Safety valve switching system

Abstract

Provided is a safety valve switching system (10) comprising: a tank (1) for switching between and storing two types of liquefied gases having different pressures applied inside of the tank (1); a first safety valve (4) that prevents an excessive internal pressure caused by a first liquefied gas which, from among the two types of liquefied gases, has a lower pressure applied inside of the tank (1); a second safety valve (5) that prevents an excessive internal pressure caused by a second liquefied gas different from the first liquefied gas from among the two types of liquefied gases; a first remote valve (6) that, in order to select one of the first safety valve (4) and the second safety valve (5) on the basis of the internal state of the tank (1), closes and opens a path for a gas flowing from the tank (1) to the first safety valve (4) and performs an opening operation after a prescribed amount of time has lapsed following an opening command for opening being received; and a second remote valve (7) that is connected in series with the first remote valve (6), that is closed when there is a possibility that the first remote valve (6) may be opened by an erroneous operation, and that is in an open state when the first remote valve (6) is operating normally.

Description

Safety Valve Switching System

[0001] The present invention relates to a safety valve switching system for switching a safety valve of a tank.

[0002] In recent years, a safety valve switching system that can switch between two safety valves of a tank according to the type of cargo stored in the tank has been disclosed. For example, the cargo is liquefied natural gas (LNG, liquefied natural gas), liquefied petroleum gas (LPG, liquefied petroleum gas), or liquefied carbon dioxide (see Patent Document 1).

[0003] However, when two safety valves corresponding to the type of cargo are provided in a tank, if a safety valve that does not correspond to the type of cargo is used due to an erroneous operation, the safety valve may be damaged.

[0004] Japanese Unexamined Patent Application Publication No. 2024-155347

[0005] An object of an embodiment of the present invention is to provide a safety valve switching system that switches between a plurality of safety valves and prevents damage to the safety valve due to erroneous operation.

[0006] A safety valve switching system according to an aspect of the present invention comprises: a tank that alternately stores two types of liquefied gas that are subjected to different internal pressures; a first safety valve that prevents excessive internal pressure caused by a first liquefied gas, which is one of the two types of liquefied gas subjected to a lower internal pressure inside the tank; a second safety valve that prevents excessive internal pressure caused by a second liquefied gas, which is different from the first liquefied gas among the two types of liquefied gas; a first remote valve that closes and opens a gas path flowing from the tank to the first safety valve to select either the first safety valve or the second safety valve based on an internal state of the tank, and performs an opening operation after a predetermined time elapses after receiving an opening command for opening; and a second remote valve connected in series with the first remote valve, wherein the second remote valve is closed when there is a possibility that the first remote valve is opened by erroneous operation, and is in an open state when the first remote valve operates normally.

[0007] Figure 1 is a configuration diagram showing the configuration of a vessel to which the safety valve switching system according to the first embodiment of the present invention is applied. Figure 2 is a schematic diagram showing the arrangement of sensors according to the first embodiment. Figure 3 is a transition diagram showing the state transition when the contents of the tank according to the first embodiment are replaced from LNG to liquefied carbon dioxide. Figure 4 is a transition diagram showing the state transition when the contents of the tank according to the first embodiment are replaced from liquefied carbon dioxide to LNG. Figure 5 is a flowchart showing the procedure for closing the second remote valve by the control device according to the first embodiment. Figure 6 is a configuration diagram showing the configuration of a vessel to which the safety valve switching system according to the second embodiment of the present invention is applied. Figure 7 is a schematic diagram showing the arrangement of sensors according to the second embodiment.

[0008] (First Embodiment) Figure 1 is a configuration diagram showing the configuration of a ship 20 to which the safety valve switching system 10 according to the first embodiment of the present invention is applied. The same parts in the drawing are denoted by the same reference numerals, and redundant explanations are omitted as appropriate.

[0009] Ship 20 is a tanker that transports LNG and liquefied carbon dioxide by exchanging them. For example, ship 20 transports LNG on its way to its destination and liquefied carbon dioxide on its return trip. This allows LNG to be transported from its source to its destination, and the carbon dioxide emitted from the use of LNG can be returned to the LNG source.

