The consultancy was hired to redesign a refinery's natural gas letdown and distribution system to improve reliability and save costs. Through concept design, engineering studies, and detailed process design, they created a new configuration with separate letdown systems and improved equipment. A dynamic simulation model validated the new design's reliability under various disturbances without requiring plant trials. It informed commissioning and ensured high reliability when converting to natural gas, saving $3 million per year by removing the costly LPG backup system.

1. Consultancy.
Project Delivery.
Innovation.
Case Study: Natural Gas Letdown Reliability
Dynamic Simulation to ensures unit reliability and saves £3m/year
Client: Refinery run by Multinational Oil & Gas Corporation

2. 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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3. Project Background
Refinery needed to convert to natural gas as primary fuel source to
replace LPG / Gas Oil to reduce energy costs (£3m/year), manage
sulphur emissions and operate a new CHP plant
Decision taken to import natural gas from national grid
Initial third party installation of let-down systems had major safety and
reliability issues
Decision to completely redesign with new knowledge and check out
design using dynamic simulation to validate reliability
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4. Our Responsibilities
Concept Design, avoiding pitfalls of original design. Issue report and
help client to make best decision
FEED, including detailed design basis document, HAZOP study and
control & trip analysis
Detailed Process Design, specification of new equipment, hydraulic
network models and relief and venting study
Dynamic Simulation to validate reliability and test operability
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5. Old Let-Down Configuration
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6. Old Reliability Issues
All critical consumers dependent on a single let down
Single trip takes down the whole site
Second stage let down station unstable, inlet pressure too high and often
trips from a disturbance in downstream fuel gas pressure. This then
causes an upstream trip at first let down station, taking down the site
Natural gas make up into fuel gas critical to keeping process
heaters/steam boilers running. Fuel gas pressure depletes very quickly
if natural gas is lost
Backup LPG vaporiser too slow to act and has to keep flowing at low
rates to ensure availability at short notice
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7. First Let Down Station
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8. New Let-down Configuration
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9. Hardware Improvements
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Separate Let Down
Systems create
diversification
Reduced inlet pressure to
second let down station &
improved margin between
operating and trip settings
Second let down station
provides diversification
Electric heater provides online
back up to steam preheater

10. Dynamic Simulation to Check Reliability
Build high fidelity dynamic simulation of gas network from 94 barg supply
to 3.5 barg fuel gas system
Include all mechanical features of system, valve closing times, detailed
valve trim characteristics, preliminary controller settings, heat transfer
from heaters and heat loss from pipes.
Set up multiple initial conditions, to represent minimum and maximum
operating conditions to examine valve positions, turndown on heaters,
stability to system noise.
Look at single disturbance scenarios such as heater trip, valve trip and
assess affects on pressure and temperature.
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11. Example Scenario – Back up Preheater Changeover
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Verify that electric heater picks
up the temperature on loss of
steam before low temperature
trip activates

12. Example Scenario - Disturbance on Turbine Stream
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Pressure recovers on parallel control valve
for fuel gas stream after initial drop
Turbine stream peaks but does not trip on
flow disturbance

13. Summary
Complete redesign of natural gas let down and distribution system
including a full concept study, design basis and detailed process design
Developed a dynamic model that demonstrated high reliability for a
variety of plant disturbances under different modes of operation without
the need to perform actual plant trials
Dynamic model informs the commissioning team of controller settings,
and operating conditions
Costly LPG backup system was removed saving £3M/year
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14. Summary
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Consultancy.
Project Delivery.
Innovation.
Please get in touch to find out how we can help your
business today.
E: enquiries@flexprocess.co.uk W: flexprocess.co.uk
T: +44 1454 629 689