A screw pump is a positive displacement pump common in many water industry uses. We used SPM HD technology to measure the bearings in a screw pump.
Figure 1: A plant with screw pumps
Our online system was measuring the bearings on the xxx and the gearbox. The gearbox and main bearing on the top motor were monitored too, along with the bottom bearing, which was submerged about two meters underwater. We had to work with another company to provide specialized seals for our equipment on this bearing.
The speed of the main bearing on a screw pump is often between 50 and 80 RPM. The speed is fixed but depends on the screw dimensions and the flow required. The speed of the bearing we measured with a proximity probe was between 30 and 40 RPM. Our measurements included the gearbox and motor.
If this pump were to fail, the water company would have to pay huge fines for pollution to the local area, so the stakes are high.
Figure 2 shows the measurements we took over time using the Intellinova® online condition monitoring system. The top graph shows the SPM HD reading at 32 kHz. The bottom graph shows the HD ENV reading with Filter #4 between 5 and 40 kHz.
Figure 2: SPM HD and HD ENV readings
We also measured low frequencies, but because of the speeds, nothing was shown. From 14 March to 23 May, the acceleration, velocity, and normal envelope were all flatlining. When you’re running these trials, it is easy to assume that there is nothing to show.
But at the end of May, the reading started to rise into the amber zone, and then severely into the red. This was shown initially in the SPM HD. The HD ENV jumped into the red at the beginning of June. This happened because the SPM HD is more sensitive at low speeds and can therefore detect very early warning signs. In the HD ENV, Filter 4 can detect early warning signs, and Filter 3 detects the next level.
Because the bearings could not be inspected, it was decided to lubricate them. The pink dots indicate the dates the bearings were lubricated, the first time being 15 June. The readings dropped after the first lubrication, but rose again afterward. This is shown more clearly in the SPM HD reading.
After the second lubrication, the readings remained at acceptable levels. After the third routine lubrication at the end of July, the readings became erratic, dropping and rising repeatedly.
Figure 3 shows the time signals, with the SPM HD at the top. The time signal is showing 16 revolutions, and each impact is 0.41 orders. The HD ENV at the bottom is also showing one impact for 0.41 orders.
Figure 3: SPM HD and HD ENV time signals
The bearing was not inspected, but we were fairly confident that there was a bearing issue. The next stage is for one of the companies to remove the screw and inspect the bearings.
Figure 4: Removing the screw pump
People sometimes ask why we use 32 kHz for the SPM HD technology. SPM, which stands for Shock Pulse method, was founded in 1970, and the patented frequency that was used was 32 kHz. Now all of SPM’s transducers are tuned to 32 kHz. A lot of scientists have since acknowledged that this is the best frequency to detect various faults such as lubrication degradation and very early-stage bearing damage.
