NDT TESTINGS: RADIOGRAPHIC EXAMINATION
In
this article we discuss about procedure and requirements, acceptance criterion
of Radiographic Testing (RT) for welds for discontinuities.
- Applicable
standards:
I) ASME
SEC. V ARTICLE 2 (Non-destructive examination code).
II) ASME
SEC. VIII DIV. 1 appendix 4.0.
III) ASME
SEC. VIII DIV. UW- & UW-51.
Radio graphic
examination is employed for find internal discontinuities such as porosity,
cracks, voids, inclusions, inadequate penetration, lack of fusion etc.
- Radio-graphic
Procedure Required data :
The procedure for RT
examination (Radiographic Examination) contain following information,
1) Type of material.
2) Thickness of material.
3) Film designation.
4) Type of screen.
5) Maximum source size.
6) Main isotopes type
7) Minimum source to film
distance.
The density of RT film checked by
Densitometer, sometimes step wedge also used. The selection of film is major
task because the image quality requirements depends on the film selection. one
penetrameter is used for each RT film The area of uniform density determine by
densitometer within minus 15% or plus 30%.In case of spherical vessels the
source is located at center of vessel and minimum penetrameters shall be spaced
approximately 120 degree apart. Unexposed film shall be stored in such a way
that film not expose to light, pressure, humidity, heat etc.
The
film processing is major process which done by equipment, with help of washing
agent, developer, fixer, drying equipment. Basically, processing of film in
which following steps involved,
1) Development of film.
2) Stop bath
3) Fixation
4) Washing of film
5) drying
- IQI (Image Quality Indicator)
Radiography
shall be performed with sufficient sensitivity to display the penetrameter
image and also specify the type of penetrameter or IQI. Generally, IQI consist
of two types
1) Hole type IQI.
2) Wire type IQI.
- Interpretation:
Discontinuities
in welds is interpreted on film and acceptance of that defect or
discontinuities done as per specification & codes.
ACCEPTANCE STANDARD(as per
code):
- For Full Radiography –
1) Any type of cracks and
incomplete fusion of penetration not acceptable.
2) Any elongated indication
which has length greater than,
1) ¼ inch (6.4mm) for t upto ¾
inch (19mm) include
2) 1/3 t for t from ¾ inch to 2
¼ inch (19mm to 57 mm) include.
3) ¾ inch (19mm) for t over 2 ¼
inch (57mm)
Where t is thickness of high thickness plate.
4) Align indications having
length grater than t in length of 12t unless minimum distance between
successive indications exceeds 6L.
5) Rounded indications is
acceptable as per code.
For Full Radiography –
1) Any type of cracks and
incomplete fusion of penetration not acceptable.
2) Weld in which slag
inclusions, cavities not acceptable.
3) Rounded indications are not
factor in acceptability of weld not required to fully radio graphed.
The acceptance criterion shown code as shown detail in code.
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