|NDT.net August 2003, Vol. 8 No.08|
The purpose of this research is to study the panoramic method of x-ray radiography for detection of welding defects in the main gas pipeline extending from Khoms to Tripoli, according to the ASTM [ASTM American Society for Testing and Materials.] standards related to the x-ray radiography detection and also according to the quality specifications API-1104 [API-1104 ± American Petroleum Institute.] related to the analysis of x-ray images, where the panoramic radiographic method were applied to specimens of the welded joints of pipes used in this pipeline in the regions [the kilometer 42] and [the kilometer 60] [[The kilometer zero] region is considered as the starting point that is located at Khoms city.], and the resulting x-ray images were analyzed.
The strict conditions imposed on the quality of modern industry related to the specifications and quality, which require a high degree of accuracy in the processes of welding, were the main reason to use nondestructive tests in the detection processes of the welded joints, such as the cooling pipes in the nuclear reactors, pipes of airplanes fuel, pumping pipes of petroleum and natural gas, because of the dangerous flowing materials and the high pressure in these pipes. The gas pipeline from Khoms city to Tripoli is of length 150.8 km and made of pipes with length 12 m (diameter 34 inches and 9.52 mm thickness)[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) welded joints for each kilometer, and more than fourteen thousands of welded joints for the whole project. Different nondestructive testing methods were used to detect all of the welded joints, the most important one is the panoramic method using x-ray tube of voltage 180-300 kV.
This method is used to detect welding joints in pipes which are more than 10 inches in diameter, where it is possible to have a complete circular image for the circumference of the pipe in the region of welding, by putting the source of x-ray in the center of the pipe from inside where two pipes join in the welding part, i.e. at the point F=D/2 where D is the diameter of the pipe, provided the angle of deviation between the direction of the x-ray and the surface of the welding region does not exceed 5o. The welded region is surrounded completely from outside by a film strip 70 mm in width (Fig- 1). Also four metal strips (penetrameters) are put with the film on four oppositely oriented poles* in order to measure the penetration ability of the ray through the material of the pipe . These penetrameters are used as a basic reference to test the quality of the x-ray film, after developing, according to the quality specifications API-1104.* The circumference of the pipe is divided into several oppositely oriented poles with positions as given in the clock such as (12 - 6,3 - 9) or (1 - 7,4 - 10) or (2 - 8,5 - 11)
|Fig 1: Panoramic Circular Radiographic method using a mechanical moving x-ray source (CRAWLER)|
The process of nondestructive tests for the welded joints of the natural gas pipeline using panoramic x-ray mode [article II] needs to put specified conditions in order to get an x-ray image, which could be analyzed so that it reflects all the points of detection, these conditions are:
|Fig 2: The relation between the exposure time (t) and thickness of metal (T) at voltages between 150-300 kV.|
The tools and equipments for testing the welding qualities used for the detection of the welding 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 panoramic circular detection method, which is the subject of this research, these are :
For the analysis of the radiographic images, a standard test piece is usually included in every radiograph as a check on the adequacy of the radiographic technique. The test piece is commonly referred to as a penetrameter, the penetrameter is made of the same material (or a radiographic similar material) as the specimen being radiographed.
The penetrameter used in this research is a hole type penetrameter (ASTM penetrameter) which consists of a small rectangular piece of metal containing three holes of diameter T, 2T, 4T, where T is the thickness of the penetrameter (Fig.-3) . The thickness T is related to the thickness of the metal layer of the pipe, and each penetrameter is identified by a lead number showing the thickness in thousandths of inch as shown in table (1).
|Fig 3: Schematic diagram of Penetrameter.|
|Pipe Wall Thickness, mm||Maximum Penetrameter Thickness, mm||Identifying Number|
|Table 1: Thickness of pipe versus thickness of penetrameter.|
The hole type penetrameter indicates whether or not a specified quality level has been attained, and according to the quality specifications API-1104, it 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 percent of the thickness of the specimen; 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.
Because of the high pressure (40-54 bar) in the natural gas pipeline, and the high inflamability of the gas and its ability to explode, in addition to that, the pipeline may be near some residential regions, it was necessary that the radiographic sensitivity level must be 2-1T or 2-2T as a lowest acceptable level of radiographic sensitivity for this project .
