MRO Magazine

Sound Waves: Ensure equipment health with ultrasound


Industry

March 29, 2010
By PEM Magazine

In today’s environment, generating revenues for any industry is important. Profit margins are shrinking and often the difference between a profit and a loss can be as simple as improving efficiencies. Locating sources of energy waste, identifying failure conditions in electrical and mechanical systems all contribute to helping improve the bottom line.

This is why many industries around the world have incorporated some form of condition monitoring. As opposed to the other forms of maintenance, such as reactive in which a failure condition has occurred and maintenance personnel must "react" to the problem; or preventive where maintenance activities are performed on a set schedule, condition monitoring is used to check the health or "condition" of operating equipment. Any change in monitored fields can alert maintenance personnel of potential failure and allow the repair to be performed on a scheduled, controlled basis.

CONTENTS on this page:
Intro: The importance of condition monitoring
Introducing ultrasound
About sound
Advantages of ultrasound

Times have changed, however, and it’s important to re-think your approach to shutdowns. All parts of the organization now come with budget scrutiny. As a result, we’re running our shutdowns under tremendous pressure.

There is no magic bullet for successful condition monitoring. There are many technologies that are very effective for preventing unplanned downtime. While there are maintenance personnel who have developed a favourite technology and lean on that more than others, ideally the approach should be to integrate as many different technologies as possible to be truly effective.

Advertisment

The analogy would be to go for an annual physical and have your doctor use only one instrument, such as a stethoscope, to listen to your heartbeat. Would you be satisfied that he/she did everything possible to make sure you were healthy? Of course not; you would expect the examination to include a multitude of instruments and technologies.

The same is true of your plant equipment. The more technologies used, the more effective the program will be in diagnosing and predicting potential problems and keep the assets up and running as needed.

The most common technologies used for condition-monitoring programs are often: vibration, thermal imaging, oil analysis and ultrasound. Depending on the plant and maintenance goals, two others should be considered as well. They are motor current analysis and laser alignment (there are other methods of alignment, but this is the most popular form).

Vibration technology has been around for a long time and has gradually become the mainstay of many maintenance departments. Usually used for mechanical inspections, they look at frequencies that can disclose potential faults by examining the speed (acceleration) and displacement of these vibratory emissions. Frequency analysis can disclose a number of potential issues ranging from soft footing and gear issues to failed bearings.

Thermal imaging involves cameras sensitive to infrared waves that are usually associated with heat emissions. These cameras are also very popular and used for a wide variety of inspections. Most often they are used to inspect electrical equipment for potential fire or other failure conditions, such as flashover. They have also been used to inspect heat build up in motors and bearings and identify faulty valves and steam traps.

Introducing ultrasound
Ultrasound inspection offers the most useful and thorough position for condition monitoring as both a "stand-alone" inspection technology and as an effective screening tool, which can speed up the inspection process and help inspectors determine effective follow-up actions for mechanical, electrical and leak applications, or what’s commonly referred to as trending.

Whether you refer to proactive inspections as predictive maintenance (PdM) or condition-based monitoring (CbM), the goal is the same; to note a deviation from a normal or baseline condition in order to determine whether or not to take corrective action in a planned orderly manner and to prevent an unplanned incident.

The ideal end result is to maintain asset availability, reduce maintenance overhead and improve safety conditions. Not one technology can cover everything. The recommendation is to incorporate as many technologies as possible into inspection procedures to assure reliable results.
Ultrasound technology

Airborne/structure borne ultrasound instruments receive high frequency emissions produced by operating equipment, electrical emissions and by leaks. These frequencies typically range from 20 kHz to 100 kHz and are beyond the range of human hearing. The instruments electronically translate ultrasound frequencies through a process called heterodyning, down into the audible range where they are heard through headphones and observed as intensity and or dB levels on a display panel.

The newer digital instruments utilize data management software where information is data logged on the instrument and downloaded to a computer for analysis. Some instruments contain onboard sound recording to capture sound samples for spectral analysis. New instruments are now being developed to house all data within the instrument itself—acting as an entire inspection system in the palm of your hand  (i.e. you won’t have to download to a computer).

Sounds are received two ways: through the air and through solid surfaces (structures). Airborne sounds, such as leaks or electrical emissions, are received through a "scanning" module. The structure borne ultrasounds, generated by bearings or leaks through valves, are sensed through a wave-guide or "contact" module.

What makes airborne ultrasound so effective? All operating mechanical equipment, electrical emissions (arcing, tracking, corona) and most leakage problems produce a broad range of sound. The high frequency ultrasonic components of these sounds are extremely short wave in nature. A short wave signal tends to be fairly directional and localized. It is therefore easy to separate these signals from background plant noises and to detect their exact location. In addition, as subtle changes begin to occur in mechanical equipment, the subtle, directional nature of ultrasound allows these potential warning signals to be detected early, before actual failure.

Most of the sounds sensed by humans range between 20 Hertz and 20 kilohertz (20 cycles per second to 20,000 cycles per second). The average human high frequency threshold is actually 16.5 kHz. Low frequencies tend to be relatively large when compared with the sound waves sensed by ultrasonic translators. The lengths of low frequency sound waves in the audible range are approximately 1.9 cm (3/4") up to 17 m (56′), whereas ultrasound wavelengths sensed by ultrasonic translators are only 0.3 cm (1/8") up to 1.6 cm (5/8") long. Since ultrasound wavelengths are magnitudes smaller, the "ultrasonic environment" is much more conducive to locating and isolating the source of problems in loud plant environments.

The high frequency, short wave characteristic of ultrasound enables users to accurately pinpoint the location of a leak, electrical emission or of a particular sound in a machine.

The basic advantages of ultrasound and ultrasonic instruments are:

• Ultrasound emissions are directional;
• Ultrasound tends to be highly localized;
• Ultrasound provides early warning of impending mechanical failure;
• The instruments can be used in loud, noisy environments; and
• They support and enhance other PdM technologies or can stand on their own in a maintenance program.

When used as part of a condition-monitoring program, ultrasound instruments help improve asset availability and save energy.


This is an edited article provided by UE Systems, Inc. Alan Bandes is vice-president, marketing. You can contact him by email: abandes@att.net. Also visit www.uesystems.com.