De la migración masiva a la Lima Conurbana

In circumstances where there is doubt as to the nature of a radiographic image it is often necessary to visually inspect the component or to cross check the radiographic results using another NDT method.

12.2 VIEWING CONDITIONS

The success or failure of radiographic interpretation is highly dependent upon the film viewing conditions. The eye is very sensitive to small variations in film density once it has developed ‘night vision’. Anyone carrying out radiographic interpretation should therefore not begin to view radiographs until ‘night vision’ has developed. Since this cannot be achieved in a brightly lit room it is important that the films are viewed in low ambient light. ‘Night vision’ takes several minutes to develop and so the films should not be viewed immediately upon entering the viewing room. Five minutes is the recommended period that should elapse before critical interpretations are made. It is also important that film is

properly masked on the viewer so that the light falling on the eye comes from the radiograph only. If the film is not adequately masked the eye will be blinded by the bright light coming from around the film.

Radiographs are easily damaged, therefore the viewing room must be clean and dry and the radiographs must be handled with care. The viewer should be mounted on a table or bench large enough to allow the films to be spread out without the danger of them falling to the floor. A well shielded reading lamp will allow reports to be read or notes to be made, without unduly increasing the overall ambient lighting.

The radiographs should be viewed at a normal reading distance ( normally less than 400 mm). A low power magnifier (2 or 3X) may occasionally be helpful, but it should not be necessary for routine examination. In accordance with PCN and CSWIP requirements the visual acuity of the radiographic interpreter must be J1 in at least one eye (corrected or uncorrected).

The viewing of radiographs is often undertaken in the dark room where the film was processed. This is satisfactory provided that the viewing bench or table is clean and well away from the processing tanks. Under normal circumstances films should NEVER be viewed whilst wet. There are two reasons for this:

(a) The film emulsion is swollen with water and the images are not as clear as when the film is dry.

(b) The emulsion is very delicate and any attempt to mask the film will result in scratches or marks on the film, effectively ruining it.

National standards generally require that the illuminance of a radiographic film viewer be sufficient to produce a transmitted light intensity of at least 30 and preferably 300 candela per square metre (cd/m²). This means that a viewer suitable for viewing radiographic film with a density of 3.0 must have an illuminance of at least 30,000 cd/m² with as much as 300,000 cd/m² being desirable. BS EN 25580 requirements for radiographic film viewers are given by table 16 below. Note that these are minimum requirements.

Film Density Minimum Screen

luminance (cd/m²) Transmitted light Illuminance (cd/m²)

Table 16. BS EN 25580 requirements for radiographic film viewers 12.3 REPORTING

The initial interpretation of a radiograph should always be undertaken by the manufacturer or his designated representative. Other interested parties should be presented with a report which includes an interpretation of each film. It is their job to check this and to agree or disagree with it. The radiographic report should contain the following information as a minimum:

(i) Identification of the item radiographed.

(ii) The date of manufacture.

(iii) The date of radiography.

(iv) Exposure details including the type of equipment used and the tube voltage for x-ray techniques and the type of isotope for gamma x-ray techniques.

(v) The type of film used.

(vi) The type and thickness of the intensifying screens used.

(vii) Geometric details, particularly the FFD or SFD and the effective focus or source dimension.

(viii) Details of the component being radiographed, including the type of material and method of manufacture, the thickness, the heat treatment condition and the repair status.

should be rejected and reshot. The manufacturer’s interpreter may, for economic reasons, not be inclined to reject radiographs which do not meet the minimum quality standards.

Therefore any third party viewing the radiographs should be extremely careful to correctly assess the quality of the radiographs prior to endorsing the relevant report. Otherwise the third party will be open to criticism should the film become the subject of any subsequent legal inquiry. When assessing a film for quality a number of items must be considered.

These are discussed below.

12.4.1 COMPONENT IDENTIFICATION

All radiographs must be permanently and uniquely marked with sufficient information so as to permit their identification with the component radiographed at a later stage. It is often useful to include such items as the date of test and heat treatment or repair status of the component in the identification. Radiographic identification could appear on the radiograph as a radiographic image but there is usually no reason why it should not be added by any other suitable means. A written procedure should be in force describing the standard method to be used for identifying radiographs.

12.4.2 LOCATION MARKERS

Location markers on a radiograph serve two functions: they permit the radiograph to be identified with the area of the component radiographed and they serve to prove that the component has been fully covered by the technique used. Refer to the sections above on radiographic techniques for details. Wherever possible location markers should permanently identify the radiograph with the area radiographed. Items such as pressure vessels are usually hard stamped with a permanent radiographic datum. A written procedure should be in force which describes the standard method used for the placement of location markers.

