Patent Description:
As known, flame monitoring is a critical issue in gas turbine engines, because unexpected flame extinction at one or more burners may lead to loss in efficiency and, in the worst case, cause unburnt fuel in the exhaust and possibly explosion.

In annular combustors, flame monitoring is often accomplished by optical flame detectors, which include optical wave guides facing the flame region and image detectors coupled to the wave guides. The mentioned monitoring systems fulfil the requirements set by the standards and are capable of effectively preventing dangerous conditions which may result in continued fuel supply in the absence of flame, but the cost for implementation are high, especially when it comes to operate gas turbine engines with can or can-annular combustor structure. This is all the more true in consideration of the severe operating conditions of parts exposed to hot combustion gas. Relatively rapid ageing of hardware components may lead to false negative detections, meaning that the response of the flame detectors indicates flame extinction, while flame is actually present, instead. Even though such events do not normally result in imminent danger for structural integrity of the gas turbine engines, nevertheless unnecessary plant trips may be triggered and in any case relatively frequent maintenance is required, with severe increase of cost.

It has also been proposed to base flame monitoring on circumferential temperature distribution of exhaust gas using dedicated temperature sensors installed in the exhaust frame of gas turbine engines between the last expansion stage and the exhaust struts. Such a system relies on increased spread in the exhaust gas temperature in case of failure of one or more burner and positioning of the temperature sensors immediately downstream of the turbine aims at reducing exhaust gas mixing and maximizing positional correlation of burners and temperatures sensors. The solution, however, on the on side requires installing not only an additional set of temperature sensors, but also corresponding supporting structures to hold the temperature sensors in place in the exhaust gas flow within the exhaust frame of the gas turbine engine. Electrical connections need be provided as well. While providing an additional set of temperature sensors is not generally an issue in itself, positioning and electrical connection can be, because intervention on the structure of exhaust system is required. On the other side, known flame monitoring systems based on exhaust temperature suffer from possible false positive, since failure of a temperature sensor may not be distinguished from flame absence at a burner and would yield similar result in terms of detected temperature spread.

<CIT> discloses a gas turbine engine comprising a combustor assembly with a plurality of burners, an exhaust frame provided with a plurality of radial struts and an exhaust diffuser extending along a longitudinal axis downstream of the exhaust frame. Temperature sensors are arranged inside the exhaust diffuser downstream of the exhaust frame and of the struts at respective angular and radial positions with respect to the longitudinal axis. A control system is configured to detect flame failure at one or more of the burners based on responses of the temperature sensors in the exhaust diffuser and of additional sensors at the burners. The detection is based on determining a first and a second parameter as functions of the temperatures detected in the exhaust and at the burners, respectively, and on comparison of the parameters with respective thresholds.

Other examples of known gas turbine engines are disclosed in <CIT> and <CIT>.

It is thus an object of the present invention to provide a gas turbine engine and a method of operating a gas turbine engine that allow to overcome or at least attenuate the above described limitations.

According to the present invention, there is provided a gas turbine engine comprising:.

The size of the exhaust diffuser offers many more opportunities to accommodate temperature sensors with required supports and connection. Thus, not only sensors arranged in the exhaust diffuser are normally not critical in designing gas turbine engines and do not have severe impact on the structure, but also installation and maintenance are simplified. Moreover, temperature sensors may be arranged in the exhaust diffusers also for the purpose of control, as exhaust temperature is an indicator of power delivered by the gas turbine engine. Therefore, it is possible either to exploit a single set of temperature sensors both for control and for flame monitoring, or to provide separate sets of dedicated temperature sensors sharing supports and connection paths. The sets of sensors may also have specifically selected characteristics, based on their function.

The use of redundant logic in groups of adjacent temperature sensors avoids or at least markedly reduces the risk of false positive detections of flame failure. In fact, malfunctions of individual temperature sensors result in low temperature readings and are not distinguishable from the effects of flame failure at one of the burners of the combustor. However, the temperature sensors may be easily placed in accordance with design preferences such that a cold spot deriving from an off burner affects a group of burners due to mixing in the hot gas path and in the upstream part of the exhaust diffuser. Since the control system assesses the temperature sensors in groups with a redundant logic, the effect of malfunctioning temperature sensors which individually would not meet flame presence criteria may be compensated by the other temperature sensors of the group. In other words, if the majority of temperature sensors in a group meets flame presence criteria (e.g. sensor response above a temperature threshold) in accordance with the redundant logic, possible temperature sensors which are lower than the flame presence criteria may be correctly identified as malfunctioning. Unnecessary plant trips may be thus avoided, without affecting safe operation of the gas turbine engine.

