GB2468686A

GB2468686A - System and method for detecting abnormal movement in a gas turbine shaft

Info

Publication number: GB2468686A
Application number: GB0904651A
Authority: GB
Inventors: Nigel Philip Turner
Original assignee: Weston Aerospace Ltd
Current assignee: Weston Aerospace Ltd
Priority date: 2009-03-18
Filing date: 2009-03-18
Publication date: 2010-09-22
Also published as: WO2010106334A3; WO2010106334A2; GB0904651D0

Abstract

A sensor 7 for use with a gas turbine to detect abnormal turbine shaft motion. The sensor has a frangible sensor element 10 and a cutter or separator coupled to and moveable with the turbine shaft to break or cut through the frangible sensor element on axial movement of the shaft. The frangible sensor element comprises an insulated cable having an elongate metal sheath enclosing an insulating material and conductive wires 9 running inside and substantially parallel to the longitudinal axis of the sheath. The wires are welded, brazed or soldered together at the distal end of the sensor element to define a single continuous conductive path running from the proximal end of the frangible sensor element to its distal end and then back to its proximal end. The cable may be mineral insulated and may contain two or four wires. Also disclosed is a method for making a frangible sensor.

Description

System and method for detecting abnormal movement in a gas turbine shaft The present invention is concerned with a system for detecting abnormal movement of a gas turbine shaft, and a method for making a sensor element for use in such a system. Abnormal movement of a gas turbine shaft is normally associated with the breaking of the shaft and the risk of so-called "turbine over-speed". When the shaft of, for example, a jet engine breaks, the compressor mass is lost to the rotating system so the shaft and turbine then rotates significantly more quickly. The movement of the turbine can be sufficiently fast to cause the turbine to fly apart and break.

Gas turbine engines (e.g. jet engines) include a rotating shaft having compressor and/or turbine blades mounted thereon and rotating therewith. Backing or rearwards axial movement of the shaft relative to the remainder of the engine is considered to be an abnormal movement and indicative of engine failure (e.g. shaft breakage). Detection of axial movement of the shaft relative to the remainder of the engine can therefore be used to detect engine failure and used to prevent further damage to the engine by activating a shut off of the engine.

It is known to detect abnormal movement of a gas turbine shaft relative to the engine casing by providing a circuit breaking element which is fixed to the shaft and moves therewith if and when the shaft moves in an axial direction to break a circuit and thereby produce a signal.

US 5,411,364 discloses an electro optic sensor for sensing unwanted or abnormal axial movement of turbine blades or rotors of a gas turbine. The sensing arrangement includes a pair of fibber optic wave guides interconnected through a frangible member disposed axially adjacent the turbine blades. Upon axial movement of the blades or rotors away from their normal position, the frangible element is broken to open the optical circuit associated with the wave guides. Associated electronic circuitry generates an output signal indicative of failure of the gas turbine rotor.

US 6,607,349 discloses a broken shaft detection system and a method which uses a detector assembly mounted downstream of a power turbine wheel of a gas turbine engine to detect rearward axial motion of the wheel and thereby a broken shaft event. The detector assembly has a plunger positioned to be axially displaced against a link connected in an electrical circuit. The link may be broken when the plunger is displaced thereby creating an open circuit that may be detected by a detection and test element. The breaking may be communicated to an over-speed circuit that controls a shut off switch that interrupts fuel flow to the engine.

The link may be connected to the detection and test element by two pairs of parallel wires to facilitate monitoring of circuit function and to detect failures that are not broken shaft event failures.

US 2007/0241 921 discloses a frangible sensor element which is cut by a separating tang mounted on and moving axially with a gas turbine shaft when the shaft fails. The frangible sensor element includes a longish, mechanically severable sensor element, which is severed by the separating tang when this moves as a result of shaft failure. One embodiment of US 2007/0241921 has a circuit formed by two wires connected at the distal or free end of the sensor element by a resistor of a defined value, and another embodiment has a circuit in which two pairs of wires are looped or bent at the free or distal end of the sensor element to define a single continuous conductive path running from the proximal end of the frangible sensing element, to its distal end, then back to its proximal end before returning to its distal end and then returning to its proximal end.

