Control method for a cryogenic tank, corresponding cryogenic tank and underwater vessel

The present invention relates to a method for monitoring a cryogenic tank comprising an enclosure containing a liquefied gas, the liquefied gas forming a liquid phase and a gas phase in the enclosure, the method comprising the following step: measurement of a parameter representative of a pressure of the gas phase and of a parameter representative of a pressure in the liquid phase.

The invention also relates to a cryogenic tank and to an underwater vessel comprising such a tank.

Anaerobic propulsion systems (or Al P, short for Air Independent Propulsion) are a relatively recent type of power generation system for submarines that can operate for a long time without using outside air. This prevents the anaerobic submarine from exiting its fresh air tube (or snorkel), thus limiting the need to rise close to the surface, and therefore its vulnerability.

The anaerobic propulsion system brings to conventional submarines a notable improvement in their diving autonomy (a few days, against a few dozen hours for a conventional propulsion submarine) and consequently their discretion.

Anaerobic propulsion systems include in particular conventional combustion engines using standard fuel, such as diesel engines, and as oxidant oxygen stored in liquid form on board the submarine, in a cryogenic tank. Liquid oxygen can also be used to renew part of the air breathed by the crew, which helps to further increase autonomy.

The control of the quantity of liquid oxygen contained in the tank, as well as the performance of the insulation, are essential to evaluate the autonomy and the safety of the underwater vessel. In addition, a cryogenic tank may require purges to the exterior of the building, which need to be anticipated as part of a mission requiring stealth.

Thus, the management of a cryogenic tank poses a certain number of difficulties, in particular in an underwater vessel constituting a confined environment and requiring a high degree of discretion to be maintained.

Existing tank management methods are based on measuring the internal pressure of the tank to estimate the mass contained in the tank and to detect deterioration of the insulation.

However, it would be advantageous to further improve the precision and reliability of these methods.

Thus, one aim of the invention is to allow more precise and more reliable management of a liquefied gas tank, in particular within an underwater vessel.

To this end, the subject of the invention is a control method of the aforementioned type, in which the method also comprises the following steps:

- measurement of a parameter representative of a temperature of the liquid phase; and

- Determination of a total mass of the liquid phase from parameters representative of the temperature of the liquid phase, the pressure in the liquid phase and the pressure of the gas phase.

According to particular embodiments, the method according to the invention has one or more of the following characteristics, taken in isolation or in any technically feasible combination:

- the method also comprises the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase;

- Carrying out a first test of the parameter representative of the temporal evolution of the temperature of the liquid phase and obtaining a first result; and

- signaling of a need for maintenance of the cryogenic tank according to the first result;

- the parameter representative of the temporal evolution of the temperature of the liquid phase is a temporal derivative of the parameter representative of the temperature of the liquid phase, the first test being a comparison of the temporal derivative of the parameter representative of the temperature of the phase liquid with a predetermined threshold value, and the need for maintenance being signaled if the time derivative of the parameter representative of the temperature of the liquid phase is greater than the threshold value;

- the method also comprises the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase;

- Carrying out a second test of the parameter representative of the temporal revolution of the temperature of the liquid phase and obtaining a second result; and

- signaling of a loss of thermal insulation of the enclosure according to the second result;

- the parameter representative of the temporal evolution of the temperature of the liquid phase is a temporal derivative of the temperature of the liquid phase, the second test comprises a calculation of a parameter representative of the continuity of the temporal derivative of the temperature of the liquid phase and the comparison of the parameter representative of the continuity with a predetermined threshold, the loss of thermal insulation being signaled if the parameter representative of the continuity is greater than the threshold;

- the method also comprises the following steps:

- comparison of the temperature of the liquid phase with a predetermined threshold; and

- signaling of a need to purge the cryogenic tank if the temperature of the liquid phase is above the threshold;

- the method further comprises the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase;

- Calculation of an expected date of need for purging the cryogenic tank from the temperature of the liquid phase, the parameter representative of the temporal revolution of the temperature of the liquid phase and the purge threshold; and or

- the cryogenic tank is located in an underwater building propelled by a propulsion system comprising an energy production system.

