Plant : Shree Cement Limited

Location : Ras

Site : Unit – 8 Kiln Section

Equipment : Cooler ESP Fan Induced Draft Fan 750 rpm – variable frequency drive

Case : On-site shaft solution

ABSTRACT

This paper elaborates on-site machining procedure carried out for the first time in entire Shree Group. This case is related to one of the ID fan in kiln section, at Line 8. Fan non-drive end bearing responded to irregular vibrations in July 2014. Detailed analysis of non-drive end bearing was conducted; concluding bearing was loose on shaft and housing. Considering plant downtime, bearing was gripped on shaft using metal putty and shims were added to grip bearing outer race on housing. However this was not a long term solution for the problem. Within time, the shaft was taken for machining and the procedure was carried out at site itself without any external assistance. It outlines the innovation and unconventional ways used by SCL for productivity improvement, to reduce downtime and cost cutting.

Plant Overview Shree Cement Ras Plant is located in the village Ras, Pali District, around 150 kms from Jodhpur. It has eight cement lines (Line 3 to Line 10) and three grinding units , two ball mills and one vertical roller mill: Line 3 to Line 8 are designed for annual production of 1 MT each and Line 9 & 10 both are designed to have an annual capacity of 2 MT. Two ball mills are designed with annual capacity of 4 MT each and vertical roller mill of 3 MT per year.

Case History –

In July 2014, during shutdown inspection of Cooler ESP Fan bearing, non-drive end bearing was found loose in housing and bearing clearance was also observed in upper limit range. So decision was made to change the bearing. Further it was also noted that bearing inner was loose on shaft i.e. bearing inner race was running freely on the shaft and on the other hand shaft size was also reduced at the bearing seat. When shaft dimensions were measured, shaft dimension were down by 0.7mm with actual shaft dimension of 100mm.

Fig 1 – Bearing loose in housing and shaft

Considering plant down time, metal putty was applied on the shaft to ensure adequate gripping action between shaft and bearing inner race. Also shims were added to hold bearing outer race with bearing housing. Although this was not a long term solution for the problem but considering production loss, temporary arrangement was done.

Observations –

Since July 2014, Cooler ESP fan was kept under close surveillance. Scheduled vibration readings were done along with

regular physical inspections. However no abnormality was observed till October 2014. On 22nd October 2014, vibrations went up to 3.8 mm/s in horizontal direction in non-drive end bearing. Detailed analysis was conducted for fan non-drive end bearing and it was suggested to inspect fan non-drive end bearing internally in the next available opportunity.

Bearing Inspection –

Soon there was a shut at Line-8 due to excess clinker stock, which presented an opportunity for us to inspect fan impeller conditions and bearing condition. When non-drive end bearing was inspected internally, it was seen that the shaft wear had increased to 1.4 mm.

Fig 2 – Bearing seat found wear A meeting was held with Mr. S.C. Maheshwari (Joint VP, Operations) to discuss the fault and possible maintenance solutions. Taking out the impeller and machining it at workshop involved long time, manpower and expertise guidance.

As a result, it was decided to make necessary arrangements at site to re-build the shaft in-situ, without taking out the impeller.

On-Site Machining –

a. Shaft holding: First of all Shree Cement made the necessary arrangements to install a bearing at the location between non drive end bearing seat and fan casing, to support the rotation of shaft while machining is performed at non drive end bearing seat. Here the shaft size was 120mm, so we decided to use bearing (22224 CC) and bearing housing was designed as per availability of space. Along with this extra bearing, a jack bolt arrangement was also equipped with bearing housing base plate to align the impeller shaft to obtain precise machining.

Fig – 3 Alternate bearing setup

b. Variable Frequency Drive: In order to rotate the shaft at 20-80 rpm for the purpose of machining and building up, we needed a variable frequency drive. Fortunately the fan motor is

designed as a variable frequency drive; as a result we need not to install an external variable frequency drive.

c. Mounting of tool post: In order to machine the shaft, a lathe machine tool post was mounted. To ensure the accuracy and precision of the shaft after the machining process, it was ensured that the movement of tool post was parallel to the axis of the shaft with the help of the dial indicator.

Fig – 4 Tool post mounted at site

d. Rough machining: To re-build the

shaft, it was essential to machine it for a level and smooth surface. Shree cement started machining the shaft at the maximum possible shaft speed of 80 rpm and with a depth of cut of 0.10 mm at a feed rate of 0.1mm/rev. The machining process continued until a smooth and level surface was achieved on shaft.

After cutting, emery paper was used to smooth the shaft surface texture. On completion of the rough machining process, the shaft was 98 mm in diameter, with an under-cut of 2 mm.

Fig 5- Rough machined shaft

e. Rebuilding the shaft: However Shree Cement had performed machining on shaft in past times but on-site this was the first time in the entire group. It was a challenge to uniformly make a build up at the undercut portion of shaft. However the electrode material compactable to shaft material was not available in local market. It was decided to carry the built-up job with Mild Steel Welding Electrode 6013 3.0mm. The surface of the shaft was pre-heated to a temperature of 90oC before commencement of welding. Welding was done in radial direction at a uniform rate. Engineers were continuously monitoring the shaft temperature and cooled compressed air was used to keep the shaft temperature optimum as per welding operation.

After 7 layers of welding shaft had a diameter of 105 mm.

Fig 6 – Actual shaft dimension

Fig 7 – Built-up on shaft

f. Final machining: Final machining was conducted on the shaft using a precise cutting tool. After attaining a shaft size of 100.10mm machining was finalized. Emery paper was used to get the shaft diameter to 100mm with a tolerance of +0.02 mm.

Fig 8 - Final machining

g. Measurements: Shaft diameter was measured at different positions with tolerance range between +0.02 to +0.03 mm was obtained.

Fig 9 – Shaft after final machining

Fig 10 – Micron reading on shaft

h. Mounting of bearing: The bearing was heated to 150oC and was mounted on shaft. The housing was fixed in the position. When the temperature of the bearing reduced to ambient temperature, the clearance was measured with a feeler gauge.

i. Operational test: After final fitment of the bearing, fan was running on its bearing drive. Vibrations reading were observed in all three directions and were found satisfactory.

Results –

After the job was successfully completed and the bearing mounting was done, the vibration reading measured at 95% fan speed were –

Motor

Drive End Non Drive End Horizontal: 0.46 mm/s 0.46 mm/s Vertical: 0.31 mm/s 0.35 mm/s Axial: 0.35 mm/s 0.26 mm/s

Fan

Drive End Non Drive End Horizontal: 1.08 mm/s 1.60 mm/s Vertical: 0.32 mm/s 0.50 mm/s Axial: 0.54 mm/s 1.30 mm/s Conclusion – Overall this challenging job was performed successfully throughout, within the plant itself without any external assistance. The successful maintenance of the shaft resulted into morale boost up of maintenance engineers and thus marking an assurance for the company to withstand future challenges.

on site machining

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