EP1735560A2

EP1735560A2 - Connecting system for cryogenic tanks

Info

Publication number: EP1735560A2
Application number: EP05733688A
Authority: EP
Inventors: Harry Robert Van Ootmarsum; Gijsbertus Cornelis F. Roovers; Bastiaan Andreas D'herripon
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-04-05
Filing date: 2005-04-05
Publication date: 2006-12-27
Also published as: WO2005098305A3; WO2005098305A2

Abstract

A cryogenic tank (200) is described, comprising an outer tank (210) with an outer tank side wall (211) and an outer tank bottom (212), and an inner tank arranged inside the outer tank having an inner tank side wall (221) and an inner tank bottom (222); wherein the outer tank side wall (211) has a passage opening (214) for a fluid pipe (230) for supplying cryogenic fluid to the inner tank (220) and/or discharging cryogenic fluid from the inner tank (220); wherein the fluid pipe is connected to a connection opening (238) in the inner tank side wall (221); and wherein the fluid pipe has a flexible tangential pipe part (237; 253; 354) between the inner tank (220) and the outer tank (210).

Description

Title: Connecting system for cryogenic tanks

Field of application of the invention The invention relates in general to cryogenic tanks, by which tanks are meant which are intended for the storage of very cold fluids .

Present art In the case of the existing cryogenic tanks for the storage of gasses such as LNG (Liquid Natural Gas) , ethylene, propane, butane and the like in liquid form, according to present insights, the supply and discharge pipes are led through the roof of the tank. The reason for this is that there is no risk of liquid gas getting outside the tank in case of a pipe breaking. The tank is implemented as a full containment tank, this means that there is a completely closed inner tank and, if it might collapse, a completely closed outer tank which assures that no cryogenic fluid escapes. For discharging fluid, an existing tank is provided with a discharge pump, which is implemented as plunger pump and is installed at the bottom of trie discharge pipe, at the bottom of the tank. To that end, the energy supply to the pump has to be led inside the discharge pipe in the tank. In order to perform maintenance to the pump, the discharge pipe at the roof of the tank has to be opened, after which the pump can be lifted out of the tank. In order to perform these activities, hoist and installation apparatus is needed on top of the tank. To that end, the tank roof is provided with a work platform. In some cases, especially with tank charges of different composition, there exists the danger of forming multiple layers of LNG with different boiling point and different specific weight in the tank. A sudden mixing of these layers leads to strong gas formation. In order to prevent sudden mixing of these layers, the tank content is mixed by inducing a vortex in the tank by directing the flow from the supply pipe. This can occur during filling of the tank, but also by pumping around fluid through a pipe system from the discharge pipe back to the supply pipe with the plunger pump. To that end, shutters and pipes are installed which can lead the fluid flow from the pump back to the supply pipe. Another effect of mixing the tank content is that thereby the temperature at the surface of the tank content can be kept lower,, by which the production of boil-off gas can be reduced, which improves the efficiency of the tank.

The state of the art is further complete i with a tank which is provided with a side discharge according to patent application PCT/US00/02039. In this patent application, a side discharge is shown which also offers full containment. This full containment is obtained by connecting an extra space to the tank in which the discharge pipe, the shutters and the pump are accommodated. This space is provided with a metal lining and insulation in order to prevent the construction from possible cracking by thermal tensions in the case of a leakage of cryogenic fluid. Thus, this extra space is in fact part of the containment part of the tank.

Disadvantages of the existin-g art The existing art of cryogenic tanks has several disadvantages. The existing way of pumping cryogenic fluid is very expensive. Furthermore, the method also has disadvantages for installation, maintenance and energetic efficiency.

* The pump with pipe construction is very expensive because: - The plunger pump is positioned at the bottom of the discharge pipe in the tank and operates rLn the fluid at a very low, cryogenic temperature. The usual temperatures are from -45 °C to -165 °C. The materials needed for this and the desired operating security and the safety requirements make the pump very expensive. - The roof of the tank has to be dimensioned to the extra weight for the work platform, for the pipes and valves, for the spare pump which is placed on the roof, for the lifting device which is needed to lift up the pump and the like. The construction of the work platform and roof also has to be adapted to a possible spilling of cryogenic fluid, for example by means of a stainless steel spatter protection and discharge -

- The costs and construction time until operation of the complete tank increase because of the complexity of the complete supply and discharge pipes.

- Long pipes are needed for the supply and the discharge of cryogenic fluid through the roof of the tank (see figure 1) ; in this context, it is noted that the height of the tank may typically amount in the order of 40 m.

- For maintenance, the pump must allow to be lifted up from the pipe. To that end, the pipe must be able to be opened. When the pipe is opened in order to lift up the pump, the tank can no longer be operational .

- The electric supply for the pump must be led to inside the tank.

The pump produces heat which is transferr-ed to the tank content, which leads to heating of the cr-yogenic fluid and production of extra boil-off gas.

Homogenising the tank content by means ofT pumping around fluid with the plunger pump requires a hi- gh fluid velocity and, thus, also a considerable pressure of the pump. This enlarges the required pump power and leads to an increase of the heat production and extra energy loss .

If the pump might fall, there exists a da-nger of causing a leak in the bottom of the tank.

Because of the limited possibilities to h-omogenise the tank content, there exists the danger of a "rollover" in which large amounts of LNG are suddenly mixed, which leads to very strong gas formation. In order to withsta-nd this gas formation safely, the tank needs to be im-plemented more strongly and heavily. * The construction according to patent PCT/US00/02039 has as important drawback that this construction does not allow relative movement of the piping relative to the tanks. In case of. seismic shocks and movements, the rigid connection of the pipe to the tank may lead to breaking, because this construction does not show a possibility to compensate for the relative movements. Another drawback of this construction is the additional complexity and construction volume and associated costs of such a construction. Also the possibility of cracks in the tertiary housing itself in oase of seismic load is an extra risk.

The invention wants to overcome a number of or all drawbacks of the present art.

