Antisurge system

Figure 2.112 illustrates how two compressors can be proportionally loaded and unloaded. Because of age, wear, or design differences, no two compressors are identical. A change in load will not affect them equally, and each should therefore be provided with its own antisurge system. Another reason for individual surge protection is that check valves are used to prevent... 

The first objective of the antisurge control system is to protect the compressor. This can be accomplished for some disturbances by using the PI algorithm with a large value of bj. However, it is also necessary to maximize the region in which the compressor can operate with the recycle valve closed. This increases the efficiency of the compressor at lower throughputs. Steady-state operation with recycle is extremely inefficient. Therefore, from this perspective, small values of bj are highly desirable. 

While protecting the compressor from surge is the most compelling control problem, it is not the only requirement. The compressor throughput must be adjusted to match its load. Capacity control interacts with surge protection, which reduces the effectiveness of the antisurge control system if they are not decoupled. 

It is essential that all compressor systems be designed to avoid possible surge operation. This is done by incorporating some type of antisurge control system. This is to ensure that the flow into the compressor is sufficient to maintain stability at the required pressure drop across the compressor. The surge control system should be used only for surge control and must not be linked with other functions of the process control system otherwise, this could result in the damage of the compressor. 

At the current level of technology MVC packages do not scan frequently enough to handle very fast processes. For example it would be unwise to apply one to compressor antisurge control unless there is a backup fast scanning system to recover from surge. 

---