System for storing and / or transporting a gas in the liquid state

Abstract

The present invention relates to a storage and / or transport system (1) comprising a tank (3) which delimits a main space (4) and a first layer (5), the system comprising a venting device (9) and a discharge branch (7), characterized in that the system comprises a pressure relief branch (10) and a pressure relief device (11), the pressure relief device (11) maintaining a leaktightness of the first layer (5) when a pressure prevailing inside the first layer (5) is lower than a first pressure threshold, the pressure relief device (11) bringing the first layer (5) and the venting device (9) into fluidic communication when the pressure prevailing inside the first layer (5) is higher than the first pressure threshold.

Description

[0001]DESCRIPTION Title of the invention: System for storing and / or transporting a gas in the liquid state The present invention relates to the field of floating structures for storing and / or transporting a cargo of gas in the liquid state, and more particularly relates to a system for storing and / or transporting said gas in the liquid state present within such floating structures. Gas in the liquid state is frequently stored and / or transported by sea in a floating structure, more particularly within at least one tank housed inside said floating structure. The tank makes it possible to maintain the gas in the liquid state at a very low temperature thanks to an assembly of wall and insulating layer ensuring sealing and thermal insulation. Each tank delimits a main space which is intended to contain the gas in the liquid state. The tank comprises at least one thermally insulating layer of the main space, advantageously two thermal insulation layers,the second layer extending around the first layer. The thermally insulating layer is separated from the main space and / or another thermally insulating layer by a sealing membrane. Each insulating layer is subject to its own pressure. A pressure increase may occur within the insulating layer, for example in the event of a leak and subsequent evaporation of gas within said insulating layer. Overpressure of the insulating layer relative to the main space may cause deformation of the membrane and thus cause significant and costly damage to the tank. To prevent overpressure of said insulating layer, pipes equipped with safety valves and connected to the atmosphere are connected to each insulating layer. Thus, in the event of excessive pressure within an insulating layer,the safety valve opens and allows the gas to be released into the atmosphere in order to lower the pressure of the insulating layer before damage to the tank occurs. However, in certain situations, for example during maintenance operations or during a testing phase of the tank properties when the floating structure is at the quayside, overpressure may occur, for example, following incorrect handling by an operator, at a time when the valves are not functional or absent. Thus, there is a need to protect the tank in the event of overpressure,and this regardless of the circumstances. The present invention falls within this context and proposes as such a system for storing and / or transporting a gas in the liquid state comprising at least one tank which delimits a main space intended to contain the gas in the liquid state and at least one first thermally insulating layer extending around the main space and constituting a means of thermal insulation of the main space, the storage and / or transport system further comprising at least one venting device and at least one discharge branch connected to the first layer and extending to the venting device, characterized in that the storage and / or transport system comprises at least one safety branch connecting the first layer to the venting device, the safety branch being arranged in parallel with at least part of the discharge branch,and a safety device positioned on the safety branch, the safety device maintaining a seal of the first layer when a pressure prevailing inside the first layer is lower than a first pressure threshold, the safety device putting the first layer and the venting device into fluidic connection when the pressure prevailing inside the first layer is higher than the first pressure threshold. Thus, in the event of a pressure rise in the first layer,the discharge branch allows an opening of the first layer towards the atmosphere by fluidically linking said first layer to the venting device. The fluid responsible for pressurizing the first layer can thus circulate in the discharge branch thanks to the pressure differential with the atmosphere. Thus, in the event of overpressure of the first layer not solvable by any means making it possible to detect this increase in pressure and to lower said pressure in response to such an increase in pressure, it is the safety device which ensures an evacuation towards the atmosphere of the gas causing the increase in pressure within the first layer, thus avoiding an overpressure source of damage within the tank. Preferably, the safety device breaks when the pressure within the first layer increases. In other words, when the pressure prevailing inside the first layer is higher than the first pressure threshold,the safety device connects the first layer and the venting device fluidically by breaking said safety device. Thus, in the event of a break, the safety device is individually replaceable, which requires a much less time-consuming and costly operation than repairing a membrane. The tank is suitable for storing gas in the liquid state and thus has various properties optimizing such storage, such as maintaining it at very low temperatures, for example at -163°C, as