Note: Descriptions are shown in the official language in which they were submitted.
81796885
SHAFT FAILURE DETECTION USING PASSIVE CONTROL METHODS
This application claims the benefit of priority of U.S. provisional
application Serial
No., 62/194,582 filed on July 20, 2015.
FIELD OF THE INVENTION
This invention relates to a gas turbine engine fuel control methods and
related apparatus.
BACKGROUND OF THE INVENTION
A gas turbine engine fundamentally consists of one or more compressors,
combustion
chambers, and one or more turbines, all displaced along an axis of rotation.
Shafts connect the
turbines to corresponding compressors, thereby providing a mechanism to
transmit the
mechanical power required to operate the compressor. In many engines,
including those used in
aircraft, at least one shaft connects one of the turbines to a fan that
provides propulsive thrust to
the aircraft.
One rare mode of failure in a gas turbine engine is a failure of one or more
of the shafts.
When one of the shafts fails, the load on the turbine driving the shaft can be
substantially
reduced, thereby resulting in a turbine overspeed. The turbine overspeed can
undesirably result
in disc burst or high energy blade release. Accordingly, the industry has
developed strategies for
addressing the risk of disc or blade release failures subsequent to a shaft
failure.
In the past, mechanical and/or electrical sensing techniques have been used to
detect a
shaft failure. Control mechanisms have been used to cut off the fuel supplied
to the engine based
on the detected failure. However, care must be taken to ensure that the fuel
supply is cut off
early enough to avoid or at least substantially reduce the possibility of
liberation of high energy
debris. Accordingly, early shaft failure detection schemes may be employed.
However, because
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purposely cutting off fuel to an engine is normally undesirable, the engine
control systems must
not prematurely react before a shaft failure is confirmed. Observable
phenomena that occur
during shaft failure can also occur due to other factors in which cutting off
fuel to the engine
would be undesirable.
As a consequence, there is a need for additional protections against high
energy debris
release upon shaft failure that also reduces the likelihood that the fuel
supply is cut off to a
healthy engine that has not had a shaft failure.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gas turbine engine
having additional
protections against the negative consequences of shaft failure.
In at least some embodiments, the above described object is achieved by a
control
scheme that employs extensions of current control methods to quickly detect
potential shaft
failure signatures and perform graduated corrective action upon initial
detection. The use of
graduated corrective action allows for immediate mitigation even before shaft
failure has been
fully confirmed.
A first embodiment is a method for use in a turbine control system that
includes
controlling the fuel supply to a gas turbine engine at least in part using a
fuel supply limit
determined as a first function of a rotational speed of a shaft of the gas
turbine engine. The
method also includes obtaining a first value representative of a rotational
speed of the shaft, and
differentiating the first value within a processing unit. The processing unit
determines an
adjusted fuel supply limit as an adjusted function of the first value. The
adjusted function is
based on the first function and the differentiated first value. The method
further includes
controlling the fuel supply to the gas turbine engine at least in part using
the adjusted fuel supply
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limit.
This embodiment effectively adjusts the model for the fuel supply limit as a
function of a
sudden change in rotational speed. Using the adjusted model, the amount of
fuel supplied to the
engine can be even more greatly reduced as the rotational speed of the turbine
increases than it
would have been under the normal operational model. This additional reduction
thus offers
greater protection in the event of a shaft failure than would have been
possible using the normal
fuel supply limit model whilst ensuring no risk of spurious shutdown.
Another embodiment involves adjusting a fuel supply derate value that is
otherwise used
to protect a gas turbine engine upon fast windmill restart. In sensed
conditions within the range
normally associated with fast windmill restart, the fuel supply derate value
is calculated as
previously known. In more extreme sensed conditions found to be more likely
associated with a
shaft failure, the fuel supply derate value is increased.
In yet another embodiment, a model for estimating rotational speed in the
absence of a
properly operating speed sensor includes an adjusted portion that artificially
lowers the estimated
speed. The adjusted portion of the model coincides with conditions associated
with possible
shaft failure. The artificially lowered estimated speed (in possible shaft
failure conditions) can
be combined with other existing control operations that otherwise reduce fuel
when a low engine
speed is detected. By artificially reducing the estimated speed, the fuel
reduction of existing
control operations is accelerated.
