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Diagnostic Technologies for Predictive Maintenance

Companies still relying on a preventative rather than a predictive approach cannot benefit from the foresight to operate, budget, and plan its manpower needs. Its impossible to plan your next move when your constantly fire-fighting.
Technological Revolution is Underway

Today's focus is on increased speed, efficiency, and accuracy. To justify an investment in specialized diagnostic technologies for predictive maintenance the tool must produce results fast, and with greater accuracy and efficiency than older, proven methods. It must be simple to use and produce results the operator can use with confidence. Technology is changing at a revolutionary pace. Look at the ways we communicate with each other today, versus ten, five, or even one year ago. Similar technological advances are being made in predictive maintenance tools.

One technology that has benefited from the technology revolution is ultrasonic detection. Ultrasonic detection has existed for over 25 years and is widely used throughout continuous process operations for a variety of predictive maintenance applications. Thanks to revolutionary advancements in the last three years, maintenance personnel are specifying ultrasonic detection with increased regularity for everything from acoustic vibration monitoring to leak detection and electrical problems.

Versatility of Ultrasound

Mechanical Wear

The theory behind ultrasonic detection is logical. All plant machinery produces sound patterns - both sonic and ultrasonic. Characteristics of those patterns change relative to the health of the machine. Subtle changes in the ultrasonic range indicate early wear signs, changes in lubrication, and structural degradation of mechanical components (bearings, couplings, gears, valves, etc...). Ultrasonic detection has proven useful for determining the onset of incipient bearing failure before conventional methods like vibration, temperature, or wear particle analysis. Recent evolutions in ultrasound detection include dynamic memory capacity, digital data processing, the use of True RMS technology for enhanced repeatability, and two way data transfer between instrument and PC.

Leak Detection

Compressed air leaks create noise that has both an audible and ultrasonic component. The audible component is not useful for leak detection due to its non-directional properties and ambient plant noise that can exceed 130 decibels. However the ultrasonic component of a leak is very useful for leak detection because of its directional properties and the ability of a quality detector to filter out ambient plant noise. Rising energy costs make compressed air single most expensive utility in most manufacturing facilities. Combined with the high rate of leakage found in most plants (average leak rate of 40 - 50% was common in 100 plants surveyed), cost justification is often based solely on implementation of a compressed air leak program.

Steam Leaks

Like air leaks, it is an alarming reality that maintenance personnel routinely ignore steam leaks. Steam leaks can exist anywhere in a system and finding became much easier with ultrasonic detection. Steam passing through an orifice generates ultrasonic turbulence that is picked up with the contact probe. Place the probe upstream and downstream of the trap and compare ultrasonic readings. Louder levels downstream indicate the trap or valve is open. Lower level indicate its closed.

Determining faulty traps relies on the operator knowledge of the trap's internal function. Using a comparative method between several similar traps will confirm any suspicions of failure. Ultrasonic Detection can also be used to check solenoids, relief valves, check valves, control valves, needle valves, and air actuated cylinders.

Corona Discharge

Traditional use of infrared cameras reveals "hot spots" that the naked eye otherwise misses. But corona, arcing, and tracking do not always generate significant increases in temperature. Ambient high temperatures can also mask them from the camera. They do however generate distinct noises in the ultrasonic range and are detectable using ultrasonic listening equipment. Ultrasonic detection systems complete the picture by extending the range of human hearing to listen for ultrasounds produced by faulty insulators, line bushings, transformers, potheads, and arresters.

