A P&ID is a schematic drawing of a process plant system. It has no relationship to scale, relative dimensions or true shape or character. It has no relationship to north, south, east or west. It has only a marginal relationship to up and down. However, through the use of standard symbols, line conventions and notes a level of understanding must be achieved and communication must happen so that what is shown on these P&ID drawings in a schematic way is converted to reality.
This document is intended to provide various members of the Piping Department with suggestions of what to look for and what questions to ask during a P&ID (Piping and Instrument Diagram) Review. The three key Piping entities that should be present during a P&ID Review are:
Piping Material Engineering – Focus on all data and comments that will impact the development and or modification of the Piping Material Line Class Specifications, The need for and the details of “SP” Items, Insulation Specifications, etc.
Piping Design – Focus on all data and comments that relate to the actual routing of piping systems, including process requirements, operations, maintenance and safety.
Pipe Stress Engineering – Focus on all data and comments that relate to factors that impact pipe stress including temperatures for both normal operations and any upset condition that might occur.
Each of these individuals (and groups) will have distinct but interrelated duties and responsibilities during the ongoing Detailed Design Phase of the project that will follow the P&ID Review. The person who will represent each of these groups (or functions) should be provided with (or obtain) ahead of time a copy of all diagrams to be reviewed. These copies should be studied and marked with questions and or comments before the review. To be prepared is to be forearmed.
Here is something everyone needs to remember. A P&ID is a schematic drawing of a process plant system. It has no relationship to scale, relative dimensions or true shape or character. It has no relationship to north, south, east or west. It has only a marginal relationship to up and down. However, through the use of standard symbols, line conventions and notes a level of understanding must be achieved and communication must happen so that what is shown on these P&ID drawings in a schematic way is converted to reality.
Areas of interest and suggested questions or items of interest:
P&ID Lead Sheet:
Does the project have a P&ID Lead Sheet? If possible the review should start with the Lead Sheet that lists all the common General Notes and may include a list of all P&ID’s and Utility Diagrams for the Unit or Project depending on the project size. It should have project applicable acronyms with definitions.
Are all the Notes clear and understandable?
Are all the notes applicable to the project?
What else is on this sheet and what does it mean to this project?
P&ID Symbology Sheet:
Does the project have a P&ID Symbology sheet? If there is not a Symbology Sheet, then ask why not? If there is a Symbology Sheet:
Are all the symbols consistent with the ISA standard and known to all present?
Are there any new symbols required for this project?
What are these new symbols and what do they mean?
P&ID Standard Detail Sheet:
P&ID’s on any project contain many situations that are repetitive and very detailed. Many times there are details that are similar but have small differences. These details, if repeated in full detail at every occurrence can cause congestion and confusion to the person reading the P&ID. So a full detail is drawn once and then a smaller and simpler symbol is placed at the use point.
These common details might include:
High Pressure Steam Trap Station – with bypass
High Pressure Steam Trap Station – without bypass
Medium Pressure Steam Trap Station – with bypass
Medium Pressure Steam Trap Station – without bypass
Low Pressure Steam Trap Station – with bypass
Low Pressure Steam Trap Station – without bypass
Utility Station Requirements – for Grade
Utility Station Requirements – for elevated platforms on Vessels
Utility Station Requirements – for at Heaters
Safety Valve (SRV, PSV, PRV, etc) Manifold with bypass
Safety Valve (SRV, PSV, PRV, etc) Manifold without bypass
Steam-Out Connection – Hard piped, with double block valve, spec blind, check valve and bleed
Steam-Out Connection – Block valve w/ blind flange, swing ell to valved steam line
P&ID Sheet Format:
P&ID formats are normally fairly standard and consistent with-in a specific company. However the Client may ask for special additions, deletions and or modifications that are new and different for a project.
Is there anything new and or different about the P&ID Format for this project?
If so, what are they and what do they mean?
