Change of cargo grades or preparation for drydock – LPG tanker procedure
Changing non compatible grades
- 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 vapour with the compressors and the upper distribution line. The compressed vapour is passed through the vaporiser and returned to the tank via the lower distribution line. The Vaporiser may be heated by thermal oil steam or sea water.
- Vapour is drawn from the tanks via the vapour 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 vapour from the compressors through the vaporiser prior to returning it to the sumps via the stripping line. Overheating the vapour 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, 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 cargo 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.
- NB No inerting is to be carried out until it is confirmed that boiling off is completed.
Venting excess vapour
- During the above vaporisation processes the vapour pressure will slowly increase. This excess pressure may be reduced by the following methods:
- If the excess vapour 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 vapour to flare or shore storage tank, either using a compressor or by pressure transfer direct from the tank.
- If at sea on passage, vapour 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 vapour entering either accommodation or machinery spaces.
- 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 vapour or by dilution of the cargo vapour.
- 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 vapour from the top distribution line, or vice versa.
- Therefore, the ratio of the specific gravities between the inert gas/air and the cargo vapour, 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 vapours. 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 vapour 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 vapour 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 degree C; propanol is used as a de-icer down to -108 degree C, below which temperature no deicer is effective
- Inerting by Dilution : In the dilution method of inerting the incoming gas mixes with the vapour already in the tank. This can be done in several ways depending on the type of vessel:
- Repeated pressurisation : Dilution can be achieved by a process of repeated pressurisation 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 pressurisation levels than by fewer repetitions using the higher pressurisation 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 vapour 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 used, and in the overall time, by inerting two or more tanks in series. This procedure also provides a ready way of inerting pipework and equipment at the same time.
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 Procedure
- Gas freeing can take place with tanks connected in series or in parallel, and with a vapour 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 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. The air, being heavier than ammonia vapour, is fed to the bottom of the tanks and ammonia vapour displaced from the top, being released up the mast. Flushing through with air must continue until the concentration of ammonia vapour is reduced to below 20 ppm. The tanks can then be considered gasfree.
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.
- The correct procedure will depend on the tank washing system fitted to individual vessels. Refer to the builders Cargo 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 vapour 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