Bowl mill in thermal power plant

Bowl Mill is driven by AC motor, coupled with flexible coupling to the worm shaft. The worm shaft drives worm gear, mounted on the Main Vertical Shaft. A bowl mounted on the top of the shaft rotates at a speed of 40 to 65 rpm. Hot primary air for drying and carrying pulverized coal, enters through insulated Mill side and Liner Assembly.

1. Bowl Mill in Thermal Power Plant XRP 1003 BOWL MILL

2. The Bowl Mill Bowl Mill is driven by AC motor, coupled with flexible coupling to the worm shaft. The worm shaft drives worm gear, mounted on the Main Vertical Shaft. A bowl mounted on the top of the shaft rotates at a speed of 40 to 65 rpm. Hot primary air for drying and carrying pulverized coal, enters through insulated Mill side and Liner Assembly. The journal assembly consisting of Grinding Rolls is mounted on Separator Body. Grinding force is applied by pressure spring adjustment. The roller assembly can be moved up and down depending upon the roll condition and coal bed. Separator Body consisting of linear assemblies deflects the air stream to the centre of bowl. Mill is equipped with the stationery classification with adjustable vanes for adjustment of coal fineness. The coarser particles in the in the pulverised coal are separated in the classifier and returned back to the bowl for further grinding. Bowl mills are employed to pulverize the pre-crushed raw coal to the required fineness before it is admitted into the boiler furnace for combustion. The operating characteristics of bowl mills make it the best mill available for the direct firing system of the coal. The best features of all the pulverisers have been incorporated in the design of the bowl mill. The mill output can be easily varied, as per the turndown ratio from its minimum to maximum load and it will operate for long periods without mechanical problems or stoppages for cleaning, oiling or adjusting.

3. Crushed raw coal at a controlled rate is fed into the revolving bowl of the Bowl Mill. Centrifugal force feeds the coal uniformly over the replaceable grinding ring where independently spun rolls exert the required grinding pressure. The rolls do not touch the grinding ring even when the mill is empty. Therefore, the Bowl Mill operates silently without the rumble that is characteristic of the mills in which there is metallic contact between the grinding elements.

4. The XRP 1003 Mill X – 3 Phase, 50 Hz Supply, R – Raymond (Inventor of the mill), P – Pressurized, 100 – Nominal dia. of the Bowl (inches) 3 – Number of the Grinding Rolls Manufacturer - Bharat Heavy Electricals Ltd., Haridwar

5. TYPE OF MILL XRP 1003 MILL BASE CAPACITY(T/Hr) * 67 MOTOR SPEED (RPM) 985 TOTAL WEIGHT OF MILL (T) (including motor) 130 TYPE OF LOAD Moderate Shock Load ROTATING WEIGHT OF MILL(T) 22.88 SPRING RATE(Kg/cm) 5359 SPRING PRE-LOAD(Kg) 9500 MILL OUTLET TEMPERATURE RANGE 65ºC -90ºC AIR FLOW 102 T/Hr FINENESS 70% Through 200 Mesh

6. Major Assemblies of Mill a) PLANETARY GEAR BOX AND BOWL HUB ASSEMBLY : Planetary gear box is used to give required torque to the bowl & bowl hub assembly which is directly mounted and bolted to the gear box top table. The direction of torque transmission in planetary gear box changed from horizontal to vertical. The bowl assembly consists of Bull ring assembly (mounted on the bowl), Skirt & Scrapper Assembly and vane wheel assembly (attached to the bowl).

7. b) SEPARATOR BODY ASSEMBLY: The separator body holds the components that give necessary grinding pressure (journal spring assembly), journal assembly and also the components that direct the coarse sized coal particles back to the bowl for re grinding.

8. c) ROLLER JOURNAL ASSEMBLY : The roller assembly consists of journal shaft, journal bearings, and journal housings, Grinding roll, journal head, trunnion shaft assembly.

9. e) MILL DISCHARGE VALVE ASSEMBLY: The mill discharge valve assembly consists of four multiport outlet and mill discharge valves mounted on the multiport outlet plate. Air cylinders operate the flaps in the mill discharge valves. Solenoid valves and limit switches are provided to effect and indicate the open or close position of the flap. This assembly does the distribution of pulverised fuel into four pipes and also it isolates the mill from boiler when required.

