What to Look for in a Vibration Consultant By Alan Friedman
The following provides an introduction to vibration analysis technology and methodology with a focus on what to look for in a vibration analysis consultant. Many corporations today are looking to outsource machinery condition monitoring programs, while others wish to have an outside vibration consultant develop a monitoring program that can later be brought in-house. In either case, the more you understand the underlying technology, the better the decision you can make when selecting a service provider.
Buy it here: Audit it. Improve it! Getting the Most from Your Vibration Monitoring Program by Alan Friedman
The Most Basic Form of Vibration Analysis
The most basic form of vibration analysis is called an overall reading. This reading provides a single number that describes the total amount of vibration energy being emitted by a machine. The idea is that more vibration indicates a problem. A number of tables and guides have been developed to explain what levels are acceptable for various machine types. This technology is inexpensive, but it can also be inaccurate and inconclusive. As an example, a pump experiencing turbulence or flow noise will have a very high overall level, although there are no mechanical faults. On the other side of the coin, the pump may have a bearing problem that is serious, but emits little vibration energy in comparison to the energy emitted by the shaft and the flow noise. Thus, the bearing problem may not be evident in the overall reading. Finally, because the overall level provides only one number, it cannot differentiate between faults. In other words, one will not know if there is imbalance, misalignment, a bearing problem, a foundation problem, etc.
Overall readings were and are used today simply because the devices needed to collect this information are inexpensive and once were the only thing available. Unfortunately, many people today have incorrect concepts of what vibration analysis is and how it works because this simplistic approach is the only experience they’ve had with the technology.
Narrow Band Vibration Analysis
As computers became widely available, so did the capacity to collect narrow band vibration data, or vibration spectra. A vibration spectrum separates measured vibration into small frequency bands. Different machine components and different faults will produce vibration and vibration patterns at specific frequencies. Thus, using a vibration spectrum, one can relate individual peaks and patterns in the spectrum to individual machine components and specific machine faults. In order to do this, one must have information about the machine, such as the number of fan blades, impeller vanes, and gear teeth, as well as shaft speeds and type of bearings (rolling contact or sleeve). The vibration consultant should be asking you for this information.
Trending Vs. Trouble Shooting
Using vibration analysis in the context of condition monitoring is easy, accurate, efficient, and inexpensive. Using vibration analysis for troubleshooting is more difficult, often less accurate depending on the vibration consultant, and more expensive. Here is the difference: In condition monitoring, one routinely monitors the machine under repeatable test conditions and looks for changes. If the machine is not failing, the vibration patterns won’t change. If it is failing, the patterns will change, and it will be easy to determine what has changed and what fault the machine has. How much and how quickly the pattern is changing tells how severe the problem is and indicates when action should be taken. Vibration analysis is sensitive enough to find some faults a year or more before they progress enough to require attention. It will take an experienced analyst no more than a few minutes to compare a new set of data to an older or reference set of data and point out what has changed and what the problem is.
Troubleshooting is the process of taking measurements on a machine one time and trying to determine if it has a fault or not. In order to troubleshoot a machine correctly, one must take quite a bit of data, including phase measurements and possibly structural measurements, in addition to spectral data. This can be very time consuming and expensive. The results a vibration consultant provides will depend on his experience and the type of tests performed. It can take a whole day, or more, to accurately troubleshoot a machine. Beyond the technical differences between the approaches, it is also widely accepted that condition based or predictive maintenance makes plants more efficient and saves money. Therefore, it is in the plant’s interest to use the approach that best facilitates this type of maintenance management. Troubleshooting a machine is different from monitoring it, and the latter offers greater rewards.
What to Look For In a Vibration Consultant
With the basic information provided in the last few paragraphs, you already have an idea of what to look for in a vibration consultant and how to tell if he is doing things correctly or not. The key information lies in the paragraph on narrow band analysis and in the description of using vibration analysis in the context of predictive maintenance. Here are the key points:
* To conduct vibration analysis, information is needed about the machines such as gear tooth counts, shaft rates, fan blade counts, etc. The vibration consultant will ask for this information or look through technical manuals to try to acquire this information. This process will likely be ongoing.
* Condition monitoring relies on repeatable test conditions (speeds and loads), therefore the vibration consultant should attempt to define and document these test conditions. * Repeatable test conditions also include repeatable test locations. The vibration consultant should define these locations and preferably mount sensor test “studs” on the machines to insure good repeatable data.
* Condition monitoring is most beneficial when it is seen in a long-term context. Although you may see results and benefits immediately, the real payoff comes when monitoring is incorporated into normal plant procedures so you know the condition of the plant and are able to plan repairs. Both you and the vibration consultant should view this as a long-term commitment.
* Because vibration can tell specifically what component of a machine has a problem, the reports you receive should have a concise description of the fault and its severity, as well as a recommendation of what action to take. Saying a machine is “in alarm” or has “high vibration” is meaningless with the technology readily available today. The vibration consultant should be able to explain how he came up with his recommendation.
* Because vibration analysis is not a “science,” you need some sort of baseline in order to monitor how machines change over time. You should ask the vibration consultant how he plans to do this. You should also note that baselines should be updated after machines have been overhauled. Although standards and guidelines are readily available, the best baselines come from your own machines because you want to know if their conditions are changing.
Conclusion
Vibration analysis for machinery condition monitoring is not magic, nor is it a hard science or simply a matter of taking an overall vibration reading and comparing it to an alarm. Although experienced vibration consultants can troubleshoot machines if they take enough measurements, it is far more cost effective and beneficial to use vibration analysis in the context of predictive maintenance and trend machines over time. The most essential and difficult part of setting up a predictive maintenance program is compiling machine information, defining standard test conditions, collecting good and repeatable data, and setting up baselines.
The next most important part is managing all of this information in a database as machines are repaired and replaced. One can judge a vibration consultant by how these tasks are approached and by what sort of equipment is used to facilitate these tasks. Because these tasks are the most time consuming and important, once they are completed by the vibration consultant the plant should be able to take over the program if it is so desired. If a vibration consultant doesn’t ask you for machinery information, doesn’t work with you to define repeatable test conditions, doesn’t mark or place pads on test locations on the machines, doesn’t maintain or manage a good database with machine information, doesn’t create baselines, and doesn’t discuss long term plans for developing and managing the monitoring program, then he probably is not giving you good results either. The more you understand vibration analysis technology, the better chance you’ll hire a competent vibration consultant and incorporate condition monitoring into maintenance management.
