FR2550333A1 - Apparatus using a source of radiation to measure the wall thickness of pipes during manufacture - Google Patents

Abstract

The apparatus comprises a source of radiation 21 formed by a row of sources of radiation and a detector 22 also in linear form. The radiation from the source passes through a collimator which shapes it into parallel beams. These pass transversely through the tubular component 11 whose wall thickness is to be determined. The length of the source 21 and the radiation detector 22 is greater than the diameter of the component 11 so that the radiation traverses the whole cross-section of the component. The mean wall thickness of the component may be determined from the attenuation recorded by the detector 22.

Description

 The present invention relates to a wall thickness meter of tubular parts and more specifically a meter using a source and a detector of radiation - of radiation there for example - to determine in particular the wall thickness of steel tubes with or without welding in the final stage of manufacturing and without interrupting manufacturing.

 In the conventional manufacture of tubes by rolling in the steel industry, the wall thickness of the tubes must generally be measured with great precision.

In order not to reduce productivity, it is essential that this measurement is carried out on the production line and without interrupting the flow of products. In addition, since the rolling operations take place in part at least at very high temperatures, it is desirable that the measurement of the wall thickness is carried out not only without contact but also at the greatest possible distance from the tube. .

 Figure 1 shows a conventional system for measuring the wall thickness of tubular parts. I1 comprises sources of rays r, designated by 1, 2 and 3, and radiation detectors or sensors 4, 5 and 6. The sources li, 2 and the detectors 4, 5 are mounted on a fixed frame 7, while the source 3 and the detector 6 are mounted on a mobile frame 8. The tubular part 11, the wall thickness of which is to be measured, is transported, transversely to the measuring radii, on a conveyor 9.

 The relative positions of radiation sources and detectors are important factors in such a system. The movable frame 8 must be positioned so that the vertices of the equilateral triangle EFG formed by the measurement beams are located, as can be seen in FIG. 2, on a circle whose diameter represents the average of the nominal outside diameter and the nominal inside diameter of the tubular part 11. The principle of this measurement is described in detail in Japanese patent application 46406/1981 submitted to the Public Inspection. Since, moreover, it is not essential for understanding the invention, it will not be described here.

While the tubular part 11 advances on the conveyor 9, it is permanently subjected to vibrations along the axes Z1 ~ Z2 and Z3 - Z4, see Figure 2. Even adding
2 3 shows an anti-vibration roller (not shown) to the conveyor rollers 9, it is extremely difficult to position the vertices of the equilateral triangle EFG exactly on the circle of average diameter.

Since the additional measures, such as the addition of an anti-vibration roller, to reduce or eliminate the vibration of the tubular part 11 are technically complicated and moreover very expensive, it is generally sufficient, with a conventional system like that shown in FIGS. 1 and 2, to take account, in the measurement results, of an error due to the vibration and called "error by default misalignment".

Japanese patent application submitted to the Inspectorate
Public 114263/1979 describes another method for measuring the wall thickness of a steel tube by means of radiation. According to this method, based on the fact that radiation applied from the outside to a steel tube is attenuated to the maximum when it is tangential to the internal wall of the tube and that it is attenuated to the minimum when the radiation is tangential to the external surface of the tube, the maximum and minimum attenuation points are detected and the wall thickness of the tube is determined from the distance between these two points.

 However, when this method is applied to steel tubes with a wall thickness of 5 or 6 mm to 40 mm, even if the radiation source uses a radioactive material of 30 Curies, the measurement requires at least 20 ms to 1 s because the radiation from the source is generally split. As the steel tube must be stopped during this time, this method is not applicable to the measurement of the wall thickness during the running of steel tubes which are subjected to vibrations during their transport. when taking the image of a steel tube exposed to radiation by a television camera with the radiation emitting slit adjusted to a width of approximately 2 mm, it will be understood that the accuracy of the thickness measurement wall thickness of steel tubes according to this method is several tens of micrometers lower than the accuracy of a steel plate thickness gauge, because the resolution of the television camera is only about 1 mm.

 The invention provides a wall thickness gauge of tubular parts allowing very precise measurements on moving parts, even if the part is subjected to vibrations, or to offsets of another nature, because its principle of measurement is independent of any misalignment.

 According to the invention, a wall thickness meter of tubular parts, comprising a radiation source and a radiation detector, is characterized in that the source and the detector are arranged face to face, with the tubular part between them, and in that the source and the detector have a length, transverse to the axis of the tubular part, which is greater than the external diameter of the part, the arrangement being such that parallel beams of radiation emitted by the source and reaching the detector cross the entire section of the tubular part, which makes it possible to determine the average wall thickness of the part from the attenuation of the radiation detected by the detector.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows of a particular preferred embodiment which is in no way limiting, as well as from the appended drawings, in which
- Figure 1 schematically shows a conventional system for measuring the wall thickness of a tubular part (already described);
- Figure 2 schematically shows a detail of the system of Figure 1;
- Figures 3a and 3b are schematic representations used to explain the principle of the meter according to the invention;
- Figure 4a is an axial section of a linear radiation source according to an exemplary embodiment;
- Figure 4b is the front view of a collimator like that used with the radiation source of Figure 4a;
- Figure Sa is a side elevational view of a linear detector;
- Figure 5b is the front view of a collimator like that used on the linear detector of Figure 5a;
- Figure 6 is a diagram for explaining the functional relationship between the wall thickness of a tube and its position;
FIG. 7 is a graph indicating the relationship between the wall thickness and a measurement quantity supplied by the detector;
- Figure 8 is a schematic perspective view of an exemplary arrangement of a meter according to the invention with respect to a tube whose wall thickness is to be measured;
- Figures 9a and 9b are a side elevation respectively a schematic front view of a stretch-reducer with two rollers per station for the manufacture of tubes, as an example of application of a meter according to the invention; and
- Figures 10a and lOb are similar views of a reducer stretcher with three rollers per station.

