Apparatus and method for monitoring steam turbine shroud clearance

A turbine blade shroud clearance monitoring system is comprised of a plurality of sensors for measuring the clearance between the stationary portion of the turbine and the shroud segments in each turbine blade row. The measurements are averaged by a microprocessor to produce an average clearance value for each shroud segment. The microprocessor then analyzes the average clearance values for each shroud segment to determine when the clearance between a shroud segment and the stationary portion of the turbine is approaching a critical value. Output responsive to the foregoing analysis is then generated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed generally to monitoring operating 
parameters in a steam turbine generator and, more specifically, to the 
monitoring of the clearance between the turbine shroud and the turbine 
seal. 
2. Cross Reference to Related Application 
The present application is related to U.S. application Ser. No. 201,225 
entitled SHROUDED TURBINE BLADE VIBRATION MONITOR filed 6/2/88, which is 
assigned to the same assignee as the present invention (W.E. 54,116). 
3. Description of the Prior Art 
Apparatus exist for measuring various operating parameters in steam turbine 
generators. One such apparatus is disclosed in U.S. Pat. No. 4,644,270 
wherein a plurality of proximity sensors measure steam flow guide to 
turbine blade top distance at various points around a flow guide. However, 
such an apparatus is not easily adaptable to a shrouded turbine blade 
arrangement. 
A typical prior art shrouded turbine blade row 10 (FIG. 1) includes the 
following components: rotor disk 11, blades 12, shroud segments 13, tenons 
14 and seal 15. The tenons 14 are integral parts of the blades 12 that 
serve to fasten the shroud segments 13 to the blades 12. The seal 15 
reduces the amount of steam that passes around instead of through the 
turbine blades 12. 
The clearance between the seal 15 and the shroud segments 13 must not be so 
great as to allow an excessive amount of steam to pass between them and 
thereby reduce the efficiency of the turbine. On the other hand, any 
contact between the shroud segments 13 and the seal 15 will result in both 
components being destroyed. Because of the dynamic nature of a steam 
turbine and the forces and temperatures involved, the clearance between 
the shroud segments 13 and the seal 15 varies during operation. Therefore, 
it is desirable to monitor this clearance on a real-time basis in order to 
prevent damage to the rotating machinery. 
In an attempt to reduce the number of unscheduled outages in steam turbine 
generator systems and protect the integrity of the components of the 
turbine generator, use is made of monitoring systems which detect and 
alert operators of abnormal operating conditions. However, because of the 
physical differences between shrouded and unshrouded turbine blades, 
monitoring systems used on the latter are not easily adaptable for use on 
the former. Accordingly, a device is needed that will indicate when the 
clearance between the shroud segments 13 and seal 15 is approaching a 
minimum limit established by the risk of contact. The present invention 
fills the need for a turbine blade shroud clearance monitoring system. 
SUMMARY OF THE INVENTION 
The present invention is directed to a turbine blade shroud clearance 
monitoring system comprised of a means for measuring the clearance between 
the stationary portion of the turbine and the shroud segments in each 
turbine blade row a number of times along each shroud segment under 
dynamic conditions. The clearance measuring means is preferably a 
plurality of sensors. The system also includes means for averaging these 
measurements to produce an average clearance value for each shroud 
segment, means for analyzing the average clearance values for each shroud 
segment to determine when the clearance between a shroud segment and the 
stationary portion of the turbine is approaching a critical value, and 
output means responsive to the means for analyzing. 
One embodiment of the present invention is directed to a turbine blade 
shroud clearance monitoring system in which the means for analyzing 
compares the average clearance value of each shroud segment to the average 
clearance values of the two adjacent shroud segments in the same row. A 
change in the clearance value of any one shroud segment is, thus, 
detected. 
Another embodiment of the present invention is directed to a turbine blade 
shroud clearance monitoring system in which the means for analyzing 
computes an average row clearance value by averaging the average clearance 
values for each shroud segment in that particular row. The average 
clearance value for each shroud segment is compared to the average row 
clearance value in order to detect a reduction in clearance of a 
particular shroud segment. 
According to another embodiment of the present invention, the analyzing 
means of a turbine blade shroud clearance monitoring system compares the 
average clearance value of each shroud segment to a predetermined critical 
clearance value. The approach of the average clearance value of any one 
shroud segment to this critical value is, thus, detected. 
The present invention is also directed to a method of monitoring turbine 
blade shroud clearance by measuring the clearance between the stationary 
portion of the turbine and the shroud segments in each turbine blade row a 
number of times along each shroud segment under dynamic conditions. This 
method also includes the steps of averaging these measurements to produce 
an average clearance value for each shroud segment, analyzing the average 
clearance values for each shroud segment to determine when the clearance 
between a shroud segment and the stationary portion of the turbine is 
approaching a critical value, and outputting responses to this analysis. 
