Cargo loading operation - Chemical tanker guideline
Loading of various noxious liquid chemicals involved numerous hazards. It is important to exercise safety during all stages of cargo loading. If, at any stage during or immediately after the loading operation, a non-conformance (which may include cargo quantity, quality, temperature and colour etc.) is believed to exist, all operations should be suspended until such time as the situation is resolved.
Commencement & execution of loading
Immediately prior to commencing the loading the valve setting and the lines on all tanks have to be checked once more. The responsible officer must be satisfied that the cargo system is, in all respects, ready and all the information of the cargo has been received. The following precautions must be observed:
Handling of high vapour pressure cargoes
When handling high vapour pressure cargoes, particularly in high ambient temperatures, high rates of vapour generation may occur during either loading or discharging. As a high vapour pressure petroleum cargo enters an empty tank there is a rapid evolution of gas, as a result it may be necessary to reduce the loading rates. During the loading of high vapour pressure cargoes a very high concentration of hydrocarbon gas, approaching 100% by volume, may be vented to atmosphere.
Therefore special precautions regarding handling of high vapour pressure cargoes recommended by ISGOTT are to be followed. When discharging every effort must be made to discharge and completely strip a tank in one operation. Difficulty can be experienced in re-gaining suction from a tank with a low level of cargo.
The main types of cargoes that can cause problems due to high vapour pressure are: -
When, after completion of a product, the shore pipelines are to be cleared by the use of air or inert gas (blow through) or by use of a line scraper (pigging), the responsible officer must ensure that there is sufficient space in the tank or tanks to accommodate the quantity of product in the shore pipeline, otherwise cargo overflow from a tank may occur.
Blowing through or pigging could cause an increase in pressure, and the responsible officer must monitor the operation carefully in order to avoid tank overpressurisation. The risk of large volumes of nitrogen or air, that has been under pressure in the shore line, escaping into the cargo tank must be taken into account . The same possibility exists for an abrupt and dramatic increase in the tank filling rate , when pressing a chemical out of shore tanks. During a line clearing operation it is important that terminal staff react promptly when the scraper is caught in its trap, in order to avoid all the compressed propelling gas entering a loaded cargo tank.
Use of compressed gas
Compressed gas is sometimes used by a terminal to press products out of shore tanks (such as railway wagons) into the ship, and there is an inherent risk of overpressurisation of the ship's cargo tank. The gas pressure used for these operations varies, but can range between 2.5 and 5 bar. The point of greatest is when the supply into the ship’s tank changes from liquid to compressed gas, causing an abrupt and dramatic increase in the tank filling rate from liquid at a few hundred cubic metres per hour to gas at several thousand cubic metres per hour.
Overpressurisation of a closed tank can occur in seconds, especially when the distance from the manifold to the tank is small or the vapour space in the tank is limited. A crew member stationed at the manifold will be best placed to detect and react to any indication that the flow in the system has changed from liquid to gas.
Fig: Chemical tanker high level alarm mounting on deck
Care must be taken as tanks become full, especially when loading a product into more than one tank simultaneously, due to the increased risk of an overflow while topping off. High level alarms and tank overflow control alarms are safety critical items, and loading should be stopped if it is suspected that either is not working correctly.
The responsible officer must ensure that tanks that have been topped off are properly isolated from tanks still being loaded. Cargo tanks which have been topped off should be checked frequently during the remaining loading operations to detect changes in liquid level, and to avoid an overflow.
When nearing completion of loading the shore should be notified and, if necessary, the loading rate reduced.
Clearing cargo hoses
When clearing the line after loading a static accumulator cargo, it is desirable to minimise the introduction of gas into the tank which will bubble up through the cargo
If nitrogen is used to clear the cargo hose after loading a cargo treated with an inhibitor that depends on oxygen, care should be taken to minimise the volume of nitrogen entering the cargo tank. Not only may bubbling the nitrogen through the liquid in the tank deplete the dissolved oxygen and affect the inhibitor by requiring it to take oxygen from the atmosphere in the ullage space, but it is also possible that excessive nitrogen will linger in the ullage space.
The loading line is always blown through with either compressed air, steam or nitrogen, depending on the cargo, to empty it between manifold and terminal. This is necessary to ensure risk-free release of loading arm or hose connection. If possible the vessel’s line system used should also blown empty this way. With heated or solidifying cargoes this is a must.
In case of high viscous cargo it is important to be aware that after line blowing the cargo may contain an air-bubble and therefore the ullage measured immediately after blowing is less than actual (Vegetable oils) giving higher volume of cargo onboard than actual. Pre-blowing and after-blowing ullage measurements should give a good indication of this. The tank in question has to settle before a correct ullage can be taken. If this is not possible or cannot be ascertained, the ullage report to be remarked of entrapped air in the cargo giving higher volume in ship tank.
At some installations the landline from tank farm to jetty manifold or part of it are pigged.
It is possible that large amounts of cargo are pressed into a cargo tank at high rate and high pressure. Method and required tank volume necessary for this shoreline emptying must be agreed upon in the pre-loading conference. Especially one must be aware of the high pressure which could be created when pigging and take precautionary measures to prevent overpressurising the tank or creating an overflow, by venting the tank during this operation and throttling manifold valve to control flow.
Fig: cargo handling gears
Completion of loading & final measurements
When loading is completed the final loading measurements have to be carried out. In order to clear the shore and vessel’s cargo line free from product, the lines are blown from the shore. Cargos sensitive to Oxygen are given a nitrogen blanket following loading. The surveyor together with an officer will take the ullage and cargo temperature of the tank(s) concerned. In the presence of an officer cargo samples are taken sealed and labelled. Upon completion:
Disconnection of cargo hoses
After the transfer of a chemical cargo is complete, established procedures should be followed to minimise residues in the line, and especially in the cargo hose or cargo arm between ship and shore. Disconnection must only take place after draining of cargo residues and relief of any pressure, even before emergency disconnection if at all possible.
Disconnection of the hose or cargo arm at the ship's manifold is a time when the cargo containment system is deliberately breached. Although hose disconnection is a routine operation that must be performed, it should be regarded as comparable to opening up any other cargo pipeline on deck. Personnel engaged in hose disconnection should wear protective equipment appropriate to the hazards of the cargo involved which, for a highly toxic cargo, will include a full chemical resistant suit and breathing apparatus.
Preparation for sea voyage
In general, a round over deck should be made by a responsible officer to check that the cargo area is seaworthy in all respects.
Technical readiness prior loading operations
Voyage planning and related considerations
Cargo sampling safety precautions
Signing a bill of lading and related guideline
Preparation for cargo operation
Preparing a cargo tank atmosphere
Cargo unloading operation safety precautions
Liaison between ship and shore
Cargo care during transit
How to prevent cargo pipeline leakage
Ship shore safety checklist while alongside a terminal
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