This invention relates to methods and apparatus for substantially preventing precipitation and/or substantially removing free water in the fuel tank of an aircraft. Background

Fuel in tanks on aircrafts may contain water. The origin of the water in the fuel system is either from the fuel load, and can be either dissolved water or a combination of dissolved water and free water, or the vent system of the fuel tank wherein water ends up in the tank because the moisture from the vent air condenses in the fuel tank, likewise leading to dissolved and free water in the fuel.

Free water in aircraft fuel tanks may be undesirable because at low temperatures this water will form ice which will deposit on the fuel system components, such as the fuel tank and valves, and adversely affect fuel flow and engine performance. Under certain

circumstances this may even result in failure of the engine.

The quantity of water dissolved in aircraft fuels is determined by the partial pressure of water in the vapour space above the fuel. When this vapour is saturated with water at a given temperature (i.e. 100% humidity) the water dissolved in fuel at equilibrium will reflect the saturation values shown in Figure 1. At relative humidity less than 100% the amount of water dissolved will be correspondingly less than saturation values in accordance with Henry's Law. Water will therefore precipitate from the fuel as it is cooled and, if sufficient, coalesce to form free water. Therefore separation of free water from fuel during loading will not be effective at eliminating free water from tank, if conducted at normal ambient temperature, as further free water will precipitate from the fuel as the temperature of the fuel falls during climb and cruise.

Therefore, there is a need for a system in which, during normal operational conditions, little or no free water precipitates, thus obviating water drain requirement, for aircraft.

In this specification, references to 'substantially preventing precipitation' and

'substantially removing' water from the fuel tank etc are used to mean that, throughout the normal operating regime of the aircraft there is little or no free water in the fuel tank, with the amount of free water being less than 100 parts per million or a sump level well below the feed pump inlet. With current aviation fuel types, dissolved water in equilibrium in the fuel at the loading temperature will precipitate out and sink to the bottom of the tank and may freeze within the normal aircraft minimum ambient operating range (typically down to -60 ⁰ C). Therefore, the fuel and or ullage gas or mixture is treated to ensure that the concentration of dissolved water in the fuel is below the concentration at which free water would precipitate out, throughout the operating regimes of temperatures and pressures to which the vehicle is exposed in use.

Description of the prior art

It is generally known to manually drain the accumulated water in the fuel tank periodically. The fuel tank is provided with a valve at a low point in the tank. The free water lying beneath the fuel is drained from the tank with a bucket until fuel starts flowing out of the tank. Especially in countries where ambient temperatures are relatively low, draining the tank can be difficult.

GB2442309 discloses a method for generating auxiliary power in which aircraft fuel is passed to a fuel reformer to generate hydrogen which is then supplied to a hydrogen fuel cell which generates electrical power. Carbon dioxide is also produced as a by-product of the fuel reforming and/or the fuel cell and this is passed via a conditioning system to be routed back to the ullage space. The conditioning system removes impurities of which carbon monoxide and moisture are mentioned. GB2379609 discloses a fire control system for an aircraft fuel tank in which the exhaust gases of an aircraft engine are passed via a desiccation chamber and a condenser to the ullage space of a fuel tank.

GB2330303 discloses a fire control system in which the exhaust gases from an aircraft engine are passed via a separator to remove water content and thence to the ullage space of the fuel tank. GB 1395691 discloses a fuel tank inerting system in which fuel and air are passed to a catalytic converter to produce reaction gases which are cooled to remove water vapour before being passed to the fuel tank.

US2008/128048 discloses a fuel tank inerting system in which fuel is evaporated to provide a fuel vapour which is mixed with air in a catalytic reactor to produce an inert gas comprising carbon dioxide and water vapour. The water vapour is removed and the inert gas is passed to the fuel tank.

In each of the above an inert gas produced as a reaction product in a combustion or catalytic conversion process is subjected to a water or moisture removal process and then deposited in the ullage to reduce the explosion risk. These documents teach removal of water content to avoid introducing water into the tank with the inert gas. They do not tackle the problem of removal of water content already in the fuel to a level where little or no free water will be precipitated when the fuel temperature is reduced and/or the ambient pressure reduced.

Summary of the invention

In one aspect this invention provides a method of substantially preventing or reducing occurrence of free water in an aircraft fuel tank operating in use within predetermined temperature and pressure ranges, which includes the step of controlling the concentration of dissolved water in said fuel to be below a preset threshold, typically corresponding to the minimum saturation concentration of dissolved water in fuel in said predetermined temperature and pressure ranges.

In this manner, there should be no significant precipitation of free water at the operating limits of temperature and/or pressure, as the water/fuel solution is maintained at less than 100% saturation of water throughout the operating temperatures and pressures. In practical terms the concentration of dissolved water at loading temperatures of e.g. 20 ⁰ C is preferably less than 0.01% and more preferably less than 0.001%. Control of the concentration may include the step of causing transfer of water out of the fuel into the ullage thereby to reduce or maintain the concentration of dissolved water to control it to be below said minimum saturation concentration.

