Fuel tank vent system monitor

Abstract

Disclosed is a fuel tank vent system monitor configured to facilitate determination of a performance of a vent system 6 for a fuel tank 3. The vent system 6 comprises a vent line 65a, 65b comprising a valve 61 that is openable to place the fuel tank 3 in fluid communication with a region external 20 to the fuel tank 3. The fuel tank vent system monitor is configured to: cause fluid to be supplied to the fuel tank 3; thereafter, obtain a parameter indicative of a characteristic of contents in the fuel tank 3; and based on the parameter obtained, cause an action to be performed to facilitate determination of the performance of the valve 61. The fluid used may be fuel and the characteristic obtained may be; fuel level, rate of change of fuel level, pressure. The system may also comprise a further second valve 62 as well as testing both the valves 61, 62 cracking pressures.

Description

TECHNICAL FIELD

[0001] The present invention relates to a fuel tank vent system monitor, a fuel system, an aircraft, and a method of determining a performance of a fuel tank vent system. BACKGROUND

[0002] Aircraft comprising engines which are powered by hydrogen fuel may comprise pressurised cryogenic fuel stored in a fuel tank of the aircraft. A temperature of the hydrogen fuel in the fuel tank may increase over time. This may particularly be the case when no external cooling system is provided to maintain the hydrogen fuel under cryogenic conditions.

[0003] Such heating of the hydrogen fuel may cause evaporation of some of the hydrogen fuel, particularly when the fuel is at or near its saturation temperature and / or pressure. This, in turn, may cause a reduction in density and / or an increase in pressure in the fuel tank. SUMMARY

[0004] A first aspect of the present invention provides a fuel tank vent system monitor configured to facilitate determination of a performance of a vent system for a fuel tank. The vent system comprises a vent line comprising a valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank. The fuel tank vent system monitor is configured to: cause fluid to be supplied to the fuel tank; thereafter, obtain a parameter indicative of a characteristic of contents in the fuel tank; and based on the parameter obtained, cause an action to be performed to facilitate determination of the performance of the valve.

[0005] By causing the action to be performed to facilitate determination of the performance of the valve on the basis of the parameter obtained after causing fluid to be supplied to the tank, it may be possible to determine the performance of the valve without requiring additional sensors associated with the valve. For instance, it may be possible to determine the performance of the valve without requiring pressure or temperature sensors in the vent line, or optical sensors for determining whether gas is discharged through the vent line. This may reduce a weight, complexity and / or cost of the vent system.

[0006] The fuel tank vent system monitor may be configured to cause the fluid to be supplied to the fuel tank during routine operations carried out on the fuel tank. In this way, the fuel tank vent system monitor may facilitate determination of the performance of the vent valve during such routine operations, without requiring a separate test to be performed. This may provide an efficient way to determine the performance of the valve.

[0007] Optionally, the valve is a unidirectional valve, such as a check valve. Optionally, the valve is configured to permit a flow of at least some of the contents of the fuel tank to flow from the fuel tank to the region external to the flow tank, but to prevent or hinder any flow in the opposite direction, from the region to the fuel tank. In this way, the valve may be openable to cause a reduction in pressure in the fuel tank. Optionally, the valve is a rupturable valve, such as a rupture disc valve. The rupturable valve may comprise a disc, or membrane, that is caused to rupture in response to a pressure difference across the valve exceeding a threshold. Such a rupture may be irreversible. A rupturable valve may provide a simple and low cost solution for reliving pressure in the fuel tank.

[0008] Optionally, the action comprises the fuel tank vent system monitor determining the performance of the valve on the basis of the parameter obtained. This may allow the performance of the valve and / or the fuel tank vent system as a whole to be determined without requiring human interaction or interpretation of the parameter. As such, the fuel tank vent system monitor may provide a convenient and efficient way to determine the performance of the vent system.

[0009] The action may comprise any one or more of: causing the parameter obtained to be saved to memory; causing the parameter obtained to be displayed on a screen; causing the parameter obtained to be transmitted to another system; causing an alert to be issued; and causing a maintenance operation of the vent system to be scheduled. Optionally, the fuel tank vent system monitor may be configured to perform a further action in response to a determination of the performance of the valve. Optionally, the further action comprises any one or more of: causing the performance determined to be saved to memory; causing the performance determined to be displayed on a screen; causing an alert to be issued; causing a maintenance operation of the vent system to be scheduled; and causing the performance determined to be transmitted to another system.

[0010] For instance, if the fuel tank vent system monitor determines that the valve has experienced a loss in performance, the fuel tank vent system monitor may cause issuance of an alert, such as an alert to prompt personnel to schedule and / or perform maintenance to improve or restore the performance of the valve.

[0011] The fuel tank vent system monitor determining the performance of the valve may comprise the fuel tank vent system monitor determining whether the valve opens as expected, and / or whether the valve unintentionally permits fluid to pass therethrough, i.e., whether the valve is leaky. Where the valve is a rupturable valve, the fuel tank vent system monitor determining the performance of the valve may comprise the fuel tank vent system monitor determining whether the valve, or a part thereof, such as a membrane or disc, has ruptured.

[0012] Optionally, the fluid caused to be supplied to the fuel tank comprises fuel. Optionally, the fuel tank vent system monitor is configured to cause the fuel to be supplied to the fuel tank. In this way, the fuel tank vent system monitor may facilitate the determination of the performance of the valve system during a fuelling or refuelling operation of the fuel tank. This may allow the performance of the valve and / or the valve system to be determined more regularly, and / or more efficiently, such as without requiring a separate test to be performed to determine the performance of the valve and / or the valve system. The fluid may comprise a fluid other than fuel, such as an inert gas for increasing a pressure in the fuel tank. This may facilitate the determination of the performance of the valve and / or the valve system in situations other than a fuelling operation, such as during routine maintenance of the fuel tank.

[0013] Optionally, the fuel tank is a cryogenic fuel tank for storing cryogenic fuel. Optionally, the cryogenic fuel is hydrogen, such as cryogenic liquid hydrogen. In this way, the fuel tank vent system monitor may be configured to cause cryogenic fuel, such as cryogenic liquid hydrogen fuel, to be supplied to the fuel tank. Optionally, the fuel tank is an aircraft fuel tank configured to store fuel to be supplied to an engine of the aircraft, and the fuel tank vent system monitor is an aircraft fuel tank vent system monitor.

[0014] Optionally, the characteristic comprises a fluidic property of the contents in the fuel tank. Optionally, the fluidic property comprises a thermofluidic property of the contents of the fuel tank. The fluidic and / or thermofluidic characteristic may comprise a pressure and / or temperature of the contents in the fuel tank. This may facilitate the determination of the performance of the valve by using a pressure sensor and / or a temperature sensor in the fuel tank, which may already be provided in the fuel tank. In such a case, the parameter may be, or may comprise, a value of the characteristic.

[0015] Alternatively, or in addition, the parameter may comprise a value of a characteristic of the fluid in a fluid source from which the fluid is caused to be passed to the fuel tank. In such a case, the parameter may be used to infer the value of the characteristic of the contents of the fuel in the tank. For instance, the parameter may be a pressure of fluid in the fluid source, which may be used to determine a pressure in the fuel tank when the fluid is caused to be supplied to the fuel tank from the fluid source.

[0016] Optionally, the contents comprise fuel in the fuel tank, and the characteristic comprises a fuel level of the fuel in the fuel tank. This may facilitate determination of the performance of the valve using an existing fuel level sensor in the tank. This may provide an improve an efficiency and / or simplicity of the fuel tank vent system monitor and / or reduce a cost and / or weight of the fuel tank vent system monitor.

[0017] Optionally, the fuel tank vent system monitor comprises a fuel level sensor for sensing the fuel level of fuel in the tank. Optionally, the fuel level sensor comprises at least one capacitive sensor. Optionally, the fuel level sensor comprises two capacitive sensors located in different parts of the fuel tank. This may allow the fuel level of fuel to be determined regardless of an orientation of the fuel tank, which may improve an accuracy and / or reliability of the fuel tank vent system monitor This may provide an accurate way of measuring the fuel level of fuel in the tank.

[0018] The parameter may comprise a fluid level of fluid in the fluid source, where provided. The fluid level in the fluid source may relate to the fuel level in the fuel tank when the fluid is caused to be supplied to the fuel tank. As such, determining the fluid level in the fluid source may allow the fluid level in the fuel tank to be inferred, and may facilitate the determination of the performance of the valve.

