Antisurge controller

The essence of surge protection is determining when and how much to open or close the recycle valve. In the simple application of Figure 6-45, the antisurge controller (UIC) will position the recycle valve using a control algorithm based on AP, AP, and data on the location of the surge limit. 

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. 

Some consideration of surge characteristics should be made, and unless your own plant design includes antisurge controls, it should definitely be mentioned in the specification. Possibly, the vendor can quote antisurge equipment with his offering. At a later date, the specification will serve as a good reminder to both the vendor and purchaser if some mention of compressor control is made. The details may be worked out at any convenient point, such as at the bid review or possibly later in a coordination meeting. 

Rana, S. A. Z., Understanding Multistage Compressor Antisurge Control, Hydro. Proc., p. 69, March (1985). 

Centrifugal compressors can go unstable if the flowrate of gas through the compressor is reduced too far. To avoid this, an antisurge controller is used that prevents the flowrate through the compressor from dropping below some minimum. Recycling from compressor discharge to compressor suction is used to keep the load on the compressor (see Fig. 7.3). 

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. 

FIGURE 25—2 A motor-driven compressor with antisurge controls... 

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. 

This is then a measure of the minimum suction flow. This value could be used as the SP of a minimum flow type antisurge controller. Or it could be subtracted from the measured flow to give the margin to surge in flow units... 

This would then be the PV of the antisurge controller with the operator entering the... 

In addition to antisurge control, it is common to have a surge recovery scheme. This relies on the detection of rapid changes in pressure or flow and then overrides the antismge scheme to fully open the recycle valve. It can also increase the safety margin used by the antisurge controller so that, when the antisurge scheme is permitted to take back control, the same situation should not recur. 

The antisurge controller will interact with other schemes on the compressor. For example, it opening the recycle valve will cause the discharge pressure to drop and hence also the flow to the downstream process. The flow (or pressure) controller will then to take corrective action. To avoid this interaction propagating this controller should be tuned to act significantly slower than the antisurge scheme. 

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