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Automatic Wall Thickness Estimation on Insulated Pipes Using Digital Radiography H.Thiele, H.-J.Friemel AMI Advanced Mobile Imaging GmbH, Eschborn Contact

1. Summary

The digital radiography system AMICA-401 offers the possibility of estimation of wall thickness and diameter of insulated and not insulated pipe systems and its evaluation on stream by its integrated software. The technical structure of the system and the method of working are discussed in detail.

The radiographs generated by a digital detector are stored, evaluated and archived in the directly connected notebook. All object and exposure data which are necessary for a following complete printout of a test report can be collected by the integrated software.

The software enables the evaluation of the tangential penetrated wall thickness area at any position in the tested area as well as the evaluation of the residual wall thickness in recognisable corroded areas in the vertically penetrated wall area and the estimation of the pipe diameter.

The measured data are added to the object data and classified according to predefined rules so that a clear statement upon required repairs is available.

The main advantages are related to the immediate evaluation of the radiographs at the site of inspection directly after the exposure, to the extreme short exposure times and resulting from this aspect the low radiation doses and to the integrated extensive image processing functions. Further no film processing chemicals are needed. The system is applicable mobile as well as stationary. Further application possibilities and limits of application are discussed in detail.

2. Principle and structure of the system

The test arrangement for estimation of wall thickness of insulated pipe systems is the same as for the well known arrangement for taking projection radiographs using film cassettes (see picture 1). The film cassette is replaced by a flat detector which is directly connected to a notebook.

Pic 1: Schematic structure of the inspection system

Pic 2: The components of the inspection system

The software on the notebook records the object data for the generation of test report, controls the detector and performs the estimation of wall thickness automatically.

The main element of the detector is a diode array from amorphe silicon. The principle and performance of such detectors has been reported already several times so that it will not be discussed here once more. Picture 2 shows the components of the inspection system. At the left side of the picture the detector, at the right side the notebook and between in the background the power supply unit of the detector. The length of the connection cable between computer and detector by be up to 200 m, in most cases 16 m are chosen.

The software of the system consists of the documentation part, the archiving part and the test software itself.

3. Documentation software

The documentation software has the task to record all information of inspection performance which are necessary for complete reporting and for the order and object related archiving of data.

The inspection order related data are structured in a 4 level hierarchy (test order, test object, test area, test activity, see picture 3). The several levels are combined by 1:n relations. From the level "test activity" the programme moves to the level of the single exposure ("result"). Just one digital exposure is assigned to each entry in this level and recorded with his unique file name.

Pic 3: Structure of the documentation software

In the case of new exposures first in the level "result" the test arrangement and the geometrical data are recorded and the used radiation source is selected from the general radiation source table (see picture 4). In the case of radioactive sources the actual activity of the source is calculated automatically from the activity and date of delivery of the source. After this operation the exposure is started.

If the radiograph is already done and recorded it will be re displayed on the screen.

In the documentation part of the software the general tables which are displayed in selection windows when recording inspection data may be altered under the menu item "basic data" (e.g. orderer, plant, results, radiation sources, components, report definitions, etc.).

Evaluations related to recorded results and to pipes which are not ok may be asked for directly from the reported information.

The import of pre defined test orders and the export of results from and to general data acquisition and administration software systems is possible. The interfaces have to be optimised individually to the structure of those general systems.

4. Inspection software

After starting of the exposure (see picture 4) the inspection software is activated. At to the end of exposure the radiograph is displayed on the notebook immediately. The original file is saved automatically in the picture path so that the original file is available permanently. Other evaluation or image processing activities are always deviated from the original file.

For evaluation of tangential penetrated wall areas the adequate mode is selected and the wall section to be evaluated is marked by a cursor line perpendicular to the wall section (see picture 5). The density profile along this marking is displayed automatically. Two evaluation markings have to be positioned in this density profile by cursor click, first at the inner side and then at the outer side of the pipe. The density profile then is calculated automatically and the estimated wall thickness is shown on the lower border of the density profile image.