Summary of the invention According to an important aspect of the present invention, a fluid pipe is connected to the side wall of th-e inner tank, at relatively short height above the bottom thereof, and this fluid pipe is led outwards through a passage opening in the side wall of the outer tank, wherein this fLuid pipe between the inner tank and the outer tank is at least partially implemented as a flexible pipe. According to a further important aspect of the present invention, this fluid pipe in the insulation space between the inner tank and the outer tank is substantially directed horizontally. According to a further important aspect of the present invention, the cryogenic fluid can be stirred by pumping around fluid through several of such side connections. According to a further important aspect of the present invention, no shutters inside the outer tank are present in the fluid pipe, and the first shutter is located outside th-e outer tank. According to a further important aspect of the present invention, the fluid pipe is fixed relative to the outer tank, and is provided with an insulation collar around the fluid pipe, which insulation socket is made of a fluid-tight and vapour-tight material and connects in a fluid-tight and vapour-tight manner on the one hand to the fluid pipe and on the other hand to a metal lining on the inside of the concrete tank wall (Thermal Protection System) . According to a further important aspect of the present invention, at least one mechanical stirrer is installed in the tank, for example in the form of a propeller with a vertical drive axle, which is driven by a motor situated on the roof of the tank.

Advantages of the invention are:

- The safety is brought to a similar level as in case of a discharge through the roof of the tank by application of securities: a flexible connection between the inner and outer tank, realising the pipe passage outside the most heavily loaded zone of the inner tank.

- The side passage is resistant against the occurring displacements by shrink and pressure load, and of OBE (Operation Based Earthquake, the normal operation of the tank does not need to be disturbed by this) and SSE (Save Shutdown Earthquake, these quakes are so heavy that the normal tank operation has to be stopped and can possibly be resumed only after inspection) earthquakes because of the application of a flexible pipe between the inner and outer tank which allows enough movement to compensate for the maximum possible displacements of the tank.

- The pump may be positioned at the outside of the tank, the pump is of a much easier design with standard materials, the electricity supply does not have to be led in the tank and repair and maintenance can take place normally after the shutter between tank and pump has been closed. - Because the pump is situated outside the tank, the heat development of the pump is not converted into extra boil-off gas .

- The side passage may be used for both discharge and supply, a simplification of the piping system is obtained herewith.

- Through the side discharge the tank, after a breaking, may be pumped empty through the (repaired) pipe, after the automatic shutter is opened again.

- A mechanical stirrer can mix the tank content better than what is possible based on pumping around. As a result, less BOG arises, the operators can pump a broader range of kinds of gas into the tank, and the safety is increased.

- The roof of the tank is relieved and may be implemented in a lighter fashion. No measures are needed for run-off and splash prevention of cryogenic fluid. Only light steps are needed on the tank, necessary for Boil-off Gas discharge, instrumentation level readers, pressure sensors, temperature readers, readers of s.m. (specific mass) of the fluid and the like.

- The bottom of the tank does not need to have provisions for a falling pump.

- By the occurring simplifications and simpler construction of the tank, a considerable cost saving can be achieved.

Short description of the drawings These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numbers indicate same or similar parts, and in which: figure 1 schematically illustrates a tank according to the state of the art; figure 2 schematically illustrates a tank according to the present invention; figure 3 is a block diagram schematically illustrating a control circuit for a stirring motor according to the present invention; figure 4 shows a schematic perspective view of the side passage; figure 5 shows a schematic top view of an exemplary embodiment of the side passage; and figure 6 is a schematic cross sectional view illustrating details of the insulated passage.

Detailed description of the invention The invention relates to a safe and cost-saving way to realise a side supply and discharge pipe in a double-wall tank by means of a flexible pipe which is attached tangentially and substantially in horizontal direction between the inner and outer tank, wherein one end is connected with the inner tank through a connecting flange in the inner tank wall and the other, lower end is connected with the supply or discharge pipe through an insulated wall passage. The pipe properties are adapted to the displacements which can occur between the ends of the pipe by thermal shrinkage, by mechanical load of the tank and by seismic forces. The required displacement is dependent on the application, the thermal shrinkage is in the order of 0.2 m, but the deflections which may be caused by seismic shocks are dependent on the region where the tank is located, and the locally occurring maximum possible seismic shocks. In general, it can be stated that the maximum deflection which can occur with the occurrence of heavy earthquakes may be 0.5 to 1 metre. In the state of the art, these deflections can not be compensated for, and lead to cracks. By applying a flexible pipe according to the present invention, these deflections can be compensated for without the occurrence of leakage. Because the pipe between the inner tank and the outer tank is substantially horizontal, at least has a horizontal component, the length of this pipe can be sufficiently large to be able to compensate for mutual displacements of its ends by elastic deformation: the pipe then behaves as flexible pipe. By manufacturing the pipe of a suitable material with enlarged flexibility, the required length of this pipe part may be reduced, which offers a reduction in supports and the like . The flexible pipe may be manufactured of synthetic material which can endure a relatively large deformation, of corrugated metal tube or of metal which needs a much larger length for the same deformation in order to realise a same displacement with elastic and safe plastic deformation. If the pipe is manufactured from a metal tube, the length will be made so large that by elastic and plastic deformation all requirements are met. If the pipe is constructed from flexible synthetic layers or corrugated metal tubes with synthetic casing (such as for pipes which are applied for jibs for the transfer of LNG) the length may be much shorter while still enough flexibility exists for the occurring maximum required displacements. The connection is attached to the side of the inner tank, as low as responsibly possible in relation to the allowable load of the material of the side wall. At the inner side of the inner tank, a tube may be attached to the connecting flange, which tube reaches down in order to be able to suck up the fluid down to a lower level in the tank.

It is favourable to have the pipe decline from the level of the passage height in the inner tank to the level of the pump, so that possible gas formation in the pipe is led back to the tank, where it may possibly escape through a hole in the supply tube at the inner side of the tank at the highest point. If for constructive reasons it is not possible to install the pipe in a declining way, then collected gas can be led back in the tank through a (automatically operated shutter) and a pipe. For example, this function may be implemented with a float construction, in order to counteract escaping of fluid.

