Cargo Tank Management is crucial for ensuring the safe and efficient handling of various liquid cargos. Effective management includes processes such as vaporizing residual liquids and adjusting for non-compatible grades during transfers. Total tank heating plays a significant role in maintaining optimal conditions for cargo.
Additionally, proper venting of excess vapor and rigorous inerting procedures help prevent hazardous situations. The gas freeing procedure is essential for preparing tanks for maintenance or repairs. Overall, implementing these practices ensures safety and compliance in cargo operations.
Introduction
Compatible cargoes are those substances which can be loaded consecutively without prior need to gas free the tanks. However, care must be taken to fully comply with Charter Party, shippers or other stated requirements for the cargo changeover, as these may require more stringent procedures.
On completion of the discharge the amount of liquid stripped from the tanks will depend on the information available on the next cargo. If no prior information is received then normal stripping will be carried out manually.
The conditions then required in the cargo tanks for loading the next grade of cargo will depend on the degree of contamination acceptable to the shippers/charterers between the last cargo and the next.
If the degree of contamination is of no importance, it may only be necessary to retain the remaining liquid and maintain the Essential Discharging Cargo Procedures for Maritime Operationcargo tank temperatures on the ballast passage by use of the reliquefaction plant. If contamination has to be kept to a minimum it may be necessary to remove the remaining liquid in the sumps by boiling off and by venting the remaining vapour to atmosphere whilst on passage.
It is emphasized that the final decision depends on the requirements of the charterer/shipper and the required maximum level of contamination. See Appendix 1 for cargo compatibility.
Vaporising of Residual Liquid
The deep well pumps can normally be used to remove liquid residues, but if the level is not sufficient for them to operate satisfactorily the residues can be cleared by drawing vapor from the tank, heating it in the LPG compressor or vaporizer, and returning it to the tank sump through the stripping line or the puddle heating line. The warm vapor heats the liquid collected in the sump which quickly vaporizes. An increase in the temperature of the tank sump signals the completion of vaporization.
THIS PROCEDURE MUST NOT BE HURRIED. During boiling off it is important to obtain a positive temperature reading on the sump thermometer. Any subsequent drop in temperature indicates that liquid may still be present hence it is important to monitor the temperature even after the flow of hot gas has stopped.
Note: Commercial propane contains some butane, similarly commercial butane cintains some propane. Both may contain impurities such as ethane and pentane, depending on their pemmitted commercial specification.
Changing Grades – Non Compatible Grades and Preparing for Repairs or Dry Dock
Gas freeing is necessary when changing between incompatible cargoes, and when preparing the ship for dry dock or repairs. Liquid residues can be removed by the total tank heating procedure or by boiling off procedure.
Total Tank Heating
A total tank heating process may be achieved by drawing off the vapor with the compressors and the upper distribution line. The compressed vapor is passed through the vaporizer and returned to the tank via the lower distribution line. The Vaporizer may be heated by thermal oil steam or sea water.
Boiling Off
Vapor is drawn from the tanks via the vapor suction line, compressed, where it receives a moderate degree of superheat, and returned to the sump through the stripping line. A higher degree of superheat can be achieved by passing the compressed vapor from the compressors through the vaporizer prior to returning it to the sumps via the stripping line.
Overheating the vapor can be counter productive and must be avoided as it has a tendency to blow the liquid out of the sumps on emerging from the stripping lines. Completion of the boiling off process will be indicated by an increase in the temperature of the sump.
On completion of boiling off NB No inerting is to be carried out until it is confirmed that boiling off is Completed.x, it is important to obtain a positive temperature reading in the sump before shutting off the flow of hot gas as any liquid remaining will greatly increase the time taken to complete the following inerting stage. The temperature should continue to be monitored to ensure that there is no liquid present.
During this period any remaining liquid is to be drained from the Pipelines in Marine Terminals: Key Considerations for Handling Liquefied Gascargo pipe system via the drain system. If during this process, liquid quantities in excess of the designed amount flow through the drain system to the vent mast, a level alarm will be activated and cause the cargo plant to shut down.
Venting Excess Vapor
During the above vaporization processes the vapor pressure will slowly increase. This excess pressure may be reduced by the following methods:
- If the excess vapor is to be retained it can be drawn off by using the compressors, reliquefied and returned to another tank, or to the deck tank.
- Returned ashore as liquid.
- Returned ashore as vapor to flare or shore storage tank, either using a compressor or by pressure transfer direct from the tank.
- If at sea on passage, vapor can be vented to atmosphere via vent masts.
During this process wind conditions must be taken into account to ensure that there is no possibility of vapor entering either accommodation or machinery spaces.
Inerting
Note: Inert gas must not be used with AMMONIA due to the chemical reaction between it and the CO2 contained in the inert gas. Dry air must always be used.
Inerting is achieved by displacement of the cargo vapor or by dilution of the cargo vapor.
Inerting By Displacement. Displacement is the most economical method and is achieved by the introduction of the inert gas/air with the higher density to the tank bottom distribution line and displacing the lighter cargo vapor from the top distribution line, or vice versa.
Therefore, the ratio of the specific gravities between the inert gas/air and the cargo vapor, and the temperature difference between the two has an important influence in determining the efficiency of the interface created.
Upper and lower distribution lines are provided in order to distribute the incoming inert gas/air and to collect the outgoing cargo vapors. This process must be started slowly to avoid high velocity at the inlet nozzles which will cause turbulence, and prevent the formation of the interface.
