Ch15 tank cleaning

ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL 15 15.1 Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R TANK CLEANING Changing of Grades Prior to loading it must be verified that the tanks are clean enough to receive the cargo without any risk of obtaining an out of specification product Tank cleaning is a significant and important part of the gas operations and may require a considerable number of working hours It is essential that the operation is carefully planned and an attentive monitoring and follow-up is performed in order to ensure the desired degree of cleanliness and effectiveness is achieved The tank cleaning procedure should not be limited to the removal of liquid, vapour or inhibitor from the last cargo only If a cargo tank during inspection is found to be particularly rusty it should be hard brushed or cleaned by high pressure water wash all over where accessible before the methanol wash The table Changing of Grades contains by means of codes guidelines for the minimum requirements with maximum allowable content of last cargo Normally the vessel will receive the charterer's requirements when changing grade(s) in the fixture telex The following codes have been used: 1: As stated in the Guidelines for CARGO HANDLING 2: Visual inspection by ship's crew 3: Tanks heated to loading temperature of the next cargo 4: Tanks heated to liquid free and pressure 1.5 bar if possible 5: Methanol wash 6: Purging with nitrogen only 7: Purging with inertgas or nitrogen 8: Gassing up with vapour next cargo 9: Charterer's additional requirements as advised by operations department 10: Not permitted The visual inspection may require some or all of the following: Tanks liquid free by means of heating of tanks to ambient temperature or by stripping by means of pumps or ejector - purging with lnertgas to below LEL -purging with fresh air to below TLV and approximately 21% oxygen - vacuum cleaning - spot cleaning with methanol Propylene Oxide must not be loaded into tanks that have held Ammonia on the last three previous loads If a grade not mentioned is to be loaded thoroughly, instructions will be received from ZMAL /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL 15.2 Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Gas Freeing of Cargo Tanks Preamble During the initial phase of tank cleaning when inerting is commenced a substantial amount of vapour is released to the atmosphere Objective To avoid harmful concentrations of vapour that are released during tank cleaning from entering the accommodation and engine room Lining Up The Cargo Tanks to be gas freed and connected to the forward vent stacks if possible 1.1 Connecting the Inert Gas Line The Inert Gas Line shall only be connected to the Cargo Tanks when the tanks are ready for inerting and the Inert Gas Plant is ready to produce inert gas of the requested quality The time for connecting and disconnecting the Inert Gas Line to the Cargo Tanks shall be noted in the vessel's official log book Venting Off Vapour The gas concentration in the cargo tanks is highest during the initial hours of venting off and will gradually decrease as the atmosphere in the cargo tanks is diluted by inert gas (or air if cleaning after ammonia.) To ensure that the vapour vented off during the initial hours is not drifting towards the accommodation and air intakes to the engine room it may be necessary: In port Terminal or port regulations regarding venting or purging of cargo vapour must be observed Such regulations may require that vented cargo vapours should be led to a flare, or vent stack or for use in the shore plant If the cargo vapour is not led ashore advance notification should be given to the port authorities and permission obtained before starting the operation If it is required to reduce the nitrogen pressure of a cargo tank containing cargo such nitrogen /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R should be regarded as cargo vapour and port regulations regarding venting cargo vapour must be observed En route to alter the course steered if the relative wind direction is unfavourable, at anchor to heave up the anchor and steam to find most favourable relative wind direction If vapour in harmful concentrations cannot be avoided at the accommodation and engine room air intakes, it may be necessary to postpone the tank cleaning in which case the Profit Centre shall be advised immediately Re-circulating Air-Conditioning The air-conditioning plant should be changed to recirculating during the initial hours of venting off vapour Personnel working in the engine room shall be informed when venting off vapour is commenced As the air intakes to the engine room cannot be completely closed during tank cleaning, the engineers will often be the first to notice any detectable gas Flexible Hoses It should be avoided to use flexible hoses to vent off vapour over the stern of the vessel as the air flow around the aft part of the vessel is very turbulent and the vapour therefore may enter the accommodation and engine room However, prevailing weather conditions and the ship's destination may favour the use of flexible hoses for venting cargo tanks over the stern Venting Ammonia Usung Flexible Hoses Ammonia will when in contact with water generate a intense heat which is able to melt the hose and ignite any remains of previous cargoes clinging to the inside of the hoses This has been experienced in a gas carrier when venting after the carriage of ammonia The generated heat melted the hose connected to the manifold immediately after opening the manifold valve and ignited the polymerised butadiene residues As a means to prevent dangerous reactions the following precautions should be observed Before being used the hoses should be checked for cleanliness from remains of other cargoes from previous tank cleaning operations It may be necessary to place the hoses on deck and leave them expanded for a few hours in order to allow the hoses to attain ambient temperature and to ensure the hoses are dry After being used the hoses should be cleaned Prior being collapsed and stored the hoses should be left on the deck in expanded condition for some