Scanning apparatus and method for inspection of header tube holes

An apparatus for scanning header tube holes includes a probe assembly designed to engage the surface of the header encircling a header tube hole. The probe assembly includes a spring mounted sensor, in a preferred arrangement, an eddy current probe. A mechanism is provided for supporting and positioning the probe assembly. The mechanism is adapted, in operation, with resilient rings engaging the tube stub. A drive device is provided for imparting axial and circumferential movement to the probe relative to the surface of the hole which is to be inspected.

BACKGROUND OF THE INVENTION 
The present invention relates to the inspection of header tube hole 
surfaces and, more particularly, to a new and improved scanning apparatus 
and method for inspecting surfaces of header tube holes. 
Headers are used extensively in steam boilers as a means for joining fluid 
circuits and for distributing fluid to fluid circuits. Such headers 
typically comprise a large-diameter, heavy-walled cylindrical shell, 
circular in cross section, having multiple straight or bent tube stubs 
which extend partially into holes formed through the wall of the header. 
The tube stubs are fixed to the header via rolled tube joints or by welded 
tube connections. The end on the portion of each tube stub extending 
outside of the header is designed to be connected to a component of a 
fluid circuit, usually in the form of a tube which is welded to the tube 
stub. 
Cracks that initiate from the header surface surrounding the tube holes in 
steam headers of boilers in fossil fuel burning electrical power plants 
can lead to failures which can cause costly unscheduled plant outages. If 
cracks are detected in early stages, however, plans can be made to repair 
or replace the header during scheduled maintenance and repair outages. 
Detection and characterization of cracks near the surface surrounding 
holes in structural components, by nondestructive testing, is often a key 
factor in assessing the condition and remaining useful life of a 
component. Crack characterization, i.e., measurement or estimates of 
depth, length and location of cracks that initiate from the surface of 
holes in headers is used with additional information to predict remaining 
life and to plan repair or replacement of the headers. 
Cracks have been detected with boroscopes that use fiber optics technology. 
Access to the hole is obtained by cutting and removing a section of the 
tube or tube stub that leads into the header. The boroscope is manually 
inserted into the hole and rotated to inspect the entire circumference. 
However, proper focusing of the boroscope is difficult to achieve due to 
instrument sensitivity to changes in probe-to-surface distance. Therefore, 
quality and reliability of data is less than adequate without a mechanism 
which permits a controlled scan that maintains a relatively constant 
position and alignment of the probe. The same, as well as other 
difficulties, arise with regard to the use of other sensors including 
ultrasonic transducers and eddy current probes. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a novel apparatus for 
supporting and positioning an inspection probe or the like within a header 
tube hole. 
Another object of this invention is to provide means for examining 
successive portions of the surface of the header surrounding the header 
tube hole with an inspection probe or the like. 
It is a still further object of this invention, to provide a method of 
inspecting header tube holes by performing rotary or axial scans, or both, 
to detect cracks and to calculate the depth of the cracks. An eddy current 
inspection technique is preferably utilized to obtain data for such 
calculations. 
In accordance with a preferred embodiment of the apparatus of this 
invention, a probe is mounted to a tube stub mounting mechanism, connected 
to a drive which rotates and translates the probe within the header tube 
hole. The mounting mechanism is interconnected between the probe assembly 
and the drive. The mounting mechanism is designed to be detachably 
connected to the tube stub for supporting the inventive apparatus. The 
probe includes spring means for pressing a probe sensor into engagement 
with the surface of the header surrounding the header tube hole. 
In a preferred embodiment, the tube mounting mechanism includes a central 
hollow cylinder connected to the spring means. The spring means, and hence 
the sensor, is moveable responsive to movement of the hollow cylinder. A 
flanged tube is slidably disposed on the hollow cylinder. The tube 
mounting mechanism further includes a pair of expansible rings on the 
tubes and means on the tube for expanding the rings into supporting 
engagement with the tube stub. The hollow cylinder may be constructed of a 
flexible material for scanning bent tube stubs. The drive includes means 
for rotating and axially translating the hollow cylinder and the sensor 
therewith. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming part 
of this specification. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawing and descriptive matter in which 
there is illustrated and described a preferred embodiment of the invention 
.

