The present work has analytically integrated the right hand side of equation (1), presented by three ordinary differential equations of the Moore and Greitzer model that give rise to modelling the compression system: the first for the non-dimensional total-to-static pressure rise across the compression system; the second for the amplitude of mass flow rate fluctuations m; and the third for the non-dimensional, spool moment of inertia . The two first equations of (1) are equivalent to the model of . Note that the Moore-Greitzer model does not attempt to explain what physical mechanism triggers these instabilities.
Surge in industrial compressor gas pipeline
Given the potential risk of damage due to violent surge oscillations, compression systems are usually operated at a safe distance from the surge line. The imposed safety margin limits the achievable pressure rise and ability to operate at off-design conditions , , , . Surge initiation measurements are performed to determine the stability of the compression system. In Figures 4 and 5 the initiation of surge is shown. By closing the throttle valve of anti surge (Figure 2), the mass flow is decreased, and surge is initiated, as can be seen in the pressure ratio. The initiation of surge is defined as the point at which the amplitude of the pressure trace starts to grow, and a distinct oscillation frequency appears, which is pointed out in the lower right plot. Upon surge initiation, a surge development is recognized in which the fully developed surge occurs in the gas pipeline system (Figure 6). Due to a constant shaft power, the reduction of mass flow results in a slight increase of the mean rotational speed, N. The figure also shows that while the throttle valve is kept at a certain value, the rotational speed still increases due to inertia of the rotating parts.
Figure 4: Instability (surge) of centrifugal compressor
Figure 5: Detailed pressure ratio at surge initiation
The development of the surge is sketched in a compressor map in Figure 7. As the throttle is closed, the compressor performance follows the steady-state characteristic. (For simplicity a constant rotational speed scenario is followed.) At a certain pressure ratio the surge limit line is reached, and a surge cycle is initiated. Before reaching the limit cycle, some smaller cycles occur, as is seen in the pressure ratio. The stability line of the compressor is determined also from the transient measurements. Since for transient measurements into surge, the instrumented orifice cannot be used for reliable mass flow measurements, the relation between the throttle valve position and the mass flow, Equation (1), is used to determine the surge initiation mass flow. First, for different rotational speeds, the surge initiation points, i.e. pressure ratio, rotational speed and throttle valve position at initiation, are determined. Then the mass flow is calculated. As can be seen, the determination of the mass flow via the throttle valve position is reasonable, since the surge initiation points are close to the lowest measured steady mass flows , .
Figure 6 : Gas pipeline system
Figure 7: Sketch of the surge cycle
SCADA in Compression Gas Pipeline Systems
The SCADA system used in surge control of centrifugal gas compressors uses a high-performance PC-based data acquisition system, including Ethernet I/O, data loggers and programmable automation controllers (PACs). The system is shown in figures 8, 9, 10 and 11. PC-based data acquisition systems ready to fit a total solution for containing the constituents of distributed control system. Easy logic processes are effortlessly executed without involving the master computer because of the presence of a smart PLC or ladder logic , . Ladder logic is a block programming language usually used to create programs running on remote terminal units (RTUs) or PLCs , .
Figure 8: supervisory control
Figure 9: PC-based data acquisition system
Figure 10: Ethernet I/O, data loggers
Figure 11: Programmable automation controllers
A different software environment managed by a UNIX system is found in the SCADA center. SCADA system applications of the compressor station remote supervision and control to remote supervision of all pipeline facilities are shown in Figure 12.
Figure 12: supervision of compressor station
Our proposal for detecting and isolating surge using the SCADA system approach to surge control follows. The idea is to control the compressor speed with feedback from mass flow so the compressor can operate in a stable mode even to the left of the surge line and thereby avoiding the unstable operation demonstrated in the results above.