By introducing a new regime of condition monitoring from scratch, a steel rolling mill has not only avoided a number of costly breakdowns but also gained a greater understanding of its plant and improved the perception of condition monitoring across the site.
Located at Skinningrove near Saltburn-by-the-Sea in Northern England, the plant is part of the Corus Group and manufactures hot-rolled steel profiles for a wide range of industries, including earthmoving equipment, materials handling, shipbuilding and mining.
Monitoring of the six pairs of spindle support bearings on the two-stand reversing mill was previously carried out by physical inspection on routine maintenance shutdown days. This involved two craftsmen stripping down the bearing housings and manually inspecting the bearings. If no defects were found, the same bearings were then reassembled into the original housings and refitted into the mill. The entire process is both labour and overhead crane intensive. This task was done in rotation so that each bearing pair was inspected every six weeks.
This maintenance regime presented the engineering team with a number of challenges. As Scott Boyd, Operations Engineer, Mechanical points out: ''It is not good practice to tie up valuable labour for a full day disassembling and then re assembling bearings when they could be working on other areas of the plant and freeing up valuable resources.''
Not only were these increased manning levels an issue for the engineering team, but there was also the risk of introducing new problems as a result of human intervention. Boyd continues: ''Due to the harsh environment in which the mill operates, there is an increased risk of particulate contamination within the bearings during the dismounting and remounting process. We also had to ensure that when we refitted the bearings we did so to the correct bearing clearance, and this all takes time.''
Following a request from Skinningrove at the beginning of 2008, Corus' Plant Condition Monitoring (PCM) team carried out patrol monitoring on the bearings. By using a portable vibration monitoring device to obtain data on bearing defect frequencies they were able to report on some interesting findings. ''We found that we could obtain some very reliable data from our handheld device which allowed us to understand the condition of the six pairs of bearings and determine when a failure was imminent,'' comments Ian Taylor, Business Development Engineer, PCM at Corus.''However, for health and safety reasons, the manual approach was not practical in the long term,'' adds Taylor. ''Rather than sending our engineers into the working steel mill to obtain this data on a regular basis, it was agreed to install an online condition monitoring system.''
In September 2008, an eight-channel FAG DTECT X1 vibration monitoring system from Schaeffler UK was installed on the rolling mill. One sensor was fitted to each of the six spindle support bearing sets, with a plan to connect the two spare sensors to the pinion box.
''By using the online condition monitoring system we have been able to gain a much better understanding of the plant,'' says Taylor. ''When we began to analyse the first set of data, we realised that interference from other general plant equipment was masking the true bearing defects. We therefore needed to eliminate this in order to obtain true readings.''
Once this issue had been resolved, the FAG DTECT X1 system discovered two bearing faults on two separate occasions within the first week of operation. The engineers were able to plan the outage of the failed bearing and avoid a breakdown, which would have resulted ina significant plant stoppage on each occasion to allow time for stripping down and replacing the failed bearings.
As Boyd points out: ''By preventing two potential plant stoppages, the system has already paid for itself. Early detection means that we are in control of our plant. Not only are we spending less time and manpower removing and replacing perfectly good bearings, or trying to identify which bearing has failed and why, but we are also avoiding the added risks that human intervention brings.''
Asked what the future plans are for condition monitoring at Skinningrove, Boyd confirmed that the company intends to further utilise such techniques and technology on other critic al pieces of plant. ''People now believe in this technology,'' concludes Boyd. ''This has changed the perception of condition monitoring as they can now clearly appreciate the benefits to be gained from using the system. We are still on a learning curve, having only introduced this regime earlier in 2008, but I am confident that we will obtain a greater understanding of the bearings.''
The DTECT X1 series is a cost effective range of online monitoring devices that enables permanent frequency-selective monitoring. The advantage of the DTECTX1 system is that it includes features that are usually only available in far more expensive online systems. The basic model is available as a two-channel or eight-channel version with external multiplexer. All commonly used acceleration, speed and displacement sensors can be connected to the system. Depending on the version used, process quantities such as speed, temperature, torque and pressure can be recorded.
The signal measured by the sensor is broken up into its frequency components using Fast Fourier Transformation (FFT), which enables monitoring of amplitudes for previously specified threshold values within very narrow,defined frequency bands, including the triggering of an alarm if these values are exceeded. Therefore, damage can be detected at an early stage by means of frequency-selective monitoring.