DETAILED DESCRIPTION 
Referring now to the drawings in detail and in particular to FIG. 1, there 
is shown a cylindrical header 20 which has a plurality of holes 22 
extending through the thickness of the header wall from the inner surface 
16 to the outer surface 18 of the header 20. A plurality of tube stubs 24 
are received within the holes 22 and are fixed to the header 20 by 
welding, as illustrated by welds 26 in FIG. 2. Alternatively, the tube 
stubs 24 may be fixed to the header by formation of a rolled joint. An end 
28 of each tube stub 24 is located within the tube hole 22 at a position 
intermediate the inner and outer surfaces 16, 18 of the header 20 and 
distant from the inner surface 16 of the header 20. 
The opposite ends 29 of each tube stub 24, which are located outside of the 
header 20, are designed to be connected to tubes (not shown) of an 
external fluid circuit to provide fluid communication between the header 
20 and the fluid circuit for joining fluid streams passed to the header 20 
or for distributing fluid streams passed from the header 20. 
On occasion, it becomes necessary to disconnect the tubes of the fluid 
circuit from the tube stubs 24 and to inspect the peripheral surface 14 
surrounding the header tube hole 22 and located intermediate the inner 
surface 16 and the end 28 of the tube stub 24 which is seated within the 
wall of the header 20. 
Referring to FIG. 3, an inspection apparatus, according to a preferred 
embodiment of the invention, includes a probe assembly 30 which is capable 
of being inserted through the tube stubs 24 of the header 20 and into the 
header tube holes 22. The probe assembly 30 is mounted to a tube stub 
mounting mechanism 40 which is designed to engage one of the tube stubs 
24. Both the probe assembly 30 and the tube stub mounting mechanism 40 are 
connected to a drive carriage 50 which is operable to move the probe 
assembly 30 as is more particularly described hereafter. 
The probe assembly 30 is provided with sensors 31, 32 supported for 
movement on one end of substantially parallel spring bars 33, 34, 
respectively. Each spring bar 33, 34 at its opposite end is secured, in 
cantilevered fashion, by a support plate, referred to herein as clamp 
block 35 composed of a number of plates which are bolted together. The 
clamp block 35, which is located at the leading end of the probe assembly 
30, in turn, is mounted to an elongated support bar 36 which extends, away 
from the leading end of the probe assembly 30, between and substantially 
parallel to the spring bars 33, 34 and terminates beyond the spring bars 
33, 34 in a C-shaped stop plate. The stop plate is composed of a crossbar 
37 which extends substantially perpendicularly relative to the elongated 
axis of the support bar 36 and stop lugs 38, 39 at opposite ends of the 
crossbar 37 which overlap the ends of the respective spring bars 33, 34. 
An internally-threaded, tubular socket 27 is provided on the side of the 
stop plate opposite the support bar 36. 
The tube stub mounting mechanism 40 is connected to the probe assembly 30, 
in the illustrated embodiment, by threaded engagement of the threaded end 
of a hollow cylinder 42 and the socket 27. As shown in FIG. 3, a flanged 
tube is mounted about the hollow cylinder 42. The hollow cylinder 42 may 
be moved freely relative to the flanged tube. The flanged tube has a 
flange 25 located at an end near the end of the hollow cylinder 42 which 
is coupled to the socket 27. Resilient locking rings or collars 43, 47, a 
sleeve 45, and spacer washers 44, 46, 48 are slidably received upon the 
tube 41 of the flanged tube intermediate the flange 25 of the flanged tube 
and an adjusting knob 49. The adjusting knob 49 is threadably engaged to 
the tube 41. The resilient locking collars 43, 47, in the preferred 
embodiment, have a diameter slightly larger than the sleeve 45, flange 25 
and spacer washers 44,46, 48 and is designed to approximate the internal 
diameter of the tube stubs 24. 
The drive carriage 50 comprises a pair of generally parallel mounting 
plates 51, 52 with guide rods 53, 54 which extend parallel to each other. 
The end of the flanged tube opposite the flange 25 is engaged to mounting 
plate 51 provided as part of the drive carriage 50. The hollow cylinder 42 
freely passes through mounting plate 51 and is operatively mounted to 
mounting plate 52 in a manner more particularly described hereafter. 