[0019] Optionally, the characteristic comprises a change, or rate of change, in the fuel level of fuel in the tank. In particular, when the fluid caused to be supplied to the fuel tank is fuel, the fuel level in the tank may tend to increase. The performance of the valve may affect the extent to which the fuel level in the tank increases. For instance, a pressure in the fuel tank may increase as fuel is caused to be supplied to the fuel tank. This may cause the valve to open to relieve pressure in the tank by allowing the contents of the tank to pass through the valve. If the valve is performing as expected, a rate of the contents of the tank passing through the valve may be higher than if the valve has experienced a loss in performance, which may allow the fuel level in the tank to increase faster than if the valve has experienced a loss in performance. As such, determining the change or rate of change in the fuel level of fuel in the tank may provide a convenient and efficient way to determine, or facilitate determination of, the performance of the valve.

[0020] Optionally, the change in the fuel level is a change in the fuel level over a predetermined time period. This may be considered to be a rate of change of the fuel level in the fuel tank. The valve may be considered to be functioning properly if there is an expected change in the fuel level over the predetermined time period. For instance, if the valve is fully functional, the fuel level in the fuel tank may be expected to increase by a certain amount over the predetermined time period. Determining such a change in fuel level over the predetermined time period may provide a convenient and efficient way to determine, or facilitate determination of, the performance of the valve.

[0021] The rate of change in the fuel level may be determined by dividing a change in the fuel level by a time over which the fuel level changes, such as by dividing the change in the fuel level over the predetermined time, or by continuously determining the rate of change in the fuel level.

[0022] Where the fluid supply is provided, the parameter may comprise a change, or rate of change, of the fluid level in the fluid supply. This may be used to infer a change, or rate of change, of the fuel level in the fuel tank, and may allow a performance of the valve to be determined, or may facilitate determination of the performance of the valve, on the basis of the parameter and / or the characteristic.

[0023] Optionally, the characteristic comprises a time taken for the fuel level in the fuel tank to change by a predetermined amount. Optionally, the predetermined amount is up to 2%, up to 3%, up to 5% or greater than 5% of a volume of fuel that can be stored in the fuel tank. This time taken for the fuel level of fuel in the fuel tank to change by the predetermined amount may be considered a measure of a rate of change in the fuel level in the fuel tank. In other words, determining the rate of change in the fuel level, where provided, may comprise determining the time taken for the fuel level of fuel in the fuel tank to change by the predetermined amount.

[0024] The valve may be considered to be performing as expected if the time taken for the fuel level to change, such as to increase, by the predetermined amount is as expected. For instance, if the valve is fully functional, the fuel level in the fuel tank may be expected to take a certain amount of time to change by the predetermined amount. Determining such a time taken and using this to facilitate determination of the performance of the valve may provide a convenient and efficient way to determine, or facilitate determination of, the performance of the valve.

[0025] Where the fluid supply is provided, the parameter may comprise a time taken for the fluid level of fluid in the fluid source to change, such as reduce, by a predetermined amount. This may be used to infer a corresponding change, such as an increase, in the level of fuel in the fuel tank, particularly where the fluid is fuel that is caused to be supplied to the fuel tank. This may provide a convenient way to determine, or facilitate determination of, the performance of the valve using sensors associated with the fluid source, rather than, or in addition to, providing such sensors in the fuel tank.

[0026] Optionally, the causing the fluid to be supplied to the fuel tank comprises the fuel tank vent system monitor: causing a pressure of fluid in a fluid source to exceed a threshold pressure; and causing the fluid source to be fluidically connected to the fuel tank.

[0027] The fluid source may be the fluid source referred to above. The fluid source may initially be isolated from the fuel tank, such as by an isolator. The causing the fluid source to be fluidically connected to the fuel tank may comprise causing the isolator, where provided, to open. The isolator may provide a simple way for the fuel tank vent system monitor to cause the fluid to be supplied to the fuel tank.

[0028] Causing the fluid source to be fluidically connected to the fuel tank may cause a pressure of the contents of the fuel tank, such as fuel in the fuel tank, to tend towards the pressure of the fluid in the fluid source. Optionally, the threshold pressure is higher than a pressure in the fuel tank before the fuel source is fluidically connected to the fuel tank. It will be appreciated that an increase in pressure in the fuel tank may cause the valve to open, which may result in pressure in the fuel tank remaining lower than the threshold pressure, despite being driven towards that pressure by supplying fluid to the fuel tank via the fluid source. This may provide an indicator of functionality of the valve. For instance, if the pressure in the fuel tank never reaches the threshold pressure, it may be determined, by the fuel tank vent system monitor or otherwise, that the valve is performing as expected.

[0029] Optionally, before or while the causing the fuel source to be fluidically connected to the fuel tank, the fuel tank vent system monitor is configured to cause the contents of the fuel tank to be prevented from flowing out of the fuel tank except through the vent line. This may be by the fuel tank vent system monitor causing a return gas isolator to close. The return gas isolator may be operable to selectively fluidically couple the fuel tank to or isolate the fuel tank from a location external to the fuel tank to which gas in the fuel tank is normally supplied whilst liquid fuel is caused to be supplied to the fuel tank. Causing the isolator to close may cause a pressure in the fuel tank to increase towards the threshold pressure when the fluid source is caused to be fluidically coupled to the fuel tank. This increase in pressure may cause the valve to open, which may allow the performance of the valve to be easily and repeatably determined, for instance as part of a refuelling operation of the fuel tank.

[0030] Optionally, the valve is configured to open when a pressure upstream of the valve exceeds a cracking pressure of the valve, and wherein the fuel tank valve system monitor is configured to cause the threshold pressure to be greater than the cracking pressure. If the valve is functioning correctly, causing the threshold pressure to be greater than the cracking pressure of the valve may cause the pressure in the fuel tank to exceed the cracking pressure of the valve when the fluid source is caused to be fluidically connected to the fuel tank, thereby causing the valve to open. Causing conditions in the fuel tank which cause the valve to open may allow the fuel tank vent system monitor to determine, or facilitate determination of, the performance of the valve, such as an operability of the valve. Optionally, where the valve is a rupturable valve, the cracking pressure may be a pressure that causes a part of the valve, such as a membrane or disc in the valve, to rupture.

[0031] Optionally, the valve is a first valve having a first cracking pressure and the vent line comprises a second valve having a second cracking pressure. Optionally, the first valve is fluidically connected in series with the second valve in the vent line. Optionally, the facilitating determination of the performance of the valve comprises facilitating determination of the performance of the first valve and / or the second valve. Optionally, the action comprises the fuel tank vent system monitor determining the performance of the first valve and / or the second valve on the basis of the parameter obtained.

[0032] Providing the first and second valve in series with each other may provide redundancy in the event of a failure of one of the valves. The fuel tank vent system monitor determining, or facilitating determination of, the performance of one or both of the valves may improve a safety and / or reliability of the fuel tank and / or vent system.

[0033] Optionally, the second cracking pressure is greater than the first cracking pressure. Optionally, to facilitate determination of the performance of the second valve, the fuel tank vent system monitor is configured to: cause the threshold pressure to be in a first range of from the first cracking pressure to the second cracking pressure; and determine whether there is a level of flow of fuel through the second valve.

[0034] In other words, the characteristic may comprise a level of flow through the second valve. In this way, when the fuel source is fluidically connected to the fuel tank, the pressure in the fuel tank may tend towards a pressure between the first and second cracking pressures, particularly when the contents of the fuel tank are prevented from flowing out of the fuel tank except via the vent line as discussed above. In such a case, if the second valve is functioning properly, the second valve will remain closed, regardless as to whether the first valve is caused to open. If the second valve remains closed, a flow rate of fuel to the fuel tank may be reduced, such as to a zero or to a significantly lower flow rate than if the second valve were to be caused to open, or partially open.

[0035] Thus, the fuel tank vent system monitor may determine, or may facilitate a determination, that the second valve is performing as expected if a change, or rate of change, in the fuel level in the fuel tank is below a threshold change or rate of change. This may provide a convenient, reliable, and / or accurate way to determine or facilitate determination of the performance of the second valve.

