Ch17 inerting purging and cooling

ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL 17 17.1 Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R INERTING, PURGING AND COOLING Inerting On gas carriers certain requirements to the atmosphere in the cargo tanks have to be met The requirements may refer to the oxygen content, dew point, traces from previous cargo, carbon dioxide, carbon monoxide, sulphur, etc In order to meet some of the requirements, the cargo tanks may be inerted using the vessel's inert gas plant The inert gas on board is produced by burning off Diesel or Gas oil in the Inert gas Generator and thereby reducing the Oxygen content The produced inert gas is subsequently washed, cooled and dried, further on vessels able to produce Nitrogen, the C02 and the CO produced during the incineration is removed by filtering With inert gas it is possible to reduce the oxygen content and the dew point in the cargo tanks On vessels equipped with C02-strippers, it is also possible to avoid excessive amounts of carbon dioxide and carbon monoxide, which is normally present in the inert gas produced It may however be necessary to continue the inerting with nitrogen supplied by the shore installation, if the inert gas plant is unable to produce sufficiently clean inert gas or nitrogen Regular measurements shall be taken during the inerting, using Servomex and Teledyne oxygen analysers From these measurements, graphs shall be drawn The graphs will clearly indicate the progress of the inerting, thereby enabling early intervention, should the inerting not proceed as planned Furthermore, these graphs will indicate, the time at which the inerting can expect to be completed In order to maintain the best possible stratification the cargo tank pressure shall be kept to a minimum and all tanks shall be inerted in parallel Before Loading 1.1 lnert gas The inert gas is introduced into the cargo tanks via the loading line (in H- and J- class) and the lower spray line (in the S-class) The air is vented off through the vapour line, which shall be connected to the vent -stacks 1.2 Nitrogen If the inert gas plant is producing nitrogen, the nitrogen is introduced into the cargo tanks via the vapour and upper spray lines The nitrogen shall be heated in the nitrogen heater before being introduced into the cargo tanks /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL 1.3 Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Preheating Pipes In order to maintain the stratification as long as possible (during inerting with nitrogen only), the pipes shall be preheated with heated nitrogen before the valves to the cargo tanks are opened (not possible on the J-class) The compressors may be inerted during the heating of the pipes 1.4 Inerting Cargo Tanks in Series Inerting cargo tanks in series has proved unsuccessful and should therefore not be used unless under special circumstances, in which case, the Gas Tanker Department will instruct the vessel 1.5 Disconnecting Inert gas Pipeline After inerting is completed, the inert gas deck pipeline shall be disconnected from the supply line from the engine room, as this connection must only be established, when the plant is in operation and inert gas is supplied to deck The times for connecting and for disconnecting shall be entered in the deck log book After Discharging 2.1 lnert gas After the cargo tanks have been heated to the required temperature, inerting is commenced As the vapour from most cargoes is heavier than inert gas, the inert gas is introduced into the top of the cargo tanks through the vapour line and the upper spray line The gas is vented off through the liquid line, which shall be connected to the vent stacks 2.2 Ethylene If inerting after ethylene, the inert gas shall be introduced into the bottom of the tanks though the loading line 2.3 Nitrogen Displacement Discharge If cargo tanks are to be further inerted after Nitrogen Displacement Discharge the inerting shall be performed according to Section E 13 in these Guidelines Inerting with Nitrogen from Shore 3.1 Before Loading The nitrogen shall always be introduced into the cargo tanks via the vapour and upper spray lines In order to ensure the stratification in the cargo tanks the nitrogen shall be heated to approximately 800C at the manifold, either by using shore heater or vessel's nitrogen heater /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Nitrogen at higher temperatures may damage the insulating material on the pipelines Unheated nitrogen which may be colder than the atmosphere in the cargo tanks shall also be introduced into the cargo tanks via the vapour and upper spray lines 3.2 Preheating Pipes In order to maintain the stratification for as long as possible, the pipes shall be preheated with heated nitrogen before the valves to the cargo tanks are opened (not possible on the J-class) The compressors may be inerted during the heating of