Rotating biological contactors (RBCs) are normally found as single units, but some plants have multiple RBCs together. In the images below, you can see a group of five RBCs, a group of two RBCs, and a single RBC. They are normally found in remote locations, such as villages in the countryside.
Figure 1: RBCs in groups and as single units
In this particular case, we used a portable instrument, the Leonova Emerald®, to take measurements from the non-drive-end bearings of the RBCs, and fixed SPM adapters on the drive-end bearings. It is very important to use adapters.
The fixed speed on machines of this sort normally range from 0.9 to 2 RPM, occasionally up to 2.2 RPM.
We started by measuring the main bearings. In Figure 2, the top image shows the drive-end bearing with a small gearbox. The distance between the drive end and the non-drive end, shown in the bottom image, is between 15 and 25 meters, so the machine is quite large.
Figure 2: The drive end and non-drive end
Regarding the software settings, we made numerous trials with the number of LOR and orders using SPM HD; for example, we tried measuring with 3200 and 6000 lines. We found that the best setting to start with is 100 orders and 800 lines. Because this is a slow-moving machine, it takes a long time to take good measurements, so we had to balance the time needed to take the reading with the amount of detail we wanted to see. If the Shock Pulse levels rise, we increase the LOR.
Figure 3 shows the software setup for the case I’m about to describe. We see the bearing number near the top; it is a Cooper split bearing. The shaft diameter is 160 mm, which is the most common size for these machines. So these bearings are quite large. When we add the turning speed, the software can provide an initial value, representing what we expect the bearing to be doing.
Figure 3: Our initial software settings
Again, we must use fixed-point adapters. Figure 4 shows the series of measurements we took, and you can see that the initial readings, taken with the portable device, are very high. This was quite suspicious compared to the later readings taken with adapters, which were acceptable.
Figure 4: Measurements from a portable device vs. measurements from adapters
We installed fixed-point adapters around 21 or 22 September, 2020, indicated by a red square on the graph, and from there we obtained acceptable readings. You may have experienced similarly poor readings when performing vibration analysis with a stinger.
On 26 August, 2020, we measured 30 or 40 bearings. You can see the initial readings in Figure 5. One bearing was showing high readings, and we decided to investigate further. The first readings we took were high, so we took another measurement 15 minutes later. The readings were still high.
Figure 5: Bearing #1 readings
At 11:39, indicated by the red square, the bearing was lubricated. The next few measurements shown were taken on the same day. Approximately one hour later, there was no change in the reading. Because this bearing is so large and slow, it takes a long time for the lubricant to completely cover it.
One month later, we returned to measure the bearing again. The readings had dropped, but they were still much higher than those of the other bearings. Because of these readings, it was decided to inspect the bearing.
During the inspection, they found a large amount of scoring around the shaft as well as actual impacts in the bearing itself. Of course, this was a split bearing, so some cracks are expected, but this bearing was clearly damaged.
Figure 6: The damage on Bearing #1
Because it takes two to three months for spare parts to arrive, they decided to repack the bearing and keep it running. The following month, the readings dropped after the bearing was packed with grease, and then they started to rise again. Luckily, the new bearing arrived and was installed on 16 December, and from then on the readings remained in good condition.
Figure 7 shows a reading from another bearing. The initial readings were acceptable, and it was decided to amend the alarm levels. As you can see, the readings started to rise, and the bearing was inspected.
Figure 7: Bearing #2 with reading and time signal
They found that the bearing had not been installed correctly, and the collar was protruding. They found various additional problems with this bearing. The bearing was replaced, and the readings dropped to acceptable levels and remained that way.
The time signal in Figure 7 shows a significant impact corresponding to the high readings. In this case, there were eight revolutions on the shaft, with one massive impact. This was an unusual reading, in our experience, and obviously showed poor installation.
We extended the readings to another area. The readings in Figure 8 were taken from the drive-end bearing. We’re actually seeing the signals coming from the gearbox, so initially we were wondering whether or not the issue was with the bearings.
Figure 8: Readings from Bearing #3 and gearbox
In the space between A1 and A2 in the time signal, we can see one order between each main impact. It’s the same all along the time signal, so these are synchronous signals. Because of this, we were reluctant to suggest bearing damage. There was obviously something else going on.
They decided to replace the bearing, but it did not make much difference in the readings. Over time, they began to realize there were issues with the gearbox itself. They increased the oil in the gearbox and performed some general maintenance, and the readings started to drop.
As you can see, the Leonova Emerald and SPM adapters provided accurate readings for these ultra low-speed bearings and even the gearboxes.