Normally, the outer tank has a thermally insulated and fluid- and vapour-tight metal shielding (Thermal Protection System) in its bottom corners, that, in case of a breaking of the inner tank, protects the outer tank against a thermal shock and crack formation as a result of the material load associated with it. The pipe passage through the outer wall is implemented in an insulated fashion and is implemented as anchor point in such a way that the occurring pipe loads, impacts as a result of pressure waves and the like can be absorbed by the wall passage. In order to guarantee the fluid- and vapour-tightness of the passage, a metal bellows is attached as a collar between the pipe and the metal wall protection. The space behind this bellows is filled with a flexible insulation material, for example glass wool blanket. The bellows is attached in a fluid-tight manner, for example by a welding connection, to the pipe and the metal wall protection. The bellows is preferably made of a cold-ductile steel, i.e. a steel which has sufficient ductility at the cryogenic operating temperatures, such as for example a nickel steel. It offers advantages if the cold resistant material of the bellows is equal to or similar to the material of the wall protection. An example of such a construction is shown in figure 6. A further simplification and cost reduction is that the discharge and the supply can make use of the same common side pipe.

If the tank is filled with different charges (with other specific weight, evaporation temperature and the like) , the danger of layer formation exists. If such layers are suddenly mixed, one speaks of a rollover. This causes a very large sudden evaporation of the cryogenic fluid, which can lead to considerate pressure increase and to damage of the tank. An evaporation of the cryogenic fluid is caused by heat leakage of the tank. The loss of evaporated gas can be reduced by constantly mixing the tank content, by pumping around, so that the temperature at the fluid surface remains as low as possible, equal to the average temperature of the total tank content . In the case of a tank with top connections and a plunger pump, a mixing can be established by pumping the fluid around with this plunger pump. Some of the disadvantages of the use of the plunger pump is that, by using the pump, extra heat is led into the tank because of the heat losses of the pump, as well as the heat supply by the diversion tubes. The consumed power of the plunger pump may be more than 100 kW, at an estimated total efficiency of 75% this means a heat input of 25 kW which directly leads to production of boil-off gas. Beside this, also the use of the expensive plunger pump is not desired because the lifetime is hereby reduced and causes extra maintenance.

In a tank which, according to the present invention, is provided with a side connection while the pump is arranged outside the tank, some of the said disadvantages are already cancelled. Mixing of the fluid in the tank may then be performed by pumping around if at least two side passages are present, one for the supply and one for the discharge. According to a further aspect of the present invention, it is possible that one single side passage suffices if use is made of mechanical stirring members. However, such mechanical stirring members may also be used in tanks with multiple side passages, and may even be applied in tanks which, according to the state of the art, are provided with top connections with a plunger pump. In a preferred embodiment according to the present invention, the cryogenic tank is provided with a stirrer/ mixer/stirring propeller and procedures to minimise the production of BOG (boil-off gas) and to avoid rollover of fluid layers of different specific weight. In order to realise the homogenisation of the tank content, one or more mechanically driven stirrers are hung up in the tank for this purpose, such that the mixing is realised with a propeller or screw-shaped mixer. This has as advantage the low installation costs, no maintenance, and very low energy supply so that the boil-off is reduced. This saves costs in the electricity costs of the motor and also in the reduction of the boil-off by an on average lower surface temperature and a lower energy supply to the tank. A further efficiency improvement can be achieved by controlling the stirrer based on the fluid temperature at different heights between the fluid surface and the bottom of the tank. The stirrer can optionally turn counter-clockwise or clockwise, so that a flow can be induced which is optimal for the combination of fluids in the tank, for example if the incoming fluid has a lower or a higher specific weight than the fluid already present in the tank.

The side passage exists, for example, of the following parts (or a combination thereof, depending on the practical requirements which are imposed on the specific application by, among others, the medium to be transported, the geographical situation, the seismological character of the location and the like) :

- a flange in the inner tank, to which the flexible pipe is attached. The flange serves for the reduction of the material load in the tank wall. The flange marks the passage height of the pipe through the inner tank and is preferably attached in the tank wall as low as allowably possible. This flange is positioned in such a way that the most heavily loaded zone of the side wall is spared and no material tension increase occurs in the corner connection between the bottom and the side wall of the tank. The bottom side of the flange may typically be situated at a height of 0 to 2 meter above the bottom of the inner tank. the flange is preferably dimensioned in such a way that the load level of the tank wall at the flange is equal or lower than on a spot which is not reinforced.

If the flange in the side wall is situated at a height which is higher than the lowest fluid level desired in the tank, a lead-up tube may possibly be provided which reaches in the inner tank from the level of the flange up to that desired lowest fluid level.

Possibly a hole at the highest point of the lead-up tube in order to led gas escape so that no vapour bubble can develop in the pipe, which obstructs the suction of the pump. A welded-in tube passage in the flange of the inner tank. This tube is preferably manufactured of equal material as the tank wall and the flange in order to avoid material tensions resulting from unequal shrinkage.

A flexible connection between the inner tank and the outer tank wall. The flexible connection may exist of, for example, a synthetic hose with metal end pieces, which is welded between tube of the inner tank and the outer tank wall. Another possibility for a flexible connection exists of a corrugated tube or a long metal tube which can compensate for the occurring displacements by elastic deformation. As a metal tube extends over a larger part of the circumference of the tank, it will more easily be able to compensate for a relative displacement of its two ends (connections) by elastic deformation. It is even possible that the tube extends over approximately 360° or more, but in practice it will not be necessary that the tube extends over more than 90°. - A static insulated fluid-tight passage of the pipe through the outer wall. This static passage is implemented in such a way that all forces which expectedly might be exerted by the external piping may be absorbed by the passage construction.

- An isolation sheath between the tube wall and the (concrete) outer tank.

