poster

1. K. Chatterjee, 75643 Center for NDT P 00 Rv 02 Self study material.
Double Wall Exposure Techniques :
When it is not practical to use a single wall technique,
double wall technique is selected.
Single Wall viewing : For materials or welds, a technique may
be used in which the radiation passes through two walls and
only the weld [ material ] on the film side is viewed for
acceptance on the radiograph. When complete coverage is
required for circumferential weld [ material ], a minimum of three
exposures taken 1200
to each other shall be made. The IQI
shall be placed on the film side with a lead letter “ F “ adjacent
to or on the IQI without obscuring the 2T hole for a hole type
IQI for each radiograph.
Set Up [ for three exposures method ] :
Visual examination : Any visually detected surface imperfections
which will produce images on the radiograph shall be rectified
before shooting.
Segment marking : The circumference is divided into three equal
segments and marked A, B, C / 0, 1, 2 etc, such that the marks
remain till the joint is examined and accepted. Lead location
markers are to be fixed at this spots.
Film length : The length of each film is selected such that
minimum 1” of overlap with next film is available at each end.
The width shall be sufficient to completely record the weld, the
markers and the penetrameters.
Selection of IQI : The IQI is selected by calculating 2% of single
wall thickness to be examined. The IQI may also be selected
from tables as per applicable specifications / procedures. The
single wall reinforcement in the weld must be included in IQI
selection.
Fixing Numbers and IQI : On the outside wall, at the ends of
the segment under examination, lead location markers are
attached with adhesive tape. Job identification numbers are
generally attached at the center of the segment. The applicable
IQI is fixed near one of the location markers such that the 2T
hole or the thinnest wire in the IQI is closest to the marker.
The lead letter ‘F’ is then attached near or on the IQI without
obscuring the essential hole to be recorded. All the numbers /
IQI shall be arranged such that their images do not fall on the
image of the weld. The film is wrapped over the set up
ensuring good contact with the examination surface and with at
least 1” projecting beyond the location markers at each end.
The width of the film shall be sufficient to record the weld, all
the markers and the IQI with identifications.
Allignment of Source point : The exposure point of the source
guide tube is then fixed to the surface on the location mark
opposite to the selected segment to be exposed. Source point
may touch the part surface or may be away from the surface
to meet the SFD requirements based on applicable unsharpness
value. The exposure point should be positioned such that the
weld on the source side is not in line with the central beam of
the radiation field being used for imaging. If the test parameter
requires the source point to be away from the part surface,
Minimum 4 exposures will be required to complete the
examination.
Safety precautions during radiography work : Use of personal
monitoring device [ TLD ] and a dosimeter, while working with a
radiation source is a must. A calibrated and properly functioning
radiation survey meter must be used to monitor the radiation
levels at the boundary of the radiation zone, when the source is
driven out of the source projector and during the withdrawal of
the source into the projector. High radiation level indicates that
the source is outside the projector. Ropes / Tapes with radiation
warning symbols must be used to separate the radiation zone
and unauthorized entry into the radiation zone must be
prevented. Red blinking light placed near the radiation source for
safety awareness is recommended. Except when operating the
projector, the radiation workers must be at a safe distance from
the radiation source.