by Doug Waetjen
National Sales Manager, UE Systems, Inc.
Albemarle Corp., in Orangeburg, South Carolina, is a specialty chemical and pharmaceuticals company spread over 200-300 acres with multiple small buildings that run individual operations. In one of these buildings there is a lethal-service 30,000 gallon tank that has two vertical-mounted internal pumps.
“Many of us know that excess lubrication shortens the life of ball bearings and eventually destroys them. It can blow out their seals and result in contamination. So we were suspicious when a pump manufacturer recommended that we lubricate the bearings on these pumps every two weeks and send them in for overhaul in two years,” said Arthur Sweatman, Senior Electric and Instrument Analyst, with the Albemarle Corp.
According to Sweatman, the manufacturer recommended 42 pumps of grease (a full ounce). “Because of the lubrication issue, we decided to use a portable ultrasonic instrument (an Ultraprobe 2000 manufactured by U.E. Systems, Inc.), to monitor the bearings,” Sweatman continued. “I set up an inspection route and trained the maintenance team in the use of ultrasonics, which enabled them to monitor the bearings in action. We found that all it took was six or seven pumps once every six months, saving us manpower and time, and extending the life of the equipment. When we sent the pumps in, as prescribed by the manufacturer, the repair shop informed us that the next rebuild cycle could be extended an additional six months because the pumps were in better shape than they had expected!”
How Ultrasonics Works
Airborne ultrasound instruments, often referred to as “ultrasonic translators,” provide information two ways: qualitatively through their ability to “hear” ultrasounds through a noise-isolating headphone and quantitatively via incremental readings on a meter/display panel.
Although the ability to gauge intensity and view sonic patterns is important, it is equally important to be able to hear the ultrasounds produced by various equipment. These instruments allow inspectors to confirm a diagnosis on the spot by being able to clearly discriminate among various equipment sounds.
This is accomplished in most ultrasonic translators by an electronic process called “heterodyning” that accurately converts the ultrasounds sensed by the instrument into the audible range where users can hear and recognize them. This high-frequency, short wave characteristic of ultrasound enables users to accurately pinpoint the location of a leak or of a particular sound in a machine.
Most sounds sensed by humans range between 20 Hertz and 20 kHz (20 cycles per second to 20,000 cycles per second). The average human high-frequency threshold is actually 16.5 kHz. These frequencies tend to be relatively gross when compared with the size of sound waves sensed by ultrasonic translators. Low frequency sounds in the audible range are approximately 1.9 cm (3/4″) up to 17 m (56′) in length, whereas ultrasounds 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 instrument is much more conducive to locating and isolating the source of problems in loud plant environments — a major contributing factor to its popularity.
Monitoring Lubrication Levels
Ultrasound inspection provides early warning of bearing failure, detects lack of lubrication, prevents over lubrication and can be used to diagnose high-speed as well as low-speed bearings. There are two ultrasound methods — comparative and historical — that are commonly used to monitor bearings.
The first step is to establish a baseline reading by comparing similar bearings for potential differences in amplitude and sound quality. To do this, an inspector makes a reference point on a bearing housing or grease fitting and touches this reference point with the contact (stethoscope) probe of an ultrasonic instrument. Next, he tunes the instrument to 30 kHz and adjusts the sensitivity to hear the bearing sounds more clearly and to bring the intensity levels on the meter/display panel into range if the received sound amplitudes are either too high or too low. Then he compares this base reading to other similar bearings.
An 8 dB gain over baseline indicates pre-failure, or lack of lubrication, and will be accompanied by white noise which will be similar in sound quality to that of a loud gas leak. A 12 dB increase establishes the very beginning of the failure mode and will sound rough. A 16 dB gain indicates advanced failure condition, while a 35-50 dB gain warns of catastrophic failure. Once a series of bearings have been tested and a baseline set, data is recorded and then compared to future readings for historical trending and analysis.
In most cases the inspection technique is simple and straightforward, but this was not the case for Albemarle Corp. Since the tank housing the pumps are located in a lethal service environment, any inspection of bearings is labor-intensive and time-consuming. To perform the task, a two-person maintenance team (a worker and an observer) have to suit up in “B” Level gear — full containment, pressurized suits to guard against contamination.
“Furthermore,” said Sweatman, “the pumps are vertically-mounted inside the tank, so it’s not a simple job of pulling the pump and taking it into the shop. It takes a couple of days of downtime to clean out the tank so that the pumps can be removed for repairs.”
Sweatman’s solution was to use an extension cable to remotely mount the stethoscope module of the ultrasonic instrument inside the building and locate the cable outside the building. As a result, only one man had to stop by weekly to take a lubrication reading, which took five minutes! Sweatman established a baseline for each pump and using a 7-8 dB increase from baseline established a lubrication alarm point. When a mechanic experienced a lubrication alarm point, he would get an additional mechanic to suit-up and they would enter the building and lubricate the pump’s bearings.
Incidently, Sweatman discovered that each lubricator applied a different hand pressure with the stethoscope module which resulted in false readings, another reason why he decided to use the extension cable to remotely mount the stethoscope module. “The surface contact of the stethoscope module is the same regardless of who’s doing the monitoring. It’s consistent and repeatable,” Sweatman explained.
“To determine the appropriate lubrication level, a technican monitors dB levels as he pumps in the grease. He continues until the level comes up and returns to his starting point. Then he gives it another shot or two of grease making sure that the level goes no higher.”
“It was our hope that the ultrasonics survey would quickly and accurately indicate which bearings needed lubrication, how much lubricant to use, and how frequently the work needed to be done,” Sweatman concluded. “We accomplished what we set out to do, learning a good deal about predictive maintenance in the process.”