To prevent overcrowding and a confusing process flow, 25% to 50% of the space on drawings should be allowed for future equipment. Disorderly P&IDs may impede the design process, and can be a liability during plant upsets—when quick comprehension is important.
D-size (22-inch by 34-inch) drawings are commonly used for P&IDs because they are a manageable desktop size; however, some systems would be seriously overcrowded on a single D-size drawing. For a “grouped” P&ID (see Section 222) a longer R-size (28-inch-by-unlimited) drawing ensures that all closely related processing equipment is included on the same P&ID. The longer drawing may be avoided by separating stand-alone process, utility, or package systems and placing them on their own major equipment or auxiliary P&IDs.
Arrangement of Elements
The initial arrangement of each process P&ID is submitted for owner/operator approval. These P&IDs include equipment, piping and instrument manifolds, instrument symbols (or reserved areas for them), piping runs (or reserved areas—horizontal and vertical), auxiliary systems and subsystems.
Arrangement of equipment and piping should follow a sequence that flows logically across the sheet from left to right; for example, feed comes in on the left, products go out on the right. The main flow lines should be heavier than secondary process lines and utility lines, and should not double back. Lines should be spaced evenly, with a minimum of lines crossing. In general, the P&ID should be kept readable.