Porfirio,
Having conducted many tests on recip/centrifugal compressors and ic/gas turbine engines, i'm curious as to your situation.  although i'm not familiar with HydroCOM control systems, the principle's are the same for unloaders.  suggest contacting the oem of compressor and have them furnish compressor performance curves for all cylinders for all stages and at various cylinder clearances.  that way, you can determine the most efficient process of changing cylinder clearances.  yes, if there are several unloaders AND the process flow / pressure varies, then process control can be a challenge with which cylinder to activate/deactivate.  are process variables, mainly pressure, duty, flow, etc. continuously changing to warrant cylinder clearances?  i would not think so, but ... -pmover

Well, thanks indeed for your anwser.
Neverheless, Hydrocom capacity control is a stepless one, where in function of the capacity required, f.e. 53%, or 67%, the unloaders are partially open during the compresing stroke. This way part of the displaced gas is not compressed. Simply gets out back of the cylinder. Reverse flow control.
So it ain't quite a matter of volumetric efficiency as per variable volume pockets. You directly save power.

The way to control the valves is through an interface to the DCS. And the procedure to control and the logic is by vendor. No way demanding curves to asses where to actuate.

Finally, FYI, I've been informed that due vibration and metal to metal contact, so far is not possible to use HYdrocom for cryogenic applications. Unloaders' valves can not be SS.

Thanks for your answer, again

Regards from spain