Cargo Change
In case your vessel will have to change cargo there are several operations which have to be carried out depending of the nature of the products to be swapped.
The operations to be carried out when changing cargoes can be divided into several individual tasks before the replacement cargo can be loaded on board the vessel. These tasks will typically involve below operations:
Stripping/ heating
Inerting
Air ventilating
Tank inspection
Purging
Gassing up
In certain cases the existing cargo vapour in the tanks will be purged with the vapour of the next cargo after inerting, and the cargo residue mixed with the next cargo to be loaded. This may only be done in such cases where the cargoes are compatible and allowed by the charterers.
Stripping/ heating
Once the vessel has discharged as much as possible of the cargo liquid by means of her cargo pumps, there will still be some remaining liquid in the tank which it is not possible to remove by the cargo pumps. Depending on the vessel – the remaining liquid will typically amount to approx 10 – 1000 liters.
This liquid shall be removed before any other operation is carried out.
On board vessels having a stripping system this operation can be carried out by pressurizing the cargo tanks and let the remaining liquid flow from the bottom of the cargo tanks via the stripping line to either the vent mast or the liquid manifold and from there overboard via the bazooka. (Extension pipe fitted on the manifold flange)
Cargo remains under pressure - drained from cargo tanks
On board vessels NOT equipped with a stripping line system the remaining cargo liquid can only be removed by evaporating the liquid. Cargo residues are normally removed by help of the cargo compressors. These will suck vapour from the tanks top and return the heated vapour to the tanks bottom. The heat is given to the vapour by compressing to a higher pressure (higher pressure = higher temperature) in the compressor. As the hours go by the tank pressure will increase as more and more of the liquid in the tank bottom will boil off. It is an advantage to keep the tank pressure as high as possible, but not all the way up to the safety valve opening pressure. Excess pressure is therefore released through the gas mast, to shore line or through the reliquefication plant to the pressurized deck storage tank.
Cargo remains evaporated to mast by means of varm vapour
Inerting / Gasfreeing
The Gas Carriers in the Eitzen Gas fleet are all fitted with independent tanks type C. This meaning that the tanks can be pressurized from 4.2 bar up to 18 bar gauge.
Often the following procedures have to be carried out:
· Removal of cargo residues.
· Heating tanks.
· Inerting of tanks and systems with inertgas or Nitrogen.
· Purging of tanks, pipelines and reliquefication plant with vapour from the next cargo.
Inerting and gasfreeing normally take place at sea, either before dry-docking or due to change of cargo grade. As only a few ships have enough nitrogen on board to inert all the cargo tanks, the inerting is carried out by inert gas produced in a combustable inert gas generator or a Nitrogen generator . The combustable inert generator consists of a fan, combustion chamber, a washing tower and a cooling facility.
The oil is burnt in the inert gas generator with as low surplus air as possible, avoiding formation of soot. About 0,5% O2 is possible. The inert gas is wet on leaving the washing tower, (100% relative humidity), and must therefore be dried in a dryer. This is either done by a cooler or absorption dryer. After drying, the inert gas is send to the consumer.
Previous experience will then help you to decide how to proceed with the inerting. Shall we begin with one tank ‘on its own’ or several tanks together? This may be different from ship to ship.
In general it can be said that there are three ways to carry out the inerting. It can be done in parallel, in series and as a combination of parallel and series.
Purging in parallel is effective to archieve the so-called ''Piston effect'' and thus save time and product.
Purging the tanks in series (from tank to tank) is maximizing the use of the inert gas introduced.
Depending on numbers af cargo tanks and the interconnected pipe systems the cargo operator will carry out the Inerting taking available time or max consumption into consideration.
If we are to enter the inert gas at the top or the bottom of the tank depends on the density difference of the gases. If the gas in the tank is lighter than inert gas, then the inert gas should be introduced at the bottom of the tank, thus pressing the cargo vapour out through the vapour line at the tank top, either to the gas mast, x-over to shore or via the reliquefication plant to the storage tank on deck.
We can measure the inert gas concentration in the tanks after a while. If everything has gone as planned, we should have a sharp division between the inert and the cargo vapour. This effect is what is called the "piston effect". If the inert gas is allowed into the tank too fast, we will easily get a mixture of inert gas and cargo vapours. This is unfortunate as the operation will then be unnecessarily longer.