[0010] This section primarily describes the exchange of two types of liquefied gases, LNG and liquefied carbon dioxide, but the same configuration can be used for the exchange of any two types of liquefied gases. For example, LPG may be used instead of LNG. Furthermore, the liquefied gas may also be liquid ammonia, which is ammonia gas that has been liquefied.

[0011] The vessel 20 is equipped with a safety valve switching system 10. The safety valve switching system 10 comprises a tank 1, a control device 2, an exhaust tower 3, a first safety valve 4, a second safety valve 5, a first remote valve 6, a second remote valve 7, a temperature sensor 11, and a liquid level sensor 12.

[0012] Tank 1 is a tank that stores LNG and liquefied carbon dioxide by exchanging them with each other. For example, Tank 1 is installed in the hull of a ship, like the tanks on an LNG carrier. The liquefied gas stored in Tank 1 during transport is either LNG or liquefied carbon dioxide.

[0013] For example, tank 1 is a Type-C tank. LNG is subjected to a pressure of 25 to 70 kPaG during transport. Liquefied carbon dioxide is subjected to a pressure of 420 to 1100 kPaG during transport. Here, the unit PaG represents gauge pressure. A pressure of 70 kPaG is the upper limit of pressure permitted for Type-B tanks and membrane tanks according to the IGC Code, and in the case of Type-C tanks, even higher pressures are permitted. However, increasing the upper limit pressure of the first safety valve 4 reduces the amount of LNG that can be loaded, so in this embodiment, even in Type-C tanks, the pressure is controlled to 25 to 70 kPaG. A pressure of 420 kPaG is the triple point of carbon dioxide, and below this pressure, carbon dioxide will not be liquefied.

[0014] LPG and ammonia are typically transported in Type A or Type C tanks. If Tank 1 is a Type A tank, the LPG or ammonia is subjected to the same pressure as LNG during transport. If Tank 1 is a Type C tank, a pressure of 300 to 700 kPaG is applied. In this embodiment, since Tank 1 is a Type C tank, a pressure of 300 to 1000 kPa may be applied. However, even in this case, it is desirable to control the pressure at 25 to 400 kPaG in order to increase the LPG load capacity.

[0015] The control device 2 is a device for controlling the safety valve switching system 10. The control device 2 includes a computer for performing various calculations. For example, the control device 2 determines the internal state of the tank 1 using various sensors 11 and 12 and selects the safety valve to be used from the two safety valves 4 and 5. The control device 2 may also have monitoring or inspection functions. For example, the control device 2 is an integrated automation system (IAS) and is installed in the cargo control room (CCR). The various commands issued by the control device 2 may be manually determined by an operator (seafarer, etc.).

[0016] The exhaust tower 3 is connected to the tank 1 via piping. When the internal pressure of the tank 1 increases, the exhaust tower 3 releases the gas inside the tank 1 to the outside of the tank 1 via safety valves 4 and 5.

[0017] Safety valves 4 and 5 are valves that maintain the internal pressure of the tank safely. When the internal pressure of tank 1 exceeds a preset pressure, safety valves 4 and 5 release the gas inside tank 1 to the outside of tank 1. This prevents the internal pressure of tank 1 from becoming excessively high. For example, the pressure value set for the safety valves is determined based on the IGC Code (International Gas Carrier Code, International Rules on the Structure and Equipment of Vessels for the Bulk Carriage of Liquefied Gas).

[0018] The first safety valve 4 is installed in the piping connecting the tank 1 and the exhaust tower 3. The first safety valve 4 is a safety valve used when LNG is stored in the tank 1. When the internal pressure of the tank 1 exceeds a set pressure, the first safety valve 4 releases the gas (BOG, boil-off gas) from the vaporized LNG in the tank 1 to the exhaust tower 3. For example, the upper limit pressure of the first safety valve 4 is set within the range of 25 to 70 kPaG.

[0019] The second safety valve 5 is installed in the piping connecting the tank 1 and the exhaust tower 3, separate from the piping where the first safety valve 4 is installed. The second safety valve 5 is a safety valve used when liquefied carbon dioxide is stored in the tank 1. When the internal pressure of the tank 1 exceeds a set pressure, the second safety valve 5 releases the vaporized gas (carbon dioxide) from the liquefied carbon dioxide in the tank 1 to the exhaust tower 3. For example, the upper limit pressure of the second safety valve 5 is set within the range of 420 to 1100 kPaG. The pressure set for the second safety valve 5 is higher than that of the first safety valve 4.