time in order to enabling them to be thoroughly dry /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R 15.3 High Pressure Water Wash After the carriage of cargoes containing inhibitor (VCM, CC4, Butadiene) it may be necessary to wash the tanks, filters, valves and pipes with water Before the high pressure water wash is carried out the tanks must be ventilated to not less than 20 percent oxygen and zero percent LEL The KEW high pressure pump placed at the engine entrance aft is connected to the special high pressure hoses and operating handle For water cleaning the following equipment is employed: piece KEW high pressure pump piece sewage nozzle x 80 m half inch pressure hose piece transfer pump Quick-lock connections and gaskets for above mentioned equipment The high pressure pump which is not intrinsically safe and therefore not safe for use on deck must be placed in engine opening aft Procedure for cleaning with high pressure water A The manifold valves should be high pressure washed with water The valves shall be opened and closed several times enabling residues to be removed from the ball valves B The filters are to be opened both at the top and the bottom and the filter elements to be removed and brushed with methanol prior the high pressure water wash The filter housing should be brushed with methanol and thereafter washed with water as well All surfaces must be clean and without discoloration C The cargo pipes must be inspected as far as possible and the necessity of cleaning evaluated Only if cleaning is needed the sewage nozzle should be fitted to the high pressure hose and the hose pushed as far as possible into the pipe connections Care should be taken that water cannot come into areas where it cannot be removed If water may enter such areas methanol should be used for cleaning The high pressure washing is initiated in the tank dome and all pipes and reachable structures should be thoroughly washed The tank bottom and tank sides are to be high pressure water washed as high up as the equipment permits In order to obtain an efficient cleaning of the tank steel the nozzle must be kept at a distance of approximately 10 cm from the tank wall The washing is carried out ensuring that no part of the tank wall is left /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R untouched as this will leave dark stripes on the steel The water is removed by using the membrane pump E When the high pressure water wash has been completed, the tank(s) should be dried immediately using dry air from the inert gas blower via the spray lines or with inert gas if a subsequent methanol wash is required 15.4 Methanol Wash with Portable Sprinklers After the carriage of cargoes containing inhibitor (VCM, CC4, Butadiene) it may be necessary to wash the tanks, filters, and pipes with methanol Before being washed with methanol the tanks must be inerted to below LEL and ventilated with fresh air to below the TLV value for the cargo carried The tank is to be vacuum cleaned prior to the commencement of the methanol wash Special attention is to be paid to the slush bulkhead which may entrap inhibitor or polymerised cargo residues and loose rust The valves and filters at the manifold should be cleaned as mentioned in the procedure for cleaning with high pressure water wash If the initial inspections shows the tank to be particularly dirty or rusty it should be cleaned by high pressure water wash or hard brushed all over where accessible before the methanol wash sprinklers are to be fitted on the lower spray line and if available a 5th sprinkler should be rigged at the top platform The 5th sprinkler to be used to clean the tank dome and vertical pipes in the tank Each sprinkler is fitted with a hose leading to the portable manifold, which is placed on deck close to the tank to be washed The portable manifold is connected to the methanol pump in the compressor room using a inch high pressure hose Membrane pump fitted with suction is placed at the pump sump and secured Additionally, discharge hose and air hose are connected to the pump and properly secured The air valve on the pump is opened fully and secured in order to make it possible to operate the pump at the air valve on deck In order to avoid the blocking of rust in the sprinklers or hoses the portable manifold should be soaked in a bucket of methanol for a period of not less than 12 hours prior being used Test of the manifold should be done by connecting it to the fresh water supply at the methanol pump and using the methanol pump toflush the manifold by opening one valve at a time to try to dislodge any flakes of rust that may clog the sprinklers whilst they are in use down the tanks If the methanol pump cannot create a high enough pressure half inch ball valve should be connected to the manifold at the sprinkler connections to regulate the pressure travelling through the manifold When the test of the manifold is completed the sprinklers should be connected to the manifold and their free rotation tested The tanks must be inerted to max 5.5 percent Oxygen prior to the commencement of the /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R methanol wash The cleaning equipment For the methanol wash the following equipment is employed: Vessel's fixed methanol pump 75 m inch high pressure hose I portable manifold for pieces of half inch hoses pieces 30 m half inch hoses pieces of sprinklers transfer pump Procedure for methanol wash Inerting to the tank to be ceased Using the fixed methanol pump the methanol spraying is conducted for a period of minutes Sprinklers are opened simultaneously in order to achieve a nozzle pressure of approximately 12 bars and thereby sufficient range If a fifth sprinkler is fitted spraying through this is conducted for a period of 1.5 minutes After a total spraying time of minutes (5.5 minutes if the fifth sprinkler is fitted) the methanol pump is stopped and the tank is left for a period of not less than 60 minutes The methanol's solvent effect is thereby utilized on the remains of inhibitor A sample of the methanol should be taken and kept During the waiting time the other tanks are sprayed with methanol using the same procedure and The sample pipes are flushed with methanol This is done by connecting the permanently installed methanol pipe above the dome to the sample cocks using a flexible hose and pump methanol through the pipes The sample cocks placed at the tank dome and at the pump discharge pipe are to be dismantled and inhibitor or polymerized cargo residues removed At least one non return valve at the pump discharge pipe must be dismantled and inspected The necessity of cleaning and dismantling the other non return valves is then to be evaluated The fresh water line on deck is connected to the methanol pump through the pump's suction strainer using a 3/4 inch air hose with claw connection /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R The tank is now sprayed with fresh water through I nozzle at a time for a period of two minutes The necessity of resuming the inerting should be evaluated Check that the pump sump is filled with liquid Start the deepwell pump briefly (a few seconds only) and flush the discharge line from the pump Tank is now to be stripped for liquid using the membrane pump To facilitate the stripping of the tanks a limited positive trim of approximately 0.5 m should be achieved In tank and liquid will be placed aft of the pump only If a positive trim of less than 0.4 m is obtained liquid is placed forward of the pump sump increasing to the whole length of the tank when at even keel In tank I and liquid will be placed in or around the pump sump if a positive trim up to 1.0 m is obtained The stripping is to be continued throughout step to 12 keeping the pump sump as empty as possible The tank is now sprayed with fresh water through max nozzles at a time The spraying is to be continued until a clear sample of water is obtained 10 The tank is now sprayed with methanol using the same procedure as under step but this time limited to a period of 1.5 minutes A sample of the methanol pumped up is to be taken Together with the sample taken under step an evidence of the cleaning efficiency is hereby achieved The samples should be presented to the surveyor inspecting the tank(s) prior to the loading The sample should be kept until the discharge of the cargo loaded is completed 11 The tank is now sprayed with fresh water as mentioned under step The spraying to be continued until a clear sample of water achieved and for a period of not less than 10 minutes in order to clear all hoses and pump for methanol 12 Step 10 and 11 are to be repeated until a clear sample of methanol has been achieved 13 After the final spraying which must be with water the tanks are dried out using fresh air from the Inert gas blower During the final inspection the necessity to vacuum clean, spot cleaning with methanol or high pressure water wash is to be evaluated Tank flannel should not be used for a possible spot cleaning as its fluffing properties will create a downy surface which is very difficult to remove Hazard and health data for Methanol Methanol is a flammable liquid which at 'room' temperatures gives off a vapour which is both toxic and when mixed with air, explosive within certain limits If liquid is spilled on clothing and allowed to remain it may cause smarting and reddening of the skin Vapour may be absorbed through skin and inhalation may cause dizziness, unconsciousness, and sighing respiration The TLV value is 200 ppm /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Exposure procedure: remove victim to fresh air Remove contaminated clothing Flush affected skin or eye gently with water for at least 15 minutes If breathing has stopped or is weak or irregular give artificial resuscitation or oxygen as necessary Obtain medical advice or assistance as soon as possible Safety measures It is not allowed to enter the tank without chemical suit and full breathing protection if the concentration exceeds 200 ppm The methanol concentration is measured by Drager Multigas detector or Drager Tubes The methanol concentration must be well below 200 ppm on arrival at the loading terminal since the terminal staff will not enter the tanks if breathing apparatus has to be worn 15.5 Methanol Wash With Fixing Machines After carriage of cargoes containing inhibitor it may be necessary to clean tanks, filters and pipes with methanol It may also be stipulated in the charter party that cleaning with methanol is required prior to loading Propylene Oxide Before methanol wash is commenced, tanks and lines must be inerted For vessels equipped with fixed washing machines, it may not be required to enter tanks prior to commencement of methanol wash It is however, important that all cargo including vapour is removed from tanks by the use of inert gas prior to commencing cleaning When inerting is completed, tanks are pressurised to a minimum of 1.5 Bars overpressure by use of inert gas The ejector will not be effective until at least 1.25 Bars overpressure has been reached Spool pieces are inserted from methanol wash system to tanks which are to be washed as per piping diagram The methanol pump is started for five minutes and tank washed, one tank half at a time Discharge pressure from the methanol pump should be 8.0 Bar After an initial wash of five minutes, the cargo tank must be drained by use of the methanol driven ejector, which is also used throughout the entire washing The cargo tank is washed four times in total, each time for approximately five minutes If cargo tanks are expected to be very dirty, the cycles may be repeated During the second washing period, the cargo pump is started when the methanol level reaches approximately centimetres The methanol is recirculated through the upper spray line and the filling line Washing with methanol machines is continued until just before the cargo pump is stopped /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R During final washing period, the cargo pump is again started, and the methanol is then transferred to the manifold, from where it is drained to the methanol storage tank using the ejector Flushing of the manifold as above should be repeated until satisfactory cleanliness of pipes is reached The colour of methanol from the manifold is checked by draining some methanol to the drip tray During washing and stripping of cargo tanks, the vessel should maintain a stern trim of at least one metre When cleaning is completed, the cargo tanks are ventilated to fresh air until atmosphere less than 200 PPM methanol before the tanks are entered for inspection, and eventual manual cleaning/sweeping or vacuuming 15.6 Product Washing With Fixing Machines In order to optimise the cleaning of the cargo tanks, some gas carriers are equipped with fixed washing machines, which apart from being used for methanol washing are also capable of washing with product Product washing shall be conducted if cargo tanks are expected to be extremely dirty, or when carrying inhibited products which are suspected of leaving deposits in the cargo tanks i.e Butadiene Before product washing is carried out, permission from the Nautical Department must be obtained The Nautical Department will liase with Gas Tanker Department before issuing permission Product washing must under no circumstance be performed when carrying Propylene Oxide Procedure 1.1 Preparation The specially manufactured 1.5 inch pipes connecting the washing machines to each individual cargo tank system shall be inserted on all tanks to be product washed prior to arrival at the discharge port 1.2 Planning The discharge operation should be planned so as to complete individual tanks at intervals sufficient to product wash each tank without reducing the overall discharge rate 1.3 Commencement The washing is commenced when the surface of the cargo is below the washing machines at approximately metres above the bottom of the tanks Washing is per-formed by recirculating /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R some of the liquid through the washing machines via the specially manufactured pipes 1.4 Low Discharge Pressure In order to maintain sufficient pressure to the washing machines, if the discharge pressure is below Bars, the pneumatic discharge valve is closed during the product washing The valves to the washing machines shall be opened prior to closing the pneumatic discharge valve In order not to damage the deepwell pump, the recirculation valve on the liquid line shall be opened slightly during washing with the closed pneumatic valve When one cycle is completed, the discharging is resumed in the normal way Liquid will remain in the bottom of the cargo tanks, when washing with the closed pneumatic discharge valve, and the bottom of the cargo tank will not be washed 1.5 High Discharge rate If the discharge rate is too high to complete one cycle with washing machines (approximately 10 minutes) from the time the liquid surface is below the washing machines until the cargo tank is dis-charged, then the discharge rate from the individual cargo tank to be washed shall be reduced during the washing by opening the recirculation valve on the liquid line slightly Discharge should be continued from the other cargo tanks at full rate 1.6 Washing of the tank bottom In order to optimise washing of the cargo tank bottom, the washing should, if possible, continue until the final stripping of the cargo tank is commenced 1.7 Vapour remaining on board Due consideration should be given to the amount of vapours produced when the cooler liquid comes in contact with the warmer tank walls Product washing shall thus not be performed if the bill of lading figure is higher than the cargo quantity actually on board, unless it is possible to recondense the vapour after completion of discharging the liquid 1.8 Inhibited cargoes If product washing is performed with a cargo containing inhibitor, it shall always be followed by methanol washing in order to avoid any inhibitor being left in the washing system 1.9 Nitrogen Displacement Discharge If efficient discharge with introduction of nitrogen to cargo tanks is used - according to Section E 13 in these guidelines - product washing shall not be per-formed 15.7 Tank Cleaning After Inhibited Cargoes On a number of occasions after cleaning of inhibited cargoes it has been observed that /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 10 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R remains from polymerised product have been left in the tanks This is extremely difficult to remove It is very likely that polymerisation - which takes place in the presence of oxygen - has occurred during the initial stages of ventilating with atmospheric air following the inerting to LEL At this time butadiene or VCM vapours are present simultaneously with the introduction of oxygen In order to avoid or at least reduce the amount of polymerised product the ventilation should not be started until no gas can be detected in the tank atmosphere, using Riken interferometer or Drager test tubes It should be noted that the interferometer will also react on the inert gas showing a concentration of about 1.5% Furthermore the venting off from each tank must only be conducted through one line, and test pipes should only be opened during checks on the tank atmosphere, until an oxygen concentration of 20.8% has been reached, thus ensuring that the risk of product polymerising in the spray lines and test pipes will be minimal Tank Cleaning and Ventilating after Ammonia on the Semi–Refrigerated Vessels When tank cleaning after an ammonia cargo, special measures are required Evaporation of Liquid Heel The compressors are used for heating up the liquid heel (which then evaporates), the ammonia vapours, and the tank steel It is very important to heat up over the whole length of the tank, but heating of the pump sump should be emphasised As a rule of thumb, this sump should be heated for an additional I to hours after all the liquid heel has been evaporated in order to prevent re-condensation of ammonia vapours against the cold tank steel Experience has shown that it is advantageous to heat up to approximately + 200C The extra time consumed for heating in comparison to heating to a lower temperature is more than saved during the following phases of tank cleaning For reasons mentioned later in "Sub-cooling", heating to temperatures lower than + 200C should be avoided in order to prevent possible damage to the tank steel Removing the Overpressure The