|NDT.net Nov 2003, Vol. 8 No.11|
The polar g-ray radiography method was studied extensively, g-ray from Ir192 source was used to detect weld defects in the main gas pipeline extending from Khoms to Tripoli. g-ray radiographic inspections were carried out according to the ASTM [American Society for Testing and Materials] standards, and the radiographs were analyzed according to the quality specifications API [American Petroleum Institute] standard-1104. The polar g-ray mode has been applied to specimens of weld joints of pipes used in this pipeline in the regions [the kilometer 118 [[The kilometer zero] region is considered as the starting point that is located at Khoms city.]] and [the kilometer 123], and weld joints in the SLR 7 station in the region [the kilometer 125]. The results obtained from g-rays have been discussed and analyzed.
Gamma-ray, Radiography, Weld joint, Polar inspection, Weld defects
Radiography today is one of the most important, most versatile, of all the nondestructive test methods used by modern industry. Employing highly penetrating x-rays, gamma rays, and other forms of radiation that do not damage the part itself, radiography provides a permanent visible film record of internal conditions, containing the basic information by which soundness can be determined. Radiography is important in the detection of the weld joints, such as the cooling pipes in the nuclear reactors, pipes of airplanes fuel, pumping pipes of petroleum and natural gas, where the pressure is very high and the flowing materials are too dangerous, that necessitate a high degree of accuracy in the welding processes.
Radiography with gamma rays has the advantages of simplicity of the apparatus used, compactness of the radiation source, and independence from outside power.
The present work is intended to study gamma-ray polar radiography for detection of welding defects in the natural gas pipeline extending from Khoms city to Tripoli. The length of the pipeline is 150.8 km and made of pipes each 12 m long [diameter 34 inches and (9.52-14.27) mm thick] [All technical references use the inch for measuring the diameter and the millimeter unit to measure the thickness, and so we shall follow this procedure.], this requires (83-100) weld joints for each kilometer, and more than fourteen thousands of weld joints for the whole project. Defects appeared in some of these weld joints, in some selected regions will be discussed.
For the radiographic inspection of weld joints, three main exposure geometries are used depending on the pipe diameter (Maximlyuk, 1999):
In this research the second geometric mode was studied, (the diameter pipe is 8 inches) where three exposures are required to radiograph the whole weld joint, in each one, an arc of 120o is irradiated and an image of only one third of the pipe circumference in the weld region can be obtained. In each stage of exposure a film strip (70 mm wide) is placed outside on the weld joint along the whole arc, while the radiation source is installed outside the pipe (i.e. the focus distance F=D where D is the pipe outer diameter) at one of the three poles [ex: at poles 12,4,8 (Fig.1)] so the film subtends a central radiation angle of 120o in each stage, provided the angle between radiation direction and the vertical to the weld seam plane dose not exceed 5o. Also two metal strips (penetrameters) are placed on each film, one is at the center of the film, the other at one of its ends, in order to measure the ability of gamma-ray to penetrate the pipe material. These penetrameters (Quinn & Sigl, 1980) are used as a basic reference to test the quality of the radiograph on the film (after development) according to the specifications API-standard 1104
Fig 1: The three stages of polar radiography for one weld joint.
The process of nondestructive tests for weld joints of natural gas pipeline using gamma-ray polar radiography (see article II) needs to put specified conditions in order to get an image that could be analyzed so that it reflects all the points of detection, these conditions are:
III 1 Gamma- Ray Source
It is difficult to give specific recommendations or the choices of gamma-ray emitter and source strength. These choices depend on several factors, among which are the type of specimen radiographed, allowable exposure time, storage facilities available, protective measures required, and convenience of source replacement. Table (1) gives the radioactive sources used in industrial radiography, the half-life of the source and gamma-rays energy are also included (Pherigo, 1980) & (Quinn & Sigl, 1980).
|Radioactive Source||Half-Life||Energy of Gamma-Rays (MeV)||Applications and Approximate Practical Thickness Limits (mm)|
|Iridium-192||73.83 days||0.137 to 0.651*||6.35 to 76.2 steel or equivalent|
|Cesium-137||33 years||0.66||25.4 to 76.2 steel or equivalent|
|Cobalt-60||5.3 years||1.17 and 1.33||25.4 to 177.8 steel or equivalent|
|Table 1: Radioactive sources used in industrial radiography.|
From this table, it is noticed that gamma-rays from Co60 have relatively great penetrating power and can be used, under some conditions, to radiograph sections of steel 177.8 mm thick, or the equivalent. Radiations from other radioactive sources have lower energies; for example, Ir192 can be used to radiograph sections of steel with thickness 6.35 to 76.2 mm, or the equivalent, so it is suitable for pipes used in the natural gas pipeline with thickness in the range 2 to 8.18mm.