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In a further embodiment, there is provided a method for use in a turbine
control
system, the method comprising: a) controlling fuel supply to a gas turbine
engine at least
in part using a fuel supply limit determined as a first function of a
rotational speed of a
shaft of the gas turbine engine; b) obtaining a first value representative of
a rotational
speed of the shaft; c) differentiating the first value within a processing
unit; d) employing
the processing unit to determine an adjusted fuel supply limit as an adjusted
function of the
first value, the adjusted function based on the first function and the
differentiated first
value; and e) controlling the fuel supply to the gas turbine engine at least
in part using the
adjusted fuel supply limit.
In a further embodiment, there is provided a method for use in a turbine
control
system, the method comprising: a) obtaining at least one pressure sensor value
representative of pressure differential through at least one compressor of a
gas turbine
engine; b) using a first model to determine an estimated rotational speed of a
shaft of the
gas turbine engine based on the at least one pressure sensor value, where the
first model
has a first portion that represents a correlation between pressure sensor
value and
rotational speed of the shaft, and a second portion that differs from the
correlation, such
that the estimated rotational speed of the first model in the second portion
is less than a
rotational speed of the shaft for the corresponding pressure sensor value; and
c)
determining a fuel supply limit as a function of estimated the rotational
speed; and d)
controlling fuel supply to a turbine at least in part using the fuel supply
limit.
In a further embodiment, there is provided a method for use in a turbine
control
system, the method comprising: a) controlling fuel supply to a gas turbine
engine based on
a plurality of sensed values to generate a fuel supply value; b) obtaining a
differential
temperature value representative of a difference between an expected
temperature at an
exit of a compressor of a gas turbine engine and a measured temperature at the
exit; c)
generating a derate value if the differential temperature value exceeds a
first threshold, the
derate value determined as a function of the differential temperature value,
wherein the
derate value increases at a first average rate as a function of the
differential temperature
value when the differential temperature value is less than a second threshold,
the derate
value increases at a second average rate less than the first average rate when
the
differential temperature value is greater than the second threshold and less
than a third
threshold, and the derate value increases at a third average rate greater than
the second
3a
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average rate when the differential temperature value is greater than the third
threshold; d)
using a controller to reduce the fuel supply value based at least in part on
the derate value.
The above-described features and advantages, as well as others, will become
more
readily apparent to those of ordinary skill in the art by reference to the
following detailed
description and accompanying drawings.
3b
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BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described, by way of example, with reference
to the
accompanying drawings in which:
Fig. 1 shows a sectioned side view of the upper half of a gas turbine engine
in accordance
with the present invention;
Fig. 2 is a sectioned side view of a portion of the combustion apparatus of
the gas turbine
engine shown in Fig. 1;
Fig. 3 is a schematic diagram of a controller that may be used in a fuel
control system of
the gas turbine engine shown in Fig. 1;
Fig. 4A shows a graphical representation of the fuel supply limit model with a
vertical
axis representative of the maximum fuel level and the horizontal axis
representative of rotational
shaft speed.
Fig. 4B shows a flow diagram of operations performed by the controller of Fig.
3 in
accordance with at least one embodiment;
Fig. 4C shows a graphical representation of the relationship between a fuel
supply limit
adjustment and change in rotational speed;
Fig. 5 shows a functional flow diagram of a control scheme for limiting fuel
under
conditions of fast windmill restart and potential shaft failure employed by
the controller of Fig. 3
in another embodiment;
Fig. 6 shows a graphical representation of a modified model of rotational
speed as a
function of sensed engine parameters according to one or more embodiments.
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DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 shows a representative, fragmentary cross-section of a ducted fan gas
turbine
engine generally indicated at 10. Only the top half of the engine 10 is
illustrated for clarity of
exposition. As shown in Fig. 1, the engine 10 comprises a fan 11 contained
within a fan duct 12,
intermediate and high pressure compressors 13 and 14 respectively, combustion
apparatus 15,
high, intermediate and low pressure turbines 16, 17 and 18 respectively and an
exhaust nozzle
19. A first shaft 20 couples the high pressure turbine 16 to the high pressure
compressor 14, a
second shaft 21 couples the intermediate pressure turbine 17 to the
intermediate pressure
compressor 13, and a third shaft 22 couples the low pressure turbines 18 to
the fan 11.