overpressure is reduced during heating in the usual manner through the vent masts under close supervision of the thermometers, especially thermometers in the pump sump /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 11 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Experience shows that ammonia vapours can enter the accommodation aided by the vacuum which often builds up aft of the superstructure When possible the forward masts should be used, as by using these, it is easier to compensate for wind driving the gas towards the accommodation If possible, alteration from the planned course is often a practical method to avoid inconveniences caused by the ammonia vapours Ventilating It is a special requirement for ammonia that inert gas must not be used to replace it, as various components in the inert gas react with ammonia vapours forming saline products, which block up valves, pipes, etc As ammonia has explosion limits between 16 and 25 percent it is difficult to ignite, and also with a low heating value, it can be considered non flammable under normal circumstances These facts make it possible to safely ventilate the tanks without any intermediate inerting However, before ventilating is commenced it must be ensured, that the tanks are sufficiently heated, as the freeing of ammonia from the tank steel is restrained considerably at low temperatures The amount of ammonia and water contents remaining in the tank will thereby be tied to the tank walls and more time is required for tank cleaning At a tank temperature of +450C, ammonia is freed about 100 times faster than at 00C and about 1000 times faster than at -450C If the tank steel is not sufficiently heated, the ammonia vapours will form a thin layer of cold ammonia against the steel This layer is fairly stable and difficult to penetrate by the warmer vapour inside When the ventilation is started, the air will, aided by turbulence, penetrate the cold layer of ammonia and the remaining liquid against the tank steel will begin evaporating Threshold Limit Value Special health precautions must be observed because of ammonia's high affinity with water A 2,000 ppm concentration will severely hamper the respiration and cause death after approximately 30 minutes Even in small concentrations ammonia will violently irritate the skin and the mucous membranes In practice, it is necessary to reduce the concentration to about 10 ppm in the tanks, before they are entered, as otherwise working will be adversely effected The smell threshold is as low as 5-10 ppm This concentration is harmless even during long lasting exposure in the tanks If the tanks are not sufficiently heated before ventilating, it could prove extremely difficult to reach this concentration Sweeping and Vacuum Cleaning /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 12 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R During sweeping, additional ammonia is released, and a subsequent vacuum cleaning will in combination with continued ventilation effectively lower the concentration further Alternative Methods It sometimes happens that ventilating alone is not sufficient to reduce the concentration of ammonia to the required level - e.g I ppm - therefore other methods must be applied A subsequent water washing will at the concentration reached effectively consume the remaining ammonia and thereby reduce the concentration further, in some cases below the measurable range Unless the tanks are to be water washed for other reasons, this method is discouraged After water washing, the IG-blower should be used for ventilating and drying Compressors can also be used, if the weather is calm with a relative humidity less than approximately 60 percent The Owners' permission to use the compressors for this purpose must be obtained It will be necessary to change the lubricating oil in the compressors after use, because the oil absorbs water from the air pumped through As this oil costs about £5 per litre, the time saved using this method must be significant to make this procedure acceptable When the concentration cannot be further reduced by continued ventilation another viable alternative, which was previously almost universally adopted is: The compressors are - after the Owners' permission has been obtained - utilised to build up the tank pressure to an over-pressure of e.g I or bars A higher overpressure requires comparatively longer time When the predetermined overpressure is reached, the compressors are stopped and the overpressure reduced to zero The procedure is repeated as many times as necessary, and the ammonia concentration is measured each time It is quite normal to read a considerably higher concentration after the first time This could be caused by ammonia remaining in the compressors and associated pipelines, but another phenomenon can also be considered as a contributory cause to this effect: As the tank pressure is increased, the tanks are expanded slightly, and ammonia remains which are trapped in the rust and dust, are released, and this will give cause to the increased concentration After the first time, the concentration will be reduced to half and then a third every time the tanks are pressurised Lubricating oil used in the compressors is water absorbent and accumulates some of the moisture from the atmospheric air This means that oil in the compressors must be changed after they have been used as described above Measures to Facilitate Tank Cleaning It is possible to prepare the cargo tanks in order to ease the tank cleaning after carriage of ammonia /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 13 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R A thorough cleaning and drying of cargo tanks and pipelines incl filters before loading of ammonia will to a large extent prevent ammonia from being trapped in the rust layer making it possible to ventilate the tanks more quickly following discharge By combined operation, the IG-blower can be connected to the liquid line and compressors to the three spray-lines keeping the lower line fully open and the upper lines partly open Thus a natural flow is secured with the heavier air entering below the lighter ammonia, which