Dedicated first temperature sensors for the function of flame monitoring may be added to second temperature sensors already in place for gas turbine engine control. Besides sharing supporting and connection structures, characteristics of the first and second temperature sensor may be separately optimized based on the respective functions. First temperature sensors may be designed to respond quickly, because prompt detection of flame failure is needed to ensure timely triggering of protection measures and, ultimately, safe operation of the gas turbine engine. For the purpose of load control, instead, the need for accuracy of the second temperature sensors prevails over rapid response, because typical transients of gas turbine engines are not particularly demanding under this standpoint and are compatible with most sensors normally used, such as thermocouples.

An example of M-out-of-N redundant logic is <NUM>-out-of-<NUM> (2oo3) logic, whereby in groups of three sensors flame failure is detected only in case two or three first temperature sensors fail to meet flame presence criteria. Flame failure at one combustor burner reflects in a cold spot that spreads along the hot gas path and exhaust diffuser and affects groups of adjacent first temperature sensors, here three. Bad response of a single first temperature sensor is not sufficient to conclude for flame failure. Rather, bad response from a single first temperature sensor may be safely interpreted as sensor malfunctioning if in none of the groups of first temperature sensors flame failure is detected based on the <NUM>-out-of-<NUM> redundant logic. The above may be generalized to N-M malfunctioning first temperature sensors and M-out-of-N redundant logic.

According to an aspect of the invention, the exhaust diffuser comprises an outer casing and supporting rods extending radially inwards from the outer casing, wherein the first temperature sensors and the second temperature sensors are arranged on respective supporting rods.

In practice, the first temperature sensors are mounted to the same supports that are already provided for the second temperature sensors dedicated to load control, without adding to design complexity of the gas turbine engines.

According to an aspect of the invention, the supporting rods are located at a common axial location and are circumferentially distributed around the longitudinal axis And at least some of the supporting rods hold a respective one of the first temperature sensors and a respective one of the second temperature sensors each, preferably all the supporting rods holding one of the first temperature sensors hold also one of the second temperature sensors each.

Optimal positioning of the first temperature sensors may be thus achieved to ensure that groups of first sensors are consistently affected cold spots in exhaust gas caused by flame extinction even at a single burner.

According to an aspect of the invention, all the supporting rods holding one of the first temperature sensors hold also one of the second temperature sensors each.

Accordingly, only existing structures are exploited for mounting the first temperature sensors.

According to an aspect of the invention, the control system is configured to detect flame failure based on comparison of first responses of the first temperature sensors with a temperature threshold and the temperature threshold is determined from an average of the first responses of the first temperature sensors and a temperature offset.

The temperature threshold may be iteratively determined to account for dependence of the current average temperature e.g. on load conditions. The average may be calculated over a single sample of the first responses of the first temperature sensors as well as over a period of time, in accordance with design preferences.

According to the present invention there is also provided a method for controlling a gas turbine engine, the gas turbine engine comprising:.

The present invention will now be described with reference to the accompanying drawings, which illustrate some non-limitative embodiments thereof, in which:.

With reference to <FIG>, number <NUM> defines a gas turbine engine provided with a control system <NUM> and comprising a compressor <NUM>, a combustor assembly <NUM>, a turbine <NUM>, an exhaust frame <NUM> and an axial exhaust diffuser <NUM>, all extending about a longitudinal axis, which is indicated by A in <FIG>. In one embodiment (not shown), the diffuser may be a radial diffuser. The gas turbine engine also comprises a fuel supply system <NUM> controlled by the control system <NUM>.

The compressor <NUM> (<FIG>) feeds the first combustor assembly <NUM> with a flow of compressed air drawn from outside. Air supply to the compressor <NUM> is controllable by the control system <NUM> by adjusting orientation of inlet guide vanes <NUM> of the compressor <NUM>.

In one embodiment (<FIG>), the first combustor assembly <NUM> is a sequential combustor and comprises a plurality of can combustors <NUM> circumferentially distributed about the longitudinal axis A. Each can combustor <NUM> comprises respective first stage burners <NUM> and second stage burner <NUM>. However, the combustor assembly could be of a different type, for example a single stage can or can-annular combustor or a sequential or single stage annular combustor.

The can combustors <NUM> admix air from the compressor <NUM> and fuel from the fuel supply system <NUM> to form a mixture for combustion. The fuel may be gaseous, for example natural gas or syngas, or liquid, for example gasoil. The gas turbine engine <NUM> can be structured to use different types of fuel, both gaseous and liquid. Fuel supply is controllable by the control system <NUM> through the fuel supply system <NUM>.

The turbine <NUM> receives and expands a flow of hot gas from the combustor assembly <NUM> to extract mechanical work, which is transferred to an external user, typically an electric generator, here not shown.

