CASE STUDY

Analytical Ferrography Diagnosed Early Warning of Raw Mill

Grinding Roller Bearing Failure at Ras Plant

Ferrography helps to examine wear rate of the system internals irrespective of

wear particle size along with material composition effectively and provides early

warning of machine failures.

UNIT – III RAW MILL GRINDING ROLLER

Plant : Shree Cement Limited

Location : Ras

Lubricant : BERU - 680

Equipment : RAW MILL GRINDING ROLLER

Roller Installation : June 2015

Running Hours : 7000 hours (approx.)

ABSTRACT

This paper contains case study of bearing damage in Raw Mill Grinding Roller at Shree Cement Ltd., Ras. In a large scale industry oriented, to minimize downtime and improve production; machine downtime have to be reduce. This case study focuses on one such case, where the machine failure was identified at an early stage and corresponding measures were taken to avoid machine failure. The case highlights the symptoms shown when the problem was identified and the corrective actions taken within time. Also the study focuses on how oil analysis practice has been useful to reduce the downtime, running cost and improve plant efficiency. Specifically in the case of grinding rollers in VRM’s, vibration analysis is practically impossible. Here the only pro-active maintenance that can be carried out is oil analysis. This case specifies that constant monitoring of equipment as soon as a basic indication is obtained can be beneficial for plant life. Particularly in this case symptoms were generated months back and since then changes in the equipment were noticed till the situation got alarming. Equipment was kept under micro level monitoring since past six months. Finally criticality of fault was identified within time and proper actions were taken to save the equipment from other damage.

Case Analysis –

From October 2015, grinding roller rating was marginal due to presence of bearing wear particles. However the ferrous ppm in oil was within range. This was an indication of abnormal wear development in roller bearing. Further in January 2016, bearing wear particle size went to 200 micron from 60 microns in Oct. 2015. But the ferrous ppm in oil was in the same range. Due to increase in wear particle size and wear rate, oil change was recommended. In February 2016, bearing wear particle of size 300 microns were observed in grinding roller oil sample. This was an alarming stage and machine condition was declared critical here forth. From Feb. 2016 onwards, machine was kept under regular monitoring.

Problem Diagnosis –

Ferrography helps us to observe & trend generation of wear particles in machines. Specifically for this case, ferrography did showed increasing trend of bearing wear particles. Figure below shows the overall trend of severity as observed in ferrography which is seen out to be positive slope curve.

Fig 1 – Ferrography trend pattern

Bearing wear particle size trend also proved to be a major tool for confirmation of bearing damage in the grinding roller. In December 2015 bearing wear particle of size up to 60 microns were observed on ferrogram which gradually kept growing and in April 2016 it went up to 400 micron. Table below shows wear particle size trending for past six months.

Bearing Wear Particle Size

Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16

60 200 300 150 400 400

Table 1 – Bearing particle size trend

Bearing wear particle were observed from October 2015 on regular basis. There was a noticeable rise in the wear particle size in grinding roller oil. Initially starting from 60 microns in December 2015, it gradually kept growing and went up to 400 microns in May 2016. This was a critical situation and immediate inspection of grinding roller bearing was suggested. Figure below shows increasing trend of bearing wear particle observed on ferrogram.

Fig 2 – Particle size trending

Also the ferrous ppm trend in the case was an important indication for failure analysis. Ferrous ppm trend can help us predict pre-mature failure and abnormal wear in equipment. Ferrous ppm trend of the grinding roller kept on rising with running hours. In March 2016, it went on an alarming situation. At this stage oil change was suggested to avoid secondary wear due to presence of excessive wear particles in oil. However in May 2016 along with increase in particle size, ferrous ppm also went to 2176. At this stage management decided to change the grinding roller and accordingly a shutdown was planned.

Ferrous PPM Trend

Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16

85 100 134 337 438 2176

Table 2 – Ferrous ppm trend

Actions Taken –

Based on criticality of machine as per ferrography and oil analysis report by Condition Monitoring Cell, it was decided to inspect roller bearings on priority basis. Hence a shutdown was planned considering the severity of the case and the grinding roller was replaced to avoid production loss and other damages.

Inspection –

After replacement of the grinding roller, old roller was sent to mechanical workshop for overhauling. Bearing inspection was carried out. Pitting was observed on bearing inner race and bearing roller. Bearings were replaced and the entire grinding roller was reassembled. Images below show bearing inner race and bearing roller condition in grinding roller.

Benefits –

This timely identification of failure prevented the breakdown at Line – 3 Raw Mill which saved production loss and additional damage. This was all possible due to efforts and support added by our management to promote oil analysis practices in our plant and make us equipped with the latest technologies for oil analysis in-house.

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