For additions, the existing instrument numbering system is usually extended to include new instruments. For new construction, the following methods are available:
• Functional orientation. This system was developed for large distributed control systems. Its restricted identifier size accommodates electronic databases. It is organized as follows:
– 100 to 899. Except for safety relief devices, major instrument loops take their identifiers from this block. An alphabetical suffix is added where more than one of the same component is present in a loop, as with split range control valves. Temperature points that are part of the control display system use this block
– 900-999. Safety relief devices, relief valves and bursting disks have 3-digit groups from this block
– Four-digit groups. Minor instruments have 4-digit identifiers
• Hardware orientation. This system was developed when individual controllers were mounted in control panels. Seven blocks of numbers are used:
– 100 to 399. Loops monitored or controlled from the control center
– 400 to 499. Field controlling, recording and indicating loops, including dial thermometers
– 500 to 599. Field contacts for alarms
– 600 to 699. Pressure gages
– 700 to 799. Level gages
– 800 to 899. Board temperature points, including test wells
– 900 to 999. Relief valves and bursting disks
• Major equipment orientation. This system provides much information to the plant operator. However, it is unwieldy and is not recommended except where already used