well as total sealing. The main space is delimited by a membrane, which also participates in the delimitation of the first thermally insulating layer. It is in particular this membrane which risks being damaged due to a potential overpressure of the first layer. Alternatively, an intermediate layer may extend between the main space and the first layer,in which case it is the membrane delimiting a separation between the intermediate layer and the first layer which risks being damaged in the event of overpressure of the first layer. The venting device may for example be a vent or any other element capable of releasing a fluid into the atmosphere. Such a venting device advantageously comprises a discharge opening positioned high up in order to avoid releasing potentially harmful fluid near operators present within the floating structure comprising the storage and / or transport system. The safety branch and the safety device guarantee a drop in pressure of the first layer,including in the event of a malfunction of the discharge branch preventing its use to lower the pressure of the first layer. The safety device may be in the form of a disc arranged across the safety branch to ensure the sealing of the first layer. This disc may be capable of breaking if the first pressure threshold is exceeded, thus allowing the circulation of fluid within the safety branch and thus lowering the pressure within the first layer. Like the discharge branch, the safety branch is connected to the venting device in order to evacuate by pressure differential the fluid responsible for the pressure increase within the first layer. Thus, even in the event of constraints preventing the discharge branch from functioning, for example a maintenance or test operation,it is always possible to implement a fluid connection to the atmosphere via the safety branch. According to such a configuration, when the pressure inside the first layer exceeds the first pressure threshold, the safety device breaks irreversibly in order to prevent a further pressure increase. However, as mentioned previously, the safety device can be easily replaced, without having to replace the safety branch or any other element of the storage and / or transport system. According to a characteristic of the invention, the first pressure threshold is 45mbarg + / - 9mbarg. Beyond this threshold value, the pressure in the first layer risks causing significant damage to the membrane separating the main space and the first layer. According to a characteristic of the invention,the discharge branch comprises at least one safety valve configured to fluidically connect the first layer and the venting device when the pressure inside the first layer is greater than a second pressure threshold, the second pressure threshold being lower than the first pressure threshold. The safety valve ensures the sealing of the first layer with respect to the discharge branch. The safety valve can open in the event of pressure measured within the first layer greater than the second pressure threshold, then close once the pressure in the first layer has fallen below the second pressure threshold. The safety valve thus constitutes the main means of regulating pressure within the first layer. However, it may happen that, during maintenance or test phases,this safety valve is removed or rendered inoperative. It is particularly in this type of configuration that the safety device performs its function. The second pressure threshold, i.e. the pressure threshold causing the safety valve to open, is lower than the first pressure threshold, from which the safety device is triggered. This allows the safety valve to be implemented as a priority. In the case where the safety device is a disc intended to rupture, this avoids recurring replacement of the safety device. According to a characteristic of the invention, the second pressure threshold is between 10mbarg and 35mbarg. This range of values ​​depends on the type of tank used,the structural characteristics varying depending on the type of tank considered. The value of the second pressure threshold is nevertheless always lower than the value of the first pressure threshold. According to a characteristic of the invention, the safety branch is connected to the discharge branch between the safety valve and the venting device. Connecting the safety branch and the discharge branch makes it possible to limit the piping connected to the venting device. This configuration thus ensures the compactness of the storage and / or transport system while guaranteeing a connection between the first layer and the venting device, regardless of the configuration of the safety valve. According to a characteristic of the invention, the storage and / or transport system comprises a second thermally insulating layer extending around the first layer. Advantageously,the thermal insulation layers as well as the membranes ensuring the sealing of the tank are doubled in order to reinforce the structure of the tank and to limit the risks of malfunction which may occur. In the case where the tank comprises a plurality of thermally insulating layers, the pressure prevailing within each of said layers must be taken into account to prevent any risk of overpressure of the tank. As mentioned previously, the objective of the safety branch and the safety device is to be able to fluidly connect a thermally insulating layer of the tank to the atmosphere, said layer being able to extend directly around the main space of the tank or being separated from it by an intermediate layer. Thus, when the tank of the storage and / or transport system comprises two thermally insulating layers,the safety branch and the safety device may be configured to connect the first layer