Trinity NDT - Brief basics of Visual and Optical Testing(VT)
Visual Testing(VT) basic requirements
The tool that is used to perform VT is the human eye. The following are the requirements for the performance of VT:
Visual Acuity - Personnel who perform VT must pass an annual
eye examination in accordance
with industry
standards. The
eye examination
checks for conditions such as visual
acuity, color blindness, and depth
perception.
Distance - The
examiner’s eye
should be
located within
24 inches
and at
an angle of
not less
than 30
degrees to
the surface
of the
weld that
is being
examined to
conduct a direct visual
examination. Mirrors can
be used
to improve
the angle of vision.
Access - If the area to be examined is not directly accessible, an examination aid can be used.
Examination
aids are covered later in this Module.
Lighting - A flashlight or other additional lighting should be used to sufficiently illuminate
the area that is to be examined. A minimum of 35 foot candles of
light should be available
for normal visual weld examinations.
When performing VT for
small indications,
a minimum
of 50
foot candles
of light
should be
available. If
required by
procedure, a
light meter
can be
used to
determine the exact amount of illumination that is available.
Examination Aids
Examination aids
sometimes are
used to
facilitate visual
examinations. The following
are examples of commonly used visual examination aids:
1. Mirrors
2. Portable Lighting
3. Flashlights
4. Light Meters
5. Straight Edges and Rulers
6. Magnifying Lenses
7. Boroscopes
8. Microscopes
9. Video Cameras
10. Weld Gages
When the use of visual testing/examination
aids (such as boroscopes) are necessary to perform remote examinations, the image resolution
must be
at least
equal to
the image resolution that
is attainable by direct visual examination.
Several types of weld inspection gages are available to simplify the measurements of complex weld configurations when
conducting VTs.
The
following are
some of the gauges that are generally used:
Cambridge Gage - This gage can measure undercut, weld reinforcement, bevel angle, root opening, fillet weld size, and joint mismatch.
GAL “Hi-Lo” Gage - This gage can measure joint mismatch, root opening, and weld reinforcement.
AWS Type Gage - This gage also can measure various sizes of standard fillet welds.
Common Applications
The following are common applications for visual weld examinations
- To determine the size and length of fillet welds on structural members
- To inspect the weld joint fit-up including bevel angle, root opening, land, and cleanliness of piping welds.
- To inspect in-process welds and completed welds prior to additional NDT.
- To inspect the proper fit-up of socket weld fittings on small diameter pipe.
The following
are the common discontinuities that can be detected during a visual
examination:
- Cracks
- Slag
- Porosity
- Undercut
- Cold Lap
- Arc Strikes
- Many more surface discontinuties
Purpose of Visual Inspection method The purpose of a visual weld examination (VT) is to detect visible surface discontinuities on a weldment. VT is the most frequently used method of examination, and welders and welding
inspectors continuously use VT during welding operations to make better quality welds. VT
often will identify
problems during welding that can be repaired “in process” to prevent the
discovery of a discontinuity by a subsequent nondestructive test.
Advantages of Visual Inspection method VT is the quickest and most cost-effective method of NDT for use in the identification of a surface discontinuity on
a weld.
Because the VT
method of
NDT requires
the fewest
technical and interpretive skills, VT also is the simplest NDT method to learn.
The use of VT throughout the welding
process significantly
improves the
success of
subsequent NDT
and reduces
the cost of repairs.
Comments