Antisurge

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

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 set point of the antisurge eontroller is represented on the eompressor map by a line parallel to the surge limit line. The eontroller ealeulates the deviation of the operating point from this surge eontrol line by the equation ... 

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. 

Attention to noise may be required beeause antisurge valves usually have large pressure drops. Appropriate valve style, noise attenuation trim, and in-line deviees, as well as attention to line veloeities and pipe sehedules, ean eliminate or minimize the noise. 

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. 

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. 

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). 

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. 

But, as we reduce N, we are moving toward the surge point in Fig. 28.3. If we try to save too much electricity, by forcing the FC to close too much, then the centrifugal compressor may be forced into surge. That is why we call the FC spillback valve the antisurge valve. 

Compressors can be connected in series to increase their discharge pressure (compression ratio) or they can be connected in parallel to increase their flow capacity. When driven by different shafts, they require separate antisurge... 

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... 

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. 

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