12.4.3 FILM DENSITY

It is important that the film density is within the specified range since a film having low film density will also have inferior film contrast. BS EN 1435 requires a minimum film density of 2.0 for class A radiography and a minimum of 2.3 for class B. ASME V Article 2 requires a minimum of 1.8 for x-ray techniques and minimum of 2.0 for gamma ray

techniques. In most cases (including BS EN 1435 and ASME V article 2) the minimum figures for film density apply to the area of interest (the diagnostic area) on the radiograph.

In weld radiography, for example, film density should generally be measured on the weld

area between the location markers (which identify the ends of the diagnostic film length).

Density can be assessed either by comparison with a calibrated density strip (which

preferably should have been made using film identical to that of the radiograph) or by using a measuring device known as a densitometer. Anyone accepting radiographs which do not meet the applicable density requirements is open to criticism at a later stage should litigation follow a component failure.

ASME V article 2 requires that the film density within the area of interest must not vary by more than minus 15% or plus 30% from the value measured through the body of the IQI. If necessary additional IQIs can be used in order to satisfy this requirement for exceptional areas.

Occasionally an upper limit is specified for film density. ASME V article 2, for example, specifies an upper limit of 4.0.

12.4.4 RADIOGRAPHIC SENSITIVITY

Radiographic sensitivity is not directly related to the minimum detectable defect size. However, a radiograph that meets the applicable code requirement for radiographic sensitivity, is much more likely to provide good defect sensitivity than a radiograph which fails to meet the code requirements.

The sensitivity of a radiograph depends upon the parameters chosen to produce that

radiograph (see the section above on the production of a radiograph). If any of the relevant parameters are altered the sensitivity will be affected. It is therefore essential to use Image Quality Indicators (IQIs) in order to prove that adequate radiographic quality has been attained. Except in the case of the ‘panoramic technique’, which has been described above, at least one IQI should generally appear each radiograph.

Anyone viewing radiographs should be careful to check that the radiographic sensitivity meets the requirements of the applicable code. Anyone who fails to do is open to criticism should litigation follow a component failure.

12.4.5 ARTEFACTS AND OTHER UNWANTED IMAGES

In film radiography an artefact can be defined as ‘any image resulting from a cause that is not directly associated with the object that has been radiographed.

Artefacts can be produced by mechanical or chemical damage to the film and by damaged or dirty intensifying screens. Sometimes radiographic images may be formed by things such as debris on the internal of a pipe. These images, while they are strictly speaking not artefacts, can also interfere with the proper interpretation of the radiograph.

When radiographs are being produced on a commercial basis it is not possible for every film to be free from all artefacts. An artefact only becomes significant when it cannot be

identified as being an artefact or when it hinders the interpretation of the film. These two

radiograph for artefacts which are not within the diagnostic area. The following sections attempt to give a description the various types of image which may be seen on a radiograph.

The ability to successfully identify all radiographic images is a skill which can only be perfected with time and experience.

12.6 ARTEFACTS

12.6.1 PRESSURE MARKS (CRIMP MARKS)

These are produced by careless film handling. If the film is crimped or buckled either before or after exposure crescent shaped images in the processed radiograph will result.

Light marks indicate crimping before exposure. Dark marks indicate crimping after exposure but before film processing.

It is usually possible to identify crimp marks by viewing the film in reflected light.

They should appear as indentations in the surface of the film. Lead screens which have been crimped should be discarded.

12.6.2 SCRATCHES: ON THE FILM

Radiographic film emulsion is delicate, it is easily damaged if handled carelessly at any stage during the production of a radiograph. Areas used for film handling must be free from dust and films must be handled carefully at all times. Depending upon how severe and when or how formed film scratches may produce either light or dark images. Film scratches can usually be identified using reflected light.

12.6.3 SCRATCHES: ON LEAD INTENSIFYING SCREENS

These may appear as either light or dark images which cannot be seen in reflected light. If the intensifying screens used to make the radiograph can be positively identified then it may be possible to trace the shape and position of such an image to a scratch on the screens. Even if this can be done it will probably be necessary to reshoot the radiograph.

Scratched lead screens should be discarded.

12.6.4 DIRT: ON THE FILM OR SCREENS

Dirt which finds it’s way between the film and the screens will in general produce a light image on the resultant radiograph which is not visible in reflected light. Greasy fingers will produce dark marks on a finished radiograph which can easily be seen in reflected light.

Greasy fingers before development produce light marks.

