US3088027A

US3088027A - Reference gage for radiographic examination of tubing

Info

Publication number: US3088027A
Application number: US841419A
Authority: US
Inventors: Martin B Graham
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-09-21
Filing date: 1959-09-21
Publication date: 1963-04-30
Anticipated expiration: 1980-04-30

Description

April 30, 1963 M. B. GRAHAM REFERENCE GAGE FOR RADIOGRAPHIC EXAMINATION OF TUBING Filed Sept. 21, 1959 2 Sheets-Sheet 1 lllll LU CAUBRATION CURVE INVENTOR. Mwmv 5. flaw/1M ,4 7'7'0/P/VE Y5 2.0 40 mm THKKNESS EEMMNWG l. 0 HEE C523 April 30, 1963 M. B. GRAHAM REFERENCE GAGE FOR RADIOGRAPHIC EXAMINATION OF TUBING Filed Sept. 21, 1959 2 Sheets-Sheet 2 1477KB/s675 United States Patent O 3,088,027 REFERENCE GAGE FOR RADIOGRAPHIC EXAMINATION OF TUBING Martin B. Graham, 214 Broadway, Wakefield, Mass. Filed Sept. 21, 1959, Ser. No. 841,419 3 Claims. (Cl. 250-65) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes withoutthe payment of any royalties thereon or therefor.

This invention relates to a method of nondestructively inspecting for internal pitting, boiler tubes or other boiler parts.

When a boiler is in operation, erosion pits develop on the internal surfaces of the tubes and deepen in time till they are serious. When the thickness of boiler tube wall remaining beneath any pit or pits is so small that it lacks the strength to withstand operating pressure inside the tube, the weakened part of the tube ruptures. The severity of the pitting is not readily predictable because it is in part a function of the time distribution of operating temperature and pressure, impurities in the feed water, frequency of peak loading and overloading of the boiler and other widely ranging variables. It

.has been common practice to carry out a periodic routine boiler inspection by cutting sample or representative tube sections from the boiler, sectioning them lengthwise, removing the mineral matter that deposited on the inside surface of the tube, and then measuring the depths of the severest pits with a micrometer. This method is very expensive and time consuming and as a direct consequence the boiler tube sampling cut from the boiler is small, often too small.

The degree of care that needs to be exercised in maintaining marine boilers generally exceeds that for stationary boilers to minimize breakdowns at sea. Ships are routinely overhauled periodically and during each overhaul the boilers are inspected for evidence of latent weaknesses and incipient failures. The conventional inspection method, namely, that described above, for determining the severity of pitting, costs on the order of several thousand dollars per ship, but for want of a better method has continued in use for a long time.

An object of this invention is to provide an objective non-destructive, and inexpensive method of inspecting the inside surfaces of boiler tubes for dangerously deep pitting without removing tubes or other sections from the boiler.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawingswherein:

FIG. 1 shows a pair of flat bars, face-to-face, of the same-material and thickness as the boiler tubes to be inspected, one of the bars having a row of flat bottomed holes of graduated depths,

FIG. 2 shows a section of an unused boiler tube in perspective, of the same material and wall thickness as the boiler tubes to be inspected, partly broken away, and having a row of flat-bottomed holes of graduated depths,

FIG. 3 shows the relationship between boiler tubes, X-ray machine, and film,

FIG. 4 shows the foil shielding on the film, and

FIG. 5 shows a plot useful for accurately measuring a pit.

This invention utilizes radiography for boiler parts inspection. The radiography equipment may include V assaoz'z Patented Apr. 30, 1963 ice portable X-ray equipment or a radioactive material as the source of penetrating rays. One obstacle that has heretofore appeared insurmountable to the application of radiography to this problem was that mineral matter deposited on the inside walls of boiler tubes would attenuate the penetrating rays appreciably and since the deposited material is nonuniform in character and thickness, a radiographic recording would manifest misleading variations in density on the recording tending to show pits where there were none and no pit where a deep one existed. I discovered by experiment that the deposited material does not interfere with radiographic inspection. Experiments were carried out as follows. Material that had been deposited on the inside walls of boiler tubes was obtained and was pulverized. Pairs of

rectangular fiat bars

11, 12 of material from which boiler tubes are fabricated were utilized. One bar 12 of each pair was machined to form a row of flat-bottom holes of graduated depth according to a predetermined schedule and ranging from zero to near percent of the bar thickness. The holes of the one bar 12 were filled with the pulverized material and another bar 11 without holes was attached over the face of bar 12 formed with the holes with an adhesive type tape to hold the pair of bars together. The pair of

bars

11 and 12 as described above and another pair of identical bars but whose holes were free of any solid materialwere radiographed side-byside simultaneously in the direction of the holes using one energy source and one recording film. Corresponding hole images were compared with a densitometer. There was no significant difference. Succeeding experiments confirmed the finding that the material deposited on the inside of boiler tubes would not interfere with radiographic examination. A commercial densitometer, namely, an H and D Densitometer, manufactured by Eastman Kodak Company was used. The hole diameter in the bars was made about A" to facilitate densitometer readings.

