Steady State Compressor Model as a Basis for Anti-Surge Control Action

Abstract

Dynamic compressors are known to suffer from surge, which can severely damage compressor components and disturb production. Surge may arise by the occurrence of disturbances (e.g. compressor discharge valve closure) that would bring its operating point to a region at low flows delimited by the so called surge line (SL). Therefore, dynamic compressors are always equipped with anti-surge mechanisms: typically a fast actuating recycle valve controlled by a PI anti-surge controller. Since surge develops extremely fast, the compressor is usually not allowed to operate too close to the surge line. A surge safety margin is considered, which is the region between the SL and a surge control line (SCL), which may be defined as a line parallel and to the right of the SL. Once the compressor crosses the SCL towards the SL, PI controller action starts. Depending on a number of factors the recommended surge margin adopted may vary. This control objective (keep the compressor away from the SL) is conflicting with energy efficiency requirements, since higher efficiency operating points are located close to the SL. Therefore, it is desirable to operate the compressor using the smallest possible surge margin that still guarantees anti-surge action is effective. In this paper we propose a method for triggering the compressor anti-surge action, aiming at a faster action than traditional PI control, which could enable the adoption of reduced surge margins and operation with higher efficiency. Given a typical single compressor system topology, the possibility of a compressor reaching the surge region coming from a stable operating point can be retrieved through the state of the system actuators. Considering a certain combination of values of the system actuators (compressor suction valve opening, discharge valve opening and motor drive torque), if the steady state operating point (calculated based on a nonlinear variable speed compressor system model) lies to the right of the SCL, then no anti-surge action is necessary. If the resulting steady state lies to the left of the SCL, anti-surge action is deemed necessary and therefore triggered. The proposed anti-surge control method relies on an offline computation of necessary recycle valve openings for each possible combination of the system actuators positions mentioned above, considering a predefined discrete set of values from the actuators positioning ranges. This generates a look-up table for online use. The values from the look-up table are used to trigger a “steady state control action” for the recycle valve that would keep the system stable after transients vanish. They are also used to identify the necessary compressor flow set-point for a feedback controller, which is responsible for ensuring that the system trajectory goes from the state upon anti-surge activation to the desired steady state. The control action from this feedback controller is added to the steady state control action so that its contribution should vanish after transients vanish. A PI and a sliding mode controller are used in feedback control action and results are compared to the traditional anti-surge control approach.