high chrome rings and high chrome balls.
• Note: For the motivation of this modification, only look at the ball cycle material costs. All Maintenance service interventions will be left out of the financial equation at this point in time. This approach to be able to determine the impact of the new grinding media.
I.1
Steel ball and high chrome ring cycle regime (current
philosophy)
I.1.1 Background on the Current Ball Cycle Regime:
With the current steel cast ball cycle regime, an average wear rate of between 105 – 115 hrs per millimetre is realized. This wear rate is empirically determined from measurements on mills balls in service. For calculation purposes, a conservative wear rate of 105 hrs/mm on the cast steel balls will be used.
I.1.2 Calculation:
In the steel ball and high chrome ring cycle regime, the critical ball dimensions are 768 (new size), 690 (filler ball size) & 640 (ball scrap size for the steel balls). The times between the sizes are as follow:
• 768 - 690 = (768mm-690mm)×105hrs/mm
= 8 190 hrs (continuous running hours)
• 690 – 640 = (690mm-640mm)×105hrs/mm
= 5 250 hrs (continuous running hours)
To deplete a rings life (60 000 hrs), the mill will operate up to cycle 8. The calculation for ball life as follow:
Cycle 0 to 3 = 4 x 8 190hrs Cycle 4 to 8 = 5 x 5 250hrs
= 5 910 hrs
The used amount of grinding balls will be:
• 4 sets (11 off balls per set) of 768mm balls (44 balls in total)
• 16 off 690 mm balls. The 768 mm balls “re-cycled” from cycles 0 to 3 are used (44 off), but to reach cycle 8, 60 grinding balls are required.
I.2
High chrome ball and ring cycle regime (proposed ball cycle
philosophy)
I.2.1 Background on the High Chrome Ball Cycle Regime:
With the high chrome mill grinding media test, empirical results on the ball and ring wear rate were obtained during mill inspections and/or maintenance interventions. Wear rates in the range of 301 hrs/mm down to 254 hrs/mm were realised on the mill balls. For calculation purposes on the 10.8E mill, a conservative wear rate of 250 hrs/mm on the high chrome mill balls will be used.
I.2.2 Calculation:
The ball sizes are the same as for the steel ball cycle regime. The times between the sizes are as follow:
• 768 to 690 = (768 mm-690 mm)×250 hrs/mm
= 19 500 hrs (continuous running hours)
• 690 to 640 = (690 mm-640 mm)×250 hrs/mm
= 12 500 hrs (continuous running hours)
Cycle 0 to 1 = 2 x 19 500 hrs
Cycle 2 to 3 = 2 x 12 500 hrs = 64 000 hrs
For a complete ring cycle, 2 sets of 768 mm & 2 off 690 mm filler balls are expected to be utilized to get optimum life out of the ring. This implies 2 ball changes and 1 ball add during a ring’s life. At 60 000 hours (end of ring life), there will be 4 000 hours left on the set of balls installed, but this life left will be scrapped.
I.3
Ring Life
The average mill running factor at Kriel Power Station is 72%. This implies that for 72% of the year a mill will be in service. The reason for this is because of the extra redundant mill, mills are swopped in operation for maintenance and operational interventions on a frequent basis.
For the 2 comparing materials (steel balls vs. high chrome balls), the ring lives are assumed to be the same, which equates to ≈ 60 000 ÷ 0.72 = 9.5yrs. This can change over time if the coal sources change. For the purpose of the financial model that will be simulated for this modification, an average life of 9 years for the steel balls as well as the high chrome balls will be assumed.
I.4
Mill Grinding Media Prices
• 10.8E High Chrome Ring = R207 023.00 per ring (2 x for set)
• 768 mm cast steel ball = R21 222.24
• 690 mm cast steel ball = R17 553.00
• 768 mm high chrome ball = R31 398.00
• 690 mm high chrome ball = R29 495.00
I.5
Financial Model Inputs (Theoretical) – Current Steel Ball Cycle
Regime
Year Reason for expenditure Costs for
grinding media
Service @ yr Mill Cycle
0 1 sets of 10.8E rings & 11 off
768 mm balls (steel) (207 023 × 2) + (21 222.24 × 11) 0 0 1 11 off 768 mm balls 11 × 21 222.24 1.3 1 2 11 off 768 mm balls 11 × 21 222.24 2.6 2 3 11 off 768 mm balls 11 x 21 222.24 3.9 3
5 12 off 690 mm balls (11 off balls
from cycle 3 + 1 from cycle 0)
Zero 5.2 4
6 12 off 690 mm balls (11 off balls
from cycle 2 + 1 from cycle 0)
Zero 6.0 5
6 12 off 690 mm balls (11 off balls
from cycle 1 + 1 from cycle 0)
Zero 6.9 6
7 12 off 690 mm balls (8 off balls
from cycle 0 + 4 off new 690- mm balls)
4 x 17 553 7.7 7
8 12 off 690 mm balls (12 off balls
new 690 mm balls
12 x 17 553 8.5 8
End of ring life at year 9.4
I.6
Financial Model Inputs (Theoretical) – High Chrome Ball Cycle
Regime
Year Reason for expenditure Costs Service @ yr Mill
Cycle
0 1 sets of 10.8E rings & 11 off
768 mm balls (high chrome)
(207 023 × 2) + (31 398 × 11)
0 0
1 Zero
2 Zero
3 1 sets of 10.8E rings & 11 off
768 mm balls (high chrome)
31 389 x 11 3.1 1
4 Zero
5 Zero
6 1 off 690 mm filler ball 29 495 x 1 6.2 2
7 Zero
8 1 off 690 mm filler ball 29 495 x 1 8.2 3
End of ring life at year 9.4
Table 17 - Proposed grinding media financial inputs
I.7
Added Benefits:
Some of the benefits that can be derived from the modification were not captured in the financial model. They are
• Labour savings: With the proposed mod, only 2 ball changes and 1 ball add is estimated, compared to 8 ball changes and 1 ball add with the current maintenance philosophy. The savings are obvious.
