Sensor-Mounting Solution Grows PdM Program

Manufacturer of industrial gases found ‘quick’ relief for an unusually tough vibration-monitoring challenge.

Air-separation facilities use a wide variety of rotating and reciprocating machinery. Predictive maintenance for this equipment involves monthly monitoring at hundreds of measurement points within a plant. The resulting vibration data, including high-frequency measurements in the 20-kHz range, provides valuable indications of machinery health. For vibration analysts at a manufacturer of industrial gases, however, the data-collection process presented a dilemma.

The analysts had concluded that a comprehensive, accelerometer-based predictive-maintenance (PdM) program linking a network of plants across the country would be ideal for their company—except for the problem of mounting requirements for high-frequency measurements.

Conventional wisdom held that accelerometers needed to be stud-mounted to provide accurate readings at high frequencies. For the analysts, stud-mounting an accelerometer at hundreds of measurement points would be a task so time-consuming and tedious that it would pose serious concerns about cost-effectiveness and data accuracy.

To successfully implement the program, the analysts needed a quick-connect/disconnect mounting method capable of providing accurate readings at high frequencies.

In cooperation with sensor manufacturer PCB Piezotronics (pcb.com, Depew, NY), they developed and tested a magnet-mounting technique that met all requirements for ease of use and accuracy at high frequencies.

Maintenance history

The industrial-gases manufacturer had been actively using predictive and proactive maintenance strategies for many years. Before implementing its new program, the existing preventive-maintenance (PM) program called for overhauling every major machine at regular intervals and replacing critical components, regardless of condition. While this approach represented an improvement over reactive maintenance, it led to considerable expense in terms of downtime, labor, and materials—especially if equipment was healthy.

In addition to that particular maintenance approach, personnel at individual plants routinely monitored the overall vibration level of major pieces of rotating equipment using permanently installed proximity probes. The industrial-gas manufacturer’s vibration analysts, in turn, used the data from those probes to assess machinery health. (They typically visited each plant two to four times a year to collect the data.) Although this practice was seen as a step toward PdM, it suffered from several shortcomings:

• Data was acquired infrequently. The intervals between the periodic vibration surveys were too long for good trending.

• Data management was inefficient. Machine histories weren’t shared among plants and the lack of standardized measurement procedures made one-to-one comparisons difficult.

• Manpower resources were not used effectively. Highly trained vibration analysts spent more time traveling between plants than analyzing data.

• The vibration analysts decided that the most effective way to eliminate these shortcomings was to implement system-wide condition monitoring/PdM. The goals were to:

• Reduce costs and improve profitability by early detection of impending problems.

• Increase the interval between turnarounds.

• Share information in a system-wide database that would improve problem analysis, diagnosis, and corrective capabilities.

Given the wide variety of rotating and reciprocating equipment running in air-separation facilities, effective PdM at these sites translates to monthly vibration monitoring at hundreds of measurement points. Improved sensors and mounting methods are quickly changing the nature of this formerly time-consuming, tedious task.

High-frequency data

An underlying principle of the new system was that the addition of high-frequency measurements to the low-frequency proximity probe data would provide a more complete picture of machinery health. High-frequency vibration is particularly useful in diagnosis and analysis of several classes of problems in high-speed turbomachinery. The most widely known phenomenon involves the set of signals generated by a pair of meshing gears in a gearbox. A high-frequency accelerometer is the preferred sensor for measuring this important information.

The general guideline for gearbox analysis is to look at three harmonics of gear mesh frequencies and their sidebands. Increases in gear-mesh harmonics and natural gear frequency are primary indications of a gearing problem, such as gear wear, misalignment, eccentricity, or improper backlash. The presence of and changes in these signals are important diagnostic indicators in assessing the cause and cure of gearing problems.

Frequency response and mounting

Vibration literature had previously explained that the frequency response of an installed accelerometer was highly dependent upon the method used to attach the sensor to the structure.  The contention was that, of the four common methods for attaching sensors to mounting locations for PdM (stud-mounted, adhesive-attached, magnet-mounted, and hand-held), the first approach—stud-mounting—provided the widest frequency response and the most secure and reliable attachment. The other methods reduced the upper frequency range of the sensor.

Yet, despite its performance under ideal conditions, stud-mounting was not a realistic option for a vibration program that demanded measurements at hundreds of points on a monthly basis. The drawbacks were related to the time- and labor-intensive nature of this technique.

Search for a ‘quick’ solution

As the vibration analysts saw it, their program required a quick-connect/disconnect strategy that provided accurate readings at high frequencies. In support of their interest and needs, PCB offered a variety of sensors for evaluation, as well as access to its own test facilities.

PCB’s inventory included low-mass, high-response accelerometers with frequency ranges exceeding 20 kHz, when properly installed, and rare-earth magnets for accelerometer mounting. Despite the general opinion that magnet mounts were useful only in low-frequency applications (less than 3 kHz), the vibration analysts felt it was possible to achieve the desired high-frequency response by:

• combining a high-response accelerometer with a low-mass, high-strength rare-earth magnet

• carefully controlling the quality of the metal-to-metal surface between the magnet and the structure

• providing a layer of lubricant to act as a coupling medium across the magnet-to-structure interface.

To validate this hypothesis, the investigators ran a series of tests using a PCB accelerometer and rare-earth magnet. The magnet pull was only 2.5 lb., but the low mass (2.3 grams) of the accelerometer/magnet system resulted in a high resonant frequency when mounted. During each of the three test phases, various configurations were evaluated to assess the effect of surface preparation, attachment methodology, target designs, and different glues and lubricants. To assure repeatability and uniform results, personnel prepared at least three test samples of each configuration and obtained multiple readings from each. As the best configurations emerged from the process, additional samples were prepared and re-tested to confirm the original results.

Ultimately, three sets of tests were performed. The third and final set was conducted at an actual air-separation facility. In addition to revealing the efficiency of magnet mounting, the testing procedures clearly demonstrated the importance of controlling the metal-to-metal surface between the magnet and the structure. Machine surfaces in the real world don’t provide the ideal flatness, smoothness, cleanliness, and surface finish necessary to achieve high-frequency response with a magnet-mount accelerometer.  The presence of a lubricant as a coupling medium across the mechanical interface between the magnet and the target proved to be an extremely crucial element of the attachment system.

Implementation

Resolution of key technical issues regarding magnet-mounting techniques has paid off well for the industrial-gas manufacturer by improving vibration monitoring at multiple sites—and on many different types of air-separation equipment. Benefits have included:

Reduction in unscheduled downtime: Almost every week, the program identifies a condition that, if not corrected in a timely manner, could have caused a plant to go down.

Increased equipment availability: Since the program has been in place, equipment availability at some plants has increased by several percentage points.

More-efficient use of personnel: At each measurement location, the quick-connect/disconnect mount saves at least a minute compared with stud-mounting. Multiply this by hundreds of measurement points at hundreds of plants every month and the result is a considerable manpower savings over the course of a year.

Better use of vibration resources: Because the improved PdM program runs smoothly, vibration analysts are able to spend their time resolving critical problems or doing proactive work to make the entire network more efficient. EP

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