[0036] Optionally, the second cracking pressure is greater than the first cracking pressure. Optionally, to facilitate determination of the performance of the first valve and / or the second valve, the fuel tank valve system monitor is configured to: cause the threshold pressure to be greater than the second cracking pressure; and facilitate determination of the performance of the first valve and / or the second valve by determining whether there is a level of flow of fuel through the respective first valve and / or second valve.

[0037] Causing the threshold pressure to be greater than the second cracking pressure may cause a pressure in the tank to tend towards a pressure that is greater than the second cracking pressure, and thus also greater than the first cracking pressure. If each of the first and second valves is performing properly, then each of the first and second valves should be caused to open. This may lead to a predictable flow rate of contents of the fuel tank through the vent line as the fluid is caused to be supplied to the fuel tank from the fluid source. If the fuel level in the fuel tank does not increase, or if it increases slower than expected, this may mean that either the first valve or the second valve has not opened, and so is not performing correctly.

[0038] As such, the fuel tank vent system monitor may determine that the first valve and / or the second valve is performing correctly in response to a change or rate of change in the fuel level in the tank exceeding a threshold change or rate of change, or being within an expected range of such a change or rate of change in the fuel level.

[0039] Optionally, the first valve is upstream of the second valve. In this way, when the second valve has a higher cracking pressure than the first valve, the second valve may provide redundancy in the event of a failure of the first valve, particularly when the first valve is stuck open, or if the first valve is defective so as to reduce the first cracking pressure of the first valve. Alternatively, the second valve may be upstream of the first valve. In this way, when the second valve has a higher cracking pressure than the first valve, the first valve may provide redundancy in the event of a failure of the second valve, such as a failure that causes the second cracking pressure to be less than the first cracking pressure, or when the second valve is stuck open.

[0040] Optionally, the vent system comprises a further vent line comprising a further valve that is openable to place the fuel tank in fluid communication with a further region external to the fuel tank. Optionally, the fuel tank vent system monitor is configured to facilitate determination of the performance of the further valve. Optionally, the action comprises the fuel tank vent system monitor determining the performance of the further valve on the basis of the parameter obtained.

[0041] Optionally, the further region comprises, is comprised in, or is otherwise fluidical 1 y connected to the region external to the fuel tank. In other words, the vent line may be configured to vent the contents of the fuel tank to the same or an adjacent region to the further vent line.

[0042] Providing the further vent line may improve a redundancy of the vent system. For instance, the further vent line may permit a flow of the contents of the fuel tank to the further region in the event of a failure of the vent line, or the first and / or the second valve, where provided. This may particularly be the case if the first and / or the second valve is stuck closed, in which case the contents of the fuel line may be permitted to flow to the further region via the further vent line and the further valve. Alternatively, the vent line may provide redundancy in the event of a failure of the further vent line.

[0043] The fuel tank vent system monitor being configured determine, or facilitate determination of, the performance of the further valve may therefore improve a safety and / or reliability of the vent system, such as by allowing any reduction in performance of the further valve, and thus a reduction in performance of the further line, to be determined.

[0044] Optionally, before or while causing the fluid source to be fluidically connected to the fuel tank, the fuel tank vent system monitor is configured to cause the contents of the fuel tank to be prevented from flowing out of the fuel tank except through the vent line and / or the further vent line. This may be by the fuel tank vent system monitor causing the return gas isolator, where provided, to close. This may cause a pressure in the fuel tank to increase, as discussed above.

[0045] Optionally, the further valve is configured to open when a pressure upstream of the further valve exceeds a further cracking pressure of the further valve. Optionally, the fuel tank valve system monitor is configured to cause the threshold pressure to be greater than the further cracking pressure.

[0046] In this way, the fuel tank vent system monitor may determine, or facilitate determination of, the performance of the further valve by causing conditions in the fuel tank that would be expected to cause the further valve to open. This may provide a convenient, efficient and / or reliable way to determine, or facilitate determination of, the performance of the further valve.

[0047] Optionally, the further valve is a first further valve and the further cracking pressure is a first further cracking pressure. Optionally, the further vent line comprises a second further valve having a second further cracking pressure. Optionally, the first further valve is fluidically connected in series with the second further valve in the further vent line. Optionally, the fuel tank vent system monitor is configured to facilitate determination of the performance of the first further valve and / or the second further valve. As mentioned above with respect to the first and second valves, this may provide redundancy in the event of a failure of one of the first further valve or the second further valve.

[0048] Optionally, the action comprises the fuel tank vent system monitor determining the performance of the first further valve and / or the second further valve on the basis of the parameter obtained. The fuel tank vent system monitor determining, or facilitating determination of, the performance of one or both of the first further valve and the second further valve may improve a safety and / or reliability of the fuel tank and / or the vent system.

[0049] Optionally, the second further cracking pressure is greater than the first further cracking pressure. Optionally, to facilitate determination of the performance of the second further valve, the fuel tank vent system monitor is configured to: cause the threshold pressure to be in a second range of from the first further cracking pressure to the second further cracking pressure; and determine whether there is a flow of fuel through the second further valve.

[0050] In this way, when the fuel source is caused to be fluidically connected to the fuel tank, the pressure in the fuel tank may tend towards a pressure between the first and second further cracking pressures, particularly when the contents of the fuel tank are prevented from flowing out of the fuel tank except via the vent line or the further vent line, as discussed above. In such a case, if the second further valve is functioning properly, the second further valve will remain closed, regardless as to whether the first further valve is caused to open. If the second valve remains closed, a flow rate of fuel to the fuel tank may be reduced, such as to a zero or to a significantly lower flow rate than if the second further valve were to be caused to open.

[0051] Thus, the fuel tank vent system monitor may determine or facilitate a determination that the valve is performing as expected if a change, or rate of change, in the fuel level in the fuel tank is below a threshold change or rate of change. This may provide a convenient, reliable, and / or accurate way to determine or facilitate determination of the performance of the second further valve.

[0052] Optionally, the second further cracking pressure is greater than the first further cracking pressure. Optionally, to facilitate determination of the performance of the first further valve, the fuel tank vent system monitor is configured to: cause the threshold pressure to be greater than the second further cracking pressure; and facilitate determination of the performance of the first further valve and / or the second further valve by determining whether there is a flow of fuel through the respective first further valve and / or second further valve.

[0053] Causing the threshold pressure to be greater than the second further cracking pressure may cause a pressure in the tank to tend towards a pressure that is greater than the second further cracking pressure, and thus also greater than the first further cracking pressure. If each of the first and second further valves is functioning correctly, then each of the first and second further valves should be caused to open. This may lead to a predictable flow rate of contents of the fuel tank through the further vent line (and / or the vent line, if the first and second valves are also caused to open, as discussed below) as the fluid is caused to be supplied to the fuel tank from the fluid source. If the fuel level in the fuel tank does not increase, or if it increases at a lower rate than expected, this may mean that either the first further valve or the second further valve has not opened, or has opened a reduced amount, and so has reduced performance.

[0054] As such, the fuel tank vent system monitor may determine or facilitate a determination that a performance of the first further valve and / or the second further valve is above an acceptable performance of the first valve and / or the second valve in response to a change or rate of change in the fuel level in the tank exceeding a threshold change or rate of change, or being within a predetermined range of such a change or rate of change in the fuel level. The threshold change or rate of change may be a change or rate of change above which the further valve and / or the second further valve is considered to have an acceptable performance.

[0055] Optionally, the first further valve is upstream of the second further valve. In this way, when the second further valve has a higher cracking pressure than the first further valve, the second further valve may provide redundancy in the event of a failure of the first further valve, particularly if the first further valve is stuck open, or if the first further valve has failed so as to reduce the first further cracking pressure of the first further valve. Alternatively, the second further valve may be fluidically coupled upstream of the first further valve. In this way, when the second further valve has a higher cracking pressure than the first further valve, the first further valve may provide redundancy in the event of a failure of the second further valve, such as due to a failure that causes the second further cracking pressure to be less than the first further cracking pressure, or when the second further valve is stuck open. Thus, by determining or facilitating determination of the performance of the first and / or the second further valve, the fuel tank vent system monitor may improve a safety and / or reliability of the vent system.