• UCLF improvement: With less ball changes on a mill, it implies less maintenance interventions. This improves the availability of the mill and reduces the risk of UCLF’s on the milling plant.
KRIEL POWER STATION Nominal Rate: 14.0% Data Revision: 0 Project 'Year Zero' 2003
PROJECT ECONOMIC EVALUATION Model Revision: 0
Calculation Base Year 2003
PROJECT NO:
PROJECT TITLE: High chrome mill grinding media ALTERNATIVE: Discount Rate 1.1400 TAX RATE 30% Benefit to Improve Performance [ RM ] = - Payback Period [ Years ] = 1 Escalation Rate 1.0600 Benefit to Reduce O&M Costs [ RM ] = 0.964 Internal Rate of Return =151.737% NET DISC. RATE 1.0800 Depreciation Period Costs to Implement Project [ RM ] = 0.733 Cost / Benefit Ratio =1.314
Capital : 5 years NET PRESENT VALUE [ RM ] = 0.231 Non-Capital : 1 year
BASE : Sent-out Capability = 475 MW / set Overall Efficiency = 34.606% Boiler Input = 1372.587734 MW / set Available Plant Load Factor = 88.00% Year Efficiency UCLF / PCLF Project Project Project Status Quo Status Quo Status Quo ERA Approval Depreciation Project Operating Maintenance Additional Additional Additional Net Benefit ( Cost ) Net Benefit ( Cost ) Cumulative
Improvement % Improvement % Implementation Operating Maintenance Replacement Operating Maintenance Amount (for Tax purposes) Tax Cost Cost Energy Energy Energy Operating in Constant 2003 2003 ( Deterioration % ) ( Deterioration % ) Cost [ RM ] Cost [ RM ] Cost [ RM ] Cost [ RM ] Cost [ RM ] Cost [ RM ] Nominal [RM] [RM] Savings [RM] Savings [ RM ] Savings [ RM ] [ GWh p.a. ] Benefit [ RM ] Cost [ RM ] Rands [ RM ] Value [ RM ] N P V [ RM ]
1991 -12 - - - 0 - - - - - 1992 -11 - - - 0 - - - - - 1993 -10 - - - 0 - - - - - 1994 -9 - - - 0 - - - - - 1995 -8 - - - 0 - - - - - 1996 -7 - - - 0 - - - - - 1997 -6 - - - 0 - - - - - 1998 -5 - - - 0 - - - - - 1999 -4 - - - 0 - - - - - 2000 -3 - - - - - 0 - - - - - 2001 -2 - - - - - 0 - - - - - 2002 -1 - - - - - 0 - - - - - 2003 0 - - - - - 0 - - - - - 2004 1 0.759 0.647 0.797 - - - - 0 - - (0.112) (0.104) (0.104) 2005 2 0.233 - - - - - 0 - - 0.233 0.200 0.096 2006 3 0.233 - - - - - 0 - - 0.233 0.185 0.282 2007 4 0.345 0.233 0.417 - - - - 0 - - (0.112) (0.082) 0.200 2008 5 - - - - - 0 - - - - 0.200 2009 6 - - - - - 0 - - - - 0.200 2010 7 0.029 0.041 - - - - 0 - - (0.029) (0.017) 0.182 2011 8 0.070 - - - - - 0 - - 0.070 0.038 0.220 2012 9 0.029 0.211 0.045 - - - - 0 - - 0.181 0.091 0.311 2013 10 - - - - - 0 - - - - 0.311 2014 11 - - - - - 0 - - - - 0.311 2015 12 - - - - - 0 - - - - 0.311 2016 13 - - - - - 0 - - - - 0.311 2017 14 - - - - - 0 - - - - 0.311 2018 15 - - - - - 0 - - - - 0.311 2019 16 - - - - - 0 - - - - 0.311 2020 17 - - - - - 0 - - - - 0.311 2021 18 - - - - - 0 - - - - 0.311 2022 19 - - - - - 0 - - - - 0.311 2023 20 - - - - - 0 - - - - 0.311 2024 21 - - - - - 0 - - - - 0.311 2025 22 - - - - - 0 - - - - 0.311 2026 23 - - - - - 0 - - - - 0.311 2027 24 - - - - - 0 - - - - 0.311 2028 25 - - - - - 0 - - - - 0.311
Rev. No. 0 Date 2008/08/15 Total Project Cost in Nominal Rands (m) : 1.301(ERA Approval Amount) THIS PROJECT IS A FINANCIAL 'GO'
2004 2005 2006 2007 2008 Future Cash Flows Escalation Factors (CPI) - For Period2003 5.00% 4.80% 4.80% 4.80% 4.90% 4.90%
EVALUATION RESULTS PRESENT VALUES
ECONOMIC PARAMETERS TAX PARAMETERS