58
LNG
INDUSTRY
SEPTEMBER
2014
Almost everyone in the industry will agree that ageing control
systems can provide adequate functionality for decades.
However, recognised best practices tell a different story. Systems
should be updated before they reach mature life for the following
reasons:
Electronic components cannot escape the pre-written destiny
of theWeibull distribution curve, as their likelihood to fail
increases exponentially as it approaches wear out life.
1
Old systems are far less capable than current ones. Imagine
using 20 year old computer or mobile phone devices, or
trying to use 1980s desktop computers while trying to
function in today’s world. Technology improves quickly and
owners/operators must leverage its power to run plants.
There is little or no choice but to update a plant’s systems
at ever shortening time intervals. This is especially true for
critical applications such as turbomachinery control, vibration
monitoring, and safety shutdown devices.
On the other hand, owners/operators cannot rely on
technological advances alone to run plants. The most successful
installations of turbomachinery control systems are the ones that
result in the successful integration between technology,
application expertise, and process know-how. While this may
seem like a basic requirement, examples where at least one of
these ingredients is missing often occur, with costly
consequences for the end user. When investing in a new system,
it is important not to make technology the only criterion, and to
remember that domain knowledge is as indispensable as ever.
To further illustrate the point, this article describes a selection
of recent case histories. These are all real life scenarios, reported
by Compressor Controls Corp. (CCC)’s field engineers. Names
and locations are omitted for obvious reasons. The objective of
the case studies is to illustrate typical shortcomings and outline a
more complete approach to turbomachinery controls.
Involve the controls systems
partner from specification to
completion of commissioning
An LNG plant in Southeast Asia took a proactive approach to
ensure a smooth commissioning and start-up process. The plant
had new compressor configurations that had not been used
before for this particular LNG process. CCC was asked to help
integrate its emulation software with the plant controls in order
to predict possible process upset scenarios.
The plant’s challenge was to ensure that the automated
start-up and shut-down procedures of parallel units were in
accordance with changing operational requirements and
production. The controls emulation system contained all of the
same detailed routines and instructions that would eventually be
commissioned in the turbomachinery trains. This would be a
difficult feature to replicate in the overall plant simulator, so the
emulator technology by itself provided excellent value. But even
more important was the pre-start-up engineering support
provided by the CCC field engineer. Working side-by-side with
the end user and the EPC contractor, the team tested an array of
start-up and shut-down scenarios as well as possible process
upset scenarios, ensuring that the overall plant and machine
control routines were completely synchronised prior to
commissioning of the plant.
The established baseline configuration reduced the time to
commission the plant, and the upfront simulation allowed the
team to thoroughly test for many scenarios in multiple occasions,
without any actual start-ups taking place.
The final outcome of this exercise provided a dual benefit.
First, the potential risk of process upsets affecting the compressor
was greatly reduced. Second, the overall commissioning and
start-up procedure went smoothly without delay. Engaging the
right people and using updated technology up front in the
process paid off with immediate results.
Application knowledge
Sometimes operators have conservative control measures in
place for what they believe to be good reasons. During a service
visit to a refinery, CCC learned that an operator had severely
damaged a fluid catalytic cracking unit (FCCU) axial compressor
on two separate occasions. Given that this unit was critical
to the operation of the plant, it was imperative to identify the
fundamental cause and implement a corrective action. During the
investigation, it was discovered that the plant had set parameters
in place to eliminate potential operator error. While the original
idea was to protect the compressor at all costs, the operator did
not realise that this approach was costing themmillions of dollars
in lost revenue.
The root of the problem originated when the plant set the
surge limit line (the minimum flow point belowwhich the
compressor becomes unstable) without the required application
and process expertise. This set point is critical for a plant’s overall
process efficiency. When an operator error led to a process
disturbance, actions were taken to adjust the minimum and
maximum flow control parameters very conservatively. While
this seemed like a reasonable business decision, the plant did not
realise that this decision would cost them in terms of lost overall
efficiency. The newly adjusted control parameters had been set
using baseline values that had been collected during a single
surge test, and were no longer a true representation of current
operating conditions.
During a scheduled outage routine visit, CCC performed the
step of verifying the surge line. The compressor was run on the
surge control line (SCL) between the minimum and maximum
inlet guide vane (IGV) parameters. The compressor operating
windowwas re-adjusted to be more in line with current operating
conditions. This improved safety and throughput at the same
time. A simple tuning exercise resulted in millions of dollars in
increased production. Furthermore, ‘system optimisation’ should
be an ongoing process rather than a single event.
Audit critical control set points
A plant asked CCC to tune its suction pressure control response
after refurbishing the suction throttling valve. As part of its
Figure 2.
Engineers can plan for possible process scenarios
using emulator software.
