Marine Maintenance Technology International - April 2016

NDT (^) ⠿
MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL 2016 ⠿ 07
A
ll objects, cold or hot, radiate heat in the form of infrared
energy. As an object increases in temperature, it radiates
more energy and the wavelength gets shorter. Infrared
radiation, visible light and ultraviolet light are all forms of energy
in the electromagnetic spectrum. The only difference is their
wavelength or frequency.
Infrared thermography measures the heat emitted by a body,
so it only picks up the external temperature of the object.
Since different materials have different emissivity, calibration
of equipment and an understanding of the basic physics
involved is essential to any interpretation of results. Emissivity is
calculated on a scale of 0 to 1, with smooth shiny metal having a
lower emissivity than rough metal, or matt black paint.
The human eye can only see a narrow range of wavelength
in the electromagnetic spectrum. These wavelengths range in
length from 0.4μm to 0.7μm. If the temperature of an object gets
hot enough however, above 525°C the energy from that object
will radiate energy in the visible spectrum and we will see it.
This is when we see an object like the burner on an electric
stove ‘glowing’ a cherry red.
Most cameras can image temperatures from
-20°C to 500°C; and can be extended down
to -40°C and up to 2,000°C. The camera
converts this invisible infrared energy
into a two-dimensional visual image and
displays this on a standard TV monitor.
Most industrial cameras can also make
temperature measurements, with
accuracies to around ±2% at 30°C.
ALL ON THE SURFACE
ABOVE: Remote
monitoring of a small
vessel engine room
using a fixed thermal
imaging camera
BELOW: Detecting
electrical faults
before trouble arises
is a key use of
thermography (R).
Handheld cameras
are easy to transport
and store (L)
He says, “We survey as much equipment
as is feasibly possible, looking at engines
and gearboxes. For example, it can be useful
for a comparison test between two similar
engines operating under similar conditions,
with our thermographic reports highlighting
those exceptions from the norm, be it
electrical or mechanical.”
Always something to report
Thermographer Keith Budden of KCM
Services, based in Hull, UK, says that in
the 25 years since he started doing tanker
inspections, initially for Mobil, he has never
done a survey without having something to
report. “We would usually do an electrical
inspection two to three months before
dry dock. The usual problems would be
things like loose or poor terminations in
switchboards, transformers or generators.
I have seen temperatures as high as 440°C
in some cases.
“Onboard engineers can use simple
equipment to help monitor engines and
gearboxes. Even without detailed analysis, it
is valuable to simply plot temperatures and
see if something is changing,” says Budden.
Seeing through glass
In fiberglass-hulled vessels thermography
is used to check the integrity of the hull,
and in wood-hulled vessels, the technology
will detect problems such as water ingress
around metal fasteners.
Jack Allinson, owner of Allinson
Infrared Inspection Services in Florida, is
now kept busy with smaller boat surveys,
but he was an early pioneer of marine
use of the technology. He is now a Level
3 thermographer, but when he started in
the field in 2003 there were no accepted
standards. In 2011, after the use of unskilled
thermographers in the medical field led to a
rash of lawsuits, Allinson helped to develop
the professional training levels that are now
accepted internationally. Level 1 training
costs about US$2,000, at which point an
operator can use the equipment and have
some understanding of the results. Skilled
interpretation is taught at Levels 2 and 3.