GB2080474A - Fuel pipes - Google Patents

Abstract

A fuel pipe (10) is enclosed by a slightly larger diameter second pipe (16) so that a substantially annular gap (17) is defined between them. The arrangement is such that any fuel leaked from the fuel pipe (10) is contained by the second pipe (16). The fuel pipe (10) is provided with a coating (21) of polytetrafluoroethylene which serves to prevent frettage between the fuel and second pipes (10) and (16) and also reduces distortion of the second pipe during pipe bending as well as generally functioning as a lubricating member between the pipes. Alternatively the coating may be provided on the internal surface of pipe 16. <IMAGE>

Description

SPECIFICATION Improvements in or relating to fuel pipes This invention relates to fuel pipes and in particular to the prevention of the ignition of fuel which has leaked from such pipes.

There is always a danger that pipes which are intended to carry fuel will develop leaks and that fuel leaked therefrom will ignite. This is a particularly troublesome problem in fuel pipes which are adapted for use in aircraft gas turbine engines. If such fuel pipes develop a leak, ignition of the fuel could result in an engine fire, which would, in turn, jeopardise the safety of the aircraft.

There have in the past been two main methods of preventing the ignition of fuels which has leaked from gas turbine engine fuel pipes. The first entails positioning the engine's fuel pipes in situations where, during engine operation, there is an airflow over the pipes which is of such a velocity that the leaking fuel never reaches a concentration' in the air which is sufficiently high to support combustion. Whilst such measures are usually effective in preventing the ignition of leaking fuel, they frequently involve the use of air which is tapped from the compressor section of the engine. This is clearly undesirable since the loss of such air has an adverse effect upon the efficiency of the engine.

The second method entails enclosing each fuel pipe in a slightly larger diameter pipe so that the two are in a generally coaxial relationship with a small gap between the outer surface of the fuel pipe and the inner surface of the outer pipe. If a leak develops in the fuel pipe, the outer pipe prevents the leaked fuel from mixing with air in combustible proportions and subsequently igniting. Thus the fuel leaks into the small gap between the pipes and is contained therein. It is usual to provide a drainage point for the outer pipe in order that any leakage may be readily detected.

Again whilst this second method is usually effective in preventing the ignition of leaking fuel, it does nevertheless present difficulties. The first arises during the manufacture of such concentric pipes. It is frequently necessary to bend fuel pipes in order that they may be properly fitted upon a gas turbine engine. Bending of concentric pipes which are radially spaced apart inevitably results in pipe distortion and consequent contact between the pipes. This being so, when such pipes are fitted upon an engine, vibration during engine operation results in frettage at these contact points with the eventual result that one or both of the pipes is worn through.

The second difficulty arises as a result of thermal differences between the fuel pipe and its outer pipe which frequently occur during engine operation. These thermal differences, which can be in the order of 1 000C, may result in pipe distortion, which again leads to contact occurring between the pipes with the consequent problem of frettage.

It is an object of the present invention to provide means adapted to prevent the ignition of fuel leaked from a fuel pipe, which means substantially avoid the aforementioned difficulties associated with known means for preventing fuel ignition.

According to the present invention, a pipe adapted for the passage of fuel is enclosed by a second pipe of larger diameter so that a substantially annular gap is defined therebetween, at least one of the radially outer surfaces of said fuel pipe and the radially inner surface of said second pipe being provided with a coating of a high temperature resistant polymer having a low coefficient of friction, said polymer coating being of such a thickness that it occupies only a portion of the radial distance across said annular gap.

Said at least one polymer coating preferably occupies approximately 30% of the radial distance across said substantially annular gap between said fuel pipe and said second pipe. Said polymer is preferably polytetrafluoroethylene.

Said fuel pipe is preferably attached to means adapted to either direct fuel into or receive fuel from said fuel pipe, said second pipe being so configured and disposed that any fuel leaked from the junction between said fuel pipe and said fuel directing or receiving means is contained within said substantially annular gap defined between said fuel pipe and said second pipe.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:~ Figure 1 is a sectioned side view of part of a fuel pipe in accordance with the present invention.

Figure 2 is an enlarged view of a portion of the fuel pipe shown in Figure 1.

With reference to Figure 1 a pipe 10 adapted for the passage of fuel is provided at one end with two annular features 1 1 and 12 which are adapted to receive two O-shaped cross section sealing rings 13 and 14 respectively. The sealing rings 13 and 14 provide a fluid tight seal between the fuel pipe 10 and the internal surface of a union 1 5 which is adapted to direct fuel into the fuel pipe 10. It will be appreciated, however, that the union 1 5 could alternatively be for the reception of fuel which has passed through the fuel pipe 10.

