Introduction
Industrial gas turbines are the heart of opera-
tions where electric power generation, cogen-
eration of electricity and steam, gas compres-
sion, propulsion in marine applications, or a
combination of these is necessary for a plant
or vessel to operate. While these turbines
are incredibly dependable, they have regular
maintenance schedules and occasionally
forced outages where remote visual inspec-
tion (RVI) is required to determine if indus-
trial gas turbines are fit for service, or if addi-
tional repairs and maintenance are required.
The nondestructive technique of RVI, which
is a discipline within the visual testing (VT)
method, allows for indirect visual inspections
of areas of the fan, compressor, combustion
section, and power turbine with minimal dis-
assembly. Auxiliary components and balance
of plant (BOP) items such as piping, valves,
vessels, and machinery are also inspected
with RVI during these outages. When surface
indications or discontinuities are detected,
extremely accurate indication sizing and
3D analysis are now possible with RVI. The
benefits are minimized downtime, increased
safety, and maximized return on investments
for the operation. This article provides insights
on how the proper implementation of RVI
technology, and accurate interpretation of
the data obtained during an RVI event, can
provide valuable diagnostic information on the
internal health of a gas turbine.
Direct visual examinations to determine
the safety of a situation or the quality of
assets have been around as long as eyesight.
Visual testing (VT) is thought of as a foun-
dation of nondestructive testing (NDT). VT
in industrial applications began in the early
1920s. It was not until 1988 that VT became
a certified testing method in ASNT’s SNT-
TC-1A. However, it was not widely accepted by
industry until the European Union Standards
Committee incorporated VT in the EN 473
certification standard in 2001. EN 473 was
subsequently replaced by ISO 9712:2021(en):
Nondestructive Testing – Qualification and
Certification of NDT Personnel.
To perform effective direct visual examina-
tions, the recommended distance and angle
for viewing is to have the eye within 600 mm
60º to 90º =good viewing angle
30º to 60º =noncritical
angle of view
0º to 30º =poor
viewing angle
600 mm
(24 in.)
Test
surface
Figure 1. Direct visual testing viewing angle and
distance.
J U L Y 2 0 2 4 • M A T E R I A L S E V A L U A T I O N 41

(24 in.) of the object and positioned at an angle not
less than 30° to the inspection surface, as shown in
Figure 1 [1].
Making a direct visual examination to deter-
mine the condition of internal components in a gas
turbine is physically impossible without significant
amounts of downtime and disassembly. As seen
in Figure 2, a technician can readily inspect the
internal components of a large frame gas turbine.
When knowing their internal condition is required,
this is where RVI becomes indispensable. It is
interesting to note that while RVI is a subdiscipline
of VT in SNT-TC-1A and ISO 9712:2021(en) visual
testing (methods), both direct unaided visual tests
and visual tests conducted during the applica-
tion of another NDT technique are excluded. This
accentuates the importance and value of qualified
and certified NDT personnel who are specifically
using RVI.
RVI enables the visual inspection of otherwise
inaccessible areas or surfaces. The earliest examples
were endoscopes that began to be used for medical
purposes in the early 18th century. With the advent
of cannons, artillery operators would lower a candle
on the end of a stick into a cannon bore to deter-
mine its condition prior to use. You might see why
this could be problematic for the inspector! People
soon realized they could only see in straight lines,
but if mirrors or fiberoptics were used, the light and
image could “go around” corners. From this discov-
ery, the borescope and borescope technology have
evolved.
Dr. George S. Crampton developed the first
industrial borescope, which was used by the
Westinghouse Co. for examining internal turbine
components. Inspecting internal surfaces of a
turbine rotor were some of the first RVI applica-
tions on industrial turbines. While Crampton was
a mechanical “MacGyver” of sorts, he used optical
instruments in his medical practice as an ophthal-
mologist and tinkered with optical instruments in
his spare time. His work led to the founding of the
Lenox Instrument Co. [2].
Today, typical RVI applications with borescopes
are inspecting internal components on aviation and
FEATURE
|
REMOTEVT
Figure 2. Visual
inspection of
a GE Vernova
large frame
gas turbine’s
fuel lines.
60º to 90º =good viewing angle
30º to 60º =noncritical
angle of view
0º to 30º =poor
viewing angle
600 mm
(24 in.)
Test
surface
Figure 1. Direct visual testing viewing angle and distance.
42
M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 4
CREDIT:
GE
VERNOVA