10. Pulverized Coal Sampling The content of the sampling jar(s) should be placed on a heavy sheet of paper or plastic and thoroughly mixed by lifting the corners to roll the coal back and forth along one diagonal and then along the other diagonal. The sample should then be coned, quartered and remixed until it is reduced to approximately 150 grams. Fifty grams of this sample should then be placed on the top screen of nested 50 mesh, 100 mesh and 200 mesh sieves. The sieves should be shaken on a shaker for a total of 15 minutes. After 5 and 10 minutes stop the shaker, remove the collecting pan, brush the bottom of the 200-mesh sieve and discard the contents of the pan. After 15 minutes the amount of coal left on each of the sieves is individually weighed and fineness is calculated as per the following example:

11. It should be remembered that with increasing fineness there is a decrease in the capacity of the pulveriser in tons per hour and an increase in the power required per ton of pulverised coal. Sample Calculation Example % on 50 mesh (2 x Wt. on Sieve) 2 x 0.5 = 1.0%     % through 100 mesh 100 - 2 (Wt. on 50 + on 100) 100 - 2 (0.5 + 4.4) = 90.2%     % though 200 mesh 100 - 2 (Wt. on 50+on 100+on 200) 100 - 2 (0.5 + 4.4 + 9.9) = 70.4%

12. MILL WEAR LIFE ASSESSEMENT: The objective of this test is to realize the maximum wear life of grinding element. Attach wear measurement gauge to journal assy. It is important to hold the gauge securely against lower journal housing and roller back face to ensure accurate reading as shown

13. THE MAINTANANCE AND OVERHAULING PULVERISER WEAR IS A FUNCTION OF THE NUMBER OF HOURS A PULVERISER IS GRINDING COAL, NOT NECESSARILY THE AMOUNT IN TONS OF COAL PULVERISED. A PULVERISER WILL LAST SOMEWHAT LONGER WHILE OPERATING AT REDUCED CAPACITIES BUT WILL GRIND MUCH LESS TONNAGE BEFORE PART REPLACEMENT BECOMES NECESSARY.

14. General Maintenance The following list describes the pulveriser parts normally subject to wear: 1. Bull Ring and Rolls: Reduced pulveriser capacity and increased spillage even after carrying and ring roll setting periodically are indications of excessive wear requiring part replacement. Contour templates, made when the equipment is new, may aid in checking the degree and location of wear as it progresses. In case sand accumulation is noticed on the bowl which would also stop the mill output, carryout “Mill Purging” periodically. Stop coal feeding and air supply to mill and run it empty for 5 minutes till all the sand from bowl is removed by centrifugal action of the rotating bowl.

15. 2. Airport Ring: Check and maintain a vertical clearance of 5 to 11 mm between the top of rotating vanes and the bottom of the body liner segment. 3. Bearings: Unusual noise and operating temperatures consistently above normal are indications of impending bearing failure. Bearings should be inspected at the first opportunity. 4. Planetary Gearbox: Planetary gearbox components usually give long, trouble-free service. Proper lubrication and its parameters, avoiding entry of large size extraneous materials into the mill are to be taken care off during operation.

16. PREVENTIVE MAINTENANCE Preventive maintenance is conducted to extend the life of the equipment. It involves the regular inspection, testing and replacement OR repair of equipment and operational systems. Preventive maintenance takes proactive approach to the problems before they occur and also saves money from mill breakdowns. Mill Clearance: i ) Inverted Cone Clearance: The inverted cone prevents the pulverised coal from by-passing the classifier. Inverted cone clearance is the perpendicular distance between the inverted cone and the classifier inner cone. The gap between inverted cone and inner cone is 100mm (±5 mm) and the gap between centre feed pipe lower and inner cone is 114mm (±5 mm).If the clearance is too small, bridging of coal between the inverted cone and the classifier inner cone can occur, which would result in carry over large coal particles to the boiler furnace leading to other related problems periodically. If the clearance is too large, the primary air can carry large particles out of the mill by passing the classifier. This results in poor pulverised fuel fineness.

17. ii) Grinding Roll-To-Bull Ring Segment Clearance: The roll-to-ring clearance should be set to 6 to 8 mm parallel for the entire length of the roll. This clearance setting affects the performance of the mill upon wear.

18. iv) Air Port Ring Clearance: Vertical Gap between body liner segment and rotating vane segment of Air Port assembly is between 5 to 11 mm.