Panoramic X-Ray Mode for Testing Weld Quality
of Natural Gas Pipeline from Khoms City to Tripoli
Dr. Ayad M. S.,
Head of Physic Department
Nasser University-Faculty of Sience,
Libya-Khoms
E. Saad M. A.,
Sin. Equipment Inspector
Sirt Oil Company,
Libya - Sirt
Corresponding Author Contact:
Email: shahout@hotmail.com
Abstract:
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.
I- Introduction:
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.
II- Concept of Panoramic Circular Radiographic Method.
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 [1]. 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)
|
III- Standards and Conditions of Detection:
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:
- The voltage of x-ray tube should be 180-300 kV.
- X-ray film kind D4 or D5 (Agfa-Gevaert) must be used.
- The optical density for the x-ray film should be between 2-4 unit optical
density.
- The exposure time should be proper, so that it is suitable for the
thickness of the pipe and the x-ray tube voltage. Fig.-(2) shows the
relation between the exposure time and pipe thickness at different voltages of the x-ray tube, because the exposure time of the welded joint
during x-ray radiography will determine the quality and clearness of the x-
ray image in the quality test.
Fig 2: The relation between the exposure time (t) and thickness of metal (T) at voltages between
150-300 kV.
|
IV- Tools and Test Equipments for Radiographic Inspection.
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 [2]:
- Mechanical moving apparatus inside the gas pipe (Crawler-see Fig.-1)
equipped with x-ray tube working at 180-300 kV voltage.
- Laboratory-containers for manual and automatic development of the
radiographic film.
- X-ray film kind D4 (Agfa-Gevaert).
- Negatiscope for reading and analysis of x-ray images.
- Densitometer for determination of the optical density of the x-ray films
kind Radix-D
- Identification symbols (numbers and letters) made from lead or graphite
in order to classify the x-ray film after developing, according to its
place, its number in the kilometer under test and its date ….. etc. For
this reason, when giving numbers or letters to the x-ray film, data must
be written from left to right according to the following:
- Code of the company responsible for the project SC[Sirt Company]
- Date of inspection dd / mm / yy.
- Pipe diameter (inches).
- Wall thickness (mm).
- Steel grade of the pipe.
- Code the main gas pipeline EKT [El-Khoms to Tripoli main gas pipeline.]
- Reference kilometer of the joint.
- Code of the joint's type and number of the joints.
- Codes of a tie-in, repair (R) cutout (N1).
V- Analyses of Radiographic Images.
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) [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
|
| 0-6.35
| 0.127
| 5
|
| > 6.35-9.52
| 0.19
| 7
|
| > 9.52-12.7
| 0.254
| 10
|
| > 12.7-15.88
| 0.317
| 12
|
| > 15.88-19.05
| 0.381
| 15
|
| > 19.05-22.22
| 0.444
| 17
|
| > 22.22-25.4
| 0.508
| 20
|
| > 25.4-31.75
| 0.635
| 25
|
| > 31.75-38.10
| 0.762
| 30
|
| > 38.10-50.80
| 0.889
| 35
|
| 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 [4].
VI-Standards of Welding Acceptability According to
API-1104 Standard.
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.
VII Results and Discussion.
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.
Steel Grade
X-60 |
Inspection Technique PANORAMIC |
Source: X-RAY |
Equipment CRWLER |
Weld
Joint
No. |
Pipe |
Film
Type |
Sensitivity |
Density |
Description
and
New Location of Defect |
Final
Conclusion |
Dia.
inch |
W T.
(mm) |
OK
R
RX
CO
| Acceptable
Repair
Re-Shoot
Cut Out |
| DNA 01 |
34 |
9.52 |
D5 |
2T |
3.0 |
ESI,SP,IU,IC |
OK |
| NA 02
| 34
| 9.52
| D5
| 2T
| 3.5
| ESI,SP,IU,EU
| OK
|
| DNA 03
| 34
| 9.52
| D5
| 2T
| 2.7
| ESI,SP,IU,IC,HB
| OK
|
| NA 04
| 34
| 9.52
| D5
| 2T
| 3.4
| ESI,SP,IU,EU,IF
| OK
|
| DNA 05
| 34
| 9.52
| D5
| 2T
| 2.7
| ESI,SP,IU,IC
| OK
|
| NA 06
| 34
| 9.52
| D5
| 2T
| 3.5
| ESI,SP,IU,EU,IF
| OK
|
| DNA 07
| 34