The hot gas is then conveyed through the exhaust frame <NUM> and the exhaust diffuser <NUM>. The exhaust frame <NUM> is arranged immediately downstream of the turbine <NUM> and comprises an inner casing <NUM>, an outer casing <NUM> and a plurality of struts <NUM>, for example ten. The struts extend radially from the inner casing <NUM> to the outer casing <NUM>, shown in <FIG> and are uniformly spaced in a circumferential direction. One of the struts <NUM>, e.g. extending in a vertical plane in an upper portion of the exhaust frame <NUM>, defines a reference for angular position ϕ about the longitudinal axis A of the gas turbine engine <NUM>.

The exhaust diffuser <NUM> extends along the longitudinal axis A downstream of the exhaust frame <NUM> and comprises an inner casing <NUM> and an outer casing <NUM> that define a portion of a flow path for exhaust gas.

The exhaust diffuser <NUM> also comprises a plurality of temperature sensors, which are therefore arranged downstream of the exhaust frame <NUM> and of the struts <NUM> at respective angular and radial positions with respect to the longitudinal axis A. Specifically, the temperature sensors comprise first temperature sensors <NUM> for the purpose of flame monitoring and second temperature sensors <NUM> for the purpose of load control (Temperature After Turbine or TAT measurements). The first temperature sensors <NUM> and the second temperature sensors <NUM> are mounted on supporting rods <NUM> that extend radially inwards from the outer casing <NUM> at a common axial location, also defined TAT plane. The supporting rods <NUM> are distributed in a circumferential direction, not necessarily in a uniform manner. In one embodiment, each supporting rod <NUM> holds a respective first temperature sensor <NUM> and at least one second temperature sensors <NUM>, while special supporting rods <NUM> longer than the others hold three second temperature sensors <NUM> each. It is however understood that some of the supporting rods <NUM> my hold only first temperature sensors <NUM> or second temperature sensors <NUM>. The first temperature sensors <NUM> are arranged at a first common radial position at the same first distance from the longitudinal axis A; likewise, the second temperature sensors <NUM>, possibly except those on the longer supporting rods <NUM>, are arranged at a second common radial position at the same second distance from the longitudinal axis A. In any case the number and angular and radial positions of the first temperature sensors <NUM> are selected such that a irregularities in temperature distribution in circumferential direction caused by flame failure at any one of the burners <NUM>, <NUM> (cold spots) affects a respective group of adjacent first temperature sensors <NUM>.

The characteristic and specifically a first response of the first temperature sensors <NUM> and a second response of the second temperature sensors <NUM> may be selected based on the specific functions for which the temperature sensors are provided. In particular, the first response of the first temperature sensors <NUM> (for flame supervision purpose) is faster than the second response of the second temperature sensors <NUM>, whereas the second response of the second temperature sensors <NUM> (for load control purpose) is more accurate than the first response of the first temperature sensors <NUM>.

The control system <NUM> is configured to operate the gas turbine engine <NUM> in accordance with received load request. The definition "control system" as used herein is to be broadly understood as meaning a system supervising all functions and operation of the gas turbine, including at least control or regulation functions, such as load control, determining set-points and driving actuators to reach the set-points, primary and secondary frequency control, and protection functions, including flame monitoring and protection against flame failure. In particular, a control system may comprise a first subsystem for control functions and a second subsystem for protection functions.

The control system <NUM> determines set-points for the gas turbine engine <NUM> so that the load request may be met and, based on determined set-points and feedback signals from selected sensors and/or detectors, including the second responses of the second temperature sensors <NUM>, it drives the inlet guide vanes <NUM> of the compressor <NUM> and the fuel supply system <NUM>.

The control system <NUM> also implements a flame monitoring function at any one of the burners <NUM>, <NUM> based on the first responses of the first temperature sensors <NUM> and on a redundant logic. Specifically, the control system <NUM> is configured to detect flame failure based on a M-out-of-N redundant logic in groups of adjacent first temperature sensors <NUM>, wherein N is a number of first temperature sensors in each group and M is an integer lower than or equal to N-<NUM>. In one embodiment, N is <NUM>, M is <NUM> and the redundant logic is a <NUM>-out-of-<NUM> logic.

As illustrated schematically in <FIG>, where the first temperature sensors <NUM> are individually further identified as TS<NUM>,. TSK, the control system <NUM> defines K groups G<NUM>,. , GK of N adjacent first temperature sensors <NUM>, wherein K is the overall number of first temperature sensors <NUM> in the exhaust diffuser <NUM>. Each group G<NUM>,. , GK includes N respective adjacent first temperature sensors <NUM> and each first temperature sensor <NUM> belongs to N different groups G<NUM>,. In other words, each sensor <NUM> is the first member by angular position ϕ of a respective group G<NUM>,. , GK: TS<NUM> is the first member of group G<NUM>, that includes first temperature sensors TS<NUM>, TS<NUM>, TS<NUM>; TS<NUM> is the first member of group G<NUM>, that includes first temperature sensors TS<NUM>, TS<NUM>, TS<NUM>; and so on until TSK-<NUM>, which is the first member of group GK-<NUM>, that includes first temperature sensors TSK-<NUM>, TSK, TS<NUM>; and TSK, which is the first member of group GK, that includes first temperature sensors TSK, TS<NUM>, TS<NUM>.