The gas turbine engine 10 functions in the conventional manner so that air
drawn into the
engine 10 by the fan 11 is divided into two flows. The first flow is exhausted
through the fan
duct 12 to provide propulsive thrust. The second flow is directed into the
intermediate pressure
compressor 13 where compression of the air takes place. The air then passes
into the high
pressure compressor 14 where additional compression takes place prior to the
air being directed
into the combustion apparatus 15. There the air is mixed with fuel and the
mixture combusted.
The resultant combustion products then expand through, and thereby drive, the
high,
intermediate and low pressure turbines 16, 17 and 18 respectively before being
exhausted
through the nozzle 19 to provide additional propulsive thrust.
The shafts 20, 21 and 22 are hollow and concentric, and transmit torque from
the turbine
sections 16, 17 and 18 to components 14, 13 and 11, respectively.
As also shown in Fig. 1, the gas turbine engine 10 includes a pressure sensor
31 operably
coupled to the fan duct 12 upstream of the fan 11 to measure an inlet pressure
value P20. Other
similar pressure sensors, not shown, are positioned in same axial location but
at different annular
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positions. Similarly, the gas turbine engine 10 includes a temperature sensor
32 operably
affixed to the fan duct 12 upstream of the fan 11 to measure the inlet
temperature T20. Other
similar temperature sensors, not shown, are positioned in same axial location
but at different
annular positions. Furthermore, the gas turbine engine 10 includes a pressure
sensor 33
operably coupled to an inlet of the combustion chamber 15 to measure the
pressure P30 at the
outlet of the high pressure compressor 14. Other similar pressure sensors, not
shown, are
positioned in same axial location but at different annular positions.
Fig. 2 shows a fragmentary cutaway view of the upper half of the combustion
apparatus
15. The combustion apparatus 15 is of generally conventional configuration
comprising an
annular combustion chamber 24 having a plurality of air inlets 25 at its
upstream end. A fuel
injector 26 is provided in each air inlet 25 to direct fuel into the
combustion chamber interior
where the combustion process takes place.
The combustion chamber 24 is surrounded in radially spaced apart relationship
by a part
28 of the casing of the engine 10. The thermocouple 23, which is one of three
such similar
thermocouples, is located in the casing part 28 so as to protrude into the
annular space 29 defined
between the casing part 28 and the combustion chamber 24. The remaining two
thermocouples
(not shown) are similarly located to protrude into the annular space 29 so
that all three
thermocouples are equally circumferentially spaced apart from each other. The
thermocouples
23 measure the temperature T30 of the air which operationally flows through
the annular space
29 in order to provide cooling of the combustion chamber 24. Apertures 30 in
the combustion
chamber 24 wall permit air from the space 29 to flow into the combustion
chamber 27 to provide
further cooling and to take part in the combustion process.
The air which flows into the annular space 29 is part of the air flow
exhausted from the
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high pressure compressor 14 and most of it flows into the combustion chamber
24 through its
apertures 30. The remainder of the air flows, as previously stated, into the
combustion chamber
24 through its upstream end air inlets 25. Consequently the air flowing
through the annular space
29 is representative, in terms of the temperature, of the air which is
exhausted from the
downstream end of the high pressure compressor 14. Therefore, the output
signal T30 from the
thermocouple 23 will be representative of that air temperature.
Speed control of the gas turbine engine 10 is carried out primarily through
the control of
fuel to the combustor apparatus 15. To this end, Fig. 3 shows a schematic
representation of a
controller 100 for the gas turbine engine 10, located on the aircraft, not
shown, in which the gas
turbine engine 10 is located. The controller 100 includes processing elements
configured to
control the amount of fuel delivered to the engine 10 as a function of various
inputs. As will be
discussed below, these inputs can include the temperature values T20, T30, and
pressure values
P20, P30, among others.
In at least some embodiments, the controller 100 includes a digital processor.
In such
cases, the controller 100 also includes a memory 101 that stores control
programs and/or
software that carry out the operations attributed to the controller 100
herein. The memory 101
may be any suitable memory device, or combination of memory devices, such as
RAM and/or
non-volatile memory devices. It will be appreciated that the controller 100
may suitably include
digital and/or analog components.
For control of fuel feed rate, among other things, the controller 100 includes
pressure
sensor inputs 102, temperature sensor inputs 104, engine speed sensor inputs
106, other sensor
inputs 108. The controller 100 may also include a set point input 110 for
receiving a signal
indicative of a desired aircraft speed (or desired acceleration/deceleration)
and/or a desired
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engine speed corresponding to a desired aircraft speed. The controller 100 may
also include
bleed valve control outputs 112, variable stator vane actuation outputs 113,
and fuel metering
valve outputs 114.
