⠿ NDT
(^78) ⠿ MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL 2016
Ultrasonic testing can be an effective diagnostic tool leading to safe
and reliable condition monitoring for rotating and electrical parts
Walter Vervloesem, SDT International
I
n SDT International’s experience, using
ultrasound for condition monitoring on
board ships enables detection of most
failures that create impact, turbulent
flow and friction. These include rotating
bearing-related issues, compressed air
leaks, valve leakage, hydraulic power pack
problems, steam leaks, vacuum leaks,
tightness testing, electrical discharge
detection and cavitation.
A major advantage is that ultrasonic
readings – when correctly taken, trended
and interpreted – give operators valuable
and instantaneous information about
the health of onboard equipment, which
enhances safety and extends service
intervals, and also avoids unnecessary
opening up of equipment.
Cranes can be found on any type of
ship, in any size, on deck and in the engine
room – their uses can be extremely varied.
The importance of cranes is that without
one, nothing can be loaded on board or
discharged from most types of ship.
Maintenance of cranes has always been a
challenge. They are not an environment that
is nice to work in, with their cramped spaces
and slippery surfaces (due to hydraulic oil or
Fault detection
grease spills) and most of the time they are
either too hot or too cold.
In many cases, having fully operational
cranes on board is a prerequisite for charter
parties, and crane failure generally results
in expensive downtime and claims – reason
enough for monitoring to ensure their
reliability and availability.
Hydraulic systems are ideal candidates
for ultrasound monitoring. These systems
generally operate under very high pressure,
and the combination of high pressure and
rather small leaks creates a highly turbulent
flow and hence a clearly identifiable
ultrasonic signature.
Ultrasonic case study
A test was carried out on a crane with
three identical hydraulic motors, one for
each of the movements – hoisting, luffing
and slewing. The unit in question had no
apparent problems during operation. It was
decided to check the hydraulic motors with
an ultrasound monitor.
After selecting the correct parameters
on the ultrasonic receiver, the hoisting
motor, which is usually the one under the
greatest strain, was heard at 12-14dBμV with
a smooth continuous sound. The slewing
motor was even quieter, with measurements
in the range of 8-12dBμV and it sounded
even smoother.
The luffing motor, on the other hand, was
found to be very noisy at 20-24dBμV. This
test gave impressive results and enabled
the operator to easily and simply locate the
internally leaking pistons at a specific point
on the motor.
Electrical arrangements
As well as malfunctioning electrical
components causing operational problems,
they can also trigger a fire. Self-combustion,
heat, and sparks generated by electrical
failures can quickly turn a crane into a
burning torch. As mentioned, crane turrets
are not maintenance-friendly spaces and
it is not unusual to find oil- and grease-
soaked rags lying variously about the
crane, or pools of spilled oil on turret floors.
Carelessly disposed of packaging and
cleaning materials further increases the fire
hazard risk.
Because failing electrical components
create typical ultrasonic sounds caused by
ionization around the failing part, ultrasound
Crane maintenance
A slewing bearing is
checked on a crane base