The measuring principle according to the invention will be explained below with reference to Figures 3a and 3b. FIG. 3a represents a row of ray sources y forming a linear radiation source 21, as well as a row of detectors forming a linear detector 22, which are arranged on either side of a tubular piece 11. The length X of the linear source 21 and of the linear detector 22 is clearly greater than the outside diameter of the tube 11. The detector 22 makes it possible to determine the attenuation by the part 11 of the radiation y emitted by the source 21, from where can be drawn the average thickness of the wall of the tube 11 in the section through which the radiation passes. In FIG. 3b, the bottom rectangle represents the total radiation N detected by the linear detector 22 in
o the absence of a tube between the source 21 and the detector 22. The figure above this rectangle represents the total radiation N detected
s (counted) by the detector 22 when the tube 11 is interposed between the source 21 and the detector 22. The average thickness of the wall of the tube 11 can be determined from the two values N and N
bone
When the length x of the source 21 and of the detector 22 exceeds the outside diameter of the tube 11 to a sufficient extent so that the tube remains inside the zone between the source and the detector when it is subjected to vibrations , the total radiation detected will not be influenced by the vibrations of the tube.

The invention therefore makes it possible to measure the average thickness of the wall of a tube without taking account of an error by misalignment.

 The linear radiation source 21 can be produced as shown in FIG. 4a. A source holder 211 is placed in a cavity 216 of a linear source container 210. The source holder 211 contains a row of capsules 212 constituting sources of radiation, of caesium 137 for example. In the cavity 216 of the container 210 is also arranged a rotary shutter 213. The container 210 is coupled to a collimator 214 having a number of parallel collimator holes 215. The shutter 213 can be rotated by a rotation mechanism (not shown) to place the shutter plate 217 in the position shown parallel to the plane of the drawing to allow the radiation of the capsules 212 to pass through the collimator 214 during a measurement, and to place the plate perpendicular to the plane of the drawing to cut the radiation outside the measurement periods. The capsules 212 emit radiations radially but these are transformed by the holes 215 of the collimator 214 into parallel beams of radiation.

 Figures 5a and 5b show an embodiment of the linear detector 22. This is combined with a collimator 220 having a rectangular hole 224. The collimator is followed by a scintillator t21 made of polyvinyltoluene. To this scintillator is connected a light guide 222 made of acrylic. To the light guide is coupled a photoelectron multiplier tube 223, the output signal of which is applied to an amplifier (not shown). As described above, the holes 215 of the collimator 214 form parallel beams of the radiation emitted by the source 21. After passing through the tube 11, the parallel beams penetrate into the hole 224 of the collimator 220 of the detector. The single rectangular hole 224 of this collimator can also be replaced by a certain number of collimator holes arranged in parallel lines, like the holes 215 of the collimator of the source 21.

Those skilled in the art know that the operation of a thickness gauge working by transmission of radiation is based on the following equation
N = no exp. (- 1ut) (1) where N is the measurement value supplied by the detector when the radiation passes through an object having the thickness t, N0 is the reference value supplied at the detector output in the absence of an object ( when the thickness t equals 0), and and is a constant or absorption coefficient.

FIG. 6 represents two orthogonal axes x and y and a section of the tube 11. It can be seen that a wall thickness t of the tube 11 in the direction of the y axis can be expressed as a function of the distance x. along the x axis as follows
t = f (x) (2) so that the measurement value N supplied by a linear detector
s 22 like the one in figure 3a can be expressed as follows

FIG. 7 represents a graph indicating the wall thickness t at different distances along the x axis and the corresponding measurement values N obtained at the outlet of the
s detector.

 As can be seen from the graph in FIG. 7, when the value Qn (Ns / No) on the y axis is reduced by half, the variation S of the wall thickness (hereinafter called half-attenuation thickness ") is approximately 4.5 mm.

 In general, if the thickness of the attenuation is very large or very small compared to the thickness to be measured, the measurement becomes difficult. The thickness of half attenuation for an ordinary flat sheet metal measured by means of a thickness gauge of the transmission type, which is a device used on a large scale, is approximately 11 mm. Since the value of 4.5 mm mentioned above represents approximately half of this value of 11 mm, it can be provided that the apparatus according to the invention for measuring the wall thickness of tubular parts has a measurement accuracy which is substantially the same as that of the gauge indicated above for sheets.

