Can Uslu │ Principal, PdMechanics
Induced draft (ID) air fans are critical assets in the cement industry for the continuous operation of the rotary kilns. ID fans handle hot flue gases and discharge them into the atmosphere through a chimney. The dust and dirt in the environment cause build-up material on the fan blades. Studies show that the main factors affecting the rate of build-up accumulation are flue gas temperature, fan speed, blade design and dust’s moisture content.
During the operation, fan speed and flue-gas temperature variations cause the build-up to unevenly break-off from the blades, and this situation generates unbalance. ID fan build-up is a widespread problem in the cement industry, causing downtime, production losses and mechanical damages. Continuous vibration monitoring is crucial for the ID fans in a cement plant since urgent actions may be needed to avoid severe failures and ensure the personnel’s safety.
A recent case in a cement plant in which PdMechanics provides online diagnostics services demonstrated the importance of the subject. On 8 February, after the reoperation of the I.D. fan upon a short outage, overall horizontal vibration values of around 3 mm/s rms were observed at both sides of the I.D. fan, which was relatively higher than the previous measurements (Figure 1). PdMechanics continued to monitor the vibration trends closely. In the meantime, frequency analysis indicated that the vibration at the shaft rotating speed (1X) was the dominant frequency (Figure 2).
Figure 1 – Free End (—) and Drive End (—) vibration velocity trends (mm/s rms)
Figure 2 – Drive End (—) vibration frequency spectrum (mm/s rms)
It is important to emphasize that various malfunctions may manifest themselves with 1X vibration. A precise diagnosis requires a good understanding of the asset’s working principle, failure modes, and maintenance records. In our case, several factors pointed to unbalance: The asset was an I.D. fan handling hot exhaust air with dust, there was a short-term outage which could affect the build-up distribution, and contrary to the radial vibrations, axial vibrations didn’t exhibit an increasing trend. The overall vibration values remained stable for nine days, and on 17 February, they started to increase rapidly, reaching 7 mm/s rms in the following two days (Figure 1). PdMechanics issued an unscheduled status report with the diagnosis of fan unbalance and recommended the maintenance team take action. The fan was stopped the same day. The plant’s maintenance team successfully performed balancing works, and the vibration values reduced down to 1 mm/s rms.
PdMechanics’ continuous condition monitoring enabled the timely detection of the rapid vibration increase. The diagnosis of unbalance led the site team to focus directly on the malfunction without any trials to figure out the issue. The repair works were completed in a day. The good communication between the PdMechanics’ remote diagnostics engineers and the plant’s operation team was the most important factor of the successful result.