This may be done by introducing into the ullage a gas or mixture thereof (such as e.g. ambient air) with a low relative humidity, whereby an amount of dissolved water passes from the fuel into the ullage, thereby to reduce or maintain the concentration of water below said minimum saturation concentration, and optionally causing said low relative humidity gas or mixture thereof to pass through said ullage. In one example said gas or mixture may be dried before it is introduced into the ullage. Also the temperature of the gas or mixture thereof may be relatively high, preferably higher than 10 ⁰ C and more preferably greater than 15°C. Fuel may be recovered from the gas or mixture flowing out of the fuel tank, optionally by drying the gas or mixture, and separating a fluid component obtained by separation into water and fuel and returning the fuel to the tank.

Many aircraft fuel tanks are designed to allow inward venting of gas. In such tanks said inwardly vented gas or mixture may be selected to have a low relative humidity. This may be done by removing water content from the gas or mixture thereof before being inwardly vented. Alternatively, a source of a gas or mixture thereof, other than air, at relatively low humidity may be provided for being inwardly vented.

In other methods, the fuel is treated to ensure that the concentration of dissolved water in said fuel is below said minimum saturation concentration, for example by removing water from the fuel at, prior to, or after the point of loading. This may be done by using at least one of the steps of:

- cooling the fuel,

- subjecting the fuel to a low water vapour pressure

- passing the fuel through a desiccant, and

- passing the fuel through a separator for separating water from fuel, said separator optionally comprising a coalescent, a cyclone separator or a desiccant.

Conveniently, the rate of water removal is adapted to prevent water accumulation in a sump of the fuel tank.

The water removed from said fuel may be disposed of, or stored, by one or more of:

- disposal from a vehicle housing the fuel tank

- storage in a container for later disposal, and

- consumption by apparatus on board the aircraft housing the fuel tank.

The invention also extends to an aircraft fuel tank system for a vehicle which in use operates within predetermined temperature and pressure ranges, including:

- a aircraft fuel tank, and

- a water content limiter for the concentration of dissolved water in said fuel below the minimum water saturation content in said pressurized temperature and pressure ranges,

said water content limiter optionally including one or more of

- a source of gas or a mixture at low relative humidity for supply to the ullage of the fuel tank, and

- a drier for reducing the concentration of dissolved water in said fuel prior to, during or after loading. In another aspect, this invention provides a method of preventing or reducing water in a fuel tank of an aircraft which comprises passing a gas through an ullage of the tank wherein this gas has a low relative humidity.

Preferably ambient air is used as the gas. More preferably the ambient air is dried before it is brought into the tank. The temperature of the gas may be relatively high, preferably higher than 10 ⁰ C, more preferably higher than 15°C. Fuel may be recovered from the gas flowing out of the fuel tank, preferably by means of drying the gas, separating the obtained moisture from the drying step into water and fuel, and returning the fuel to the tank.

In another aspect this invention provides a method of preventing or reducing water in a fuel tank of an aircraft which comprises removing water from the fuel at the point of loading.

In this aspect, the fuel may be cooled and/or subjected to a low water vapour pressure. Additionally or alternatively, the fuel may be passed through a desiccant.

In another aspect, this invention provides a method of preventing or reducing water in a fuel tank of an aircraft which comprises removing water from vent air before entering the vent air through the vent system into the fuel tank. The vent air may be dried before it enters into the fuel tank.

In yet another aspect, this invention provides a method of preventing or reducing water in a fuel tank of an aircraft which comprises filling the tank with a gas to prevent inward venting of ambient air. The gas may be dried before it is entered into the tank.

In yet another aspect, this invention provides a method of preventing or reducing water in a fuel tank of an aircraft which comprises circulating fuel in the tank through a separator for separating water from the fuel. The rate of circulating the fuel through the separator is preferably adapted to prevent water accumulation in tank sump. A coalescer, cyclone and/or a desiccant may be used as separator. The separated water may be disposed overboard; temporarily stored in a storage tank for later disposal; or used on-board, such as to flush aircraft toilets. Accordingly the present invention in its various aspects provides several solutions for managing the water content in a fuel tank in an aircraft. The solutions focus on two generic approaches:

1. Prevention: prevent the entry of moisture to the system, or

2. Managing: managing moisture once within the system.

A combination of these approaches will also be possible.

Whilst the invention has been described above, it extends to any inventive combination or sub-combination of the features set out above or in the following description or drawings.

Short description of the drawings

The above and other aspects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which:

FIG. 1 is a graph showing water solubility in aircraft fuel vs. temperature. FIG. 2 is an illustrative picture of a fuel tank of an aircraft.

FIG. 3 shows schematically an embodiment in accordance with the first approach.

FIG. 4 shows schematically another embodiment in accordance with the first approach.

FIG. 5 shows schematically a further embodiment in accordance with the first approach. FIG. 6 shows schematically still a further embodiment in accordance with the first approach.

FIG. 7 shows schematically an embodiment in accordance with the second approach.

