P & ID and PFD- what is the difference? 3

A PFD (Process Flow Diagram) describes the basic flow through a system. This is a generalised document.
A P & ID (Sometimes called either a Piping and Instrumentation Diagram, or a Process and Instrumentation Diagram) illustrates all details related to the control of flow through the system. This would show all pumps. valves instruments, in-line equipment, vents/drains and control loops for a system. It should also provide details of the equipment illustrated on the drawing - ie; as a minimum, tank volumes (total and active), Design & operating temperatures, Design and operating pressures. Pumps, - type, motor power & details, discharge flow and pressure, exchangers - type, max & min oper & design temps and pressures for hot and cold sides etc. All piping diameters should be shown, as should the piping materials or a reference to them, insulation & heat tracing if required,

There is little that can be added to the excellent description by quantas.

PFD's nowadays also contain mass and enthalpy balances. These are shown by tag-numbering the different streams, process and utilities, describing, in tabulated form, all their flow rates, relevant physico-chemical properties, composition, states of aggregation quantified, and the operating parameters.
If the plant is meant to operate on various feedstocks, all cases, or those considered critical or extreme, are presented separately.

In P&IDs, instrumentation hookups, including analyzers, are detailed, also in cases of cascading or any other interaction among instruments. Lines and equipment, not used in normal operation are fully described. Elevations of lines and vessel internals, if critical, are also detailed.
Details are so many, that frequently P&IDs are divided in several sections (pages); for example, it is justified to dedicate one page when dealing with a process heater, and another page when describing an LPG holding and distribution center. At every page border, the continuation of each line on other pages is duely shown for easy finding.

A good reference for your question is in "Piping Handbook" by Nayyar. Not only does this book tell you what is in a P&ID and PFD, it also discusses what should not be in them.

P&MD's (Process & Mechanical drawings, but usually called P&ID by convention)should be added to the list. Similar to P&ID's but with less instrument detail.

This is a very important subject to all engineers involved in process work, design or operations.

The P&ID (by the way, major E&C companies will confirm that this means PIPING and Instrumentation Diagram -not process) is the document of record with OSHA in their PSM program. The P&ID is based on the PFD (Process Flow Diagram), which is the representation of the scope of work and a formal heat and mass balance that is to be used as the design criteria for the process in question - especially the piping. The PFD should serve as the basic data source for the mechanical design of the process. This, in essence, ties both documents together in a logical sense.

The P&ID contains, as has been stated previously, detailed design information and sizing AS WELL AS LOGIC. Do not fail to consider that such is the importance of the P&ID that it is the basic document on which a formal HAZOP is based -and without which a process cannot be started up. Some major companies (Dupont, Carbide, DOW, BP, Shell, etc. demand that all released P&ID's be formally sealed and signed by professional, registered engineers to confirm the accuracy and veracity of the contents.

Additionally, I would remind all that OSHA requires that the P&IDs be absolutely accurate in their description of the process that is being operated by any company in the USA. This makes the P&ID a legally-important document and one that cannot be dealt with in an uncontrolled fashion. All pressure relief devices, for example, MUST BE SHOWN and identified in the P&ID or OSHA will have your hide.

Many contractors and operating companies use the P&ID as the means to transfer project construction instructions and details to their field crews. Therefore, these documents must be controlled and very, very accurate.

In short, its importance to a design or E&C company or to an operating company is paramount and may mean the difference between an operating permit or a trip to the jail house. It is a very serious document these days.

Art Montemayor