[0056] Optionally, when the second cracking pressure is greater than the first cracking pressure, the second further cracking pressure is greater than the second cracking pressure. In this way, the second further valve may only open if the second valve is already open, or in the event of a failure of the second valve that causes the second valve to remain shut. As such, the second further valve may provide redundancy in the event that the vent line is unable to vent the contents of the fuel tank out of the fuel tank at a high enough rate to prevent a pressure in the fuel tank increasing above the second further cracking pressure, such as in the event of a failure of the second valve. Thus, the fuel tank vent system monitor being configured to determine or facilitate determination of the performance of the first valve, the second valve, the first further valve, and / or the second further valve may improve a safety and / or reliability of the vent system.

[0057] Optionally, the first further cracking pressure is between the first cracking pressure and the second cracking pressure. In this way, as a pressure in the fuel tank is caused to increase, the first valve may be caused to open first, followed by the first further valve, then the second valve, and finally the second further valve. In other words, cracking pressures of each valve may take a set of values such that second further cracking pressure is greater than the second cracking pressure, which is greater than the first further cracking pressure, which is greater than the first cracking pressure.

[0058] Optionally, where the fuel tank vent system monitor is configured to determine the performance of the second further valve, the threshold pressure is caused to be between the first further cracking pressure and the second cracking pressure. This may provide conditions in the fuel tank that cause the first valve and the first further valve to open, while the second valve and the second further valve remain closed, when each of the valves is performing correctly. When the threshold pressure is set as such, the fuel tank vent system monitor may determine or facilitate a determination that both the second valve and the second further valve are performing correctly if there is no, or a reduced increase in fuel level in the fuel tank, as discussed above. Setting the cracking pressures as such may therefore allow the fuel tank vent system monitor to confirm that there are no leaks through either the vent line or the second vent line, or across the respective second valve or second further valve. In this way, the fuel tank vent system monitor may be used to improve a safety and / or reliability of the vent system.

[0059] It will be appreciated that, when the cracking pressures of each valve take the set of values discussed above, the fuel tank valve system monitor may be configured to determine the performance of the second valve by causing the threshold pressure to be between the first cracking pressure and the second cracking pressure, but less than the first further cracking pressure. This may ensure that the first further valve remains shut, so that only the performance of the second valve is tested, and not also the performance of the second further valve. This may allow the fuel tank vent system monitor to separately determine, or facilitate determination of, the performance of the second valve and the second further valve, which may provide a more accurate and reliable determination of the performance of the vent system as a whole. For instance, in the event of a failure of one of the second valve and the second further valve, the fuel tank vent system monitor separately determining or facilitating determination of the performance of the second valve and second further valve may facilitate a determination as to which one of the second valve and the second further valve has failed. This may reduce a requirement for further sensors, such as optical sensors, for determining which of the vent line and further vent line is discharging the contents of the fuel tank to the region, to identify which of the vent line and further vent line has experienced a failure or drop in performance. This, in turn, may lead to a simpler and lighter fuel tank, vent system, and / or fuel tank vent system monitor.

[0060] It will also be appreciated that when the cracking pressures of each valve take the set of values discussed above, then a pressure in the fuel tank above that of the second further cracking pressure would cause each of the first valve, the second valve, the first further valve, and the second further valve to open, if each of the valves is performing correctly. As such, when the threshold pressure is caused to be greater than the second further cracking pressure, the rate of change in the fuel level in the tank may be increased when all valves are functioning correctly compared to when the first and / or the second further valves have failed so that they remain shut. This is because, when both vent lines are open, there is a greater flow rate of the contents of the fuel tank to the region and / or further region, than when one of the vent lines (e.g., the further vent line) is closed.

[0061] In such a case, the fuel tank monitor may be configured to determine or facilitate a determination that the first and / or the second further vent valve is performing correctly if a change or rate of change in the fuel level in the tank is above a threshold value, which is itself greater than a value that would be expected if the first and / or the second further vent valve were to remain shut.

[0062] A second aspect of the present invention provides a fuel system comprising: the fuel tank vent system monitor of the first aspect of the present invention; the fuel tank for receiving the fluid; and the vent system comprising the vent line comprising the valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank.

[0063] It will be appreciated that the fuel system of the second aspect may comprise and / or benefit from any of the optional features and / or advantages ascribed to the first aspect.

[0064] A third aspect of the present invention provides a method of determining a performance of a vent system for a fuel tank. The vent system comprises a vent line comprising a valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank. The method comprises: causing fluid to be supplied to the fuel tank; thereafter, obtaining a parameter indicative of a characteristic of contents in the fuel tank; and determining the performance of the vent system by determining a performance of the valve on the basis of the parameter obtained.

[0065] It will be appreciated that the method of the third aspect of the present invention may comprise any of the actions performed by the fuel tank vent system monitor of the first aspect of the present invention. Similarly, the method of the second aspect of the present invention my comprise and / or benefit from any of the optional features and / or advantages of the first aspect and / or the second aspect of the present invention.

[0066] A fourth aspect of the present invention provides a non-transitory computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to perform the method of the third aspect of the present invention.

[0067] It will be appreciated that the non-transitory computer-readable storage medium of the fourth aspect of the present invention may comprise and / or benefit from any of the optional features and / or advantages ascribed to any of the first to third aspects of the present invention.

[0068] A fifth aspect of the present invention provides an aircraft comprising the fuel tank vent system monitor of the first aspect of the present invention, the fuel system of the second aspect of the present invention, or the non-transitory computer-readable storage medium of the fourth aspect of the present invention.

[0069] Optionally, the aircraft comprises the non-transitory computer-readable storage medium, and the processor configured to execute the instructions stored on the non-transitory computer readable storage medium.

[0070] It will be appreciated that the aircraft of the fifth aspect of the present invention may comprise and / or benefit from any of the optional features and / or advantages ascribed to any of the first to fourth aspects of the present invention.

[0071] Features of any of the above aspects may be combined with other features of any of the above aspects in any suitable way as appropriate to particular implementations of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0073] Figure 1 shows a schematic view of an example aircraft;

[0074] Figures 2 and 3 show an example fuel system and supply system associated with the aircraft of Figure 1;

[0075] Figure 4 shows the parameters used in example tests to determine the performance a vent system of the fuel tank of Figures 2 and 3;

[0076] Figure 5 shows an example method of determining a performance of the vent system of the fuel tank of Figures 2 and 3; and

[0077] Figure 6 shows a further example method of determining a performance of the vent system of the fuel tank of Figures 2 and 3. DETAILED DESCRIPTION

[0078] Figure 1 shows an example aircraft 1. The aircraft is shown on-ground and comprises a fuel system 2 for storing and supplying fuel to an engine 5 of the aircraft 1. The fuel here is hydrogen fuel, which is specifically stored in the aircraft fuel system 2 as cryogenic liquid hydrogen. It will be appreciated, however, that the disclosures herein similarly apply to fuel systems configured to store any other suitable type of fuel, such as conventional aviation fuel, such as kerosene.

[0079] The aircraft 1 comprises a tail 8 and wings 7. The engine 5 is located on one of the wings 7. The fuel system 2 comprises a fuel tank 3 which is here located towards the tail 8 of the aircraft, but in other examples may be in any other suitable location, such as in one or more of the wings 7 of the aircraft.

[0080] Shown in Figure lisa supply system 4 for supplying fuel to the fuel tank 3 during a fuelling operation of the aircraft 1. The supply system 4 is specifically a ground-based supply system 4 that is removably fluidically connectable to the fuel system 2, and specifically the fuel tank 3, during a fuelling operation of the fuel tank 3. The fuel system 2 comprises a vent system 6 for venting gaseous hydrogen fuel in the fuel tank 3 to a region 20 external to the fuel tank 3 and the aircraft 1.

[0081] Figures 2 and 3 show schematic diagrams of the supply system 4 connected to the fuel system 2. For clarity, reference numerals to components of the fuel system 2 and the supply system 4 shown in Figure 3 are omitted in Figure 2, while boxes outlining the fuel system 2, the supply system 4, and the region 20 shown in Figure 2 are omitted in Figure 3.

