Compressed Air Vibration Monitoring Best Practices

For most manufacturers keeping an operation up and running, downtime is what keeps them up at night.  And well it should.  Interruptions to operations can be expensive and have consequences beyond lower output – from supply chain and inventory impacts, to workforce challenges to investor and customer relations.

A key element of most (approximately 80%) of manufacturing is air power.  Considered the fourth utility for its ubiquitous role, keeping compressed air flowing is critical to an overall operation.  To ensure air power is reliable, proper maintenance of a compressed air system is imperative.  A step beyond proper maintenance is predictive maintenance.  Knowing the compressor’s health throughout its life span helps keep operations running smoothly.

One of the easiest and most cost-effective predictive maintenance steps is vibration analysis.  When done regularly, vibration analysis monitors the health of a compressed air system from installation and provides an in-depth view of what is ailing the system – and more importantly, what might cause problems in the future.

What is vibration analysis?

Vibration analysis is a predictive way of determining a machine’s health and is the testing and tracking of an air compressor’s vibration profile to aid in diagnosing any current or future faults.  Vibration can come from electrical or mechanical faults, poor installation or may be caused by normal functioning of the machine.

In this way, vibration can be normal or indicate an issue.  Every compressor has its own vibration signature and documenting and tracking that unique vibration allows the owner to “see” a number of performance issues.  As the compressor runs, bearings in the air end and motor as well as gears and other mechanics emit a vibration frequency.  That frequency, when tracked over time, tells a story.

At certain known or documented frequencies, a skilled analyst can tell (and then predict) when a bearing or gear is failing.  The analyst can even discern details like the difference between the rolling element, the inner or outer race, or the cage in a bearing that has or is beginning to fail.  In addition to bearings, the issues that may be diagnosed through vibration analysis include mass unbalance, misalignment, mechanical looseness, gear issues, motor faults and more.

In addition to helping prevent catastrophic compressed air machine failure, there are many benefits to vibration analysis such as reduced maintenance costs, less standby equipment, lower energy costs and enhanced safety.

Capturing vibration analysis data

Vibration analysis is ideally started as soon as the air compressor is installed.  Collecting a base-line vibration profile will help the analyst see, over time, when the integrity of the various components begins to break down.  It can also determine if the compressor was installed correctly and identify any balance issues, etc.  And, while vibration analysis can begin after installation and even midlife (or later) of a compressor, the readings are best if trended from installation startup.  Collecting readings from machines in a faulted state doesn’t allow the analyst time to alert a customer on possible downtime from what’s creating the issue as base-line readings are captured on an already faulted machine.

After the initial baseline analysis, a vibration profile should ideally be collected at quarterly preventive maintenance visits – depending on the usage of the air compressor, but at a minimum annually.  The route and data collection usually only takes about 20 to 30 minutes to complete.

To conduct the analysis, a small handheld vibration tool with an accelerometer is placed (via magnet) to various areas of the air end as well as the motor and other select areas of the compressor.  The technician will then take readings in up to three directions – vertical, horizontal and axial along the measuring points – and may even take several readings from one measuring point.  Some vibration accelerometer devices read in all three directions – vertical, horizontal, and axial – at the same time, but some analysts prefer using a single direction as it tends to be more accurate.  Additionally, the data collected can be more dependable if the measuring points of the routes are marked so that the future routes are taken in the same locations.  Once the machine reaches its operating temperature and stabilizes, the compressor will then need to run at rated pressure and full load for the entire time the readings are being taken – with the usual environmental conditions and temperature.  The handheld analyzer along with the accelerometer measure the machine’s vibration in the Fast Fourier Transform (FFT) spectrum and converts the data gathered into the FFT spectrum by the algorithm.  This FFT data is recorded at each measuring point of the compressor and route and sent to an analyst to evaluate the readings.

The analyst will view the collected data on spectrum and time wave form plots and render a report.  The vibration levels recorded on the graph will indicate what components are performing normally, which are changing (if historical data is available), and what is beginning to fail.

Every component from the ball bearings to the gears and rotors has its own frequency.  By reviewing the frequency readings and studying the peaks and valleys on the graph, analysts can pinpoint an issue by a shift or abnormal component frequency as vibration peaks will become more pronounced or elevated when there is a problem.  The process is extremely accurate.  If the analyst can’t determine exactly what is at issue, they can come very close to providing a strong indication of the issues brewing.

The final vibration report provides an overview of the health of the compressor and includes a detailed summary of where the various parts and components are in their life cycle.  Most importantly, the report can inform the user when an issue will because serious or a failure is likely.  This will allow the user to schedule maintenance at a time most convenient to the overall operation.  Instead of a bearing failing during a busy or critical production window, plant management can shut down operations at a pre-determined time and coordinate repairs to other areas of the factory at the same time.  Scheduling this kind of maintenance also ensures that the parts needed for the repairs are in-house.  Unplanned shutdowns can stretch into days or even weeks if parts are unavailable.

Oil sampling’s role in vibration monitoring

Oil sampling is another predictive maintenance tool that, when used with and at the same time as vibration analysis, can provide a detailed and accurate picture of the health of a rotary screw compressed air system.  Both tools are inexpensive and, when performed on a regular schedule, can predict and prevent costly and untimely breakdowns.  With oil sampling metals and other particulates found in the machine’s lubricant can signal an issue.  Combine that with vibration analysis and it’s easier to identify the culprit before catastrophe strikes.  Oil sampling is also often necessary to keep a compressed air system in warranty.

Users also should examine the compressor’s foundation and the addition of non-shrinking epoxy grout to help reduce vibrations from the frames of the machines.  Some compressor foundations are not designed and reinforced properly to carry the compressor’s load.  It’s also possible the compressor’s foundation has weakened over time.  Ensuring the foundation has not been compromised after years of heavy, vibrating machinery sitting on top can help reduce the overall vibration of the air compressor and reduce repairs and extend the life of the compressor.  Vibration analysis is an important tool to regularly utilize to ensure keep air compressors maintained and running as efficiently as possible.  When paired with oil sampling and grout upkeep and performed consistently, it can provide invaluable information and provide the user peace of mind.