The hollow cylinder 42 is operatively attached to a gear 55 for rotation 
via a bearing mounted on the mounting plate 52. The gear 55 is connected 
to a rotational drive motor 60 or other means for imparting rotation to 
the hollow cylinder 42 via an intermediate gear 56. 
Mounting plate 52 is slidably disposed on the guide rods 53, 54 and has a 
pair of bearings 57, 58 which are slidable along the guide rods 53, 54, 
respectively, to allow the mounting plate 52, with the hollow cylinder 42, 
gears 55, 56 and motor 60, to move rectilinearly and parallel to the guide 
rods 53, 54. 
A pinion gear 61 drives a drive rack 62 to rectilinearly advance and 
retract the mounting plate 52. The drive rack 62 is connected to mounting 
plate 52 and freely extends through mounting plate 51. The pinion gear 61 
is mounted on a drive shaft 63 which may be driven by suitable means, for 
example, a hand crank or stepping motor. In addition, a pair of pinion 
gears 61 may be provided on opposite sides of the carriage assembly, 
interconnected by drive shaft 63, for engagement with parallel drive 
racks. 
FIG. 4 is a schematic representation showing the apparatus of the invention 
in position for inspecting the peripheral surface 14 of the header tube 
hole 22 of header 20. 
In operation, the tube stub 24, which is seated within the hole 22 which is 
to be inspected, is disconnected from the fluid circuit to which it is 
normally attached. The probe assembly 30 is inserted through the tube stub 
24 and into the hole 22 until the tube stub mounting mechanism 40 is 
positioned with the tube stub 24. As the probe assembly 30 passes through 
the tube stub 24, the spring bars 33, 34 which resiliently urge the 
sensors 31, 32 outwardly of the support bar 36 are compressed toward the 
support bar 36. On passing tube stub end 28, however, the spring bars 33, 
34 expand outwardly to press the sensors 31, 32, against the peripheral 
surface 14. The outward movement of the spring bars is restricted by 
contact with the stop lugs 38, 39 of the C-shaped stop plate. The stop 
lugs 38, 39 restrict lateral movement of the spring bars 33, 34 beyond the 
inner diameter of the tube stub 24 so that the probe assembly can be 
retrieved from the header 20 without sustaining damage to the probe and 
sensors. The sensors 31, 32 in the preferred embodiment, comprise housings 
having tapered edges to further facilitate retrieval of the probe assembly 
30. 
The adjusting knob 49 is then tightened. The tightening of the adjusting 
knob 49 causes displacement of the sleeve 45, and spacer washers 44, 46, 
48 toward the flange 25. This causes the resilient, expansible rings or 
locking collars 43, 47 to diametrically expand into supporting engagement 
with the tube stub 24. The circumferential contact of resilient locking 
collars which may, for example, be rubber rings, with the tube stub 24, 
facilitates support and reference for the apparatus. 
As illustrated in FIGS. 3 and 4, the hollow cylinder 42 comprises a rigid 
tubular member composed, for example, of type 304 stainless steel. 
In an alternate embodiment, the hollow cylinder 42 may be a flexible 
tubular member, for example, a braided steel tube. Braided steel flexible 
tubes are typically flexible to bending but extremely rigid to torsional 
forces. Utilization of a flexible hollow cylinder will permit the probe to 
be inserted through bent tube stubs. In such case, the tube stub mounting 
mechanism can be modified, as shown in FIG. 5, to omit the central spacer 
washers, and alternatively to include two short sleeves 65, 66. 
The electrical connections to the probe, e.g., electrical lines 21, 23, are 
contained within the hollow cylinder 42. The gears 55, 56 mounted at the 
end of the cylinder 42, link the probe assembly 30 to the rotational drive 
motor 60. The cylinder 42 rotates in the bearing 59 within the mounting 
plate 52. A second motor 64, illustrated in FIG. 4, is operable to drive 
the pinion gear 61 and drive rack 62 to slide the mounting plate 52 along 
the guide rods 53, 54 so that the probe assembly 30 moves in the axial 
direction of the header tube hole. The pinion gear 61 is preferably 
mounted to the mounting plate 51. Control can be provided for both motors 
60, 64 for a coordinated sequence of rotational probe scans at various 
axial positions along the length of the header tube hole. When stepping 
motors are used, relatively precise rotational and axial positioning of 
the probe assembly sensors 31, 32 can be obained. 