The turbine blade shroud clearance monitor of the present invention may be 
used in any steam turbine utilizing shrouded turbine blades. In a typical 
steam turbine comprised of rows of high pressure, intermediate pressure 
and low pressure blades, all rotating rows with the exception of the last 
row in the low pressure turbine have a shroud at their outer periphery. 
The need for such a system exists because of the importance of maintaining 
the proper clearance between the shroud segments and the seal, which is 
the stationary portion of the turbine. The reduction of this clearance may 
cause contact between the shroud and the seal which results in their 
destruction. The turbine shroud clearance monitoring system will alert an 
operator of a critical condition in the clearance between the shroud and 
the seal and, thus, prevent the destruction of the turbine components. 
These and other advantages and benefits of the present invention will 
become apparent from the description of the preferred embodiments 
hereinbelow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 illustrates a shrouded turbine blade row 10 in which the method and 
apparatus of the present invention to monitor the turbine shroud clearance 
can be employed. Like reference numerals are employed among the various 
figures to designate like elements As previously illustrated in FIG. 1, 
the turbine blades 12 are connected to a rotor 16 by means of the rotor 
disk 11. The tenons 14 are integral parts of the blades 12 and serve to 
fasten the shroud segments 13 to the blades 12. The seal 15 reduces the 
amount of steam that passes around instead of through the turbine blades 
12. Also illustrated in FIG. 2 is a sensor 21, which is the means for 
measuring the clearance between the shroud segments 13 and the seal 15. 
The sensor 21 may be a variable reluctance sensor or may incorporate any 
practical method of sensing distance, including, but not limited to, 
microwave or optical methods. One type of sensor which can withstand the 
rigors of the interior of a steam turbine is described in U.S. Pat. No. 
4,644,270. As further illustrated in FIG. 2, a reference sensor 19 is 
additionally provided. The sensor 19 in conjunction with indicia 20 on the 
rotor 16 is operable to provide an output signal once per rotor 16 
revolution and is the means for identifying a particular shroud segment 
13. Such a reference signal is commonly known in the turbine art. 
One sensor 21 is provided for each turbine blade row 10 of the steam 
turbine 18 illustrated in FIG. 3. The rotor 16 carries a plurality of 
turbine blade rows 10 consisting of blades 12 of differing sizes 
corresponding to the high pressure, intermediate pressure and low pressure 
stages of the turbine 18. The turbine seal arrangement 15 encompasses all 
blade rows 10. 
A cross-sectional view of a portion of the turbine blade row 10 arrangement 
of FIG. 2 is shown in FIG. 4. The sensor 21 is situated in the plane of 
the blade row 10 and directly above the shroud segment 13 and tenon 14 
surface. The placement of the sensor 21 outside of the direct flow path of 
the steam through the turbine blades 12 prevents significant erosion of 
the sensor 21. Because the position of the seal strips 15 relative to the 
sensor 21 does not change and the sensor 21 measures the distance from its 
edge to the shroud segment 13, the shroud segment 13 to seal 15 clearance 
can be determined. 
The turbine shroud clearance monitoring system 30 of the present invention 
is illustrated in FIG. 5. The clearance between the shroud segment 13 and 
seal 15 as measured by sensor 21 of FIG. 2 is converted to a digital 
signal by the analog to digital converter 33 which is then sampled by the 
microprocessor 31. The microprocessor 31 is the means for both averaging 
and analyzing these measurements and is also capable of displaying shroud 
clearance data to operating personnel as well as initiating turbine 
protective action through the output interface 32. 
The operation of system 30 shown in FIG. 5 may be implemented as 
illustrated in the flow chart of FIG. 6. The flow chart begins at step 40 
where the microprocessor 31 of FIG. 5, through known data acquisition 
techniques, samples the shroud clearance measurements a number of times 
circumferentially along each shroud segment 13 under dynamic conditions 
through sensor 21 of FIG. 2. The measurements sampled will be indicative 
of the shroud-tenon-shroud-tenon-shroud-gap pattern that is characteristic 
of the shroud segment 13 surface of FIG. 2. Because the height of the 
tenons 14 is greater than the height of the shroud segments 13 and because 
the shroud segments 13 may deform sufficiently to contact the seal 15 
without getting higher than the tenons 14, the minimum clearance measured 
is not indicative of the actual clearance between the shroud segment 13 
surface and the seal 15. Thus, a number of measurements per shroud segment 
13 must be made and an average clearance value (L.sub.N) representative of 
the distance between the shroud segment 13 surface and the seal 15 is 
calculated by the microprocessor 31 in step 41 of the flow chart in FIG. 
