WELCOME TO TECHNICAL PAPER TECHNICAL P APER PRESENTATION ON
“PERFORMANCE BASED LINER
REPLACEMENT IN TUBE MILLS FOR EFFICIENT, COST EFFECTIVE OPERATION - UNCHAHAR EXPERIENCE”
ANUJ NIGAM Sr. Engineer (O&M) FEROZ GANDHI UNCHAHAR THERMAL POWER PROJECT
NTPC LTD (A GOVT OF INDIA INDIA NAVA RATNA ENTERP ENTERPRI RISE SE ) ENTERPRIS E
UNCHAHAR, RAEBARELI DISTT. UP
NTPC IS THE LARGEST AND BEST POWER UTILITIES POWERING INDIA’S GROWTH LIGHTING 1/4 Th OF INDIA. NTPC/UNCHAHAR IS ONE OF THE PRESTIGIOUS POWER PROJECT OF NTPC.
B IO D A TA O F A U TH O R Shri ANUJ NIGAM, presently posted as Sr. Engineer in coal handling plant of NTPC/Unchahar, is a graduate in Power Engineering from BITS, Pilani, Bureau of Energy Efficiency, GOI certified Energy auditor having PG Diploma in Management & pursuing his MBA in Human Resources Management from Indira Gandhi National Open University, New Delhi. He has worked in Uttar Pradesh Rajya Vidyut Utpadan Nigam Ltd. from 1989 to 1992. He joined NTPC in the year 1992 as a result of takeover of Unchahar project by NTPC during the process of clearing it’s dues by the Govt. of UP. During his 20 Yrs of service he has worked both in Operation & Maintenance of large thermal power station.
INTRODUCTION Like in many process industries, Pulverizers are one of the major auxiliaries in a coal fired thermal power station. These are used for grinding raw coal. The selection of right type of grinding media and machinery is often difficult as it depends on many complex parameters. There are number of models and varieties in the market. The ball mill is one of the oldest preliminary machines that are still working in many industries.
b
Often Bowl Mills were preferred over initial Ball Mills because of their anticipated flexibility in operation, easier maintenance and low power consumption. But with deteriorating coal quality, the performance in certain cases proved contrary to expectations, leading to poor availability of these pulverisers and with maintenance problems on bowl mills a peak in the beginning of eighties, alternate systems were considered for installation.
bOne
such alternative is the balltube mill manufactured by M/s STEIN Industries, of France, which has greater adaptability to modern grinding requirement than any type of grinding equipment.
b NTPC-Unchahar
Stage-II units are provided with BBD 4760 Tube mills. These mills are energy intensive equipment and even a minor reduction in % of power consumption would certainly reduce the consumption of precious Energy. This technical paper is a sincere effort in this direction
OBJECTIVE b To
reduce the auxiliary power consumption of mills. b To reduce the consumption of grinding media. b To increase mill loading. b To
run the mill in auto.
BALL TUBE MILL SYSTEM b A
ball Tube mill may be described as a cylinder made of steel plate having separate heads or trunion attached to the ends with the trunion resting on suitable bearings for supporting the machine. The trunion are hollow to allow for the introduction of discharge of the materials undergoing reduction in size. The mill shell is lined with chilled iron, carbon steel, manganese steel, High Chrome liners attached to shell body with counter sunk bolts. These liners are made in different shapes so that the counter inside surface of the mill is suited for requirement of application.
b As
indicated by the name the grinding in the ball and tube mill is produced by rotating quantity of steel balls by their fall and lift due to rotation of tube. The ball charge may occupy one third to half of the total internal volume of the shell. The significant feature incorporated in the stein mills is its double end operation, each end catering to one elevation of a boiler. The system facilitated entry of raw coal and outlet of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit.
PULVERISING ACTION IN BALL TUBE MILLS
Impact of falling balls on the coal. b Attrition as particles sliding over each other as well as over the liners. b Crushing as balls roll over each other. b Crushing as balls roll over liners with coal particle in between. b
PRINCIPLE OF GRINDING IN BALL TUBE MILL
ATTRITION
IMPACT
NEXT
FACTORS AFFECTING BALL TUBE MILL PERFORMANCE Q = f (HC, HR, FC, PB, DB, NB,LG) b b b b b b b b
Q = Flow of Pulverised coal (T/hr) HC = Moisture content of coal (%) HR = Hard grove grind ability index FC = Coal fineness at mill outlet PB = Weight of balls in the mill. DB = Inside diameter of the mill. nB = Rotational speed of mill. LG = Shell liner geometry.
