|1.||Cargo heating system for oil tankers and product carriers equipped with turbine operated cargo pumps, wherein the turbine has been modified so as to be used both for discharge of the cargo as well as for circulation heating of the same, c h a r a c t e r i z e d in that the steam exhaust channel of the turbine is equipped with a stop valve to the condensor and a valve and pipeline from the turbine outlet to a deck mounted heat exchanger to which the steam is led after first having passed through the turbine operating the pump that circulates the oil in the tank through the deck mounted cargo heater.|
|2.||Cargo heating system for oil tankers and product carriers equipped with turbine operated cargo pumps as per claim 1, c h a r a c t e r i z e d in that the turbine already from the beginning has been built for back pressure and that the deck mounted heat exchanger is used both for circulation heating of the cargo and as condensor for the turbine during discharge, by means of sea water thereby being circulated on the oil side of the heat exchanger by a separate pump.|
|3.||Cargo heating system according to any of the above claims, c h a r a c t e r i z e d in that the heat content of the exhaust steam from the turbine is utilized during discharge by means of sea water for cooling of the heat exchanger (condensor) being circulated in a tank heated to about 80°C, the sea water then being used for tank cleaning.|
|4.||Cargo heating system for tankers and product carriers with modified turbine as per claims 1 and 2, c h a r a c t e r i z e d in that the pump is indirectly operated via a hydraulic system or an electrical system.|
The present invention relates to a cargo handling system for tankers where the cargo pumps are directly or indirectly operated by steam turbines.
The directly operated systems are applied to almost 100 percent in all large oil tankers (VLCC and ULCC). The turbine operated pumps are normally four, located in a separate pump room and each pump is connected to 3 - 4 cargo tanks. If any systems for heating the cargo oil are applied at all, these as a rule consist of tube coils in the tanks, heated by steam directly from the steam boiler via a pressure reducing valve. The turbine operated pumps are used only for discharging the cargo oil since the efficiency of the turbines is very low due to the fact that the major part of the heat content in the steam is absorbed in the cooling water through the condensor.
Thus, it is not possible with the existing systems to heat the cargo by circulating the same through a heat exchanger in order to reduce the viscosity and make the cargo pumpable without simultaneously getting a low efficiency and bad economy.
The indirectly operated cargo handling systems are used in up to 50 percent of all product and chemical carriers. In this case, the pumps are placed submerged, one in each tank and mostly in combination with a deck mounted heat exchanger for heating the cargo by circulation through the heat exchanger. The power for the indirectly operated pumps is normally acquired by diesel/electricity, diesel/hydraulic power pack, turbine/hydraulic power pack or a combination thereof. The steam for heating the cargo in the cargo tanks is taken directly from the steam boiler via a reducing valve and the power for running the pumps is normally taken from the hydraulic power pack with the best efficiency
(diesel/hydraulic), unless the boiler has to be in operation to cover the demand for inert gas to the cargo tanks when no heating is required. In the latter case the turbine-hydraulic power packs are used, otherwise these are only used for discharging the cargo.
The purpose of the present invention is to make it possible by means of a modified design of the turbine to use the turbine in an economical way, also in the original design, for unloading the cargo at full capacity and for heating the cargo oil when required by circulating the cargo oil through deck mounted cargo heaters. Unlike the prior art design with separate steam lines to the turbine and the heat exchanger, in the present invention the steam line from the boiler is first led to the turbine wherein the steam gives off the energy required for circulating the cargo oil quantity through the heat exchanger. From the turbine outlet the steam is led to the heat exchanger where the heat content of the steam is transferred to the circulating cargo oil. After having given off energy to the turbine, the steam still has 95 percent of its heat content left to be used in the heat exchanger. This energy would otherwise have been cooled off with sea water in the condensor.
