CO2 Compressors- IGV control Versus VSD control

[Sawsan311]

Dear All,

As part of the design of wet CO2 compressors (MW 43) with suction pressure = 0.3 barg while discharge pressure at last stage is 40 barg. I have gone through the possible control mean with a a variable IGV against performance control through motor speed variation. I understand that IGV performance curve matches with the 100% speed curve of a VSD machine, however, I have a technical concern on its robustness to cover the system resistance curve for the pipeline application. Additionally, IGV cause more drooping of the efficiency curves unlike VSD which maintains flatter efficiency at the different speeds.

I need your views on the advantages and disadvantages of IGV against VSD motor control specially for such a heavy gas CO2 application.

Regards,

 

[georgeverghese]

The IGV only enables slightly better efficiency at lower capacity. It is by no means a replacement for antisurge control. A VSD would enable better energy eff at lower speeds, lower capacity, since IGV has limited capacity in this respect.
As far as I know, large compressor schemes I have seen use
a) IGV for initial response to feed flow variations
b) At higher turndown on suction flows, VSD kicks in

The same concept would apply for any gas.

 

[Sawsan311]

Thans alot Mr. georgeverghese, Yes, antis-ruge control is always required integrated either with the perfomance controller through the common IGV or VSD. I also think that for parallel trains operation and the need to maintain network pressure, VSD would be better than IGV at each machine to act as a master performance controller.

 

[georgeverghese]

IGV has some uses - it can reduce the extent of reduction in polytropic eff at low flows when discharge pressure remains the same, thus saving some power and reducing the escalation in discharge temp also.
It is important for plant owner process and controls engineers to pay attention to these loop decoupling features that are typically set up between performance - capacity controller and individual AS controls on each train by compressor controls systems engineers for parallel compressor operation - they are quite complicated and some times DONT work, so you need a backup controls strategy also.

 

[Sawsan311]

Thanks Mr.George, I agree with you. I think you meant power increase upon efficiency reduction at low flow rates

Regards