This paper presents a surge avoidance controller (SAC) for compressor systems operating near maximum pressure.The main result is a SAC structure characterized as an affine state-feedback control law in terms of the surge line and a directional parameter. This paper shows that with this new SAC characterization combined with existing compressor models, the existence of performance limiting jo-axis zeros. A 4-step gain SAC design procedure and an example =presented.
I . IntroductionFor the compressor's optimum performance, operation is desired near the maximum pressure rise which places the machine near instability where a control is needed. With our approach to SAC, the control is shown to be an affine state-feedback based on a minimum distance between the operating point and a straight line approximation to the maximum pressure ridge. In applications, most SAC systems consider the surge line and the control laws drive the state of the machine toward this line. In this work, we show that the general SAC system presented encompasses many existing SAC control laws.
I I . System Description A. SAC BackgroundFigure 1 illustrates the compressor with an applied load and Fig. 2 illustrates the compressor characteristics (constant speed curves) and the load. Steady-state operation or equilibrium is determined by the pressurdflow load curve and compressor characteristic intersection. The straight line (least-squares) passing through the maximum pressure rise points is the surge line.Figure 1. Compressor (vc), and load (KL). r L \ I x2 0 mas flow Figure 2. Compressor, load, surge l i e and surge l i t cycle. t Centre d'Energ6tique Ecole des Mines de Paris 60 Boulevard Saint-Michel, 75272 Paris Cedex 06, France
B . Compressor Surge CyclingIf the load is increased, an operating point in the stable E region in Fig. 2 approaches the surge line. As the operating point crosses the surge line, the mass flow rapidly decreases along the horizontal A into the unstable region. The machine follows B until the mass flow reaches zero (r = 1) where it rapidly increases along C to the maximum compressor capacity. The machine then follows the decreasing mass flow D to E where the process repeats.The compressor blades undergo violent rotation accelerations and substantial damage occurs on the first cycle and complete break down after a few cycles. The main objective of a surge avoidance controller is to avoid entering the unstable region: left side of the surge line.
C. Compressor with SAC RecycleThe approach to surge avoidance control is to augment the compressor by recycling illustrated by Fig. 3 where a portion of the gas from the outlet is fedback to the input by the recycle valve, K , . This increases the mass flow through the compressor and avoids the surge cycling.
