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Liquid
Penetrant Testing |
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Liquid
penetrant (PT) testing is a nondestructive testing method used
to locate discontinuities that are open to the surface of solid
materials. PT is often described as an aid to visual inspection
because it reveals flaws that are open to the surface of a
material. Cracks and other defects become easier to see when
either colored (visible) or fluorescent dye is carried into the
defects through a process called capillary action. After
penetrant has soaked into the discontinuities, the excess
penetrant is removed, a developer is applied to the material's
surface and the penetrant out of flaws. The developer provides a
contrasting surface to view the defects.
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Visible dye
penetrant systems make it approximately 6X easier to see
discontinuities than without the use of liquid penetrant.
Fluorescent penetrant, viewed under black light, increases
discontinuity visibility approximately 300X. Visible and
fluorescent penetrant systems are further classified by the
method used to remove excess penetrant from the materials
surface: water washable, solvent removable, and
post-emulsification, each referring to a different type of
penetrant.
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PSA uses
both visible and fluorescent dye penetrant systems and either
water washable or solvent removable processes to locate and
identify cracks and flaws as small as 1/16th of an
inch. Water washable fluorescent penetrant systems are generally
used to examine a boiler floor or large areas of pressure
vessels or storage tanks. Visible solvent removable dyes are
usually used to examine smelt spout openings, air ports, and
other openings in the lower furnace area of a boiler, as well as
weld areas on boiler tubes, piping systems, and fans.
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Magnetic
Particle Testing |
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Magnetic
Particle (MT) testing is a nondestructive testing method used to
locate discontinuities at or near the surface of ferromagnetic
materials. MT is performed by magnetizing the area to be
examined and applying a high-permeability magnetic powder. The
powder will be attracted to flaws in the area by magnetic lines
of force leaving the material at discontinuity sites. Magnetic
lines of force leave the magnetized material at the
discontinuities because typically flaws such as cracks,
incomplete fusion, or slag inclusions cannot hold as many
magnetic lines of force as solid ferromagnetic material. Thus,
once magnetic powder is applied to the surface of a
magnetized ferromagnetic material it will be attracted to
leakage fields
produced by
defects at and above the material's surface.
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Powdered
magnetic particles may be either colored or fluorescent and are
dusted over the surface or poured over the surface suspended in
a liquid carrier. Fluorescent particles viewed under black light
offer much higher contrast than colored particles and particles
suspended in a liquid carrier are smaller and more mobile than
dry powder particles. So when looking for small discontinuities
on the surface of a ferromagnetic material optimum sensitivity
can be achieved by using wet fluorescent particles.
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Magnetic
particle examination can also be classified by the type of
magnetism used to magnetize the material. PSA usually uses an
alternating current magnetic yoke to induce a continuous
longitudinal field in the area to be examined. This is
considered best for locating small flaws at the surface of
ferromagnetic materials. We can also induce longitudinal
magnetization using coils and circular magnetization using
prods, contacts, or central conductors.
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PSA uses
alternating current yokes with wet fluorescent particles to
examine structural welds on boiler fan rotors and
circumferential and longitudinal seams on deaerating feedwater
tanks and heaters.
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Ultrasonic
Thickness Testing |
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Ultrasonic
Testing (UT) is a nondestructive testing method utilizing
high-frequency sound waves to perform thickness measurements,
corrosion checks, and internal discontinuity scans in welds,
forgings, castings, and state-of-the-art composite materials.
Defects or weak areas, such as flaws or wall thinning, are found
by observing reflected sound energy (pulse echo testing) or by
measuring energy lost as the ultrasonic sound waves travel
through the material (transmission testing) under examination.
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The major
advantages of using the ultrasonic testing method are the
ability to locate flaws in thick materials, accurately determine
defect area and depth, test when restricted to one side of the
material, and electronic output capability that makes it
relatively easy to automate ultrasonic testing and recording
systems. Ultrasound can penetrate more than 100" of solid steel
and provide accurate test results. UT can measure thickness up to an accuracy of within ±
0.001”.
Thousands of readings can be automatically recorded and stored
for later review and evaluation.
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PSA uses
both manual and automated ultrasonic test systems. Manual
systems employ either longitudinal waves, typically used to
measure tube wall thickness in boilers at elevations that are
prone to wall thinning, or shear waves, used to inspect critical
welds in pressure vessels, pipes, or tubes. PSA’s automated
systems, IRIS and
OmniScan™ / AccuScan™,
provide 100% coverage (360º of mapping) on boiler tubes and
their near drum areas. Link to the IRIS
and OmniScan™ / AccuScan™
pages for detailed descriptions.
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