[0020] The first remote valve 6 and the second remote valve 7 are valves that can be operated, such as opening and closing, by remote operation commands. For example, the remote valves 6 and 7 are opened or closed based on an open command or a close command transmitted from the control device 2. The first remote valve 6 and the second remote valve 7 are connected in series. The remote valves 6 and 7 connected in series are installed in the piping between the tank 1 and the first safety valve 4 and are connected in series with the first safety valve 4. In Figure 1, the first remote valve 6 is installed on the tank 1 side and the second remote valve 7 is installed on the exhaust tower 3 side, but the positions of the first remote valve 6 and the second remote valve 7 may be swapped.

[0021] The first remote valve 6 opens and closes the gas path (piping, etc.) from tank 1 to the first safety valve 4. When tank 1 is filled with LNG, the first remote valve 6 is open. When tank 1 is filled with liquefied carbon dioxide, the first remote valve 6 is closed.

[0022] When the first remote valve 6 is opened by an open command from a remote location (e.g., control device 2), it performs the open operation after a predetermined time (delay time) has elapsed since receiving the open command. The delay time is the time required to delay the start of the open operation in order to ensure a predetermined time between the receipt of the open command and the start of the open operation. The delay time may be set to any desired time. The delay time is the time required to ensure that the second remote valve 7 is closed if the first remote valve 6 receives an incorrect open command. For example, the delay time is 30 seconds, but it is desirable to have at least 3 seconds. When the first remote valve 6 receives a close command, it performs the close operation without delay. The delay time may be disabled (e.g., set to 0 seconds). In addition, if an operator performs the open operation of the first remote valve 6 without the judgment of the control device 2, the open operation of the first remote valve 6 may be performed instantaneously without delay.

[0023] The pressure set in the first safety valve 4 is lower than the pressure set in the second safety valve 5. Therefore, when the first remote valve 6 is open, the first safety valve 4 functions as the safety valve for tank 1, and the second safety valve 5 effectively does not function as a safety valve. When the first remote valve 6 is closed, the first safety valve 4 is not used, and the second safety valve 5 functions as the safety valve for tank 1. In this way, depending on the opening and closing of the first remote valve 6, one of the two safety valves 4 and 5 is selected as the effectively functioning safety valve.

[0024] The second remote valve 7 is a valve provided as a countermeasure against malfunctions of the first remote valve 6. Under normal circumstances, the second remote valve 7 is open. The second remote valve 7 is closed when the first remote valve 6 is unable to be closed due to a malfunction or other circumstances, or when an incorrect command to open the first remote valve 6 is issued. Therefore, under normal operation of the first remote valve 6, the second remote valve 7 is open regardless of the open / closed state of the first remote valve 6.

[0025] Figure 2 is a schematic diagram showing the arrangement of sensors 11 and 12 according to this embodiment. The arrangement of sensors 11 and 12 described here is just one example, and they may be arranged in any other way.

[0026] The temperature sensor 11 is a sensor that detects the temperature inside the tank 1. The temperature sensor 11 outputs the detected temperature to the control device 2. At least one temperature sensor 11 is required, but any number of temperature sensors 11 may be provided. The temperature sensor 11 is installed on the inner wall of the tank 1 or on a structure inside the tank 1. For example, the temperature sensor 11 is a thermometer commonly used in tanks of ships and the like.

[0027] For example, the temperature sensor 11 is installed on the inner wall of the tank 1 near the equatorial EQ, where it bulges most horizontally, and at multiple locations on the vertically extending pump tower 21 located inside the tank 1. The pump tower 21 is equipped with a cargo pump and the like for lifting the liquid stored in the tank 1. For example, the temperature sensor 11 is installed on the pump tower 21 at multiple heights, such as near the top and near the bottom. As a result, the temperature sensor 11 detects the temperature at multiple heights inside the tank 1.