- A doubly implemented automatically operated shutter at the outside of the outer tank which closes the discharge pipe in case of a leak.

- If desired, a flow rate operated shutter may be included in the pipe, which instantaneously closes the pipe in case of a pipe breaking.

- A vapour discharge pipe with automatically operable shutter in order to discharge vapour bubbles in the supply pipe to the pump.

- The side pipe may be used for both discharge and supply. With this, the piping is further reduced.

- A pump arranged on the outside of the tank.

Also patent is asked for a system for a single side discharge or single side supply, and a system for a combined side supply and side discharge of cryogenic fluids to the tank:

- a system which makes use of a single side passage pipe for supply of fluid to the tank or discharge from the tank, or a single side passage pipe which is usable for both supply of fluid to the tank or discharge from the tank, as well as stirring means to prevent layer formation in the tank and to keep the temperature in the tank as homogeneous as possible. The system comprises a side passage pipe, a stirrer attached in the tank, supply or discharge pipes attached outside the tank, pumps and shutters and the required control members. - a monitoring system to detect a leakage of the inner tank, the flexible pipe or the connection of the flexible pipe to the outer tank. Such a warning system may be based on temperature measurement near the bottom of the gap between the inner and outer tank.

- A system to energetically optimise the operation of the stirrers, comprising different temperature sensors and/or sensors for determination of the specific mass, installed at different heights between the fluid surface of the tank and the bottom of the tank wherein the switching on and speed of the stirrers is set based on the differences between the measured signals.

- Mechanical stirrers which bring the tank content into circulation. Such a stirrer may be implemented as a propeller attached to a rotating drive axle. The stirrer may be suspended cardanically, so that no load is exerted on the tank roof in case of external forces on the stirrer, while further the drive axle is able to withstand large deflections, which is favourable at the occurrence of an earthquake. With this, the operating security in seismically active areas is guaranteed under all circumstances. The stirrer may optionally turn counter-clockwise or clockwise in order to establish an optimal mixing of the different fluids .

- The system has reduced energy use for homogenising the tank content, a reduction of the boil-off gas and cheaper install and maintenance of the tank.

During construction of the tank, an entry opening is made in the inner and outer tank through which all materials are brought into the tank. It is logical and easy to install the side passage in this opening, hereby an extra processing during construction of the tank is avoided. The safety can be brought at the same level as in the case of a tank with plunger pump and fluid supply and discharge through the top side of the tank by application of one or more of the following constructional measures:

- A flange on the wall of the inner tank which is implemented in such a way that the material load at the spot of the tube passage is equal to or lower than in the rest of the wall.

- a double, automatically operated shutter at the outside of the outer tank,

- a flexible connection between the inner and the outer tank,

- realising the tube passage outside the most heavily loaded zone of the inner tank. Flow forces in the double bypass in the plane of the tank wall and can be absorbed well,

- the flexible pipe is placed tangentially in the annular opening between the inner and outer tank, and is slightly slanting, so that gas accumulation in the pipe is prevented.

- The connections in the inner and the outer tank are connected fluid-tightly with the wall by means of a. welding connection.

Practical preferred embodiment details of the invention 1. The inserted tube in the wall of the inner tank is preferably made of material which is weldable well with the material of the inner tank and which has the same expansion coefficient. In case the tank is intended for LNG, the inner tank is usually made of 9% Ni steel. The tubes are usually of RVS 304 and the like and this is poorly suitable to combine with 9% Ni steel with regard to expansion coefficient. In the proposed invention therefore, the tube part which is welded in the tank wall is made of 9% Ni steel, to which then the other components manufactured from RVS 304 are attached. The length of the 9% Ni tube part is sufficient to absorb the tension resulting from the differences in thermal shrinkage between RVS 304 and 9% Ni, an indication of the required length is 5 to 10 cm. 2. The passage of the pipe through the outer wall is implemented as an anchor point wherein the pipe is anchored in the outer wall in an isolated way, and wherein, at the inner side, a metal membrane of the same material as the thermal corner protection is welded between the pipe and the wall lining.

3. One of the methods in the application exists of continuously mixing the present fluids by application of one or more stirrers (propellers ) , which may be driven from the roof of the tank counter-clockwise or clockwise as desired.

4. In the use of one side passage for both supplying and discharging fluid, it is desirable to provide the tank with stirrers to make sure that the temperature of the tank content is equalised and that rollover of fluid layers resulting from too large temperature and/or weight gradients is prevented.

5. The system is preferably implemented in such a way that the pipe ascends from the pump, which is placed outside the tank, to the opening in the inner tank. This prevents that a gas bubble is formed in the pipe which may impede the sucking in of the fluid.

6. A suction tube may be attached on the opening in the inner tank. On the inner side of the tank, a hole is made in the suction tube whereby accumulation of gas in the pipe is prevented, so that trie priming of the pump does not cause problems, among others when starting to use the installation. Figure 1 schematically illustrates a tank design according to the state of the art. A tank assembly 100 comprises a tank 110 with a roof 111. In the tank 110, a vertical filling pipe 113 and a vertical exhaust pipe 114 are arranged, which extend through the roof 111. A plunger pump 115 is arranged at the bottom of the exhaust pipe 114; cabling for the plunger pump 115 is attached inside the exhaust pipe 114, but is not shown. A supply pipe 121 situated at bottom level connects to the vertical filling pipe 113 through a vertical supply pipe part 121a, at a location on the roof 111, where also a supply valve 123 is arranged. A discharge pipe 122 situated at bottom level connects to the vertical exhaust pipe 114 through a vertical discharge pipe part 122a, at a location on the roof 111, where also an exhaust valve 124 is arranged. In order to be able to stir the cryogenic fluid 112 in the tank 110, a coupling pipe 116 with a coupling valve 117 is arranged on the roof 111, that can couple the filling pipe 113 and the exhaust pipe 114 with each other; stirring occurs then by activating the plunger pump 115. Large disadvantage of this construction are the required vertical pipes, the heavy installation on the roof 111,, the necessity that the plunger pump has to be arranged at trie bottom of the cryogenic fluid, and the fact that the plunger- pump in operation produces heat, which leads to heating of the fluid and as a result to gas formation, i.e. loss of fluid.