the pipes 3.3 Rate To ensure stratification, the initial rate shall be approximately 7% of the total volume to be inerted When the oxygen content in the top of the cargo tank is less than 1% the rate is gradually increased to approximately 20-25% of the total volume to be inerted If the supplied quantity is insufficient to maintain a rate of 20-25% of the volume to be inerted, the volume shall be reduced accordingly by purging a smaller number of cargo tanks at a time This rate is maintained until maximum 1% oxygen is measured in the bottom of the tanks As the cargo tanks at this stage contain mainly pure nitrogen, no stratification can be expected and the rate is now increased to approximately 50% In order to control the flow into each cargo tank, the cargo tanks may be divided into two systems The nitrogen used to inert first tank(s) shall not be reused to inert the next tank(s) When the cargo tanks are divided into two systems, the pipes shall be heated prior to changing the inerting to the second system 3.4 Stratification If the stratification is disrupted e.g by too high a rate or by changing temperatures, it can often be re-established by stopping the inerting completely for an hour or two Inerting is then resumed at low rate and high temperature The sooner turbulence is detected, the easier stratification may be re-established 3.5 Compressors and Piping If inerting of compressors and piping has not been completed during heating of pipes, the compressors and piping should be inerted simultaneously with the inerting of the cargo tanks When inerting the compressors, it is import-ant to turn these frequently as otherwise the associated piping will not be inerted 3.6 Change of Compressor Oil /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R If unable to reduce the content of previous cargo in the compressors after ventilating the oil sump, the oil should be changed The oil should not be changed on request from shippers or receivers unless so specified by the Gas Tanker Department Nitrogen from tanks must not be reused in void spaces 17.2 Purging and Cooling - Gas carriers with Cargo Deck Tanks Preamble Before loading it is necessary to replace the Inert Gas atmosphere with product gas to enable cooling of the cargo and cargo tanks At some installations replacement (Purging) of the atmosphere is performed during loading by use of the vapour return line Purging during loading shall only be performed if the temperature difference between the cargo and the cargo tanks is within the maximum allowed temperature difference (6oC on fully ref gas carriers 150C on semi-ref gas carriers) otherwise the steel may be stressed beyond design limits Where installations are not equipped with vapour return or not able to deliver vapour to the vessel it is necessary to load a purge parcel and purge the cargo tanks at sea In order to avoid calling at an additional port for loading a purge parcel, the fully ref gas carriers are equipped with deck tanks for loading and/or storing of purge parcels The larger deck tank of 210 m3 is normally used for storing LPG The smaller deck tank of 60 m3 is normally used for storing ammonia, however, if vessel is not likely to be in the ammonia trade, the tank may be used for storing additional LPG for cooling purposes The semi-ref gas carriers are equipped with a deck tank of 210 m3 only Before purging is commenced a number of conditions have to be taken into consideration, such as: Number of tanks to be loaded with the product in question Estimated product loss Do the deck tanks contain sufficient product to accomplish requirements Purging with LPG and Chemical Gases 1.1 Purge Parcel /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R In order that there is sufficient product to perform a full conditioning of the cargo tanks, as much cargo as possible should be transferred to the deck tank(s) during normal operation in the LPG trade (when changing grades etc.) If possible the large deck tank should always contain Propane 1.1.1 Loading Purge Parcel If the deck tank(s) does not contain sufficient product to perform a full purging of the cargo tanks, additional product shall be loaded either in the deck tank(s) or in a cargo tank if already conditioned 1.2 Purging 1.2.1 Transferring Liquid to the Vaporiser Deck tank pump should be used to transfer the liquid if the deck tank pressure is below bar 1.2.2 Vaporiser Starting procedure for the vaporiser is described in the LPG operation manual 1.2.2.1 Steam Supply During automatic operation the steam supply should be constant This is to prevent any loss of stem pressure which will result in liquid filling the vapourizer and adjacent piping, and in the worst case, rupturing the vapourizer 1.2.3 Flow Direction of Vapour The cargo tanks shall be purged in parallel as this will minimise the flow through each cargo tank and thus increase the stability of the stratification The additional