials have limited operating time, the stock onboard must be adjusted to the expected consumption the next month Before adding new insulation in place, corrosion and pitting must be controlled The steel must be protected from corrosion before new insulation is put to place 393 Gas Tankers Advance Course 14.1.6 The qualities of the insulation material compared with other materials Thermal Density Pressure Heat Fire Diffusion conductance firmness capacity qualities resistance W/m K kg/m3 kg/cm2 J/kg K Stationary air 000 0,024 1,3 on/20oC CO2 0,015 2,0 840 Freon 22 0,012 4,7 090 Polyurethane foam, R11 Polystyrene foam (Isophor) Carbamide foam Phenol foam (Bakelite) Glass wool plates 0,023 40,0 2,00 260 0,033 25,0 0,30 340 0,035 10,0 0,041 32,0 0,035 20,0 840 Rock plates 0,035 45,0 840 Expanded perlite 0,035 50,0 840 Iron Steel (12 Cr) Stainless steel (19 Cr/10Ni) Water of/ 20 o C Ice of/ -20 oC 80 25 17,3 860 612 020 452 460 510 ¥ ¥ ¥ 0,56 998 180 ¥ 2,4 920 950 ¥ wool 1,10 394 Selfquenching Selfquenching 50 70 260 340 Flammable above 700oC flammable above 700oC Non flammable 1,4 1,4 1,2 Gas Tankers Advance Course 14.2 CALCULATION OF THERMAL TRANSFER The cooling plant on a gas carrier is dimensioned by calculated heat transfer to cargo tanks and systems when the ship is new The insulation is exposed for wear and tear and is on many gas carriers partly strong reduced When the insulation on cargo tanks gets inferior, it will have influence on the capacity of the vessel Its construction and choice of components give the cooling plant’s capacity Systematic maintenance will hold this capacity The amount of heat transferred to cargo tanks and cargo pipes dependent on the insulation’s state, surrounding temperature, heat radiation and movements Before taking a closer look at the condition around heat transfer on gas carrier, it is useful to form a picture of the heat balance It is indicated easily by following illustration: Heat is transferred from the surroundings to the cargo and systems for cargo because of the temperature difference The transmission heat to the cargo tanks, Qtr.tank is the total transferred heat to the cargo tanks with cargo, steel and insulation Transmission heat to cargo pipelines QTr.pipe is the total transferred heat to cargo in the pipes, pipe and insulation around the pipes The heat of compression, QTr.Compr is the heat supplied to the gas in the compressor and the heat of condensation QCond is the heat transferred to the seawater in the loading condenser When the cooling plant is driven to keep the temperature of the cargo constant, the heat balance is expressed as: QTr.tank + QTr.pipe + QTr.Compr = Qcond or as: Qtr.tank = Qfrom cargo tank- Qreturn to cargo tank The actual amount of heat transferred from the surroundings to a cargo tank or a system can be quantified in several methods At first we will look on how this can be 395 Gas Tankers Advance Course done onboard, how to evaluate the result and what results the eventual effectuated effort will have Most gas carriers are equipped with a graphic description of the calculated heat transfer to the cargo tanks This is a theoretical calculated description that does not necessarily give the right image of the heat transfer The older the ship is the larger probability that the calculated heat transfer DOES NOT coincide with reality Control of the reel heat transfer can be executed onboard If a loaded cargo tank is closed and isolated from the cooling plant over a period of time, the transferred heat from the surroundings can be measured The heat transfer is thereby quantified and is comparable with what it was or should be Before looking on a concrete example, it is of importance to emphasise that when accomplish such measuring, one must evaluate the results from the accuracy of the instruments As the measuring instruments onboard has normally no more accuracy than + 10%, the period of measure should be as long as possible Further it is important that if comparing repeated actual measurements, the measures has to be made at the best possible equal condition We will now take a look at different examples and what we can 14.2.1 Example A 12 year old smaller intermediate pressure gas carrier with cargo tanks is loaded with ethylene and has just moved the sailing area from Europe to SEA The captain rapport that the ship capability to cool down ethylene is perceptible inferior The cooling rate is now at the lower edge of 0,3oC per day when the temperature of the cargo gets lower than minus 102oC Inspection of the loading tanks indicate ice more than usual round all of the tank foundation and tank no.2 and has many “ice spots” Several “ice spots” than before are also observed on suction lines and condensate lines on deck The cooling plant is checked and driven at optimum, but the cooling rate is more than halved at the same pressure in proportion to when the ship sailed in European waters As the cooling plants condition is verified good and the plant is verified optimum driven, the bad cooling rate must be the result of the insulation has been worse during the years The heat transfer has probably increased gradually through the years, but the influence has not been operational visual before the ship altered sailing area It is obvious that the insulation on tank no and plus the insulation on the lines on deck are mostly reduced Repair of the insulation is necessary, but the question is which areas have most influence on the cooling capacity Clarification of this is of importance when planning and priority of the insulation repair The amount of heat transfer to the cargo tanks and pipelines must be concretised and compare with the repair costs before making the right decision Cargo tank no 2, and are the same type and size To find out how much heat that is transferred to the “bad” loading tanks in proportion to one of one of the “good”, the tanks are shut for 24 hours 396 Gas Tankers Advance Course