[0082] The supply system 4 comprises a fuel supply tank 40 comprising fuel for the fuel tank 3, an inert gas tank 41 comprising an inert gas, and an inert gas valve 42 fluidically connecting the inert gas tank 41 to the fuel supply tank 40. The inert gas tank 41 here comprises helium gas, but may comprise any other suitable inert gas in other examples. The fuel supply tank 40 is fluidically connected to the fuel tank 3 by a fuel supply valve 51 of the fuel system 2. During fuelling of the fuel tank 3, the inert gas is stored in the inert gas tank 41 at a pressure greater than that in the fuel supply tank 40. The inert gas valve 42 can be opened to permit the inert gas to pass into the fuel supply tank 40 to increase a pressure of the fuel in the fuel supply tank 40. When the pressure of the fuel supply tank 40 is greater than a pressure in the fuel tank 3, the fuel supply valve 51 can be opened to permit the pressurised fuel from the fuel supply tank 40 to pass into the fuel tank 3.

[0083] The fuel system 2 comprises a return gas valve 52, which is here connected to a vent stack 43 of the supply system 4. The vent stack 43 is configured to vent return gas, which here is hydrogen gas, from an ullage space 33 in the fuel tank 3 to an atmosphere 45 external to the fuel tank 3 and the supply system 4. In particular, during fuelling of the fuel tank 3, the return gas valve 52 can be opened to allow the gas to pass to the vent stack 43 as a fuel level 31 of fuel in the fuel tank 2 increases in the direction shown by the arrow 35 in Figure 2.

[0084] The fuel system 2 comprises two capacitive sensors 32a, 32b for determining a fuel level 31 of fuel in the fuel tank 3. To allow accurate determination of the fuel level 31 in the fuel tank 3, the capacitive sensors 32a, 32b are located in different locations in the fuel tank 3. Although not shown in Figure 2, for clarity, the capacitive sensors 32a, 32b each comprise a respective pair of electrodes, each pair of electrodes defining a respective interspace therebetween. Each electrode in each pair of electrodes is formed by a respective one of two concentric tubes. The fuel in the fuel tank 3 is received in the interspace between each of the pair of concentric tubes, which affects a capacitance of the respective capacitive sensors. The fuel tank vent system monitor 70 can determine the fuel level 31 in the fuel tank 3 based on the capacitance of each capacitive sensors 32a, 32b. Spacing the capacitive sensors 32a, 32b apart in the fuel tank 3 allows the fuel level 31 in the fuel tank 3 to be accurately determined irrespective of an orientation of the fuel tank 3.

[0085] The vent system 6 comprises a first vent 60a, a second vent 60b, and first and second vent lines 65 a, 65b connecting the respective first and second vents 60a, 60b to the ullage space 33 in the fuel tank 3 . By venting gaseous hydrogen fuel from the fuel tank 3 via the vent system 6, the fuel system 2 is able to release excess pressure in the fuel tank 3. In this case, no cooling system is provided for maintaining a cryogenic temperature of the fuel in the fuel tank 3. As such, the fuel in the fuel tank 3 will heat up over time, such as during a flight, causing some of the liquid hydrogen in the fuel tank 3 to transition into gaseous hydrogen, expanding in the process. Such expansion of gases in the fuel tank 3 increases a pressure in the fuel tank 3. The gaseous hydrogen can be vented through the first and / or the second vent 60a, 60b to maintain a pressure in the fuel tank 3. The first and second vents 60a, 60b are here located in the tail 8 of the aircraft 1. It will be appreciated, however, that in other examples first and / or the second vent 60a, 60b may be located in any other suitable location, such as close to the engine 5, or the wings 7.

[0086] The first vent line 65a comprises a first valve 61 and a second valve 62 fluidically connected in series in the first vent line 65a. The second vent line 65b comprises a third valve 63 and a fourth valve 64 fluidically connected in series in the second vent line 65b. The first vent line 65a and the second vent line 65b are fluidically connected in parallel to each other between the fuel tank 3 and the region 20 external to the fuel tank 3. It will be appreciated that, when the aircraft 1 is on the ground, the region 20 comprises the atmosphere 45 discussed above in relation to the vent stack 43. The first to fourth valves 61, 62, 63, 64 are pressure relief valves configured to open in response to a particular pressure upstream of the respective first to fourth valves 61, 62, 63, 64, thereby to release gas from the fuel tank 3 through the respective first and second vent lines 65a, 65b to the region 20.

[0087] The first to fourth valves 61, 62, 63, 64 are configured to open in response to different respective upstream pressures. The pressure at which a valve is configured to open may be referred to herein as a “cracking pressure”. Here, a fourth cracking pressure of the fourth valve 64 is greater than a second cracking pressure of the second valve 62, which is in turn greater than a third cracking pressure of the third valve 63, which is in turn greater than a first cracking pressure of the first valve 61. In other words, taking Pn to be a cracking pressure of the ‘n’th valve, the following relationship (A) applies: P4> P2> P3> Pl (A)

[0088] By providing the second valve 62 downstream of the first valve 61 in the first vent line 65a, and by ensuring P2 >Pl, the second valve 62 provides a backup in the event that the first valve 61 has failed such that it is stuck open or has a reduced first cracking pressure. Similarly, by providing the fourth valve 62 downstream of the third valve 63 in the second vent line 65b, and by ensuring P4 >P3 the fourth valve 64 provides a backup in the event that the third valve 63 has failed and is stuck open or has a reduced third cracking pressure. Moreover, as the fourth cracking pressure (P4) is greater than the second cracking pressure (P2) and the third cracking pressure (P3) is greater than the first cracking pressure (Pl), the second vent line 65b provides a backup in the event of a failure of the first vent line 65a, such as a failure of the first and / or the second valves 61, 62.

[0089] Figure 2 also shows a fuel tank vent system monitor 70, which is configured to determine a performance of the first to fourth valves 61, 62, 63, 64 by performing various tests to determine the performance of the first to fourth valves 61, 62, 63, 64. As will be described in more detail below, the tests comprise the fuel tank vent system monitor 70 causing fuel to be supplied from the fuel supply tank 40 to the fuel tank 3 at a particular pressure which causes one or more of the first to fourth valves 61, 62, 63, 64 to open. The fuel tank vent system monitor 70 then monitors a fuel level 31 in the fuel tank 3 to determine whether the first to fourth valves are operating correctly.

[0090] To determine the performance of the first, second, third and fourth valves 61, 62, 63, 64, the fuel tank vent system monitor 70 is communicatively coupled to the inert gas valve 42, the fuel supply valve 51 and the return gas valve 52, so as to be able to cause operation of the inert gas valve 42, the fuel supply valve 51, and the return gas valve 52, respectively. The fuel tank vent system monitor 70 is also communicatively coupled to each capacitive sensor 32a, to permit the fuel tank vent system monitor 70 to determine the fuel level 31 in the tank, and to a supply pressure sensor 44 in the fuel supply tank 40, which allows the fuel tank vent system monitor 70 to determine a supply pressure in the fuel supply tank 40. The “communicative coupling” described herein is to be understood as a wired coupling, but in other examples may be a wireless coupling.

[0091] As noted above, the fuel tank vent system monitor 70 is able to perform tests to determine the performance of the first, second, third and fourth valves 61, 62, 63, 64. To do this, the fuel tank vent system monitor 70 is able to cause the inert gas valve 42 to open when the fuel supply valve 51 is closed to increase a pressure in the fuel supply tank 40. Once a desired supply pressure (PS) in the fuel supply tank 40 is reached, the fuel tank vent system monitor 70 can then cause the inert gas valve 42 to close, and cause the fuel supply valve 51 to open, which in turn causes fuel to be supplied to the fuel tank 3 from the fuel supply tank 40. It will be appreciated that the supply pressure (PS) is initially higher than a pressure in the fuel tank 3, so as to cause fuel to move from the fuel supply tank 40 to the fuel tank 3.

[0092] If the return gas valve 52 is closed, then is it is possible to determine the performance of the first, second, third, and / or fourth valves 61, 62, 63, 64 by comparing a rate of increase of fuel in the fuel tank 3 to an expected rate of increase for a particular supply pressure (PS) in the fuel supply tank 40 prior to the fuel supply valve 51 being opened. This is because, when the return gas valve 52 is closed, a build-up of pressure in the fuel tank 3 can only be relieved by a release of gas through the first, second, third and / or fourth valves 61, 62, 63, 64. If the valves are not performing as expected, then for a given supply pressure (PS) in the fuel supply tank 40, a flow rate of fuel through the first and second vent lines 61a, 60b may change, which can in turn affect a rate of increase of the fuel level 31 in the fuel tank 3.