Detailed description of exemplary embodiments

The graph of Figure 1 shows that the capacity to dissolve water in fuel reduces as fuel temperature is reduced. As fuel cools during ascent, excess water will precipitate from fuel. Below 0 degrees Celsius the water will precipitate as ice. This is illustrated in Figure 2. Figure 2 shows a fuel tank 1 containing fuel 2 with water in suspension and water 3 in liquid and/or solid phase accumulated at the bottom of the tank 1. Reference number 4 indicates the ullage. Several solutions to substantially prevent or manage water in the fuel tank 1 of an aircraft will now be described. In each of these, the concentration of dissolved water is caused to be below the minimum saturation concentration for the fuel experienced throughout the operation ranges of temperature and pressure to which the aircraft is subject. This can be done by treating the fuel directly or indirectly to maintain the water concentration below the requisite level.

The following embodiments address the issue of water originating from fuel load.

Dry fuel at loading (Figure 3)

According to this embodiment of the fuel drying concept, dissolved and free water is removed from the fuel at the point of loading. Dissolved water at loaded temperature is removed by either cooling the fuel, subjecting to a low water vapour pressure, or passing the fuel through a desiccant. Ground and/or aircraft mounted equipment can be used to perform these actions. When using ground equipment, airborne weight is reduced. In particular embodiments, the fuel is exposed to an atmosphere selected to have a low water vapour pressure and a high fuel vapour pressure. Dry fuel in aircraft (Figure 4)

According to this embodiment of the fuel drying concept, dissolved water is removed from the fuel during turn-around and/or in-flight, whereby the fuel is already present on the airplane.

The ullage 4 is maintained with air at very low relative humidity. This will cause the water in the fuel 2 to be drawn out of the fuel into the air. Since the water content of air decreases with temperature, air therefore becomes drier with increasing altitude. Advantageously, ambient air could therefore be pumped through the ullage 4 at cruising heights. As an additional advantage, this approach could be combined with an ullage scrubbing approach to fuel tank inerting. Means are provided for recovery of fuel vapour in the air flowing out of the tank 1.

Preferably further means are provided for separation and discharge of water from the air downstream of the fuel tank 1.

Figure 4 shows again the fuel tank 1 provided with on-board equipment for drying the fuel 2. Ambient air is brought through a drier 5. Water being extracted from the air is exhausted from the drier 5 as indicated with arrow 6. The dry air is then pumped through the ullage 4 to draw dissolved water from the fuel 2. The wet, fuel laiden air is brought through a second drier 7. The drier 7 has two outlets: a first outlet 8 for the dried air and a second outlet 9 for the liquid phase which will consist of water and condensed fuel vapour that has been removed from the tank 1 together with the water. The fuel vapour must be recovered to prevent excessive fuel loss. Recovery of fuel vapour will take place by means of a separator 9 which will separate the water from the fuel. The fuel is returned to the tank 1 through outlet 11 of separator 10. Water is discharged from the separator through another outlet 12.

The temperature of the dry air that is brought into the tank 1 for removing the water out of the fuel 1 is preferably relatively high. Air of relatively high temperature has a relatively large water capacity which is beneficial for removing water out of the fuel. Preferably air of more then 10 degrees Celsius, more preferably 15 degrees Celsius is being used.

An important advantage of this embodiment is that it can be combined with an ullage sweeping approach to fuel tank inerting. Instead of ambient air other gases, preferably free of or decreased in oxygen content, for example nitrogen, potentially stored in a reservoir can be used in the arrangement shown in Figure 4 to remove water from the fuel and at the same time provide for fuel tank inerting. The following embodiments address the issue of water originating from the venting system of the fuel tank.

Dry inward venting air (Figure 5) According to this embodiment, moist vent air will be dried at point of entry into the vent system of the fuel tank.

The main advantages of this embodiment are: compatibility with explosion mitigation approaches to fuel tank flammability e.g. reticulated foam, expanded metal products, active suppression.

Prevent inward venting of moist air (Figure 6)

In this embodiment the fuel tank 1 is filled with an alternative dry gas (e.g. nitrogen, dry air) through inlet 12 of tank 1 to prevent inward venting of moist ambient air at 13.

Also this embodiment is advantageously compatible with fuel tank inerting and ullage sweeping flammability reduction methods. Separate and dispose (Figure 7)

According to this embodiment the fuel 2 in the tank 1 is circulated through a separator 14 to remove free water and ice crystals as the fuel cools. The removal is done at a sufficient rate to avoid water accumulation 3 in the tank sump. The following separator technology can be used: coalescer, cyclone and/or desiccant. The water removed by the separator 14 can be disposed as follows:

- Storage tank 15 for disposal at turn-around via valve 16,

- Overboard, and/or

- Used elsewhere e.g. toilet flush.

A motive flow 17 by using for example a jet pump is preferably added to the feed line to the separator for enhancing the flow of fuel 2 to the separator 14. An advantage of this embodiment is that it avoids sending water to the engine. Thereby, it will prevent water accumulating and freezing in the tank sump.