Rakuday,
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"centrifugal compressors are constant pressure, variable flow machines"
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As already mentioned, this is not correct.If you are on production you would maintain a given constant pressure against huge variation of suction pressure, MW, and flow whatsoever. In a downstream environment, the discharge pressure can be floating (e.g refineries process) whereas in this case the suction conditions are well known. That really depend on the application.
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In VSD compressors, CV’s are generally not used as the motor can control the capacity. In some cases where capacity modulation is needed below 65% of the rated capacity, then CV & VSD can be used together. As you have rightly mentioned in your second post, “the variable speed unit "could" be operated at lower flow, same head condition”.
As seen in the performance curves of centrifugals, above the design point, when flow reduces, the pressure increases and the machines will surge, in such cases the CV’s are modulated to stabilize the compressor.
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Control valve are there anyway, at least for anti-surge protection, and quite commonly are used nowadays for continuous recycle except where special considerations apply (heavy duty recycle, etc) imposing some further checks or whereas an operator does not want recycle and anti-surge protection done by the same valve. On some situations you would omit completely without the control valve (hydrogen recycle service is an example). So it is not only a matter of VSD or not VSD.
