What is a P&ID?

P&ID Introduction?

If you’ve ever asked, “What is a P&ID?”, you’re not alone. A Piping and Instrumentation Diagram (P&ID) is a fundamental document used to represent how fluids or gases move through a site’s process system. It outlines the connections between piping, equipment, and instrumentation—acting like a functional map for industrial sites such as oil refineries, chemical plants, and storage terminals.

Unlike a floor plan, a P&ID doesn’t show the physical layout. Instead, it focuses on how things work—how materials flow, how control systems are configured, and where safety devices are positioned.

These diagrams are used daily by process engineers, site operators, and maintenance personnel to:

• Visualise flow paths and system controls
• Diagnose faults and plan maintenance
• Ensure operational safety and regulatory compliance

Should Every Site Have P&IDs?

In short—yes. Any facility running pipelines or processing materials is expected to have accurate, up-to-date, as-built P&IDs.

But after 25 years in the field, I’ve learned not to take that for granted. Often, when a client says “we have P&IDs,” what they actually have is:

• Outdated drawings, sometimes decades old
• Hand-sketched diagrams taped to control room walls
• Inconsistent drawing styles across departments
• Manufacturer-provided documentation that doesn’t reflect site changes
• Or confusion between P&IDs and PFDs

Another common issue: instrumentation is often missing—especially remote sensors or transmitters—leaving the drawings incomplete and out of sync with what’s actually on site.

These gaps aren’t just frustrating—they can be dangerous. Poor documentation is a major compliance risk and can make fault-finding during downtime significantly harder.

What’s the Difference Between a PFD and a P&ID?

It’s easy to confuse the two, but they serve different purposes.

Process Flow Diagram (PFD)

A PFD provides a simplified, big-picture view of the process. It shows:

• Main process equipment (tanks, pumps, exchangers)
• General material flow paths
• Basic temperature, pressure, and flow data

These are typically used during early process design or feasibility stages.

Piping and Instrumentation Diagram (P&ID)

A P&ID, on the other hand, offers a detailed representation of how everything is connected and controlled.

Expect to see:

• All process and utility piping (with diameters, specs, and routing)
• Valves, vents, drains, reducers, and connections
• Instrumentation tags, control loops, and interlocks
• Relief systems and safety components

It’s the go-to document for anyone operating, maintaining, or upgrading a process system.