to the atmosphere or the second layer to the atmosphere. According to a characteristic of the invention, the safety branch and the safety device are respectively a first safety branch and a first safety device, the storage and / or transport system comprising a second safety branch and a second safety device, the second safety branch fluidly connecting the second layer to the first layer, the second safety device maintaining the seal between the first layer and the second layer when a pressure differential measured between the first layer and the second layer is less than a differential threshold,and fluidically connecting the first layer and the second layer when the pressure differential measured between the first layer and the second layer is greater than the differential threshold. Like the first layer, the second layer includes its own safety device in the event of overpressure of the second layer relative to the first layer, in order to avoid damaging the membrane separating the two thermally insulating layers. Unlike the first safety device, which is triggered in the event of pressure that is too high compared to atmospheric pressure, the second safety device is triggered in the event of a pressure difference that is too high between the pressure prevailing in the first layer and the pressure prevailing in the second layer, more particularly when the pressure prevailing in the second layer is too high compared to the pressure prevailing in the first layer. In this configuration,the second safety device is triggered, thus fluidly connecting the two insulating layers in order to balance the pressures between them. It is also possible for the two safety devices to be triggered simultaneously or successively in the event of the first pressure threshold and the differential threshold being exceeded simultaneously or successively. Like the first safety device, the second safety device may be a rupture disc that ruptures in the event of overpressure and can be replaced individually. According to a characteristic of the invention, the differential threshold is 45mbarg + / - 9mbarg. In other words, the second safety device is triggered when the pressure in the second insulating layer is 45mbarg + / - 9mbarg higher than the pressure in the first insulating layer. According to a characteristic of the invention,the second safety branch is connected at a connection point arranged on the first safety branch, between the first layer and the first safety device. This configuration allows a fluid connection both between the two thermally insulating layers and between the first thermally insulating layer and the venting device, without either of the safety devices interfering at such connections. According to a characteristic of the invention, the first safety branch comprises a control member arranged between the first layer and the connection point with the second safety branch. According to a characteristic of the invention,the second safety branch comprises a control device arranged between the second layer and the second safety device. The control member and the control device may for example be valves capable of switching between an open position and a closed position. The control member and the control device are useful when replacing the safety devices, for example when they break. They make it possible to isolate the safety device to be replaced from the first layer and / or the second layer. The position of the control member and the control device is advantageous insofar as these two elements alone are capable of isolating the two safety devices. According to a characteristic of the invention, the safety device comprises at least one element capable of breaking if the first pressure threshold or the differential threshold is exceeded, preferably said element is a seal. In such a case,the safety device comprises a first flange and a second flange connected to each other by assembly means, the cover being interposed between the first flange and the second flange, the first flange and the second flange jointly delimiting a channel which at least partially houses the cover. When the cover breaks, the channel is then completely open and a circulation of fluid can take place therein for the purpose of lowering pressure. The invention also covers a method for securing at least one tank of a floating structure implemented by a system for storing and / or transporting a gas in the liquid state as described above, carried out during a maintenance operation of said storage and / or transport system,characterized in that the securing method comprises a step of maintaining a seal between the first layer and the venting device when a pressure prevailing inside the first layer is lower than a first pressure threshold, and in that the securing method comprises a step of placing the first layer in fluidic connection with the venting device when the pressure prevailing inside the first layer is higher than the first pressure threshold. Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several exemplary embodiments given for informational and non-limiting purposes with reference to the appended schematic drawings on the other hand, in which: [fig 1] schematically represents a system for storing and / or transporting a gas in the liquid state comprising at least one tank provided with a thermally insulating layer,[fig 2] schematically represents the storage and / or transport system comprising at least one tank provided with two thermally insulating layers. Figure 1 illustrates a storage and / or transport system 1 containing a cargo of gas in the liquid state 2 and which can be arranged within a floating structure capable of storing and / or transporting said cargo. This floating structure can for example be a ship for transporting gas in the liquid state, such as an LNG tanker, responsible for delivering the cargo to a destination point, for example a port. Alternatively, the storage and / or transport system 1 can be integrated within a land-based tank. To do this, the storage and / or transport system 1 comprises at least one sealed and thermally insulating tank 3, said tank 3 allowing the storage of the gas in the liquid state 2. In Figures 1 and 2, only one tank 3 is shown,but the storage and / or transport system 1 may comprise a plurality of tanks 3 in order to increase the storage capacity of the floating structure. The tank 3 illustrated in FIG. 1 comprises a main space 4, a first thermally insulating layer 5 and a membrane 6 separating the main space 4 and the first layer 5. The main space 4 contains the gas in the liquid state 2 stored and / or transported and the first layer 5 extends around the main space 4. The membrane 6 ensures the sealing of the main space 4, while the first layer 5 ensures thermal insulation of the cargo of gas in the liquid state 2. The latter can thus be kept at a temperature low enough to be maintained in the liquid state. The pressure prevailing within the first layer 5 is constantly monitored. Indeed, an overpressure within the first layer can create significant damage, in particular at the level of the membrane 6,whose repairs can be substantial and costly. To avoid such overpressure within the first layer 5, the storage and / or transport system 1 comprises an evacuation branch 7 provided with a safety valve 8, as well as a venting device 9. The evacuation branch 7 extends between the first layer 5 and the venting device 9. The venting device 9 may for example be a vent or any other means of access to the atmosphere. Advantageously, the venting device 9 is positioned high above a deck of the floating structure so that the occupants of the latter are not in contact with a potentially toxic fluid released into the atmosphere from the first layer 5. Thus, it is possible, in the event of an increase in the pressure of the first layer 5,to open a fluid connection between the latter and the atmosphere in order to lower the pressure of the first layer 5 and thus avoid creating irreversible damage within the tank 3. The safety valve 8 is kept closed to ensure the sealing of the first layer 5, but can open temporarily in the event of excessively high pressure prevailing within the first layer 5. Damage resulting from overpressure within the first layer 5 is thus avoided. The pressure prevailing within the first layer 5 can for example be measured in real time by a pressure sensor not shown. However, it happens that, in certain situations, the safety valve 8 is not functional,preventing a potential opening of the discharge branch 7 and therefore a connection between the first layer 5 and the venting device 9. This may be the case for example during certain maintenance operations when the floating structure is stopped, rendering the safety valve inoperative and thus preventing any discharge to the atmosphere during this maneuver. Although such maintenance is highly controlled, and alternative measures are implemented to avoid overpressure of the first layer 5, human error can still occur and can generate overpressure within the first layer 5 resulting in significant damage within the tank 3 because the pressure within the first layer 5 cannot be lowered via the discharge branch 7 and the safety valve 8. To overcome such a situation,the storage and / or transport system 1 comprises a safety branch 10 provided with a safety device 11. The safety branch 10, like the discharge branch 7, makes it possible to connect the first layer 5 to the atmosphere via the venting device 9. The safety device 11 is arranged across the safety branch 10 and ensures the sealing of the first layer 5 as long as the pressure prevailing within the first layer 5 is lower than a first pressure threshold. If the pressure prevailing within the first layer 5 becomes higher than the first pressure threshold, then the safety device 11 is configured to trigger, thus generating a fluid connection between the first layer 5 and the atmosphere via the venting device 9, and thus ensuring a drop in the pressure prevailing in the first layer 5. The tank 3 thus remains protected from potential overpressure phenomena,including when the safety valve 8 is not functional. Advantageously, the fluid connection thus generated between the first layer 5 and the atmosphere is made by breaking the safety device 11. In a manner not illustrated, the safety device 11 may be in the form of a rupture disk comprising an element such as a seal capable of breaking if the first pressure threshold is exceeded. According to this example, the safety device 11 comprises a first flange and a second flange connected to each other by assembly means, and the seal is interposed between the first flange and the second flange. The first flange and the second flange jointly delimit a channel which at least partially houses the seal. When the seal breaks, the channel is then completely open and a circulation of fluid can take place therein for the purpose of lowering pressure. When the safety device 11 breaks,this can subsequently be replaced easily and affordably, since it is sufficient to simply replace the safety device 11 with another unbroken safety device 11, without having to replace any other element of the storage and / or transport system 1. In Figure 1, the first layer 5 extends directly around the main space 4. In a manner not illustrated in the figure, an intermediate layer can be arranged between the main space 4 and the first layer 5. In order to facilitate the replacement of the safety device 