Control of Gas-Liquid Cylindrical Cyclone (GLCC©) Separators
—A Predictive Strategy*
Shankar B. Earni, Master’s Candidate,
Mechanical Engineering
Slug flow is a highly complex type of flow
with an unsteady nature. Two-Phase flow often results in large flow variations where slugs
of liquid occur in the flow lines which are connected to separation, treatment and pumping
equipment. Conventional approach to managing the slug flow is based primarily on the
prediction of maximum slug flow conditions, and this usually leads to over-designing
facilities.
A method to mitigate the slug measures the
available volume in the GLCC© and the volume of the liquid slug
moving towards the GLCC© to effect proportional throttling of
the outflow from the GLCC.© It is expected that early detection
of terrain slugging (slug length, slug velocity and holdup) and control of the liquid
level in GLCC© using feed-forward mechanism can improve the
operational range of GLCC,© by decreasing the gas carry under
and liquid carry over, and thereby decreasing the control valve dynamics. A liquid slug
may be detected by an ultrasonic or a conductance probe sensor interposed in the flow line
upstream of the GLCC.© The conventional feedback control loops
can seldom achieve perfect control considering the impact of huge slugs that are keeping
the output of the process continuously away from desired set point value. The reason is
simple: a feedback controller reacts only after it has detected a deviation in the value
of the level from the set point. Unlike the feedback systems, a feed forward control
configuration measures the disturbance directly and takes control action to negate the
effect of the disturbance on liquid level in the GLCC.©
Therefore, feed forward control system has the theoretical potential for perfect control.
A model has been developed for predictive
control system integrating feedback and feed forward control systems. The feedback
controller has been designed in frequency domain; this approach has some inherent
advantages when dealing with slugs and slug frequencies. A feed forward controller has
been designed using an analytical approach, for integration with the feedback control
system.
Based upon the design, the predictive
control model has been simulated in MATLAB-Simulink and the results obtained demonstrate
that the proposed strategy is a viable approach for GLCC©
predictive control.
*This work is supported by Tulsa
University Separation Technology Projects (TUSTP).
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