12.6.5 STREAKINESS OR MOTTLING: POOR DEVELOPMENT

This is usually caused by insufficient agitation in the early stages of development and is due a process known as ‘bromide streaming’. Reaction products from the chemical interaction of the developer with the silver halides in the film emulsion tend to build up around high film density zones. These reaction products slow down the action of the

developer. Since they are relatively heavy they will tend to flow down the surface of the film leading to a light coloured streak in the finished radiograph. Under or over development usually leads to a mottled effect on the finished radiograph. A similar effect will be produced by developer which has passed the end of its service life. In less severe cases such artefacts may not be a cause for rejection of the radiograph but darkroom procedures should be reviewed in order to prevent a recurrence or a further deterioration in radiographic quality.

12.6.6 DEVELOPER SPLASHES

These will appear as dark spots on the film and indicate poor dark room practice.

Such marks are usually visible in reflected light.

12.6.7 FIXER SPLASHES

These will appear as light spots on the film and again indicate poor dark room practice. Such marks are usually visible in reflected light.

12.6.8 WATER SPLASHES

These may appear as either light or dark images on a radiograph. Water splashes before exposure tend to cause light marks. Water splashes after exposure tend to cause dark marks. Such marks are usually visible in reflected light.

12.6.9 WATER MARKS

These are easily seen on the radiograph in both transmitted and reflected light and are due to uneven drying. They commonly occur where a dry or partially dry film is wetted locally either by splashing or by excess water running down from a film clip. The

appearance of water marks can be reduced or eliminated by the use of a squeegee to remove excess water or by the use of a final wash that contains a small amount of detergent (i.e. a wetting agent).

12.6.10 AIR BELLS

not occur in gamma radiography because of the absence of low energy beam components.

Diffraction can be used to advantage. It is the basis for the study of metal crystals by x-ray crystallography.

12.6.12 STATIC MARKS

Penetrating radiation is by definition ‘ionising’. It always causes the build up of an electric charge on the film during exposure but under normal circumstances this is not a problem because the charge quickly flows to earth. In dry climates, however, a static charge may remain on the film up to the point where it is unloaded in the darkroom, whereupon it flows to earth suddenly in a manner which could be painful for the radiographer. Such a sudden dissipation of electrical energy leads to the emission of a sudden burst of light. This light produces dark tree-like marks on the finished radiograph. Static marks can be avoided by careful film handling.

12.6.13 DICHROIC FOGGING

Radiographs affected by dichroic fog will appear reddish when viewed using transmitted light and greenish in reflected light. Dichroic means two-coloured. This artefact is caused when the development process continues during the fixing process. It happens when the fixer solution has become insufficiently acidic to stop the development process.

The use of an acidic stop bath between the development and fixing processes will in general prevent the occurrence of this seldom seen artefact.

12.6.14 RETICULATION

This artefact appears on the radiograph as an orange peel like mottling effect. It is caused when the film emulsion is subjected to a temperature shock at any stage during the film processing. A sudden change in temperature causes the film emulsion to wrinkle. It will not generally occur as long as the sudden change in temperature is less than 10°C.

12.6.15 FILM FOGGING BY X OR GAMMA RAYS

If radiographic film is not stored well away from sources of ionising radiation then it is likely to become ‘fogged.’ Films which have been fogged in this way will produce

reduced radiographic contrast (fogging has much the same effect as scattered radiation which

is explained in a section above). If it is suspected that the film is fogged then the fog level can be checked by processing a piece of unexposed film. Film which has a density due to fogging of 0.3 or more is not suitable for use in high quality industrial radiography.

12.6.16 LIGHT FOGGING

Exposure to light other than that from darkroom safelights (actually prolonged exposure to safelights will cause also fogging) at any stage prior to fixing the film will cause the film to become fogged. Such fogging may be localised or general. Localised fogging is not a problem unless it encroaches onto the diagnostic film area. General fogging by light has the same effect as fogging due to exposure to ionising radiation.

12.6.17 FILM FOGGING DUE TO INADEQUATE STORAGE CONDITIONS Film stored at too high a temperature or which has been exposed to chemical fumes may become fogged. The fog level of all film increases with age, even under ideal storage conditions, therefore all film boxes are marked with an expiry date. High speed films deteriorate more quickly than do slower films.

12.6.18 SOLARISATION

Image reversal due to extreme over exposure to x or gamma rays or caused by exposure to light during film development.

12.6.19 A FINAL WORD ON ARTEFACTS

It should be stressed again that artefacts are cause for rejecting the film only if they interfere with interpretation. A large number of artefacts present on the radiographs indicates poor practice and the interpreter should take time to inspect the radiographic facilities and review darkroom procedures.