[0093] Specifically, when the supply pressure (PS) is set to a pressure between the first cracking pressure (Pl) of the first valve and the third cracking pressure (P3) of the third valve (i.e., P3 >PS >Pl), then upon opening the fuel supply valve 51, the pressure in the fuel tank 3 will increase towards PS. As it does, the first valve 61 will be expected to open, but the second, third and fourth valves 62, 63, 64, which each have a respective cracking pressure equal to or greater than the third cracking pressure (P3), will be expected to remain closed. In other words, it is expected that fuel will be unable to leave the fuel tank 3, and the fuel level 31 in the fuel tank 3 should either remain constant, or should increase only due to compression of fuel (e.g., gaseous hydrogen fuel) in the fuel tank 3. As such, if a rate of increase (R) in fuel level in the fuel tank 3 exceeds a first expected rate of increase (RI) in fuel level when each of the first to fourth valves 61, 62, 63, 64 is closed, for the particular supply pressure (PS), then it can be inferred that there has been a loss or reduction in performance of the second valve 62. Such a loss, or reduction, in performance could be due to a failure of the second valve 62 that reduces the second cracking pressure (P2) of the second valve 62. This could cause the supply pressure (PS) to exceed the reduced second cracking pressure (P2) and cause the second valve 62 to open. Alternatively, the second valve 62 may have experienced a loss in performance that causes a leak through the second valve 62; for instance, the second valve 62 may be partially open at all times, or partially open when a pressure upstream of the second valve approaches the supply pressure PS. The test for determining such a loss in performance of the second valve 62 may be referred to as a “second-valve leak test”.

[0094] In a similar way, if P2 >PS >P3, then when the fuel supply valve 51 is opened, the first and third valves 61, 63 should also open when functioning correctly, while the second and fourth valves 62, 64 (which each have cracking pressures greater than or equal to the second cracking pressure (P2)) should remain closed. In this way, if the second valve 62 has not experienced a loss in performance, as determined using the second-valve leak test, then when P2 >PS >P3, it can be inferred that the fourth valve 64 has experienced a loss, or reduction, in performance if the rate of increase (R) in fuel level in fuel tank exceeds a second expected rate of increase in fuel level (R2). Such a loss, or reduction, in performance may be one that causes a reduction in the fourth cracking pressure (P4) of the fourth valve 64, or causes a leak through the fourth valve 64. As such, this test may be referred to as a “fourth-valve leak test”.

[0095] If P4 >PS >P2 when the fuel supply valve 51 is opened, then the first, second, and third valves 61, 62, 63 should open, but the fourth valve 64 should remain closed. This will permit a flow of gaseous hydrogen from the fuel tank 3 through the first vent line 65a via the first and second valves 61, 62, which will cause a level of fuel in the fuel tank to increase. If there has been no reduction in performance of the fourth valve 64 as determined by the fourth-valve leak test, the gaseous fuel will be prevented from flowing through the second vent line 65b through the fourth valve 64. The rate of increase (R) in fuel level in the fuel tank 3 will therefore depend upon a flow rate of gas through the first vent line 65a. In this way, it is possible to determine a third expected rate of increase in fuel level (R3) in the fuel tank 3, based on the supply pressure PS, when fuel is able to flow through the first vent line 65a but not the second vent line 65b. If there has been no or an acceptable reduction in performance of the fourth valve 64, the fuel tank vent system monitor 70 can determine that there has been a loss, or reduction, in performance of the first valve 61 and / or the second valve 62 if the rate of increase (R) in fuel level is lower than the third expected rate of increase in fuel level (R3). This would mean that the first valve 61 and / or the second valve 62 is either not opening, not opening fully, or that there is some other restriction to flow (e.g., due to debris) in the first vent line 65a. As such, this test may be referred to as a “first vent line function test”.

[0096] Finally, if PS >P4 when the fuel supply valve 51 is opened, then each of the first, second, third and fourth valves 61, 62, 63, 64 should also open if they are operating correctly. This will cause gaseous fuel in the fuel tank 3 to vent through each of the first and second vent lines 65a, 65b. This, in turn, will lead to a greater rate of increase in the fuel level 31 in the fuel tank 3 than when only the first vent line 65a is open. As such, it is possible to determine a fourth expected rate of increase in fuel level (R4) in the fuel tank 3 based on the supply pressure PS when gaseous fuel is able to flow through each of the first and second vent lines 65a, 65b. When no or an acceptable reduction in performance of the first vent line 65a has been determined by the first vent line function test, then a loss, or reduction, in performance of the second vent line 65b can be determined when the rate of increase (R) in fuel level is less than the fourth expected rate of increase (R4). This might occur, for example, when the third valve and / or the second valve is either not opening, not opening fully, or when there is some other restriction to flow (e.g., due to debris) in the second vent line 65b. As such, this test may be referred to as a “second vent line function test”.

[0097] The parameters employed in the above-described tests are summarised diagrammatically in Figure 4. Each box in Figure 4 shows the specific supply pressure (PS) caused to be achieved during a respective test in relation to the first to fourth cracking pressures (Pl, P2, P3, P4). Each box also shows the rate of increase (R) in fuel level in the fuel tank 3 that would lead to a determination of a loss, or reduction, in performance of the vent system 6 in relation to the first to fourth expected rates of increase (RI, R2, R3, R4). Here, RI = R2, and R4 >R3.

[0098] It will be appreciated that the fuel tank vent system monitor 70 is configured perform each of the tests described above, by causing operation of the inert gas valve 42, the fuel supply valve 51, and the return gas valve 52, and by monitoring fuel level 31 in the fuel tank 3 using the capacitive sensors 32a, 32b. The fuel tank vent system monitor 70 also uses the supply pressure sensor 44 to achieve a specific supply pressure PS in the fuel supply tank 40 during the respective tests.

[0099] Shown in Figure 5 is an example method 400 implemented by the fuel tank vent system monitor 70 to perform the tests described above. The method 400 comprises: causing 410 the fuel to be supplied to the fuel tank 3; determining 420 a rate of increase (R) in fuel level in the fuel tank 3; and determining 430 a reduction in performance of one or more of the first to fourth valves 61, 62, 63, 64 based on the rate of increase (R) in fuel level. The determining 430 the reduction in performance is by comparing the rate of increase (R) in fuel level to an expected rate of increase (RI, R2, R3, R4) in fuel level for the particular test, and for a given supply pressure (PS) of the fuel supplied to the fuel tank 3 during that test.

[0100] Figure 6 shows a more specific example method 500 for performing the second vent line function test. The method 500 comprises causing 510 the supply pressure (PS) to be between the fourth cracking pressure (P4) and the second cracking pressure (P2), where P4 >P2. This is specifically by the fuel tank vent system monitor 70 closing the fuel supply valve 51, opening the inert gas valve 42, monitoring the pressure in the fuel supply tank 40 using the supply pressure sensor 44, and closing the inert gas valve 42 when the desired supply pressure (PS) has been reached. The method 500 then comprises causing 520 the fuel to be supplied to the fuel tank 3. This is specifically by the fuel tank vent system monitor 70 opening the fuel supply valve 51.

[0101] The fuel tank vent system monitor 70 then determines 530 the rate of increase (R) in fuel level using the capacitive sensors 32a, 32b. by determining a time taken for the fuel level 31 in the fuel tank 3 to increase by 5%, where the time taken represents the rate of increase of fuel in the fuel tank 3. In other examples, the fuel tank vent system monitor 70 may alternatively determine an increase in the fuel level 31 over a given time period, so that the increase in the fuel level 31 in that time period is representative of the rate of increase in the fuel level 31. Alternatively, the fuel tank vent system monitor 70 may determine the rate of increase in the fuel level 31 directly, such as by using the time taken for the fuel level 31 to increase by 5%, or by using the change in fuel level 31 over the given time period, or in any other suitable way.

[0102] Returning to Figure 6, the method 500 comprises the fuel tank vent system monitor determining 540 a reduction in performance of the first vent line 65a if the rate of increase (R) in fuel level is below the third expected rate of increase (R3). In response to such a determination, the fuel tank vent system monitor 70 is configured to cause 550 an alert to be issued to personnel to facilitate maintenance and / or replacement of the first and / or the second valve 61, 62, or the first vent line 65a as a whole.