In general, the controller 100 employs the various inputs 102, 104, 106, 108
and 110 to
generate control signals for the bleed valve control outputs 112, variable
stator vane outputs 113,
and the fuel metering valve outputs 114. The bleed valve control outputs 112
are operably
connected to one or more bleed valves, not shown, but which are used to
redirect air from the
engine to other components of the engine 10 and/or aircraft for various
reasons known in the art.
The control of bleed valves is well known. The speed sensor inputs 106 are
operably connected
to speed sensors 124. The speed sensors 124 are operably connected to detect
the rotational
speed of one or more of the shafts 20, 21, 22 and/or turbines 16, 17 and 18 or
other hardware
connected to the rotating shafts. In one embodiment, the speed sensors 124
provide
measurements NEI, NI, and NL of rotational speeds of the respective shafts 20,
21 and 22.
The fuel metering valve outputs 114 are operably coupled to fuel valve
actuators 120 that
control the amount of fuel delivered to the combustion apparatus 15 of Figs. 1
and 2 via the fuel
injectors 26. The general control of fuel metering valves to achieve desired
aircraft and/or
engine speed is known in the art. In addition to using the fuel control to
modulate engine thrust,
there exist other fuel supply control schemes that operate under abnormal
circumstances. For
example, the fuel supply can be controlled to limit or avoid stalling upon
restart of the engine.
Other necessary control schemes for "surge" and other conditions are known.
In accordance with at least some embodiments of the present invention, certain
existing
control schemes are modified or altered to perform a separate function of
providing early
detection and remediation of a possible shaft failure scenario (failure of any
of the shafts 20, 21
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and 22). Preferably, these schemes are implemented in conjunction with the
prior art protection
scheme of shutting off the fuel to the engine upon conventional detection of a
shaft failure.
Thus, the modified control schemes discussed herein perform a preliminary
protective operation
prior to conventional detection of shaft failure. In the event that shaft
failure is ultimately
detected, the preliminary protective operation helps reduce the possibility of
disc burst prior to
conventional detection of the shaft failure. In the event that the shaft has
not failed, then the
engine 10 may recover and premature shut down of the engine is avoided.
In the current embodiment and referring generally to Figs. 4A, 4B and 4C, one
of the
known control schemes monitors the rotational speed NH of shaft 20 to employ a
maximum fuel
level WFmax based on that shaft speed. Those of ordinary skill in the art will
recognize that a
similar approach which monitors rotational speed NL, shaft 22, or NI, shaft
21, can be used in a
similar fashion to control a maximum fuel flow level. In particular, under
normal circumstances,
the controller 100 employs a fuel supply limit as a function of speed to limit
compressor
operating line migration during engine transients. In particular, it is known
to employ an upper
limit on fuel supply at lower speeds to prevent problems during engine start.
As will be
discussed below, one embodiment of the invention performs an adjustment on the
operation of
the fuel supply limit function based on the possible detection of a shaft
break.
In particular, Fig. 4A shows a graphical representation of the fuel supply
limit model 400
with the vertical axis representative of the maximum fuel level as a function
of pressure of the air
exhausted from the high pressure compressor, WFõ,a, /P30, and the horizontal
axis representative
of rotational shaft speed NH. The speed NH is obtained from speed sensors 124,
which may
suitably be disposed just in front of the HP shaft thrust bearing, not shown,
at the front of the
high pressure compressor 14. However, in other applications, the speed sensor
124 may be
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placed in an accessory gearbox, not shown, but which would be known to those
of ordinary skill
in the art, where the speed sensor 124 is operably coupled to measure speed
from a driven
component, for example, a permanent magnet generator (PMG). In accordance with
an
embodiment of the present invention, however, this known model 400 may be
adjusted upon
detection of a possible shaft break.
To this end, a shaft break is typically characterized by a sudden decrease in
rotational
speed of the compressor and released hardware (due to the disconnection of the
shaft from the
drive/turbine). In the detection of an unusually sudden decrease in rotational
speed NH, the
controller 100 effectively adjusts the NH value used to determine WFmax in the
model 400
depicted in Fig. 4A. The controller 100 preferably stores information
representative of the model
400 in memory 101.
