Boosting success
If an application is suitable for enveloping, several factors will ensure better results. Accelerometers to measure the low level signal should first be selected carefully – in the proper frequency range – to suit the needs of the particular machine or application. At the same time, they should be mounted close to the component being monitored on a flat, clean surface to guarantee consistent results.
Accurate sensor mounting is essential to ensure true readings. Mounting a sensor via a drilled and tapped hole direct to the machine housing is likely to produce the best results. A good way of ensuring that the vibration sensor is mounted into the rotating machine correctly – especially if the housing is not flat – is by using a spot facing kit that includes a tapping drill, taps, tap wrench and spot facing tool.
To correctly determine any number of defects – be it imbalance, misalignment, mechanical looseness or rubbing – the sensor must be in a position to measure horizontal, vertical and axial movements effectively.
When reading horizontal displacement in a pump motor, for example, sensors should be mounted on the two motor or pump bearings. For vertical measurements, the sensors should be located on the motor pump or pump drive end bearings. Axial measurement uses the same measurement criteria to detect misalignment between the components, such as the motor and fan.
For a typical crusher motor, accelerometers are mounted radially on the Drive End and Non-Drive End, in order to monitor the motor bearing condition. Within a gearbox, a sensor will be mounted radially on input and output shafts. This enables the condition of the bearings (g) and fan (velocity) to be monitored. An optional axial accelerometer on the input shaft can give a good indication of the thrust on the shaft.
Cement manufacturing applications typically use 4-20mA accelerometers connected to PLC systems. Each sensor is connected to a local junction box; then multicore (screened twisted pair) cable is connected to the PLC system for data trending and alarming.
Once accelerometers have been installed and calibrated, readings should be taken at regular intervals over a period of time to allow accurate trend analysis to be performed. This allows a steadily deteriorating condition to be identified, for example.
It is important to understand that the information provided is not a simple ‘yes/no’ answer, and requires a certain level of skill and experience to interpret: the amplitude of a worsening condition can actually reduce over time, for example, as the imperfection becomes slightly smoother.
Conclusion
The potential benefits of acceleration enveloping are clear: it helps to pick out the vital signs of component failure that may eventually develop into serious problems. This is especially important in the punishing environment of cement production, where dust and dirt can penetrate into bearings and other components – and, potentially, lead to catastrophic failure and unscheduled downtime.
However, a word of warning: it is not appropriate to rely on acceleration enveloping alone. It is far more effective to use it as part of a wider monitoring and analysis regime, helping plant engineers maximise the performance of all assets within a cement manufacturing facility.
This is part two of a two-part article written for World Cement’s September issue and abridged for the website. Subscribers can read the full September issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the September 2016 issue here.