 It can be seen in the graph in Figure 7 that the attenuation curve is linear in the wall thickness range from 3 mm to 15 mm (which roughly corresponds to the range from 0.03 to 0.1 in the ratio t / d of the wall thickness t to the diameter D), so that it is not necessary to carry out the required correction when the curve is not linear. In other words, the average wall thickness of the room can be determined directly from the value supplied at the output of the linear detector.

 By applying the measurement principle described above, one can use a measurer like that shown in Figure 8 to determine the wall thickness of a tubular part. The rollers 9 serve to transport the tubular part 11 transversely to the measurer. The length x of the linear radiation source 21 and of the linear radiation detector 22 clearly exceeds the outside diameter D of the tube 11. Such an apparatus has a high response speed and makes it possible to measure the wall thickness of a tube without come into contact with it, while the tube is advancing and whatever the vibrations to which the tube is subjected during its transport. The wall thickness thus measured can be applied, for example, to the upstream rolling mill producing the tubular part to trigger, in the case of a deviation from a set value, a speed or temperature variation in the rolling mill in order to maintain the desired characteristics and the quality of the tube.

In the system according to the invention, the thickness of half-attenuation is approximately half that of a flat sheet, as indicated above. This means that a change in tube wall thickness of around 0.5 mm changes the intensity of the radiation received as much as a change in thickness of 1 mm from a flat sheet. The invention therefore makes it possible to measure the wall thickness of a tube with at least as much precision as it is possible to measure the thickness of a flat sheet, which corresponds to a very high degree of precision.
The measurer according to the invention is particularly advantageously applicable to a drawing-reducer, that is to say to a machine (rolling mill or bench) used in the finishing rolling in practically all the manufacture of seamless tubes of relatively small diameter. Due to its high degree of efficiency, a reducer is also used in the final stage of manufacturing relatively small diameter welded tubes. In such a machine, fourteen to twenty cages each comprising two or three rollers are arranged one after the other along a tube. While the rollers exert a rolling action on the external surface of the tube, the rollers of neighboring cages or stations are driven at different circumferential speeds, so that the tube is stretched longitudinally while it is rolled, with variation in its wall thickness.

Such a machine makes it possible to process tubes of different kinds and to produce tubes of different diameters. FIGS. 9a and 9b schematically and partially represent a stretch-reducer of the type with two rollers per station, while FIGS. 10a and 10b are similar views of a stretch-reducer of the type with three rollers per station These figures represent rollers 31 acting on a tube 32 during stretching. As the elongation of a tube on such a machine also changes its wall thickness, to improve the handling of the machine, it is essential to detect the average wall thickness of a tube in the longitudinal direction, rather than detecting any irregularities in the wall thickness in the section of the tube, in particular in the case of a welded steel tube made from a flat material d uniform thickness.

 In the case where, on such a machine with multiple drawing and reduction stations, the speed is changed to change the traction exerted on the tube in order to adjust its wall thickness, it is desirable that the response speed of the measurer wall thickness is high. A meter according to the invention applied to a reducer-reducer will generally be placed at the inlet or at the outlet, where the tube is exposed to considerable vibration.

However, in these places, there is no place to mount anti-vibration pinch rollers or rollers. The invention eliminates this problem, by eliminating the need for anti-vibration devices, by a measurer whose measurements are not affected by the vibration of the tube and whose response time is also very short.

 Although the conventional systems for measuring the wall thickness also make it possible to measure the variation in the wall thickness in the direction of the section of the tube, these systems are very expensive since it requires at least three sources of radiation and three detectors. The apparatus according to the invention, on the other hand, only has one radiation source and one detector, so that it can be much more economical.

 Although the invention has been described with reference to steel tubes, it should be noted that the basic concept of the invention is generally applicable to the measurement of a wall thickness of tubular parts and that the y-rays such as x-rays, p-rays, ultraviolet rays, visible rays and infrared rays can be used, in particular depending on the material of the tubular parts, which can be made of metal but also of plastic, glass, cement and so on.

 The invention is therefore in no way limited to the specific embodiments and those skilled in the art may make various modifications to it, without going beyond its ambit.

Claims (3)

1. Apparatus for measuring the wall thickness of tubular parts, comprising a radiation source and a radiation detector, characterized in that the source (21) and the detector (22) are arranged face to face, with the tubular part (11) between them, and in that the source and the detector have a length), transverse to the axis of the tubular piece (11), which is larger than the outside diameter of the piece, the arrangement being such that parallel beams of radiation emitted by the source (21) and reaching the detector (22) pass through the entire section of the tubular part, which makes it possible to determine the average wall thickness of the part from the attenuation of the radiation detected by the detector. 2. Apparatus according to claim 1, wherein the radiation source (21) and the radiation detector (22) are each combined with a collimator (214, 220) to form parallel beams of radiation. 3. Apparatus according to claim 2, wherein the tubular part (11) is transported transversely to said parallel beams, in particular continuously on a line for manufacturing tubular parts (11).