Analyze Lubrication Needs

All rotating equipment produces ultrasonic or acoustic vibration regardless of the state of lubrication. By measuring and trending this energy with a digital ultrasonic detector lubricators trend deviations from normal baselines and determine specific lube needs. Lubrication absorbs energy created by friction between the balls and raceway of a bearing. Acoustic vibration is low when the bearing is properly lubricated but as the lubrication film breaks down this energy proportionally increases even though the bearing may not have any significant wear. This energy can be measured and trended on the digital readout of the SDT 170. An increase of 8 to 10 decibels over historical baseline indicates a need for lubrication. This is confirmed by listening to the bearing's acoustic qualities in the headphones, or by viewing the waveform on a spectrum analyzer. Bearings lacking lubrication will sound louder, with a rough growl, compared to the relatively smooth whirring noises of a well-greased bearing. The waveform on an oscilloscope will show inconsistent peaks if the bearing is lacking grease.

Heat Exchangers

Pinpointing leaks in heat-exchanger tubes and tube sheets is an important use for ultrasonic diagnostic equipment. Labor intensive methods include flooding the tube walls with water and watching for leaks but its time consuming and not always effective.

Ultrasonic detection can be carried out using air pressure or with an ultrasonic transmitter. When pressurizing the exchanger with air the method used is the same as if detecting a compressed air leak. The hissing rush of air at the leak reveals quickly the source of the problem and the directional nature of a quality ultrasonic detector will allow the leak to be pinpointed immediately.

The other off-line method involves flooding the tube shell with ultrasound from an ultrasonic transmitter. The low energy ultrasound generated by the transmitter is not strong enough to penetrate the steel walls of the tube but easily pass through hairline cracks. Using a flexible airborne sensor the integrity of each tube can be quickly verified.

With this method it is extremely important that the transmitter used works in a bi-sonic mode. That is to say that the ultrasonic transmitter emits two separate, overlapping frequencies. Some products may claim to use an alternating sound pattern but in reality do not. The problem that arises is a phenomenon known as standing waves. The outgoing wave hits a surface and reflects back to the source. On its travels it meets more source waves with the same amplitude and frequency. The resulting sine waves cancel each other out creating silent dead zones.

Features to Consider

Choosing the right tool is dependent on need and budget. Never compromise quality for price and be sure to do your homework with the following suggestions: schedule a live demonstration with a manufacturer's representative, visit trade-shows, call references, rent or borrow equipment for trial, purchase with a risk-free refund clause. Finally, choose the tool with the features that are right for your applications, and with the ability to expand. Companies at the forefront of the technology revolution are producing single platform hardware. You purchase that platform and have access to all the features and functionality without having to pay for them until you need them. For instance, you can purchase a simple basic system that can be upgraded at any time to a premium system simply by purchase a software key that unlocks additional applications. At the same time, any system enhancements that are made after your purchase can be added over the Internet with a simple program (usually covered by warranty). Avoid suppliers with 5 or 6 models that do the same thing. In the end, you are stuck with what you bought with no chance to upgrade without doubling your initial capital expense.

In the world of ultrasound detectors look for equipment that provides a digital readout on a known measurement reference scale. The input signal should be processed digitally and the use of True RMS Averaging is a must to ensure stable linearization. An internal memory of no fewer than 1000 points is standard. Premium models offer multiple route storage and software that allows for individual route customization and two way uploading of data.

The unit must be lightweight and feature one-hand operation. All the buttons should be accessible by the fingers that hold the unit (the other hand must be free to hold accessory sensors such as the flexible airborne sensor, contact probe, and parabolic dish). The plant environment is a rugged one and requires a robust detector that will withstand abuse. Aluminum is an excellent housing material that is strong and lightweight. Look for rubberized covers to add extra shock protection.

Sensors must connect with rigidity to avoid damage. The airborne sensor is normally internal to the unit and hardwired. It should also be protected with aluminum to avoid shock. Avoid interchangeable modules. It is common for the pins to snap off inside the detector leaving an expensive repair bill.

Your vendor is as important as the equipment he sells. Look for equipment that can be serviced locally (without having to deal with international customs). How long has he been in business and how good is his warranty? This may be the time to call on some references.

For more information about specialized diagnostic technologies, contact:

Mr. Allan Rienstra
SDT North America 
Toll Free (800) 667-5325

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