Please read your question again and ask yourself "if someone asked me this, what would my reaction be?" We don't know anything about your field (oil?, gas?, gas and condensate? oil and casinghead gas? much water? what temperature? are the wells pumped or free flowing? etc). Even people who have been doing this stuff for decades might struggle with your cryptic question (I do not have a clue what a "PS separator" might be)
All of these questions have an impact on the pressure you want to run the test separator at. Bottom line, you should ask the reservoir what backpressure maximizes production.
This appears to be your first post, but you've managed to annoy someone who has a wealth of knowledge and is very helpful if you had actually read his reply and responded to his request for more information form you. Remember we know nothing other than what you tell us so less than one line of text and no drawing isn't going to be a lot of use. This is like someone asking you out of the blue "how much oil is my well going to produce?" when you know nothing about his well, its history, what pressure it is at etc etc - See what I mean?
On this site if you respond in a civil and professional manner, then others will treat you with respect - if you don't then they won't. Think of us like talking to your supervisor / technical manager in your own company or organisation.
If you can't frame your question in English then there is help on this site or better still just draw / sketch things and add numbers - that's fairly universal....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