Fig. 4B shows an exemplary set of operations that may be carried out by the
controller
100 to implement this operation. In step 402, the controller 100 obtains the
rotational speed
value NH from the sensor(s) shown in Figure 3. In step 404, the controller 100
differentiates the
value NH to obtain the value NH. The differentiation may be carried out
digitally or via analog
processes. In step 406, the controller identifies any adjustment value ADJ
that corresponds to the
value &NH. To this end, the graph of Fig. 4C may be employed, which shows ADJ
as a function
of reduction of SNH. Information representative of the relationship depicted
in Fig. 4C may also
be stored in memory 101.
In step 408, the controller 100 employs the ADJ value to modify the perceived
NH speed
and thereby reduces the maximum fuel limit function 400 of Fig. 4A. To this
end, for example,
the controller 100 may suitably subtract the adjustment value ADJ from the
actual speed value
NH to obtain an adjusted speed value NHADJ, and identify the value WF,,,õ that
corresponds with
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the value NHADJ using the function 400 of Fig. 4A. As shown in Fig. 4A, this
adjustment results
in a lower WF,õ,õ" than would normally occur at the speed NH. The controller
100 thereafter in
step 410 controls the fuel supplied to the fuel valve actuators 120 using the
maximum fuel limit
value WFõ,ax identified in step 408.
It can be seen from Figs. 4A and 4C, that sudden and severe decrease in speed
NH will
result in a bigger value of ADJ. As a consequence, the reduction in the fuel
supply limit
is accelerated.
It will be appreciated that the flow diagram of Fig. 4B merely illustrates the
generalized
operations carried out by the controller 100 to perform the preliminary shaft
failure detection and
remecliation, and is not intended to define the detailed software (or
hardware) operations of the
controller 100. In any event, the operations of Fig. 4B are repeated in an
ongoing manner, which
allows for possible recovery if the speed value NH recovers. If a shaft
failure is subsequently
detected by other, conventional means, then the controller 100 may completely
cut off the fuel
supply.
Thus, the controller 100 in this embodiment first obtains a first value (e.g.
NH)
representative of a rotational speed of one or more of the shafts of a
turbine. The controller 100
differentiates the first value to obtain the rate of change. The controller
100 then adjusts the
model of Fig. 4A based on the differentiated first value. The controller 100
thereafter carries out
its normal control mechanisms, such as controlling fuel supply based on the
adjusted version of
NH based on the model in Fig. 4A.
By adjusting the model, it is meant that the controller 100 may suitably move
the speed
value back (or curve of Fig. 4A forward) by a value corresponding to the
differentiated first
value. Thus, when the shaft speed NH exhibits rapid deceleration, the curve
400 will effectively
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move to the right by a value corresponding to the deceleration rate. This
results in the fuel
supply limit being even lower (at any particular rotational speed NH) than
would result from
normal operation at that rotational speed NH.
In yet another embodiment of the invention, another known control feature may
be
manipulated to provide a preliminary detection and response to a possible
shaft break. In
particular, the controller 100 in this embodiment employs a known control
scheme
that limits the amount of fuel that can be supplied to an engine upon restart.
In particular, it is
known that an in-flight restart of a gas turbine engine, referred to as a fast
windmill restart, can
cause issues due to residual heat in components within the engine. Thus, it is
known to control
the fuel supply such that during conditions indicative of a fast windmill
restart, the amount of
fuel is limited to the engine. The details of an exemplary embodiment of this
control scheme are
shown and discussed in U.S. Patent 5,628,185.
In this embodiment of the present invention, the start bias control scheme is
modified to
provide even further fuel feed reduction when excessively high temperatures,
those typically not
associated with fast windmill restart, are detected. Such excessively high
temperatures can be
indicative of a shaft failure. Accordingly, the modified start bias control
scheme provides early
detection and mitigation of a possible shaft failure without completely
shutting off the fuel.
Referring to Fig. 5, a modified version of the start bias control scheme
described in U.S.
Patent 5,628,185 is shown. In general, the control scheme of Fig. 5, which is
carried out by the
controller 100, provides start bias (fuel modifier) control similar to that of
the prior art, but also
provides early detection and response to a possible shaft failure. In general,
if an engine stall
occurs, then the controller 100 reduces the fuel to a degree which will avoid
overheating issues
as the engine restarts and begins to push cooler fresh air
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through the heated components. If, however, conditions indicate that a shaft
20, 21, 22 may have
failed, then the controller 100 reduces the fuel to a greater degree to reduce
the probability of
disc burst or high energy blade release. Nevertheless, the fuel is not
completely cut off and the
engine 10 may recover if it is ultimately determined that the shaft has not
failed.
