GB2305975A - Fuel system for a gas turbine engine - Google Patents

Abstract

A fuel system for a gas turbine engine comprises a metering valve (8) which supplies fuel via a pressure raising and shut-off valve (10) to a fuel manifold of the engine. A manifold drain valve (20) has a control line (24) connected to the outlet of the metering valve (8) so as to connect the manifold to a drain tank when the pressure at the outlet is below a threshold value and so as to prevent draining of the manifold when the pressure at the outlet is above the threshold value. The manifold does not, therefore, drain unless the metering valve is closed, thereby avoiding complete engine shut-down when only a momentary curtailment of fuel supply is required (eg. to correct an overspeed condition). Details of individual valve constructions are also disclosed (figures 2-5).

Description

FUEL SYSTEM FOR GAS TURBINE ENGINE The present invention relates to a fuel system for a gas turbine engine.

EP-AO 388 046 discloses a fuel control system for a gas turbine engine.

The fuel is supplied to the engine via a pressure raising and shut-off valve. The pressure raising and shut-off valve carries a stem arranged to open a manifold drain valve when the pressure raising and shut-off valve is at a closed position. Consequently, closure of the pressure raising and shut-off valve automatically opens the manifold drain valve, thereby connecting the engine manifolds to a drains tank.

It may be desirable to inhibit fuelling of a gas turbine engine without draining the engine manifold, for example, in response to detection of an engine overspeed condition. Under such circumstances, only a momentary loss of engine power is required so as to control the engine speed, but a full engine shut-down is not required.

According to the present invention there is provided a fuel system for a gas turbine engine, comprising a metering valve, a pressure raising and shut-off valve and a manifold drain valve, the manifold drain valve having a control line connected to an outlet of the metering valve so as to hold the manifold drain valve closed when a pressure at the outlet of the metering valve is greater than a first threshold pressure.

It is thus possible to provide a fuel system in which the engine manifold will not be drained unless the metering valve is at a closed position.

Such a system prevents accidental draining of the manifold when, for example, the pressure raising and shut-down valve is operated so as to inhibit engine fuelling in response to a detection of engine overspeed.

Preferably the pressure raising and shut-off valve is responsive to fuel pressure on a control line thereof to close the valve.

Advantageously the manifold drain valve comprises a piston slidable within a cylinder. The piston may carry a resilient member for blocking a fluid flow path through the manifold drain valve. Advantageously the piston is biased to an open position such that engine manifold is in fluid flow communication with a drains tank when the pressure in the manifold drain valve control line falls below the first threshold pressure.

Preferably the connection between the manifold drain valve and the gas turbine engine is via a fuel flow path within the pressure raising and shut-off valve, the path only being open when the pressure raising and shut-off valve is at a closed position, preventing fuelling of the engine.

The present invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an engine fuel system constituting an embodiment of the present invention; Figure 2 is a schematic diagram of a pressure drop spill valve; Figure 3 is a schematic diagram of a pressure raising and shut-off valve; Figure 4 is a schematic diagram of a shut-off solenoid valve; and Figure 5 is a schematic diagram of an engine manifold drain valve.

The fuel system shown in Figure 1 comprises a first fuel pump 2 for supplying fuel to the inlet of a gear pump 6 via a fuel filter 4. Fuel from the outlet of the gear pump 6 is supplied to an engine manifold (not shown) via a metering valve 8 and a pressure raising and shut-off valve 10 arranged in series. A spill valve 12 is arranged to monitor the pressure difference acting across the metering valve 8, and to spill fuel from the outlet of the gear pump to the inlet of the gear pump so as to maintain a constant pressure across the metering valve 8. A pressure relief valve 14 is connected in parallel with the gear pump 6 and is arranged to spill fuel from the outlet of the gear pump in the event that the fuel pressure becomes excessive.

A manifold drain valve 20 has a fuel inlet 21 connected to the engine (not shown) via the pressure raising and shut-off valve 10. An outlet 22 of the manifold drain valve 20 is connected to a drains tank (not shown).

A control port 24 of the manifold drain valve 20 is connected to the output of the metering valve 8 via a flow wash filter 26. A two-way solenoid valve 28 also has an input connected to the outlet of the metering valve 8 via the flow wash filter 26. A first outlet of the twoway solenoid valve 28 is connected to a control port 30 of the pressure raising and shut-off valve 10 and also to a low pressure line 32 via a pull down orifice 34. A second outlet is connected to a pressure sensing port of the spill valve 12.

The metering valve 8 has a variable metering aperture, the pressure drop across which is held nominally constant by the regulating spill valve 12.

Under such conditions, the rate of fuel flow through the metering valve 8 is a function of the size of the metering aperture. The construction of such a valve is known, and a suitable description can be found in EP-A-0 388 046.