11,the safety branch 10 comprises a control member 12 arranged between the first layer 5 and the safety device 11. The control member 12 may for example be in the form of a valve. The control member 12 is mainly in the open position in order to fluidly connect the first layer 5 to the safety device 11 in the event that the latter should break in the event of high pressure in the first layer 5. When the safety device 11 must be replaced, the control member 12 is closed in order to isolate the safety device 11 from the first layer 5 and to be able to replace the broken safety device 11. It should be noted that the discharge branch 7 comprises a control element 13 arranged between the first layer 5 and the safety valve 8, in the open position during normal operation of the storage and / or transport system 1. In order to reduce spatial congestion by limiting the piping of the storage and / or transport system 1,the safety branch 10 can be connected to the discharge branch 7 instead of being connected directly to the venting device 9. The connection is implemented so that the safety valve 8 is positioned between the first layer 5 and the connection with the safety branch 10, so that the safety valve 8 and the safety device 11 are both operational without interfering with each other. It should be noted that the safety valve 8 opens when the pressure inside the first layer 5 becomes higher than a second pressure threshold, the latter being lower than the first pressure threshold,and this so that in normal operation of the storage and / or transport system 1 it is the safety valve 8 which ensures the function of reducing the pressure of the first layer 5 rather than generating the rupture of the safety device 11. The second pressure threshold can be between 10mbarg and 35mbarg depending on the model of the tank 3. The first pressure threshold is set at 45mbarg + / - 9mbarg. Figure 2 also represents the storage and / or transport system 1 according to the invention, this time comprising a tank 3 with two thermally insulating layers. Only the structural and functional aspects specific to a tank with two thermally insulating layers will be discussed here and reference will be made to the description of Figure 1 for the structural and functional aspects common to the two representations of Figures 1 and 2. In Figure 2, the tank 3, in addition to the first layer 5,comprises a second layer 15 extending around the first layer 5. The second layer 15 constitutes an additional thermal protection, optimizing the safety of the tank 3 compared to a single thermally insulating layer. The first layer 5 and the second layer 15 are separated from each other by an additional membrane 16 which also risks suffering irreversible damage in the event of overpressure within the second layer 15. In order to avoid this,the storage and / or transport system 1 comprises a discharge line 17 on which an additional valve 18 is arranged. The discharge line 17 extends between the second layer 15 and an additional venting device not shown in FIG. 2. The discharge line 17 can however be connected to the venting device 9. The additional valve 18 opens in the event of excessive pressure within the second layer 15. A safety device must also be deployed at the level of the second layer 15 when maintenance operations render the additional valve 18 inoperative. This is why the safety branch 10 and the safety device 11 shown in FIG. 1 are here a first safety branch 10a and a first safety device 11a,while the storage and / or transport system 1 comprises a second safety branch 10b and a second safety device 11b arranged on the second safety branch 10b. The second safety branch 10b extends between the second layer 15 and the first safety branch 10a, at a connection point 14 located between the first layer 5 and the first safety device 11a. This second safety branch 10b therefore allows a connection between the first layer 5 and the second layer 15. The second safety device 11b is structurally identical to the first safety device 11a. On the other hand, the second safety device 11b is configured to break when a pressure differential between the first layer 5 and the second layer 15 is greater than a differential threshold. The breaking of the second safety device 11b thus allows the fluidic connection between the first layer 5 and the second layer 15,and thus to rebalance the pressure of the two layers 5, 15 relative to each other. Too high a pressure differential, i.e. a pressure value prevailing in the second layer 15 that is too high compared to a pressure value prevailing in the first layer 5, risks seriously damaging the additional membrane 16. The differential threshold is for example 45mbarg + / - 9mbarg. Depending on the situation, it is possible that the two safety devices 11 rupture simultaneously or successively. For example, assuming that the safety valve 8 and the additional valve 18 are not functional, the second safety device 11b may rupture following a rise in the pressure prevailing in the second layer 15 when the differential threshold is exceeded. This results in a balancing of the pressures in the layers 5, 15,and therefore an increase in the pressure prevailing within the first layer 5. If this increase in pressure within the first layer 5 causes the first pressure threshold to be exceeded, then the first safety device 11a will also break to open the first layer 5 to the atmosphere. The second safety branch 10b further comprises a control device 19 which may be in the form of a valve, and which makes it possible to isolate the second layer 15 from the second safety device 11b in order to replace the latter. Thus, thanks to only two elements, the control member 12 and the control device 19, the two safety devices 11 can be isolated from the layers 5, 15. For this, the control member 12 is precisely positioned on the first safety branch 10a, between the first layer 5 and the connection point 14. Of course,the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. The invention, as it has just been described, achieves the aim which it had set itself, and makes it possible to propose a storage and / or transport system comprising a tank and a safety device making it possible to permanently avoid overpressures which could damage said tank. Variants not described here could be implemented without departing from the context of the invention, provided that, in accordance with the invention, they comprise a storage and / or transport system in accordance with the invention.,