Pic 4: Definition of test geometry

Pic 5: Evaluation of tangential penetrated pipe wall

Pic 6: Evaluation of corrosion hollows

The estimated value may be taken or deleted. The positions of the taken values are marked and numbered automatically. In one exposure as much positions as necessary may be evaluated in this manner. If different nominal wall thickness values are related to those positions the relevant nominal values have to be recorded to each evaluated position additionally. In the case of unique nominal wall thickness the value from the documentation module is transferred automatically into the inspection module and used as reference value.

For evaluation of corroded areas in the perpendicularly penetrated sections of the pipe wall the adequate evaluation modus is chosen as well and the interesting area has to be marked with the cursor (see picture 6). The lowest existing wall thickness in this area and the dimension of the corroded area are estimated and displayed automatically. The evaluated areas may be taken as well or deleted. They are numbered automatically in the same manner as mentioned above. The basis for this evaluation is the knowledge of a material specific factor which represents the radiographic material behaviour. This factor is defined in the documentation part of the software and linked to the definition of the material. When starting the inspection software this factor is transmitted from the documentation part into the inspection part automatically.

For both evaluation possibilities an additional picture file is stored together with the marked positions of evaluation which may be re displayed and added every time.

Further possibilities of evaluation are the measurement of density in any area of the exposure and the estimation of distances in the exposure. The measurement of density is interesting for definition of correct exposure time. The estimation of distances may be used for dimensioning of any details in the projection plane of the detector.

In the case of exposures with a bigger amount of object extent density and contrast may be changed by the integrated image processing functions. Because of the linear characteristic of the detector and the dynamic of 16 bit/pixel a density range of D=4,8 can be covered by one exposure and displayed in the visible density range without a drop of information. Therefore it is not required to expose additional radiographs with different density or to use multi film techniques (higher film expenses).

Further electronic filters and colouring possibilities are available for special evaluations or for extreme density situations. Parts of the exposures may be marked and zoomed as well.

The picture files can be transferred into "BMP" files so that they may be sent by mail to other locations and displayed and printed out using normal standard graphical software. To avoid losses of quality additionally a special viewer software is available which can be mailed together with picture file. This viewer contains all possibilities of image processing from the main programme but files cannot be saved in changed versions. It shall be a tool for remote experts for optimised evaluation of digital radiographs.

After leaving the inspection software the programme returns to the documentation part and transfers the results of evaluation (tangential wall thickness and/or corrosion hollows) into the documentation of the exposure.

The list with the transmitted data can be called for displaying from the level "result" (see picture 7). Depending on the estimated value the wall thickness is marked as "ok" or "n.ok". The criteria for this evaluation is the value of the field "acceptable wall thickness reduction" in percent.

Pic 7: Transmission of estimated values into the documentation software

Pic 8: Test report

Pic 9: Searching possibility in the documentation software

This evaluation shall give an orientation about the status of the pipe system. This value is used too for the generation of the list "not acceptable pipes (advise for repair)". After finishing the inspection work the inspection report may be printed out immediately on the site if desired (see picture 8).

For the print out of documents a MS Word interface is used. The selected data are transferred into a pre defined Word document which may be printed out using the integrated standard functions of WORD.

The form and definition of the Word document may be changed by the user itself if the test report shall include other information or shall have another structure.

The data which shall be transmitted into the report form are defined in the printing module of the software which can be called via the programme branch "general data". The definition can be changed via this way. New lists and new forms may be defined for printing data using this printing module as well.

For searching special exposures out of the existing the searching function is used (see picture 9).

After inserting any search information all fitting data sets (exposures) are shown on the display. If the corresponding exposures files are already available they are displayed in small size in the searching mask for orientation. In this case the original exposure is shown and additionally, if existing, the evaluation of tangential wall thickness and the evaluation of corrosion areas in the perpendicular penetrated pipe areas.

5. Possibilities of application and limits

The used detector system is applicable for the whole range of x-ray energies, SE 75 and Ir 192 radiation sources. The gray scale dynamic, as already mentioned, is 16 bit/pixel so on the same picture a density range of D=4,8 is covered and can be evaluated.

