Operating pressures of multistage separation systems can very across the wide range. The maximum OP for of a separator will be no higher than sales gas pressure accept in the case of gas lift requirement where in the pressure is higher than sales gas pressure.
In case of intermediate stages the gas will need compression therefor optimization of intermediate separator can be derived from equation below:
R = (Pd / Ps)1/n
where: R = Ratio per stage
Pd = Discharge pressure
Ps = Suction Pressure
n = number of stages
The minimum pressure for the lowest pressure stage can be between 20 to 50 psig. This pressure should be set such that there is enough suction head for the downstream pumps.