Bowl Mill-feb 2009

FEBRUARY 2009

FEEDBACK - 1 Project : SIPAT 2X500MW Problem: BOWL MILL-XRP 1003 PROBLEMS FACED AND REMEDIAL MEASURES TAKEN

1. Mill Labyrinth seals failure: BHEL Hyderabad supplied 10 numbers of XRP 1003 mills for each of the unit. The grinding of coal is being carried on bull ring segments assembled on the bowl and retained by retainer ring with the hub of the mill. The hub in turn mounted on planetary gear box driven by bevel pinion arrangement. This bevel pinion shaft is being driven by motor. Refer Fig.1 & 2

Hot primary air of about 320 º C enters at the bottom of the mill, moves upwards around the periphery of the bowl through the air port assembly lifting the pulverized fuel on the bowl and escapes through the classifier assembly, enters into the coal pipes, finally reaches the furnace. To avoid hot air coming in direct contact with the hub and heating hub shaft, which in turn will transfer heat to the planetary gear box, upper and lower skirts are fixed on the hub forming a hollow chamber to be pressurized with cold seal air supplied through seal air pipe provided below bottom liners of the mill. To sweep the spilling coal through air port assembly into tramp iron port two swinging scrappers are provided with the lower skirts at 180 º apart. Between the mill bottom insulation cover plate and scrapper bottom an axial gap of around 16 to 19mm is maintained. Between bottom skirt plate and mill bottom insulation a vertical gap of 19 to 21 mm is provided. To

maintain minimum radial running clearance for mill bowl hub, two labyrinth seals are provide above (inside the hot air box) and below the mill bottom plate for maintaining a radial clearance of 0.3±0.05mm. Cold seal air enters through seal air pipes provided below bottom insulations at 180º and maintains a higher pressure than mill inlet pressure inside labyrinth seals, small quantity of air escapes constantly into hot air and at the bottom of the mill. Thus seal air prevents coal dust entering into the labyrinth seals and damaging the mill shaft and labyrinth seals. Small iron filling, iron rods and other small solid particles escaped through the bottom skirt clearance and entered the labyrinth seal and machined the mill bowl hub and damaging the labyrinths seals as well. Thus mill 4E and 4J small iron rods have entered and damaged the seals. To over come above problem a mall ring was formed to match inner radius of bottom skirt with a 5X50mm iron strip and welded to both the halves of bottom skirts to maintain a axial gap of around 5mm between ring and the mill bottom insulation (liners) and then lower skirt were bolted to the upper skirts. Refer 2 This was carried out in all mills and no subsequent failures of labyrinths seals were reported. Conclusion: The Axial clearance may be reduced in consultation with BHEL/Hyderabad by decreasing the Gear box pedestal casting by 16mm or suitably so that this problem may be avoided.

2. Failure of Planetary Gear box Bevel Pinion Shaft of Mill-5F: The Pulveriser lube oil system (PLOS) consists of an oil tank, 2X100% Screw pumps & motor filter, oil cooler etc. Lube oil pump capacity is 150 LPM. Filtered and cooled oil, to the required temperature range of 30 to 50 deg C, is supplied to the gearbox thrust pad bearing, input shaft bearings and gears. The return line from the gear box is connected to the tank. A heater provided in the oil tank keeps the oil temperature in the required range, provided the pump is ON. Lube oil supply line to the above three points is coming as header in assembled condition with a dummy at the supply end. Oil lines after flow meter of the lube supply skid to this supply line point and return line from the planetary gear box to the tank were site fabricated, dismantled acid cleaned, passivated, dried, wetted with oil and reassembled prior to oil flushing of the lines. During oil flushing, the end supply connection to input shaft bearing, thrust pad bearing and gears were disconnected and ensured oil flow through the lines. After completion of oil flushing, the old oil was removed and new oil was filled. After checking all interlocks, all six RTDs of Planetary gear box, Mill motor BEARING RTDs and winding temp RTDs healthiness were checked, trip was confirmed by bringing

trip value less than cold temp readings of these RTDs. After moralizing and ENSURING all protection, Mill-5F lube oil pump was started and flow was maintaining around 130 LPM (Adequate flow is 118 LPM) and oil was vented from the line supplying oil to the bevel pinion shaft. Mill-5F was started from remote. After few minutes, there was heavy rattling sound heard from the planetary bevel pinion shaft end. So the mill was tripped from local. Mill was decoupled to open planetary gear box for inspection. When the oil supply line removed from the nipple of the supply end point to pinion shaft, it was noticed was like material blocking the hole (3 to 4mm) of the nipple. It was noticed that Paired taper roller bearing 31324 crushed due to poor oil supply to the bearing while spherical roller bearing towards gear box end was intact. New bearing was received from the customer and replaced in the Bevel pinion shaft. Lube pump was started and flushed the line supplying oil to pinion shaft through the nipple end and through vent for few minutes. Small hole was again cleaned in the nipple with a wire and connection was reestablished. Mill was taken trial run successfully. Since the line supplying oil to the pinion shaft was is about 10mm having an ID of around 6mm, end supplying oil to the pinion bearings less than 4mm, any blockage of this will not affect the total flow of lube oil to the planetary gear box. Conclusion: Hence we made it a procedure to open the line to this connection and clean the nipple with a small wire before first start up and during starting of the mill after a prolonged shut down to ensure oil flow through this small line.