The process of quality specifications for the analysis of x-ray images according to API-1104 standard for welded joint, comes after developing the film and determining its optical density where it must be not less than 1.8 and does not exceed 4 unit optical density, together with the radiographic sensitivity level which must be equal to 2-1T or 2-2T. If both conditions are not fulfilled in the film, it will be not possible to start the process of analysis of the x-ray image and see if it is in accordance with the standards of welds acceptability. In this case, it is important to re-shoot this joint in order to obtain a clear picture of the x-ray image that can be analyzed according to quality specifications API-1104 standard.
Table-2, contains all the acceptable defects for the weld efficiency, if the defect appear in the welded joint with dimensions, and shapes as mentioned in the table, they are considered as acceptable ones, otherwise, defects of larger dimensions are not allowed.
|Table 2: Standards of Welds Acceptability.|
The panoramic circular radiographic method has been used for testing the weld quality for successive specimens of welded joints in the regions the kilometer 42 and the kilometer 60 for two kinds of joints (NA)[NA- A code indicating that the weld joint process was carried out manually near the gas pipeline trench.] and (DNA)[DNA – A code indicating that the weld joint process was carried out automatically in the company technical shop]. After developing the film, which belongs to each welded 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 -2). 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 group:
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 its 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 welded joint must be cut out and should be rewelded and retested again, and the defects are given the symbol [CO]. Table- (3) shows the results obtained in the kilometer 42 region, with welded joints from (DNA 01) to (NA 30). The results of the kilometer 60 region are included in table- (4) with welded joints from (NA 46) to (NA 57). In table- (5), only the unacceptable defects have been discussed where it shows the dimensions of these defects, their features, and their positions. And comparison has been made with the quality specifications API-1104.
|Inspection Technique PANORAMIC||Source: X-RAY||Equipment CRWLER|
New Location of Defect
|NA 08||34||9.52||D5||2T||3.5||CP 196-198||R|
|NA 12||34||9.52||D5||2T||3.5||EU 140||R|
|NA 16||34||9.52||D5||2T||3.1||SP 147||R|
|NA 20||34||9.52||D5||2T||3.6||SP 145||R|
|NA 28||34||9.52||D5||2T||3.4||CP 193-195;230-239;254-255||R|
|NA 30||34||9.52||D5||2T||3.5||CP 185-187;210-213;229-231||R|
|Table 3: Radiographic Inspection Report EKT 42.|
|Inspection Technique PANORAMIC||Source: X-RAY||Equipment CRWLER|
New Location of Defect
|NA 46||34||9.52||D5||2T||2.8||IC 207-208||R|
|Table 4: Radiographic Inspection Report EKT 60.|
|Joint No.||Defects & Locations||Description|
|NA 08||CP 196-198||The defect is Cluster Porosity in the region 196-198 mm in the circumference of the welded circle. It is a swell caused by the entrance of air during the welding process in the Cap Pass region. A circle of diameter larger than 12.7 mm is observed on the film, so it is not accepted in the quality specifications API-1104.|
|NA 12||EU 140||External Undercut in the region 140 mm in the circumference of the welded circle. It represents a case of discontinuity between the metal of the pipe and the welding from outside in the Cap Pass region. It appeared in the film as a long continuous line of length 54.6 mm. This exceeds the allowed length (50.8 mm).|
|The defects are Spherical Porosity in the region 147 mm in the first welded joint and in the region 145 mm in the second welded joint in the circumference of the welded circle. These 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 4 mm.|
|CP 193-195; 230-239;254-255|
|These are the same defects as in the region NA 8 but it is repeated in several close regions. The total length is larger than 12.7 mm as an upper limit in the total length unit (304.8 mm)|
|NA 46||IC 207-208||The defect is Internal Concavity in the region 207-208 mm in the circumference of the welded circle. It looks as a concave up cavity in the Root Pass region. This is caused as a result of deficiency of the welding material in this region. It is noticed in the film as a black dark line of length 15.7 mm. This defect is not controlled by length, but by the optical density. The black dark line appeared darker than the metal of the pipe, and so, it is unacceptable|
|Table 5: The unacceptable defects that appeared in the results of the kilometer 42 and kilometer 60.|