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A Model for Sequencing and

Optimizing Steel Melt Shop

Operations

Using Iterative Hierarchical Decomposition

based Discrete Event Simulation

Steelmaking

The Production Process Flow at the Client’s

Plant

2

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Blast

Furnace

Basic

Oxygen

Furnace

Basic

Oxygen

Furnace

PIG Casting

Machine

Slag

Dumping

Yard

Ladle

Refining

Furnace

Vacuum

Degassing

Unit

Continuous

Caster

Ho

t m

eta

l la

dle

by lo

com

otive

s

Liq

uid

Ste

el L

ad

le b

y

Tra

nsfe

r C

ar

and

Cra

ne

Liq

uid

Ste

el Ladle

by

Tra

nsfe

r C

ar

and

Cra

ne

Liq

uid

Ste

el L

ad

le b

y

Cra

ne

Sla

g p

ot b

y

Tra

nsfe

r C

ar

Scrap

Yard

Scra

p B

ox

by C

rane

Liquid Steel Ladle by

Transfer Car

and Crane

Ladle

Preparation

Aisle

Empty Ladle

by Crane

Ironmaking

Objective: Overall Throughput Improvement

3

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Minimize tap-to-tap time of

furnace

Maximize utilization while

maintaining heat sequence

Optimal resource utilization

• Cranes

• Ladle cars

Basic Oxygen Furnace

(BOF)

Ladle Refining Furnace (LRF)

&

Vacuum Degassing Unit (VD)

Caster

Unit Optimization

Operations Synchronization

Overall Throughput Improvement

Bottleneck Analysis Using Operational Laws

4

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Melt Shop

Operations

Use a queuing

model

Maintain job flow

balance

Apply utilization

law

BOFService Time

LRFService Time

VDUService Time

CasterService Time

Pro

port

ional

The unit with the

highest total

service time can

be a limiting

factor in

achieving higher

throughput

Reduce the cycle time to the minimum level possible at the

bottleneck unit

BOFUtilization

LRFUtilization

VDUUtilization

CasterUtilization

Iterative Cycle Time Optimization

5

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Reduce the cycle time to

the minimum level possible

at the bottleneck unit

Sim

ula

te th

e m

odel

Again

with

the re

duce

d

cycle

times Stack rank the new

service times of the units

in the production chain

Bottle

neck s

hifts

during

itera

tion

s o

f

impro

vem

ent

Continue iterating

till there is no

further

opportunities for

practical cycle time

improvement

Iterative Capacity Utilization Improvement

6

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

• Further scope for improvement might exist across

production units, service units and buffer units

• Observe the behaviour of capacity utilization of the units

which could potentially create a bottleneck

• Capacity utilization of units might change and tend to

move in a way affecting its performance

• Adding unit capacity and/or buffer capacity can further

improve system performance

Addition of unit capacity needed to be guided by • Economics

• Cost Benefit

Cycle Time

Approach Using Iterative and Hierarchical

Decomposition

7

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Unit 3

Unit 1

Unit 2

Capacity Utilization

Unit 1

Unit 2

Unit 3

Progressive hierarchical decomposition of the chain and iteration based on

the two heuristics drives the simulation towards an optimal solution

Baseline Simulation

8

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

• One year operation data was collected for

the BOF, LRF, VD and Caster

• Fitted into distributions and best fit

distribution was selected with the help of

goodness of fit tests

• The model was run with the existing

operation parameters

Results

Average heats per day 21

Average BOF tap-to-tap time 65 mins

LRF utilization 79 %

Caster utilization (using 2

strands) 53 %

VD utilization 48 %

Caster heat sequence Maintained

Snapshot of 10 days with existing

parameters after attaining stabilization

BOF Basic Cycle Time Distribution

9

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Activities Time*

Scrap charging 1 to 2

Hot metal pouring 4

Blowing 16 to 18

Deslagging 2 to 3

Temperature & Analysis 5

Tapping 4 to 6

Slagging off 2

Slag coating 5

Ba

sic

activitie

s in

BO

F

*Figures in minutes

Weibull

Mean:43 ; Standard Deviation: 0.7

Average delta of 22 mins between Tap-to-Tap and BOF basic

cycle time

BOF Cycle Time Improvement Strategy

10

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Event Average

Time*

Mouth jam cleaning and dozing after

every 8 heats 90

Tap hole changing after about 15

heats 45

Other maintenance after 75 heats 180

Current Maintenance Downtime

Practices

Event Average

Time*

Mouth jam cleaning and dozing after

every 15 heats 45

Tap hole cleaning and other

maintenance after 75 heats 180

Proposed Maintenance Downtime

Practices

*Figures in minutes

Use T

ap h

ole

sle

eves

Improved BOF Tap-To-Tap Time Distribution

11

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Lognormal

Mean: 59; Standard Deviation: 4.6

Skewness: 1.21585

Although average tap-to-tap time improved to 59 mins,

non trivial positive skew indicates potential BOF

blockage due to downstream

*Figures in minutes

Capacity Utilization Analysis

12

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Results

Average heats per day 23

Average BOF tap-to-tap time 59 mins

LRF utilization 92 %

Caster utilization (using 2

strands) 60 %

VD utilization 54 %

Caster heat sequence Maintained

Snapshot of 10 days with proposed

parameters after attaining stabilization

• Probable downstream blockage in the

LRF

• Removing downstream blockage can

further reduce BOF tap-to-tap time

• Make use of the blocked BOF capacity

as well as the additional caster capacity

with three strands in operation

?

45 60 65 (time in minutes)

13

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Lognormal

Mean: 51; Standard Deviation: 0.8

Skewness: 5.93932e-002

Resulting improvement in BOF Tap-To-Tap

Time Distribution

Reduced skewness and more symmetrical

pattern indicates lesser system wide congestion

Additional Capacity along with Cycle Time

Improvements Increases Throughput Significantly

14

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

• Average BOF tap-to-tap could be brought down further to an

average of 51 minutes

• The caster could be operated with three strands while

maintaining the heat sequence

• The overall throughput increased by about 20%

The increased overall steel production decreased the diversion of hot

metal to the pig casting machine improving the profitability of the melt

shop

Cost Benefit Analysis

15

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Unit Costs & Operational Parameters

Tap hole sleeves 200 mm, 10 sleeve pack $3000

Sleeve change interval 75 heats

Electrode consumption in LRF 12 gm/KWH

Electricity consumption in LRF 0.5 KwH/Degree Celcius/ton

Average heating in LRF 5 Degree Celcius

Cost of Electricity 10 C/ KWH

Investments

1X35 Ton LRF 8 MM$

Product Price

Average Billet Price $480 /ton

Average Pig Iron Price $ 400/ton

Marginal Revenue R 5.3 MM$/year

Marginal Cost C (Sleeve Cost + Electrode

Consumption + Power Consumption) ~$400,000

Additional profit , P 4.9 MM$

Payback Period for Investment 1 Year and 7 Months

Future Work

16

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

Large scale plant wide throughput improvement through

Cycle time reduction

Capacity addition

Include operational elements like intra-plant logistics, facility layout re-engineering,

plant wide inventory & movement buffers, routing sequences and material

allocations

Use hybrid models of discrete event and system dynamic simulation for overall

performance improvement

17

Atanu Mukherjee, Arnab Adak

M.N. Dastur & Company

16.06.2015

M.N. Dastur & Company (P) Ltd Consulting Engineers

www.dastur.com