Referring now to Fig. 5, the actual temperature T30 of the air exhausted from
the high
pressure compressor 14 is monitored by a plurality of thermocouples, one of
which 23 can be
seen adjacent to the combustion apparatus 15 if reference is now made to Fig.
2. If the
theoretical expected output signal of the thermocouples 23 in a given
situation is designated
T3Osynth, then
T30syõth/T20 =F(P30/P20)
This relationship is depicted as 534 in Fig. 5. This processing of the input
signals T20,
P30 and P20 provides an output signal 535 of T30h/T20. That output 535 is then
processed by
the multiplication unit 536 which also has T20 as an input signal to provide
an output signal 537
which is representative of T30;,. The T30õ,h output signal 537 is then
directed to a summing
and subtraction unit 538 which is adapted to provide an output signal 539
which equals the
difference between two input signals. The second input signal to the unit 538
is the output signal
T30 of the thermocouples 23, which is representative of the actual temperature
of the air
exhausted from the downstream end of the high pressure compressor 14. The
output signal from
the thermocouple 23 is designated T30,. Thus the output signal 539 of the unit
538 which is
designated AT30, equals T30meas -T30synth=
The signal AT30 is therefore representative of the difference between the
actual
temperature of the air exhausted from the downstream end of the high pressure
compressor 14
and the theoretical temperature of the air which would be expected under
normal engine running
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conditions. As will be discussed below, within a first range of values, this
differential could be
due to temperature increase resulting from a compressor stall. However, within
a second range
of values, a much greater difference could be due to a temperature increase
and other parameter
changes that are indicative of a shaft failure.
Referring again specifically to Fig. 5, the AT30 signal 539 is employed to
determine a
fuel derate value, WF/P30derate, as depicted in the relationship at 540. WF is
representative of the
rate of flow of fuel to the engine 10 and P30 is, as previously stated,
representative of the
pressure of the air exhausted from the high pressure compressor 14.
The derate value relationship depicted at 540 is selected such that if AT30
does not
exceed a minimum threshold, then there is no WF/P3Oderate output. However, if
AT30 exceeds
the minimum threshold, then there is a WFIP3Oderale output signal 541 which
increases with AT30
up to an intermediate maximum or constant derate value. The minimum to maximum
threshold
range represents the probable range relating to a restart after an engine
stall (called a quick
windmill relight) or other low idle condition. In accordance with the present
invention, however,
if AT30 further exceeds the maximum threshold, then the WF/P30derate signal
increases even
further. The choice of this threshold for the start of the second range of
increase may be altered,
but in general is intended to exceed any change in compressor temperature
typically associated
with quick windmill restart.
The WF/P30derate signal 541 is then directed into a multiplication unit 542
which
multiplies the signal 541 by a derate factor signal 543. The derate factor
signal 543 is derived
from a relationship depicted at 544 between derate factor and NH/'J 020 which
is an expression
representative of the speed of rotation of the engine 10. Essentially, if the
rotational speed of the
engine is less than 50% of what it should be under cruise conditions, then the
derate factor is 1.
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Consequently, under these circumstances the output signal 545 of the
multiplication device is the
same as the input signal 541 which is WF/P3Oderaõ. However, as the engine
speed increases to
60% of cruise, the derate factor 543 progressively decreases to zero.
Consequently, if the speed
of the engine 10 is greater than 60% of cruise speed (where 60% NH is
representative of the
minimum idle speed for the representative application), the output 545 from
the multiplication
unit 542 is set to zero.
The output 545 of the multiplication unit 542 is directed into a summing and
subtraction
unit 546 which subtracts the output 545 from 1.0 to provide a final output
signal 547. The final
output signal 547 is then directed to the main fuel control function 548 of
the controller 100.
The control function 548 then adjusts the normal fuel control signal provided
to the FV actuators
120 of Fig. 3 by multiplying that signal by the final output signal 547.
It will be appreciated that the various "units" 536, 538, 542 and 546 may be
simply
mathematical operations carried out by a programmed processor within the
controller 100, or
may be carried out by analog circuitry. Similarly, it will be appreciated that
information
representative of the relationships 534, 540, and 544 may be stored in any
suitable format (e.g.
piece-wise linear equation, mathematical function, or look-up table) in memory
101 associated
with the controller 100, shown in Fig 3. Alternatively, the relationships may
also be
implemented purely in software steps.
