Where plant numbers are used, they should be incorporated into equipment numbers, and, often, into the instrument numbers. Where plant numbers are not used (such as for offshore platforms) many prefer that the instrument numbers relate to uniquely numbered equipment. This method is used on platforms built to API RP 14C to associate safety devices with the equipment they protect. Some facilities incorporate a plant number into the equipment number and associate instruments with equipment.
Most facilities also use alphanumeric systems with a letter prefix that indicates the type of equipment involved. For instance, the prefix MAF designates a 7-tray glycol contactor for an offshore platform (see API RP 14C). The same equipment in a downstream major processing plant would be denoted by a C for column.
The prefix system allows the number series to be restarted for each type of equipment, so that numbers can usually be limited to two digits. Equipment is numbered serially or in decade steps for major equipment. Thus, for plant 20, three sequential columns might be numbered 2010, 2020, 2030. P-2021 might be the reflux pumps for column C-2020. Skipped numbers are acceptable in a system such as this.