Figure 2 schematically illustrates a tank design according to the present invention. The cryogenic tank 200 comprises an outer tank 210 with a roof 213, an outer tank side wall 211 and an outer tank bottom 212, and an inner tank 220 arranged within the outer: tank 210 with an inner tank side wall 221 and an inner tank bottom 222. A cryogenic fluid 201 is present in the inner tank 220. In the sketched preferred embodiment, the cryogenic tank 200 has a stirring system 240, which comprises a vertical drive axle 241 having a propeller 242 at its bottom end, and of which the top end extends through the roof 213 and is driven by a motor 243 arranged on the roof 213. Such a stirring system 240, which is also applicable to a tank according to the state of the art, has, among others, the advantages that the motor is not submerged in the cryogenic fluid, and that the heat produced by the motor is not given off to the cryogenic fluid. Alternatively, the drive axle could 3oe directed horizontally, and could pass the side walls of the inner tank 220 and the outer tank 210 through passages, but then complicated passages would be necessary, -which have to be designed to prevent leakage of cryogenic fluid; a passage through the roof is simpler, while further, the gravity, acting on the propeller works in the longitudinal direction of the drive axle. Instead of one single propeller, the drive axle may be provided with multiple propellers, on axial distance relative to each other. The stirring system 240 may also coiαprise two or more drive axles next to each other, each with accompanying propeller (s) and motor: driving multiple smaller propellers may be energetically more favourable than driving one single large propeller. Then, it is also possible that propellers are driven in mutually different directions o f rotation, in order thus to realise an optimal flow pattern. The stirring motor 243 may be a simple motor, which can only turn with one single speed in one single direction. Preferably however, the stirring motor 243 is adapted to work with variable number of revolutions. Further, the stirring engine 243 is preferably adapted to selectively rotate counter clockwise or clockwise. Figure 3 shows a block diagram which schematically illustrates a control circuit for a stirring motor 243. In the inner tank 220, a system of temperature sensors 245 for measuring the temperature at different heights in the cryogenic fluid is installed, in order thus to be able to determine the temperature distribution in the cryogenic fluid; in figure 3, four of such temperature sensors are shown. Further, a system of density sensors 246 for measuring the density (specific mass) at different heights in the cryogenic fluid is installed in the inner tan-k 220, in order thus to be able to determine the density distribution in the cryogenic fluid; in figure 3, four of such density sensors are shown. Since suitable temperature sensors and density sensors intended for cryogenic circumstances are known per se, a further discussion thereof is not necessary here. The stirring motor 243 is controlled by a control member 244, that receives the measurement signals of the temperature sensors 245 and the density sensors 246. The control member 244 is adapted to switch on the stiirring motor or not, and/or to select the direction of rotation of the stirring motor, and/or to select the speed of rotation of the stirring motor based on the received measurement signals.

According to an important aspect of the present invention, the tank 200 has a side connection for a fluid pipe 230, at a level which is situated jiust a little higher than the level of the bottom 222 of the inner tank 220. Figure 2 shows a supply pipe 231 with an entry valve 233 arranged outside the outer tank 210. Figure 2 further shows a discharge pipe 232 with an exhaust valve 234 arranged outside the outer tank 210. Beyond the exha"ust valve 234, a pump 235 is installed in the discharge pipe 232. It is a big advantage that the pump 235 is installed at bottom level, and that the pipes 231 and 232 do not have to be led to the roof level through vertical pipes. In the sketched embodiment, both the supply pipe 231 and the discharge pipe 232 are connected to the fluid pipe 230, thus offering a common passage. Depending on the position of the valves 233 and 234, the fluid pipe 230 functions as entry passage or as exhaust passage. It is also possible that there are two passages present, one for the supply and the other for the exhaust, but that is not shown.