product used for purging in parallel compared to purging in series is marginal Vapour is directed from the vaporiser to the cargo tanks through the liquid filling line Nitrogen or inert gas is removed from the cargo tanks through the vapour lines which shall be connected to the vent stacks with elbows I.2.4 Pressure In order to maintain the best possible stratification the cargo tank pressure shall be kept to a minimum 1.2.5 Monitoring and Recording It is essential to monitor the progress closely during the entire purging operation with regular /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R measurements taken to ensure uniform distribution of vapour in all tanks From these measurements, graphs shall be drawn The graphs will clearly indicate the progress of the purging, thereby enabling early intervention, should the purging not proceed as planned Furthermore, these graphs will indicate, at which time the purging can expect to be completed When purging with VCM, the Gascope must not be used as the measuring filament of the instrument will be destroyed by VCM vapours 1.2.6 Starting Compressors Before starting the compressors, it is good practice to continue the purging for 15 minutes after 100 % reading is reached 1.2.6.1 Blow off Should the atmosphere in the cargo tanks not be 100 % pure gas, the inert gas/nitrogen may be vented off via the condensers until the top of the cargo tank is 100 % gas 1.3 Cooling On completion of purging, the liquid remaining should be transferred to the liquid line It is then used to cool the cargo tanks via the upper spary line 1.3.1 Pressure If the tank pressure falls too quickly during cooling there may be insufficient pressure to complete the operation Provided there is sufficient liquid in the cargo tanks and the pressure is not above 0.25 bar, the cargo pumps should then be started and liquid circulated through the upper spary line 1.3.2 Temperature The change in temperature shall always be within the recommended limits This will be complied with by uniformly cooling the tank through the upper spray line Purging with Ammonia 2.1 Purge Parcel If the purge parcel is loaded in one of the cargo tanks, then the cargo tank shall always be purged to 100% ammonia prior to starting loading liquid 2.2 Purging Ammonia reacts with various components in the inert gas forming saline products It is therefore not allowed to purge with ammonia in tanks containing inert gas /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Due to ammonia's narrow range of flammability and high ignition temperature it does not generally constitute a fire hazard Therefore it is considered safe to purge ammonia into tanks containing fresh air However, some terminals require that the cargo tanks are inerted to a low oxygen level before loading Inerting shall in these circumstances be conducted with nitrogen Please also refer to paragraph 2.2.5 below 2.2.1 Transferring liquid to the Vaporiser As the small deck tank does not have a transfer pump, any cargo stowed in this tank should be as warm as possible Cargo should then be transferred to the cargo tank via the vaporiser to prevent any fully refrigerated product entering the tank 2.2.2 Vaporiser Please refer to paragraph 1.2.2 in this chapter 2.2.2.1 Steam Supply Please refer to paragraph 1.2.2.1 in this chapter 2.2.3 Flow Direction of Vapour The vapour is directed from the vaporiser into the cargo tanks via the vapour line and the upper spray line The manual valve between the liquid line and the condensate line shall be closed and prior to initiating purging it shall be ensured that this valve is closed securely Air or nitrogen is displaced from the tanks through the liquid filling line, which shall be connected to the vent stacks with elbows 2.2.4 Monitoring and Recording Please also refer to paragraph 1.2.5 in this chapter When purging with ammonia, barrett-testers are used to measure the ammonia concentration These testers use fresh water and not require the use of any special chemicals 2.2.5 Stress Corrosion Cracks There is a possibility on the fully ref vessels of stress corrosion cracks when carrying ammonia is present According to experience stress corrosion cracking may occur in cargo tanks containing ammonia, depending on type and treatment of steel /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R To minimise the risk of stress corrosion cracking, the following guidelines issued by SIGTTO shall be followed: 2.2.5.1 Temperature The carriage temperature shall be kept below -300C 2.2.5.2 Water content Water will act as an inhibitor against stress corrosion and it is recommended by shore installations that the ammonia should contain a minimum 0.2% by weight of water If unable to receive confirmation that the water content is a minimum 0.2%, the Nautical Department shall be contacted prior to commencing purging/loading ammonia 2.2.5.3 Oxygen content Ammonia does not cause