Dynamics Seminar
Section 9 Risk Assessment
John Edwards, P&I Design Ltd
August, 2014
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Simulation for Risk Assessment
Process Simulation Dynamic Modelling and Control Section 9
HAZOP Study
Methodical application of key words and deviations
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Assess implications of deviation or condition under consideration
Apply mitigations
Dynamic simulation of validated model allows deviations to be
considered under predicted real conditions
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Wash Water Heat Recovery
Process Simulation Dynamic Modelling and Control Section 9
Steam
Superheater Boiler
Economiser
D9.01 page 197
Water Heater
Flue to Atm
GT Exhaust
Burner
Natural Gas
BFW
Water Temperature
Control Loop
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Wash Water
10000 kg/h
60°C
Transfer Pump
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Pressurised
Water Loop
GRP Wash Water
Tank 100m3
Design 90°C
Town’s Water
Circulation Pump Circulation Pump
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Wash Water Heat Recovery
Operational Objectives
•
•
Produce plant wash water at 60°C and 10000 kg/h
GRP tank design temperature of 90°C not to be exceeded
Design Objectives
•
•
•
•
•
Process and control system design
Equipment and safety system recommendations
Develop a dynamic simulation
Test transient behaviour for specified operating conditions
Stress test abnormal conditions and failure modes
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Risk Assessment
•
•
Storage tank temperature rise response to valve failure
Control System response stability to anticipated process changes
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Simulation of Boiler
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Boiler Heat Balances
TG1
TG2
TW3
TSat
TG3
TW2
TG4
TW1
Item
Heat Duty
OHTC
Pinch
Approach
Factor K
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SIMULATION
SUITE
CHEMCAD
SOFTWARE
Units
Superheater
Generator
Economiser
MJ/h
3.27E03
3.71E04
1.05E04
W/m2 °K
106
158
489
(TG3 - TSat)
= 244.6 – 208 = 36.6
(TSat - TW2)
= 208.1 – 208 ≈ 0
(TG1 - TG3)/(TG1 - TG4)
= (539 – 244.6)/(539 – 153) = 0.76
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Typical Process Conditions
Steam
17.3 te/h
17 barg 252°C
Superheater Boiler
515°C
539°C
Economiser
229°C
Water Heater
153°C 130/221°C
95°C
Flue to Atm
GT Exhaust
100 te/h
468°C
Burner
Natural Gas
182 kg/h
208°C
208°C
104°C
BFW
60/70°C
Water Temperature
Control Loop
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Wash Water
10000 kg/h
60°C
Transfer Pump
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Pressurised
Water Loop
100/160°C
GRP Wash Water
Tank 100m3
Design 80°C
Town’s Water
Circulation Pump Circulation Pump
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Basic Control System
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Control Valve Calibration
100
ANY
CHARACTERISTIC
90
80
EQUAL %
60
VALVE COEFFICIENT (Cv)
VALVE STROKE (%)
70
LINEAR
50
QUICK
OPENING
40
30
20
VALVE
POSITIONER
10
0
0
10
20
30
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40
50
60
FLOW %
SIMULATION
SUITE
CHEMCAD
SOFTWARE
70
80
90
100
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Controller Simulation
Control mode selection
Split range control valves
Cascade control
100
𝑚=
𝑒+𝑏
𝑃
100
𝑚=
(𝑒 + 1/𝑅
𝑃
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SIMULATION
SUITE
CHEMCAD
SOFTWARE
𝑒𝑑𝑡)
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Process Flow Diagram
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Sealed, pressure compensating recirculating purified water
Wash water temperature control satisfied then balance to CHP2 BFW
Fast response temperature control with adequate failure mode protection
Thermal pressure relief not required as no trapped sections of pipe/equipment
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Simulation Conditions
Process Conditions
Pressurised water loop pump head set at 3 bar
Circulation tank initial level set at 10 m with control set point at 5 m
Circulation flow in pressurised water loop set at 40000 kg/h
Circulation flow in tank recirculation loop set at 40000 kg/h
Heat Exchangers
Water heater in flue set at calculated U 407 W/m2-°K with A 60.6 m2
Hot water heat exchanger U 5500 W/m2-°K and A 3.8 m2
CHP2 boiler feedwater heat exchanger U 850 W/m2-°K and A 15 m2
SIMULATION
SUITE
CHEMCAD
SOFTWARE
Controllers & Control Valves
Hot water heat exchanger control valve equal % with Cv 25 - dp 1.75 bar
CHP2 feedwater heat exchanger control valve equal % with Cv 50 - dp 1.75 bar
Temperature controller control modes PI only with PB 50% and I 15 min
Level controller control modes P only with PB 15%
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Simulation Action Matrix
D9.01 page 197
Valve Fail Open
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Control System Performance
D9.01 page 198
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Control System Performance
D9.01 page 198
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Control System Performance
40000 kg/h Circulation Rate
D9.01 page 198
Peak at 95°C
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Control System Performance
20000 kg/h Circulation Rate
D9.01 page 198
Peak at 84°C
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Dynamic Column Relief Study
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Dynamic Column Relief Study
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Dynamic Column Relief Study
SIMULATION
SUITE
CHEMCAD
SOFTWARE
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Dynamic Column Relief Study
REBOILER CONDITIONS
INCREASING COLUMN PRESSURE REDUCES TEMPERATURE ∆T
DUTY REDUCES FROM 3.9E06 TO 2.5E06 KCAL/H AT RELIEF
ESTIMATION OF PROCESS CONDITIONS
LATENT HEAT INCREASES FROM 89.9 TO 125 KCAL/KG
THIS EFFECT REDUCES RELIEF FLOW BY FACTOR 0F 1.4
RELIEF LOADS
DUTY 3.9 E06 KCAL/H REQUIRES
35141 KG/H
SIMULATION
SUITE
CHEMCAD
SOFTWARE
DUTY 2.5 E06 KCAL/H REQUIRES 24269 KG/H
DYNAMIC SIMULATION PREDICTS ≈ 6000 KG/H
BENEFITS
DYNAMIC SIMULATION GIVES BETTER UNDERSTANDING OF
DISTILLATION COLUMN BEHAVIOR AT RELIEF CONDITIONS
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