TECHNICAL FIELD

The present invention relates to venting arrangement for a vehicle comprising a liquid natural gas (LNG) operated fuel system and to a vehicle comprising the arrangement as defined in the appended claims.

BACKGROUND ART

Natural gas, which primarily comprises methane, can be used as fuel in vehicles, e.g. heavy vehicles such as trucks or buses. The vehicle comprises a fuel system, which is configured to utilize the natural gas either as compressed (CNG) or liquefied natural gas (LNG). Liquefied natural gas is stored at a low temperature, approximately -120 °C, inside a cryogenic tank, which is mounted to the chassis of a vehicle. The tank contains both a liquid and vapour phase of liquefied natural gas and a pressure of 10-16 bar is provided inside the tank. Instead of a pump, the pressure of the vapour may be the principal method of supplying fuel to the engine.

The cryogenic tank is a highly insulated thermos comprising two vessels, a first inner vessel and a second outer vessel. The inner vessel is surrounded by the outer vessel, and a low- conducting ceramic material is arranged between the vessels to provide insulation, while a vacuum is drawn between the vessels. By storing methane as liquefied natural gas in a cryogenic vessel, the density of the fuel can be increased. The density of the liquefied natural gas is lower than the density of petroleum-based fuels e.g. diesel. Two tanks containing liquefied natural gas are therefore commonly used in heavy vehicles, one on each side of the chassis of the vehicle. Thus, the liquefied natural gas fuel is sufficient for a driving distance of approximately 1000 km with two tanks.

In order to protect the tank and retain a set pressure, vapour is discharged from the tanks. The discharge of this vapour phase from the storage vessel is called venting. The speed of the pressure build-up depends on the quality of the insulation and the amount of the liquefied natural gas in the tank, e.g. there is a slower build-up if more fuel is present. In addition, the tanks are subjected to ambient air and the warmer ambient temperature will cause heat transfer to the tank. The effect of this heat input is warming of the liquefied natural gas, which will result in a vaporization of liquefied natural gas and thereby an increase of the pressure inside the storage vessel. This vaporized liquefied natural gas resulted by warming is referred to as boil-off gas. In order to avoid an excessive pressure in the storage vessel when the vehicle engine is not running, the cryogenic tank is provided with a venting unit comprising a primary relief valve and a secondary relief valve. The primary relief valve will open when the pressure in the storage vessel exceeds a lower first threshold value, for instance in the order of 16 bar, in order to release vapour phase of the liquefied natural gas from the storage vessel and thereby lower the pressure therein. The secondary relief valve will open when the pressure in the storage vessel exceeds a higher second threshold value, for instance in the order of 24 bar, in order to release vapour, e.g. boil-off gas from the storage vessel in a situation when the primary relief valve malfunctions.

To fill the tanks, a fuel hose is connected to one of the tanks and liquid fuel is pushed into the tank. The tanks can be connected with fuel piping to each other. The tank pressure should be preferably of about 9-10 bar when the fill-up is started. As the tanks fill up the pressure increases and at 16 bar, the fill-up is stopped. If the pressure is higher than 9-10 bar, a venting connector can additionally be used to reduce the pressure in the tanks. The filling and venting connectors may comprise a check valve, which is actuated when connecting occurs. Thus, for example in case of venting connector with a check valve, the check valve normally preventing vapour from escaping to the atmosphere can be actuated such that venting of the tanks is allowed via the venting connector. The filling connector may additionally have a check valve downstream of a connector, which prevents the fuel leaking out from the tank if there is a fault with the connector. However, the venting connector may not have an additional check valve, as it would then block a flow out of the tank. Instead, a manual venting valve, which needs to be manually opened, is fitted on each tank. If both manual valves are left open during normal operation, then vapour in the tanks will be in direct contact with each other. If the tanks are separated, i.e. when the manual valves are closed, then the pressure will drop in the empty tank and liquid will instead be drawn from the other tank, which is the intended function of the fuel system. The manual valves function as a safety arrangement for the fuel system in which liquefied natural gas is used as fuel, but there is still a need to improve the safety and user friendliness of venting systems.

SUMMARY OF THE INVENTION

There are problems associated with the existing venting arrangements and especially the manual valves used in the venting arrangements. For example, when filling up the tanks, manual valves in connection with both tanks have to be opened for ventilation of the tanks. After the fill-up is finished, the manual valves need to be closed. Thus, the manual valves need to be actuated both before and after filling, as well as before and after each servicing, which is laborious. Furthermore, there is a need to walk from one side to the other of the vehicle twice during each filling, i.e. before and after, which is an extra step and makes it more cumbersome to fill up the tanks compared to a diesel driven vehicle. In addition, the manual valves may be dirty and hard to reach.