The linear characteristic of the gray scale resolution enables the electronic pass through of the total density range of the exposure without any loss of information. The displayed density range may be reduced as well. This corresponds to an increase of contrast on the display. Big advantages result from these functions for the evaluation of the total density range of the exposure compared to the evaluation of film radiographs.

The active area of the detector used at this time is 204 x 204 mm in dimension with a weight of 8,5 kg. The outer dimensions of the cover are 330 x 335 x 52 mm.

Depending on the pixel dimension of 400 mm the system may not yet be used for welding inspection.

The high sensitivity of the system shortens the required exposure time considerably compared to film exposures. In the average the required times are reduced by a factor of 10 (related to D5 film). So the radiation dose for the environment is reduced considerably as well when performing radiography. Regarding the general reduction of dose rate limits which is going on in legislation at this time there is a big advantage for this system compared with film radiography. Furthermore in many cases inspection can be performed in plants during running production without the need to deactivate the radiation sensitive measuring devices for plant control because the exposure times may be reduced below the reaction dose of the control devices.

Related to the immediate display of the exposure required corrections of the test arrangement are obvious at once and the area may be re examined after arrangement correction immediately. Any time delay caused by required film development or reading out processes in the case of other systems are completely avoided.

The deviation related with wall thickness estimation are +- 5-10% for wall thickness > 2 mm.

Initially the system was developed for the estimation of wall thickness in the tangentially penetrated area, for the estimation of residual wall thickness in corroded areas in the perpendicularly penetrated area and the estimation of diameter of insulated pipe systems in co operation with BASF. Besides these applications further interesting possibilities of application are obvious.

Basically the system is applicable for radiographic applications where a pixel size of 400 mm is sufficient. Among others these may be the recognition of sedimentation of solid mater in pipe systems during running production (special advantage of the on line exposure display), the inspection of castings with high dimension differences, the inspection of plastic components looking for the internal structure, the estimation of dimensions in the case of inacccessibility, the estimation of position and dimension of iron reinforcement components in concrete structures.

If micro focus x-ray equipment combined with direct magnification is used the limits in application related to the pixel size of the detector used at this time concerning weld inspection can be neutralized partly or totally so that applying this technique weld inspection can be performed in stationary operation as well.

Related to the advantages of the digital system in the future further application possibilities will be opened up where nowadays radiography does not seem to be suitable.

6. Performance of inspection

The practical application of the mobile system is shown in the pictures 10 and 11. Picture 10 shows the fastening of the detector within a protection cassette mounted at an insulated pipe by a clamping band, picture 11 shows the complete arrangement for the case of projection radiography.

Pic 10: Fastening of detector at the pipe system

Pic 11: Test arrangement for projection exposures

7. Practical examples

The following pictures represent examples from the first applications of the inspection system.

Pic 12: Pipe 93 x 2,6mm, broken weld at pipe junction, Ir 192, 25 Ci, 6 sec	Pic 13: Corrosion area in an insulated pipe system, 116 x 5,3 mm, Ir 192, 20 Ci, 12 sec
Pic 14: Pipe system blocked by washing powder, 160 x 4 mm, , Ir 192, 25 Ci, 15 sec	Pic 15: Radiograph from picture 14 after image processing (changed density and contrast)

In picture 14 und 15 the advantages of changeable density and contrast are obvious. Both pictures show the same exposure. Picture 14 represents the original exposure. Only a not clearly definable suspicion of a fault along the pipe wall may be guessed. After just changing density and contrast by image processing two longitudinal cracks in the wall are clearly detectable (see picture 15).

Pic 16: Wheel hub from aluminium with unacceptable shrinkage, wall thickness approximately 100/60/30 mm, X-ray, 200 kV / 10 mA, 8 sec

Pic 17: Turbine blade approximately 100 x 40 mm, micro focus tube, 100 kV / 0,2 mA, 8 sec, enlargement approximately 5 times

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