Difference between a PFD and P&ID

Both PFD (Process Flow Diagram) and P&ID (Piping/Process & Instrumentation Diagram) are chemical / process engineering drawings. These drawings are very useful as they convey the right amount of process information as needed during various stages of bidding, engineering design, procurement, construction, operating & commissioning phases of the process.

Process Flow Diagram (PFD)

A process flow diagram shows the relationships between the major equipment’s (columns, vessels, pumps, turbines, heaters etc) in the system. This diagram shows flow of chemical fluids and the equipment’s involved in the process with the properties of flowing chemical fluids (like temperature, pressure, fluid density, flow rate etc). Generally, a PFD does not show minor components (like pressure, temperature and flow instruments), piping systems (like control loops, by-pass lines, drain lines etc), pipe properties (like size, spec, ratings etc) etc.

PFD gives an overview of the complete chemical plant processes. PFD is a simplified version of P&ID’s. Process engineers are responsible for designing PFD based on the chemical process and thermodynamic properties of the fluid in question.

Piping / Process & Instrumentation Diagram (P&ID)

A Piping and Instrument Drawing (P&ID) includes more details than a PFD. It includes both major and minor details of the chemical process. It show all major equipment’s, piping details (like service, size, spec, rating, insulation etc), instrumentation details (like all the pressure, temperature and flow instruments, control valves, pressure safety valves, meters etc) and the actual pipe routing conditions (like free flow, slope, minimum distance, tapping from top/bottom requirements etc).

P&ID is a detail form of PFD. Process engineers, with coordination of piping, instrumentation, electrical and safety engineers, are responsible for designing P&ID’s. P&ID plays as the basic documents for piping engineers for purpose of material procurement and deciding of pipe routing.

A P&ID can be best understood with the help of a legend sheet given by the P&ID developer. Legend sheet contains information regarding the nomenclature, tag philosophy, symbols, a lot more.

PFD include,

1. Main equipment
2. Main process piping
3. System ratings and operational values ( pressure, temperature, flow, density)

P&ID include,

1. Pipe line number, size, material, insulation
2. Every instrument and its pneumatic/ electric signal
3. Detailed control loops
4. Utility lines, drain and vent lines, sampling lines etc
5. Parallel and spare equipment’s and connected piping
6. Isolation and shut off valves.

In both kinds of drawings (PFD and P&ID), arrows indicate the flow of fluid and various symbols represent pumps, tanks, valves, and other equipment’s. The symbols used vary somewhat from organization to organization.