The quest to minimise breakdowns
Published: 08 April, 2015
Swedish integrated board mill BillerudKorsnäs in Gävle recently got proof that gearbox condition monitoring is a reliability investment that pays for itself.
Excess wear or damage in industrial gears and gearboxes may result in catastrophic failure as well as secondary damage to other vital machinery, causing production losses, high repair costs and expensive downtime. Due to their prevalence in rotating machinery, condition monitoring of gearboxes is therefore fundamental in the quest to minimise breakdowns and unplanned downtime.
Condition based maintenance at the BillerudKorsnäs Gävle mill
Swedish integrated board mill BillerudKorsnäs Gävle has a condition based maintenance strategy for much of its plant equipment. The mechanical condition of some equipment rated extremely critical, such as the Digester 3 gearbox in this case, is continuously monitored using online systems with the patented SPM HD shock pulse measurement technology from SPM Instrument.
The continuous digester is at the heart of the chemical pulping process. In the digester, wood chips are exposed to heat and chemicals to separate cellulose from lignin. Besides low RPM, the digester is a fairly straightforward application from a condition monitoring perspective. The drivetrain components are at the bottom of the digester tower, from which pulp is continually discharged by a rotating bottom scraper run through an electric motor and a large, four-stage gearbox running at low speed (6-8 RPM).
Digesters are generally built robustly and maintenance problems on this equipment is relatively rare, but their critical importance to the production process still makes them prime candidates for condition monitoring. BillerudKorsnäs rates the digester gearbox highly process critical; if the continuous digester malfunctions, pulp production immediately halts.
The SPM HD method used to monitor the digester is an extremely sensitive measuring technique, providing the earliest possible detection of deteriorating mechanical condition. In the BillerudKorsnäs gearbox case, a single shock pulse transducer mounted on the output shaft picked up signals originating from the intermediate shaft.
Tim Sundström, technology and applications manager at SPM Instrument, commented: “Gearboxes are a common target for condition monitoring among our customers. Implementing condition monitoring on gearboxes is generally very satisfactory, since our methods are highly efficient for detecting faults and damage, most often with very long forewarning times. The gearbox at BillerudKorsnäs is a schoolbook example of the impact of condition monitoring.”
The Digester 3 gearbox case
Continuous monitoring on the digester gearbox started in the summer of 2011. A year later, measurements with the SPM HD shock pulse monitoring technology started picking up signals from an intermediate shaft indicating deteriorating bearing condition. After some time with increasing shock pulse levels, a period of more stable readings followed, after which levels again started to rise. The temporarily more uniform levels are most likely attributed to the fact that at a certain stage of the damage process, bearing damage typically “level off”, resulting in shock pulse signals with lower energy content.
The distance between the shock pulse transducer - mounted on the output shaft - and the damaged bearing is comparatively large, which made the signals in the spectrum hard to interpret. The fact that the only bearing and speed data available in the system at this time related to the output shaft also contributed to making spectrum analysis of the readings difficult. The shock pulse trend from the gearbox however spoke for itself, showing steadily and consistently increasing levels (see Fig 1).
The online measurements were the only monitoring method indicating developing damage. Temperature measurements showed nothing, nor did oil samples. The decision to change the gearbox was therefore based solely on the measured trend, which was considered a strong enough argument because of its steady increase over a long period of time.
In the spring of 2014, after closely monitoring the progression of the damage for 22 months, the decision was made to plan for the replacement of the Digester 3 gearbox with a spare during the upcoming autmumn maintenance stop. Once the digester was back in operation after a four day replacement job, the shock pulse readings immediately dropped to normal, “good condition” levels. When the shock pulse spectrum was later compared to the actual bearing data for the damaged bearing, clear matches on its known damage frequencies were confirmed. Inspection of the damaged bearing revealed a cracked inner ring (caused by an intermediate shaft diameter out of tolerance;, scratches in the rolling elements, and considerable spalling.
Thomas Björklund, preventive maintenance manager at BillerudKorsnäs Gävle, commented: ”The gearbox replacement took place at precisely the right time. A breakdown most likely would have occurred in the near future. Longer stops such as the autumn stop only occur once a year in our plant. If we had not replaced the gearbox when we did, it is unlikely that the bearing would have lasted until next year’s planned maintenance stop. Thanks to our condition based maintenance with online shock pulse monitoring, we avoided the risk of an unplanned and therefore much more expensive stop”. He continued: “We trusted SPM technology and experience to help us detect potential problems and determine when to replace the gearbox. It proved to be the right decision and we are very content with this condition monitoring solution and the results so far, having saved vital production and hence deliveries to our end customers.”
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