[0028] The liquid level sensor 12 is a sensor that detects the height (liquid level) of the liquefied gas inside the tank 1. The liquid level sensor 12 outputs the detected liquid level to the control device 2. The liquid level sensor 12 is installed in the upper part of the tank 1. For example, the liquid level sensor 12 is installed near the top of the pump tower 21. The liquid level sensor 12 is a radar-type liquid level gauge. The liquid level gauge emits radar waves onto the liquid surface inside the tank 1 and detects the height of the liquid level based on the reflected radar waves. Note that the liquid level sensor 12 is optional.

[0029] In addition to the temperature sensor 11 and the liquid level sensor 12, any other sensors may be provided to understand the internal state of the tank 1. For example, a pressure sensor may be provided inside the tank 1. This would allow the internal pressure of the tank 1 to be determined.

[0030] Figure 3 is a transition diagram showing an example of the state transitions when the contents of tank 1 according to this embodiment are replaced from LNG to liquefied carbon dioxide. The first state ST1 is the initial state inside tank 1. The seventh state ST7 is the final state inside tank 1. When replacing LNG with liquefied carbon dioxide, the inside of tank 1 transitions sequentially from the first state ST1 to the seventh state ST7.

[0031] In the first state ST1, tank 1 is filled with LNG, the internal temperature is -162 degrees Celsius, and the internal pressure is 20 kPaG.

[0032] When methane gas is injected from the first state ST1 and LNG is discharged, the system transitions to the second state ST2. In the second state ST2, the internal temperature of tank 1 is -162 degrees Celsius and the internal pressure is 20 kPaG.

[0033] When heated methane gas is injected from the second state ST2 to heat the inside of tank 1 to an appropriate temperature, the third state ST3 is reached. In the third state ST3, tank 1 is filled with methane gas, the internal temperature is -70 degrees Celsius, and the internal pressure is 20 kPaG.

[0034] When carbon dioxide is injected from the third state ST3 and methane gas is discharged, the fourth state ST4 is reached. Carbon dioxide is injected into the tank 1 in a way that prevents the injected carbon dioxide from turning into dry ice. The injected carbon dioxide is heated to a higher temperature than the inside of the tank 1. In the fourth state ST4, the internal temperature of the tank 1 is -65 degrees Celsius and the internal pressure is 20 kPaG.

[0035] From the fourth state ST4, if carbon dioxide is further injected while maintaining the internal pressure and methane gas is discharged, the state becomes the fifth state ST5. In the fifth state ST5, the internal temperature of tank 1 is -50 degrees Celsius and the internal pressure is 20 kPaG.

[0036] From state ST5 to state ST6, carbon dioxide gas is injected to increase the internal pressure of tank 1. In state ST6, tank 1 is filled with carbon dioxide gas, and the methane gas has been completely discharged. In state ST6, the internal temperature of tank 1 is -50 degrees Celsius, and the internal pressure is 400 kPaG.

[0037] From state ST6, liquefied carbon dioxide is injected while increasing the internal pressure of tank 1 to reach state ST7. State ST7 is when tank 1 is filled to capacity with liquefied carbon dioxide. In state ST7, the internal temperature of tank 1 is -50 degrees Celsius and the internal pressure is 1000 kPaG.

[0038] Next, we will explain the timing for the control device 2 to switch from the first safety valve 4 to the second safety valve 5. That is, we will explain the conditions under which the control device 2 outputs a closing command to close the first remote valve 6.

[0039] The timing for switching safety valves 4 and 5 is determined by the following four conditions. The output conditions for the closing command of the first remote valve 6 by the control device 2 may be determined by any one of these four conditions, or by a combination of two or more conditions. Furthermore, these four conditions are just examples, and any condition that determines any state between the first state ST1 and the seventh state ST7 may be adopted. In addition, these conditions may be used as interlock conditions to prevent erroneous operation of the first remote valve 6.

[0040] Furthermore, the final decision to output a closing command by the control device 2 may be made manually by the operator. For example, the control device 2 may display on the monitor that the conditions for closing the first remote valve 6 have been met, and the operator may check the display on the monitor and output a closing command.

