U.S. patent application number 10/158954 was filed with the patent office on 2003-12-04 for turbine blade clearance on-line measurement system.
This patent application is currently assigned to Siemens Westinghouse Power Corporation. Invention is credited to Twerdochlib, Michael.
Application Number | 20030222638 10/158954 |
Document ID | / |
Family ID | 29582779 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030222638 |
Kind Code |
A1 |
Twerdochlib, Michael |
December 4, 2003 |
TURBINE BLADE CLEARANCE ON-LINE MEASUREMENT SYSTEM
Abstract
A monitoring system for measuring the gap between a turbine
blade and a stationery component of a turbine, includes an
insertion probe having a sensor which is moved radially within the
gap to a pre-selected distance from the turbine blade as measured
by the sensor's output. The distance the sensor moved from a
reference point to arrive at the pre-selected distance from the
blade is also monitored and the gap between the fixed turbine
component and the blade is determined from these measurements.
Inventors: |
Twerdochlib, Michael;
(Oviedo, FL) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
186 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Westinghouse Power
Corporation
|
Family ID: |
29582779 |
Appl. No.: |
10/158954 |
Filed: |
May 31, 2002 |
Current U.S.
Class: |
324/207.16 ;
324/207.26 |
Current CPC
Class: |
G01B 7/14 20130101 |
Class at
Publication: |
324/207.16 ;
324/207.26 |
International
Class: |
G01B 007/14 |
Claims
What is claimed is:
1. A system for monitoring the clearance between a plurality of
turbine blades and a stationary portion of the turbine, comprising:
sensor means having an exterior tubular support structure with an
axial dimension, that supports a centrally disposed drive shaft
that is reciprocally mounted within the support structure to move
along the axis thereof, said sensor means positioned within said
stationary portion of the turbine and aligned so that the
reciprocal movement of the drive shaft moves a forward end of the
drive shaft, closest to the turbine blades, towards and away from
the turbine blades; a proximity sensor that is responsive to being
positioned substantially a preselected distance from the turbine
blade to provide a given output representative of the preselected
distance, supported at the forward end of and coupled to said drive
shaft so that said proximity sensor moves with the drive shaft; a
motor coupled to said drive shaft and operable to move said drive
shaft a given distance either towards or away from the turbine
blades and responsive to such movement to provide a position output
from which the sensor position can be determined relative to its
distance from a predetermined calibration point; and means for
monitoring the sensor output and controlling the motor movement, so
the sensor is substantially positioned the preselected distance
from the blade determined by monitoring for the given output, and
for monitoring the position output to determine the distance of the
blade form the calibration point.
2. The monitoring system of claim 1 wherein the proximity sensor
comprises an eddy current search coil.
3. The monitoring system of claim 2 wherein the search coil inside
diameter is between approximately 0.020 and 0.050 inch (0.508 to
1.27 mm).
4. The monitoring system of claim 1 wherein the preselected
distance is 0.010 inch (0.254 mm).
5. The monitoring system of claim 1 wherein the drive shaft is
axially movable over a distance of approximately, at least 0.250
inch (6.35 mm).
6. The monitoring system of claim 1 wherein the predetermined
calibration point is located at the location along an axis of
movement of the proximity sensor that places a surface of the
proximity sensor, opposite the blades, flush with an interior
surface of the stationary portion of the turbine.
7. The monitoring system of claim 1 including an electrically
conductive slug strategically positioned in fixed relation to the
tubular support structure at a location that provides a unique
output at the proximity sensor when the proximity sensor is at the
calibration reference point.
8. The monitoring system of claim 7 including an insertion probe,
positioned within the support structure, said insertion probe
having a first end affixed to the forward end of the drive shaft
between the drive shaft and the proximity sensor which is affixed
to the axially opposite end to said first end.
9. The monitoring system of claim 8 wherein said insertion probe
has an elongated, tubular, axially extending, hollow core and said
electrically conductive slug is stationarally supported within said
core relative to said support structure.
10. The monitoring system of claim 9 wherein said electrically
conductive slug is supported within said core by a pin that extends
transverse to the axis of the support structure, through said slug,
through axially extending slots in said insertion probe and in to
said support structure.
11. The monitoring system of claim 9 wherein the electrically
conductive slug is formed as a hollow cylinder.
12. The monitoring system of claim 1 wherein said insertion probe
is constructed from thin walled stainless tubing having a wall
thickness of approximately 0.20-0.375 inch (5.08-9.525 mm).
13. The monitoring system of claim 12 wherein the thin walled
stainless steel tubing comprises an epoxy or ceramic filling.
14. The monitoring system of claim 1 including means for monitoring
the continuity of the search coil.
15. The monitoring system of claim 1 wherein the stationary portion
of the turbine is a blade ring.
