When pump professionals discuss condition-based maintenance (CBM)—or predictive maintenance (PdM) as it is referred to in most circles—vibration monitoring is generally what comes to mind. While it may be a popular topic of discussion, vibration monitoring is arguably the least understood PdM technology out there. It is a complex science made even more complicated by a focus on the tools rather than the fundamental reason the tools are used.
Much of the discussion surrounding vibration monitoring focuses on what the different technologies can and cannot provide while ignoring the importance of a comprehensive maintenance program that can improve the dynamic condition of rotating equipment.
When meeting with pump professionals who are interested in starting a vibration monitoring program, I begin by asking the following question: What is your motivation behind obtaining a vibration monitoring program? What do you want this PdM technology to accomplish?
I would love to hear people say, “I want to monitor the dynamic condition of my rotating equipment and enhance overall machine condition over time. This will improve my bottom line by minimizing the probability of failure. It is an investment in good maintenance practices.”
Instead, this is what I often hear: “I want to find bad bearings.”
The few who provide the first answer likely have a management style of preventing or mitigating failure. Those who give the second answer are usually in a reactive mode. At this point, they are not looking to prevent failure or extend service life; they just want to build a schedule around equipment failure.
The attitude behind the second answer is predominant on an international scale, crossing disciplines in all industries.
When CBM/PdM monitoring goals are failure-based, the technology provides only partial benefit. With the limited goal of detecting failure, operators only maintain profitability by managing production outages. In other words, a facility still has the same number of outages with the sole benefit of fewer unplanned events.
Embracing failure prevention as a business goal, however, extends service life and allows facilities to reduce the number of outages. This goal improves profitability.
Understanding Vibration Monitoring Technology
Regardless of a facility’s goals—either failure prevention or failure detection—understanding the following categorizations of vibration monitoring technology can help users save significant amounts of money on monitoring equipment and training. This allows the facility to focus its resources on developing a structured and effective maintenance program.
Please note that the following discussion is focused on equipment that uses rolling element bearings and situations where vibration is measured at the bearing cap. This discussion is not specific to machines that use sleeve-type bearings, where shaft vibration is generally measured directly using a proximity probe.
While bearing type and measurement locations differ depending on the application, discussions about measurement technology typically still apply.
Vibration monitoring technology is broken down into two basic frequency ranges: low-frequency and high-frequency.
Low-Frequency Monitoring: 0 to 2 Kilohertz (kHz)
This range of the frequency spectrum is used to determine equipment condition. Specifically, alignment, balance and structural deficiencies (including resonance) are identified in this range. These conditions shorten bearing life, crack housings and cause ergonomic issues.
Generally, a thorough spectrum analysis within this frequency range is completed early in the machine’s life (for example, at startup or acceptance, after installation). Any corrected deficiencies are considered to be one-time efforts.
A quick check of overalls (peak or root mean squared [RMS] signal amplitude, commonly called broadband or filter out) should be completed semi-annually, after major repairs and alterations are made to the machine or its associated system. This task ensures that any material changes do not inadvertently cause vibration problems. A thorough or full spectrum analysis need only be repeated when a large jump in the overall is detected.
High-Frequency Monitoring: 5 to 40 kHz
Several technologies are specific to this frequency range. Two predominant commercial technologies are shock-pulse and high-frequency enveloping (signal demodulation). Both are based on pulse theory. Simply stated, pulse theory looks at a constant amplitude across an infinite frequency range caused by a single impulse in the time domain.
Monitoring this range of the frequency spectrum using these technologies is specific to identifying rolling element bearing defects and, in some cases, may be applicable to gear train faults. Depending on the technology used, bearing faults can be detected as early as 12 months in advance of an actual audible indication.
Shock-pulse and simple envelope amplitude monitoring are easy to teach, work well with formal lubrication programs and fit within most autonomous maintenance efforts. At a minimum, bearing condition should be evaluated by one of these technologies on a monthly basis.
The next important concept to understand is the different types of devices that can monitor in these frequency ranges.
For low-frequency monitoring, there are two basic choices: a digital box or an analog device. There is a major difference in cost, training and the experience required to use each.