5 research outputs found
Fault tolerant control system design for distillation processes
PhD ThesisThe complexity and sophistication of modern control systems deployed in the
re nery operation, particularly the crude distillation unit as a result of increasing
demand for higher performance and improved safety, are on the increase.
This growing complexity comes with some level of vulnerabilities, part of which
is the potential failure in some of the components that make up the control
system, such as actuators and sensors. The interplay between these components
and the control system needs to have some built-in robustness in the
face of actuator and sensor faults, to guarantee higher reliability and improved
safety of the control system and the plant respectively, which is fundamental
to the economy and operation of the system. This thesis focuses on the
application of frugally designed fault tolerant control systems (FTCS) with
automatic actuator and sensor faults containment capabilities on distillation
processes, particularly atmospheric crude distillation unit. A simple active
actuator FTCS that used backup feedback signal, switchable references and
restructurable PID controllers was designed and implemented on three distillation
processes with varying complexities { methanol-water separation column,
the benchmark Shell heavy oil fractionator, and an interactive dynamic crude
distillation unit (CDU) to accommodate actuator faults. The fault detection
and diagnosis (FDD) component of the actuator FTCS used dynamic principal
component analysis (DPCA), a data-based fault diagnostic technique, because
of its simplicity and ability to handle large amount of correlated process measurements.
The recon gurable structure of the PID controllers was achieved
using relative gain array (RGA) and dynamic RGA system interaction analysis
tools for possible inputs { outputs pairing with and without the occurrence
of actuator faults. The interactive dynamic simulation of CDU was developed
in HYSYS and integrated with MATLAB application through which the FDD
and the actuator FTCS were implemented. The proposed actuator FTCS is
proved being very e ective in accommodating actuator faults in cases where
there are suitable inputs { outputs pairing after occurrence of an actuator
fault.
Fault tolerant inferential controller (FTIC) was also designed and implemented
on a binary distillation column and an interactive atmospheric CDU to accommodate
sensor faults related to the controlled variables. The FTIC used
dynamic partial least squared (DPLS) and dynamic principal component regression
(DPCR) based soft sensor techniques to provide redundant controlled
variable estimates, which are then used in place of faulty sensor outputs in
the feedback loops to accommodate sensor faults and maintain the integrity
of the entire control system. Implementation issues arising from the e ects of
a sensor fault on the secondary variables used for soft sensor estimation were
addressed and the approach was shown to be very e ective in accommodating
all the sensor faults investigated in the distillation units. The actuator
FTCS and the FTIC were then integrated with the DPCA FDD scheme to
form a complete FTCS capable of accommodating successive actuator and
sensor faults in the distillation processes investigated. The simulation results
demonstrated the e ectiveness of the proposed approach.
Lastly, fault tolerant model predictive control (FTMPC) with restructurable
inputs { outputs pairing in the presence of actuator faults based on preassessed
recon gurable control structures was proposed, and implemented on
an interactive dynamic CDU. The FTMPC system used a rst order plus dead
time (FOPDT) model of the plant for output prediction and RGA and DRGA
tools to analyse possible control structure recon guration. The strategy helped
improve the availability and performance of control systems in the presence
of actuator faults, and can ultimately help prevent avoidable potential disasters
in the re nery operation with improved bottom line { Pro t. Overall,
the proposed approaches are shown to be e ective in handling actuator and
sensor faults, when there are suitable manipulated variables and redundant
analytical signals that could be used to contain the e ects of the faults on the
system.University of Lagos, Nigeria & Petroleum Technology Development
Fund (PTDF) for the Scholarship award at the later stage of my research
programme