Presuming there is no process safety function (i.e it is purely for process controls and it is not meant for ESD depressuring either), I'd go for FC since the hazard imposed by a PCV to flare failing full open are much worse than if the valve were to go FC (flow over load to flare system; high liquid entrainment in vapor stream to flare).
Besides, you've probably got other safety trip loops to address high pressure events on this separator, and you'd most likely want this PCV to go to close position during an ESD, given that one of the causes for an ESD could also be low pressure on the plant main instrument air header ??