What is a P&ID?
A. The acronym stands for Piping and Instrument Diagram. This is a document used to show a graphical display of a complete system or process with-in a plant. A system is a self contained portion of a manufacturing plant.This would include all piping, instruments, valves and equipment that make-up that system. An example might be the fuel gas system with-in a refinery.
B. The P&ID shows primarily the mechanical parts of the system. There is some electrical detail shown because of the process controls schemes which are depicted;such asthe controllers, loops, etc.
C. The P&ID is generally revised by the process or controls engineers before the construction documents are revised or generated.
What detail does the P&ID provide?
A. As stated previously, the P&ID provides primarily mechanical details of a given system or process. The system is depicted in a one line or schematic view. This means it shows simply the way the system is laid out or connected and does not provide any type of construction or installation detail.The electrical detail that does appear is for the control loops. This detail simply shows the logical flow of the information in the loop.
B. You will see piping information such as sizes and material specifications.
C. You will see continuation flags leading to the connecting P&ID’s.
D. Flow arrows are shown.
E. DCS alarm types are shown.
F. The connection types from the controllers to the field devices and device to device are shown.
What are the industry standard symbols?
In the instrument and controls business, we use the ISA standard. ISA is the standard which most companies will attempt to use but it should be noted that many companies have their own symbols that are used along with ISA. Because of this, the symbols may vary somewhat client to client.
ISA addresses specific symbols to be used to denote various types of devices,connections and device locations,
A. DCS/PLC (Controllers)
B. Local Field Devices
C. Panel Mounted Devices
D. Connection Types
E. Valves Types
H. Device Tagging Convention
These are the ISA symbols for
- The DCS controller or console display.
This symbol represents both the physical DCS cabinet or the DCS software point. In either case , it implies operator console control from a control room.
- The PLC controller or console display.
Again, this symbol represents both the physical cabinet or software point and implies console control.
A PLC or Programmable Logic Controller is similar to a DCS(HPM) in that it can be used for both analog and digital controls. It differs in that it does not have the point capacity nor display interface capabilities of the DCS.
You will see these symbols anytime a control loop is shown on the P&ID.
The symbol on 2 is for field mounted instruments and devices. By field mounted we mean devices that are physically located in the plant adjacent to equipment or piping runs and devices in-line of the pipe. Examples would be transmitters, switches, local indicators,valves,thermocouples, etc.
The symbol on the right is for a local or field indicating light. Notice this is still a circle meaning “Field Mounted.”
These symbols are sometimes referred as “Bubbles” when addressing the P&ID. These circles represent the location and tag number for devices
These symbols3 6 represent field devices, but those mounted in panels. Since both symbols are still circles, we have field mounted. The line or lines through the circle imply panel. As you will recall, the DCS symbol was a circle with the single line but it had a square around it. From this we could deduce that the DCS symbol means primary panel mounted also. This is correct, but the addition of the square means shared display or control console.
The circle with a single line would be quite common in a controls system where pneumatic displays are still used. Pneumatic displays were typically brought in to the control room and mounted on large panels for operator convenience.
The circle with two lines represents a panel located in the field. These are common when you want your field operators to have a single location to view multiple displays.
Here we see three of the impulse lines or connection types. These symbols represent the most common connection types you will probably see.
Equipment symbols are not addressed by ISA and as such it is the responsibility of the user to define these symbols. Generally, the client will have equipment symbol libraries they have previously developed and you would use their symbols.
How to Tag Instrument
ISA follows a very specific format for the development of instrument and device tag numbers. We will see in the following tables how tag numbers are developed.