The fuel pipe 10 is enclosed by a second pipe 16 which is of larger diameter than the fuel pipe 10 so that an annular gap 17 is defined between them. The end of the second pipe 1 6 is provided with an annular feature 18 similar to those 1 1 and 12 on the fuel pipe 10, which is adapted to receive an O-shaped cross-section sealing ring 19. The sealing ring 19 provides a seal between the second pipe 16 and the enlarged diameter end portion 20 of the union 1 5.

The outer surface of the fuel pipe 10 is provided with a coating of polytetrafluoroethylene 21 which can be seen more easily in Figure 2. The polytetrafluoroethylene coating 21 is of such a thickness that it does not completely occupy the annular gap 17 but instead occupies approximately 30% of the radial distance across the annular gap 17. Thus in one particular application of the present invention a fuel pipe 10 of 1" external diameter enclosed by a second pipe 16 with a 0.020" radial gap between them was provided with a 0.008" thick coating 21 of polytetrafluoroethylene.

The polytetrafluoroethylene coating 10 may be applied by an convenient means. For instance, we have found that it may be easily applied by heat shrinking an appropriately dimensioned tube of polytetrafluoroethylene on to the outer surface of the fuel pipe 10.

It will be seen therefore that the second pipe 16 serves to contain any fuel which may leak from the fuel pipe 10. In practice most leaks are likely to occur at the junction between the fuel pipe 10 and the union 15 through failure of the sealing rings 13 and 14. However as long as the sealing ring 19 between the second pipe 16 and the end portion 20 of the union 1 5 functions satisfactorily then any leaking fuel will be contained within that portion of the annular gap 17 between the polytetrafluoroethylene coating 21 and the second pipe 16. If on the other hand the fuel pipe 10 fails, the polytetrafluoroethylene coating 21 may contain the leakage although if it does not, the leaking fuel will still be contained by the second pipe 16.

The polytetrafluoroethylene coating 21 serves two important functions. Firstly when the fuel pipe 10 is adapted for fitting to a gas turbine engine, it is almost certain that some portion of it will have to be bent. This being so, the polytetrafluoroethylene coating 21 serves to both act as a lubricant during the bending operation and also space apart the fuel pipe 10 and the second pipe 16 when the bending operation has been completed. Thus by spacing the two pipes apart in this manner, distortion of the second pipe 16 as a result of the bending operation is not as severe as would have been the case if the polytetrafluoroethylene coating 21 had not been present.

The second function performed by the polytetrafluoroethylene coating 21 is to function during engine operation as a lubricant between the fuel pipe 10 and the second pipe 16. Thus those areas of the fuel and second pipes 10 and 16 which would have been subject to frettage through engine vibration are instead spaced apart by the polytetrafluoroethylene coating 21.

Frettage is thereby eliminated so that the lives of both the fuel pipe 10 and the second pipe 16 are considerably extended.

It will be appreciated that although the present invention has been described with reference to the use of a polytetrafluoroethylene coating 21, there are other alternative polymers which could be used. Thus mention may be made of high temperature resistant nylons, polysulphones and polyurethanes. Moreover, if it is so desired the coefficient of friction of the polymer may be improved if it is filled with an additional lubricant such as graphite or carbon fibre.

Whilst the present invention has been described with reference to a fuel pipe 10 having a coating 21 of polytetrafluoroethylene on its outer surface, it may be convenient to alternatively provide a polytetrafluoroethylene coating on the inner surface of the second pipe 16. A further possible configuration is one in which both the outer surface of the fuel pipe 10 and the inner surface of the second pipe 16 are provided with a polytetrafluoroethylene coating.

Although the present invention has been described with reference to a fuel pipe for use on a gas turbine engine, it will be appreciated that the present invention is generally applicable to fuel pipes which are required to operate in areas which are subject to vibration and in which a fuel leaked from the fuel pipe could ignite.

Claims (5)

1. A pipe adapted for the passage of fuel and enclosed by a second pipe of larger diameter so that a substantially annular gap is defined therebetween, at least one of the radially outer surface of said fuel pipe and the radially inner surface of said second pipe being provided with a coating of a high temperature resistant polymer having a low coefficient of friction, said polymer coating being of such a thickness that it occupies only a portion of the radial distance across said annular gap.

2. A pipe adapted-for the passage of fuel as claimed in claim 1 wherein said at least one polymer coating occupies approximately 30% of the radial distance across said substantially annular gap betwen said fuel pipe and said second pipe.

3. A pipe adapted for the passage of fuel as claimed in claim 1 or claim 2 wherein said polymer is polytetrafluoroethylene.

4. A pipe adapted for the passage of fuel as claimed in any one preceding claim wherein said fuel pipe is attached to means adapted to direct fuel into or receive fuel from said fuel pipe, said second pipe being so configured and disposed that any fuel leaked from the junction between said fuel pipe and said fuel directing or receiving means is contained within said substantially annular gap defined between said fuel pipe and said second pipe.

5. A pipe adapted for the passage of fuel substantially as hereinbefore described with reference to and as shown in the accompanying drawings.