·Table of Contents
·Industrial Plants and Structures
Evaluation of Corroded Pipelines Wall Thickness Using Image Processing in Industrial Radiography
Drai Redouane, Kabir Yacine, Azmane Amal,
Abdat Farid & Benchaala Amar
Laboratoire de traitement de signal et de l'image du Centre de Soudage et de Contrôle. Route de Dély-brahim - BP64 - Chéraga - Algérie.
Tél. & fax. 02 36 18 50
In this paper we propose to apply radiographic tangential method for
corrosion and deposit evaluation in pipes. Automatic method based on image
processing is developed in order to improve wall thickness measurements. The
application of this method on small and large pipes diameters (2 inches to 6
inches) is shown and results are discussed.
The reliability and the safety of industrial equipment in the petroleum
industry and power plants are substantially influenced by degradation
processes such as corrosion, erosion, deposits and blocking of pipes, which
might reduce production, cause leaks, fires or unpredictable and costly
shutdowns due to repair and replacement.
The condition of critical
components in these industries can be monitored by the proper use of NDT
inspection methods even while the plant is in operation, thus making possible
the planning of components replacements, repairs, deposit removal and
shutdowns. Preventive and corrective maintenance averts the environment and
the public from excessive risk of industrial disasters.
One of the most
important parameters in a piping or pipe-line to be monitored and measured is
the wall thickness. Only radiographic method assures inspection without costly
removal of insulation material during operation of the plant. An additional
advantage is that these techniques can be even applied in high temperature
environments. Radiographic evaluation of the deposits/scales in pipes
(insulated or not) is also an effective NDT technique.
This radiogram is
examined by radiography interpreters whose the task consists in measure and
quantify wall thickness. Because of the bad contrast and high unsharpeness of
the radiogram quality, make difficult their job, the wall thickness evaluation
in these conditions is submitted to human judgement.
The paper objective
is to propose :
- To apply radiographic tangential method for corrosion and deposit
evaluation in pipes.
- To develop automatic method based on image processing in order to
improve wall thickness measurements.
- To show application of this method on small and large pipes diameters (2
inches to 12 inches).
1. Tangential method principle
We describe the tangent method of X and gamma ray to determine tubes
thickness. This very delicate method requires a meticulous setting of
parameters of cliché hold (time of exposure, distance source-film, energy,
The principle of this method is described on the figure 1. being
given a radioactive source used in industrial radiography as the iridium 192.
It is about exposing tube to this source to inspect while holding in amount of
1.1 termination of source-film distance:
The source is put
far from the tube in order to project the two walls of this tube on the film.
Because of dimensions of the radioactive source, an effect of fuzzy geometric
appears on the film. The relation that expresses this parameter, is given by:
SFD : source film distance
Ug : geometric unsharpeness
OD : external diameter of tube
d : Insulation thickness
s : source
To be able to decrease the effect of fuzzy, studies showed
that the operator should use SFD superior to 8 times tube diameter.
1.2. Determination of a :
Holding amount of SFD, the operator must put the tangent points of
the tube in order to be able to put in evidence risk areas damaged by
corrosion. The relation that expresses the angle a as
shown in figure 1, is given by:
This angle is function of SFD and
2. Thickness evaluation
Once parameters of shoot applied and film developed, it will be proceeded
to its interpretation and evaluation of tube thickness. Figure 1 shows the
apparition of two walls with the presence of different types of corrosion
(deposits, uniform corrosion or ). We will give examples of detected
2.1. Geometrical evaluation :
This method is based on
geometric concept, we can demonstrate that the remaining thickness
Tw is given by :
Where Ta is the measured thickness
on the radiogram.
2.2 Method based on density :
The calculation of tube
thickness is achieved while using the step being on film. Laws of rays
absorption in materials are exponential type, it is necessary to calculate an
inverse logarithmic law permitting to read tube thickness according to the
density in every point of radiogram.
3. Image analysis
The exploitation of radiograms can be done from the interpretation of
film on negatoscope by an operator. This one determines external and internal
diameter limits of the pipe in order to measure, manually, thickness in areas
of maximal corrosion.
The encountered problems that affect the measure
precision and its repeatability, are the followings :
- Effect of geometric projection of the fact that the film is not plated
on the tube (insulated).
- Effect of unsharpness on edges (geometric unsharpness and diffusion of
- Appreciation of edge limits different from an operator to another
This, bring us to develop techniques of images analysis in view
of palliating to these problems and to give back a method more efficient and
The image processing chain used includes here the following
- Digitizing of radiograms.
- Enhancement of image quality.
- Extraction of corroded zone (Segmentation).
- Quantitative analysis in the evaluation of thickness profile.
Once radiogram developed, is digitized by a specialized
scanner of AGFA type on 256 levels of grey. On the images digitized, one can
only distinguish about fifty Grey nuances when the image is displayed on a
The enhancement of contrast permits us to choose a small
range of grey and to amplify differences of luminance between them, improving
so the visibility of weak contrast details.
The extraction of the zone
corroded is the most difficult step to achieve since it is often based on the
complex mathematical concepts. Our research team have developed a very robust
segmentation algorithm permitting to put in value zones of the image reached
by the corrosion. Of a brief manner, the method used so-called co-operative
, takes in consideration advantages of the segmentation in regions and in
contours. What makes, the detected zones are well defined and exact
The last step consists to measure thickness and to draw its
profile according to the tube length.
4. Experimentation and results
We have prepared and made two types of samples. Several tubes are cut up
in 300 mm of length with 1, 2, 4 and 6 inches diameter. Internal and external
grooves (longitudinal and transversal) are manufactured to permit a simulation
corrosion. Other tubes have sudden a local attack by an acid to permit the
obtaining of a local corrosion. All alternatives are taken in amount: straight
or bent, reinforced tubes either no, insulated either no.
In this article,
we have taken steel as material of work. The second type of tube has been
appropriated from industrial installations containing a real corrosion:
erosion, deposits or pits. We constituted a share of samples permitting
different tests and holding amount of all cases that we can find.
to present results, we chose some films among so many others.
shows the radiogram of a tube presenting a deposit in which has been
manufactured some grooves. Figures 3 shows radiogram of a tube containing a
real corrosion "pits".
In order to measure automatically tube thickness,
we digitized radiograms and stored data in a computer memory. We applied
segmentation techniques that permit us the measurement of thickness variation
in tube. As example, we show the following results represented by figure 4:
This work is a co-ordinate research project between our Research Centre
and International agency of Atomic energy. It is a group of work regrouping
several countries in the goal to proceed to tests of the so-called method and
to propose a inspection procedure and evaluation thickness of corroded tube
without removing the insulating.
- W.S. Burkle. "Application of the tangential radiographic technique for
evaluating pipe system erosion/corrosion.". Material evaluation. Vol. 47,
- S. Lee, Y. H. Kim. "Determination of pipe thickness using tangential
radiography and film density-thickness correlation.", Rapport AIEA, Syria
- SIRAC/HORUS, France "Outil d'assistance numérique à l'analyse de
radiogrammes". Rapport AIEA, Istanbul 1997
- Y. Kabir "Segmentation des images dédiée au contrôle non destructif par
radiographie.". Master Thesis. University of Blida. (Algeria) - 1999