stress corrosion cracking unless it is contaminated by oxygen When manufactured ammonia has an extremely low oxygen content, less than ppm by weight During cool down an oxygen content of 0.5% by volume is generally considered safe However, the oxygen content should be less than 0.025% by volume prior to introducing liquid to the cargo tanks 2.2.6 Starting Compressors As measurements are not always absolutely accurate, it is a good practice to continue the purging for 15 to 30 minutes after 100 % ammonia concentration in the bottom of the cargo tanks has been measured before starting the compressors 2.2.6.1 Blow-off During the first few days of operation of the reliquefaction plant, the incondensible gases which accumulate in the condensers, i.e the air remaining in the cargo tanks, shall be vented off 2.3 Cooling Please refer to paragraph 1.3 in this chapter 2.4 Calculation of the Time Required for Precooling The steel mass of the cargo tanks on a 35,000 m3 fully ref gas carrier is estimated to be 2698 metric tons The steel has to be cooled from + 250C to - 400C The cargo tanks are purged with 60 /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R mt of propane vapour at + 250C The following quantity of heat must be substracted: From the steel mass: 2,698,000 kg x 0.12 x (25 + 40) = 21,044,400 Kcal 0.12 is the specific heat of the steel From the propane vapour: 60.000 x (195.9- 178.9) = 1.020.000 Kcal 195.9 and 178.9 are the enthalpy of the propane vapour at + 250C and - 400C Total capacity of reliquefaction plant; 1,035,000 Kcal Time required: about 22 hours The above calculation not take into consideration the heat flow through the tanks and should be regarded as a guidance only 17.3 Purging and Cooling - Gas Carriers without Cargo Deck Tanks Purging with LPG and Chemical Gases 1.1 Loading with Vapour Return to Shore At some installations replacement of the atmosphere (Purging) is performed during loading by use of the vapour return line 1.1.1 Product Temperature Purging during loading shall only be performed if the difference between the cargo temperature and the cargo tank temperature is within the maximum allowed (iSoC) as the steel otherwise may be stressed beyond the construction limits If the difference between the cargo temperature and the cargo tank temperature is outside the maximum allowed limit, and it is impossible to load via the cargo heater and heat the cargo to within the maximum allowed limit, the cargo tanks shall be purged with vapour and subsequently cooled down prior to commencing loading liquid 1.2 Purging without Vapour Return to Shore Where installations are not equipped with vapour return it is necessary to load a purge parcel and purge the cargo tanks at sea 1.2.1 Loading Purge Parcel The purge parcel shall be loaded into one of the cargo tanks and a number of conditions have to be taken into consideration when deciding the quantity of the purge parcel to be loaded: /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL - Number of tanks to be loaded with the product in question - Estimated product loss during purging - Do tanks need to be fully refrigerated before loading Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R It is recommended to load the purge parcel in cargo tank No I as this cargo tank is the smallest and thus easier to prepare for loading the purge parcel 1.2.1.1 Purging prior to loading Purge Parcel Prior to loading the purge parcel it may be necessary to purge the cargo tank into which the purge parcel is to be loaded, depending on the temperature of the purge parcel If the temperature difference between the purge parcel and the cargo tanks is outside the maximum allowed limit, the cargo tank must be purged with vapour and subsequently cooled down prior to commencing loading the purge parcel 1.2.1.2 Loading via Cargo Heater If the sea water temperature is low and sufficient energy to evaporate the purge parcel during purging may not be extracted from the cargo heater, it may be advantageous to load the purge parcel via the cargo heater and thereby add heat to the purge parcel, which in turn will increase the efficiency of the subsequent purging of the remaining cargo tanks If the cargo tanks require cooling prior to loading, the purge parcel should not be loaded through the cargo heater, unless the sea water temperature is extremely low 1.3 Purging The cargo tanks shall be purged in parallel as this will minimise the flow through each cargo tank and thus increase the stability of the stratification The additional product used for purging in parallel compared to purging in series is marginal 1.3.1 Transferring Liquid to Cargo heater The deep well pump is started with the recirculation valve on the liquid line open 1.3.1.1 Transferring via Condensate Line The liquid is pumped through the condensate line to the dome on cargo tank No and from here via a flexible hose from the stripping line to the