In an ideal displacement process the interface between the incoming and outgoing gases would be perfect, with no mixing above or below the interface. The minimum amount of inert gas/air required to displace the outgoing cargo vapor would then be equivalent to one tank volume.
The higher the degree of mixing that occurs at the interface, the higher will be the quantity of incoming inert gas/air required to achieve satisfactory inerting. Any liquid cargo remaining in the sump from the boiling-off process will greatly increase the time taken and the quantity of inert gas/air required to complete the inerting process.
Tanks can be inerted in series or in parallel, with series inerting being the more economical of the two. The outgoing cargo vapor is sent either to shore flare or, if at sea, vented to atmosphere via the vent mast.
When inerting in series with gas from the on board inert gas generator, the sequences must be in accordance with gas plant suppliers manual. The inert gas generator may also be used to blow air for purging. During inerting frequent checks of the dew point are to be made as near to tank entry as possible. Maintaining a low dew point will help prevent the formation of water and ice during the subsequent cool down.
Drying can be accomplished simultaneously with inerting either using nitrogen from shore or, alternatively, the inert gas generator on board. The generator is provided with drying facilities. Whichever method is used, time and care must be spent on the drying operation. Malfunction of pumps and Valves due to ice or hydrate formation can often follow from an inadequately dried system and, while methanol addition facilities are available to allow freezing point depression at deep well suctions, etc., this may not be regarded as a substitute for thorough drying. Methanol is only used on cargoes down to -48 °C; propanol is used as a de-icer down to -108 °C, below which temperature no de-icer is effective.
Inerting By Dilution. In the dilution method of inerting the incoming gas mixes with the vapor already in the tank. This can be done in several ways depending on the type of vessel:
Repeated pressurization: Dilution can be achieved by a process of repeated pressurization of the tank with inert gas using a compressor, followed by a release of the compressed contents to atmosphere. Each repetition will bring the tank contents nearer to the oxygen concentration level of the injected inert gas. Thus to bring the tank contents to a level of 5 % oxygen within a reasonable number of repetitions, an inert gas quality better than 5 % oxygen content is required.
Quicker results will be achieved by more numerous repetitions each at a lower pressurization levels than by fewer repetitions using the higher pressurization levels of which the tank and compressor may be capable.
Continuous Dilution: Inerting by dilution can be a continuous process. An increased flow of inert gas, hence the better mixing and a reduction in overall time may be achieved by maintaining the tank under vacuum by passing the diluted efflux through the compressor. Care must be taken to ensure continued good quality inert gas under the increased output flow conditions of the inert gas generator.
The locations of the inert gas inlet and tank outlet are not important provided that good mixing is achieved. It is generally found more satisfactory to introduce the inert gas at high speed through the vapor line and exit through the liquid loading line. Where several tanks are to be inerted it may be possible to achieve a reduction in the total quantity of Inert Gas Systems – Design, Operation, Control Mechanismsinert gas used, and in the overall time, by inerting two or more tanks in series. This procedure also provides a way of inerting pipe work and equipment at the same time.
Gas Freeing Procedure
Gas freeing can take place with tanks connected in series or in parallel, and with a vapor flow within the tanks from either top or bottom or bottom to top. A table of relative densities is included in appendix 1. The procedure is continued until the tanks are completely gas-free, i. e. the oxygen content is restored to 21 %.
Gas-Freeing With Air. When the following cargo is not compatible with the previous one it may be necessary for the tanks to be gas freed after inerting as part of the process of preparing the tanks for next cargo. This is commonly the case when loading chemical gases such as VCM, ethylene, butadiene, etc. When preparing tanks for dry docking or repairs gas freeing with air after inerting is always required. A table in Appendix 1* provides guidelines for the required tank condition when changing cargoes.
Gas Freeing Tanks Containing Ammonia. Ammonia is always gas-freed with fresh air, and this is swept through the cargo system once tank temperatures have increased above the dew point of the air so as to avoid condensation.
Read also: Gas Freeing of Cargo Tanks on Liquefied Natural Gas Carriers
The air, being heavier than ammonia vapor, is fed to the bottom of the tanks and ammonia vapor displaced from the top, being released up the mast. Flushing through with air must continue until the concentration of ammonia vapor is reduced to below 20 ppm. The tanks can then be considered gas-free.
Cargo Tank Washing
Water washing cargo tanks will be required when changing from certain cargoes, particularly ammonia and polypropylene oxide, and before dry docking. Washing can commence after gas-freeing.
In the context of the requirements of MARPOL Annex II*, provided the tanks have been properly stripped and ventilated, any water introduced into the tank for preparing the tank to receive the next cargo can be regarded as being clean, and not subject to the discharge restrictions included in Annex II.
Procedure. The correct procedure will depend on the tank washing system fitted to individual vessels. Refer to the builders Cargo Handling Manual for the correct procedure.
Washing Tanks Which Have Contained Ammonia. Unlike other cargoes ammonia requires considerable quantities of water to effectively wash the tanks. Special attention should be paid to the avoidance of tank corrosion.
When water is sprayed into a tank containing ammonia the tank pressure will fall rapidly due to the ammonia vapor being absorbed in the water, and the temperature will rise as a result of exothermic reaction. To avoid a vacuum forming in the tank, the upper distribution lines must be opened to atmosphere. Ships staff must keep clear of ammonia venting to atmosphere.
