64
LNG
INDUSTRY
SEPTEMBER
2014
to proceed in a straightforward and timely manner, and brings
the process online sooner. However, simulation provides
future benefits for plants as well. The same tools of simulation
that smooth out start-ups and train operators can also enable
ongoing operational improvements and testing for future control
upgrades.
For example, engineers at a US refinery used simulation to
verify start-up and shutdown procedures. This plant’s air blower
system for a fluid catalytic cracking unit (FCCU) had four
compressors arranged in three circuits (A, B and C) (see Figure 6).
Units 1 - 3 were centrifugal compressors driven by electric motors,
and unit 4 was an axial compressor driven by a steam turbine.
Because the downstream units had to be operating to provide
steam, unit 4 had to be started last. This systemwas particularly
sensitive to the start-up and shut-down sequencing.
The previous start-up procedure required sequential start-up
of each unit to its maximum capacity to provide maximum surge
margin. The required flowwas controlled by venting the snort
valves to the atmosphere. Overall, this was a safe but highly
inefficient procedure.
Employing dynamic simulation, engineers used decoupling
controls to balance the capacity and anti-surge controllers.
Simulation enabled them to evaluate various scenarios to
determine decoupling algorithms that provided maximum
efficiency and safe surge margins (see Figure 7).
Based on simulation results, plant engineers added a master
flow controller with load sharing to control the target flow
required by the process. Aminimum position was required on
suction valves to keep motors from tripping due to high current
spikes. This new procedure dramatically improved start-up
efficiency.
Conclusion and application
This article has looked at how using dynamic simulation can
reduce costs, save time, improve safety and increase availability
for turbomachinery. Before start-up and training, simulation
makes a difference by providing data. For example, plant
engineers often face questions about their specific systems that
are difficult to answer in the course of day-to-day operations. If
data is available from simulation, common questions such as the
following can be answered:
Howwill this upgrade affect the current system?
Will the start-up go smoothly, or will many adjustments need
to be made to tuning in the process?
Is the recycle valve fast and large enough?
Is a hot bypass valve required?
Is the motor strong enough for a hot restart?
Will the controls respond to emergencies such as a trip?
Are the operators adequately trained?
In providing tangible answers to these weighty concerns,
dynamic simulation proves its merit. With simulation it is possible
to accurately predict how future upgrades will affect an existing
system or how a new systemwill perform. Simulation can affect
how smoothly a start-up goes and enable the examining of
technical specifics, such as motor strength and valve
performance. One can watch simulated scenarios to see how the
controls will respond to emergencies and operators can be
trained to perform successfully in both normal and emergency
situations.
By allowing plants to ‘test drive’ their turbomachinery control
systems, high fidelity dynamic simulation provides valuable
insight. It helps substantiate design, provides a means for
verifying functionality, maintains high efficiency by protecting
turbomachinery and saves significant expense over time.
References
1. O’Brien, L., ‘Why we need a better approach to procedural
automation’, J&H Marsh & McLennan, Inc., cited by: ARC,
September 2010.
2. Lorenzo, D. K., ‘A Manager’s Guide to Reducing Human
Errors: Improving Human Performance in the Chemical
Industry’, Washington DC: American Chemistry Council,
1990.
Figure 6.
Air circuit configuration.
Figure 5.
Operator response skills over time.
Figure 7.
Verifying start-up and shut-down procedures.
