Loading pressurised ships from refrigerated storage
The cargo tanks on fully pressurised ships are made from carbon steel which is only suitable for a minimum temperature of between 0°C and -5°C. In contrast, LPG when stored in the fully refrigerated condition are maintained at the temperatures given in Table 2.5. Consequently, some refrigerated cargoes require considerable heating prior to loading on such ships. Given that fully pressurised ships may not have cargo heaters fitted on board, all heat input must be provided by pumping through heaters fitted on shore.
Of course, on a pressurised ship, having loaded a cargo at close to 0°C, the cargo may warm up further during the voyage in accordance with ambient conditions. The Gas Codes only allow cargo to be loaded to such a level that the tank filling limit will never be more than 98 per cent at the highest temperature reached during the voyage. This means that, during pre-loading discussions, tank topping-off levels must be established to allow sufficient room for liquid expansion into the vapour space while on voyage.
Loading semi-pressurised ships from refrigerated storage
The cargo tanks on semi-pressurised ships are usually constructed of low temperature sheets able to accommodate fully refrigerated propane at temperatures of between -40°C and -50°C — or even for ethylene carriers at -104°C. Refrigerated cargoes can therefore be loaded directly to such ships without heating. In addition, these ships can usually maintain fully refrigerated temperatures on voyage and this is often done to gain more space so that a greater weight of cargo can be carried. The tank pressure must however always be maintained slightly above atmospheric. Temperatures of sub-cooled products under vacuum conditions can reach levels much lower than what is acceptable for the tank material. However, when discharge to pressurised storage is planned, this is conditional on the ship having suitable equipment to warm the cargo. On semi-pressurised ships, the cargo is occasionally allowed to warm up during the loaded voyage and in this case, a similar procedure to that described for fully pressurised ships applies.
Terminal pipeline system and operation
Where a terminal can expect to load fully pressurised ships not fitted with their own heaters, in-line equipment fitted to terminal pipeline systems is needed. This usually comprises the following:—
· Shore tank
· Cargo pump
· Booster pump
· Cargo heater
· Suitably sized loading arm
When considering a refrigerated terminal loading a fully pressurised ship, given that loading temperatures on these ships are limited to about 0°C, loadings can normally be managed by pumping through the refrigerated pipelines rated at 19 bar.
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Operation of the system takes the following form: Firstly, until back pressure starts to build up from the ship, loading is carried out by pumping only through the cargo heater then, as the back pressure increases, the booster pump is also brought into operation.
At the start of loading, the pressure in a ship's tank should be at least 3 bar. This pressure will limit flashing-off and sub-cooling as the first liquid enters the tank. At this time, in-tank cargo temperatures should be carefully watched. Practical observation is also of value, with the sighting of ice formation on pipelines acting as a warning that temperatures on board the ship are falling below safe levels. In such cases, loading must be stopped until temperatures increase and the problem is resolved.
Small ship problems at large berths
A primary concern for the loading of small ships is that refrigerated storage is most often designed for large ship/shore operations. At the jetty, this means that mooring plans must be properly adapted to accommodate the very different mooring patterns from small ships and that loading arms or hoses are of a size suited to the operation.
Large loading arms can introduce difficulties on small ships. If the berth is in an exposed area, a small ship (being more sensitive, than a larger ship, to the sea state) may roll and pitch at the berth. The loading arm has to keep pace with these fast movements and this is quite a different question from any slow changes (say tidal) which may be accommodated under normal design considerations. Here, the inertia of the loading arm has to be taken into account. At present, such dynamic forces are not considered in loading arm design and manufacturers leave this for terminal managers to address in operational procedures. In such cases, a possible solution is the use of cargo hose.
7.5.4 Bulk loading
Depending on the efficiency of the earlier gassing-up operation, significant quantities of incondensible gases may be present in tank atmospheres and, without vapour return to shore, these incondensibles will have to be vented via the ship's purge-gas condenser (where fitted) or, alternatively, from the top of the cargo condenser. Figure 4.17 shows a purge-gas condenser arrangement. Care must be taken when venting incondensibles to minimise venting of cargo vapours to the atmosphere. As the incondensibles are vented, the condenser pressure will drop and the vent valve should be throttled and eventually closed.
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A close watch should be kept on the ship's cargo tank pressures, temperatures, liquid levels and interbarrier space pressures, throughout the loading operation. Monitoring of liquid levels may present difficulties when the reliquefaction plant is in operation. This is because the liquid in the tank is boiling heavily at these times and, as a result,
vapour bubbles within the liquid increase its volume, thus giving false readings when using float-type ullage gauges. Accurate level monitoring can be achieved by suppressing boiling and this can be done by temporarily closing the vapour suction from the tank.
Towards the end of loading, transfer rates should be reduced as previously agreed with shore personnel in order to accurately top-off tanks. On completion of loading, ship's pipelines should be drained back to the cargo tanks. Remaining liquid residue can be cleared by blowing ashore with vapour, using the ship's compressor. Alternatively, this residue may be cleared by nitrogen injected into the loading arm to blow the liquid into the ship's tanks. Once liquid has been cleared and pipelines have been depressurised, manifold valves should be closed and the hose or loading arm disconnected from the manifold flange.
