INDUSTRIAL CASE STUDY:

THE CEMENT INDUSTRY

PREPARED BY

MAUSAM SHARMA

with assistance from

RAHUL MUNDRA

S

HR

EE

CE

ME

NT

LT

D.

Plant : Shree Cement Limited

Location : Ras

Site : Cement Mill - IV

Equipment : Cement Mill Main Drive Gear Box (Pressing Side) Model – SZN 800 Make – Flender

Case : Early warning of gearbox bearing failure

ABSTRACT

This paper contains case study of bearing damage at Cement Mill – IV in 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 as the problem was identified and the corrective actions taken within time. Also the study focuses on how pro-active maintenance practices have been useful to reduce the downtime, running cost and improve plant efficiency. 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 eight months. Finally criticality of fault was identified within time and proper actions were taken to save the equipment from other damage.

Case History –

In May 2014, during physical inspection of cement mill pressing side gearbox; minor frictional sound was observed from gearbox input non-drive end bearing. On studying the vibration bearing spectrum, bearing outer race fault frequency was observed with minor amplitude. Considering the criticality of the equipment, bearing was kept under monitoring; along with this bearing inspection was also suggested in the next possible opportunity to get assurance of bearing condition. Further in June 2014, gearbox was made available for inspection and it was found that gearbox input drive side bearing was found loose in the housing. Input drive side bearing looseness in housing was arrested using retainers and input non-drive side bearing was also inspected. Due to bearing location restriction only 50% of the bearing area was available for inspection and no abnormality was found. In July 2014 abnormal bearing wear particle of size 40 microns were observed in ferrography. Based on the vibration analysis report and ferrography report equipment was kept in close surveillance.

Case Analysis –

Since May 2014, the equipment was kept under close monitoring and scheduled physical inspections were done. However no noticeable change was observed till November 2014.

In December 2014, vibration reading got paced up to 4.5 mm/sec as measured on 17- Dec -2014. However on 20-Dec-2014, during physical inspection of the machine in normal running condition; abrupt change in noise was observed at gearbox input non-drive end side bearing. Intensity of noise itself was a confirmation tool for some damage in the bearing. Checking over the vibration, the reading had reached a level of 5.1 mm/sec. Moreover spectrum also showed slight changes in the bearing condition. On 22-Dec-2014, vibration reading went up to 7.2 mm/sec along with this the spectrum peak has also intensified. Considering change in the parameters, decision was taken to inspect the bearing at the earliest with prior planning of bearing replacement. On 23-Dec-2014 gearbox was available for inspection.

Pitting was observed in bearing inner race and outer race. Figure below show pitting marks on bearing.

Fig 1 – Pitting marks on outer race Fig 2 – Pitting marks on inner race

Problem Diagnosis –

• Overall Vibration Reading

As discussed in case analysis section, the vibration reading in the gearbox non-drive bearing were changing abruptly. This can be clearly seen through vibration monitoring graph.

Fig 3 – Graph showing vibration trending

Date Vibration reading 26-Nov-2014 2.8 mm/s 17-Dec-2014 4.5 mm/s 22-Dec-2014 7.2 mm/s

Table 1 –Table showing increasing vibration trend

• Velocity Spectrum

Velocity spectrum was also showing major changes which helped as a confirmation tool to detect the failure before breakdown. Spectrum showed a peak of 1.5 mm/s at 129 Hz (Bearing Outer Race Frequency [BPFO]) measured on 17-Dec-2014 and it went up to 4.2 mm/s at 129 Hz on 22-Dec-2014.It could be seen clearly that there were no peaks at 129 Hz on 26-Nov-14. Further on 17-Dec-14 there were new developments at 129 Hz which amplified with time till 22-Dec-2014.

26-Nov-14

17-Dec-14

22-Dec-14

Fig 4 – Velocity spectrum trend pattern

• Shock Pulse Trending

Shock Pulse is a method that tells us about the bearing surface condition and lubrication condition in bearing. Lower value of shock pulse indicates proper running and normal working conditions. However there was a sudden increase in the shock pulse value as the surface condition was getting deteriorated due to pitting and the lubrication film was also not maintained in the bearing due to surface wear and oil leakage from non-drive end seal.

Fig 5 – Shock pulse trending

• Oil Analysis using Ferrography

Oil analysis has been a very useful tool to identify machine failure before breakdown. Ferrography helps us to observe & trend wear particles present in the oil. Specifically for this case, ferrography did show bearing wear particles of size up to 200 microns in marginal quantities. Particles were present in marginal quantities as the gearbox is designed to have a particle filter arrangement along with forced lubrication system. Figure below shows the overall trend of severity as observed in ferrography which is seen out to be positive slope curve.

Fig 6 – Ferrography trend pattern

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Overall Wear Severity Trend

Also the particle size in ferrography was growing since July 2014. In August 2014 bearing wear particle size was observed 50 micron which gradually went up to 80 micron in September 2014 and by November 2014 particle size was 200 microns as depicted below in the figure.

Fig 7 – Particle size trending

Benefits –

The available vibration analysis and oil analysis (ferrography) technique has helped us to detect this failure and hence avoid a major breakdown at the most critical drive in Cement Mill Section. Secondary damage due to this could be observed on all other gear box bearing or may be on gear tooth damage. Hence timely monitoring and physical inspection along with pro-active maintenance practices has helped us to avoid a possible major breakdown, adding to plant productivity and increasing MTBF.

Conclusion –

This timely identification of failure prevented the breakdown at Cement Mill Unit which saved production loss and additional damage. This is all possible due to efforts and support added by our management to promote pro-active maintenance practices in our plant and make us equipped with the latest technologies of pro-active maintenance in-house.

Before Job

After Job

Fig 8 – Velocity spectrum

The overall vibration reading after the job (changing the bearing) went down to 2.0 mm/s and also the value for shock pulse went down as shown in Fig 3 and Fig 5 respectively. Also the velocity spectrum was in satisfactory limits once the job was done successfully. Figure below shows the change in spectrum pattern before the job and after the job was done.

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