FIELD OF INVENTION
This invention relates to an apparatus for sensing defects of magnetic boiler tube.
BACKGROUND OF THE INVENTION
With use, boiler waterwall and Super heater/Reheater tubes deteriorate due to erosion, corrosion, thinning, pitting, oxidation, cracking etc. For in-situ condition assessment of boiler tubes, presently couplant based spot ultrasonic thickness measurements of boiler tubes at certain expected locations and interval of boiler tube length are carried out in accessible region of the boiler. In practice, it has been found that the location of defects and thinning is not necessarily confined to any particular predictable location (spot) on the furnace-side periphery of the boiler tubes. Hence, a device is required which can sense defects like loss of thinning etc anywhere around the inner and outer periphery and on continuous length of the tube.
OBJECTS OF THE INVENTION
Primary object of the present invention is to provide an apparatus for sensing defects of boiler tube of magnetic material, which is simple in construction.
Another object of the present invention is to provide an apparatus for sensing defects of boiler tube of magnetic material, which is cost effective and reliable.
Further object of the present invention is to provide an apparatus for sensing defects of boiler tube of magnetic material, which is efficient.
Yet another object of the present invention is to provide an apparatus for sensing the defects on the ID and/or OD of the boiler tubes of magnetic material in in-situ condition.
STATEMENT OF INVENTION
According to this invention, there is provided an apparatus for sensing defects of boiler tubes of magnetic material from the outer surface comprising of an exciter generating a magnetic field to penetrate through the tube, which is received back by atleast a pair of receivers in close proximity with said tube, wherein said receivers are located adjacent to the exciter.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:
Fig. 1 shows apparatus of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWING
The present invention discloses apparatus for sensing qualitative defects of boiler tube of magnetic material from the outer surface, which comprises of an exciter (1) to be excited with the excitation voltage of an excitation system (11) to generate a magnetic field. The excitation system is having frequency for ex. 20 Hz to 10 KHz, upto 60V AC voltage and upto 2.5A current. The magnetic field penetrates through the magnetic tube under consideration and emerges from the other surface of said tube. Said magnetic field on its way back to the excitation system penetrates through other parts of the tube, which is received by sensors constituting receivers in close proximity with the tube. The pair of receivers can vary, which is decided by the coverage length of tube/accuracy required. The receivers in differential configuration generate voltage. This voltage is processed through signal conditioner to generate conditioned signal, which is forwarded to display unit so as to obtain difference of magnetic intensity and phase angle. The difference is displayed on the oscilloscope for analysis. The defects may be on outer and inner surfaces of tubes being scanned.
The penetrative magnetism based apparatus can be mounted on crawler for condition assessment of the boiler tubes. It is applicable for both waterwall and Super Heater /Reheater tubes, which are magnetic. The
sensor is based on penetrative magnetism to sense the defect including thickness loss along entire cross section of the tube including its ID. It senses the distortion in the incidental electromagnetic field due to changes in the wall thickness of tube on account of pits, thinning, geometrical distortion like bulging, crack, deposits or any other flaws causing inhomogeneity in magnetic field. The distorted field is picked up by the sensor and displayed on the screen of oscilloscope for interpretation and analysis. The magnetic crawler mounted with the present invention is used for continuous sensing of the tube condition for their entire length when in motion. The crawler can go vertically, up and down and even overhead on the tubes. The system sensor does not need to be in direct contact with the tube.
The present invention can be moved manually or mounted on crawler for sensing various defects on ID and/or OD of the boiler tubes.
Typical Test Results Test Setup Line Diagram
(Figure Removed)
Fig. 2: Line-diagram of Test Set-up

Tube dimensions : Tubes with OD -60mm to 65 mm with
thickness 6.0 mm to 7.0 mm
Tube material : Magnetic
Probe(Apparatus) Gap between Probe and outer
Setting surface -2 to 5 mm
No. of Sensors Variable
Position of sensors/ : Adjustable and on outer surface of the
Exciters tube
Scanning Speed : Approx. 6 meters/min (on outer surface
of the tube).