Claims

CLAIMS 1- Storage and / or transport system (1) for a gas in the liquid state (2) comprising at least one tank (3) which delimits a main space (4) intended to contain the gas in the liquid state (2) and at least one first thermally insulating layer (5) extending around the main space (4) and constituting a means of thermal insulation of the main space (4), the storage and / or transport system (1) further comprising at least one venting device (9) and at least one discharge branch (7) connected to the first layer (5) and extending to the venting device (9), characterized in that the storage and / or transport system (1) comprises at least one safety branch (10) connecting the first layer (5) to the venting device (9), the safety branch (10) being arranged in parallel with at least part of the evacuation branch (7),and a safety device (11) positioned on the safety branch (10), the safety device (11) maintaining a seal of the first layer (5) when a pressure prevailing inside the first layer (5) is lower than a first pressure threshold, the safety device (11) putting the first layer (5) and the venting device (9) into fluidic connection when the pressure prevailing inside the first layer (5) is higher than the first pressure threshold, the discharge branch (7) comprising at least one safety valve (8) configured to put the first layer (5) and the venting device (9) into fluidic connection when the pressure prevailing inside the first layer (5) is higher than a second pressure threshold, the second pressure threshold being lower than the first pressure threshold. 2- Storage and / or transport system (1) according to claim 1,wherein the first pressure threshold is 45mbarg + / - 9mbarg. 3- Storage and / or transport system (1) according to claim 1 or 2, wherein the second pressure threshold is between 10mbarg and 35mbarg. 4- Storage and / or transport system (1) according to any one of claims 1 to 3, wherein the safety branch (10) is connected to the discharge branch (7) between the safety valve (8) and the venting device (9)., 5- Storage and / or transport system (1) according to any one of claims 1 to 4, comprising a second thermally insulating layer (15) extending around the first layer (5).6- Storage and / or transport system (1) according to claim 5, wherein the safety branch (10) and the safety device (11) are respectively a first safety branch (10a) and a first safety device (11a), the storage and / or transport system (1) comprising a second safety branch (10b) and a second safety device (11b), the second safety branch (10b) fluidly connecting the second layer (15) to the first layer (5), the second safety device (11b) maintaining the seal between the first layer (5) and the second layer (15) when a pressure differential measured between the first layer (5) and the second layer (15) is less than a differential threshold, and fluidly connecting the first layer (5) and the second layer (15) when the pressure differential measured between the first layer (5) and the second layer (15) is greater than the differential threshold.7- Storage and / or transport system (1) according to claim 6, wherein the differential threshold is 45mbarg + / -9mbarg. 8- Storage and / or transport system (1) according to claim 6 or 7, wherein the second safety branch (10b) is connected at a connection point (14) arranged on the first safety branch (10a), between the first layer (5) and the first safety device (11a). 9- Storage and / or transport system (1) according to claim 8, wherein the first safety branch (10a) comprises a control member (12) arranged between the first layer (5) and the connection point (14) with the second safety branch (10b). 10- Storage and / or transport system (1) according to any one of claims 6 to 9, wherein the second safety branch (10b) comprises a control device (19) arranged between the second layer (15) and the second safety device (11b).11- Storage and / or transport system (1) according to any one of claims 1 to 10, wherein the safety device (11) comprises at. at least one element capable of breaking if the first pressure threshold or the differential threshold is exceeded, preferably said element is a seal.12- Method for securing at least one tank (3) of a floating structure implemented by a storage and / or transport system (1) for a gas in the liquid state (2) according to any one of claims 1 to 11, carried out during a maintenance operation of said storage and / or transport system (1), characterized in that the securing method comprises a step of maintaining a seal between the first layer (5) and the venting device (9) when a pressure prevailing inside the first layer (5) is lower than a first pressure threshold, and in that the securing method comprises a step of placing the first layer (5) in fluidic connection with the venting device (9) when the pressure prevailing inside the first layer (5) is higher than the first pressure threshold.

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