Figure 4 shows a schematic perspective view of the side passage construction on larger scale. The figu-re shows a part of the side wall 211 and bottom 212 of the outer tank 210, as well as a part of the side wall 221 and bottom 222 of the inner tank 210, sectioned at the passage. The figure shows that the fluid pipe 230 has a tube part 236 extending outside the outer tank 210, and a tube part 237 extending in the annular space 202 between the inner tank and tine outer tank, and which is connected to a connection opening 238 in the inner tank side wall 221. From this connection opening 238, a tube 239 leads to the inner tank bottom 222, b"ut that is not essential . In case of a side passage for discharging fluid, this tube 239 has the function of a suction tube, so that fluid can be pumped out of the tank to a level lower than the connection opening 238. In case of a side passage for supplying fluid, a mouth piece (not shown) may be formed or attaclied on the free end of that tube 239, especially designed for inducing a desired circulation in the inner tank 220, which is then useful for the purpose of stirring the fluid in the inner tank. In that last case, that tube 239 does not need to be directed towards the inner tank bottom 222. In a possible embodiment variation, the outer tank side wall is provided with at least two passage openings, each having a fluid pipe extending through it, wherein each fluid pipe is connected to a corresponding connection opening in the inner tank side wall. Such an embodiment variation, which is not illustrated, offers the advantage that it is possible to install at least one connection pipe provided "with a pump outside the outer tank, which connects the said two fluid pipes, in order to enable stirring by pumping around. In that case, the mechanical stirring system 240 may be left out , but it is also possible that the pumping around is combined " ith the mechanical stirring system 240. Preferably however, -the number of passages is as small as possible. The presence of the mechanical stirring system 240 offers the advantage that, while preserving stirring facility, one single passage suffices, which, depending on the circumstances, is used for either supply or discharge. Figure 4 further shows that the connection opening 238 is situated on a somewhat higher level than the tube part 236. The fluid pipe 230 has a passage pipe part 236 which extends through a passage opening in the outer tank side wall 211, but that opening is not visible in this figure. According to an important aspect of the present invention, the pipe part 237 which extends in the annula space 202 between the inner tank and the outer tank is a t least partly implemented as a flexible pipe. This pipe part 237 may, at least partly, extend tangentially, so that the angular position of the connection opening 238 is shifted horizontally relative to the angular position of the passage opening in the outer tank side wall 211. This is illustrated in figure 5, which shows a schematic top view of the side passage construction. The passage opening in the outer tank side wall 211 is indicated at 214. In the embodiment shown, a knee coupling piece 251 is mounted in the inner tank side wall 221 near the connection opening 238 , and also a 90° coupling piece 252 is mounted near the passage opening 214 in the outer tank side wall 211. A tangential pipe part 253 extends between these coupling pieces 251, 252. This tangential pipe part 253 may be oriented substantially horizontally, but preferably, the first en-d 253a situated near the passage opening 214 in the outer tank side wall 211 is somewhat lower than the second end 253b situated near the connection opening 238 in the inner tank side wall 221. Preferably, the tangential pipe part 253 has a substantially constant angle of inclination over its total length. In fact, going from second end 253b to first end 253a, no inclining parts occur, in order to prevent vapour accumulation. The tangential pipe part 253 is designed to be able to compensate for relative mutual displacements of its connection ends (coupling pieces 251, 252) without damage. In case of thermal shrinkage of the inner tank, the knee coupling piece 251 in the inner tank side wall 221 will mainly displace radially, which means a transverse displacement of the second pipe end 253b. In case of displacement of the inner tank 2-20 relative to the outer tank 210 resulting from an earthquake, the second pipe end 253b will be displaced axially or transversely, depending on the displacement direction. The longer the tangential pipe part 253, the more easily the tangential pipe part 253 can compensate for such shape changes. It will be easily appreciated that, if the tangential pipe part 253 has a length of 90° tank circumference, the shape change can be compensated for virtually without tensile strain, independent of the displacement direction. Depending on the choice of material of the tangential pipe part 253, such a length however is not necessary. Preferably, the tangential pipe part 253 is implemented as a flexible pipe part. The pipe part 253 can have the desired flexibility because it is implemented as a metal bellows pipe. Alternatively, the pipe part 253 may be implemented as a synthetic pipe, preferably provided with a helically wound wire armouring. In any case, the material of the pipe part 253 must be resistant against cryogenic fluid, and allow sufficient elastic deformation at cryogenic temperatures. In the embodiment shown, the 90° coupling piece 252 mounted in the outer tank side wall 211 near the passage opening 214 is a T-piece, that is connected to second connection opening 238B in the inner tank side wall 221 through a second tangential pipe part 254 and a second knee coupling piece 255. The passage opening (inner diameter) of each pipe part 253, 254 may then be smaller, whereby those pipe parts are more flexible, while the total flow capacity corresponds to that of the common passage pipe 236. The numb r of connection openings 238 in the inner tank side wall 221 per passage opening 214 in the outer tank side wall, with accompanying pipe parts, may also be more than two, with corresponding adaptations to the distribution piece 252. For the second tangential pipe part 254, the same rema ks apply as those made above in relation to the first tangentia 1 pipe part 253.

As is already illustrated in figure 2, and as is clearly visible in figure 5, the pipe parts situated inside the oute r tank 210 are free of shutter valves. Going into the direction from inside to outside, a first shutter valve 234 is situated outside the outer tank 210. This has the advantage, among others, that it is not necessary to enter the inner space of the outer tank 210 for maintenance on the shutter valve 234. Further, it is shown that preferably a second shutter valve 234B is provided beyond the first shutter valve 234, the pipe part between the first shutter valve 234 and the second shutter valve 234B being free of branches, and wherein it is preferred that a housing of the second shutter valve 234B is fixed to a housing of the first shutter valve 234. Hereby it is possible to use the second shutter valve 234B during operation, wherein the first shutter valve 234 is always open. In case of malfunctions of the second shutter valve 234B, t e first shutter valve 234 can then be closed and the second shutter valve 234B can be disassembled for maintenance, repa ir or replacement. This offers the important advantage that, if the main valve 234B must be replaced, or needs maintenance, it is not necessary to empty the entire tank. A potential risk with shutter valves placed outs ide the tank is a leakage of the fluid pipe on a location bet-ween the tank and the first shutter valve 234. In order to red ice this risk drastically, a housing of the first shutter valve 234 preferably forms a fixed entity with a passage construction near the entry opening 214 in the outer tank side wall 210, so that the first shutter valve 234 is fixed to the outer tank side wall 210 and the fluid pipe 230 is protected extensively and effectively against mechanical damages. A further protection is offered if the tank 200, as illustrated in figure 5, is provided with a protection bunker 260 fi -xed to the outer tank 210, extending around the passage construction and the at least one shutter valve 234, 234B. This p otection bunker 260 constructively forms a whole with the oute r tank 210 which is resistant against mechanical loads, and surrounds an inner space 261 in which the shutter valves 234, 234B are arranged. The protection bunker 260 only has a mechanical protection function, so it does not need to be fluid- tight. It is noted that this protection construction is also usable with tanks which are not provided with a flexible pipe part. It is further noted that this protection construction is also usable with tanks which are not provided with a thermal protection system.