One should always begin introdusing the purge gas at a slow rate to avoid getting turbulence in the cargo vapour phase. After a couple of hours the rate can be increased and the turbulence will then take place in the inert gas layer and thus preserve the obtained ''Piston effect''
At the end of the inert operation when the cargo tanks are measured with HC/O2 gas measuring instruments and the reading shows below LEL (Lower Explosive Limit), we can regard the inerting as being completed. We can then start ventilating the tank with air, but only after we have measured the air dewpoint and found this to be lower than the temperature in the tank. For the ventilation we can use fans, cargo compressors or the inert gas fan (without the burner). Care has to be taken during the gasfreeing to avoid entering the flammability zone. This can be ensured by measuring the atmosphere with the transportable HC-instrument and the O2-instrument during the whole process.
In the case of last cargo Ammonia we cannot use inert gas for gasfreeing as the Ammonia will react with the CO2 in the inert gas and form Ammonium Carbonate, which is a white powder which will stick to the cargo tank walls. Often air is used directly against the ammonia vapour for gasfreeing, but it is not really advisable. To avoid the formation of explosive mixtures of ammonia/air, nitrogen should be used. Nitrogen does not react with ammonia.
Cargo Tank Inspection
Maintenance and inspection of cargo tanks may include the following operations:
· Removal of cargo residues.
· Heating tanks.
· Inerting of tanks with inert gas or nitrogen.
· Inerting of pipe-line system with inert gas or nitrogen.
· Ventilation of tanks with air/possible tank washing with water.
· Ventilation of the pipe-line system with air/possible washing with water.
· Ventilation of the Void Space with air.
After the tank has been vented, the tank is entered to investigate if any work is necessary inside the tank. Often a thin dust layer on the tank bottom has to be cleaned up. If the tank shell is wet, it has to be dried.
When tank cleaning after a propylene-oxide cargo (PO cargo), the tanks and pipe lines have to be washed with fresh water. Thereafter they shall be dried well by use of hot air. It is very important to remove all water from bends and traps in the pipeline system.
Mixing of Gases by Diffusion
Two gases will always mix together, even if they have different densities (sp. gravity). The "piston" effect we always aim to achieve when inerting or purging tanks is only effective over a short period, and in the layer between the two gases there will be a diffusion taking place. If we shut off the tank where there is a defined layer between the gases, then the atmosphere will be a homogeneous mixture of the gases after a while. That is, the gases are completely mixed.
Basic calculation of Operation-time for Inerting and Ventilating
If we consider a homogeneous mixture of purge gas (air, inert, N2 or cargo vapour) when replacing the tank atmosphere, the necessary number of changes can be calculated with the following formula:
N = Ln (Proportional number)
Where.............................................
N = number of changes
Ln = natural logaritm
Proportional number = (O2 % before Inerting/venting divided by O2 % after inerting/venting)
Example:
Oxygen content before inerting:21 %
Oxygen requirement before loading gassing-up parcel: 0.3 %
The proportional number will be (21 divided by 0.3) = 70, and the natural Logaritm to the number 70 is 4.25, i.e. it will be necessary with 4.25 changes of atmosphere to reach 0.3 % Oxygen.
|
Oxygen content
|
Oxygen content
|
Proportional
|
|
|
before inerting
|
after inerting
|
Number
|
Shifts/Changes
|
|
21%
|
5%
|
4.25
|
1.45
|
|
|
3%
|
7
|
1.95
|
|
|
2%
|
10.5
|
2.35
|
|
|
1%
|
20
|
3.04
|
|
|
0.50%
|
42
|
3.74
|
|
|
0.30%
|
70
|
4.25
|
|
|
0.20%
|
105
|
4.65
|
|
|
0.10%
|
210
|
5.35
|
The formula is based on a homogeneous mixture of air and inert gas, hence the required number of changes will be less if a good ''Piston'' effect is obtained. As the formula above is based on Nitrogen purging, (then, if using inert gas) the Oxygen content in the inert gas is not considered. This will be of importance if the Oxygen content has to be very low.
Above calculations is only intended as a guide. Depending of cargo systems, the numbers and sizes of cargo tanks - some vessels will use a little more and other vessels a little less Nitrogen.
|
