Compressors antisurge control

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

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. 

Figure 4-81. Axial compressor antisurge and guide vane control.

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. 

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). 

Control valves form an important equipment item in process plants, and therefore, a lot of money can be saved by ensuring that the right valves are installed. Selecting the right control valve for any application starts with defining the valve s functions. For example, if an on-off interlock valve is to be used, then the primary criteria for selection are reliability and simplicity. If very fast and precise modulation is required, such as for a compressor antisurge vent, a high quality valve is chosen. 

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... 

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. 

Capacity control is more complex in compressor networks where two or more compressors operate in series or parallel. Automatic distribution of the load between compressors is required. The antisurge and load sharing control loops must be coordinated to ensure surge protection while tlie load distribution and energy consumption are optimized. 

A minimum flow controller ( antisurge ) sets the valve in the compressor bypass line to prevent the flow rate through the compressor from dropping below some minimum flow rate. 

In order to avoid surge occurrence in compressor, axial or radial one, antisurge valve is used usually. Various mechanisms have been presented for these valves which all of them are based on reduction of compressor downstream pressure and according to surge control line (to avoid surge occurrence). We use this line to compensate uncertainly in the measurements and delay in valve performance. There are some descriptions for this parameter that one of them has been shown in Eq. (3.8). 

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