Can Someone share about Specification of design Metering and Regulating Station

In design plant what part to be more specifict and detailed, and when we using PRV, PSV, Flare, or Vent Stack

Please upload tutorial example for sizing and calculating of FCV/PCV in Gas (Natural gas) that this Control Valve can be operate in main Fungtion to Control Pressure, but if needed function to Control Flow the Control Valve can change switch sensor from Control Pressure become Control Flow