Piping and Instrumentation Diagrams (P&IDs) are not just for designing a plant; they are kept up to date through all the modifications, redesigns, and revamps of the plant. It's called a "living document" - the drawings always reflect reality in the plant.
When planning a revamp or modification of an existing plant, you are going to alter a set of drawings to add "what will be" onto what is already there. You need a way to keep track of where changes are being made on the drawing: what lines are being cut, what equipment added or replaced. The tie point is a tool for that. Normally a hexagon symbol with a "tie point number" in it, the tie point shows where a change is being made. Alongside it, the tie point list explains the details of each tie point on the drawings, and where to find them.
That way, when it comes time to do the actual work, there's a practical list of what needs doing where.
But as a person marking up P&IDs, when do you add a tie point and when don't you? Hopefully you work in a company that has procedures, etc, but if not then here's what I do:
Tie point rules of thumb:
- Any small changes that affect a single point gets a single tie point. Like the addition/removal of a valve, expander, or tee, warrant one tie point
- OPTIONAL: some people prefer to do "TP-101A" and "TP-101B" on both sides of these small changes
- Adding or removing blinds (small plates that block a pipe off) warrant a tie point, even though no piping is being cut. It's still important work being done on the piping system.
- Large additions, like inserting/removing equipment, adding a lot of piping small elements in a row, or changing "long" sections of pipe require tie points at each end of the alteration. (So this could be two tie points, or more)
Changing instrumentation and control, like adding, removing, or re-routing a control signal, does NOT require a tie point
In addition to adding tie points, sometimes it helps to distinguish the new lines and equipment in some other way. Like bolding the new lines, or coloring them, or using a separate "layer" in the drafting software for all changes.