16. A method of monitoring the clearance between a plurality of
turbine blades and a stationary portion of the turbine comprising
the steps of: positioning a proximity sensor at a calibration point
relative to the stationary portion of the turbine; driving the
proximity sensor away from the stationary portion of the turbine
towards the blades while monitoring an output signal of the
proximity sensor; identifying from the output of the proximity
sensor when the proximity sensor is at a preselected distance from
the blades; monitoring a distance of movement of the proximity
sensor from the calibration point to a travel point along the path
of movement of the proximity sensor where the proximity sensor is
at the preselected distance from the blades; and determining from
the distance of movement the spacing between the stationary portion
of the turbine and the blades.
17. The method of claim 16 including the step of monitoring the
electrical continuity of the proximity sensor.
18. The method of claim 17 including the step of returning the
proximity sensor to the calibration point should the continuity
monitoring step detect a loss of electrical continuity in the
proximity sensor.
19. The method of claim 16 including the step of repeating the
method of claim 14 at least substantially every 5 minutes.
20. The method of claim 19 wherein the method is repeated
approximately every 5 seconds.
21. The method of claim 16 including the step of withdrawing the
proximity sensor substantially to the calibration point when the
monitoring step is complete.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is directed generally to systems for
monitoring the clearance between fixed and rotating parts within a
combustion turbine and, more particularly, to systems of that type
that operate on line.
[0003] 2. Related Art
[0004] The clearance between the stationery seals of a turbine and
compressor and the tips of the blades must not be so great to
permit an excessive amount of air in the case of the compressors
and combustion gases in case of the turbines to pass between them
and thereby reduce the efficiency of the turbine. On the other
hand, clearances can not be too small because high centripetal
loading and high temperatures may cause blades to lift or grow
radially. Such blade lifting or radial growth can cause blade tips
to rub the stationery seal and may eventually cause seal, and blade
tip damage.
[0005] In addition, the differences in thermal response time of the
various combustion turbine components can result in the mechanical
interference between stationery and moving parts under certain
conditions. This is certainly the case during the restart of a hot
turbine where contact between the compressor/turbine blading and
the blade ring has resulted in massive compressor and turbine
damage. Even a slight rub will destroy blade seals and reduce the
efficiency of a combustion turbine. The obvious solution is to
prolong restart until the turbine cools. This requires many hours.
However, the situation is further complicated by the competing need
to spin-cool the turbine following shutdown to prevent sagging or
humping of the rotor. Both can only be done if the blade clearance
is accurately measured, and appropriate action is taken based on
this on-line measurement.
[0006] Capacitance blade clearance probes are used to study blade
clearance patterns to establish restart and spin-cool rules. These
sensors have proven both inaccurate and unreliable as an
engineering tool and are thus even less suitable for commercial
on-line monitoring.
[0007] A number of blade clearance systems have been developed for
steam turbines, such as those described in U.S. Pat. No. 4,987,555.
These systems depend upon indicia on the blades shroud to obtain a
meaningful proximity measurement. However, the approaches do not
appear readily applicable to combustion turbine applications.
[0008] Accordingly, a need exists for an on-line combustion turbine
blade clearance monitor that can accurately measure, in real time,
the clearance between the blades and blade ring of a combustion
turbine.
SUMMARY OF THE INVENTION
[0009] An on-line, real time blade clearance monitor is provided to
meet the foregoing objectives. The clearance monitor includes an
insertion probe that is positioned within a stationery member
portion of the turbine and reciprocally moveable within a cavity,
preferably radially in line with the turbine blade at the point
where the blade is closest to the stationery member. A proximity
sensor is supported at one end of the insertion probe closest to
the turbine blade and a connecting rod is affixed to the other end
of the insertion probe. The connecting rod is reciprocally driven
by a motor such as a stepper motor or a pulsed D.C. motor and
resolver that provides a positional output to a computer. A
calibration indicia is provided that results in a unique output of
the proximity sensor when the proximity sensor is positioned flush
with an interior surface of the stationery turbine member that
faces the turbine blade.
[0010] In operation, the insertion probe starts from a point where
the proximity sensor is flush with the interior surface of the
stationary turbine member. The insertion probe is then advanced
towards the blade until the proximity sensor output is a
preselected distance from the turbine blade as represented by a
given output of the sensor and recognized by a computer controller.
The computer controller also monitors the motor drive to determine
the distance the insertion probe has been moved towards the
compressor blade. The computer controller then calculates the
clearance distance between the blade and the stationery turbine
member by adding the preselected distance to the monitored
advancement of the insertion probe as indicated by the motor drive
to the computer controller.
[0011] Preferably, the insertion probe is constructed of materials
that will not substantially damage the blade should the insertion
probe and the blade come in contact. Desirably, the electrical
continuity of the proximity sensor is monitored to determine
whether any such contact has occurred. If the sensor is disabled,
the computer controller directs the motor to withdraw the insertion
probe to the calibration point.