TUBE MILL LINER WEAR PROFILE
LEADING EDGE TRAILING EDGE
0 mm
80
94
mm
mm
77 mm
REDUCED BALL LIFTING DUE TO LINER WEAR
BALL RETURN
COMPARISION OF OLD /NEW LINERS
COMPARISION OF OLD /NEW LINERS
MILL LINER REPLACEMENT RECOMMENDED
b
The mill shell is lined with special liners for protection of shell plate. The liners are replacable components and also assist in grinding of coal by lifting of coal and forcing them to fall on coal pieces. Under this impact the coal gets pulverized besides due to attrition. For this application high chromium white cast iron castings are so selected and designed that there is minimal wear on the liners and proper lift is given during the service life.
b
The thickness of new shell liners of BBD 4760 is 94 mm and as per the manufacturers recommendations these liners are to be replaced when the wear is more than 45 mm from top. This wear is generally takes place after 40,000-50000 running hours of mill operation. However the wear of liners is a function of coal characteristics, quantity of ball charge ad abnormal operation of mills.
CASE STUDY OF LINER REPLACEMENT IN MILL 4 CD OF UNCHAHAR b
Mill Running hours as on date
41000 hours
b
Mill Running hours when liners were replaced
38658 hours.
b
Mill Power reduction due to liner replacement
300 KW
b
Annual Savings due to power reduction
2450822 KWHrs.
b
Annual Savings in power consumption
Rs 40.47 Lacs per mill
b
Annual Savings in PA fan power due to improved
Rs 8.0 Lacs per mill
b
Annual savings due to fewer requirements of balls.
Rs 15.0 Lacs
b
Total Savings due to Liner replacement
Rs 63.47 Lacs
b
Cost of Liner Set
Rs 50 Lacs
b
Simple Payback period.
Rs50 Lacs/ Rs 63.47 Lacs 9 months 14 days
b b
Mill liner wear in 35000 hours
22 mm
b
Mill liners wear rate assuming linear relationship
0.0006 mm/ 1000 hours.
b
Running hours to reach 45 mm wear
36600 hours (5 years)
b
If liners are not replaced till this year
b
Energy loss in balance of liners
b
Pay back period if this loss in Energy consumption is considered 50/380
6 X 63.47 Lacs = Rs 3.80 Crores 1 month 16 days
LINER WEAR OF BALL MILL 50
m m 40 n i r 30 a e w 20 r e 10 n i L
0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 5 5 1 2 3 3 4 5
0
Runnuing hours
CASE STUDY OF LINER REPLACEMENT IN MILL 4 AB OF UNCHAHAR(MARCH’2009) b b b b b b b b b b b b b b b
Mill Running hours when liners were replaced Mill Power reduction due to liner replacement Annual Savings due to power reduction Annual Savings in power consumption Annual Savings in PA fan power due to improved Lifting of coal Annual savings due to fewer requirements of balls. Total Savings due to Liner replacement Cost of Liner Set Simple Payback period.
44293 hours. 280 KW 2452800 KWHrs. Rs 46.60 Lacs per mill Rs 19.02 Lacs per mill Rs 2.5 Lacs Rs 68.12 Lacs Rs 60 Lacs Rs 60 Lacs/ 68.12 Lacs 10 months 15 days 27 mm 0.0006 mm/ 1000 hours. 30000 hours (3.4 years)
Mill liner wear in 44313 hours Mill liners wear rate assuming assuming linear linear relationship Running hours to reach 45 mm wear If liners are not replaced till this t his year Energy loss in balance of liners 3.4 X 68.12 Lacs = Rs 2.31 Crores Payback period if this loss in Energy consumption is considered = 60/231 3 month 3 days
CONCLUSION “Tube mills are energy intensive equipment and there is lot of scope for innovations in the strategies adopted for energy saving. Instead of blindly following recommendations, if we carry out the dynamic techno economical analysis based on performance of mill taking all related site factors in to consideration for replacement of liners, there would be a lot of saving not only in energy consumption of tube mills and PA fan power but also reduced requirement of grinding media for same output. This in turn reduces the auxiliary power consumption which is the need of the hour to maintain a sustainable future”
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