The invention will now be described in more detail with reference to the accompanying drawings, in which:
Figure 1 shows a side view of a typical cargo turbine with cargo pump and condensor, modified for cargo heating and with deck mounted cargo heat exchanger;
Figure 2 shows a diagram for the same system; Figure 3 shows a diagram for an alternative system without condensor but with deck mounted heat exchanger serving both as cargo heater and discharge condensor;
Figure 4 shows a Mollier diagram with process cycle for turbine/heat exchanger during circulation heating; Figure 5 shows a diagram for an alternative system with indirect hydraulically driven submerged cargo oil pumps located in each cargo tank;
Figure 6 shows a diagram of the system in Fig. 5 with a combination of a separate back pressure turbine for circulation heating and a separate condensing turbine for discharging the cargo. In the present invention as per Figs. 1 and 2 the cargo handling system is extended by a heating system consisting of one deck mounted heat exchanger 1 for each cargo pump, through which heat exchanger the cargo oil from each connected tank 11 is circulated by means of existing cargo oil pump 3 operated by steam turbine 2 back to each tank 11 respectively through pipeline 10 drawn back close to the tank bottom (dropline).
The steam for heating the circulating cargo oil through the heat exchanger 1 is taken from the outlet channel of the turbine through valve 6 after the steam first having passed through the turbine 2 for power supply to the circulation pump 3. Thereby the steam pressure has been reduced from about 16 bar at the turbine inlet to about 6 bar at the outlet, which is a suitable value for oil heating. The outlet opening to existing condensor 4 is thereby closed by means of additional valve 5 allowing all steam passing through the turbine 2 to flow to the heat exchanger 1 where it condensates and the condensate is subcooled below the saturation temperature (<100°C) before it is fed back to open tank 21.
Due to the modification of the turbine 3 as above two substantial advantages are achieved. Firstly, the economy of the system is considerably improved compared to the turbine 2 operating against the existing condensor 4 where the heat content in the steam flow is cooled off by sea water. Secondly, the steam pressure is reduced in the turbine 2 to a value suitable for heating the cargo oil in heat exchangers and to a pressure approved by the classification societies for steam pipelines on deck (below 8 bar). In this way, the normally required steam pressure reducing valve can be eliminated.
During discharge the cargo oil is led the ordinary way via manifold 17 to oil storage, i.e. bypassing the heat exchanger 1, and the turbine 2 is operating at full capacity with outlet valve 5 against the condensor 4 open and steam valve 6 closed.
A study of a cargo handling system for a 44 000 DWT product tanker equipped with 4 cargo pumps, each with a capacity of 1 000 m /h at a head of 140 m wc and each connected to a steam turbine with a maximum capacity of 490 kW at 15 bar steam pressure and 540 mm Hg vacuum shows that: if the turbine would be used in an unmodified design for circulating the cargo oil, i.e. operating against the condensor, the power necessary to circulate the required cargo oil quantity (about 500 m 3 /h) through the heat exchanger would be 120 kW and the steam demand would be 3.2 t/h for each of the four turbines. The required steam quantity for the heat exchangers is 16.734 t/h, i.e. totally 29.534 t/h. The total thermal efficiency for the system will thus be 56.5% and for the turbine alone only 5.6%. The total fuel cost for a voyage of 8 days with continuous heating at a boiler efficiency of 81% and an oil consumption of 2 270 kg/h, oil price 150 USD/ton, calorific value of the fuel 9 250 kcal/kg = 2 270 x 150 x 8 x 24 = 65 376 USD/trip.
At operation with the modified system with the same capacity, i.e. a circulation effect for the cargo oil pump of 120 kW , 500 m 3 /h oil in circulation, the steam pressure is reduced from 16 to 6 bar when passing through the turbine, as illustrated in the HS diagram in Fig. 4 at 31 and 32. Thereafter the steam pressure is reduced by pressure drop in the pipeline to 5 bar (at 33) at the inlet of the heat exchanger, where the steam is condensed and the condensate is subcooled to 90°C (at 34). Thereby the total steam demand for turbine and heat exchanger together will be 4.319 t/h and unit, i.e. for 4 units 17.276 t/h steam and a total thermal efficiency of the
system of 96.3%. The total fuel cost for a voyage of 8 days at continuous heating, fuel consumption 1 328 kg/h, and with the fuel cost, boiler efficiency and calorific value as above will then be 1 328 x 150 x 8 x 24 = 38 246 USD/trip.