6. The measurement sampling rate must be high enough (three or four 
samples between tenons 14) so as to both obtain a representative average 
of the shroud segment 13 surface to seal 15 distance and clearly indicate 
the edge of a tenon 14 so that these measurements are not used in the 
calculation of the average clearance (L.sub.N) 
It is important to note that the average clearance value (L.sub.N) may not 
be representative of the minimum distance between the shroud segment 13 
surface and the seal 15 in the situation where the shroud segment 13 
deforms unevenly. FIG. 11A illustrates the condition where the shroud 
segment 13 deforms evenly and the distances A are equal on both sides of 
the shroud segment 13 while FIG. 11B illustrates the condition where the 
shroud segment 13 deforms unevenly and the distance B on one end of the 
shroud segment 13 is less than the distance A on the other end of the 
shroud segment 13. In the situation where uneven shroud segment 13 
deformation occurs, the minimum shroud segment 13 surface to seal 15 
distance between any two tenons 14 on the shroud segment 13 must be used 
as the average clearance value (L.sub.N) for that shroud segment 13. The 
microprocessor 31 will determine, in step 41 of the flow chart in FIG. 6, 
whether the individual shroud segment 13 surface to seal 15 distance 
measurements indicate uneven shroud segment 13 deformation and then 
substitute the minimum shroud segment 13 surface to seal 15 distance for 
the average clearance value (L.sub.N). 
In order to discriminate between changes in the average clearance value 
(L.sub.N) due to either a change in shroud segment 13 to seal 15 clearance 
or a change in position of the turbine rotor 16, the microprocessor 31, in 
step 42, calculates the differences (.DELTA.L.sub.N-1, .DELTA.L.sub.N+1) 
between the average shroud segment clearance (L.sub.N) and the average 
shroud segment clearance values (L.sub.N-1, L.sub.N+1) for the two 
adjacent shroud segments 13. The rate of change (L.sub.R) between the 
average shroud segment clearance (L.sub.N) and the average shroud segment 
clearance values (L.sub.N-1, L.sub.N+1) for the two adjacent shroud 
segments 13 is calculated in step 43. In step 44, the time to damage 
(T.sub.D) is estimated by the microprocessor 31 using the equation T.sub.D 
=.vertline..sup.1 /L.sub.R .vertline. x L.sub.N where L.sub.R is the rate 
of change just described and LN is the average shroud segment clearance. 
L.sub.R is constrained to negative values which correspond to decreasing 
average shroud segment clearance values (L.sub.N). The absolute value of 
the clearance difference values (.DELTA.L.sub.N-1, .DELTA.L.sub.N+1), the 
rate of change (L.sub.R) and the estimated time to damage (T.sub.D) are 
output for display in step 45 in order to alert the operator of any 
further change in the difference between the average shroud segment 
clearance (L.sub.N) and the average shroud segment clearance values 
(L.sub.N-1, L.sub.N+1) of the two adjacent shroud segments 13. 
The microprocessor 31 compares, in step 46, the absolute value of the 
clearance difference values (.DELTA.L.sub.N-1, .DELTA.L.sub.N+1) to a 
critical clearance difference value (.DELTA.L.sub.CR) which corresponds to 
a deviation of the average shroud segment clearance (L.sub.N) from the 
average shroud segment clearance values (L.sub.N-1, L.sub.N+1) for the two 
adjacent shroud segments 13 at which the risk of contact between the 
shroud segment 13 and seal 15 becomes significant. Initially, the critical 
clearance difference value (.DELTA.L.sub.CR) may be set at 0.75 to 1 mm 
(30 to 40 mils). The microprocessor 31 also compares, in step 46, the 
estimated time to damage (T.sub.D) to a critical time to damage (T.sub.CR) 
which corresponds to the minimum time until damage before which protective 
action must be initiated in order for the protective action to be 
effective in preventing turbine 18 damage. The critical clearance 
difference value (.DELTA.L.sub.CR) and the critical time to damage 
(T.sub.CR) are stored in a data base structure. If either of these 
comparisons indicates a shroud clearance difference (.DELTA.L.sub.N-1, 
.DELTA.L.sub.N+1) greater than the critical clearance difference value 
(.DELTA.L.sub.CR) or an estimated time to damage (T.sub.D) less than or 
equal to the critical time to damage (T.sub.CR), the microprocessor 31 
initiates the protective actions of step 47 by generating an operator 
alarm and then returns program control to step 40. Otherwise, program 
control returns directly to step 40. This process is performed for each 
shroud segment 13 of each turbine blade row 10. 