[0041] The first condition is that the cargo pump for unloading LNG from tank 1 is started and the liquid temperature is -150°C or lower. The first condition represents the state in the first state ST1 where the cargo pump is started. For example, the control device 2 determines whether to start the cargo pump based on a start command to the cargo pump or a response signal from the cargo pump. The liquid temperature is detected by the temperature sensor 11.

[0042] The second condition is that the liquid level is below a predetermined value (for example, 90% or less) and the liquid level gradient is negative. The second condition represents a state in the first state ST1 where the amount of LNG in tank 1 is decreasing. For example, the liquid level and liquid level gradient are detected by the liquid level sensor 12.

[0043] The third condition is that the temperature near the equator EQ of the tank 1 has risen to -130°C. The third condition is a state between the second state ST2 and the third state ST3, and represents a state where the content in the tank 1 is replaced from LNG to methane gas, and the temperature of the methane gas has risen to -130°C. For example, the temperature near the equator EQ of the tank 1 is detected by a temperature sensor 11 provided near the equator EQ on the inner wall surface.

[0044] The fourth condition is that the temperature of the liquid flowing through the pipe for injecting liquid into the tank 1 is -80°C or higher. The fourth condition is a state between the sixth state ST6 and the seventh state ST7, and represents a state where liquefied carbon dioxide gas is flowing through the pipe for injecting liquid into the tank 1. For example, the temperature of the liquid flowing through the pipe is detected by a temperature sensor 11 provided on the pipe.

[0045] FIG. 4 is a transition diagram showing the transition of a state where the content of the tank 1 according to the present embodiment is replaced from liquefied carbon dioxide gas to LNG. The first state ST11 is an initial state inside the tank 1. The seventh state ST17 is a final state inside the tank 1. When replacing liquefied carbon dioxide gas with LNG, the inside of the tank 1 sequentially transitions from the first state ST11 to the seventh state ST17.

[0046] In the first state ST11, the tank 1 is filled with liquefied carbon dioxide gas, the internal temperature is -40°C, and the internal pressure is 1000 kPaG.

[0047] When carbon dioxide gas is injected and liquefied carbon dioxide gas is discharged while depressurizing the internal pressure of the tank 1 from the first state ST11, the state transitions to the second state ST12. In the second state ST12, the internal temperature of the tank 1 is -50°C, and the internal pressure is 500 kPaG.

[0048] When carbon dioxide gas is further injected and liquefied carbon dioxide gas is discharged while maintaining the internal pressure from the second state ST12, the state transitions to the third state ST13. In the third state ST13, the tank 1 is filled with carbon dioxide gas, the internal temperature is -50°C, and the internal pressure is 500 kPaG.

[0049] When carbon dioxide is discharged from the third state ST13 while reducing the internal pressure of tank 1, the state becomes the fourth state ST14. In the fourth state ST14, the internal temperature of tank 1 is -75 degrees Celsius and the internal pressure is 20 kPaG.

[0050] When methane gas is injected from the fourth state ST14 and carbon dioxide gas is discharged, the tank reaches the fifth state ST15. Methane gas is injected into the tank 1 in a state where the carbon dioxide gas inside does not turn into dry ice. In the fifth state ST15, the internal temperature of tank 1 is -75 degrees Celsius and the internal pressure is 20 kPaG.

[0051] Methane gas is injected from the fifth state ST15 to the sixth state ST16. In the sixth state ST16, tank 1 is filled with methane gas and carbon dioxide has been completely discharged. In the sixth state ST16, the internal temperature of tank 1 is -110 degrees Celsius and the internal pressure is 20 kPaG.

[0052] From state ST16, LNG is injected to bring the system to state ST17. State ST17 is when tank 1 is filled to capacity with LNG. In state ST17, the internal temperature of tank 1 is -162 degrees Celsius and the internal pressure is 20 kPaG.

[0053] Next, we will explain the timing for the control device 2 to switch from the second safety valve 5 to the first safety valve 4. That is, we will explain the conditions under which the control device 2 outputs an open command to open the first remote valve 6.

[0054] The timing for switching safety valves 4 and 5 is determined by the two conditions described below. The output conditions for the command to open the first remote valve 6 by the control device 2 may be determined by either one of these two conditions, or by a combination of two or more conditions. Furthermore, these three conditions are just examples, and any condition for determining any state between the first state ST11 and the seventh state ST17 may be adopted. In addition, these conditions may be used as interlock conditions to prevent incorrect operation of the first remote valve 6.