The subject of the invention is also a cryogenic tank comprising:

- an enclosure containing a liquefied gas, the liquefied gas forming a liquid phase and a gas phase in the enclosure;

a first pressure sensor arranged to measure a parameter representative of a pressure in the liquid phase;

a second pressure sensor arranged to measure a parameter representative of a pressure of the gas phase;

a temperature sensor designed to measure a parameter representative of a temperature of the liquid phase; and

- a control module configured to control the first pressure sensor, the second pressure sensor and the temperature sensor, and to determine a total mass of the liquid phase from the temperature of the liquid phase, from the pressure in the liquid phase and the pressure of the gas phase.

A further subject of the invention is an underwater vessel with anaerobic propulsion, comprising a propulsion system comprising an energy production system and at least one cryogenic tank as defined above, the liquefied gas contained in the cryogenic tank being oxygen and feeding the energy production system as an oxidizer.

The invention will be better understood on reading the following description, given solely by way of example and made with reference to the appended drawings, among which:

- Figure 1 is a side view of an underwater vessel according to the invention;

- Figure 2 is a schematic sectional view of a cryogenic tank of the submarine of Figure 1;

- Figures 3 and 4 are graphic representations of the temporal evolution of a temperature of the fluid contained in the reservoir of Figure 2.

In Figure 1, an underwater vessel 10 is shown plunged into a body of water 12, under a surface 14 of the body of water 12.

The term “submarine” is understood to mean that the vessel is suitable and intended to make journeys completely submerged, in particular for a period greater than or equal to one day. Such a building is also able to return to the surface.

The underwater vessel 10 comprises a hull 16 delimiting an internal space sealed off from the body of water 12, as well as a propulsion system 18 received in the internal space and at least one propeller 20 driven by the propulsion system 18, and suitable for setting the underwater vessel in motion.

The submarine building 10 is an anaerobic propulsion submarine, that is to say that the propulsion system 18 does not require outside air for its operation.

The propulsion system 18 is a thermal-electric system, which comprises, in a known manner, an electric motor 22 driving the propeller 20, an alternator 24 and a turbine 26 supplying the electric motor 22. The propulsion system 18 also comprises a steam generator 28, a condenser 30 and a heat exchanger 32 driving the turbine 26, as well as an energy production system 34, more particularly a combustion chamber, generating the heat supplying the heat exchanger 32.

The propulsion system 18 further comprises a fuel tank 36, for example containing ethanol, and a cryogenic tank 38, containing dioxygen. The fuel tank 36 and the cryogenic tank 38 supply the energy production system 34, here the combustion chamber, respectively with fuel and oxygen, which plays the role of oxidizer therein, to participate in an exothermic combustion reaction.

The cryogenic tank 38 is shown in more detail in FIG. 2. It comprises a storage enclosure 40 comprising a thermally insulating wall 42, in particular a double wall, defining an internal space for storing liquid oxygen.

The dioxygen contained in the cryogenic tank 38 forms a liquid phase 44 and a gas phase 46 in the chamber 40, separated by an upper surface 47 from the liquid phase 44.

The cryogenic tank 38 is shown in FIG. 2 in relation to an elevation direction Z-Z 'oriented in the local direction of gravity. The liquid phase 44, denser than the gas phase 46, occupies a lower part of the enclosure 40, and the gas phase 46 an upper part of the enclosure 40, relative to the direction of elevation Z-Z '.

The cryogenic tank 38 also comprises a first conduit 50 opening into the lower part of the enclosure 40 and a second conduit 52 opening into the upper part of the enclosure 40. The first conduit 50 is in particular an inlet conduit for liquid oxygen. in the enclosure 40, and the second duct 52 is in particular an outlet duct of the enclosure 40.

The first conduit 50 and the second conduit 52 are provided respectively with a first valve 54 and a second valve 56, suitable for controlling the entry and exit of fluid into the enclosure 40 through the first conduit 50 and the second conduit 52.

The first duct 50 and the second duct 52 advantageously comprise thermal insulation envelopes (not shown).

The first duct 50 and the second duct 52 comprise an internal portion extending between the enclosure 40 and the first valve 54 and the second valve 56 respectively, through the wall 42.

Advantageously, the internal portion of the first duct 50 opens out through a lower face 58 of the enclosure 40, relative to the direction of elevation Z-Z ', in order to ensure that the first duct 50 opens into the liquid phase 44 even for a low filling rate of the enclosure 40.

In one embodiment, the cryogenic tank 38 further comprises a first pressure sensor 60, located in the lower part of the enclosure 40 and a second pressure sensor 62, located in the upper part of the enclosure 40.

The first and second pressure sensors 60, 62 are suitable for measuring a hydrostatic pressure respectively in the liquid phase 44 and in the gas phase 46 of the oxygen contained in the enclosure 40.

