In part 1 we discussed why any organization that is involved -or planning to get involved- in a condition monitoring system project should work with a specialized condition monitoring system integrator only. The main reason for that was the specialized experience and vibration skillset required to ensure such systems would deliver its full potential. In parts 2 and 3 we talk in more details about some issues that may be present in existing or new condition monitoring system installations. Thus clarifying the value of such expertise.
The condition monitoring systems integrator’s (CMSI for short) responsibility covers the entire scope a condition monitoring project. It starts at application engineering and the components selection phase, to the installation phase which covers the instruments, wiring, panel design, and of course the system hardware and software configuration. And ends with the successful commissioning of the overall system, which sometimes includes a system observation period during normal operation.
Condition monitoring engineers may come from different backgrounds. They can be mechanical, instrumentation, or automation engineers. In fact nobody studies condition monitoring at the engineering school, it is knowledge that is acquired through specialized training and work experience. Condition monitoring engineers with a mechanical background may have an edge in understanding the effect of materials selection and parts design on the total asset health. Yet a condition monitoring engineer with instrumentation background may have an edge in the understanding how to make vibration instrumentation and systems work trouble-free. Both complete each other to deliver a functional system.
Any automation engineer can perhaps follow some guidelines and configure full scales, setpoints, relays, and some graphics for a vibration monitoring system. But it takes a specialized CMSI to configure the system properly, ensuring high resolution waveforms and spectrums, and meaningful FFT band alarms and machinery diagnostics insights.
Lack of expertise can lead to one or more of the below discussed problems.
The problem of noise
From a measurement point of view, vibration is very different from anything else measured in the industry. In most process sensors, reading fluctuations are usually be overlooked or treated as noise and filtered away. In the case of vibration, reading fluctuation is the name of the game! Thus making dealing with signal noise problems a much bigger problem.
Noise in vibration signals can be caused by:
- Wrong selection of cable
- Wrong connection of wiring shields
- Wrong grounding of circuit components
- Electromagnetic interference
Noise is very difficult to pinpoint and troubleshoot, because it doesn’t exist all the time, and doesn’t come with the same magnitude every time. To troubleshoot it, you would fix something and then wait for days to verify that your action stopped the noise, often to find that you were not so lucky, so you try something else and you wait again. Sometimes it takes months to completely cure an installation.
Noise filtration is not a good option either. It takes a great deal of expertise (and courage) to decide whether signal components of particular frequencies are noise or real machinery vibrations. What’s worse than noise existence is to filter away real vibration components by mistake. Unfortunately if you are not able to prevent noise and cure it, you have no other option but to use filters.
The best way to prevent noise from attacking your vibration signals is to do a proper installation in the beginning. The installation has to be supervised closely by a condition monitoring systems expert.
We would expect the system integrator working on a condition monitoring system to inspect the installation from the sensor until the HMI to ensure noise won’t be present before even powering up his system.
The problem of instruments installation
Bad instrument installation is also a source of misleading data in vibration signals. An accelerometer that is loosely tightened may resonate and create unwanted fluctuations in the measured signals, often causing machines to trip. The fun part is that the same can happen because of “over tightening” of these sensors. The same applies to piezo-velocity sensors installed in the wrong place on the machine. Using the wrong casing vibration sensor mounting pad or base can sometimes shield some important vibrations as well.
In the case of proximity sensors, the mounting bracket stiffness is a common source of wrong information, as it may resonate with the natural machine vibration. The mounting location of proximity probes as well -if not selected properly- can cause issues like cross-talk, and side view losses.
Sensors installed in bad locations may not pick up all the important vibration components. The frequency response and range of the sensor selected is also key in collecting meaningful diagnostics information. For example putting a sensor with a cut-off frequency at 5000 Hz on a gearbox might not be very smart! This also depends on the machine speed and type of course.
A trained and experienced eye can easily capture these installation mistakes, and recommend reasonable fixes to allow the commissioning of the system to proceed without delays.
Common issues caused by wrong instruments installation are:
- Cross talk
- Side view losses
- Wrong vibration readings due to:
- Resonating mounting brackets
- Limited frequency response
- Limited sensitivity
The problem of signal attenuation
Most common wired field instruments will communicate their readings/messages on a 4-to-20mA signal, operated on 24VDC power source. These are strong and stable signals, with no high frequency components. They can run in cables for many kilometers without any significant attenuation, even while using small wire diameters.
I think by now you can guess what I will be saying next… vibration is different! It’s always the case. A vibration signal is made of 2 voltage components, an AC voltage component that is often in the scale of a few millivolts, coupled with an almost stable DC voltage component in the range of 7 to 10 V DC. Each component tells different information about the machine condition. The AC component gives us information about the vibration amplitude, frequency, and phase. In case of proximity sensors, the DC component gives information about the magnitude of the gap between the machine shaft and the proximity probe while the machine is stopped, and the average distance between shaft and the probe while the machine is running.
Unfortunately this AC voltage millivolt signal can’t run in instrumentation wires for a long distance. Most vibration monitoring and system vendors will recommend that the maximum distance between sensors and system is only 300 meters (approximately 1000 feet), and in special situations the signals can run up to 500 meters.
Although this is clearly a design problem, an experienced condition monitoring SI can help you evaluate whether the signal will reach the vibration monitoring system without destructive attenuation or not, using information about the machine type, measurement location, expected failure frequencies, and of course the cable properties. And of course in many cases the CM SI may recommend the use of condition monitoring systems designed for field installation.
We will continue the discussion on other issues in part 3 of this series. We will talk about the problem of machine startup, and system health evaluation. And we are saving the best for last, a topic that is very dear to us, and what really differentiates an experienced CMSI from the rest: “Moving expertise from an expert’s mind to electronics”. See you in part 3.
Proact Engineering Services is a member of the Rockwell Automation Integrator Program (RAIP), and is an authorized system integrator in the discipline of condition monitoring. The team includes two vibration (ISO CAT IV certified), reliability, automation, and instrumentation engineers with more than 50 collective years of experience. The company focuses on supporting clients in the engineering, design, installation, commissioning, and troubleshooting of their condition monitoring systems. We are also an authorized training and consultation partner for leading organizations in the fields of vibration, lubrication, reliability, and thermography.
Leave a ReplyWant to join the discussion?
Feel free to contribute!