The measuring instruments that are utilised during the experiment is calibrated and the following sketch for heat transfer is utilised and filled in: Heat transfer for Cargo tank no.2 Date/time 30.07.94 / 10:30 Cargo tank filling ratio 31.07.94 / 10:30 97,0 % Mass cargo in MT 592,837 Ambient temperature in oC o Sea water temperature in C o Average Hold space temperature in C 24 34 28 29 23 27 Ship's movement Calm sea Calm sea Weather condition Cloudy Cloudy Average liquid temperature in oC Cargo tank pressure in mBar Enthalpy liquid in kJ/kg -103,1 -102,2 80 165 27,0 29,3 As the weight of gas is relative much less than the liquid weight, only the enthalpy change of the liquid is measured The enthalpy values exist in heat technical table and the heat transfer to the cargo in tank no is calculated to: [(mass cargo x enthalpy-change) / (time in seconds)] [(592,837 x 103 x (29,3 - 27,0)) / (24 x 60 x 60)] kW = 15,8 kW Corresponding, the heat transfer is measured and calculated to the cargo in tank no and to respectively 13,9 kW and 22,2 kW In the technical description of the ship the calculated heat transfer when the ship was built to 13,1 kW for each tank at the same surrounding temperature and seawater temperature is located A comparable table can be made and indicates as follows: Calculated transfer before: Cargo tank 13,1 kW Calculated transfer now: Percentage change: 15,8 kW 20 % 397 Gas Tankers Advance Course Cargo tank 13,1 kW 13,9 kW 6% Cargo tank 13,1 kW 22,2 kW 70% The calculations confirm the observations and presumptions made before in connection with the inspection of the tank insulation It is rather no doubt that the insulation on tank no is essential much more deteriorated than the remaining two tanks The priority at en eventual re-insulation of cargo-tanks can thereby well substantiate By comparing present operational parameters for the cooling plant with earlier registered operational data, the temperature increase on the vapour from the cargo tank to compressor has increased essential Six years ago, during almost the same condition, the vapour temperature rise from –100oC to –60oC from the cargo tank to compressor Present observed temperature increase is from –100oC to –40oC The vapour is essential now more over heated than earlier A compressor’s cold capacity is expressed as: Qnetto = m x Dh Higher temperature on the vapour into the compressor involves lower density and thereby reduced amount of vapour through the compressor per time The influence of inferior insulation on the suction lines to the cooling plant will have direct influence on the cooling capacity A comparable table indicates as follows: Density on gas v/1 bars and -60 oC Density on gas v/1 bar and -40 oC Percentage reduction in density 1,7699 kg/m3 1,6181 kg/m3 9% The reduction of the suction line's insulation involves a direct reduction of the cooling rate of about 10% in this temperature area The size of the heat loss through condensate lines and liquid lines cannot be measured directly because there will at all times be and unknown and varying mixture of liquid and gas in the pipes Judgement must at all times be adjusted and from the rapport from this ship, one must assume that the insulation here has the same condition as the suction lines If this is the case, the influence of bad insulation on the condensate lines will have maximum consequence The liquid lines are only utilised in a short period (during loading and discharging), while the condensate lines are utilised during all of the cooling period The increase of the relative heat loss through the insulation on loading tank no and is now indicated Likewise is the reduction of cooling capacity because of increased heat transfer to the suction gas established There is no doubt that both conditions have influence on the operational situation on the ship But it is difficult to compare these two directly, for thereby to establish which one of them that has the 398 Gas Tankers Advance Course strongest effect on the ship’s possibility to execute the transport commission To make the data’s comparable the alteration in the heat transfer to the suction line is quantified The ship’s three loading compressors have stated a capacity of 680 m3/h at the same operational condition The alteration of the heat transfer to the suction line in proportion to earlier years is calculated to: Vapour Ethylene Temperature at inlet compressor -60 before, T1 Enthalpy, h1 572,7 Temperature at inlet compressor -40 now, T2 Enthalpy, h2 607,4 Density, r2 1,6181 Number of compressors Capacity per compressor, V 680 o C kJ/kg C o kJ/kg kg/m3 m3/h Difference in heat transfer, DF 31,8 kW DF = ((3 x V) / 3600) x r2 x (h2 - h1) One can see that about 30 kW more heat to the suction cables is supplied now compared to earlier year Simultaneously the measures indicate that the heat transfer to the cargo is about 15 kW more than earlier (One must here emphasise that the total heat transfer to the cargo tanks will be larger because the steel in the cargo tanks with insulation is supplied heat) Heat transfer to the suction vapour in the suction pipe has a direct influence on the cooling plant's capacity In addition, too high suction temperature will have a bad influence on the plant’s operational conditions by for example that the pressure pipe temperature may be too high 399 ... A-V/1 regulations 15 - 21 Gas Tankers Advance Course 02 Actual Gas Cargoes Gas Tankers Advance Course ACTUAL GAS CARGOES In the late 1920th transportation of liquefied gases in bulk started In... Chemical gases are mainly utilised in the petro-chemical industry and rubber production 11 Gas Tankers Advance Course 2.9 LNG CONDSATION PLANT FLOW DIAGRAM 12 Gas Tankers Advance Course 2.10 OIL /GAS. .. barrier is incorporated 14 Gas Tankers Advance Course 03- Cargo Compartment Systems 15 Gas Tankers Advance Course CARGO COMPARTMENT SYSTEMS Cargo compartment systems on gas carriers are divided