liquid drain in the bottom of the cargo heater /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 10 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R The flow is controlled by the manually operated valve close to the pneumatic valve 1C361 on the stripping line, which should only be opened slightly 1.3.1.2 Transferring via Liquid Line The liquid is pumped via the liquid line through the cargo heater in reverse order compared to normal flow direction The flow is controlled by the manually operated valve at the cargo heater just before the liquid enters the cargo heater The valve shall only be cracked open 1.3.2 Vaporising Liquid It is essential that all liquid is vaporised in order to avoid the cargo heater being filled with liquid, which will obstruct the vaporisation and slow down the entire operation Further if liquid is allowed to pass the cargo heater and enter the bottom of the cargo tanks the cargo tanks may be damaged 1.3.3 Flow Direction of Vapour Vapour is directed from the heat exchanger to the cargo tanks through the liquid lines entering the tanks partly through the liquid line and partly through lower purge line for distributing the vapour throughout the length of the tank The pneumatic valves on the condensate line on the domes shall be closed if the liquid is transferred via the condensate line Nitrogen or inert gas is removed from the cargo tanks through the vapour lines which shall be connected to the vent stacks with elbows 1.3.3.1 Cargo Tank No Starboard - Condensate Line If using the Condensate Line for transferring liquid to the cargo heater, vapour to cargo tank No Starboard can only be directed through the liquid line, as the condensate line is used to supply liquid from cargo tank No I to the cargo heater 1.3.4 Pressure In order to maintain the best possible stratification the cargo tank pressure shall be kept to a minimum 1.3.5 Monitoring and Recording It is essential to monitor the progress closely during the entire purging and regular measurements shall be taken during the purging to ensure uniform distribution of vapour in all tanks /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 11 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R From these measurements, graphs shall be drawn The graphs will clearly indicate the progress of the purging, thereby enabling early intervention, should the purging not proceed as planned Furthermore, these graphs will indicate, the time at which the purging can expect to be completed When purging with VCM, Gascope must not be used as the measuring filament of the instrument is destroyed by VCM vapours 1.3.6 Starting Compressors As measurements are not always absolutely accurate, it is good practice to continue the purging for 15 minutes after 100 % gas concentration has been measured in the top of the cargo tanks before starting the compressors 1.3.6.1 Blow off Should the atmosphere in the cargo tanks not be completely pure gas, the inert gas/nitrogen may be vented off from top of condensers until the atmosphere is 100 % gas in the cargo tanks 1.4 Cooling When purging is completed, the remaining liquid is transferred from cargo tank No I to other cargo tanks to be cooled The liquid is transferred via the upper spray line and start-up line as this will assist in cooling the entire tank more evenly 1.4.1 Pressure Transferring the liquid via the upper spray line and the start-up line will also assist in increasing the tank pressure, which will make the compressors more efficient As the liquid is cooled down, the tank pressure will decrease, and if the tanktop still requires cooling, then the deepwell pumps should again be started and the liquid recirculated through the upper spray line and the start-up line 1.4.2 Temperature The change in temperature shall always be within the design limit of the cargo tanks Further the cooling shall be performed as uniform throughout the cargo tank as possible Purging with Ammonia 2.1 Loading with Vapour Return to Shore The cargo tanks shall always be purged to 100 % ammonia prior to starting loading liquid If the /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 12 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R shore is unable to deliver vapour, then liquid is directed very slowly to the cargo heater where all liquid shall be vaporised The vapour is directed to the cargo tanks via the upper spray line and the air/nitrogen is vented of via the lower spray lines The lower spray lines shall be connected to the vapour line 2.2 Purging without Vapour Return to Shore Please refer to paragraph 1.2 and 1.2.1 in this chapter 2.2.1 Purging prior to loading Purge Parcel The cargo tank into which the purge parcel is loaded shall always be purged to 100 % ammonia prior to starting loading liquid If shore is unable to deliver vapour, then liquid is directed very slowly to the cargo heater where all liquid shall be vaporised The vapour is