Figure 6 is a schematic section on larger scale, illustrating details of the passage construction. The figure illustrates that a metal, fluid-tight and vapour-tight lining 271 is attached to the inner side of the concrete outer tank side wall 211, typically in the form of a 3 mm thick plate of 9% Ni steel. Between the concrete and the metal linin-g ^'271, an insulation layer 272 is situated, typically a TPS- in-sulation. In the passage opening 214 in the outer tank side wall 211, a steel tube 281 is installed, that is anchored in the concrete 211 by anchoring pins 282. Within the steel tube 281, the fluid pipe 236 extends through the passage opening 214; this fluid pipe 236 is typically made of steel RVS304. In the space between the fluid pipe 236 and the steel tube 281, a thermally insulating material 283 is arranged, preferably a polyurethane foam with high density, which pairs good insulating properties with large strength. So the material 283 does not only provide thermal insulation between the cold fluid pipe 236 and the outer tank side wall 211, but also provides the force transmitting, mechanical anchoring of the fluid pipe 236 to the outer tank side wall 211. For the purpose of increasing the mechanical strength of this connection, the steel tube 281 at its inner side is preferably provided with protrusions 284 extending into the thermally insulating material 283, and further, the fluid pipe 235 is preferably provided with lateral protrusions 285. At the location of the passage opening 214, the lining 271 and the insulation layer 272 are interrupted. According to an important aspect of the present invention, the passage construction is fluid-tight, and the integrity of the thiermal protection of the concrete of the outer tank side wall 211 is assured, because an insulating socket 273 is attached around the fluid pipe 236 at the inner side of the outer tank side wall 211, which insulating socket 273 is made of a fluid-tight and vapour-tight material and connects fluid-tightly and vapour-tightly to the fluid pipe 236 on the one hand and the metal lining 271 on the other hand. In the inner space 276 surrounded by the insulating collar 273, preferably an insulating material is arranged, for example glass wool. The insulating collar 273 is preferably made of a cold ductile material, preferably a suitable kind of steel, such as for example a 9% nickel steel, which is cold resistant well, as well as the metal of the lining 271. A fluid-tight and vapour-tight connection between the insulating socket 273 and the lining 271 can be achieved by welding the outer circumference 274 of the insulating socket 273 to the metal lining. In a similar way, a fluid-tight and vapour-tight connection between the insulating socket 273 and the flui d pipe 236 can be achieved by welding the inner circumference 275 of the insulating socket 273 to the fluid pipe 236. An advantage of the use of a nickel steel for the insulating socket 273 is that this material is weldable well to the metal of the lining 271. However, it is possible that the nickel steel is difficult to weld to the steel of the fluid pipe 236. To avoid this problem, according to the invention, the fluid pipe 236 comprises a pipe part 236B, of which at least the outer surface is made of a nickel steel, preferably a 9% Ni steel. Then, the inner circumference 275 of the insulating socket 273 can be welded relatively easily to the outer surface of this pipe part 236B. It suffices if this pipe part 236B is only several centimetres long. It is possible to manufacture this pipe part 236B as a tube part, which is welded on end to the fluid pipe 236. It is noted that this passage construction is also u.sable with tanks which are not provided with a flexible pipe part.

It will be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention- as defined in the attached claims. For example, it is possible that the insulating sockzet is made of a polyurethane foam, which is cold-ductile to a sufficient extent, which is fluid-tight and vapour-tight, and which adheres well to the material of the lining 271 and the passage pipe 236. An example of such a suitable polyureth-ane foam, which is also thermally insulating, is a two-component polyurethane composition which is commercially available from the company TAGOS S.r.L. in Busto Arsizio, Italy, under the trade name IWR ESATEC HR 1000. In the market, this material is also known under the name IWR CRYOCOAT HR, and is commercially available from the INSU-W-RAPID B.V. in Tilburg, tlhe Netherlands . Further, it is possible that the tangential ipe has a vertical pipe part.

Claims

1. Cryogenic tank (200), comprising an outer tank (210) having an outer tank side wall (211) and an outer tank: bottom (212), and an inner tank (220) arranged inside the outer tank, having an inner tank side wall (221) and an inner tank: bottom (222); wherein the outer tank side wall (211) has a passage opening (214) for a fluid pipe (230) for supplying cryogenic fluid to the inner tank (220) and/or discharging cryogenic fluid from the inner tank (220) ; wherein the fluid pipe is connected to a connection opening (238) in the inner tank side wall (221) ; and wherein the fluid pipe between the inner tank (220 ) and the outer tank (210) has a pipe part (237; 253; 354) extending substantially tangentially.

2. Cryogenic tank according to claim 1, wherein the fluid pipe between the inner tank (220) and the outer tank ( 210) has a pipe part (237; 253; 354) which is at least partly implemented as a flexible pipe.

3. Cryogenic tank according to claim 1 or 2, wherein the passage opening (214) in the outer tank side wall is positioned at the same height as or somewhat lower than the connection opening (238) in the inner tank side wall ( 221), and wherein the fluid pipe (237; 253; 354) between the inner tank (220) and the outer tank (210) is positioned horizontally or graded, i.e. has no inclining parts from inner tank: to outer tank.

4. Cryogenic tank according to any of the preceding claims, wherein the tangential pipe (237; 253; 354) is implemented as a metal bellows pipe.

5. Cryogenic tank according to any of the preceding claims 1-3, wherein the tangential pipe (237; 253; 354) is implemented as a synthetic pipe, preferably pro-vided with a helically wound wire armouring.

6. Cryogenic tank according to any of the preceding claims, wherein at least two connection openings (238; 238B) in the inner tank side wall (221) are present per passage opening (214) in the outer tank side wall (211); wherein the fluid pipe has a passage part extending through the said passage opening; wherein fluid pipe is provided with a distribution piece (252) connected to the passage part; wherein the fluid pipe comprises multiple connection pipe parts (253; 254) , wherein each connection pipe jpart is connected to the passage part on the one hand and is connected to a connection opening (238; 238B) on the othejc hand.

7. Cryogenic tank according to any of the preceding claims, wherein the connection opening (238) in the inner tank side wall (221) is situated at a certain height above the inner tank bottom (222) ; wherein a tube (239) extending into the inner tank (220) is connected to the connection opening in the inner tank side wall (221) , of which a free end is situated at a level lower than the level of the connection opening (238) , preferably near the inner tank bottom (222) .

8. Cryogenic tank according to any of the preceding claims, wherein a tube (239) extending into the inner tank (210) is connected to the connection opening (238) in the inner tank side wall (221) , of which tube a free end is provided with a mouth piece designed for inducing a desired circulation in the fluid (201) in the inner tank (220) .