In many ports it is a requirement, before disconnection takes place, for the hard arm, hose and pipelines at the manifold to be purged free from flammable vapour.
The relief valves of some ships have dual settings to allow higher tank pressures during the loading operation. This is permissible in the absence of dynamic forces which occur only when the vessel is at sea. If relief valve settings are altered by changing the pilot spring, then the procedure must be properly documented and logged and the current MARVS must be prominently displayed. Relief valves must be reset to the seagoing position before the ship departs. When relief valve pressure settings are changed, high pressure alarms have to be readjusted accordingly.
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7.5.5 Cargo tank loading limits
Chapter 15 of the IGC Code recognises the large thermal coefficient of expansion of liquefied gas and gives requirements for maximium allowable loading limits for cargo tanks. This is to avoid tanks becoming liquid-full under conditions of surrounding fire.
The maximum volume to which any tank may be filled is governed by the following formula:—
where:
LL = loading limit expressed in per cent which means the maximum liquid volume relative to the tank volume to which the tank may be loaded.
FL = filling limit = 98 per cent unless certain exceptions apply.
rR = relative density of cargo at the reference temperature.
rL = relative density of the cargo at the loading temperature and pressure.
Thereference temperature (in the expression pR above) is defined as the temperature corresponding to the vapour pressure of the cargo at the set pressure of the relief valves. Some pressurised ships with Type 'C' tanks have a pressure capability of up to about 18 bars with relief valves being designed for this pressure. These loading limits impose a substantial cargo shut-out for fully pressurised ships loading cargo when operating in ambient conditions, well below 45°C which is the maximum operating temperature for which the pressure capabilities of such tanks are designed.
In the case of cargo tanks on fully refrigerated ships, the Gas Codes envisage relief valves set to open only marginally above the vapour pressure of the cargo at the
maximum temperature it will reach over the whole cycle of loading, transportation and discharge. Even so, the loading limit must be such that, if a surrounding fire occurs, the tank will not become liquid-full before the relief valve opens. Thus, the amount of cargo shut-out required, over and above the normal operational considerations of cargo expansion, depends upon the margin between the relief valve setting and maximum envisaged vapour pressure on the voyage.
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There are good safety reasons for minimising cargo shut-out. The concept is very simple. The fuller the tank, the longer the tank structure will be able to withstand fire conditions. The tank contents, when exposed to a fire, will boil at a constant temperature until the bulk of the liquid has been vented through the relief valve system. After this, the upper regions of the tank become exceedingly hot and eventually fail. However, the greater the mass of liquid inside the tank, the longer the tank can withstand unacceptable external temperatures.
Cargo quantities can be maximised by adjustable settings on relief valves. This brings its own problems — particularly for Type 'C' pressurised ships — where the pressure differential between saturation temperature at the maximum allowable pressure is considerable. Relief valves designed for, say, 18 barg do not perform well at the reduced pressures required to minimise shut-out. When operated at such settings, gases are ejected at velocities well below those associated with design pressures, and as a consequence, the effluent is not propelled clear of hazardous areas.
The Gas Codes permit a further alternative solution which obviates any cargo shut-out on loading beyond that of normal operational considerations of cargo temperature change. This solution requires the provision of an additional pressure relieving system with relief valves set to open at the maximum operational vapour pressure of the cargo. The system is brought into operation by the melting of fusible elements suitably located to detect surrounding fire conditions. It is not a popular or very practical solution.
Examples
Case 1
A fully pressurised ship loading propane at 20°C with relief valves set at 16 barg.
Reference temperature +49°C (corresponding to SVP of 16 + 1 =17 bar for propane)
Density of liquid propane at 49°C = 452 kg/m3
Loading temperature +20°C
Density of liquid propane at 20°C = 502 kg/m3
Therefore, the tank can be filled to 88.2 per cent of tank volume.
Case 2
A semi-pressurised ship loading propane at-42°C with relief valves set at 5 barg and having no additional pressure relieving facility fitted.
Here, since no additional pressure relief is fitted in accordance with the Gas Codes, the reference temperature must be taken as the temperature corresponding to vapour pressure at set pressure of relief valves, i.e. a temperature corresponding to an SVP of 5+1=6 bar.
Reference temperature = + 8°C
Density of liquid propane at 8°C = 519 kg/m3 Loading temperature = -42°C
Density of liquid propane at -42°C = 582 kg/m3
Thus, the tank can be filled to 87.4 per cent of tank volume.
Case 3
A fully refrigerated ship loading propane at -42°C with relief valves set at 0.25 barg.
Reference temperature = -37.5°C
Density of liquid propane at -37.5°C = 577 kg/m3
Loading temperature -42°C
Density of liquid propane at -42°C = 582 kg/m3
Thus, the tank can be filled to 97.1 per cent of tank volume.