The flow charts of FIG. 7 and FIG. 8 illustrate alternate embodiments of 
the present invention with respect to the determination of a critical 
change in the shroud segment 13 to seal 15 distance of FIG. 2. Steps 50 
and 51 of FIG. 7 and steps 60 and 61 of FIG. 8 are identical to steps 40 
and 41, respectively, of FIG. 6. In step 52 of FIG. 7, the microprocessor 
31 of FIG. 5 calculates an average row clearance (L.sub.A). The difference 
(.DELTA.L) between the average row clearance (L.sub.A) and the average 
shroud segment clearance (L.sub.N) is then calculated in step 53. The rate 
of change (L.sub.R) between the average shroud segment clearance (L.sub.N) 
and the average row clearance (L.sub.A) is calculated in step 54. In step 
55, the time to damage (T.sub.D) is estimated by the microprocessor 31 as 
previously described in relation to FIG. 6. The absolute value of the 
shroud clearance difference (.DELTA.L), the rate of change (L.sub.R) and 
the estimated time to damage (T.sub.D) are output for display in step 56 
in order to alert the operator of any further change in the diference 
between the average shroud segment clearance (L.sub.N) and the average row 
clearance (L.sub.A). 
The microprocessor 31 compares, in step 57, the absolute value of the 
shroud clearance difference (.DELTA.L) to the critical clearance 
difference value previously described in relation to FIG. 6. The 
microprocessor 31 also compares, in step 57, the estimated time to damage 
(T.sub.D) to the critical time to damage (T.sub.CR) also described in 
relation to FIG. 6. If either of these comparisons indicates a shroud 
clearance difference (.DELTA.L) greater than the critical clearance 
difference value (.DELTA.L.sub.CR) or an estimated time to damage 
(T.sub.D) less than or equal to the critical time to damage (T.sub.CR), 
the microprocessor 31 initiates the protective actions of step 58 by 
generating an operator alarm and then returns program control to step 50. 
Otherwise, program control returns directly to step 50. This process is 
performed for each shroud segment 13 of each turbine blade row 10. 
Turning now to the flow chart of FIG. 8, the microprocessor 31 of FIG. 5, 
in step 62, calculates the rate of change (L.sub.R) between the average 
shroud segment clearance (L.sub.N) and a predetermined minimum critical 
clearance value (L.sub.MIN) The minimum critical clearance value 
(L.sub.MIN) corresponds to the shroud segment 13 to seal 15 clearance at 
which the risk of contact between the shroud segment 13 and seal 15 
becomes significant. The minimum critical clearance value (L.sub.MIN) is 
stored in a data base structure. In step 63, the time to damage (T.sub.D) 
is estimated by the microprocessor 31 as previously described in relation 
to FIG. 6. The average shroud segment clearance (L.sub.N), the rate of 
change (L.sub.R) and the estimated time to damage (T.sub.D) are output for 
display in step 64 in order to alert the operator of any further change in 
the difference between the average shroud segment clearance (L.sub.N) and 
the predetermined minimum critical clearance value (L.sub.MIN). 
The microprocessor 31 compares, in step 65, the average shroud segent 
clearance (L.sub.N) to the minimum critical clearance value (L.sub.MIN). 
The microprocessor 31 also compares, in step 65, the estimated time to 
damage (T.sub.D) to the critical time to damage (T.sub.CR) previously 
described in relation to FIG. 6. If either of these comparisons indicates 
an average shroud segment clearance (L.sub.N) less than the minimum 
critical clearance value (L.sub.MIN) or an estimated time to damage 
(T.sub.D) less than or equal to the critical time to damage (T.sub.CR), 
the microprocessor 31 initiates the protective actions of step 66 by 
generating an operator alarm and then returns program control to step 60. 
Otherwise, program control returns directly to step 60. This process is 
performed for each shroud segment 13 of each turbine blade row 10. 
In either of the last two embodiments of the invention just described, it 
may be necessary to provide a second sensor 21 in a diametrically opposite 
position to the first sensor 21 in order for the microprocessor 31 to 
identify cases where the center line of the rotor 16 has moved relative to 
the center line of the turbine seal 15. A corresponding increase in the 
average shroud segment clearance (L.sub.N) of the shroud segment 13 
diametrically opposite to a shroud segment 13 experiencing a decrease in 
average shroud segment clearance (L.sub.N) indicates movement of the 
center line of the rotor 16 rather than a reduction and the seal 15. Four 
sensors 21 spaced at 90 degree intervals around the seal 15 circumference 
will detect both horizontal and vertical movement of the rotor 16. 