The control system <NUM> detects flame failure based on comparison of the first responses, that include respective monitoring signals identified as T<NUM>,. , TK in <FIG>, of the first temperature sensors <NUM> with a temperature threshold TTH, which is determined from an average of the first responses T<NUM>,. , TK of the first temperature sensors <NUM> and a temperature offset ΔT. The temperature threshold may be iteratively updated and may be calculated e.g. over a single sample of the monitoring signals T<NUM>,. , TK of the first temperature sensors <NUM> or over samples collected in a period of time.

With a <NUM>-out-of-<NUM> logic (generally M-out-of-N), the control system <NUM> detects flame failure if in at least one group G<NUM>,. , GK of first temperature sensors <NUM> the first responses, specifically the monitoring signals T<NUM>,. , TK, of <NUM> (generally M-<NUM>) first temperature sensors <NUM> of the group are lower than the temperature threshold TTH. Flame failure at one of the burners <NUM>, <NUM> causes a cold spot that results in increased temperature spread of the exhaust gas and affects measurements of adjacent first temperature sensors <NUM>, which are remarkably lower than the average. Flame failure is recognized when at least M (two in the embodiment described) adjacent first temperature sensors <NUM> give monitoring signals T<NUM>,. , TK below the temperature threshold TTH, while it not required that all the first temperature sensors <NUM> in a group fail the test. In the example of <FIG>, the monitoring signals TJ+<NUM> and TJ+<NUM> of the temperature sensors <NUM> at angular positions ϕJ+<NUM> an ϕJ+<NUM> are below and thus fail the comparison with the temperature threshold TTH and thus fail the comparison. Groups GJ and GJ+<NUM> contain both the temperature sensors <NUM> at angular positions ϕJ+<NUM> and ϕJ+<NUM>, while all the other groups contain at most one of them. In fact, group GJ contains the temperature sensors <NUM> at angular positions ϕJ, ϕJ+<NUM>, ϕJ+<NUM> and group GJ+<NUM> contains the temperature sensors <NUM> at angular positions ϕJ+<NUM> ϕJ+<NUM>, ϕJ+<NUM>. Thus, based on <NUM>-out-of-<NUM> redundant logic, the control system <NUM> detects flame failure at one of the burners <NUM>, <NUM>. In the example of <FIG>, instead, only the temperature sensor <NUM> at angular position ϕJ+<NUM>, is below the temperature threshold TTH. Based on <NUM>-out-of-<NUM> redundant logic the anomalous measurement may be identified as a false positive because it is not consistent with the measurements of adjacent first temperature sensors <NUM>.

It is finally apparent that changes and variations may be made to the gas turbine engine and method described and illustrated without departing from the scope of protection of the accompanying claims.

In one embodiment, for example, only one set of temperature sensors may be used both for the purpose of flame monitoring and load control. This solution may be particularly advantageous if the responses of the available sensors is fast and accurate enough to meet design preferences for both control and flame monitoring functions.

Claim 1:
A gas turbine engine comprising:
a combustor assembly (<NUM>) including a plurality of burners (<NUM>, <NUM>);
an exhaust frame (<NUM>) provided with a plurality of radial struts (<NUM>);
an exhaust diffuser (<NUM>) extending along a longitudinal axis (A) downstream of the exhaust frame (<NUM>);
a plurality of temperature sensors (<NUM>, <NUM>) arranged inside the exhaust diffuser (<NUM>) downstream of the exhaust frame (<NUM>) and of the struts (<NUM>) at respective angular and radial positions with respect to the longitudinal axis (A);
a control system (<NUM>) configured to detect flame failure at one or more of the burners (<NUM>, <NUM>) based on responses of the temperature sensors (<NUM>, <NUM>) and a temperature threshold (TTH) and on a redundant logic;
characterized in that the temperature sensors (<NUM>, <NUM>) comprise:
first temperature sensors (<NUM>), having a first faster response and arranged such that irregularities in temperature distribution in a circumferential direction caused by flame failure at any one of the burners (<NUM>, <NUM>) affect a respective group of adjacent first temperature sensors (<NUM>); and
second temperature sensors (<NUM>), having a second more accurate response;
and wherein the control system (<NUM>) is configured to determine flame failure at one or more of the burners (<NUM>, <NUM>) based on the first responses of the first temperature sensors (<NUM>) and on a M-out-of-N redundant logic in groups (G<NUM>, ..., GK) of adjacent first temperature sensors (<NUM>), wherein N is a number of first temperature sensors (<NUM>) in each group (G<NUM>, ..., GK) and M is an integer lower than or equal to N-<NUM>.