9. Cryogenic tank according to any of the preceding claims, wherein the outer tank side wall is provided with at least two passage openings, each having a fluid pipe extending through it, wherein the inner tank side wall is provided with at least two connection openings, wherein a first of said fluid pipes is connected to a first of said connection openings, wherein a second of said fluid pipes is connected to a second of said connection openings, and wherein at least one connection pipe provided with a pump is present outside the outer tank, connecting the said two fluid pipes, in order to enable stirring by pumping around.

10. Cryogenic tank (200), comprising an outer tank (210) having a outer tank side wall (211) and an outer tank bottom (212), wherein the outer tank side wall (211) is manufactured of concrete, wherein a metal, fluid-tight and vapour-tight lining (271) is attached to the inner side of the outer tank side wall (211), with an insulation layer (272) between the concrete and the metal lining (271) ; wherein the outer tank side wall (211) has a pas sage opening (214) with a fluid pipe (236) for cryogenic fluid extending through it; wherein the fluid pipe (236) is fixed relative to the outer tank side wall (211) ; and wherein an insulation collar (273) is arranged at the inner side of the outer tank side wall (211) around the fluid pipe (236) , which insulation collar (273) is made of a fluid- tight and vapour-tight material and connects in a fluid-tight manner and vapour-tight manner to the fluid pipe (236) on the one hand and the metal lining (271) on the other hand.

11. Cryogenic tank according to claim 10, wherein the insulation collar (273) is made of a cold-ductile and fluid- tight material, for example steel, such as nickel steel, and is fixed in a fluid-tight manner to the fluid pipe (236) and to the metal lining (271), for example welded.

12. Cryogenic tank according to claim 11, wherein the fluid pipe (236) , at least at the location of the connection to the insulation collar (273) , comprises a pipe part ( 236B) which is made of a cold-ductile and fluid-tight material, for example steel, such as a nickel steel.

13. Cryogenic tank according to any of the preceding claims 10-12, wherein a thermally insulating material (283) is arranged in the passage opening (214) between the fluid pipe (236) and the concrete outer tank side wall (211) , which thermally insulating material also provides the force transmitting fixation of the fluid pipe (236) to the concrete outer tank side wall (211) .

14. Cryogenic tank according to claim 13, wherein the thermally insulating material (283) is a PURfoa .

15. Cryogenic tank according to claim 13 or 14, w-herein, at the position of the said thermally insulating material (283), the fluid pipe (236) is provided with lateral protrusions (285) .

16. Cryogenic tank according to any of the preceding claims 13-15, wherein a steel tube (281) anchored into the concrete is arranged in the passage opening (214), which tube is provided with protrusions (284) on its inside extending into the said thermally insulating material (283).

17. Cryogenic tank according to any of the preceding claims 1-9, wherein the construction of a pipe passage in the outer tank side wall is implemented as described in any of the preceding claims 10-16.

18. Cryogenic tank according to any of the preceding claims, wherein the fluid pipe (230) is provided with at least one shutter valve (234) for closing the fluid pipe, wherein the pipe parts situated inside the outer tank (210) are free from shutter valves and, seen in the direction from ins ide to outside, the first shutter valve (234) is situated outside the outer tank (210) .

19. Cryogenic tank according to claim 18, further provided with a second shutter valve (234B) beyond the first shutter valve (234), wherein the pipe part between the first shutter valve and the second shutter valve is free from branches, and wherein preferably a housing of the second shutter valve is fixed to a housing of the first shutter valv^e .

20. Cryogenic tank according to claim 18 or 19, wherein a housing of the first shutter valve (234) forms a fixed entity with a passage construction at the passage opening in the outer tank side wall and is thus fixed to thαe outer tank side wall (211) .

21. Cryogenic tank according to any of the preceding claims 18-20, provided with a protection bunker (260) fixed to the outer tank (210), extending around the passage construction and the at least one shutter valve (234).

22. Cryogenic tank (200), preferably implemented according to any of the preceding claims, comprising a container (221) for receiving therein a cryogenic fluid (201) , wherein at least one rotatable mechanical stirring member (240) is arranged in that container, which stirring member comprises stirring blades (242) attached to a drive axle (241); wherein the stirring member (240) is provided with a stirring motor arranged outside the cryogenic fluid for driving the drive axle of the stirring member.

23. Cryogenic tank according to claim 22, wherein the drive axle (241) of the stirring member (240) is directed vertically.

24. Cryogenic tank according to claim 23, wherein the container has a roof (213) , wherein the stirring motor (243) is arranged on the roof (213) and the drive axle (241) of the stirring member (240) extends through the roof (213) .

25. Cryogenic tank according to any of the claims 22-24, wherein the stirring motor (243) is adapted to work with variable rotational speed.

26. Cryogenic tank according to any of the claims 22-25, wherein the stirring motor (243) is adapted to rotate selectively counter clockwise or clockwise.

27. Cryogenic tank according to any of the claims 22-25, wherein multiple stirring members (240) are present in the container.

28. Cryogenic tank according to any of the claims 22-26, further provided with: a system of temperature sensors (245) for measuring the temperature distribution in the cryogenic fluid (201) ; a system of density sensors (246) for measuring ttie density distribution in the cryogenic fluid (201) ; a control member (244) for controlling the stirring motor (243), wherein the control member (244) is connected for receiving measurement signals of the temperature sensors (245) and the density sensors (246) , and wherein the control member (244) is adapted to switch on the stirring motor (243) or not, and/or to select the direction of rotation of the stirring motor (243) , and/or to select the speed of rotation of the stirring motor (243) , based on the received measurement signals .

29. Cryogenic tank according to any of the claims 22-26, provided with a single supply/discharge pipe for selectively supplying or discharging cryogenic fluid.

30. Connection system for cryogenic tanks, comprising: a passage pipe through an outer tank side wall; a connection to an inner tank side wall; a flexible tangential pipe part between the outer tank and the inner tank.