| 9.52
| D5
| 2T
| 2.8
| ESI,SP,IU
| OK
|
| NA 08
| 34
| 9.52
| D5
| 2T
| 3.5
| CP 196-198
| R
|
| DNA 09
| 34
| 9.52
| D5
| 2T
| 3.1
| SI,SP,IUE
| OK
|
| NA 10
| 34
| 9.52
| D5
| 2T
| 3.1
| ESI,SP,IU,EU,IF
| OK
|
| DNA 11
| 34
| 9.52
| D5
| 2T
| 3.1
| SP,ESI,IC,HB,IU
| OK
|
| NA 12
| 34
| 9.52
| D5
| 2T
| 3.5
| EU 140
| R
|
| DNA 13
| 34
| 9.52
| D5
| 2T
| 2.8
| SP,ESI,HB,IC,IU
| OK
|
| NA 14
| 34
| 9.52
| D5
| 2T
| 3.6
| ESI,SP,IU,EU,IC
| OK
|
| DNA 15
| 34
| 9.52
| D5
| 2T
| 2.6
| SP,ESI,HB,IC,IU
| OK
|
| NA 16
| 34
| 9.52
| D5
| 2T
| 3.1
| SP 147
| R
|
| DNA 17
| 34
| 9.52
| D5
| 2T
| 2.7
| ESI,SP,IU,IC,BT
| OK
|
| NA 18
| 34
| 9.52
| D5
| 2T
| 3.3
| ESI,SP,IU,EU,IC
| OK
|
| DNA 19
| 34
| 9.52
| D5
| 2T
| 2.8
| ESI,SP,IU
| OK
|
| NA 20
| 34
| 9.52
| D5
| 2T
| 3.6
| SP 145
| R
|
| DNA 21
| 34
| 9.52
| D5
| 2T
| 3.0
| ESI,SP,IU,IC,BT
| OK
|
| NA 22
| 34
| 9.52
| D5
| 2T
| 3.5
| ESI,SP,IU,EU
| OK
|
| DNA 23
| 34
| 9.52
| D5
| 2T
| 2.6
| SP,ESI,HB,IC,IU
| OK
|
| NA 24
| 34
| 9.52
| D5
| 2T
| 3.4
| ESI,SP,IU,EU,IF
| OK
|
| DAN 25
| 34
| 9.52
| D5
| 2T
| 2.5
| SP,ESI,HB,IU
| OK
|
| NA 26
| 34
| 9.52
| D5
| 2T
| 3.4
| ESI,SP,EU,IU
| OK
|
| DNA 27
| 34
| 9.52
| D5
| 2T
| 2.9
| SP,ESI,HB,IC,IU
| OK
|
| NA 28
| 34
| 9.52
| D5
| 2T
| 3.4
| CP 193-195;230-239;254-255
| R
|
| DNA 29
| 34
| 9.52
| D5
| 2T
| 2.7
| SP,ESI,IC,BT,IU
| OK
|
| NA 30
| 34
| 9.52
| D5
| 2T
| 3.5
| CP 185-187;210-213;229-231
| R
|
| Table 3: Radiographic Inspection Report EKT 42. |
Legend
BT Burn-through ESI-Elongated Slag Inclusion IFD-Incomplete Fusion due to Cold Lap
CP-Cluster Porosity HB-Hollow Bead IP-Inadequate Penetration of Weld Root
CR-Crack IC-Internal Concavity IPD-Inadequate Penetration due to High Low
EU-External Undercut IF-Incomplete Fusion ISI-Isolated Slag Inclusion
IU-Internal undercut SP-Spherical Porosity
Steel Grade
X-60 |
Inspection Technique PANORAMIC |
Source: X-RAY |
Equipment CRWLER |
Weld
Joint
No. |
Pipe |
Film
Type |
Sensitivity |
Density |
Description
and
New Location of Defect |
Final
Conclusion |
Dia.
inch |
W T.
(mm) |
OK
R
RX
CO
| Acceptable
Repair
Re-Shoot
Cut Out |
| NA 46
| 34
| 9.52
| D5
| 2T
| 2.8
| IC 207-208
| R
|
| NA 47
| 34
| 9.52
| D5
| 2T
| 2.8
| SP,ESI,EU,IU,IC
| OK
|
| NA 48
| 34
| 9.52
| D5
| 2T
| 2.8
| CP,ESI,EU,IU,IC
| OK
|
| NA 49
| 34
| 9.52
| D5
| 2T
| 2.9
| SP,ESI,EU,IU,IC
| OK
|
| NA 50
| 34
| 9.52
| D5
| 2T
| 2.9
| SP,ESI,EU,IU,IC
| OK
|
| NA 51
| 34
| 9.52
| D5
| 2T
| 3.0
| SP,ESI,EU,IU,IC
| OK
|
| NA 52
| 34
| 9.52
| D5
| 2T
| 3.0
| ESI,SP,EU,IC,IU
| OK
|
| NA 53
| 34
| 9.52
| D5
| 2T
| 2.8
| ESI,SP,EU,IC,IU
| OK
|
| NA 54
| 34
| 9.52
| D5
| 2T
| 3.1
| ESI,SP,EU,IU,IC
| OK
|
| NA 55
| 34
| 9.52
| D5
| 2T
| 2.8
| ESI,SP,IU,EU,IC
| OK
|
| NA 56
| 34
| 9.52
| D5
| 2T
| 3.0
| ESI,SP,EU,IU,IC
| OK
|
| NA 57
| 34
| 9.52
| D5
| 2T
| 2.9
| ESI,SP,EU,IU,IC
| OK
|
| Table 4: Radiographic Inspection Report EKT 60. |
Legend
BT Burn-through ESI-Elongated Slag Inclusion IFD-Incomplete Fusion due to Cold Lap
CP-Cluster Porosity HB-Hollow Bead IP-Inadequate Penetration of Weld Root
CR-Crack IC-Internal Concavity IPD-Inadequate Penetration due to High Low
EU-External Undercut IF-Incomplete Fusion ISI-Isolated Slag Inclusion
IU-Internal undercut SP-Spherical Porosity
| 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).
|
NA 16
And
NA 20
| SP 147
SP 145
|
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.
|
NA 28
And
NA
| CP 193-195; 230-239;254-255
And
CP 185-187;210-213;229-231
| 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. |
References:
- Maximlyuk, Y. V. "Radiographic Inspection of Weld Joints". Quality
System Procedures Manual (Moscow, 1999 nd. ed.) pp. 1-21.
- Maximlyuk, Y. V. "Nondestructive Test of Weld Joints" Quality
System Procedures Manual (Moscow 1999, 1st. ed) pp. 1-17.
- "Radiography in Modern Industry", Quinn A. Richard and Sigl C. Clair
(Eds.) (Eastman Kodak Company 1980) 4th.ed.
- Evseev, V. L. "Welding of Pipeline and Related Facilities" Quality
System Procedures Manual (Moscow 1990 17th. ed) pp.1-17.
Table 2: