Module 11

Antisurge control systems of parallel compressors

When centrifugal compressors are operated in parallel, the problem of properly dividing the load is always present. The solution is particularly difficult when the compressors have dissimilar characteristics. But even when the characteristic are identical, some means must be provided to ensure that the desired load distribution is obtained.

Controls to accomplish this will not discussed here, except as they affect the antisurge control system.

Figure 11-1, shows the effect of inadequate control. With a differential pressure of 80 PSI, and both compressors running at 100 % speed, the load is equally divided and the flow through each machine is Q1.

If, however, for any reason (e.g. dissimilar governor characteristics), one compressor is running at only 98% peed and the other is at 102%, the flows will be Q2 and Q3 respectively.

Figure 11-1 compressor performance curve

If the difference in speeds is large enough, the flow through one machine will decrease sufficiently to cause surging.

From Figure 11-1, it can be seen that the flatter the characteristic curves, the more difficult the problem becomes.

For this reason/centrifugal compressors require more precise methods of load distribution than axial flow compressors, which have much steeper curves. This susceptibility to surge make it even more essential to provide positive protection for centrifugal machines during parallel operation.

The control arrangement shown in Figure 11-2, allows maximum operating flexibility with minimum supervision.

It consists of a standard surge control system for each compressor, which provides complete protection at all times whether the machines are operating one at a time or together in parallel.It is even possible to start a compressor with the surge controller in automatic while the other compressor is running.

Figure 11-2

As the newly started machine approaches its operating speed, the surge controller holds the antisurge valve open just far enough to bring the compressor up along the control line and keep it away from surging. This system could be used for any number of machines.

Figure 11-3, shows an alternate surge control system that requires less equipment. It uses only one ratio station and controller, one DP transmitter and a single antisurge control valve.When both compressors are operating, the low selector relay chooses the lower flow signal and sends it to the ratio station.

If only one machine is running, its flow signal is connected to the ratio station by a manual selector switch. This selection can also be performed automatically with relay logic, if desired.The cost saving resulting from using this system is not obtained without some sacrifice.For example, each compressor must be equipped with a manual recycle valve for starting. With one machine operating, the recycle valve cannot be used for starting the second one, so the recycle must be done manually without automatic surge protection.

Figure 11-3