FIG. 2 there is shown a short length of boiler tube stock 15, not previously used in a boiler; its inner and outer surfaces being smooth and radiographic examination having indicated that there are no significant unseen faults therein. A row of fiat bottom holes 16 of graduated depths according to a predetermined schedule are machined radially into the outer surface and in a line parallel to the axis. The tubing 15 is used as a gage as described below.

In carrying out a radiographic inspection of boiler tubes of known material and known wall thickness in accordance with this invention, either a pair of

bars

11 and 12 as in FIG. 1 of the same material and thickness as the tubes to. be inspected, or a length of boiler tube as in FIG. 2 of the same material and thickness as the tubes to be inspected is used as a gage. The

gage

11, 12 or gage 15 or other, adjacent the boiler tubing to be inspected. In

the boiler section shown in FIG; 3, the spacing between tubes is too smallto receive atube section 15 of FIG. 2 therebetween. Therefore, the gage of FIG. 1 is used. Though the gage of FIG. 2 is perhaps a more precise standard for comparison, the gage of FIG. 1 has proved to be satisfactory and has the advantage of being smaller and lighter in weight. A portable X-ray machine 20 is supported in front of the boiler tube sections and gage to be radiographed. A kv. Baltospot portable X-ray machine manufactured by Balteau Electric Company, Stamford, Connecticut, was used successfully. A radioactive material suitable for this purpose is radioisotope iridium 192. The gage is oriented so that the holes therein are generally aligned with the direction of radiation. Film 22 is supported on the side of the boiler tubing and gage opposite that facing the X-ray machine. A

sheet of lead foil 24 about .005 inch thick is supported over both surfaces of the film as shown in FIG. 4 to shield the film against scatter radiation. A flexible plastic film cassette, not shown, supports the film and foil, and the combination is supported in place against the tubes with an adhesive type of tape. The flexible. cassette lends itself to manipulation in the close quarters in a boiler. Type AA commercial film is recommended for the radiographs. Double film technique, one film over the other, maybe used so that one film may be retained with the boiler records or general maintenance records.

The following radiographing details are provided as a guide. In radiographing A or fireside tubes which have nominal wall thickness of 0.134 inch or a total metal thickness of 0.268 inch, a film cassette 14 x 17 inches is attached behind the tubes, the

reference gage

11, 12 is taped to the cassette between two tubes located in the middle of the cassette, the X-ray machine described above is supported with a 3 foot space between the X-ray tube and the nearest boiler tube and the exposure time is 200 seconds. The exposed films are developed in the standard manner at 68 Fahrenheit for 5 minutes using commercial developer.

The radiograph is inspected visually for the images of the deepest pits which are the darkest spots. The density of these spots are measured with a densitometer. Then the spots representing the holes in the gage are measured with the densitometer. By comparing pit image density with hole image densities, the depth of the pits can be approximated fairly closely.

A more accurate method is to obtain the density reading for the image of each hole in the gage, the density readings for the area around each hole, for each hole subtract from the density reading for the hole the density reading for the area around that hole, then plot a curve as in FIG. 5 with percentage wall thickness remaining beneath the hole as abscissa and with difference in density for each hole obtained as above, as ordinate. Then obtain the density reading for the spot representing the deepest pit and the density reading for the area surrounding that spot, subtract from the density reading for the pit, the density reading for the area surrounding the pit, then from the curve obtain the percent-age wall thickness re maining beneath the pit.

This method is applicable to parts of a boiler other than tubing.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. A method for non-destructively inspecting the inside surface of a portion of a boiler tube assembled in and forming part of a boiler, to detect dangerously deep pits in the tube Wall, after the boiler has been in operation for a time, where the interior of the tube is inaccessible for direct inspection by means of inserted inspection devices, and where mineral deposits on such interior surface may fill and cover such pits in such interior surface, which comprises disposing, at one side of and exterior to said tube portion a source of penetrating rays capable of penetrating completely through the walls of said tube portion, and directed toward said tube portion to penetrate the same, disposing a recording film of the type responsive to such rays exteriorly of said tube portion in a position to intercept the rays from such source which penetrate such tube portion, also disposing adjacent to said tube portion and between said source and said film in the path of some of said rays directly from said source and at approximately the same distance from said source as said tube portion, a member of the same material as said tube portion and having in the path of such rays a thickness graduated in predetermined thicknesses between about percent and 200 percent of the thickness of the wall of said tube portion, and activating said source to record images on said film of the tube portion and member concurrently, whereby upon a comparison of the recordings on said film of said tube portion and member, of the images of any random pits in the wall of said tube portion and the images of the portions of different thicknesses of said member, one can ascertain the presence of dangerously deep pits in the wall of said tube portion.