Although two springs 33, 34, as illustrated in FIG. 4, are provided to 
support two support sensors 31, 32, there are many applications where only 
one sensor is needed. In such case, a housing fixture that does not 
contain a sensor is fastened to one spring, to provide a counter force to 
balance the other sensor spring. This counter force helps keep the probe 
assembly 30 near the central axis of the hole. The forces of the leaf 
springs tend to move the sensor into engagement with the surface of the 
hole. The sensor housing can be mounted on sponge rubber pads to assist in 
alignment of the sensors. The flat spring and sensor backing, in such 
case, provide sufficient surface area for application of an adhesive bond 
to secure the sponge rubber pad. 
The apparatus of the invention provides a systematic controlled means for 
scanning one or more transducers, or other inspection devices, over the 
surface of a header tube hole. It allows good transducer alignment with 
relatively constant probe-to-tube surface distance throughout the scan. 
Moreover, the use of electronically controlled stepping motors provides 
precise rotational and axial positioning of the sensors. The use of 
separate motors for sensor rotation and axial movement offers particular 
versatility in scanning the surface of the header hole. Rotational 
scanning can be executed without slip rings and without twisting the 
sensor cables. This can be accomplished by programming the stepping motor 
control to return to starting position before repeating the scan at a new 
axial location. Stepping motors, moreover, can be associated with digital 
control to facilitate data acquisition at predesignated locations for 
repeated scans. This provides spacially coherent signal responses which 
are the basis for improved accuracy in the interpretation of sensor data. 
The device of the invention comprises a simple design which can be embodied 
in a light weight structure for ease of use. The expandable rubber rings 
hold the device in place throughout the inspection. Finally, the stop lugs 
of the probe assembly and the tapered sensor housings facilitate the 
removal of the device from a tube hole without damage to the probe, 
cabling or spring mounts. 
The apparatus of the invention has particular utility in the sensing and 
processing of eddy current signal response data to provide an estimation 
of the depths of cracks that initiate from the peripheral surface of 
header tube holes. An eddy current probe is preferably embodied within the 
one or more sensors 31, 32 of the probe assembly 30. The eddy current 
probe may be among one of several commercially available type probes and 
is operatively connected, via conventional electric lines 21, 23 or cable 
extending from the sensor housing, through the hollow cylinder 42. 
The eddy current sensor 31, 32 is scanned along a circumferential path on 
the surface 14 of the hole 22. Eddy currents are induced in the 
electrically conducting header material by applying alternating currents 
to the eddy current coils. The probe is designed so that the components of 
the induced eddy currents are in a direction that is perpendicular to the 
plane of the crack. The well known skin effect associated with the eddy 
currently phenomenon concentrates the eddy currents near the surface of 
the header material. The induced currents tend to flow along the crack 
surfaces around the bottom and ends of the crack. This change in current 
path results in a corresponding change in the alternating magnetic field. 
The change in magnetic field in turn results in a change in the amplitude 
and phase of the voltage observed at the terminals of the coil. 
Commercially available eddy current instruments detect the change in 
amplitude and phase of the voltage to provide a signal response as the 
probe is scanned over a crack. 
Eddy current signal responses caused by variables such as electrical 
conductivity, magnetic permeability and small variations in 
probe-to-material distance can be reduced or minimized by the 
incorporation of two coils. The coils can be positioned and oriented so 
that one coil exhibits a substantially different response to the crack 
than the other. For example, cross-wound coils can be designed and 
oriented so that one coil produces a minimum signal response to the crack 
while the other produces a maximum response. Since the coils are connected 
so that their respective signals subtract the signals caused by 
nondirectional variables such as electrical conductivity, magnetic 
permeability and probe-to-material distance will tend to cancel. Other 
differential coil configurations such as concentric coils and adjacent 
coils of various shapes and sizes can be used to achieve a similar 
reduction of these unwanted signal responses. 
The eddy current probe can also be designed so that a relatively high 
frequency will provide greater resolution than that of a lower frequency. 
