Assignment of instrument numbers must be coordinated with all design agencies that are developing P&IDs or subsections of P&IDs. A unique identification for each instrument is assigned in accordance with ISA Standard S5.1 (see Appendices), as follows:
Reading left to right, the first element of the code is the plant number. This is normally omitted on the P&IDs, but it is included in the instrument number for other purposes such as ordering, I. D. tags, etc. The letters that follow represent the instrument type according to ISA Standard S5.1. The final element is the loop identification common to all instruments and components in a loop. If possible, loop identifiers should be in order of their positions upstream in the process flow; that is, feed loops have the lowest numbers, and product and final effluent loops the highest.