in the beginning is ventilated out through the vapour-lines to the masts and later through the tank hatches when vacuum cleaning the tanks Sub-cooling It is an invariable rule that tanks - including tank steel - are heated to a temperature above dew point of the air used for ventilating Theoretically, it should be sufficient to heat to a temperature of e.g -250C when ventilating with the IG-blower, as this supplies air with a dew point of about -500C However this does, not apply in practise as other conditions have to be considered If warmer and especially more humid air is added to the tanks' atmosphere, this air will absorb the ammonia vapour making the ammonia liquid try to fill the tank space with vapour by evaporation The heat required for this process is taken from the nearest surroundings, e.g the tank steel, and a drastic drop in temperature can be observed Temperatures far below the steel's design temperature and a rate of decrease multiple exceeding what the tanks are designed for can be observed and thereby possibly damage the tanks The humidity in the air will also reliquify against the steel, the water will dissolve the ammonia and substantial prolongation of the duration of the tank cleaning must be faced, as this ammonia and water solution is very difficult to remove It should be a general rule to heat the tanks to not less than +200C, and special care should be taken to ensure that the pump sump is heated thoroughly 15.9 Tank Cleaning and Ventilating after Ammonia on the Fully–Ref Vessels Special precautions are necessary when cleaning tanks after a cargo of ammonia Vaporising liquid heel Compressors with vapour blowers are used to heat up the remaining liquid (which then evaporates), the ammonia vapours, and tank steel It is very important to distribute heat evenly throughout the tank although the pump sump should be heated comparatively more The pump /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 14 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R sump should generally be heated for an additional I to hours, after all liquid has been vaporised, in order to prevent re-condensing of vapour against cold steel It has at previous tank cleanings in our other gas carriers proved to be beneficial to heat up to above +200C The exact time required for this additional heating will be more than saved during the following phases of tank cleaning In the fully ref vessels it is not always possible to reach this temperature by use of compressors alone Sub-cooling It is an invariable rule that tanks - including tank steel - are heated to a temperature above dew point of the air used for ventilating Theoretically, it should be sufficient to heat to a temperature of e.g -250C when ventilating with the IG-blower, as this supplies air with a dew point of about -550C However, this does not apply in practise as other conditions have to be considered If warmer and especially more humid air is added to the tanks' atmosphere, this air will absorb the ammonia vapour making the ammonia liquid try to fill the tank space with vapour by evaporation The heat required for this process is taken from the nearest surroundings, e.g the tank steel, and a drastic drop in temperature can be observed Temperatures far below the steel's design temperature and a rate of decrease multiple exceeding that which the tanks are designed for can be observed and thereby possibly damage the tanks The humidity in the air will also reliquify against the steel, the water will dissolve the ammonia and substantial prolongation of the duration of the tank cleaning must be faced, as this ammonia and water solution is very difficult to remove It should be a general rule to heat the tanks to not less than +200C, and special care should be taken to ensure that the pump sump is heated thoroughly In order to be able to heat up the tanks to required temperatures, the fully ref vessels have been furnished with portable units with a total of blowers with steam heating of the air Using these blowers it is possible to heat up the tanks to at least +400C At this temperature it is not difficult to lower the ammonia concentration Removal of overpressure Overpressure is removed gradually from the tanks through the vent masts in the usual way closely monitoring the thermometers Ventilating With ammonia it is a special requirement not to use inert gas for diluting the gas as certain components in the inert gas react with ammonia to form saline products which block up valves, pipes etc As ammonia's explosion limits are from 16 to 25 percent and it is difficult to ignite and also with a /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 15 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R low calorific value, it can under normal circumstances be regarded as non flammable Thus, tanks can be ventilated without first being inerted Venting off ammonia with IG-blower The portable blowers are not able to work against back pressure of any significant size It is therefore necessary to ventilate ammonia by using the inert gas blower while the concentration is still too high for it to be vented off through open tank hatches, i.e while the concentration of ammonia is too high for personnel contact Air from the inert gas blower is supplied to the tanks through liquid lines over the manifold to the tanks' liquid filling lines By reason of ammonia's vapour density compared to that of air, tanks should be connected in parallel Tanks are vented off at the top, and the vented off vapour is led to the forward gas mast in order to minimise inconveniences from the ammonia It may also be necessary to adjust the course so that vapour clears the accommodation The process of dilution can be monitored closely using Barrett testers A very clear stratification can be expected Usually one change of atmosphere can be expected to be sufficient to bring the concentration below measurable limits using Barrett-testers Ventilating using the inert gas blower must be continued for approximately another 15 hours as the portable blowers cannot work against back pressure During this phase, Drager test tubes can be used to indicate, when the portable hot air blowers should