In the realm of instrumentation and control or automation, a P&ID is a schematic illustration of the functional relationship of piping, instrumentation, and system equipment components. Engineers who are creating a production process for a physical plant are often the ones who come up with them. In the upstream oil and gas business, three-phase separators are routinely used to separate oil, gas, and water streams coming from oil wells. This standard P&ID for a three-phase separator can be tweaked and applied to other separator vessels. First and foremost, as illustrated in the diagram, the proper vessel symbol should be chosen. This should be chosen from a list of equipment symbols found on a project's legend sheets.