If the valves are not closed after the fill-up, there will be an open vapour line between the tanks. Thus, there is a risk for pressure drop in the fuel system for example in a situation in which vapour should only be drawn from an empty tank. In case of an open connection, there is an access also to the other tanks vapour. This means that there is a risk that the whole system pressure is reduced, instead of switching from the empty tank to the other tank.

Additionally, since the valves need to be opened and closed manually, there is a risk that the operator/technician forgets to close the valves, which increases the risk for operational disturbances substantially. If the tank is overfilled then liquid may be trapped between the manual valves. If the manual valves are then closed, there is a risk that a pipe connecting the tanks may burst.

Furthermore, during service at a workshop, should the technician forget to open one or both manual valves to discharge vapour and/or boil-off, the vapour will be released via a normal channel, i.e. via a vent outlet, which may be positioned behind a cab of the vehicle, instead of via a ventilation tool which can be connected to a ventilation connector during fill-up or service. There will thus be a release of mainly methane inside a workshop, which is highly undesirable. The problems above are solved by the present solution as claimed in the appended claims.

The present invention relates to a venting arrangement for a vehicle comprising a liquid natural gas operated fuel system. The vehicle comprises a pair of cryogenic tanks for storing the liquefied natural gas and they are mounted to a chassis of the vehicle. The venting arrangement comprises a venting pipeline arranged for venting the tanks and connected to the respective tank, and comprises a venting connector comprising a check valve arranged to discharge vapour from the cryogenic tanks to the atmosphere. Each of the tanks is connected to the venting connector via the venting pipeline, the venting pipeline comprising a pair of manual valves connected to the respective tank and located between the respective tank and the venting connector. The venting pipeline further comprises a pair of check valve devices, each of the check valve devices being arranged between the respective manual valve and the venting connector to allow flow of vapour phase out of the tank at a first pre-determined pressure and to allow flow of vapour phase into the tank at a second pre-determined pressure, wherein the first predetermined pressure is smaller than the second predetermined pressure. The first pre-determined pressure may be from 0-1 bar, but is limited thereto. The second pre-determined pressure may be for example from 7-8 bar, but is also not limited thereto. For example, the second pre-determined pressure could be for example 5-9 bar larger than the first pre-determined pressure.

By the venting arrangement, a simple and robust venting of the tanks can be obtained while the risk for operational disturbances caused by manual valves being in a wrong position are minimized. In addition, the risk for sudden pressure drops in the tanks is minimized, whereby a more robust fuel supply to the gas engine can be provided. Also, if the tanks are overfilled, pipes will not be over-pressurized. Also, further redundancy for the pressure relief valves can be provided.

The check valve device may comprise at least one check valve arranged or configured to open at the first or second pre-determined pressure. By the use of at least one check valve, a simple and robust check valve device can be obtained. The check valve device may comprise two check valves wherein one check valve is arranged to allow flow of the vapour phase out of the tank and the other check valve to allow flow of the vapour phase into the tank, respectively. Thus, a simple and robust device can be obtained. The check valves may be passive valves. Therefore, there is no need for electrical control, whereby the arrangement may function also in case no power is on in the vehicle. Alternatively, the check valves may be electrically controllable active valves. The active check valves may be connected to a control system of the vehicle and configured to be controllable by the control system. Thus, a more accurate and flexible check valve device may be obtained.

The manual valves may be configured to be opened/closed by means of a tool configured for opening/dosing of the valves. Thereby, the risk for leaving the valves in a wrong position can be decreased. The manual valves may be arranged at an open position during the operation of the vehicle and at a closed position during the service of the venting connector of the vehicle. In this way, there is no need to actuate the valves during normal operation and filling up of the tanks, and thus the venting arrangement is less laborious to use.

The present invention also relates to a vehicle comprising the venting arrangement as described above.

Further features and advantages are described below in the detailed description with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows schematically a vehicle comprising a venting arrangement of the present disclosure in a side view;

Fig. 2 shows schematically a coupling scheme of the venting arrangement of the present disclosure;

Fig. 3 shows schematically a manual valve and a tool for opening the valve.