be brought into use At a reading of 500-700 ppm, the concentration will no longer decline gradually but instead rise and fall This is an indication that the tank steel is beginning to free trapped ammonia Using hot air blowers At a concentration of about 700 ppm, hot air blowers can be put to work It is possible to connect one unit with one blower to each of the liquid manifolds Reducers are required with dimensions fitting the spiral wound hoses with a diameter of 40 cm The hoses can be rolled over the flanges of the reducers and they are fastened with straps supplied with the equipment The bolt holes in the flanges must of course be sealed off with tape to avoid air escaping through them Alternatively, special connectors can be custom made on board Made of thin metal plate, they will both be easier to handle and without unwanted holes to be covered During the trial run with these blowers, measurements were taken with a provisional manometer made of plastic tube shaped as a U Results indicated that it was necessary to open the tank hatches to make it possible for the hot air blowers to supply air at a rate substantially higher than that of the Inert gas blower /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 16 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R To minimise the back pressure, it is necessary to open the hatches by removing all but bolts, which are only loosened By squeezing one bolt under the hatch covers, sufficient clearance is obtained to avoid too high a back pressure At a concentration of about 700 ppm, work can be carried out on the hatches without any inconvenience and without using gas masks The blowers are started and steam heating applied A marked increase in efficiency can be seen after initiating the hot air blowers After approximately hours, the concentration will have been reduced enough to make it possible to remove the hatches and rig hoses in the tanks First rigging of hoses in the tanks Ventilation is stopped during rigging to make it possible to lower the hoses into the tanks from the domes In each tank half, a metre and a 10 metre hose are required The hoses are connected using a strap, making the metre hose the lower Only spiral wound hoses should be used The lower hose of metres' length is at its lower end furnished with lanyards The loose ends are joined and connected to a line with a length of approximately 20 metres The other end of this line remains on deck during rigging First, the 10 metre hose is attached to the lower steel ring on the supporting frame, which is to be fastened to the hatch coaming Then, the two hoses are lowered slowly and carefully to prevent damage from contacting sharp edges in the tank - to a level, where the lower hose ends just above the railing of the upper platform directly below the hatch The slack of the line is now pulled up The hoses are then lowered further to give them a U-form The hose can now be manipulated to tumble out over the railing at the platform When this has been achieved, the hoses are lowered all the way down, and the line is slacked to allow the hoses to hang vertically and with the opening about metres above the tank bottom Another method for lowering the hoses was attempted The intention was to swing the hoses like a pendulum thereby forcing them over the railing, but this could not be achieved as the space through the manhole directly below the hatch prevented sufficiently large movements Making the hot air blowers ready The units are positioned one at each tank dome, all on the port side, so as to make it possible to connect the steam and condensate hoses Blowers must be positioned with the exhausts facing to starboard and at sufficient distance, that a metre long spiral wound hose can be connected from the upper blower to the port tank half, and a 10 metre long spiral wound hose from the lower blower to the starboard tank half The hoses are secured to the rims of the blowers' exhausts with the straps and on the frames on the tank hatches via the 90 elbows, which should already be positioned in the upper steel rings The hoses are secured to the elbows with straps /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 17 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Ventilating with hot air Before connecting the blowers to electric power, permission to use them should be obtained from Nautical Department, as this involves use of electrical equipment on deck Each blower can, after positioning and connection of hoses for ventilation, steam, and condensate, be started to begin hot air ventilation, while hoses are rigged in the other tanks When all blowers are running, inert gas blowers are started again Thus, air is added throughout the tanks along the bottom simultaneously with supplying hot air at a level a few metres above the bottom A continuous supply of air is thereby achieved all over the tanks, and substantial heating of the tank atmosphere and consequently of the tank steel is initiated simultaneously Approximately 12 hours later, the tank temperature will have been raised about 100C, and the ammonia concentration will have been reduced to 10 ppm which is below the Threshold Limit Value The smell of ammonia is still present, but the tanks can be entered without suffering any discomfort Rearranging hoses in the tanks At a concentration of 10 ppm, blowers are stopped, and tanks entered The line fastened to the lower hose is to be used inside the tank, so it is taken down into the tank when entering It will be used with the hose, so remains attached to it The hoses also remain connected The line is pulled through one of the middle openings in the thwartship swash bulkhead Thence, the line is used to heave the hoses forward to a position where the end is about metres from the bulkhead The line is then secured at a reinforcement structure in the forward part of the tank The hose will have its opening slightly offset from the end in the bulkhead, which is intended The air will thereby be distributed with a little more than half into the forward section of the tank, while the aft part receives air deflected by the bulkhead and sent downward and aft Final ventilation After repositioning the