A multifrequency eddy current instrument can be used to obtain separate 
responses to two or more adjacent cracks. Appropriate signal responses 
have been obtained, for example, by using a cross-wound coil that is 
shielded with a thin brass foil approximately 0.005 inch thick. The brass 
foil contains a small hole at the position where coil windings cross. 
Relatively low frequencies, e.g. 1-10 KHz, penetrate through the foil. 
With this low frequency excitation, the coil provides a good response to 
increasing crack depth but can be influenced by two or more adjacent 
cracks at a given time. This multiple crack condition causes a signal 
response of greater width and amplitude than would occur for a single 
crack having identical dimensions. Relatively high frequencies, e.g., 500 
KHz, produce a magnetic field that penetrates primarily through the small 
hole in the brass shield. This provides a high resolution indication and 
precise location of each of the cracks. 
Low frequency response to multiple cracks is dependent on the number, 
spacing and lengths of the cracks as well as their respective depths. For 
example, two or more adjacent cracks can cause depth indications that are 
twice that of only one crack. The use of additional higher frequencies 
provides a basis for compensating for the errors in measuring crack depth 
when multiple cracks occur. 
In accordance with a preferred technique for detecting and measuring cracks 
initiating from the surface 14 surrounding a header tube hole 22, the eddy 
current sensors, energized via the electrical lines 21, 23, to induce eddy 
currents in the surface to be inspected, are scanned in a circumferential 
direction by rotating the hollow cylinder 42. The scan may be repeated at 
additional axial locations by axially moving the hollow cylinder via 
operation of the drive rack 62. Signal responses are obtained at each 
location. In particular, two signal responses, such as the in-phase 
component and quadrature component or amplitude and phase is recorded for 
each frequency. 
Various other features may be extracted from the signals and recorded in 
respect of the axial position of the scan. 
The crack depth is then computed by using a formula that compensates for 
the errors caused by the proximity of two or more adjacent cracks of 
variable length and depth. 
An example of a formula for estimating crack depth is given as follows: 
##EQU1## 
In general, the formula is a function of variables as indicated by: 
EQU D=F(A.sub.km, A.sub.ki, d.sub.i, l.sub.j) 
where, 
A.sub.km =Maximum low-frequency signal component, k, acquired within a set 
of rotational scans 
A.sub.ki =Signal amplitude of one of the k components at circumferential 
location X.sub.i 
X.sub.i =Circumferential location of crack, i, determined from the 
high-frequency, high-resolution signal response 
Z.sub.ij =Axial location of the end j of crack i 
d.sub.i =Estimated lateral distance of crack at location X.sub.i, from the 
position X.sub.m, where the maximum occurs, i.e., (X.sub.m -X.sub.i) 
l.sub.j =Estimated length of an adjacent crack at lateral position j, e.g., 
(Z.sub.it -Z.sub.ib) where t represents the top end of crack i and b 
represent the bottom end. 
C.sub.km and C.sub.ki for i=1, 2, . . . N are coefficients. The designation 
k=1 indicates that the amplitudes are acquired from the first component of 
the low frequency signal reponse. 
In the case where there are no adjacent cracks, the formula reduces to the 
simple form: 
EQU D=C.sub.m (A.sub.lm).sup.2 
Similar formuli incorporating additional extracted features such as crack 
length and phase angle can be derived. Although the feature extraction and 
depth calculations can be determined by manual computation, microcomputer 
software can be designed to perform these computations with much greater 
efficiency and reliablity. The weighting coefficients, i.e., C.sub.km and 
C.sub.kj, can be determined by using the least squared error criterion and 
guided random search techniques can be used to estimate coefficients when 
sufficient test sample data are available. 
The technique generates and uses multifrequency eddy current signal 
response data to provide improved estimates of crack characteristics such 
as crack depth. 
A microcomputer can be utilized to facilitate scan control, extraction of 
the numerical value of features and estimation of crack characteristics, 
e.g., depth and length. The preferred eddy current technique permits 
detection of cracks that may be missed by other inspection techniques such 
as penetrant and magnetic particle inspections. It does not require fluid 
coupling that may contaminate components and is relatively fast compared 
to other inspection methods and associated devices.