Plant vibration monitoring - Part I: A practical introduction to predictive maintenance
February 2003, Maintenance, Test & Measurement, Calibration
Traditionally, maintenance procedures in industry have taken two routes. Firstly, during the summer holiday time, the maintenance engineers take advantage of closed or slower production cycles to fully inspect all aspects of the machinery. They will replace, as a matter of course, components of the plant, such as bearings on motors, pumps and fans, whether it is needed or not, resulting in avoidable component and labour costs.
The second approach is to simply react to plant failure as and when it happens. This means that the maintenance team has to be on call 24 hours a day, and that capital is tied up in a stock of the parts most likely to fail. The failure also incurs loss of production, with its inevitable high costs. This can result in slipping of product delivery times, which reflects on the credibility of the company image.
On reflection, this traditional method of maintenance may seem, in the present day of high technology, somewhat out of date. However, many companies, including many of the large organisations, still adopt this approach.
However, the wind of change is blowing. Making use of today's technology, a new scientific approach is fast becoming the new route to maintenance management. One of the key elements to the new approach is predictive maintenance by plant vibration monitoring.
The business of anticipating machine failure with sufficient accuracy to enable corrective action to be taken before breakdown. One thing is certain: if you run industrial plant, you can benefit from implementing a vibration based condition-monitoring strategy. The key to success is to select the most appropriate technique for the application.
All machinery in operation (having moving parts) will produce a degree of vibration, generated by its rotational or linear action. Small levels of ambient vibration are perfectly acceptable. However, higher levels and increasing trends are a symptom of machine performance abnormality.
In general, with rotating machinery, the problem is caused by misalignment of components in a drive train, worn or damaged bearings, load asymmetry due to the adhesion of debris on rotating parts (eg, particle build-up on fans), or even incorrect assembly.
Detecting these problems
This is done by the use of a device that detects and mimics the vibration level and translates it into engineering units that can be manipulated. Any device that converts a physical state (vibration, temperature, pressure, etc) into an electrical signal, is called an accelerometer, or vibration sensor.
Machinery vibration is the unwanted, small, high speed repetitive to and fro movement, generally with its major component at right angles to the axis of rotation of the load transmission shaft. Since the movement is constrained by the stiffness and geometry of the machine's structure and the way in which it is clamped to its base, the vibrating elements oscillate between two end limits, governed by the above and also the speed of rotation of the shaft concerned. It is possible to actually see the machine vibration or even its effects, but by far the best method to detect it is by contact on the offending part.
The accelerometer/vibration sensor
When an accelerometer is shaken back and forth in its sensitive plane, it generates an electrical output that is proportional to the severity of the vibration.
The piezoelectric accelerometer contains a small piezo crystal that is bonded to a seismic mass. When subjected to a vibratory force, the crystal generates an electrical signal that is proportional to the vibration level. Internal electronic circuitry converts the signal to a voltage (mV/g) output which can be read by a data collector and optionally through further processing to a 4 to 20 mA output compatible with a PLC or other process computer.
By attaching the accelerometer securely to the machine (pump, gearbox, fan, motor, etc) it will generate an electrical output proportional to the excess vibration level in the plant component. Industrial accelerometers are available which have been designed specifically for use in the maintenance field. They are designed to continue working under extremes of temperature or harsh environmental conditions, they can be permanently installed on plant machinery for many years, requiring no maintenance. A large choice of models is offered including versions for high/low temperature, waterproofed, intrinsically safe areas and radiation resistant. Various output configurations (AC, velocity, 4 to 20 mA) are available to give conformity with most proprietary monitoring systems. Each accelerometer is supplied with a calibration certificate, giving sensitivity values of either 100 mV/g or 50 mV/g.
The piezoelectric accelerometer is a dynamic device and functions within a specified frequency range, eg, 2 Hz (2 cycles per second) to 8 kHz (8000 cycles per second). To relate these to rotational speed (rpm), multiply by 60. There are accelerometers which will operate below 2 Hz, where very low speed operation is encountered, such as on ore crushers, high ratio reduction gearboxes, etc. These are known as piezo-resistive devices and use a different method of vibration detection, allowing them to operate down to 0 Hz, or DC steady state.
Applying the strategy - the choices
For many applications and as an entry level into maintenance monitoring, the regular use of a simple, but accurate handheld vibration meter will pay dividends within a short space of time.
If the continuous operation of plant is paramount, permanently installed vibration monitoring coupled with fixed sensors is advisable. The multiple sensors can be monitored by regular patrols taking readings with a vibration meter, by using a simple sensor switch box, or by continuously scanning all sensors using a data collector alarm analysis system.
Trending in vibration measurement
In condition monitoring, specific absolute vibration measurements in general are not required. More importantly, it is the method by which a pattern is developed from vibration monitoring at regular intervals on the same pre-determined position and under similar operating conditions on the machine (ie, load, speed, etc). Again, even with a fairly basic vibration meter, it is possible to build up a data history on machinery which, when plotted graphically or viewed in tabular reports, can warn of potential bearing failure. The accuracy of prediction increases in direct proportion to the pre-history of the monitoring program. See Figure 1.
Figure 1. A graph of vibration in mm/s
It should be noted that very rarely do bearings fail without giving sufficient warning for remedial action to be taken. This may, in the first instance, simply be a spur to increasing the frequency of the patrol inspections at the suspect points. But it will enable the scheduling of machine strip-down and repair to coincide with a planned production break. See Figure 2.
Figure 2. A table of guidelines on mechanical vibration
There is a wide choice of options when it comes to vibration monitoring equipment. Handheld vibration meters are the ideal choice as an entry-level instrument. Some simply display the basic measurement in mm/s RMS, which is entered into a notebook for later insertion into a spreadsheet, with subsequent graphical analysis.
Other handheld instruments can store the readings, date them and then allow the user to download into the data using dedicated software, for more sophisticated analysis. Whichever system is used, as long as it is done regularly, the cost savings will not be long in coming. It is worth remembering that even a very basic meter can provide instant comparisons where the same type of machinery is duplicated across a plant. Rogue machines can be spotted after only one measurement. The meter kit will usually include a magnetic base and a spike for attachment or contact with the machine under investigation.
For more information contact Andy Brown, Temperature Controls, 011 791 6000, firstname.lastname@example.org, www.tempcon.co.za