[0103] While not shown in the Figures, it will be appreciated that the fuel tank vent system monitor 70 can perform the first-valve leak test, second-valve leak test, and the second vent line function test described above by implementing the method 500 shown in Figure 6, but instead using the specific values of supply pressure PS and expected rate of increase (RI, R2, R3, R4) in fuel level shown in Figure 4, for that particular test.

[0104] Moreover, while the invention has been described above with specific reference to the Figures, it will be appreciated that this is for ease of understanding only, and various modifications and combinations may be made with the scope of the appended claims. For instance, in some examples, in the method 500 shown in Figure 6, the causing 510 the supply pressure (PS) to take a particular value, causing 520 the fuel to be supplied to the fuel tank 3, and / or determining 540 the reduction in performance, may be performed by personnel, rather than by the fuel tank vent system monitor 70 itself.

[0105] In some examples, the fuel tank 3 may comprise a fuel tank pressure sensor (not shown) communicatively coupled to the fuel tank vent system monitor 70. In this way, the fuel tank vent system monitor 70 may determine the performance of the valves based on the pressure in the fuel tank 3. This may be alternative to, or in addition to, using the fuel level 31 in the fuel tank. For instance, if the fuel supply valve 51 is opened when the supply pressure (PS) in the fuel supply tank 40 is greater than the second cracking pressure (P2), then if the second valve 62 is performing acceptably, the pressure in the fuel tank 3 may not exceed the second cracking pressure (P2), or may reach a pressure between that of the second cracking pressure (P2) and the supply pressure (PS), as gas is expelled through the first vent line 65a. If the second valve 62 instead remains closed, or partially closed, the pressure in the fuel tank 3 may approach, or even reach, the supply pressure (PS). The fuel tank vent system monitor 70 may thereby determine that there has been a loss in performance of the second valve 62 if the pressure in the fuel tank 3 is within a predetermined range of the supply pressure (PS). A similar process may be applied to determine the performance of the fourth valve 64.

[0106] In some examples, in addition to, or alternative to, the capacitive sensors 32a, 32b, the fuel system 2 may comprise any other suitable fuel level sensor, such as an optical sensor or weight sensor.

[0107] While the fuel supply valve 51 and the return gas valve 52 are here a part of the fuel system 2, in other examples, one or both of the fuel supply valve 51 and the return gas valve 52 may be a part of the supply system 4, or may be separately provided. In other examples, the fuel system 2 and / or the supply system 4 may comprise more than one such fuel supply valve 51 and / or more than one such return gas valve 52.

[0108] In the illustrated example, the pressure in the fuel supply tank 40 is increased by supplying helium gas from the inert gas tank 41 to the fuel supply tank 40. In other examples, the pressure may be increased in any other suitable way. For instance, the fuel supply system 4 may comprise a heater (not shown) for increasing a temperature of the fuel in the fuel supply tank. This may cause liquid fuel in the fuel supply tank 40 to transition into a gaseous phase, thereby increasing a pressure of the fuel in the fuel supply tank 40. Such a heater may take any suitable form. For instance, the heater may comprise a channel, such as a coil, within or external to the fuel supply tank 40 through which a heat exchange medium is passable to transfer heat between fuel in the fuel supply tank 40 and the heat exchange medium. Alternatively, or in addition, the heat exchanger may comprise a gas mover for moving a gas, such as ambient atmosphere surrounding the fuel supply tank 40, across the fuel supply tank 40. This may increase a rate of heat transfer between the (cryogenic) fuel in the fuel supply tank 40 and the ambient atmosphere, thereby increasing a temperature and pressure of the fuel through evaporation of the fuel. In such examples, the inert gas tank 41 and / or inert gas valve 42 may not be provided.

[0109] Where the heater is provided, the fuel tank vent system monitor 70 may be communicatively coupled to the heater. In this way, the fuel tank vent system monitor 70 may be configured to determine the performance of the first, second, third and fourth valves 61, 62, 63, 64 as part of any of the above-described tests by causing operation of the heater to cause heating of the fuel in the fuel supply tank 40 whilst the fuel supply valve 51 is closed. This may cause an increase a pressure in the fuel supply tank 40 as the fuel evaporates and expands. Once the desired supply pressure (PS) in the fuel supply tank 40 is reached, the fuel tank vent system monitor 70 can then cause the heater to stop heating fuel in the fuel tank, and cause the fuel supply valve 51 to open, which in turn may cause fuel to be supplied to the fuel tank 3 from the fuel supply tank 40.

[0110] In some examples, the supply system 4 may comprise a pump (not shown) for pumping fuel from the fuel supply tank 40 to the fuel tank 3 at the supply pressure (PS). In some such examples, the inert gas tank 41 may not be provided. In other such examples, the return gas valve 52 may be fluidically connected to the fuel supply tank 40, to return gas from the fuel tank 3 to the fuel supply tank 40, and there may be no vent stack 43 provided. Alternatively, the return gas valve 52 may be fluidically connected to a return gas tank (not shown) of the supply system 4 for receiving and storing the return gas. This may permit the return gas to be subject to further treatment, such as to re-condense the return gas, which is here hydrogen gas, into liquid hydrogen. Where the pump is provided, the fuel tank vent system monitor may be communicatively coupled to the pump, and may be configured to cause operation of the pump to cause fuel in the fuel supply tank 40 to pass to the fuel tank 3 at the desired supply pressure (PS), when performing any of the above-described tests.

[0111] The first to fourth valves 61, 62, 63, 64 described above are absolute pressure valves, in which the respective first to fourth cracking pressures (Pl, P2, P3, P4) are such that the respective first to fourth valves 61, 62, 63, 64 are caused to open in response to a pressure upstream of the respective valves 61, 62, 63, 64 exceeding the respective cracking pressures (Pl, P2, P3, P4), regardless as to a pressure downstream of the respective valves 61, 62, 63, 64. In other examples, any one or more of the first to fourth valves 61, 62, 63, 64 may be a differential pressure valve, in which the respective cracking pressure (Pl, P2, P3, P4) is such that the respective valve 61, 62, 63, 64 is caused to open in response to a pressure difference across the respective valve exceeding the respective cracking pressure (Pl, P2, P3, P4) . For instance, the first and third valves may be differential pressure valves, and the second and fourth valves may be absolute pressure valves.

[0112] It is understood that differential pressure valves are simpler and lighter in construction, whereas, to ensure operation at a set pressure, absolute pressure valves require additional means, such as reference to a constant pressure source, or a design that pressure balances the outlet pressure in a manner to make the valve operation independent of the downstream pressure. Application of absolute pressure valves at respective outlets of the first and second vent lines 65a, 65b ensures that the vent system 6 behaviour is independent of downstream ambient pressure in the region 20, while simultaneous application of differential pressure valves at inlets to the respective first and second vent lines 65a, 65b may reduce equipment weight without compromising vent system 6 functionality. It should be noted that two differential pressure valves in series are prone to oscillations as the pressure in a valve interspace between the valves depends on the valve state, and the pressure in the valve interspace also drives the valve state. Therefore, application of absolute pressure valves at the outlets of the first and second vent lines 65a, 65b, with simultaneous application of differential pressure valves at the inlets of the first and second vent lines 65a, 65b, will ensure a steady pressure in the interspaces between the respective valves and prevent oscillation.

[0113] With a differential pressure valve, the supply pressure (PS) required to cause the valve to open will change depending on a pressure downstream of the respective valve. Where the first and third valves 61, 63 are differential pressure valves, the first and third cracking pressures (Pl, P3) may be differential cracking pressures. In that case, the supply pressure (PS) may instead be set relative to first and third upstream pressures (P1U, P3U) upstream of the respective first and third valves 61, 63. These upstream pressures (P1U, P3U) may be determined, e.g., by the fuel tank vent system monitor 70, based on the first and third cracking pressures (Pl, P3) and a pressure downstream of the respective first and third valves 61, 63. Such a downstream pressure may be a downstream pressure in the respective first and second vent lines 65a, 65b, between the respective first and third valves and the respective second and fourth valves 62, 64. As such, in some examples, the fuel system 2 may comprise a vent line pressure sensors located in the respective vent lines 65a, 65b, and in communication with the fuel tank vent system monitor 70, to allow the downstream pressure in the respective vent lines 65a, 65b, and thus the first and third upstream pressures (P1U, P3U), to be determined. The first and third upstream pressures (P1U, P3U) can then be used in any of the tests described above in place of the respective first and second cracking pressures (Pl, P3).