DETAILED DESCRIPTION

An example of vehicle 1, which in the Fig. 1 is a heavy truck, comprising a liquid natural gas (LNG) operated fuel system, is schematically illustrated in Fig 1. The vehicle 1 comprises a chassis 4, which in the front is configured to support a driver's cab 5 and in the rear to support a cargo body (not shown). The vehicle 1 is provided with a gas engine 2 that powers the vehicle's tractive wheels 8 via a gearbox 6 and a propeller shaft 7. The engine 2 is powered by liquefied natural gas fuel supplied from a cryogenic tank 10 comprising a storage vessel 11, in which the liquefied natural gas to be used as fuel for the engine is stored.

The vehicle may comprise at least two cryogenic tanks 10, 10' (shown in Fig. 2) one on each side of the chassis 4, to which the tanks 10, 10' are mounted by means of suitable mounting members 9, which may be for example metallic straps but are not limited thereto. The cryogenic tanks 10, 10' are provided with a venting arrangement 20, which is connected to the cryogenic tanks 10, 10' comprising storage vessels 11, 11'.

The storage vessels further comprise a primary relief valve (not shown) and a secondary relief valve (not shown). The primary relief valve will open when the pressure in the storage vessel 11 exceeds a lower first threshold value, for instance in the order of 16 bar, in order to release boil-off gas, which is a vapour phase of the liquefied natural gas and which builds up when the temperature inside the storage vessel 11 increases, from the storage vessel 11 and thereby lower the pressure therein. The secondary relief valve will open when the pressure in the storage vessel 11 exceeds a higher second threshold value, for instance in the order of 24 bar, in order to release boil-off gas from the storage vessel 11 in a situation when the primary relief valve malfunctions. The cryogenic tanks 10, 10' are connected to a vent outlet 13, which in turn is connected to the primary relief valve, wherein boil-off gas released from the storage vessel 11 via the primary relief valve will leave the cryogenic tank 10 through this vent outlet 13, which may be located behind the cab 5 of the vehicle.

The vehicle further comprises a venting arrangement 20 of the present disclosure, in which a manual valve 14, 14' for the respective tank 10, 10' is included. By manual valve is meant a valve, which needs to be actuated manually or by means of a tool, i.e. a valve that is not automatically or electrically actuated. The manual valves 14, 14' are connected to the tanks 10, 10' for example by a valve interface 12. The manual valves need to be opened to ventilate the tanks during fill-up and/or service. The venting arrangement also comprises a venting connector 15 comprising a check valve 25 (see Fig. 2). The venting connector 15 is arranged in a venting pipeline 16 between the manual valves 14, 14' and is used to vent the tanks for example during filling of the tanks.

To fill the tanks 10, 10' a fuel connector (not shown) can be connected to one of the tanks and liquid fuel is pushed into the tank. The tanks are connected with piping to each other, and will therefore be equally filled. The tank pressure may be about 9-10 bar when the fill-up is started, and will increase as the tanks fill up and at 16 bar the fill up is stopped. If the pressure is higher than 9-10 bar, the venting connector 15 can be used to reduce the pressure in the tanks. The venting connector 15 may comprise the check valve 25 (see Fig. 2), which is actuated when connecting with a ventilation tool for venting. The filling connector may additionally comprise a second check valve, which prevents the fuel leaking out from the tank if there is a fault with the connector. By connecting the venting tool, venting of the vapour phase,, can be performed via the venting connector in a controlled manner.

Fig. 2 shows the venting arrangement 20 of the present invention in more detail. The venting arrangement 20 comprises the above-mentioned venting connector 15, which comprises the check valve 25, which during normal operation is closed, and therefore adds redundancy from boil-off gas escaping via the venting connector. Instead, the boil-off gas may be vented through the vent outlet 13 in case the temperature is increased in the tanks. However, during e.g. tank fill-up and/or service, the check valve 25 may be actuated by coupling a connector or connector tool (not shown) having a corresponding check valve which can be actuated, and thereby actuate the check valve 25 such that it opens and allows vapour to be discharged from the tanks 10, 10'.

For venting the tanks during e.g. service, the manual valves 14, 14' connected to each tank 10, 10' are opened so that the pressure insider the tanks can be decreased if desired. The manual valves are normally closed during the operation of the vehicle and opened during the service. After the service, if both manual valves 14 are left open during normal operation, there is a risk that the vapour phase in the tanks will be in direct contact with each other. Thereby, there is a risk for pressure drop in the fuel system, which may cause problems in the fuel supply. If the tanks are separated, i.e. when the manual valves are closed, then the pressure will drop in the empty tank and liquid will instead be drawn from the other tank. This is the intended function of a fuel system without a pump.

