P&ID and the ISA 5.1
Before we start talking about P&ID and the ISA 5.1, let me clear up a possible point of confusion. This article will address piping and instrumentation diagrams (P&IDs). If you expected PID control, we have an article about that over here.
I know! Two very similar acronyms for two completely different things. I got a bit confused myself in the beginning. However, both are widely used and important concepts to master in the process automation world.
Now that we’ve cleared any misunderstanding, let’s see what a P&ID actually is and why we should know it, shall we?
What’s a piping and instrumentation diagram?
I still remember my first day on an internship at a biotech company. They had processes to treat wastewater and produce biogas. When I entered the engineering department, I saw this huge whiteboard covered in symbols and notations. And people looked at it a lot. I mean, a lot.
That’s because this whiteboard held the P&ID of the project we had to work on. And a few weeks into the internship, I realized why people kept looking at it over and over again. It gave you a detailed overview of the whole process. In the first week, before I familiarized myself with the process, I found myself staring at this diagram several times a day.
A P&ID is a graphic representation, using standard symbols and annotations, of the piping, instrumentation, and system components in your process. It plays a big role in the management of a physical process.
In the planning stage, the P&ID shows you the whole process before anyone installs a single storage tank or field instrument. And after the process is running, it can help with maintenance, plant safety, and training. It was my best friend in the first month of my internship. I came back to it thousands of times so I could better understand the process and the field devices used in it.
ISA 5.1 – the standard
Not all P&ID elements are standardized, but the instrumentation symbols follow a standard set by the International Society of Automation (ISA).
The ANSI/ISA’s S5.1-1984 (R 1992) standards help an engineer communicate instrumentation, control, and automation goals consistently. This consistency makes it easier for other engineers or technicians to understand the process and all its pieces without needing to talk to the engineer.
The ISA 5.1 defines four elements with four shapes:
- Discrete instruments: circle
- Shared control/display: circle in a square
- Computer function: hexagon
- Programmable logic controller: triangle in a square
And each shape will also have standardized letters to represent the devices. The first letter tells you what kind of measuring device it is. The next letter can either modify the first letter or tell you the function of the device.
You’ll probably find numbers with the letters too, to represent device tags. No international standards exist for these, but they may follow company standards.
For example, if you have a PT04 inside a circle, then you have a pressure transmitter in that section of the plant, designated 04 for reference.
The letters make more sense than the symbols, so even if you don’t know the standard, you can guess what things mean from context. This table will help.
Okay, now we know about the devices. However, the ISA 5.1 goes beyond that. It also tells you where this instrument is installed.
In some of the shapes, you might see lines crossing them. If you don’t see a line, then that device is installed locally in the field. If you see a solid line, you’ll find the instrument on a main panel or screen. A double solid line means this device is on a subpanel or remote location. And a dotted line means this device is inaccessible, hidden, or installed behind a panel board.
Besides all the measuring instruments, you have standardized symbols for other parts of your process.
These symbols fall into six categories:
These symbols represent a variety of hardware which won’t fit in other categories, like compressors, conveyors, motors, turbines, and vacuums.
These symbols will tell you what kind of fluid flows through those pipes. You have different symbols for electrical, pneumatic, hydraulics, and so on. You’ll also find pipe elements such as reducers, caps, and flanges.
Vessels store process material. There are many types of vessels such as tanks, cylinders, columns, and bags.
In this category, you’ll find boilers, condensers, and other heat exchangers.
Pumps control your process fluid flow. Depending on your installation or even your process fluid, you’ll need a specific kind of pump. Here you’ll find all kinds of pumps as well as some fans.
Valves guide and regulate the fluid process. You can use them to build bypasses or control the flow of your process fluid. The market has a wide variety – ball, butterfly, needle and more.