[0114] In other examples, the second and fourth valves 62, 64 may comprise differential pressure valves and / or the first and third valves 61, 63 may comprise absolute pressure valves. In such a case, the “downstream pressure” of the second and fourth valves 62, 64 may be a pressure in the region 20 external to the fuel system 2. As such, in some examples, the fuel system 2 may comprise a region pressure sensor located in the region 20, to determine the downstream pressure, and subsequently second and fourth upstream pressures (P2U, P4U) required to open the second and fourth valves 62, 64. Alternatively, the fuel tank vent system monitor 70 may infer the downstream pressure based on an altitude of the aircraft 1. In other examples, where the second and fourth valves 62, 64 are differential pressure valves, the pressure downstream of the second and fourth valves 62, 64 may be assumed to be a constant pressure, or a pressure range, at ground level, such as a typical or average pressure at ground level, particularly where the tests are performed as part of a fuelling operation of the aircraft 1. In that case, the fuel tank vent system monitor 70 may not need to determine the pressure downstream of the second and fourth valves 62, 64 and the second and fourth upstream cracking pressures (P2U, P4U) may instead be considered constant.

[0115] While not shown in the figures, in some examples, the vent system 6 may comprise one or more rupturable valves, such as one or more valves comprising a rupture disc or membrane. Such a rupture disc or membrane may be configured to rupture, irreversibly, when a pressure across the rupture discs or membrane exceeds a threshold. Such rupture discs may be provided in addition to, or in place of, any one or more of the first to fourth valves 61, 62, 63, 64.

[0116] It will be appreciated that, in some examples, the vent system 6 may comprise any other suitable number of valves and / or vent lines. For instance, the vent system 6 may comprise just one valve in one or both of the first and second vent lines 65a, 65b, or more than two valves in one or both of the first and second vent lines 65a, 65b. In some examples, the vent system 6 may comprise only one, or more than two vent lines, each comprising respective valves. It will be appreciated that in examples deviating from the illustrated examples in the numbers of valves and vent lines provided, the cracking pressures of each valve may be set so as to permit the methods and tests described above to be performed to determine a performance of each valve and / or vent line.

[0117] It is to be noted that the term “or” as used herein is to be interpreted to mean “and / or”, unless expressly stated otherwise.

Claims

1. A fuel tank vent system monitor configured to facilitate determination of a performance of a vent system for a fuel tank, wherein the vent system comprises a vent line comprising a valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank;wherein the fuel tank vent system monitor is configured to: cause fluid to be supplied to the fuel tank;thereafter, obtain a parameter indicative of a characteristic of contents in the fuel tank; andbased on the parameter obtained, cause an action to be performed to facilitate determination of the performance of the valve.

2. The fuel tank vent system monitor of claim 1, wherein the action comprises the fuel tank vent system monitor determining the performance of the valve on the basis of the parameter obtained.

3. The fuel tank vent system monitor of claim 1 or claim 2, wherein the fluid caused to be supplied to the fuel tank comprises fuel, and the fuel tank vent system monitor is configured to cause the fuel to be supplied to the fuel tank.

4. The fuel tank vent system monitor of any one of claims 1 to 3, wherein the characteristic comprises a fluidic property of the contents in the fuel tank.

5. The fuel tank vent system monitor of any one of claims 1 to 4, wherein the contents comprise fuel in the fuel tank, and the characteristic comprises a fuel level of the fuel in the fuel tank.

6. The fuel tank vent system monitor of claim 5, wherein the characteristic comprises a change, or rate of change, in the fuel level of fuel in the tank.

7. The fuel tank vent system monitor of claim 5 or claim 6, wherein the characteristic comprises a time taken for the fuel level in the fuel tank to change by a predetermined amount.

8. The fuel tank vent system monitor of any one of claims 1 to 7, wherein the causing the fluid to be supplied to the fuel tank comprises the fuel tank vent system monitor: causing a pressure of fluid in a fluid source to exceed a threshold pressure; and causing the fluid source to be fluidically connected to the fuel tank.

9. The fuel tank vent system monitor of claim 8, wherein the valve is configured to open when a pressure upstream of the valve exceeds a cracking pressure of the valve, and wherein the fuel tank valve system monitor is configured to cause the threshold pressure to be greater than the cracking pressure.

10. The fuel tank vent system monitor of claim 9, wherein:the valve is a first valve having a first cracking pressure and the vent line comprises a second valve having a second cracking pressure;the first valve is fluidically connected in series with the second valve in the vent line; andwherein the facilitating determination of the performance of the valve comprises facilitating determination of the performance of the first valve and / or the second valve.

11. The fuel tank vent system monitor of claim 10, wherein the second cracking pressure is greater than the first cracking pressure, and wherein, to facilitate determination of the performance of the second valve, the fuel tank vent system monitor is configured to:cause the threshold pressure to be in a first range of from the first cracking pressure to the second cracking pressure; anddetermine whether there is a level of flow of fuel through the second valve.

12. The fuel tank vent system monitor of claim 10 or claim 11, wherein the second cracking pressure is greater than the first cracking pressure, and wherein, to facilitate determination of the performance of the first valve and / or the second valve, the fuel tank valve system monitor is configured to:cause the threshold pressure to be greater than the second cracking pressure; andfacilitate determination of the performance of the first valve and / or the second valve by determining whether there is a level of flow of fuel through the respective first valve and / or second valve.

13. The fuel tank vent system monitor of any one of claims 1 to 12, wherein:the vent system comprises a further vent line comprising a further valve that is openable to place the fuel tank in fluid communication with a further region external to the fuel tank; andthe fuel tank vent system monitor is configured to facilitate determination of the performance of the further valve.

14. The fuel tank vent system monitor of claim 13, wherein:the further valve is configured to open when a pressure upstream of the further valve exceeds a further cracking pressure of the further valve, and wherein the fuel tank valve system monitor is configured to cause the threshold pressure to be greater than the further cracking pressure.

15. The fuel tank vent system monitor of claim 14, wherein the further valve is a first further valve and the further cracking pressure is a first further cracking pressure, and wherein the further vent line comprises a second further valve having a second further cracking pressure, wherein:the first further valve is fluidically connected in series with the second further valve in the further vent line; andthe fuel tank vent system monitor is configured to facilitate determination of the performance of the first further valve and / or the second further valve.

16. The fuel tank vent system monitor of claim 15, wherein the second further cracking pressure is greater than the first further cracking pressure, and wherein, to facilitate determination of the performance of the second further valve, the fuel tank vent system monitor is configured to:cause the threshold pressure to be in a second range of from the first further cracking pressure to the second further cracking pressure; anddetermine whether there is a flow of fuel through the second further valve.

17. The fuel tank vent system monitor of claim 15 or claim 16, wherein the second further cracking pressure is greater than the first further cracking pressure, and wherein, to facilitate determination of the performance of the first further valve, the fuel tank vent system monitor is configured to:cause the threshold pressure to be greater than the second further cracking pressure; andfacilitate determination of the performance of the first further valve and / or the second further valve by determining whether there is a flow of fuel through the respective first further valve and / or second further valve.

18. The fuel tank vent system monitor of any one of claims 15 to 17, when dependent upon claim 11 or claim 12, wherein the second further cracking pressure is greater than the second cracking pressure.

19. A fuel system comprising:the fuel tank vent system monitor of any one of claims 1 to 18;the fuel tank for receiving the fluid; andthe vent system comprising the vent line comprising the valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank.

20. A method of determining a performance of a vent system for a fuel tank, wherein the vent system comprises a vent line comprising a valve that is openable to place the fuel tank in fluid communication with a region external to the fuel tank;wherein the method comprises:causing fluid to be supplied to the fuel tank;thereafter, obtaining a parameter indicative of a characteristic of contents in the fuel tank; anddetermining the performance of the vent system by determining a performance of the valve on the basis of the parameter obtained.

21. A non-transitory computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to perform the method of claim 20.

22. An aircraft comprising the fuel tank vent system monitor of any one of claims 1 to 18, the fuel system of claim 19, or the non-transitory computer-readable storage medium of claim 21.

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