I'm not a english native speaker but I'll try explain. We've several gas line, straight ones that has points where we got a psv valve, then we expericed a reduction of gas volume, that's is what we need to calculate and, also need that formula.
I dont know what this is for - but if ts for a real system then i think you should be more carefully in your work. While a internet page might help you "refresh" your mind a little you questions seems very basic and design of PSV's and flare system is VERY important stuff both from a safety and from an operational point of view.
How will you document this? You should see more professional help (inhouse or from an external consultant).
