case study: column relief study
TRANSCRIPT
Consultancy.
Project Delivery.
Innovation.
Case Study: Column Relief Study
Using Dynamic Simulation for in-depth relief study
Client: Milford Haven Refinery
Who are we?
Process engineering consultancy
Specialising in advanced modelling techniques, to give answers that
are more accurate, reliable and robust
Expertise in HYSYS Dynamics and Dynsim
Decades of process and mechanical engineering experience
Combine cutting edge simulation with real world understanding, to
provide you with solutions that are effective and practical.
Core team of eight engineers, supported by a wide network of
associates.
Extensive simulation experience across a range of industries
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Project Background
Naphtha Splitter Column throughput increased, required significant
hardware changes
Installed new relief valves and a high pressure trip as part of project
Project flare study had inconsistencies picked up by HSE audit
Recent site power failure resulted in liquid droplets in flare stack, the
suspicion was that could originate from Naphtha Splitter Column
Site decided to complete a comprehensive dynamic study of tower to
properly understand complex flare scenarios
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Column and Relief Valves
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New Pilot Relief Valves
Hardware Improvements
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New Trays
Additional Condenser Area
New Pilot Relief Valves
New High Pressure Trip
New Pressure Control
New Relief Valve
Larger Bottoms Pumps
Column Interconnection
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12 barg
(Design 13.7 barg)
1.5 barg
(Design 5.0 barg)
Pressure Transfer
Debutaniser
Naphtha Splitter
Debutaniser bottoms feeds
Naphtha Splitter Column
Dynamic Modelling
Model Developed in Dynsim 5.0
High Fidelity – included all control response, liquid and vapour holdup,
e.g. characteristics of thermosyphon reboiler
Important to model the response of pilot relief valves with significant flare
backpressure effects
Important to model all equipment directly upstream and downstream of
column
Model rating using actual plant data, reveals hidden features, such as
position of bypass valves and controller settings
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Main Relief Scenarios
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External Fire
Total Power Failure
Tube Failure
High to Low
Depressurisation
Partial Power Failure
Main Findings
Effect of pressuring the column and accumulating lights in overheads
helps to pinch reboiler and reduce relief load. Justifies the large margin
between operating and design pressure.
Liquid tends to relieve first via the reflux drum and then by main pilot
relief valves creating hazardous two phase in the flare header (which
may explain previous site event)
On power failure: Liquid accumulates in the tower as upstream column
depressurises, exceeding design pressure at column base (static head +
column top pressure)
Inadequate fire relief for reflux drum
Inadequate relief capacity for tube fracture. Complex steam stripping
effect occurs
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Main Recommendations
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Re-rate reflux drum to
prevent liquid preferentially
relieving via low point in
overheads
Increase size of relief valve
on reflux drum and design
for backpressure
Re-rate column to
withstand
accumulating liquid
during a relief case
Summary
Dynamic simulation explains complexities such as reduction in heat
transfer rate as thermosyphon recycle rate slows or the effects of steam
entering the column on tube fracture
Complex interconnected events modelled: column to reflux drum, or
column to column that are not intuitive, particularly when pressure and
composition are changing with time.
Visualisation of relief events helps all parties understand the issues
The sensitivity/robustness of the results can be checked by modifying
the assumptions
All recommendations can be checked out using the model before
implementation
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Summary
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Consultancy.
Project Delivery.
Innovation.
Please get in touch to find out how we can help your
business today.
E: [email protected] W: flexprocess.co.uk
T: +44 1454 629 689