directed to the cargo tanks via the upper spray line 2.2.2 Loading via Cargo Heater Please refer to paragraph 1.2.1.2 in this chapter 2.3 Purging Ammonia reacts with various components in the inert gas forming saline products It is therefore not allowed to purge with ammonia in tanks containing inert gas Due to ammonia's narrow range of flammability and high ignition temperature it does not generally constitute a fire hazard and it is thus considered safe to purge ammonia into tanks containing air Some terminals do, however, require that the cargo tanks are inerted to a low oxygen level before loading Inerting shall in these circumstances be conducted with nitrogen 2.3.1 Transferring liquid to Cargo Heater Please refer to paragraph 1.3.1, 1.3.1.1 and 1.3.1.2 in this chapter 2.3.2 Vaporising Liquid As the vapour is lighter than air and nitrogen, it shall be directed into the top of the cargo tanks It is extremely important that all liquid is vaporised in the cargo heater and that no liquid is allowed to enter the cargo tanks to be purged, as this will destroy the stratification and risk damaging the cargo tanks 2.3.3 Flow Direction of Vapour /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 13 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R The vapour is directed from the cargo heater to the cargo tank domes via the liquid line On the domes the vapour is directed into the cargo tanks via the upper spray line The manual valve on the liquid loading line shall be closed and prior to initiating purging it shall be ensured that this valve is securely closed Air or nitrogen is removed from the tanks through the lower spray lines The lower spray lines shall be connected to the vapour lines which in turn shall be connected to the ventstacks with elbows The manual valves on the vapour line to the cargo tanks on the domes shall be closed and prior to initiating purging it shall be ensured that these valves are closing tightly 2.3.4 Pressure Please refer to paragraph 1.3.4 in this chapter 2.3.5 Monitoring and Recording Please also refer to paragraph 1.3.5 in this chapter When purging with ammonia, barrett-testers are used for control of the ammonia concentration Measuring ammonia, barrett-testers are working satisfactorily with fresh water, as I litre of water can absorb approximately 800 litres of ammonia vapours It is therefore not necessary to use special chemicals 2.3.6 Starting Compressors As measurements are not always absolutely accurate, it is good practice to continue the purging for 15 minutes after 100 % ammonia concentration has been measured in the bottom of the cargo tanks before starting the compressors 2.3.6.1 Blow-off Should the atmosphere in the cargo tanks not be completely pure ammonia, the air may be vented off from top of condensers until the atmosphere is 100 % ammonia in the cargo tanks 2.4 Cooling Please refer to paragraph 1.4, 1.4.1 and 1.4.2 in this chapter 2.5 Calculation of the Time Required for Precooling The steel mass of the cargo tanks on a 15,000 m3 semi-ref gas carrier is estimated to be 1464 metric tons The steel has to be cooled from + 250C to - 400C The cargo tanks are purged with 30 mt of propane vapour at + 250C The following quantity of heat must be subtracted: From the steel mass: /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 14 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R 1,464,000kg x 0.12 x (25 + 40) = 11,419,200 Kcal 0.12 is the specific heat of the steel From the propane vapour: 30,000 x (195.9 - 178.9) = 510,000 Kcal 195.9 and 178.9 are the enthalpy of the propane vapour at + 250C and -400C Capacity of each cargo compressor is 155,000 Kcal Time required: about 26 hours The above calculation not take into consideration the heat flow through the tanks and should be regarded as a guidance only 17.4 Vacuum Prevention in Cargo Tanks The cargo tanks in the semi-refrigerated vessels are constructed to sustain a vacuum of 0.3 Bar while the cargo tanks in the fully refrigerated vessels can only sustain a vacuum of 0.1 Bar If the pressure in a cargo tank decreases to minus 0.1 Bar in the semi-refrigerated vessels and to minus 0.05 Bar in the fully refrigerated vessels an alarm function will activate a warning buzzer and light signal Further decompression to a vacuum of 0.2 Bar (on the fully refrigerated vessels 0.07 Bar) will automatically shut-down the cargo pumps and compressors During normal working conditions the tanks are thus protected against structural damage caused by excessive vacuum In the event of abnormal circumstances the alarm system and built-in safety measures may be unable to prevent development of a damaging vacuum Thoughtless action may result in a violent chemical-physical reaction which will be impossible to halt before reaching critical limits Ammonia If water is injected into a tank containing a rich ammonia atmosphere it will cause an implosion Unless the manhole is fully