Differential expansion, that is, a shifting of the steam turbine 18 of FIG. 
3 along its axis due to expansion of the turbine rotor 16 relative to the 
turbine seal 15 can create problems with respect to the application of the 
system 30 of this invention. The shroud segment 13 may be shifted out from 
under the sensor 21. An alternate embodiment of this invention, 
illustrated in FIG. 9, is designed to compensate for the problems 
associated with differential expansion. FIG. 9 is a cross-sectional view 
of the turbine blade row 10 arrangement similar to the arrangement 
depicted in FIG. 4. This embodiment of the invention, however, utilizes 
two sensors 21, each of them situated within the seal 15 in the plane of 
the turbine blade row 10. The positioning of the sensors 21 within the 
seal 15 ensures that at least one sensor 21 will always be positioned 
above a portion of the shroud segment 13 even in the most severe cases of 
differential expansion. Two sensors 21 are provided for each turbine blade 
row 10 of the steam turbine 18. 
The operation of system 30 shown in FIG. 5 in conjunction with the 
embodiment of the invention as depicted in FIG. 9 may be implemented as 
illustrated in the flow chart of FIG. 10. The flow chart begins at step 70 
where the microprocessor 31 of FIG. 5, through known data acquisition 
techniques, samples the shroud clearance measurements a number of times 
along each shroud segment 13 under dynamic conditions through each of the 
sensors 21 of FIG. 9. Average shroud segment clearance values (L.sub.N1, 
L.sub.N2) are calculated in step 71 corresponding to the shroud clearance 
measurements sampled by each of the sensors 21. In step 72, the average 
shroud segment clearance values (L.sub.N1, L.sub.N2) are compared to a 
clearance validity value (L.sub.V) which corresponds to a maximum valid 
clearance reading based upon the minimum outer dimensions of the turbine 
blades 12 and shroud segments 13. The clearance validity value (L.sub.V) 
is stored in a data base structure. An average shroud segment clearance 
value (L.sub.N1, L.sub.N2) greater than the blade row 10 has shifted 
completely out from underneath the sensor 21 thus rendering invalid the 
average shroud segment clearance value (L.sub.N1, L.sub.N2) corresponding 
to that sensor 21. If this comparison indicates an average shroud segment 
clearance value (L.sub.N1, L.sub.N2) greater than the clearance validity 
value (LV), program control continues with step 73 where the valid average 
shroud segment clearance value (L.sub.N1, L.sub.N2) is selected. Program 
control then continues with one of the steps 42, 52 or 62 of one of the 
flow charts of FIGS. 6, 7 or 8, respectively. 
If neither average shroud segment clearance value (L.sub.N1, L.sub.N2) is 
determined to be invalid from the comparison in step 72, program control 
continues with step 74 where the microprocessor 31 analyzes the average 
shroud segment clearance values (L.sub.N1, L.sub.N2) in order to detect a 
simultaneous change in these values. A simultaneous change in the average 
shroud segment clearance values (L.sub.N1, L.sub.N2) indicates that the 
turbine blade row 10 has moved in relation to both of the sensors 21 
within a plane perpendicular to the plane of the seal 15. This is 
significant in relation to the embodiment of the invention illustrated in 
FIG. 9 in that movement of the shroud segment 13 within a plane 
perpendicular to the plane of the seal 15 will result in one sensor 21 
detecting clearance values greater than the actual shroud segment 13 
surface to seal 15 clearance because of the placement of the sensors 21 at 
the outer portion of the shroud segment 13. If a simultaneous change in 
the average shroud segment clearance values (L.sub.N1, L.sub.N2) has 
occurred, then the microprocessor 31, in step 75, selects the lesser of 
the two average shroud segment clearance values (L.sub.N1, L.sub.N2) which 
is representative of the actual shroud segment 13 surface to seal 15 
clearance. Program control then continues with one of the steps 42, 52 or 
62 of one of the flow charts of FIGS. 6, 7 or 8, respectively. If a 
simultaneous change in the average shroud segment clearance values 
(L.sub.N1, L.sub.N2) has not occurred, the microprocessor 31, in step 76, 
averages the average shroud segment clearance values (L.sub.N1, L.sub.N2) 
in order to obtain a single average shroud segment clearance value 
(L.sub.N) Program control then continues with one of the steps 42, 52 or 
62 of one of the flow charts of FIGS. 6, 7, or 8, respectively. 
While the present invention has been described in connection with an 
exemplary embodiment thereof, it will be understood that many 
modifications and variations will be readily apparent to those of ordinary 
skill in the art. This disclosure and the following claims are intended to 
cover all such modifications and variations.