Are there additional “Notes” for a specific P&ID in addition to the “General Notes” on the common Lead Sheet?
P&ID Equipment Review:
The review for each type of equipment and each piece of individual equipment should follow a consistent pattern. If the review does not follow a pattern then things get forgotten.
Is each equipment type and piece shown by a standard and recognizable symbol?
Is this symbol consistent with what is known about the actual planned equipment?
Is the equipment identified?
Is there an appropriate Data Box at the top (or bottom) of the P&ID for this type of equipment?
Is all the Data Box information completed?
If the Data Box information is not completed, when will it be completed?
For trayed vessels, are the trays at all side entry nozzles numbered?
For Shell & Tube Exchangers, is the correct number of shells shown?
For Air Coolers (Fin Fan Coolers) is the correct number of Cells and Cell Nozzles shown?
For Air Coolers, does the inlet and/or outlet headers need to be; Balanced Flow? Symmetrical Flow? Other?
Line by Line Piping Review:
Piping on a P&ID is, as noted above, schematic. However, there are certain absolutes that rule the interpretation of the pipe lines. These rules apply to the sequence of appearance of the valves or other symbols on any given line. With only a few exceptions what the P&ID shows that is the way the finished physical piping shall be installed.
As you review each line you should look for the following:
Does the line originate and terminate on the same sheet?
If the line originates or terminates on another sheet does it have connectors? Do both connectors for the line match?
Is there a Line Identification (Line Number, Line Designation or other) for the line?
Is the commodity known?
Have the Design Conditions been set?
Is the maximum operation pressure known?
Is the maximum operating temperature known?
Is the line size known and shown?
What is the specific function for “This” line?
Are all special process required line conditions (Line Slope, Do Not Pocket, Free Drain, etc) shown or noted?
Are all valves shown?
Are the valve type symbols consistent with the actual valve type in the Piping Line Class Specification?
What is the purpose and function of each valve?
Is there a size on each valve? Or are all valves to be assumed to be line size unless noted otherwise?
If there are Line Class Spec Breaks are they on the correct side of the valve?
Are all Instruments in or on a line shown?
If a Pressure instrument and a Temperature instrument are in close proximity (10 pipe diameters or less) to each other is the temperature instrument downstream (easy way to remember is alphabetical order) of the pressure instrument?
When reducers are shown does process really want a reducer or are other common methods of pipe size changes (reducing TEE branches from a common header) acceptable?
Is the term or note “Min” used and what is specifically needed and or to be avoided?
Are all Process vents and drains shown?
Are there any odd mechanical device requirements that are not in (or will not be in) the normal Line Class Specifications? Are these items identified as “SP” Items and details available?
Utility Distribution Sheet Review:
Utility Distribution Diagrams are also schematic drawings but they differ from a P&ID in that they are somewhat Plot Plan oriented. This is required and done so the supply and return headers can be properly sized.
Utility Distribution Diagrams may have all services on one sheet or may have the services grouped by some “family” breakdown.
Steam Utilities may include:
High Pressure Steam
Medium Pressure Steam
Low Pressure Steam
Medium Pressure Condensate
Low Pressure Condensate
Water Utilities may include:
Cooling Water Supply
Cooling Water Return
Potable (Drinking) Water
Air Utilities may include:
Miscellaneous Utilities may include:
Specific items to question are:
Is the Utility Distribution Diagram plot plan oriented?
Are all Utilities shown?
Are the branches from the Utility headers in the correct sequence?
Are the sizes for all the branches all known and indicated?
Are the main Utility header sizes correct for the number, size and sequence of the branches?
Is the number of Utility Stations correct?
Are the Utility Stations numbered?
Are the Utility Stations schematically located correctly?
For Steam Headers, are steam trap manifolds shown at the correct locations?
Do the connectors from the Utility Distribution Diagram match the connector on the P&ID?
Is there a Line Identification (Line Number, Line Designation or other) for each line?
About the Author
James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.