open, the energy of the implosion will make the tank collapse If flexible hoses laid out over the stern of the vessel are used when venting off ammonia vapours, it must be ascertained that these hoses are not lowered below the water surface as ammonia has great affinity with water and may result in water being sucked up into the hose and into the tanks where more ammonia vapours will be dissolved in the water The tank pressure is thereby reduced dramatically and the tanks may collapse LPG When purging with butane vapour into a tank containing fully cooled propane, the butane vapour may condensate due to the low temperature The condensation may under special circumstances continue until a physical balance is achieved at a vacuum of 0.5 Bar, which pressure is so low that /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 15 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R risk of damaging the tank exists Prior to initiating any operation involving change-over between products, all possible risks shall be evaluated in the stage of planning in order to avoid any damage 17.5 Purifying of Atmosphere before cooling When initiating cooling of cargoes loaded in a nitrogen atmosphere, nitrogen will most often be present in too high a concentration for cooling to be possible without removing the nitrogen At some locations the cargo receivers furthermore require that the ship amves with the cargo in a pure tank atmosphere, i e with 100 % vapour without nitrogen Removing of the nitrogen can be carried out by different methods: By use of refrigeration plant When the refrigeration plant is in operation, the nitrogen can be removed through the valve on top of the condenser The valve is opened when the pressure in the condenser is rising above the normal operating pressure and partially throttled when the pressure decreases to normal operating pressure The valve is adjusted until the pressure is maintained close to the normal operating pressure The temperature in the compressor must however never exceed the allowed values, i e 600C for butadiene or 900C for VCM In order to minimise the loss of cargo, the gas concentration in the cargo tanks must be checked during the operation, using the Gascope model LV When 100 % concentration has been reached, blow off from the condenser is stopped If VCM is carried, the Gascope must not be used as the measuring filament will be destroyed by the VCM vapours Riken interferometer may be used, but only to a concentration of 30 %, which makes this instrument less suitable for this purpose If the nitrogen content is too high for the compressors to operate even with the valve on the condenser fully opened, some of the nitrogen may initially be returned to the tank through the bypass valve Blow off from the tanks If the tanks are only partially loaded, another method can be to shift the cargo from one tank to another until approximately 98 % full The safety relief valves are fitted with the highest set-point The vapour line between the tanks concerned is kept open in order to equalise the pressure If more tanks are loaded with same cargo, pumping from the partly emptied tank is continued to these tanks until the tanks are 98 % filled (97.5 % filled in the J-class), while the vapour line is kept open between the tanks concerned Then the vapour line valves are closed, and the cargo is returned to the tank containing a mixture /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 16 of 17 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R of product-vapour and nitrogen in the vapour phase until the pressure increases to appear 0.2 bar below the set point Then the vapour line from this tank is opened to the vent mast and the pressure is reduced to a level corresponding with the temperature of the cargo The transfer of cargo is continued until the tank is appr 98% full Reduction of the pressure should be carried out gradually in order to minimise the loss of cargo, which will occur due to turbulence in the tank during blow off Purifying of the tank atmosphere by this method is not complete The remaining nitrogen is removed during operation of the refrigeration plant by blowing off from the top of the condensers /storage1/vhost/convert.123doc.vn/data_temp/document/oft1554800588-1959494-15548005889199/oft1554800588.doc Page 17 of 17 ... consideration the heat flow through the tanks and should be regarded as a guidance only 17.3 Purging and Cooling - Gas Carriers without Cargo Deck Tanks Purging with LPG and Chemical Gases 1.1 Loading with... 17.2 Purging and Cooling - Gas carriers with Cargo Deck Tanks Preamble Before loading it is necessary to replace the Inert Gas atmosphere with product gas to enable cooling of the cargo and cargo... hour or two Inerting is then resumed at low rate and high temperature The sooner turbulence is detected, the easier stratification may be re-established 3.5 Compressors and Piping If inerting of