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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

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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

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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

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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

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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

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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

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• 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

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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

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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

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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

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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)

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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

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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

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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

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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



16.06.2015

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

www.dastur.com

A Model for Sequencing and Optimizing Steel Melt Shop ... · PDF fileTransfer Car and Crane...

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