The known frangible sensor elements which rely on the breaking or cutting of a wire or pair of wires at the sensor free or distal end, to indicate movement of the shaft (such as those disclosed in the second embodiment of US 2007/0241921) have a wire looped or bent back on itself at the free end of the sensor element. The space available to the sensor element inside a gas turbine such as a jet engine is small and any such arrangement would therefore have to be bent back sharply and it is therefore prone to breakage or failure.

The present invention provides system and method according to claims 1 and 4 to which reference should now be made.

The present invention provides a sensor, and method for its manufacture, particularly suited to an arrangement such as that disclosed in the second embodiment US 2007/0241921 but which avoids the need for bending or looping back of the wires and thereby reduces the likelihood of failure. Furthermore, embodiments of the invention are suitable for operation in high temperature regions such as adjacent the low pressure turbine rotors of a jet engine.

Preferred features of the invention are set out in the dependent claims 2 and 3 to which reference should now be made.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached figures.

The figures and following description are intended to exemplify the invention and it will be readily appreciated that alternative embodiments of the invention are envisaged and are covered by the scope of the claims.

Figure 1 shows a two spool turbo fan engine illustrating the location of a sensing system embodying the invention; Figure 2 is an enlarged view of the portion A of figure 1 including the sensing system; Figure 3 is a schematic illustration of a system embodying the present invention; Figure 4 illustrates the free or distal end of the frangible sensor element shown in figures 2 to 3; Ejgpre 5 is a cross-section along plane V-V of figure 4; Figures 6a to 6f are a schematic illustration of a method of making a frangible sensor element embodying the present invention; Figure 7 is an alternative view of the processing step of figure 6a; and Figure 8 is an alternative view of the processing step of figure 6d.

Figure 1 shows a typical two spool turbo fan, jet engine having an intake (3), fan (2), nacelle (22), compressors (5), combustion chamber (23), fan nozzle (24), turbines (6) and core nozzle (25).

The engine (1) includes a fan (2) across the air intake (3). The fan (2) is mounted on a shaft (4) on which are also mounted the compressor rotors or blades (5) and the turbine rotors or blades (6). A so-called backing sensor (7) is located adjacent and behind the low pressure turbine rotors (6b).

As shown in more detail in figure 2, the rotatable shaft (4) has a cutter or separator (8) mounted thereon and axially movable with the shaft.

The backing sensor (7) has a flange mounting (11) and is mounted in a hole in the engine casing, and has an elongate frangible element (10) and a connection portion (12) coupled thereto and extending through the flange mounting (11) and a fixed vane (14) of the engine. The frangible elongate element (10) projects below the fixed vane (14) opposite the cutter or separator (8) so that it is cut or separated along cutting or parting plane (15) when the shaft moves towards the back of the engine (i.e. axially to the right in direction X when looking at figures 1 and 2).

The backing sensor (7) has a connector (16) for connecting the sensor to control and sensing electronics andfor data processing. The connector (16) can also house those parts of the sensor which are heat sensitive in the manner described in US 2007/0241 921.

The portion of the turbine in which the sensor element is located is, when the jet engine is in use, a high temperature environment.

Temperatures in the engine can exceed 800 °C which means that the environment in which the free or distal end of the sensor element is located is an aggressive one. Furthermore and as can be seen from figure 2, space is limited in the engine for the sensor element.

In a preferred embodiment of the invention (see figure 3) the backing sensor has a circuit similar to that disclosed in US 2007/0241 921.

We shall therefore not describe the circuit in detail but simply refers to the disclosure of US 2007/0241921 whose contents are hereby incorporated by way of reference.

The frangible elongate sensor element (10) and connection (12) through flange (11) to the connector element (16) comprises a four core mineral insulated cable for most of its length. The cable (10, 12) includes four nickel alloy conductors (9), and an insulating powder (17) around and between the four conducting core wires (9) as shown in figure 5. The insulating powder can be an inorganic magnesium oxide powder of the type commonly used in mineral insulated cable. A nickel alloy sheath (18) surrounds the insulating powder and conducting cores therein. At its distal end below beyond the cut plane (15), the four conducting core wires are connected in pairs (see figure 4) by welds (26) (see figure 4) to define a single continuous circuit of the type described in US 2007/0241921 and shown in figure 3.