Thus a saving of 65 376 ./. 38 246 = 27 130 USD/trip is achieved.
Apart from the economical advantages also practical advantages are achieved through better service accessibility to the tanks and cleaner tanks than if the tanks were equipped with heating coils.
A further alternative aspect of the invention which is more applicable for newbuildings is shown in Fig. 3, wherein the condensing turbine has been replaced by a back pressure turbine 2a and the seawater cooled condensor has been replaced by the deck mounted heat exchanger la.
The heating process is exactly similar to the process described above for the modified turbine, i.e. the steam from the boiler 13 is initially passing through the turbine 2a which is operating the cargo pump 3 and flows thereafter to the heat exchanger la on deck for heating of the circulating cargo oil.
During discharge the deck mounted heat exchanger la operates as a condensor for the steam from the turbine. The steam in the heat exchanger la is condensated due to the circulation of sea water through the heat exchanger la by means of pump 28 from the slop tank 30 which is filled up with sea water. Thereby the temperature in the slop tank will increase during the discharge period from initially about 25°C to about 80°C. The hot water can then be used for tank cleaning and thereby the butterworth heater can be eliminated.
Further the heat in the exhaust steam from the turbine is recovered during the discharge period which further improves the economy compared to conventional systems where the tank cleaning water has to be heated
separately by means of fresh high pressure steam directly from the boiler.
Fig. 4 shows the circulation and heating cycle in a Mollier diagram wherein the working cycle in the turbine during circulation heating represents the enthalpy drop from point 31 to 32, the pressure drop in the steam pipeline between turbine and heat exchanger from point 32 to 33 and the heat transfer in the heat exchanger from point 33 to 34. As appears from the diagram, 95% of the heat content in the steam thus remains after the working cycle in the turbine, heat that would otherwise go to waste at circulation heating with the turbine operating in an unmodified design.
An alternative application of the invention .is shown in Fig. 5 where the discharge pump 3a according to prior art method is located submerged in each tank 11 respectively and operated by a built-in hydraulic engine 36 which receives its power supply via pipes 37 from the hydraulic power pack 35 connected to the turbine 2. As in Figs. 1 and 2 each pump is here connected to a deck mounted heat exchanger 1 through which the cargo oil is circulated when heating is required. The steam for heating is taken from the exhaust channel of the turbine via valve 6 after the steam first having passed through the turbine 2 supplying the power required to operate hydraulic power pack 35, as shown earlier in Figs. 1 and 2.
At discharge the turbine 2 works at full capacity with the exhaust valve 5 to condensor 4 open and steam valve 6 closed as described earlier. Since the power required for circulation heating constitutes only one fourth of the total power quantity required for discharging the cargo, the system as per Fig. 5 is most suitable in combination with further hydraulic units, diesel/electricity or turbine operated units which will then only be used at discharging of the cargo. The advantages of the system according to the invention compared to a system according to prior art
where the circulation power required is obtained by means of condensing turbine-operated or diesel/electrically operated hydraulic power packs are the considerable increase of efficiency and the improved economy described above.
A further alternative is shown in Fig. 6, where a separate back pressure turbine 2b is installed to be used only as a power source for hydraulic power pack 35 at circulation heating of the cargo and for reducing the steam pressure for the steam to the deck mounted cargo heaters. In order to achieve the power required for discharge, this system is combined with additional hydraulic power packs 35 operated by conventional condensing turbines only used for discharging the cargo. The last described system is most applicable for newbuildings and the advantage over the system described in Fig. 5 is that standard components can be used in this case and that no rebuilding of the turbine is required. The economical advantages during operation are identical to those described above.