Piping and Instrumentation Diagrams (P&IDs)
P&IDs are used throughout the industry to show the relationships between equipment and instrumentation in the system. They can also show other information to enhance clarity, such as how ammonia flows through the system. P&IDs are part of PSI, and while there are many variations of these diagrams, their purpose is the same as other diagrams in the system: provide information to assist the operators in performing their duties.
Content and Complexity
Computer software programs are often used to design P&IDs, which means there is a good chance you have the software and the ability to edit the drawings within the company. The common question here is “What do I have to include?”
The layout of P&IDs is not standardized throughout the industry – engineers are free to document their designs in any way the customer sees fit. Two common RAGAGEP citations for P&IDs are the CCPS Guidelines for Process Safety Documentation and the ANSI ISA S5.1 Instrumentation Symbols and Identification. P&IDs should contain basic design information such as:
- Vessels and other process equipment with size or capacity shown
- Lines (including size) used for the process (piping) and its associated utilities
- Instrumentation and its function
- Tag or equipment names and numbers
- Failure modes of automatic valves
- Set pressures for relief valves
- Other useful information needed to understand the process design
This P&ID will at least show the equipment’s name, pipes, expected direction of flow, valves, controls and a method of showing revision dates. It may also include safety systems, model numbers, serial numbers, capacities, etc.
Since valve designations and line designations are not standardized within the industry and vary by engineer, you will often find completely different systems of presenting P&IDs depending on who does the drawings. This can present challenges if the scheme is not standardized at your facility.
Imagine the line designation CD means Condenser Drain on one drawing and Compressor Discharge on another. This could lead to confusion when attempting to trace lines or troubleshoot system operation. This is the importance of standardization – there isn’t one correct way to document this information, but you have to pick a way and ensure all your P&IDs conform to it. Doing so will simplify your operator training and help minimize confusion when referring to the P&IDs.
Be cautious of trying to document too much information on your P&IDs. For example, some facilities document the date of installation for their relief valves. While there is nothing inherently wrong with this, it will require you to update every P&IDs with a relief at LEAST every five years when the valves are replaced.
Minimizing confusion is also a high priority – each piece of equipment should have a unique name within the system. Under no circumstances should two pieces of equipment be called the same name without adding some sort of identifier. For example, having two vessels called “Receiver” is unacceptable, but referring to them as “Receiver #1” and “Receiver #2” or “New Receiver” and “Old Receiver” would be fine.
It is important to understand that in a compliance inspection, your P&IDs will be checked for accuracy. P&IDs that do not accurately reflect the process pose a threat to safely operating the process and have led to many incidents. Because of these past incidents, inspectors are very keen to check the P&IDs against the actual equipment and piping in the facility.
Format and Distribution
One common way to provide P&IDs is on large 36×48 sheets with groups of equipment. Another, perhaps simpler, way is to split the system into individual pieces of equipment and place each on an 11×17 sheet of paper which allows operators to take the drawings out into the field with them. These individual sheets are bundled together in a single pack, prefaced by title sheets that contain information applicable to the entire group. When organized in this way there will be title sheets and then as many P&IDs as necessary to document the layout of the system.
The title sheets will include a key to following process flows, a list of the equipment covered and a method of showing revision dates, a key showing various line designations and their associated drawing colors, a table providing an explanation of the abbreviations used in naming both the piping and the valves in the system, an explanation of the pictograms used to depict valves within the drawings and a method of showing revision dates. Colored lines can make the P&IDs easier to follow. If the actual color of the pipe in the system does not match the drawing, make sure to note that the color of the line is for “illustrative purposes only”.
When the lines leave the page to go to other pieces of equipment it is important you explicitly state where they go. The arrows indicating flow on pipes denote only the expected direction of flow during normal operations – stress this in your operator training as flow direction can change during upset or maintenance conditions.
Since processes change it is necessary to have some sort of document revision control such as a version number or “last updated date” on your P&IDs. It’s good practice to record and track these changes with some sort of revision or certification document. This gives people using the program one place to go to in order to check the version of their documents.
When designing or modifying P&IDs, be sure to customize them to the individual needs of your facility while gathering input from the operators. The complexity of P&IDs can be overwhelming to a newer engineer or operator, so it’s important to remember the simple goal of P&IDs: A tool to be used in the operation of the system.