2. A method as defined in claim 1 wherein said recording film is a film transparency and further including measuring with a densitometer the densities of images representing pits in the tube and the densities of images of said graduated thicknesses.

3. A method for non-destructively inspecting the inside surface of a portion of a boiler tube assembled in and forming part of a boiler, to detect dangerously deep pits in the tube Wall, after the boiler has been in operation for a time, where the interior of the tube is inaccessible for direct inspection by means of inserted inspection devices, and where mineral deposits on such interior surface may fill and cover such pits in such interior surface, which comprises disposing a film of a type responsive to rays of the type which will penetrate the material of said tube portion, behind and exteriorly of said tube portion but adjacent thereto and behind a member adjacent to said tube portion and having thicknesses in dilferent portions thereof varying from about 100 percent and 200 percent of the thickness of the wall of said tube portion, and directing penetrating rays of said type from a source disposed exteriorly of said tube portion and said member, and at about the same distance from both tube portion and member, part of said penetrating rays through said tube portion to said film and part of said penetrating rays through said member to said film, whereby upon a comparison of the recordings on said film of said tube portion and member, of the images of any random pits in the walls of said tube portion and the images of the portions of different thicknesses of said member, one can ascertain the presence of dangerously deep pits in the wall of said tube portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,399,650 Moyer May 7, 1946 2,719,926 Procter Oct. 4, 1955 2,812,440 Hartman Nov. 5, 1957 2,957,987 Arnesen Oct. 25, 1960 2,975,281 Williams Mar. 14, 1961

Claims

1. A METHOD FOR NON-DESTRUCTIVELY INSPECTING THE INSIDE SURFACE OF A PORTION OF A BOILER TUBE ASSEMBLED IN AND FORMING PART OF A BOILER, TO DETECT DANGEROUSLY DEEP PITS IN THE TUBE WALL, AFTER THE BOILER HAS BEEN IN OPERATION FOR A TIME, WHERE THE INTERIOR OF THE TUBE IS INACCESSIBLE FOR DIRECT INSPECTION BY MEANS OF INSERTED INSPECTION DEVICES, AND WHERE MINERAL DEPOSITS ON SUCH INTERIOR SURFACE MAY FILL AND COVER SUCH PITS IN SUCH INTERIOR SURFACE, WHICH COMPRISES DISPOSING, AT ONE SIDE OF AND EXTERIOR TO SAID TUBE PORTION A SOURCE OF PENTRATING RAYS CAPABLE OF PENETRATING COMPLETELY THROUGH THE WALLS OF SAID TUBE PORTION AND DIRECTED TOWARD SAID TUBE PORTION TO PENETRATE THE SAME, DISPOSING A RECORDING FILM OF THE TYPE RESPONSIVE TO SUCH RAYS EXTERIORLY OF SAID TUBE PORTION IN A POSITION TO INTERCEPT THE RAYS FROM SUCH SOURCE WHICH PENETRATE SUCH TUBE PORTION, ALSO DISPOSING ADJACENT TO SAID TUBE PORTION AND BETWEEN SAID SOURCE AND SAID FILM IN THE PATH OF SOME OF SAID RAYS DIRECTLY FROM SAID SOURCE AND AT APPROXIMATELY THE SAME DISTANCE FROM SAID SOURCE AS SAID TUBE PORTION, A MEMBER OF THE SAME MATERIAL AS SAID TUBE PORTION AND HAVING IN THE PATH OF SUCH RAYS A THICKNESS GRADUATED IN PREDETERMINED THICKNESSES BETWEEN ABOUT 100 PERCENT AND 200 PERCENT OF THE THICKNESS OF THE WALL OF SAID TUBE PORTION, AND ACTIVATING SAID SOURCE TO RECORD IMAGES ON SAID FILM OF THE TUBE PORTION AND MEMBER CONCURRENTLY, WHEREBY UPON A COMPARISON OF THE RECORDINGS ON SAID FILM OF SAID TUBE PORTION AND MEMBER, OF THE IMAGES OF ANY RANDOM PITS IN THE WALL OF SAID TUBE PORTION AND THE IMAGES OF THE PORTIONS OF DIFFERENT THICKNESSES OF SAID MEMBER, ONE CAN ASCERTAIN THE PRESENCE OF DANGEROUSLY DEEP PITS IN THE WALL OF SAID TUBE PORTION.