In use, liquid is drawn from a liquid pipe 29, 29' inside the respective tank 10, 10', which is positioned at the bottom while vapour pipe 28, 28' and filling pipes 26, 26' are positioned in the top of the tank. The tanks will supply liquid fuel to the engine supply 33 downstream of the tanks when the pressure is below 10 bar and vapour if the pressure is above 10 bar. This ensures that the pressure does not drop too much in the tanks, which causes torque limitations or, if the pressure is allowed to drop below a minimum pressure, "vehicle of road" (VOR), i.e. an operational stop for the vehicle. Regulating valves 27 and 27' are connected to each tank 10, 10'. The regulating valve may be for example a dubbed economiser and it controls whether liquid or vapour is supplied from the respective tank 10, based on the tank pressure. When liquid is drawn from the tank and it becomes empty, then vapour is drawn instead. The tanks 10, 10' may also comprise a liquid level sensor, and the fuel supply may be additionally or alternatively based on the liquid level in the respective tank 10, 10'. The liquid level sensors may be connected to a control device of the vehicle and the regulating valves may be controlled based on the detected liquid level. In Fig. 2, the tank 10 is nearly empty and the liquid level 22 is below a level in which the liquid pipe 29 reaches the liquefied natural gas. Thus, vapour is supplied to the engine supply 33 via a supply line 30. If the pressure drops below a pre-determined pressure value, e.g. 9 bar, which can be measured by a pressure sensor 31 in an engine supply line 33, the automatic shut-off valve downstream of the regulating valve 27 shuts the flow from the tank 10 and instead, fuel will be provided from the tank 10' on the right hand side, which has a higher level of liquid natural gas via a supply line 30'. The shut-off valve and the regulating valves may be active valves, i.e. electronically controllable via the control unit of the vehicle. The control unit may control the automatic shut-off valve based on a pressure value measured by a pressure sensor 31. Thus, if the pressure from the tank 10 drops below a pre-determined value of e.g. 9 bar, the control system is configured generate a signal to the shut-off valve and shut the shut-off valve 23, whereby the liquefied natural gas is supplied from the tank 10' instead.

Fuel should principally be drawn equally from each tank, but as the pipe lengths from the tanks are of different lengths and the regulating valves have some tolerances. The practical effect is that one tank often is used more than the other tank before it is empty, and the other tank will then subsequently empty. When entering a workshop for service, there is a wish to ensure that boil-off gas is not vented into the workshop. To prevent the gas entering the workshop a venting tool which allows venting to occur at a desired time and location may be fitted to the venting connector 15. The manual valves must then be opened for the tool to be able release gas from the tanks 10, 10' via the venting arrangement 20. According to the present invention, the problems associated with the use of manual valves are decreased by the present venting arrangement 20. Each of the tanks 10, 10' is connected to the common venting connector 15 via a venting pipeline 16 arranged for venting the tanks 10, 10' and connected to the respective tank. The venting pipeline 16 extends between the tanks 10, 10' and connects the tanks 10, 10' and the storage vessels 11, 11' thereof and comprises the venting connector 15 between the tanks. The venting connector 15 comprises the check valve 25 arranged to discharge vapour phase, i.e. the vapour phase of the liquefied natural gas, from the cryogenic tanks to the location where the fill-up of the tanks takes place. The check valve 25 can be actuated to discharge vapour by connecting a tool (not shown) configured to actuate the check valve 25 and thus open the check valve 25. The venting pipeline 16 comprises a pair of the manual valves 14, 14' described above, which are connected to the respective tank 10, 10' and located between the respective tank and the venting connector 15, e.g. in the valve interface 12 (see Fig. 1 and 3). The venting pipeline 16 further comprises a pair of check valve devices 17, 17', each of the check valve devices 17, 17' being arranged between the respective manual valve 14, 14' and the venting connector 15 to allow flow of vapour phase out of the tank at a first pre-determined pressure and to allow flow of the vapour phase into the tank at a second pre-determined pressure, wherein the first predetermined pressure is smaller than the second predetermined pressure. By the present venting arrangement, the manual valves do not need to be actuated during the normal operation of the vehicle including filling up the tanks with fuel, since the manual valves can always be in an open position. Thus, when filling fuel to the tanks a user will never need to actuate the manual valves, which will make the fuel system more robust. Therefore, the risk for operational disturbances or stop caused by the manual valves being in the wrong position is considerably reduced. The manual valves need only be closed, when the venting connector 15 needs service and be removed. In that case, also the valves in the valve devices 17, 17' need to be closed.