III 2 Exposure Time
It is well known that gamma-ray sources gradually lose activity with time, the rate of decrease of activity is depending on the kind of radioactive material (see table-1). This decrease in emission necessitates more or less frequent revision of exposures and replacement of sources. The exposure calculations necessitated by the gradual decrease in the radiation output of gamma-ray source can be facilitated by the use of the decay curves.
The half-life time of Ir192 isotopes is 73.83 days, therefore every 1-2 weeks, exposure time must be increased by dividing the initial value by Ir192 correction factor - K (Table-2) (Maximlyuk, 1999). The initial value of the exposure time can be determined by control photographs, where maximum radiograph sensitivity indicates optimum exposure time, it is found to be 15 sec in this project.
|Time (Weeks)||Ir192 Factor Value K||Exposure Time (Sec)||Time (Weeks)||Ir192 Factor Value K||Exposure Time (Sec)|
|Table 2: Values of Ir192 factor (K) and exposure times for successive weeks of radiography.|
Thus the gamma-ray output is directly proportional to both activity of the source and exposure time, and hence is directly proportional to their product. The gamma-ray exposure E may be stated as E=A x t, where A is the activity in curies and t is the exposure time, the amount of gamma-radiation remaining constant so long as E remains constant. This permits specifying gamma-ray exposures in curie-hours without stating specific values for source activity or time. So the only way to change the penetrating power of gamma-ray is to change the source.
III 3 Source Film Distance
The density of any radiographic image depends on the amount of radiation absorbed by the sensitive emulsion of the film. This amount of radiation depends on the total amount of radiation emitted from gamma-ray source which is expressed by the exposure (E) and the amount of radiation reaching the specimen which is governed by the distance between the source and the specimen, varying inversely with the square of the distance. The exposure factor (Quinn & Sigl, 1980) is a quantity that combines source activity, time, and distance. Numerically the exposure factor equals . Radioactive techniques are sometimes given in radioactive isotope and exposure factor. In such a case, it is necessary merely to multiply the exposure factor by the square of the distance to be used in order to find the curie-hours required.
It is important to mention that the amount of radiation absorbed by the film depends also on the proportion of this radiation that passes through the specimen which absorbs some of gamma-ray, the absorption depends on the thickness of the specimen, density, and atomic number.
III 4 Selection of the Film
The selection of a film for the radiography of any particular part depends on the following factors (Quinn & Sigl, 1980):
The film used in this work was of kind D4 (Agfa Gevart).
III 5 Optical Density (Quinn & Sigl, 1980)
Optical density refers to the quantitative measure of the film blackening, it is defined by the equation d = log I0/It where d is the optical density, I0 is the light intensity incident on the film and It, is the light intensity transmitted. As mentioned above the optical density must be from 2 to 4.
The equipments for testing the welding qualities used for the detection of the weld defects in the project of gas pipeline Khoms-Tripoli differ according to the kind of detection which depends in the first place upon the diameter of the pipe. For this reason, we shall mention the tools and test equipments that are used in the polar detection method, which is the subject of this research, these are (Maximlyuk, 1999):
Fig 2: Schematic diagram of Penetrameter.
|Pipe Wall Thickness, (mm)||Maximum Penetrameter Thickness, (mm)||Identifying Number|
|Table 3: Thickness of pipe versus thickness of penetrameter.|
The ASTM penetrameter permits the specification of a number of levels of radiographic sensitivity, depending on the requirements of the job. For example, the specifications may call for a radiographic sensitivity level of 2-2T. The first symbol (2) indicates that the penetrameter shall be 2% of the specimen thickness, the second (2T) indicates that the hole having a diameter twice the penetrameter thickness shall be visible on the finished radiograph. However, critical components may require more rigid standards, and a level of 1-2T or 1-1T may be required. On the other hand, the radiography of less critical specimens may be satisfactory if a quality level of 2-4T or 4-4T is achieved. The more critical the radiographic examination (that is, the higher the level of radiographic sensitivity required) the lower the numerical designation for the quality level.
|Table 4: Standards of Welds Acceptability.|
Exposed radiographic films are developed in specialized laboratories with manual and automatic processing equipment. Weld radiographs accepted for interpretation should show clearly penetrameter, identification markers and measuring belt images. The optical density must be from 2 to 4 and the radiographic sensitivity level must be equal to 2-1T or 2-2T as per ASTM E142, if both conditions are not fulfilled in the film, then the weld joint must be re-shot again in order to obtain a radiograph that is accepted for interpretation. Later on it becomes possible to start the process of analysis of the radiographs where weld joints evaluation and rejection under the gamma-ray inspection results should be in strict compliance with API standard-1104.