hoses, ventilation with hot air is resumed Another 20-24 hours' ventilation will be required After this, concentration can be expected to be well below ppm On the some vessels, a concentration of 1-2 pmm was measured after 20 hours' ventilation Notably, this was achieved without removal of loose rust in the tanks' bottom Even the discoloration of the detection layer in the Drager test tubes was not the usual dark blue but rather greyish blue, and this discoloration faded away after a few minutes /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 18 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R Thus, an ammonia concentration which is detectable but unmeasurable, can be achieved by ventilating with hot air Drying tanks When hot air ventilation has been completed, hoses are brought on deck together with all other equipment which has been used The tanks must then be dried thoroughly which is done by using the inert gas blower Ventilation is continued until the required dew point has been reached It is not anticipated that the ammonia concentration will increase again Expected time required The timetable below indicates the expected time required for the whole operation The table is intended as a guide for planning on board, and discrepancies may occur: Vaporising liquid heel, heating with compressors days Reducing overpressure to zero 1,5 hours Ventilating high concentration with Inert gas blower 9,5 hours Continued ventilation with Inert gas blower 15 hours Ventilating with hot air blower through manifold 4,5 hours First rigging of hoses in tanks 3,5 hours Ventilating with hot air blowers and IG blower 12 hours Rearranging hoses in tanks 5,5 hours Ventilating to unmeasurable values 24 hours Additional time for various setting up of equipment 2,5 hours Total estimated time required days, hours 15.10 Vaporising Liquid Heel In connection with tank cleaning, the liquid heel must be vaporised before the tanks are inerted and ventilated Before heating of the tanks is commenced, the deck-piping shall be thoroughly drained of liquid into the liquid collector tanks The pipe system is blown through with warm vapour from the compressors in order to ensure that it is completely free of liquid Semi - Rrefrigerated Vessels Warm vapour from the compressors is fed to the tanks through the puddle-heating line in order to speed up the evaporation of liquid heel As this line is not dimensioned to accept all vapour the compressors can deliver, vapour must also be supplied partly through the lower spray line Thus, an even distribution of the supplied heat is ensured, while at the /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 19 of 20 ZODIAC MARITIME AGENCIES LTD GAS OPERATIONS MANUAL Form Issue Date Approved : : : : GOM 002 05-05-02 M.J.R same time increasing evaporation of liquid in the pump sump It is very important to continue heating of the pump sump after the liquid has been evaporated, as the tank steel is still cold If the heating through the puddle-heating line is stopped immediately after all liquid has been evaporated, warm vapour will reliquify by contact with the cold steel Heating of the pump sump should be continued for about hours after removal of the remaining liquid Then it is safe to assume that the tank is sufficiently heated, so that vapour cannot reliquify by contact with the tank steel To avoid unnecessary delay in heating, the overpressure must be removed at intervals It must, however, be ensured that enough overpressure is present (0.1 - 0.2 bars) to enable the compressors to draw vapour from the tanks instead of the mast To facilitate subsequent tank cleaning, it is important to heat up the tanks to a temperature substantially over the boiling point of the cargo If it is planned to ventilate the tanks by means of the cargo hold blower, the tanks must be heated to a temperature at least 100C over the ambient temperature In order to avoid too much water vapour in the tanks, the cargo hold blower must not be used at ambient humidity in excess of 60 Fully Refrigerated Vessels Vapour is sucked from the tanks by the vapour-blower feeding the compressors and heated vapour is returned to the tanks through the condensate lines The heat is evenly distributed in the tanks by using the liquid filling line To heat up and vaporise remaining liquid in the pump sump, warm vapour must also be sent down through the puddle heating-line When liquid heel has been heated and vaporised it is mandatory to continue heating the pump sump for approximately hours in order to avoid reliquefaction of vapour making contact with cold steel in the sump After this time has elapsed, warm vapour should be sent through the upper spray line in addition to through the liquid filling line To facilitate subsequent tank cleaning it is important to heat the tanks to a temperature well above the boiling point of the cargo Where it is planned to ventilate the tanks using portable water driven blowers, it is imperative to heat up the tanks to a temperature exceeding the ambient temperature by at least 100C To prevent heavy corrosion in the tanks, portable water driven blowers must not be used at ambient relative humidity over 60 percent It is necessary to blow off overpressure during heating as the tank safety valves are adjusted to open at an overpressure of 0.28 bar Generally, substantially longer heating times must be allowed for in this ship-class, compared with other ship-classes /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc Page 20 of 20 ... amount of ammonia and water contents remaining in the tank will thereby be tied to the tank walls and more time is required for tank cleaning At a tank temperature of +450C, ammonia is freed about... have been used as described above Measures to Facilitate Tank Cleaning It is possible to prepare the cargo tanks in order to ease the tank cleaning after carriage of ammonia /storage1/vhost/convert.123doc.vn/data_temp/document/nek1554800583-1959494-15548005833798/nek1554800583.doc... vapour-lines to the masts and later through the tank hatches when vacuum cleaning the tanks Sub-cooling It is an invariable rule that tanks - including tank steel - are heated to a temperature above