Radiography
When X-rays and gamma rays have been passed through
material some of the
radiation is absorbed in the material. The amount of
absorption is depend
up on thickness of material , the density of material and
the atomic number
of the absorber. Some kind of detector like a special
category film takes the
image of the material and show the discontinuity if any
in the material. This
is one of the common NDE testing in the field of construction.
This is
extensively used in structures and pressure parts welding fault
detection.
The advantage of RT over other
NDT methods is that it presents a permanent
record, simple to read out ,
relatively less costly , can be uses in sites and the
system is portable. But
it is highly dangerous to handle radiation sources
without proper training.
The isotopes ( Iridium 192 and
cobalt 60) are generally used at site
now a days. However use of cobalt 60 is
also very restricted because
it has
adverse effect on humans and only used for high thickness material.
Progress of
ultrasonic testing reduces the requirement of Cobalt 60 to
a large extent in a
thermal power station.
Iridium 192:- Produced by neutron
bombardment of Ir191 has a half life
of 75 days .The penetrating capability is
equivalent to 600kv x-ray
machine. It is relatively low energy radiation and
high specific activity
makes it an easily shielded strong radiation source of
small specific size.
Cobalt 60. :- This is also
artificial isotopes produced from Cobalt 59. This
has the half life of 5.3
years. This has penetrating capability of 2000kv x-ray machine.
Isotope Camera.:- The equipment
,which accomplishes safe handling and
storage of isotopes sources is called a
camera.
Radiographic film is normally
used as the recording medium in radiography
and resembles a photographic film.
The film consists of a ) base and
b) emulsion. The base is transparent medium
made of polyesters of
about 50 microns thick on which the sensitive emulsion is
coated.
Image quality Indicator : As a check on radiographic
technique ,a
standard test piece , called penetrameter is used in radiography.
There
are three to four type penetrameter is available however wire type or
hole type is more popular than others. Penetrameter shall be same material
as of
the job but it is sometimes found that the same material is not
available then
material with lower atomic number will suffice.
Radiographic Techniques : There are different type of
radiographic technique
for different jobs .
1. Single wall single image : Generally used for plates
,drums welds , casting of
larger sizes etc.
2. Double wall double image : Genrally used for smaller
size tubes in boiler .
3. Double wall single image : When the diameter is
above 89 mm this type of
technique is used.
Radiographic film is viewed through a viewer whose area
and intensity of light
can be varied as per requirement. Interpretation of
radiographic film is a
specialised job and to be done by experts only having
certificate of Level-II.
There are many institute in India who give training
on NDE .
The common defects noticed in Radiographic film
interpretation are gas
inclusion and porosity , slag inclusion , lack of
penetration , tungsten
inclusion, cracks and lack of fusion.
Iridium 192 has penetration capability of 5mm to 70 mm
steel.
Cobalt 60 has
penetration capability of 50 mm to 200 mm steel.
Radiographic source are only available at BARC India.
All agencies operating
in India should have registration with BARC.
Iridium 192 source having intensity of 20curie to 32
curie is best suitable for
regular job. More the intensity less the exposure time. Production will be
more.
Radiographaic
source to be handled by nominated person
only. Safety of the personnel is
prime. All the observation
of BARC to be followed word by word. Source pit to
be
made at site as per BARC design. No compromise to
be made on the recommendation.
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