Table (4) contains all the acceptable defects for the weld quality, if the defects appear in the weld joint with dimensions and shapes as those mentioned in the table, they are considered as acceptable ones, otherwise, defects of larger dimensions are not allowed.
The polar radiographic method has been used for testing the weld quality for some specimens of weld joint in the regions, the kilometer 118 and the kilometer 123 for pipes of 34 inches in diameter (SNA 54T and SNA 75T, respectively) where the panoramic circular method becomes impossible (see article II), and the kilometer 125 for pipes of diameter 8 inches in the station SLR7 [Scraper Launcher & Receiving Station] (SAZ V461 [A code related to the weld joints of SLR 7 station, and the letter V indicates that the weld joint process was carried out in the company technical shop.], SAZ V462, and SAZ V470). After developing the film, which belongs to each weld joint, measuring its optical density, and determining the level of its radiographic sensitivity (using the penetrameter), the process of analysis of each image was conducted according to quality specifications API-1104 (see table -4). And since the process of welding includes some defects, and our purpose is to know these defects and classify them as acceptable defects and unacceptable ones according to quality specifications API-1104, one can divide the defects into two groups:
Firstly Acceptable Defects.
These defects must have dimensions (or size) which are allowed by quality specifications API-1104, whether these defects were single, repeated, or mixed, in such a way that their total dimensions in a limited length (304.8 mm) does not exceed the upper limit accepted in the quality of welding, so these defects are considered as (Acceptable), and are denoted by the symbol [OK].
Secondly Unacceptable Defects.
These defects have dimensions (or size) which are larger than those allowed by quality specifications API-1104, whether these defects were single, repeated, or mixed, in such a way that their appearance as single defects or collective defects in one single welded joint is larger than the upper limit allowed by welding quality. Accordingly, these defects are classified as (Unacceptable), They must be repaired if they were contained in a narrow band in the circumference of the welded circle and denoted by the symbol [R]. Otherwise, if their spread was recurrent, then all the weld joint must be cut out and should be rewelded and retested again, in this case the defects are given the symbol [CO].
Table (5) shows the results obtained for weld joints in the kilometer 118 and the kilometer 123 regions, where three radiographic images were taken for each weld joint. No defects were found (neither acceptable nor unacceptable) in these weld joints, this means that the welding was of good quality.
|Equipment: Ir192||Source: g-RAY||Film Type D4||Inspection Technique: Polar||Steel Grade: X-60|
|Final Conclusion||Description and Location of Defect||Density||Sensitivity||Film Number||Pipe||Weld Joint No.||Location|
|W.T. (mm)||Dia. (inch)|
|Table 5: Radiographic inspection report EKT 118, and EKT 123.|
Table (6) shows the results obtained for weld joints in the station SLR7 in the kilometer 125 region, where the three radiographic images were also taken for each weld joint. A gain no defects were found (neither acceptable nor unacceptable) for the weld joints SAZ V461 and SAZ V462, while four spherical porosity defects were found in the weld joint SAZ V470, two of them were in the first image in the regions 0 mm and 3 mm in the first arc of the circumference of the welded circle, the others were in the third image in the regions 62 mm and 69 mm in the third arc of the circumference of the welded circle. These defects are spherical deformations that occur usually inside the two welded regions (the Hot Pass and the Root Pass). They appeared in the film as circles with diameter larger than 3 mm, these necessitate repairing those regions, re-welding and retesting them again. As a whole the number of defects is little, this is partly due to the good quality welding and partly due to the small circumference of the pipes that makes the probability of occurrence of defects in the weld joint small.
|Equipment: Ir192||Source: g-RAY||Film Type D4||Inspection Technique: Polar||Steel Grade: GRB|
|Final Conclusion||Description and Location of Defect||Density||Sensitivity||Film Number||Pipe||Weld Joint No.||Location|
|W.T. (mm)||Dia. (inch)|
|R||Spherical porosity 0;3||3.1||2T||1||8.18||8||SAZV470||EKT125|
|R||Spherical porosity 62;69||3.1||2T||3|
|Table 6: Radiographic inspection report for SLR7 station in EKT 125.|
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