emergency depressurisation: 690 kpa or 50% in 15 minutes?
TRANSCRIPT
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Emergency Depressurisation:
690 kPa or 50% in 15 Minutes?
Cameron Milne, Lead Technical Safety Engineer, px Limited
Duncan Talbert, Principal Consultant, MMI Thornton Tomasetti
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Overview
Objectives:
• To consider emergency depressurisation of process plant
and reduced risk of failures due to fire.
• To review generic depressurisation criteria.
• To describe a scenario specific approach.
Content:
• Requirement for depressurisation.
• Commonly applied criteria.
• “Specific Calculations”.
• Benefits of Specific Calculations over Generic Criteria.
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Emergency Depressurisation: Why?
Reduction in harm:
• People; Environment; Plant
• Immediate Harm; Escalation
Vessel Wall Strength, decreases
with increasing temperature
Load on Vessel Wall – increases with
temperature
TIME
STR
ES
S RUPTURE
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Emergency Depressurisation: How?
HOW:
EDP System:
• Actuated valves; Collection/header pipework; Flare or vent
• Design Standards: typically API 521
Vessel Wall Strength
Vessel Wall Stress
TIME
STR
ES
S
RUPTURE
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EDP: How fast?
Vessel Wall Strength
Vessel Wall Stress
TIME
STR
ES
S
FAILURE
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Depressurisation Rate Constraints
Rapid depressurisation is generally good however;
• Equipment and piping low temperature constraints.
• Equipment protection – compressor seals.
• Flare/Vent Capacity on an existing plant.
• Flare/Vent size and weight on a new plant.
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How fast?
API 521:
• Depressurise to 50 % of Design Pressure in 15 minutes
• Or 690 kPa in 15 minutes
• Or perform more specific calculations
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EDP: How fast?
API 521:
Depressurise to 50 % of design pressure in 15 minutes
• Pool fire exposure – not jet fire
• Carbon steel vessel
• Wall thickness approximately 25.4mm,
690 kPa in 15 minutes
• “Vessel leak or failure case”
• “Commonly applied for both fire and leak scenarios”
• Or perform more specific calculations
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Specific Calculations
What do we do with the answer?
• What is the organisation’s tolerance of risk?
• What’s the nature of the fire threat, pool or jet, size and
duration?
• What governs the depressurisation rate?
• How does vessel rupture under fire attack contribute to
overall risk?
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Fire Risk Assessment
• Fire type
• Fire size
• Fire duration
• Fire frequency
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Fire Sources
Fire sources are determined from:
• Installation description: 3D models, P&IDs, equipment
layouts
• Fire and Explosion Risk Assessment ,QRA
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Specific Calculations
Take cognisance of:
• Vessel, material, size, orientation,
• Vessel contents, composition, conditions,
• Contiguous pipework, etc.
• Fire type, size and duration
• Where does the fire contact the vessel
• Pressure relief fitted to the vessel
• Blowdown
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Specific Calculation Results
Vessel Wall Strength
Vessel Wall Stress
TIME
ST
RE
SS
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Governing Inventory
TARGET
VESSEL
SOURCE
VESSEL
Emergency
Depressurisation
Valves
To
Flare
1
2
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Source vessel to target
vessel separation
Target vessel fire endurance
Jet flame length versus
time
TIME
DIS
TAN
CE
Target Vessel Fails
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TIME
DIS
TAN
CE
Increase Source Vessel EDP rate to
reduce fire threat to Target Vessel
Target Vessel Survival – 1
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TIME
DIS
TAN
CE
Increase Target Vessel EDP rate to
increase Target Vessel endurance in fire
Target Vessel Survival – 2
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Optimisation
On a real plant;
• Optimisation may involve adjustment of depressurisation
rates across a number of vessels.
• It may involve reducing the depressurisation rates of some
vessels to accommodate increasing the rates on others.
• Where multiple vessels are involved a number of iterations
may be required to determine the best result.
• Optimisation is unlikely to be achieved by applying generic
rules.
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Other Mitigation
If depressurisation rates cannot be adjusted to give tolerable failure times, other measures are available;
Plant Configuration
• Separation
• Orientation
• Screening
Passive Fire Protection
• Wet applied systems
• Precast systems or boxes
Active Fire Protection
• Deluge
• Fire response team
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Conclusions
There are benefits of using specific calculations to determine
emergency depressurisation rates rather than applying generic
rules;
• Gives better understanding of the fire risk to process
vessels, and the contribution of this hazard to overall risk.
• Allows optimisation of risk reduction on an existing facility.
• Allows optimisation of depressurisation system design on a
new facility.
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Thank You
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