The first pre-determined pressure can be determined as desired, but could be for example be from 0-1 bar, but is not limited thereto. The second pre-determined pressure is larger than the first pre-determined pressure, but may be determined such that the pressure difference between the tanks does not become too large, and can be for example from 7-8 bar, but is not limited thereto. Thus, the check valve, which opens for flow out of the tank, is set to open at a very low backpressure to easily allow venting. The check valve, which opens for flow into the tank, instead is set higher, for example at about 7-8 bar, which means that the pressure differential between the tanks cannot become larger than 7-8 bar. Thus, should one tank become empty first, then the differential between the tanks will be smaller and thus no flow will take place. Should one tank leak and empty, then the other tank will only reduce its pressure to 7-8 bar, but not empty further. Even if this happens, then the fuel system will be able to deliver full power. Additionally the check valves add doubled redundancy of the pressure relief valves, as the relief valves of the other tank are used, albeit at an increased pressure differential of 7-8 bar from the check valve. Should the primary relief valve malfunction and not open, then redundancy is today lost for that tank. With the addition of the check valves described above, at 24 bar when the secondary relief valve should open, then also the primary valve of the other tank would be close to open (24-8=16 bar). Additionally, at 32 bar, the secondary relief valve of the other tank opens, should both primary relief valves and one secondary relief valve malfunction.

In Fig. 2, an example of the check valve device 17, 17' is shown. Each of the check valve devices 17, 17' comprises two check valves 171, 172 and 171', 172', respectively. The first check valves 171, 17 are arranged to allow flow of the vapour phase out of the tank and the second check valves 172, 172' are arranged to allow flow of the vapour phase into the tank, respectively. The first check valves 171, 171' are arranged to allow flow of the vapour phase out of the tank at e first pre-determined pressure, which may be from 0-1 bar. The second check valves 172, 172' are arranged to allow flow of the vapour phase into the tank at a second pre-determined pressure, which may be from 7-8 bar. Thus, the first predetermined pressure is smaller than the second predetermined pressure, thereby allowing venting of the tanks while the pressure difference between the tanks can be maintained at a higher level. Each of the check valves 171, 172; 171', 172' is configured to open at the first and/or second pre-determined pressure. Each of the check valves 171, 172; 171', 172' may comprise for example a spring or it could be replaced with other mechanical solutions, such as pressure sensitive membranes, as long as the valve opens at the desired pressure. The check valves of this type are passive valves, which are simple and robust. The check valves 171, 172; 17 , 172' may alternatively be electrically controllable, i.e. active, valves. The electrically controllable check valves may be connected to a control device of the vehicle and the incoming/outgoing vapour pressure may be measured by means of a pressure sensor arranged upstream or downstream of the check valve device to measure the pressure upstream or downstream of the check valve device The double check valves 171, 172; 171', 172' will ensure that overfilling will not hinder expansion of the fuel as it warms. In addition, it is ensured that no pipes can become over-pressured, which may cause operational disturbances or stop of the vehicle.

The manual valves 14, 14' may be arranged to be opened/closed by means of a tool 141 configured for opening/closing of the valves as schematically shown in Fig. 3. For example, a wheel 142 of the manual valves 14, 14' can be constructed so that a mating tool can grip the wheels. Thus, the tool and the manual valves can be constructed so that there is a mating connection between connecting parts and so that the tool can rotate the wheel of the manual valve. This will reduce the possibility of inadvertently having the manual valves closed. As previously described, the manual valves are arranged at an open position during the operation of the vehicle and at a closed position during the service of the venting connector of the vehicle. Only when there is a need to service the venting connector it is necessary to close the manual valves 14, 14' or the valves of the check valve device 17, 17'. This also means that the risk for discharge of boil-off gases via the vent outlet 13 situated behind the cab into the workshop is reduced, since the manual valves are already open.

The foregoing description of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described, but instead the invention is to be limited by the scope of the appended claims. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for a skilled person to understand the invention for various embodiments and with the various modifications appropriate to the intended use.