More precisely, the first and second pressure sensors 60, 62 are suitable for measuring a parameter representative of a hydrostatic pressure respectively at a point of the liquid phase 44 and at a point of the gas phase 46.

By “representative parameter” is meant that the sensor measures a physical quantity making it possible to obtain the pressure directly by a direct calculation.

Advantageously, the parameter representative of the pressure in the liquid phase 44 is the pressure in the liquid phase 44 itself and the parameter representative of the pressure of the gas phase 46 is the pressure of the gas phase 46 itself.

The cryogenic tank 40 further comprises a temperature sensor 64 located in the lower part of the enclosure 40, capable of measuring a temperature of the liquid phase 44.

Advantageously, the parameter representative of the temperature of the liquid phase 44 is the temperature of the liquid phase 44.

More precisely, the temperature sensor 64 is suitable for measuring a parameter representative of the temperature at a point of the liquid phase 44. The parameter representative of the temperature is advantageously the temperature of the liquid phase 44 itself.

The temperature sensor 64 is for example a platinum resistance thermometer of the PT100 type.

Advantageously, the temperature sensor 64 is located in the internal portion of the first duct 50, facing the lower part of the enclosure 40, so that the temperature sensor 64 is able to measure the temperature of the liquid phase 44 itself. for a low filling rate.

The first pressure sensor 60, the second pressure sensor 62, and the temperature sensor 64 are connected to a control module 66, configured to follow the respective temporal evolutions of the pressure of the liquid phase 44 at the level of the first sensor of pressure 60, the pressure of the gas phase 46 and the temperature of the liquid phase 44.

The control module 66 comprises a processor adapted to execute computer programs and a memory adapted to store data.

The control module 66 is also configured to implement a method for controlling the cryogenic tank 38 described below and to determine, as a function of the temperature and pressure measurements, a mass of the liquid phase 44 contained in the enclosure 40. , a possible need for maintenance of the cryogenic tank 38, a possible degradation of the thermal insulation of the enclosure 40 and a possible need to purge the cryogenic tank 38, as well as to predict a date of need for purging the cryogenic tank 38.

The reservoir 38 also comprises a purge module 68 comprising a pump and fluidly connected to the first conduit 50 and to the second conduit 52 through purge valves 70. The purge module 68 is suitable for sucking part of the gas phase 46, in particular through the second duct 52, in order to lower the pressure of the gas phase 46 in the chamber 40, which causes vaporization of part of the liquid phase 44. Vaporization of part of the liquid phase 44 lowers the temperature in the enclosure 40, according to the enthalpy of vaporization of the dioxygen.

The purging of the cryogenic tank 38 by the purge module 68 requires being able to evacuate the part of the gas phase 46 sucked in, which can pose a problem in a confined environment.

In an alternative embodiment, the cryogenic tank 38 comprises a differential pressure transmitter (not shown), in place of the first pressure sensor 60, located in the lower part of the enclosure 40 and of the second pressure sensor 62. , located in the upper part of the enclosure 40.

The differential pressure transmitter is for example arranged outside the cryogenic tank 38. This allows in particular that the differential pressure transmitter is not in contact with the liquid phase.

A method of controlling the cryogenic tank 38 of the underwater vessel 10 described above will now be described. The cryogenic tank 38 contains liquid oxygen forming a liquid phase 44 and a gas phase 46.

The method comprises a step of repeatedly measuring a pressure in the liquid phase 44 by the first pressure sensor 60, the pressure of the gas phase 46 by the second pressure sensor 62, and the temperature of the liquid phase 44 by the temperature sensor 64.

More generally, the pressure 60, 62 and temperature 64 sensors measure parameters representing respectively the pressure in the liquid phase 44, the pressure of the gas phase 46 and the temperature of the liquid phase 44. Advantageously, the parameters representative are respectively the pressure in the liquid phase 44, the pressure in the gas phase 46 and the temperature of the liquid phase 44 themselves.

The temperature and pressure measurements are controlled by the control module 66 and the results of the measurements recorded in the memory.

The method also comprises a step of determining a mass m of the liquid phase 44, from the temperature and pressure measurements.

A density p of the liquid oxygen of the liquid phase 44 is evaluated from the temperature of the liquid phase 44, by the control module 66.

A filling height h of the enclosure 40 is then established as a function of a pressure differential DR, the difference between the pressure measured in the liquid phase 44 and the pressure of the gas phase 46, and of the equilibrium equation hydrostatic DR = pgh, where a is gravity.

