CAMBRIDGE, UK – May 2016 – MISTRAS Group Limited, the UK subsidiary of US based MISTRAS Group, Inc. (NYSE:MG) is monitoring the condition of low speed tilt bearings on Basic Oxygen Steelmaking (BOS) plant vessels at a steel production plant.
In doing so, MISTRAS is providing valuable support to the customer helping establish the condition of the vessel bearings. The objective of the monitoring was to, over time, trend the bearing condition, provide advanced information on the behaviour of the bearing during rotation and identify any acceleration in degradation rates of these critical, high value bearings. The BOS plant uses two vessels that convert molten iron from the blast furnaces into steel. Each vessel weighs around 1,300 tonnes, supported on two, 1.75m diameter spherical roller (tilt) bearings. Each vessel is able to tilt to various angles up to 150° from the vertical during the process for charging and discharging purposes. The arduous duty generates high local contact loads, which over time, result in surface fretting, pitting and spalling of the race surfaces. Ultimately, if undetected, further degradation can result in roller fractures and consequent catastrophic failure of the tilt bearings.
In March 2015, MISTRAS installed its Sensor Highway Data Acquisition System and permanent Acoustic Emission (AE) sensors on two tilt bearings on the BOS plant vessel. During controlled vessel rotations performed by the customer’s engineers, MISTRAS online monitoring (OLM) engineers monitored the vessel bearings via remote connection from its Cambridge, UK office to the onsite data acquisition system.
The delivery lead time for bearings of this magnitude can be as long as 12-18 months, with plant downtime for a planned BOS plant vessel bearing change requiring four weeks. The bearing cost per unit becomes almost insignificant in comparison to the overall production outage costs. To help predict when the bearings are likely to fail and therefore when bearing replacement should be scheduled, we were asked by the customer to monitor the condition of the bearings on one of its BOS plant vessels.
Kirill Svartsov, Applications Engineer at MISTRAS Group UK operations
Rather than monitor the tilt bearings during normal operation, MISTRAS assessed the condition of the bearings during controlled vessel rotations. Under normal vessel operating conditions, the bearings do not rotate through a full 360-degree of rotation – they rotate around 60-75 degrees in either direction (i.e. clockwise and anti-clockwise). This means the rollers directly underneath the bearings are constantly subjected to the high loads during normal operation while others remain unloaded. MISTRAS procedure assess the condition of the bearings as they were tilted through the full 360 degrees of rotation, so as to gather a more complete assessment of the condition of the bearings.
The vessel are rotated 10 times in the clockwise direction and five times in the anti-clockwise direction (360 degrees of the trunnion rotation). This enables the rollers to be re-distributed ensuring loading is not focused on one particular set of rollers. To ensure continuity the vessel is empty during the tests, rotations are started from the vertical position and are also conducted continuously with no stops.
The AE data collected from the controlled bearing rotation tests provided features that indicate the bearing condition, these include:
- The number of hits (events above the test threshold), which indicated activity rate of the bearing.
- Energy released from the transient emissions, which indicate the severity of the source
- Amplitude (loudness) of the transient emissions.
An experience-based grading of emission level is provided by MISTRAS which grades the bearing from an ‘A’ (no significant activity) and ‘E’ (intense activity).
When the first rotation test was conducted it was found, as expected, the BOS vessel monitored had already reached an E grade with intense activity as it was known to have a problem. The objective of the monitoring was to use the additional AE features collected to track the degradation of bearing condition over time.
Over the next 10 controlled rotation tests, MISTRAS was able to trend the bearing condition (Figure 1). As Svartsov states: “As a result of these tests, in August 2015, a decision was taken by the customer to replace the drive end [DE] bearing on the BOS plant vessel, due to a clear acceleration in the deterioration of the bearing. The AE monitoring had allowed the customer to continue operation of the critical vessel while being able to manage the problem bearing and make an intervention when the assets rate of degradation was changing.”
In September 2015, following the installation of a new bearing, a further controlled vessel rotation test was carried out. As Svartsov explains: “The acoustic emission activity rate and severity received from the drive end bearing reduced significantly following the bearing replacement and returned to the expected low levels.”
Acoustic emission versus vibration monitoring
When it comes to monitoring the condition of critical, slow-moving (0.25-100rpm), high capital value plant and machinery, companies should consider the benefits of AE monitoring – particularly where there are fluctuating load conditions and where a breakdown would have a significant impact on production.
Many companies rely on vibration monitoring systems to check the condition of low speed bearings on critical plant and machinery, but this method is not as accurate as acoustic emissions monitoring, especially when it comes to slow speed, fluctuating load applications. Vibration monitoring is fine for high speed motors, fans and pumps. However, where components or machines rotate at less than 100rpm and operate under fluctuating load conditions, or only move through a part revolution such as a BOS plant vessel, it is more difficult to collect meaningful data from methods such as vibration monitoring.
Many engineers are not fully aware of how acoustic emission monitoring systems can help them reduce maintenance costs and improve plant availability. Also, many companies simply do not possess the necessary skills in-house to interpret the data from acoustics emissions monitoring, so they continue to use vibration monitoring or other devices.
Kirill Svartsov, Applications Engineer at MISTRAS Group UK operations
Tools and techniques
MISTRAS’ LSB-PAC™ system is a condition monitoring and assessment system that employs a proven inspection methodology and advanced diagnostics to rapidly assess the condition of low-speed bearings during normal operation. Effective at assessing the condition of slow rotational speed bearings down to less than one revolution per minute (RPM), the LSB-PAC™ system assesses bearings with fluctuating load conditions or when a breakdown would have a significant impact on production or operational efficiencies.
A variety of bearing defects such as corrosion of tracks or balls (emission from corrosion product break-up), insufficient lubricant (emission from surface fretting), cracking or plastic deformation of bearing material (balls, track and cage), crushing and fracture of debris in the bearing, are all target areas of focus for the LSB-PAC™ system.
Low-speed rotating or reciprocating bearing signatures are produced by the LSB-PAC™ after a few dozen assessment cycles. Further, a more detailed analysis would involve the identification and elimination of extraneous noise, grading the severity of the bearing signature, and identifying any features that may indicate the root cause of the problem. Multi-level reporting options provide tailored reports meeting customer requirements, including grading reports and time signatures.