[0012] During turbine operation or cool-down while rotation of the
rotor is maintained, the foregoing clearance measurement is
monitored at periodic intervals such as every five minutes, though
the steps of the method of this invention may be repeated as often
as every five seconds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] A further understanding of the invention can be gained from
the following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:
[0014] FIG. 1 is a plan view of a blade ring and compressor blade
with the support structure, connecting rod and insertion probe
assembly shown cut away and with the insertion probe shown in
cross-section;
[0015] FIG. 2 is a plan view of the motor/controller portion of the
monitoring system of this invention with portions cut away to
reveal the interior operation thereof; and
[0016] FIG. 3 is a graphical illustration of the proximity coil
output signal of the invention plotted over the travel distance of
the coil.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] FIG. 1 illustrates the forward sensing portion of the blade
clearance monitoring system 10 of this invention for monitoring the
clearance 16 between a turbine blade, such as a compressor blade 14
and a stationery component of the turbine such as a blade ring 12.
A very small eddy current search coil 30 is employed to provide a
short range and sensitive indication by means of a discrete voltage
signal level output, of the proximity of the blade tip and search
coil in the order of 0.010 inch (0.254 mm). A search coil 30 that
can be employed for this purpose preferably has an inner diameter
from between 0.020 to 0.050 inch (0.508-1.27 mm). The search coil
is mounted at an end of a short throw insertion probe 18. A short
throw of approximately 0.25 inch (6.35 mm) can be employed for this
purpose. In other words, the insertion probe 18 has an approximate
range of movement in the radial direction towards the blade 14 of
approximately 0.25 inch (6.35 mm). The insertion probe is slideably
supported coaxially within an outer support 20 fixed within the
compressor ring 12 by mating threads 22. The insertion probe has a
thin outer stainless steel wall 24 having a thickness of, for
example, 0.20-0.375 inch (5.08-9.525 mm). The interior of the
insertion probe 18, surrounding an elongated hollow cavity 26, is
filled with an epoxy or ceramic 28. The end of the insertion probe
wall 24 juxtaposed to the compressor blade 14 is slightly enlarged
to seat against a mating surface of the outer support 20 when the
search coil 30 is seated flush with the surface of the blade ring
opposing the compressor blade 14.
[0018] Radial translation of the insertion probe 18 towards and
away from the compressor blade 14 within the support 20 is achieved
by means of a connecting rod 34, screw action and computer
controlled stepper motor drive assembly 60 mounted outside the
turbine and shown in FIG. 2. Alternately, a pulsed D.C. motor and
resolver can be employed. The D.C. motor provides more torque than
the stepper motor and the resolver is connected to the motor's
shaft and directly measures the shaft's rotation.
[0019] Confirmation that the search coil is properly seated in the
reference position flush with the blade ring 12 surface is achieved
by means of a hollow cylindrical electrically conductive slug 36
positioned within the hollow cavity 26 of the insertion probe 18,
shown below the search coil 30 in FIG. 1. The reference slug 36 is
affixed to the outer support member 20 by a pin 38 passing
diametrically through axial slots 40 in the insertion probe 18 so
that when the probe is driven radially, the pin 38 rides within the
axial slots 40 maintaining the slug 36 in a fixed position relative
to the support structure 20. The insertion probe 18 is thus free to
translate within the fixed outer support 20 and over the fixed
reference slug 36. The reference slug length and position within
the outer support is set so the search coil provides a unique
indication that the insertion probe's intruding surface into the
gap between the blade 14 and the blade ring 12 is flush with the
inner blade ring surface, hereafter at times referred to as the
insertion probe "reference position" or "calibration point". The
signal from the proximity sensor coil 30 is communicated through
the coil leads 32 which are threaded through the connecting rod 34
to the drive assembly 60 shown in FIG. 2. The signal 32 is
connected to a search coil controller circuit 48, which
communicates the discrete outputs 52 to a computer 50. The end of
the connecting rod 34 opposite the insertion probe 18 is fitted
with a female-threaded coupling 42 which is translated by a
rotating screw 44 having a mating male thread, which is, in turn,
driven by a stepper motor 46. The stepper motor 46 is controlled by
the computer 50, which directs the number of steps to be taken and
the direction of rotation of the motor 46. The motor 46 also
preferably provides a position signal 54 to the computer 50,
confirming the steps that have been taken. The drive assembly 60
also includes a pressure seal 56 that isolates the drive assembly
60 from exposure to the high-pressure interior turbine
environment.
