The control of centrifugal compressor surge using a recycle valve

A compressor is a machine that is used to increase the pressure of various gases. How to increase the pressure depends on the compressor type. One of the functional problems of centrifugal compressors is the phenomenon of the surge, which is still an obscure and unknown phenomenon that can induce mechanical or thermal stress in the system and cause a lot of damage. This type of aerodynamic instability reduces the compression ratio at both ends of the compressor, thereby reducing the overall efficiency of the system. In general, when a surge occurs, it causes process turbulence, the degradation of overall compressor efficiency, the reduction of compressor life due to mechanical damage to seals, bearings, rotor, and impellers, and the loss of internal freedoms and sensitive mechanical parts of the system. Therefore, surge control is one of the challenges of compressor control and expands the operating range of compressor operation. A map compressor consists of two axes, horizontal and vertical, as well as a set of curves that show the horizontal axis of the flow (capacity) and the vertical axis, head or pressure. Compressors must change their speed to change the output flow. For each speed, there is a minimum point and a maximum flow point within which the compressor operation is stable and predictable. The maximum capacity point is called the stone wall point and the minimum capacity point is called the vertex point. The surge line (SL) of a compressor is formed by connecting the surge points at different speeds. If the compressor works on the right side of the surge line, it is in a steady state, but if it works on the left side of the surge line, it is in an unstable or surge state. Using methods based on active surge control, the instabilities leading to the surge can be eliminated, and the area of stable performance of the system can be extended to the surge line, and thereby the stable area of the system can be widened. This paper uses a dynamic model of centrifugal compressors with a recycle valve, as well as a proportional-integrator-derivative (PID) controller and a sliding mode controller, to control the surge phenomenon with a compressor control approach based on the surge control line. A new sliding surface is defined for controlling sliding mode, and it is controlled by using recycle valve and a compressor inlet valve. A quadratic Lyapunov function is used to ensure the stability of the intended slip surface. The two approaches of inlet and recycle valve are expressed individually and collectively. The results of simulation in MATLAB state that among the compressor with a PID controller and the compressor with sliding mode, the latter outperforms the former in controlling the compressor at different speeds. Based on the comparison of the results, the amplitude of the control signal in the sliding mode is less than PID and the system reaches a stable state with less energy consumption, which shows better control by the sliding mode than by PID