In any event, during normal engine operating conditions in which the engine 10
is
rotating at or above its idle speed (that is when NH400 is greater than 60%)
the derate factor
signal 543 will be zero and so, consequently will be the multiplication device
output signal 545.
This results in the final output signal 547 being 1.0 and therefore having no
effect upon the
operation of the main fuel control function 548.
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However, if the engine 10, operating at or near full power conditions, is shut
down and an
immediate attempt is made to restart it, then air passing through the high
pressure compressor 14
will be heated by the residual heat of the compressor 14 components. This, in
turn will result in
AT30 in the range of 60 K to 300 K. As a result, the signal 539 increases to
such an extent that
a WF/P3Oderare signal 545 will be dependent upon the rotational speed of the
engine 10 and the
magnitude of AT30. However, within that range, the fuel derate value
WF/P3Odemie is limited to
a maximum value FWRM for reasons discussed further below. In any event, the
summing and
subtraction unit 546 subtracts the output signal 545 from 1.0 to provide a
final output signal 547.
Since the final output signal 547 is now less than 1.0, it has an effect upon
the operation of the
main fuel control function 548 so that the rate at which fuel is supplied to
the engine 10 is
reduced by that proportion. The amount by which the rate of fuel supply is
reduced is arranged to
be sufficient to change the stall characteristics of the high pressure
compressor 14. The stall
characteristics are changed to such an extent that during the engine 10
restarting procedure, the
high pressure compressor 14 does not stall, thereby in turn permitting
effective engine 10
restarting.
As soon as an effective engine restart has been achieved, the engine
rotational speed
increases, thereby in turn increasing the value of MIN 020 so that the derate
factor signal 543
decreases to zero. This in turn leads to the output signal 545 from the
multiplication device 542
also decreasing to zero, thereby causing the final output signal 547 to return
to its original value
of 1Ø As a consequence of this, the main fuel control unit 548 reverts to
supplying fuel to the
engine 10 at a normal rate, thereby permitting normal engine operation.
One advantageous aspect of the control scheme of Fig. 5 is the alteration of
the fuel rate
to AT30 relationship 540 such that it has two distinct zones ¨ one associated
with fast windmill
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restart and the other associated with possible shaft failure. In the first
zone (associated with fast
windmill restart, the derate value increases at a first average rate as a
function of the differential
temperature value up until a point, at which point the derate value stays
constant (or at least
increases at lesser rate) for a range of differential temperature values. To
this end, it has been
found that over certain levels of fuel reduction, fast windmill restart can be
hindered. Thus, as
illustrated by the relationship 540, after increasing the WF/P3Oderaie to a
maximum value FWRNI
corresponding to a AT30 value of approximately 180 , the WF/P3Oderate remains
at the same
value throughout the rest of the range of temperatures associated with fast
windmill restart. In
the second zone, associated with possible shaft failure, the derate value
increases again at a more
aggressive average rate with respect to the differential temperature value.
The more aggressive
rate may be similar to the first average rate.
Thus, the Z-shaped curve of the relationship 540 allows for two-different
control
objectives to be obtained within the same control topology.
Thus, in another scenario, if the engine 10 is operating at or near full power
conditions
and a shaft fails, then air passing through the high pressure compressor 14
will be heated by the
residual heat of the compressor 14 components to a greater degree. The value
of AT30 would
increase through the range of 60 K to 180 K into the range well beyond that
associated with
windmill restart. As the 6130 exceeds 3000 K (or other suitable range above
that associated with
fast windmill restart), the WF/P30a,m, signal 545 begins increasing again to
even higher values,
beyond the intermediate constant FWRM, further reducing the fuel rate. The
controller 100
otherwise operates in the manner described above. It will be appreciated that
in the case of a
shaft failure, the AIHNO20 value will decrease well below 50%, thereby
providing a derate factor
543 of 1.0 in the relationship 544 of Fig. 5. This also accelerates the fuel
rate reduction.
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As with the other embodiments, when shaft failure is confirmed by other means,
the
controller 100 causes the fuel to be completely cut-off from the engine 10.
If, however, a shaft
failure has not occurred, and the AT30 is due other factors, then the engine
10 may gracefully
recover without shutting off the fuel completely.