[0020] Blade clearance is measured relative to the reference
position by advancing the insertion probe to a preselected radial
distance from the blade, the preselected distance, in this case is
0.010 in (0.254 mm). This is the distance where the search coil
signal peak amplitude indicates the longest blade is within the
pre-selected distance of the insertion probe. The distance of the
blade from the stationary member is then calculated from the motor
step angle and screw pitch using the equation:
Radial translation=(Number of motor steps).times.(Motor Rev Per
Step).times.(Inch/mm per thread) (1)
[0021] FIG. 3 graphically illustrates the search coil peak voltage
output as a function of the travel distance of the coil. The Y axis
denotes the peak voltage on the excited search coil and the X axis
denotes the gap or distance from the search coil to the blade tip.
As the search coil approaches the blade tip, the voltage on the
excited search coil drops. This drop rate increases as the gap
between the search coil and the blade tip becomes smaller. At a
0.005-0.020 inch (0.127-0.508 mm) gap, the slope, which shows the
rate of the decrease of the peak voltage on the excited search coil
as the gap between the search coil and the blade tip decreases, is
high providing a sensitive determination of the search coil/blade
tip gap. This coil voltage is A.sub.o (volts peak) when the gap is
a.sub.o. For example, this voltage is 6V.sub.p at a 0.010 inch
(0.254 mm) gap. When the coil voltage drops to 6V.sub.p, the
coil/blade tip gap is thus determined to be 0.010 inch (0.254
mm).
[0022] Insertion probe movement is governed by control of the
computer 50 using the algorithm:
[0023] [n is set so as to provide approximately 0.001 to 0.002"
(0.025-0.05 mm) displacement]
[0024] (find reference position)
[0025] 1 measure search coil signal peak amplitude "A" volts)
[0026] 2 withdraw search coil "N" stepper motor steps
[0027] 3 measure search coil signal peak amplitude "B" volts)
[0028] 4 compare peak amplitude A and B
[0029] 5 .vertline.A-B.vertline.<0.001 goto 1 (on flat portion
of the curve between points (1) and (2) shown in FIG. 3--need to
withdraw further)
[0030] 6 A<B go to 1
[0031] 7 B>A.sub.o go to 1
[0032] 8 stop--reference found (find blade clearance)
[0033] 9 m=0 (set motor step counter to zero)
[0034] 10 step motor into turbine n steps
[0035] 11 m=m+n
[0036] 12 measure search coil signal peak amplitude "C" volts)
[0037] 13 compare peak amplitude C and A.sub.o
[0038] 14 C>A.sub.o go to 11
[0039] (measure blade clearance)
[0040] 15 R_T=m*M_R_per_S*I_per_T+a.sub.o
[0041] 16 compare R_T and Alert_Level
[0042] 17 Alert_Level>R_T goto 1 (no rub will occur between
blade ring and blade tip)
[0043] 18 Energize Alert Relay (if not 17 rub will occur) (repeat
process)
[0044] 19 goto 1 (make next measurement)
[0045] The alert level is determined by the computer based on the
turbine condition, i.e., at turning gear, 132 minutes after a full
load trip or at 2 minutes into spin cool cycle following 31 minutes
at turning gear following full load unit trip. Under these
conditions, the assigned radial translation for the alert level
implies a rub will occur between the blade's tip and the blade ring
at or below the alert level measurement.
[0046] Electrical continuity of the search coil 30 is continually
monitored by the computer 50. Should electrical continuity or the
proximity signal be lost as a result of unplanned contact with the
blade, the insertion probe is returned to the reference position
and placed in a sleep mode. The insertion probe is constructed of
epoxy or ceramic filled 0.375 inch (9.525 mm) thin-walled stainless
steel tubing weighing a few ounces resulting in no possibility of
blade or internal turbine damage should unplanned contact with the
blade be made.
[0047] A blade gap measurement could be taken every five seconds,
but a five minute cycle time is preferable given the thermal
response time of the compressor. Probes are easily replaceable from
outside the turbine. The system can also be applied to hot turbine
blades if high temperature diamond or ceramic insulated wire and
ceramic nonconductive materials are used. The stepper motor can
easily operate in the temperature environment outside the
compressor and turbine engine, which is less than 200.degree. F.
(93.3.degree. C.). The support structure 20 can be constructed of
any compatible metal that is capable of handling the caustic
environment to which it is being exposed, such as stainless steel
or a ceramic composite.
[0048] Thus, a monitoring system of this invention can provide a
precise and durable gap measurement that is reliable. While
specific embodiments of the invention have been described in
detail, it will be appreciated by those skilled in the art that
various modifications and alternatives to those details could be
developed in light of the overall teachings of the disclosure. For
example, other proximity sensors could be employed such as a
capacitance sensor. Accordingly, the particular embodiments
disclosed are meant to be illustrative only and not limiting as to
the scope of the invention which is to be given the full breadth of
the appended claims and any and all equivalents thereof.
* * * * *