The metering valve 8 and two-way solenoid valve 28 are controlled by two independent electrical channels CHA and CHB, thereby providing a system capable of operating in the event of failure of one of the channels.

The regulating spill valve 12 is shown in greater detail in Figure 2. The regulating spill valve 12 consists of a piston 40 slidable within a sleeve 42. The piston 40 is biased to a first position by a compression spring 44 and carries a lip 46 which engages with a shoulder 48 formed in the sleeve 42 so as to prevent movement of the piston 40 past the first position. An inlet 50 of the valve 12 is connected to the outlet of the gear pump 6. Fuel pressure at the inlet acis on a first surface 51 of the piston 40 and urges the piston to move against the combined urging of the spring 44 and fuel pressure admitted into a chamber 52 via a sensing port 53. The fuel pressure in the chamber 52 acts on a further surface of the piston to urge the piston to the first position.Movement of the piston 40 away from the first position uncovers a plurality of ports 54 which communicate with an annular channel 56 formed within the sleeve 48. The channel 56 communicates with an outlet 58 of the valve 12 which is connected to the input of the gear pump 6. The amount by which the piston 40 moves to uncover the ports 54 is a function of the difference between the fuel pressure at the inlet 50 and at the sensing port 53. The spill valve 12 acts to maintain a nominally constant pressure of approximately 550 kPa (80 psi) across the metering valve 8.

The pressure raising and shut-off valve consists of a piston 70 slidable within a sleeve 72, as shown in Figure 3. A helical compression spring 74 urges the piston 70 to engage with a valve seat 76 thereby closing a fluid flow path between a valve inlet 78 and a first valve outlet 80. The valve outlet 80 communicates with an annular space 82 formed within the piston 70 when the piston is in engagement with the valve seat. The outlet 80 communicates with the valve inlet via a plurality of ports 84 which are closed by the piston 70 when the piston is in engagement with the valve seat 76.

The annular space 82 formed in the piston 70 allows fluid flow communication between the outlet 80 and a second outlet 85 only when the valve is at the closed position.

A control line 90 is in fluid flow communication with a volume 91 defined by the piston 70 and the cylinder 72. Application of high pressure to the volume 91 causes the piston 70 to move towards the valve seat 76 and thereby to close off the fluid flow path from the inlet 78 to the outlet 80. The control line 90 is connected to a first outlet 92 of the two-way solenoid valve 28.

As shown in Figure 4, the two-way solenoid valve 28 has a valve member 100 axially slidable in substantially fluid sealed engagement within a cylinder 102. An annular recess 104 is formed in the cylinder 102 and an inlet 106 of the solenoid valve communicates with the recess 104.

The valve member 100 carries an enlarged portion 110 which reaches into the annular recess 104 and which is movable to engage and substantially seal with either end of the recess 104. The first outlet 92 is located to one side of the recess 104, whereas the second outlet 112 is located at the other side of the recess 104. A spring 116 biases the valve member 100 to the position shown in Figure 4 such that fluid flow communication between the inlet 106 and the first outlet 92 is inhibited, and that the first inlet 106 is in fluid flow communication with the second outlet 112. The second outlet 112 is connected to the sensing port 53 of the spill valve 12.

The valve member 100 is movable by a solenoid 120 having two coils 122 and 124, driven from CHA and CHB, respectively, thereby allowing the solenoid valve to be operated by application of a current to either coil.

The manifold drain valve 20 consists of a piston 140 slidable within a cylinder defined by a valve body 142. The valve inlet 21 is connected to the second outlet 85 of tiie ,uresoure raising and shut-off valve 10. The outlet 22 of the manifold drain valve is connected to the drains tank (not shown). The inlet 21 communicates with a central portion of the cylinder formed within the valve body 142. The piston 140 carries a cap seal 144 which is movable to close the inlet 21. A spring 146 acts against a head 148 of the piston 140 to urge the piston 140 to a position allowing fluid flow communication between the inlet 21 and the outlet 22.The spring is held within a volume 150 which is in communication with the low pressure return line 32 thereby ensuring that the operation of the valve is not affected by variations of pressure in the low pressure line 32. The manifold drain valve control line 24 communicates with a volume 152 at the opposite side of the head 148 to the spring 146 such that pressure within the volume 152 urges the piston 140 t6 move to the right (as indicated in Figure 5) so as to close the valve.

During normal engine running, the two-way solenoid valve 28 is de-energised such that the outlet of the metering valve 8 is connected with the sensing input of the regulating spill valve 12 via the second outlet 112 of the solenoid valve 28. During engine shut-down, the solenoid valve 28 is operated so as to close its second outlet 112 and to divert fuel via the first outlet 92 to the chamber 91 of the pressure raising and shut-off valve 10. The operation of the solenoid valve causes a rise in pressure in the chamber 91 which allows the spring 74 to rapidly close the pressure raising and shut-off valve 10. The operation of the solenoid valve 28 also causes the reference pressure to the spill valve 12 to become depleted via an air vent orifice 130. This allows the spill valve 12 to "off load" the pressure at the output of the gear pump 6 during engine shut-down.This reduces the energy added to the recirculating fuel during "windmilling" of the engine, thereby minimising the heat rise in the gear pump 6.