[0055] The output conditions for the opening command of the first remote valve 6 described here do not take into account the delay time when the first remote valve 6 opens. Therefore, you may change the output conditions to be earlier than those described below to take the delay time into account, or you may not take the delay time into account at all. For example, if the delay time is about 30 seconds, there will be no problem even if you output an opening command to the first remote valve 6 using the output conditions described below.

[0056] The final decision on whether to output an open command by the control device 2 may be made manually by the operator. For example, the control device 2 may display on the monitor that the conditions for opening the first remote valve 6 have been met, and the operator may check the display on the monitor and output an open command.

[0057] The first condition is that the temperature near the equatorial EQ of tank 1 has dropped to -130°C. The second condition is a state between the sixth state ST16 and the seventh state ST17, where tank 1 is filled with methane gas and the internal temperature has dropped to -130°C. For example, the temperature near the equatorial EQ of tank 1 is detected by a temperature sensor 11 installed near the equatorial EQ on the inner wall surface.

[0058] The second condition is that the liquid level is below a predetermined value (for example, 90% or less) and the liquid level gradient is positive. The second condition represents a state in the seventh state ST17 where the amount of LNG in tank 1 is increasing. For example, the liquid level and liquid level gradient are detected by the liquid level sensor 12.

[0059] Figure 5 is a flowchart showing the procedure for closing the second remote valve 7 by the control device 2 according to this embodiment.

[0060] The control device 2 receives an open command to the first remote valve 6 (step S101). The received open command may include an open command transmitted by the control device 2 itself.

[0061] When the control device 2 receives an open command, it determines the internal state of the tank 1 (step S102). For example, the control device 2 determines the internal state of the tank 1 based on the detection results of sensors 11 and 12. The control device 2 may have information indicating the internal state of the tank 1 pre-set. In this case, the control device 2 may determine the internal state of the tank 1 without using the detection results of sensors 11 and 12. During periods when cargo exchange operations are not being carried out, such as when the ship 20 is underway, there are no significant fluctuations in the internal state of the tank 1. For this reason, information indicating the internal state of the tank 1 can be pre-set in the control device 2. The information indicating the internal state of the tank 1 may only include information indicating whether the cargo is LNG or liquefied carbon dioxide. The control device 2 only needs to determine the internal state (cargo) of the tank 1 using either the pre-set information or the determination based on the detection results of sensors 11 and 12. Furthermore, when the control device 2 determines the cargo based on both pre-set information and the detection results of sensors 11 and 12, if there is a discrepancy between the two, it is desirable for the control device 2 to issue an alarm to notify the crew, but the determination based on the detection results of sensors 11 and 12 may be given priority.

[0062] The control device 2 determines whether the open command to the first remote valve 6 was erroneous based on the result of determining the internal state of the tank 1 (step S103). For example, if the cargo indicated by the pre-set information (signal, etc.) is liquefied carbon dioxide, or if the control device 2 determines that the actual cargo is liquefied carbon dioxide based on the detection results of sensors 11 and 12, the control device 2 determines that the open command to the first remote valve 6 was erroneous. If the control device 2 determines that the cargo is LNG, it determines that the open command to the first remote valve 6 was transmitted correctly. For example, the control device 2 may make such a determination during normal operation, such as while the ship 20 is underway.

[0063] If the control device 2 determines that the open command to the first remote valve 6 was erroneous, it sends a close command to the second remote valve 7 (step S104). As a result, the second remote valve 7 is closed. The control device 2 may control the operation of the first remote valve 6 in addition to controlling the closing of the second remote valve 7. For example, the control device 2 may lock the operation of the first remote valve 6 so that the first remote valve 6 does not accept an open command, or it may output a close command to the first remote valve 6. Once the cause of the erroneous transmission is removed, the second remote valve 7 is opened. The opening operation of the second remote valve 7 may be performed based on the judgment of the control device 2, or it may be performed based on the judgment of the operator.