Both of the above reference embodiments rely on a moderately accurate speed
signal NH
to carry out their respective control operations.
It will be appreciated, however, that in many cases, and specifically those
involving shaft
failure, the shaft rotational speed sensor signal may not be available. In
particular, shaft failure
(and other events) can involve failure of the shaft rotational speed sensor
124. (See Fig. 3).
Because rotational speed sensors can fail, it is known to employ logic in the
controller 100 that
estimates rotational speed NH based on other engine operating parameters. The
controller 100
then uses the estimated speed for its control operations.
To this end, it is known to use a model of shaft speed based on various sensed
parameters
when a speed signal is not available. As will be discussed below in detail, an
alteration of that
model, which artificially reduces the estimated shaft speed for a portion of
the model, can be
used to carry out an early detection and remediation of a potential shaft
failure.
Fig. 6 shows a model 600 of estimated rotational speed as a function of the
sensed
pressure value P30, which is obtained from the sensor 33 located near the
inlet of the combustor
15. In particular, in the embodiment described here, the estimated speed NHõõ
expressed as a
percentage of shaft design speed, is shown as a function of the ratio of P30
to a value AP20. The
value AP20 is a normalizing parameter based on the Buckingham Pie
theorem/principle, which
in this embodiment is equal to P20/14.696.
As shown in Fig. 6, the solid line 602 represents the true estimated
correlation between
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the pressures sensor values P30IAP20 and the corrected rotational speed of the
shaft (e.g. shaft
20, 21 or 22), N1-II4020. The model 600 has a first portion 604 that consists
of a part of the solid
line 602, thereby representing the proper correlation between the pressure
sensor values
P30IAP20 and rotational speed NH of the shaft. In accordance with embodiments
of the present
invention, however, the model 600 has a second portion 606, represented by the
dashed line, that
differs from the actual correlation 602. In the second portion 606 of the
model 600, the
estimated rotational speed NHõ, of the first model in the second portion is
less than a rotational
speed of the shaft for the corresponding pressure sensor value
Thus, using the model 600 with the adjusted portion or segment 606, the
controller 100
operates with an estimated speed value NHest that is lower than the actual
speed that would be
expected for that pressure value. As a result, the controller 100 can employ
control mechanism
such as those shown in Figs. 4A, 4B, 4C or 5, using the artificially lowered
speed value NHest.
Referring to Fig. 4A, for example, the artificially reduced estimated speed
NHest moves
backward in the model 400 to a lower maximum fuel rate WFõ.. The model 600 is
adjusted in
very low pressure value areas (e.g. between P301 and P302) that are more
likely to be associated
with shaft failure. This again limits the fuel in the event of a possible
shaft break, thereby
limiting the risk of disc burst or blade release.
In operation, if the speed sensor(s) 124 is/are not operating, then the
controller 100
receives a speed sensor fault signal. As a consequence of receiving the fault
signal, the
controller 100 employs an estimated speed signal NHõ, for various control
operations. To this
end, the controller 100 uses the adjusted model 600 of Fig. 6 (with the
segment 606) in its
various control schemes.
For example, the controller 100 performs the maximum fuel limit control such
as shown
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in Fig. 4B, or the operations of Fig. 5, using the estimated value NHõ,. hi
the event of the shaft
breakage, the pressure P30 will rapidly decrease into the zone between P301
and P302. In that
zone, the controller 100 will use an estimated speed NHem that is below what
would normally be
expected for the pressure level value. As a consequence, for example, the
controller 100
executing the operations of Fig. 5 will use an NHI\11320 value with the lower
NHe,õ value in
determining the derate factor 543. This results in the derate factor more
rapidly increasing to
1Ø
As a consequence, the reduction in fuel is accelerated. However, in the event
that no
shaft failure has occurred and the pressure value P30 recovers to normal
levels above P302, then
the controller 100 can resume operating the engine 10 normally without the
extreme
precautionary measure of shutting the engine down completely, as would
traditionally occur
when a shaft break is detected.
It will be appreciated that the above-described embodiments are merely
illustrative, and
that those of ordinary skill in the art may readily devise their own
implementations and
modifications that incorporate the principles of the present invention and
fall within the spirit and
scope thereof. For example, the current disclosure is presented with respect
to a three spool gas
turbine engine, but the concepts and descriptions of the inventions are
equally applicable to two
spool and single spool gas turbine engines, as would be understood by one of
ordinary skill in the
art.