During a normal engine shut-down, the metering valve 8 is moved to a zero flow position at the same time as the solenoid 28 is energised to shut the pressure raising and shut-off valve 10. Thus the fuel pressure at the outlet of the metering valve, and hence the fuel pressure in the control line 24 of the manifold drain valve 20 decays to low pressure via the first outlet 92 of the solenoid valve 28 and the pull down orifice 34.

The reduction in pressure allows the spring 146 to move the piston 140 to the left as shown in Figure 5 so as to allow communication between the inlet 21 and the outlet 22 of the manifold drain valve. When the pressure raising and shut-off valve 10 returns to its closed position, such that the piston 70 abuts the valve seat 76, the first outlet 80 is in fluid flow communication with the second outlet 85 thereby enabling the manifolds to drain via the outlet 80, the annular recess 82, the annular recess 86, the second outlet 85, the manifold drain valve inlet 21, the manifold valve 20 and the manifold valve outlet 22. The fuel is pushed into the drains tank as the engine runs down by air pressure within the engine combustion chamber.

In the event of an engine overspeed being detected, the solenoid valve 28 is energised by the overspeed protection circuits but the metering valve 8 is not moved to the closed position. This causes the pressure raising and shut-off valve 10 to be operated to inhibit fuel flow to the engine, but the pressure in the manifold drain valve control line 24 remains at a minimum pressure of 80 psi above manifold pressure as set by spill valve 12 thereby keeping the manifold drain valve 20 shut.

Thus, the manifold will remain full of fuel allowing the engine to relight without having to re-prime the manifold after overspeed protection has been iniiiaittd. iiiis allows normal engine operation to the resumed without shutting the engine down, provided that the period during which the solenoid valve 28 is operated is sufficiently short. If required, engine control electronics can be arranged to close the metering valve if the engine overspeed protection circuits are invoked for a sufficient length of time to make engine shut-down inevitable.

During engine start-up, the solenoid valve 28 is de-energised thereby allowing the manifold drain valve 20 to close as the pressure at the outlet of the metering valve 8 increases. Further increase of fuel pressure will open the pressure raising and shut-off valve thereby allowing the engine to start.

It is thus possible to provide a fuel system in which the manifold drain valve is shut during starting and normal running and in which leakage to the drain is prevented by a compliant seal 144 on the end of the piston 140. Furthermore, during normal running, the pressure raising and shutoff valve is open and this closes the connection between the manifold drain valve inlet 20 and the engine manifolds. Thus, even if the manifold drain valve 20 fails open, there will be no leakage to the drains tank other than the small seepage that occurs between the piston and sleeve of the pressure raising and shut-off valve.

In the event of failure of the valve 28 such that high pressure fuel cannot be supplied to the chamber 91 of the pressure raising and shut-off valve 10, engine shut-down can still be accomplished by closing the metering valve 8.

It is thus possible to provide dll ellgillC fuel system in which the manifold drain valve is operated only in response to a normal engine shut-down.

Claims (8)

1. A fuel system for a gas turbine engine, comprising a metering valve, a pressure raising and shut-off valve and a manifold drain valve having a control line connected to an outlet of the metering valve so as to hold the manifold drain valve closed when the pressure at the outlet of the metering valve is greater than a threshold pressure.

2. A system as claimed in Claim 1, in which the manifold drain valve comprises a piston slidable within a cylinder.

3. A system as claimed in Claim 2, in which the piston carries a resilient member for blocking a fluid flow path when the manifold drain valve is closed.

4. A system as claimed in Claim 3, in which the piston is biased towards an open position.

5. A system as claimed in any one of the preceding claims, in which the manifold drain valve is connected to an outlet of the pressure raising and shut-off valve via a path through the pressure raising and shut-off valve which is closed when the pressure raising and shut-off valve is open and which is open when the pressure raising and shut-off valve is closed.

6. A system as claimed in any one of the preceding claims, in which the pressure raising and shut-off valve has a control inlet connected to the outlet of the metering valve via a control valve.

7. A system as claimed in Claim 6, further comprising a pressure drop spill valve for regulating the pressure drop across the metering valve, the control valve comprising a two-way valve having an inlet connected to the outlet of the metering valve, a first outlet connected to the control inlet of the pressure raising and shut-off valve, and a second outlet connected to a control inlet of the pressure drop spill valve.

8. A fuel system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.