1. Operating parameters
Excitation Current : £ 500 mA but less than 600 mA Excitation frequency : between 35 to 40 Hz (Sine wave)
a) Test Observations:
Boiler tube with minor change in wall thickness & free from any defect:
Display on oscilloscope shows a periodic wave shape with small change in amplitude. These small changes in amplitude were noticed because of small thickness changes, which was verified by ultrasonic thickness gauge. A change in tube thickness upto 0.25 mm was sensed by the apparatus by showing small change in amplitude on display unit/oscilloscope .
b) Test Observations :
Boiler tube with minor change in wall thickness & flat bottom holes of dia 2 mm & depths > 1 mm- 1.5 mm drilled on inner surface of the tube.
Display on oscilloscope shows a periodic wave shape with small change in amplitude in the region with no holes as (a) above. There was
significant change in amplitude as well as in phase in the region with holes (> 1.0 mm depth) in the inner surface.
2. Operating parameters
Excitation Current : > 600 mA Excitation frequency : between 25 to 35 Hz (Sine wave)
c) Test Observations:
Boiler tube with minor change in wall thickness & flat bottom holes of dia 2mm 8s depths > 1 mm- 1.5 mm drilled on inner surface of the tube.
Display on oscilloscope shows a periodic wave shape with small change in amplitude in the region with no holes as (a) above .But there was major change in amplitude as well as in phase in the region with holes (> 1.0 mm depth & 2 mm dia ) in the inner surface . The changes were greater than (b) as above.
3. Operating parameters
Excitation current : > 500 mA but less than 600mA
Excitation frequency : between 35 to 40Hz (sine wave)
Boiler Tube cut in full length and joined (welded) at one end to form one double length halved tube.
(d) Display on oscilloscope shows periodic wave shape with minor change in amplitude as at (a) above for the region without weld. In the region of weld joint there is a major change in amplitude and phase reversal of approx 180 deg.
(e) Same test as at (d) above with excitation current > 600mA and excitation frequency 25 to 35 Hz (Sine wave) showed an increase in amplitude from result at (d) above as seen on oscilloscope.
4. Operating parameters
Excitation current : ≥ 600mA
Excitation frequency : between 25 to 35Hz (Sine wave)
(f) Boiler tube cut in half in full length and defect (1.5 mm deep and 25 mm long cut circumferentially and longitudinally in the inner surface). Display on oscilloscope shows minor changes in amplitude in periodic wave for region without defect as (a) above but significant change in amplitude and phase angle in the wave shape for region with longitudinal and circumferential defect.
(g) With excitation current >500mA but less than 600 mA and excitation frequency 35 Hz to 40Hz (Sine wave) the display on oscilloscope was similar as (f) above but with decrease in amplitude.
5. Operating Parameters
(h) Excitation current: : ≥ 600 mA
Excitation frequency : between 25 to 35 Hz (sine wave)
Boiler tube with bulging defect, circumferentially crack (about 50 mm) and gradual thinning was scanned.
Display on oscilloscope shows small change in amplitude for region with no defects. For defective regions their were large change in amplitude for gradual change in thinning defect and large change in amplitude along with phase change for bulging and crack defects.
(i) The same boiler tube was subjected to excitation current ≥ 500mA but less than 600mA and excitation frequency between 35 to 40Hz (sine wave).
Display on oscilloscope showed same trace as at (h) above, but with reduced amplitude and phase angle.
It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

WE CLAIM;
1. An apparatus for qualitative sensing of defects of boiler tube of magnetic material from the outer surface comprising of an exciter generating a magnetic field to penetrate through the tube, which is received back by atleast a pair of receivers in close proximity with said tube, wherein said receivers are located adjacent to the exciter.
2. An apparatus as claimed in claim 1, wherein said exciter is excited with the excitation voltage of an excitation system.
3. An apparatus as claimed in claim 1 or 2, wherein said receivers comprise of sensors in close proximity with the tube.
4. An apparatus as claimed in claim 3, wherein said receivers are connected to an oscilloscope through signal conditioners.
5. An apparatus as claimed in claim 4, wherein said oscilloscope displays difference in magnetic intensity and phase angle.
6. An apparatus for sensing defects of boiler tube of magnetic material substantially as herein described and illustrated.