P&ID Basics: Avoid Common Mistakes in Reading a Piping & Instrumentation Diagram (Part 2)

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This series of videos will help you significantly in getting better at reading as well as DESIGNING P&IDs.

In the previous episode (Part 1), we discussed the P&ID symbols present in ISA5.1 and commenced a practical example of a Piping and Instrumentation Diagram.

I showed you how to demonstrate the electric motor winding temperature sensors and their transducers in the P&ID. We’re not done with this example, yet!

Watch or Read Part 1 over at ▶ https://upmation.com/piping-and-instr...

In this video, we will continue and complete the example we have started in Part 1.

As the Ball Mill electric motor is controlled by a Variable Speed Drive or VSD, I decided to connect its temperature transducers directly to the VSD. Therefore, I will do so in the P&ID and first consider the VSD and then connect the transducers to that.

But I also want to send the signals from the motor's transducers to a PLC card to show the real-time temperature of the motor to the operators in a monitoring system or let’s say Human-Machine Interface. I will send them via the network between the VSD and PLC.

In this way, through the Human Machine Interface or HMI, if the operator opens the related pages for the Ball Mill motor, he/she should observe the motor winding temperature indicators.
Inside the P&ID, I use a “circle-in-square” symbol to indicate that the operator will see those temperatures on his/her computer in real-time. In other words, each of these symbols in our P&ID equals one of these temperatures in the HMI.

You should already know that the dashed lines are used for electric signals or electrical connections. But to show the communication links or a network bus between the devices and HMI objects, we should use a dashed and dotted line type.

The VFD sends us some other pieces of data such as:

Is it turned on or off?
Is there a fault with the VSD?
Is it running the motor at the moment?
What is the motor’s real-time speed?

But the VSD also ‘receives’ some signals and data from the control system. Like the Start and Stop commands. Although the VSDs are provided with physical Start and Stop Pushbuttons, we might want to command the motor to start or stop remotely via the HMI as well.

Shared Display means that we have shared some data with the operator in the HMI, and Shared Control means we have shared some controls there in the HMI.

There is a very important point here:
Some instructors might tell you that if your automation system is based on Decentralized Control System or DCS you should use the “circle-in-square” symbol for a Shared Control or Shared Display object or both of them.

And if your control system is PLC based, then you have to use a “diamond-in-square” symbol. Even though this idea was true for older versions of P&IDs that were designed according to previous versions of the ISA document, it is not correct anymore.

As stated on pages 10 and 11 of the ISA documents this definition is no longer accurate. The following is exactly copied from the ANSI/ISA-5.1-2009:

The commonly assumed meanings of ‘circle-in-square’ as a distributed control system (DCS) functions and of ‘diamond-in-square’ as programmable logic controller (PLC) function are no longer accurate because they no longer reflect the currently acceptable meanings.

DCSs and PLCs can both perform continuous and binary control functions. The same functions are performed by personal computers (PC) and by fieldbus and device-bus devices.

Both ‘circle-in-square’ and ‘diamond-in-square’ symbols are classified as ‘shared display, shared control.’

‘Circle-in-square’ will depict either:
(a) primary control system choice or
(b) a basic process control system (BPCS).

‘Diamond-in-square’ will depict either:
(a) an alternate control system choice or
(b) a safety instrumented system (SIS).

Users who continue to use the symbols as in the past should change to the revised meanings as soon as possible.

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