Page 266 - Intro Predictive Maintenance

Ultrasonics 257

11.1.2 Leak Detection

The principal application for ultrasonic monitoring is in leak detection. The turbulent
flow of liquids and gases through a restricted orifice (i.e., leak) will produce a high-
frequency signature that can easily be identified using ultrasonic techniques. There-
fore, this technique is ideal for detecting leaks in valves, steam traps, piping, and other
process systems.

11.1.3 Materials Testing
Ultrasonics has been, and continues to be, a primary test methodology for materials
testing. Typical test frequencies start at 250 kiloHertz (kHz), or 250,000 cycles per
second (cps), up to 25 MegaHertz (MHz), or 25 million cps.

Testing materials generally consist of introducing an energy source into the material
to be tested and recording the response characteristics using ultrasonic instruments.
These tests may be as simple as striking the material with a hammer and recording
the results with an accelerometer and ultrasonic meter.

Ultrasonic testing relies on the measurement of time and amplitude or strength of a
signal between emission and reception. Because of a mismatch of acoustic properties
between materials, the sound will partly reflect at interfaces. The quality of reflected
energy depends on the acoustic impedance ratio between two materials. For example,
sound transmitted through steel reaching a steel/air boundary will cause 99.9 percent
internal reflection, whereas a steel/water boundary would reflect only 88 percent
within the material and transmit 12 percent into the water. If impedance ratios are
widely different, such as an open crack with a steel/air interface, then adequate reflec-
tion will occur and permit detection of the flaw. Conversely, a small crack in a com-
pressive stress field that does not have oxidized faces will yield a steel/steel boundary
and cannot be detected using this method.

11.2 TYPES OF ULTRASONIC SYSTEMS
Two types of ultrasonic systems are available that can be used for predictive mainte-
nance: structural and airborne. Both provide fast, accurate diagnosis of abnormal
operation and leaks. Airborne ultrasonic detectors can be used in either a scanning
or contact mode. As scanners, they are most often used to detect gas pressure leaks.
Because these instruments are sensitive only to ultrasound, they are not limited to
specific gases as are most other gas leak detectors. In addition, they are often used
to locate various forms of vacuum leaks.

In the contact mode, a metal rod acts as a waveguide. When it touches a surface, it is
stimulated by the high frequencies, ultrasound, on the opposite side of the surface.
This technique is used to locate turbulent flow and/or flow restriction in process piping.

Some of the ultrasonic systems include ultrasonic transmitters that can be placed inside
plant piping or vessels. In this mode, ultrasonic monitors can be used to detect areas