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Mechanical Monitoring Concepts
There are ultrasonic components in practically all forms of friction. As an example, if one were to rub
the sensor probe with a finger, an ultrasonic signal will be generated. Although there might be some
audible components to this friction, the sensor will only sense the ultrasonic components which, in this
example, will be considered a gross signal that is also amplified. In fact, due to the comparative low
amplitude nature of ultrasound, amplification is a very important feature. Although there are obvious
audible sounds emitted by most operating equipment, is the ultrasonic elements of the acoustic
emissions that are generally the most important. Ultrasound offers a predictable diagnostic capacity.
When changes begin to occur in the ultrasonic range, there is still time to plan appropriate
maintenance. According to NASA research, when a bearing enters the beginning stages of failure,
there is an amplitude increase of from 12 to 50 times over a set baseline. Not only can the early stage
of bearing failure be monitored and detected, other warning signs can also be noted such as: lack of
lubrication, advanced failure and catastrophic failure.
Cavitation
As air enters a valve or pump, the dynamics of the pressure within can create cavitation: the forming
and explosion of bubbles. Although cavitation may be present, it does not necessarily create a
problem. It becomes a maintenance problem only when the process increases to produce conditions
that will cause internal damage. By setting a baseline, the increase in cavitation activity can be
monitored to a point where an alarm can be set and preventive measures can be taken.
Monitoring: flow/no flow and leakage
Valves control fluid flow.
Whether the valve’s function is to provide a simple flow/ no flow operation
(on/off) or to regulate the amount of flow, a malfunction can be critical.
Changes in amplitude related to these conditions can be monitored and alarm levels may be set to
note or control these changes. When leak occurs, the fluid will move from high pressure (upstream),
through the valve seat, to the low pressure (downstream) side. As it reaches the low pressure side , it
expands briefly, producing a turbulent flow. This turbulence has strong ultrasound components. The
amplitude of the turbulence is related to a few basics:
1. Fluid Viscosity
Under identical environments, pressures, leak size, etc.; a lighter fluid, such as air will produce
more turbulence than a heavier fluid, such as oil.
2. Orifice Size
The more restriction on a fluid, the less amplitude generated. A smaller diameter hole will not
produce as much sound as a larger hole under similar flow conditions.
3. Pressure Differential
Given identical leak sizes, when there is a greater pressure difference between the upstream
and downstream sides, the leak with the greater difference will produce a louder signal.
4. Orifice Shape
Under the same environmental conditions, a smooth orifice will not produce as much
turbulence as a jagged edge orifice.
Solid Flow: Powders, Metal Filings, Etc.
As solid moves through a carrier, such as piping, the particles will produce friction which may be
monitored. Any flow disruption will display a drop in amplitude over a preset baseline and will be
detected. This may be set up to produce an alarm.
