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E.Storkaas, S. Skogestad and V. Alstad, "Stabilization of Desired Flow Regimes in Pipelines", Presented at AIChE Annual Meeting, Reno, Nov. 2001

Comment: This was our first paper on slugging and includes for the first time the Hopf bifurcation diagram. It also has the insight that the non-slug region becomes unstable at some valve opening, but it can be stabilized by feedback. Many conference papers followed but the first journal paper only appeared in 2007.


Abstract.
Elimination of slugging by changing the design or the operating point (e.g. increasing the pressure) has been suggested by many authors. The objective for them was to get to an operating point where the desired ow regime is (open-loop) stable. In this paper the objective is to stabilize the unstable (optimal) operating point using feedback control. A simple two-phase ow model is used for analysis and controller design. The model captures the main dynamics of severe slugging, and is rst order continuous, thus suited for linear analysis and controller design. We have shown that a system exhibiting severe slugging is a Hopf bifurcation with the choke valve position as free variable. The pressure sensor used as measurement for control should be placed in the lower part of the system. With the pressure sensor located in the riser, RHP-zeros close to the imaginary axis limits the bandwidth of the control system, making stabilization of the system dicult. The task of eliminating severe slugging in pipelines consists of two subproblems; 1. Breaking the limit cycle and bringing the system to the desired operating point 2. Stabilize the system at the desired operating point In this work we show that it is possible to break the limit cycle manually by closing the choke valve and then bring the system to its desired closed-loop stable operating point using a simple PI-controller.