New developments
In recent years IMO recognised that the problem of cargo shut-out on ships with pressurised tanks (Type 'C' tanks) had not been properly solved under the original Gas Code formulae. Either the fire protection afforded by full tanks or the ability of relief valves to project vented gases away from decks and structure is sacrificed.
Amendments to the Gas Codes in 1995 produced a solution which allows additional cargo to be loaded in Type 'C' tank ships. This concession can be granted to all Type 'C' carriers, except those designated by Chapter 19 of the IGC Codes as being 1G ships. These are specialised carriers transporting chlorine, ethylene oxide, methyl bromide and sulphur dioxide — see Appendix 2.
When the Gas Codes were first produced, it was recognised that tank relief valves were sized using empirical formulae based on experimental data from valve manufacturers. This data was based exclusively upon vapour flow. Although manufacturers had made allowances for liquid pick-up in vented gases, IMO decided tank relief inlets should never be exposed to liquid and, to this end, they required that tanks should never be more that 98 per cent full. This decision leads to the cargo shut-out illustrated by the worked examples.
Since the Gas Codes were first introduced, much work has been done on relief valve operation. It became apparent that, with a tank at 98 per cent full, relief valve operation would inevitably involve both liquid and vapour in the vented stream. Such two-phase flow occurs even when tank levels are as low as 80 per cent. This implied that existing relief valves sized using valve manufacturers' methods can cope with all conditions of two-phase flow and still provide protection against over-pressure.
A further concern was dispelled when it was demonstrated that even with a tank 100 per cent full, when relief valves open, no jetting of liquid will occur at the vent riser. Much of this work was based on theoretical analysis made possible by an increased knowledge of the physics of two-phase flow. Theoretical work was backed by practical tests.
With this knowledge, IMO decided to amend the Gas Codes as they relate to Type 'C' tanks. In Chapter 15, they added a change in the definition of the relative cargo density for this particular category of tank.
rR = relative density of cargo at the highest temperature which the cargo may reach upon termination of loading, during transport or at unloading, under the ambient design temperature conditions.
In the above definition the expression Ambient design temperature conditions is linked to the performance specification for temperature control of cargoes which states that the upper ambient design temperatures should be a sea temperature of 32°C and an air temperature of 45°C.
The Gas Codes further state that for service in especially hot or cold zones, these design temperatures should be increased or reduced, as appropriate, by the national administration.
This allows the shipowner to demonstrate to the relevant national administration the rationale for the selection of thehighest temperature.
In these new developments, IMO has retained the requirement for 2 per cent of tank volume to be a vapour space. The tank volume filling limit thus remains at 98 per cent.
Although accepting that pressure relief valves can cope with all aspects of two-phase flow, IMO recognises that relief valve performance can be affected by the piping system within which it is installed. To this end, administrations will now require shipowners to demonstrate that ships taking advantage of increased loading have tank venting systems which are adequate to deal with all aspects of two-phase flow.
Guidelines which provide a method whereby the adequacy of the vent system can be assessed are now available as an IMO publication. New ships should use the Guidelines as design criteria and, for existing ships, they will demonstrate if modification to the vent system is required.
The advantages of these concessions are easily demonstrated. Considering Case 1 of the worked examples, should the ship concerned be on a long voyage and likely to encounter seas at 32°C and air temperatures of 45°C for prolonged periods the prediction of the highest cargo temperature then becomes, say, 38°C. Under these circumstances, the Loading Limit becomes 92 per cent of tank volume. If however, it can be shown that the ship will operate in temperate waters and that the highest cargo temperature is 25°C, then, the Loading Limit becomes 96 per cent. If in Case 1, the highest cargo temperature anticipated is 20°C, then, the density ratio is unity and the Loading Limit 98 per cent. Furthermore, for cases 2 and 3 of the worked examples, the Loading Limit normally becomes 98 per cent.
For shipowners to take proper advantage of these rules, they should have performance details such that national administrations can understand how quickly a fully pressurised ship's cargo may warm up during the voyage. Additionally, a clear indication of the route which the ship will take and the ambient conditions existing along that route, will further justify the selected highest temperature.
This selection process deserves a more detailed appraisal than is possible in this book. Accordingly, IACS and SIGTTO have produced a joint publication entitled:
Application of the Gas Carrier Code Amendments to Type 'C' Cargo Tank Loading Limits (see Reference 2.27).
7.6 THE LOADED VOYAGE
Cargo temperature control
For all refrigerated and semi-pressurised gas carriers, it is necessary to maintain strict control of cargo temperature and pressure throughout the loaded voyage. This is achieved by reliquefying cargo boil-off and returning it to the tanks (see also 7.5 and 4.5). During these operations, incondensibles must be vented as necessary to minimise compressor discharge pressures and temperatures. In LNG ships, the boil-off is burned as fuel in the ship's main boilers (see also 4.6.5).
Frequently, there are occasions when it is required to reduce the temperature of an LPG cargo on voyage. This is necessary so that the ship can arrive at the discharge
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