[0064] According to this embodiment, in a tank 1 that stores two types of liquefied gases (for example, LNG and liquefied carbon dioxide) with different storage pressures by exchanging them, safety valves 4 and 5 corresponding to each liquefied gas are provided. In addition, a first remote valve 6 for switching between safety valves 4 and 5 is provided, and a backup second remote valve 7 is provided as a countermeasure against erroneous operation or failure of the first remote valve 6. This allows the two safety valves 4 and 5 to be easily switched, enabling safe operation.

[0065] Here, we will provide further explanation regarding the provision of the second remote valve 7. In Chapters 8.2.7 and 8.2.9 of the NK Rules for Steel Ships N (Liquefied Gas Bulk Carriers) established by the Nippon Kaiji Kyokai (Japan Maritime Association), it is permitted to provide one gate valve equivalent to the first remote valve 6, but a device for temporarily isolating the safety valve for repair purposes is required as an emergency isolation measure. Furthermore, regarding the emergency isolation of the safety valve, there is a principle that "no extra valve should be installed upstream of the safety valve." This is because there is a concern that the safety valve may cease to function if such a valve is not opened. Therefore, it is conceivable that simply installing a valve connected in series with the first remote valve 6 would not be permitted in ship classification inspections (Class NK), etc. Therefore, in this embodiment, by operating the second remote valve 7 in an open state as a general rule, it is possible to prevent forgetting to open the second remote valve 7.

[0066] In this embodiment, the case of exchanging two types of gas in tank 1 has been described, but it is also possible to exchange three or more types of gas. Specifically, a similar configuration can be achieved by providing safety valves according to the number of gas types, setting the pressure of each safety valve according to the gas type, and selecting the safety valve to be used according to the internal state of tank 1. Furthermore, the first remote valve 6 and the second remote valve 7 can also be provided in accordance with the increased number of safety valves, allowing them to function in the same way as the other safety valves.

[0067] (Second Embodiment) Figure 6 is a configuration diagram showing the configuration of a vessel 20A to which the safety valve switching system 10A according to the second embodiment of the present invention is applied. Figure 7 is a schematic diagram showing the arrangement of sensors 11 to 13 according to this embodiment. The arrangement of sensors 11 to 13 described herein is an example and may be arranged in any way. Note that the temperature sensor 11 and liquid level sensor 12 are the same as in the first embodiment.

[0068] The safety valve switching system 10A is the same as the safety valve switching system 10 according to the first embodiment shown in Figure 1, but with an additional gas sensor 13 installed inside the tank 1. The control device 2A determines the switching of safety valves 4 and 5 based on the various sensors 11 to 13. Other aspects are the same as in the first embodiment.

[0069] The gas sensor 13 is a sensor for identifying the components of a gas. For example, the gas sensor 13 detects methane gas. The gas sensor 13 is installed at the bottom of the tank 1 (in the lower half of the tank 1). The gas sensor 13 samples the gas at the bottom of the tank 1 and identifies the components of the sampled gas.

[0070] Furthermore, the gas sensor 13 is not limited to identifying specific components such as methane; it is sufficient if it can determine which of the two types of gas (in this case, LNG and liquefied carbon dioxide) that may be present in tank 1 is being detected.

[0071] The control device 2A determines the internal state of the tank 1 using the temperature sensor 11, the liquid level sensor 12, and the gas sensor 13, and selects the safety valve to be used from among the two safety valves 4 and 5. In other respects, it is the same as the control device 2 according to the first embodiment.

[0072] Referring to Figure 3, the timing of the control device 2A switching from the first safety valve 4 to the second safety valve 5 when the contents of tank 1 are replaced from LNG to liquefied carbon dioxide will be explained. That is, the conditions under which the control device 2A outputs a closing command to close the first remote valve 6 will be explained. Conditions 1 through 4 are the same as in the first embodiment, so here only condition 5 will be explained.

[0073] The fifth condition is that methane is no longer detected at the bottom of tank 1. The fifth condition represents the fourth state ST14, where there is no methane gas at the bottom of tank 1, and carbon dioxide gas has accumulated there. Since carbon dioxide gas is denser than methane gas, it tends to accumulate at the bottom of tank 1. Therefore, once a certain amount of carbon dioxide gas is injected into tank 1, methane gas will no longer be detected at the bottom of tank 1. For example, whether or not the fifth condition is met is determined based on the detection results from the gas sensor 13 installed at the bottom of tank 1.