Figures 6a to 6f illustrate a method of making the cable illustrated in figure 3 from four core mineral insulated cable having nickel alloy cores and a nickel alloy sheath.

What will be the distal end (19) of the cable (21) is drilled (see figure 6a) to remove the insulating material and conductors from an end of the cable to leave a space (see figure 7) between the end of the sheath (18) and the end of the insulating material (17) and conductive cores or wires (9).

The exposed end of the insulating material is then powder blasted (see figure 6b) to leave the ends of the four conductive cores exposed.

The exposed wires (9) (see figure 6c) are then converged before being welded in pairs (figure 6d). The pairs of wires are converged or pushed together by a trained operator viewihg the end of the cable through a microscope and using a dental pick like converging tool. The pairs of wires may be welded together either by plasma or TIG welding (i.e. tungsten inert gas welding). An advantage of plasma welding is that the welding process allows for a greater degree of automatisation. Plasma arc welding also allows one to determine when the two conductors are electrically connected and thereby avoids the need for a manual inspection to check the weld is complete as is necessary with TIG welding.

In a preferred embodiment, the outside diameter of the mineral insulated cable forming the backing sensor is 2mm with a 0,25mm sheath wall thickness and each of the conductors or wires (9) are 0.27 mm.

The end of the cable is then closed by closure welding the sheath (18) (see figure 6e). Again, the sheath end can be welded shut using either plasma arc or TIG welding.

The described embodiment has four wires (9) defining a single conductive loop. Other even numbers of wires could also be used. For example, a dual channel sensor could use eight conductors to define two separate continuous conductive loops.

Claims

Claims 1. A system for use with a gas turbine to detect abnormal turbine shaft motion comprising: a frangible sensor element having a distal and a proximal end, and a cutter or separator coupled to and moveable with the shaft to break or cut through the frangible sensor element towards its distal end on axial movement of the shaft, wherein the frangible sensor element comprises a cable having an elongate metal sheath enclosing an insulating material and two conductive wires running inside and substantially parallel to the sheath's longitudinal axis and through the insulating material, wherein the two wires are welded, brazed or soldered together at the distal end of the sensor element to define a single continuous conductive path running from the proximal end of the frangible sensing element, to its distal end and then back to its proximal end, and the frangible sensor element further including a connector at its proximal end for connecting the two wires to sensing circuitry.
2. A system according to claim 1 wherein the cable of the frangible sensor element has four wires wherein the four wires are connected together in pairs at the distal end of the frangible sensor element and two of the four wires are connected together at the proximal end of the sensor element so as to define a single continuous conductive path running from the proximal end of the frangible sensing element, to its distal end, then back to its proximal end before returning to its distal end and then returning to its proximal end.
3. A system according to any preceding claim, wherein the cable is a mineral insulated cable.
4. A method of making a frangible sensor element for use with a gas turbine to detect abnormal turbine shaft movement, the method comprising: i) providing a cable having a elongate metal sheath enclosing an insulating material and two conductive wires running inside and substantially parallel to the sheath's longitudinal axis and through the insulating material; ii) removing some of the insulating material and conductive wires to create an end portion of the cable having no material inside the sheath; iii) removing some of the insulating material to leave exposed ends of the conductive wires; iv) pushing or otherwise manipulating the exposed wire ends so that the two wires are adjacent each other; v) welding, brazing or soldering together the exposed ends of the wires; and vi) welding, brazing or soldering together the end of the sheath so as to enclose the exposed wire ends.
5. A method according to claim 4 comprising: i) providing a cable having a elongate metal sheath enclosing an insulating material and four conductive wires running inside and substantially parallel to the sheath's longitudinal axis and through the insulating material; ii) removing some of the insulating material and conductive wires to create an end portion of the cable having no material inside the sheath; iii) removing some of the insulating material to leave exposed ends of the conductive wires; iv) pushing or otherwise manipulating the exposed wire ends so that the two pairs each of two wires are adjacent each other; v) welding, brazing or soldering together the exposed ends of the two wires of each pair; and vi) welding, brazing or soldering together the end of the sheath so as to enclose the exposed wire ends.
6. A system substantially as hereinbefore described with reference to the attached figures.
7. A method substantially as hereinbefore described with reference to the attached figures.