The filling height h corresponds to the height separating the first pressure sensor 60 and the upper surface 47 of the liquid phase 44. In fact, the pressure in the gas phase 46 is considered to be substantially homogeneous, the gas phase 46 being much less. dense than the liquid phase 44.

The mass m of the liquid phase 44 is then calculated as a function of the filling height h and the geometry of the enclosure 40.

For example, the mass m of the enclosure 40 is obtained by a direct calculation as a function of the geometry of the enclosure 40. In the case where the enclosure 40 is of cylindrical shape with radius R, the mass m of the liquid phase is obtained by the formula: m = php R 2 .

Alternatively, in the case where the enclosure 40 has a complex geometry, the mass m is obtained from a filling chart established beforehand and recorded in the memory.

The mass m of the liquid phase 44 is displayed, for example on a screen, in order to inform an operator.

The precise and continuous measurement of the mass of liquid oxygen contained in the enclosure 40 allows more reliable control of the remaining autonomy of the submarine vessel 10.

The method further comprises a step of detecting a possible need for maintenance of the cryogenic tank 38. A temporal change in the temperature of the liquid phase 44 is determined from the temperature measurements obtained through the temperature sensor 64, and a time derivative of the temperature is calculated, repeatedly, by the control module 66.

The time derivative of the temperature is compared to a predetermined threshold value, and the control module 66 determines that the reservoir 58 requires maintenance 66 if the time derivative of the temperature is greater than or equal to the threshold value.

The need for maintenance of the cryogenic tank 38 is then signaled to an operator, for example by displaying a message on a screen.

This detection of the need for maintenance is shown in FIG. 3, which is a graphical representation of the temperature T of the liquid phase as a function of time t. The time derivative T of the temperature, that is to say the local slope of the temperature curve T, increases over time t. At a time noted t m on the graph, the time derivative T of the temperature T exceeds the threshold value T ′ max, and the control module 66 signals a need for maintenance of the cryogenic tank 58. The threshold value T ′ max of the time derivative of the temperature has an order of magnitude, for example, 0.5 K / h, more particularly is equal to 0.5 K / h.

Typically, the increase in the time derivative T of the temperature T is observed on a time scale of the order of one or more years.

The detection of the need for maintenance by monitoring the temperature of the liquid phase 44 provides more precise information than the pre-existing methods, with which the maintenance was carried out at constant intervals, without taking into account the specific aging of the cryogenic tank 38.

The method also comprises a step of detecting a possible degradation of the thermal insulation of the enclosure 40. The time derivative T of the temperature of the liquid phase 44 is calculated repeatedly as described above. The loss of thermal insulation of the enclosure 40 is detected when the time derivative T of the temperature of the liquid phase 44 increases discontinuously.

By this is meant that the curve of the temperature T of the liquid phase 44 as a function of time t has a slope discontinuity, as shown in FIG. 4 at an instant t ^ The instant ^ corresponds to a detection by the module control 66 of the degradation of the insulation of the enclosure 40, which generally requires urgent intervention.

To detect such a discontinuity in slope, the control module 66 calculates a parameter representative of the continuity of the time derivative of the temperature T of the liquid phase 44, and compares this parameter with a predetermined continuity threshold. The control module 66 detects the slope discontinuity when the parameter representative of the continuity is greater than the continuity threshold.

The parameter is for example a difference between two successive values ​​of the time derivative of the temperature of the liquid phase 44.

The degradation of the insulation is then signaled to an operator, for example by displaying a message on a screen, and / or by means of an audible warning.

The detection of the loss of insulation from the temperature of the liquid phase 44 is faster than the detection from the monitoring of the pressure of the gas phase 46, according to the methods previously used. This allows an earlier reaction to insulation degradation and reduces the risk of damage to the cryogenic tank 38.

The method further comprises a step of determining a need for purging the cryogenic tank 38, and for predicting this need for purging. The temporal change in the temperature of the liquid phase 44 as a function of time is determined as described above.

The need to purge the cryogenic tank is determined by the control module 66 when the temperature of the liquid phase 44 exceeds a predetermined purge threshold value.

The purge threshold value is for example equal to 1 10 K.

When the purge threshold is exceeded, the need to purge the tank is signaled to an operator, for example by displaying a message on a screen.

