A loop or system diagram should be provided for every instrument loop (control, indicating, protective, etc.) to illustrate its operating philosophy. This is an extension of P&IDs, which shows the components and accessories of the instrument loop, connections between devices, cable, junction box and terminal numbering codes, and identification of component action.
As a minimum, the loop or system diagram should contain the following information:
- Identification of the loop and loop components.
- Indication of interrelationships to other instrumentation loops, including overrides, interlocks, cascaded set points, etc.
- All point-to-point interconnections with identifying numbers or colors of electrical cables, conductors, pneumatic tube bundles, and individual pneumatic and hydraulic tubing. This identification of interconnections includes junction boxes, bulkheads, ports, and grounding connections.
- General location of equipment such as field, panel, rack termination cabinet, etc.
- Energy sources of equipment such as electrical power, air supply, and hydraulic fluid supply. Identify voltage, pressure, and other applicable requirements. For electrical sources, identify circuit or disconnect numbers.
- Actions or fail-safe positions of devices such as switches, control valves, and solenoid valves.
- Identification of all auxiliary components used to make up the full functionality of an instrument loop should be shown even when the equipment has been designed and provided in whole or in part by different Vendors/Contractors and is detailed across more than one P&I diagram.