Testing
& Inspection |
| NON-DESTRUCTIVE
EXAMINATION |
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RADlOGRAPHIC
EXAMINATION |
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Radiographic
examination is based on the principle that extremely high
frequency light waves, usually x-ray or from a radioactive
source such as Cobalt 60, will penetrate solid materials and,
when projected onto a photosensitive film, will reveal voids,
areas of discontinuity, and lack of homogeneity. This examination
is widely used in evaluating the soundness of welds and in
general, is limited to evaluating butt welds of parts of substantially
the same thickness and material. In the case of bellows, this
is normally limited to the evaluation of longitudinal seam
welds before forming.
Unless required by the purchaser, radiographic examination
of the longitudinal seam of a bellows need not be specified.
Examination of the longitudinal seam can be accomplished by
some other means, such as liquid penetrant examination. If
a radiographic examination is required on the longitudinal
seam of a bellows then it should be performed before the bellows
is convoluted. After the forming operation, it is usually
not possible for the source or the film to be placed to yield
a meaningful radiograph.
Radiographic examination of the bellows attachment weld should
not be specified. Interpretation of such radiographs is impractical
due to the weldment geometry, differences in thickness and
penetrability. In view of the above, and recognition of the
attachment weld as a seal weld, non-destructive examination
of this weld should be accomplished by some other means such
as liquid penetrant examination.
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LIQUID
PENETRANT EXAMINATION |
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Liquid
penetrant examination consists of cleaning a surface, coating
it with a dye, wiping the dye off and coating the surface
with a developer which after sufficient time will draw the
dye from the cracks, pin holes, and make them apparent to
the observer. Liquid penetrant examination is limited in scope
to detecting surface indications such as fine hairline cracks,
pin holes and weld roll-over. With the thin material used
in bellows, the probability of any defect remaining subsurface
is unlikely. This examination is frequently used in evaluating
bellows welds. The bellows base material may also be inspected
by this method but shall be performed prior to convolution
forming. The developer used in this procedure acts as a blotter;
therefore, when rechecking a questionable indication it is
absolutely essential to re-clean that area and reapply dye
and developer. Unless otherwise specified, examination procedures
shall conform to the requirements of ASTM-E165. |
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MAGNETIC
PARTICLE EXAMINATION |
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Magnetic
particle examination consists of coating a surface with finely
powdered iron and establishing a magnetic field in the material
being examined. The presence of discontinuities or irregularities
in the magnetic field, as indicated by the lines of powdered
iron, will indicate surface and also subsurface defects, cracks,
slag inclusions, and lack of weld penetration, This examination
is limited to magnetic material and will not indicate deep
subsurface defect. Although generally used for examination
of welds, it is possible to examine base material if there
is reason to suspect material defects such as laminated plate. |
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ULTRASONIC
EXAMINATION |
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Ultrasonic
examination uses high frequency sound waves to detect flaws,
and is useful in determining thickness, depth, and exact location
of defects. Interpretation of indications in sections of sharply
varying thickness is difficult. The examination is not limited
to any group of materials. |
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AIR
JET LEAK EXAMINATION |
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Air jet leak examination utilizes compressed air is directed
through a nozzle on to a small area between two welded parts.
A leak detector solution is applied on the opposite side of
the welded connection which will bubble up if the compressed
air is able to pass through the weld. This examination is
useful on low pressure. Expansion Joint bellows end connection
welds where other forms of examination and testing are not
practical.
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PRESSURE
TESTING |
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Hydrostatic
and pneumatic are two types of pressure tests that can be
performed on an Expansion Joint. Hydrostatic pressure testing
involves filling the Expansion Joint with a liquid, usually
potable water, while pneumatic pressure testing involves filling
the Expansion Joint with air or other gas. After the Expansion
Joint is filled it can then be pressurized to the required
test pressure. Pneumatic pressure testing is hazardous and
it is recommended that special precautions be taken.
In the Expansion Joint industry, it is frequently the practice
to hydrostatic pressure test an Expansion Joint to one and
one-half times the design pressure or pneumatic pressure test
it to 1.1 times the design pressure at ambient temperature.
Expansion Joints placed in high temperature service may require
the pressure test be performed at an adjusted pressure. It
is imperative that the test pressure does not produce a membrane
stress in excess of yield strength or cause permanent deformation
or instability (squirm) of the bellows at test temperature.
It may be necessary to reduce the test pressure adjusted for
temperature, to the maximum pressure that will not exceed
yield or cause instability.
An Expansion Joint should not be subjected to a test in the
field at a higher pressure than was used in the manufacturer's
shop without the manufacturer's knowledge. All anchors and
guides must be installed and shipping devices removed prior
to such testing. In the case of large Expansion Joints, additional
supports may be required to support the weight of the water
used during hydrostatic testing.
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