In addition, the control module 66 predicts a date on which the reservoir 38 will have to be purged, as a function of the temperature of the liquid phase 44, of the threshold value, and of the time derivative of the temperature of the liquid phase 44, calculated as before.

The expected date of the next purge requirement is signaled to an operator, for example by displaying it on a screen.

This makes it possible to plan the purge away from a mission of the submarine vessel 10 requiring a high degree of discretion to be maintained, and in particular during a rise to the surface of the submarine vessel 10.

Using the liquid phase temperature 44 to determine and predict the need to purge cryogenic tank 38 is more reliable than the gas phase pressure monitoring used previously.

CLAIMS

1 Method for controlling a cryogenic tank (38) comprising an enclosure (40) containing a liquefied gas, the liquefied gas forming a liquid phase (44) and a gas phase (46) in the enclosure (40), the method including the following step:

- measurement of a parameter representative of a pressure of the gas phase (46) and of a parameter representative of a pressure in the liquid phase (44);

characterized in that the method also comprises the following steps:

- measurement of a parameter representative of a temperature of the liquid phase (44); and

- determination of a total mass of the liquid phase (44) from parameters representative of the temperature of the liquid phase (44), of the pressure in the liquid phase (44) and of the pressure of the gas phase (46) ).

2. A method according to claim 1, also comprising the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase (44);

- Carrying out a first test of the parameter representative of the temporal evolution of the temperature of the liquid phase (44) and obtaining a first result; and

- signaling of a need for maintenance of the cryogenic tank (38) according to the first result.

3.- The method of claim 2, wherein the parameter representative of the time evolution of the temperature of the liquid phase (44) is a time derivative of the parameter representative of the temperature of the liquid phase (44), the first test being a comparison of the time derivative of the parameter representative of the temperature of the liquid phase (44) with a predetermined threshold value, and the need for maintenance being signaled if the time derivative of the parameter representative of the temperature of the liquid phase (44) is greater than the threshold value.

4. A method according to any one of claims 1 to 3, also comprising the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase (44);

- implementation of a second test of the parameter representative of the temporal evolution of the temperature of the liquid phase (44) and obtaining a second result; and

- signaling of a loss of thermal insulation of the enclosure (40) according to the second result.

5.- Method according to claim 4, wherein the parameter representative of the temporal evolution of the temperature of the liquid phase (44) is a temporal derivative of the temperature of the liquid phase (44), the second test comprises a calculation. a parameter representative of the continuity of the time derivative of the temperature of the liquid phase (44) and the comparison of the parameter representative of the continuity with a predetermined threshold, the loss of thermal insulation being signaled if the parameter representative of the continuity is greater than the threshold.

6. A method according to any one of claims 1 to 5, wherein the method also comprises the following steps:

- comparison of the temperature of the liquid phase (44) with a predetermined threshold; and

- signaling of a need to purge the cryogenic tank (38) if the temperature of the liquid phase (44) is above the threshold.

7. A method according to claim 6, further comprising the following steps:

- Obtaining a parameter representative of a temporal change in the temperature of the liquid phase (44);

- calculation of an expected date of need for purging the cryogenic tank (38) from the temperature of the liquid phase (44), the parameter representative of the temporal evolution of the temperature of the liquid phase (44) and the purge threshold.

8. A method according to any one of claims 1 to 7, wherein the cryogenic tank (38) is located in an underwater vessel (10) propelled by a propulsion system (18) comprising a production system of. energy (34).

9.- Cryogenic tank (38) comprising:

- an enclosure (40) containing a liquefied gas, the liquefied gas forming a liquid phase (44) and a gas phase (46) in the enclosure (40);

- a first pressure sensor (60) arranged to measure a parameter representative of a pressure in the liquid phase (44);

- a second pressure sensor (62) arranged to measure a parameter representative of a pressure of the gas phase (46);

- a temperature sensor (64) arranged to measure a parameter representative of a temperature of the liquid phase (44); and

- a control module (66) configured to control the first pressure sensor (60), the second pressure sensor (62) and the temperature sensor (64), and to determine a total mass of the liquid phase (44) from the temperature of the liquid phase (44), the pressure in the liquid phase (44) and the pressure of the gas phase (46).

10.- Underwater building (10) with anaerobic propulsion, comprising a propulsion system (18) comprising an energy production system (34) and at least one cryogenic tank (38) according to claim 9, the liquefied gas contained in the cryogenic tank (38) being dioxygen and supplying the energy production system (34) as an oxidizer.