Online Monitor of Critical Bearings in a Pulp & Paper Plant
In this case study we will look at how a pulp & paper plant was able to detect an issue on a critical bearing using an online monitoring solution with ultrasonic sensors.
This is a slow-speed bearing (3RPM) from a bleach decker, with a diameter of about 48 inch / 120 cm diameter.
Usually, in pulp and paper plants we will find a wash floor or wash area, where the paper comes through to be thoroughly cleaned / bleached. That job is done by a machine called a bleach decker, which is considered a critical and fundamental piece of equipment for production operations.
In this particular plant it was decided to invest in online monitoring for these machines. Each one has 4 bearings, of about 48 inch / 120 cm diameter, rotating at 3 RPM.
Ultrasonic sensors are being used to monitor the bearings: these are UE Systems Remote Access Sensors, which are permanently installed on the bearings and constantly collect decibel readings and sound recordings.
All this data is then sent to a central processing unit called the 4Cast. This unit is connected to the Internet and will alert the maintenance team (e-mail and SMS alerts) when certain decibel levels are reached.
The preference for ultrasound has to do with its obvious advantages: since the ultrasonic sensors monitor the bearings’ friction levels, any increase in friction will be noticed. This allows for a very early warning of failure.
Also, because the data from the sensors comes in the form of decibel readings, it is easy to interpret: the higher the friction, the higher the dB value.
Also, ultrasound is the most efficient technology to inspect slow speed bearings. At such slow speeds such as 3 RPM, it is generally challenging to notice any issues using technologies such as vibration analysis or thermography.
Failure detection using ultrasonic sensors and online monitoring
Everything seemed to be fine with the bleach decker at this pulp and paper facility.
However, the 4Cast received an unusual decibel reading from one of the ultrasonic sensors. The NDE (non-drive-end) bearing was registering 17dB when, normally, a bearing rotating at such slow speeds like 3RPM should simply show a 0dB reading.
This triggered the system to immediately send an alert:
We can clearly see why the alert was triggered: the 4Cast received a 17dB reading from a bearing where the threshold for a high alarm was setup at 8dB.
When an alarm level is reached, the 4Cast will also take a sound recording from the bearing for further analysis.
The peaks shown in this sound sample clearly indicate a problem with the bearing. Also, when reproducing the sound file, we could very clearly hear the impact noises.
The failure was even more obvious when the sound file was compared to a sound recording from one of the other bearings.
We can clearly see the differences. In this case, the recording sounds smooth and looks uniform, and we don’t see amplitude peaks at all.
The next step for the maintenance team was scheduling the replacement of this bearing, without disrupting production. When the bearing was dismantled, the damage was clearly visible.
The signs of impact are obvious. Also, metal fragments were found in the shaft, plus spalling, with some pitting, and slight abrasion were present in the outer race.
By detecting the issue at an early stage, the maintenance team was able to replace the bearing during scheduled downtime and without disrupting the production process.
We can imagine the consequences if the issue was not detected at this stage and the bearing was allowed to continue operating: the metal fragments would certainly affect the motor shaft, which would then also need to be replaced; and the facility would have to face unplanned downtime.
In such a situation we could be looking at a loss of around 250.000.