[0074] Referring to Figure 4, the timing of the control device 2A switching from the second safety valve 5 to the first safety valve 4 when the contents of tank 1 are replaced from liquefied carbon dioxide to LNG will be explained. That is, the conditions under which the control device 2A outputs an open command to open the first remote valve 6 will be explained. Since the first and second conditions are the same as in the first embodiment, the third condition will be explained here.

[0075] The third condition is that methane is detected from the bottom of tank 1. The third condition represents a state between the fifth state ST15 and the sixth state ST16, where tank 1 is filled with methane gas. Methane gas has a lower specific gravity than carbon dioxide. Therefore, unless a considerable amount of methane gas is injected into tank 1, it will not be detected from the bottom of tank 1. For example, whether or not the third condition is met is determined based on the detection result from the gas sensor 13 installed at the bottom of tank 1.

[0076] According to this embodiment, in addition to the effects of the first embodiment, the addition of a gas sensor 13 to the tank 1 increases the switching conditions for the safety valves 4 and 5 by the control device 2A. This increases the options for the timing of switching the safety valves 4 and 5. Furthermore, these conditions can also be used as interlock conditions to prevent erroneous operation of the first remote valve 6.

[0077] Furthermore, additional advantages and modifications may readily arise for those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific detailed and representative embodiments described herein. Accordingly, various modifications can be made without departing from the spirit or scope of the general concept of the invention as defined by the appended claims and their equivalents.

Claims

1. A safety valve switching system comprising: a tank for storing two types of liquefied gases with different internal pressures; a first safety valve for preventing excessive internal pressure caused by a first liquefied gas, which has a lower internal pressure than the first liquefied gas; a second safety valve for preventing excessive internal pressure caused by a second liquefied gas, which is different from the first liquefied gas; a first remote valve for selecting either the first or second safety valve based on the internal state of the tank, by closing and opening the gas path from the tank to the first safety valve, and opening the valve after a predetermined time has elapsed since receiving an opening command; and a second remote valve connected in series with the first remote valve, which is closed when the first remote valve may be opened by erroneous operation, and remains open when the first remote valve is functioning normally.

2. The safety valve switching system according to claim 1, characterized in that the second remote valve is closed when the first remote valve is unable to be closed.

3. The safety valve switching system according to claim 1, characterized in that it includes a control device that outputs an operation command for closing and opening the first remote valve based on the internal state of the tank.

4. The safety valve switching system according to claim 3, characterized in that when the control device determines that it has erroneously transmitted an open command to the first remote valve, it transmits a close command to close the second remote valve.

5. The safety valve switching system according to claim 3, further comprising a temperature sensor for detecting the temperature inside the tank, wherein the control device outputs an operation command for the first remote valve based on the temperature detected by the temperature sensor.

6. The safety valve switching system according to claim 3, further comprising a liquid level sensor for detecting the liquid level of liquefied gas in the tank, wherein the control device outputs an operation command for the first remote valve based on the liquid level detected by the liquid level sensor.

7. The safety valve switching system according to claim 3, further comprising a gas sensor for determining the type of gas contained in the tank, wherein the control device outputs an operation command for the first remote valve based on the detection result from the gas sensor.

8. The safety valve switching system according to claim 1, characterized in that the two types of liquefied gases are one of liquefied natural gas, liquefied petroleum gas, or liquid ammonia and liquefied carbon dioxide.

9. A ship comprising: a hull; a tank provided in the hull for storing and exchanging two types of liquefied gases with different internal pressures; a first safety valve for preventing excessive internal pressure caused by a first liquefied gas, which has a lower internal pressure than the first liquefied gas; a second safety valve for preventing excessive internal pressure caused by a second liquefied gas, which is different from the first liquefied gas; a first remote valve that, based on the internal state of the tank, closes and opens the gas path from the tank to the first safety valve in order to select either the first or the second safety valve, and opens after a predetermined time has elapsed since receiving an opening command; and a second remote valve connected in series with the first remote valve, which is closed when the first remote valve may be opened by erroneous operation, and is open when the first remote valve is functioning normally.

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