Loop Diagrams

Having looked at the PFD and P & ID Diagrams in our previous posts, we can now discuss the loop diagrams also called the loop sheets. Let's consider the diagram of a compressor surge system shown below:

From the loop diagram above we can see more additional instruments that weren’t shown in the PFD and  P & ID: We have 2 transmitters, a controller, and a valve. We also have 2 signal transducers. The first transducer  42a modifies the flow transmitter signal before it goes into the controller. and the second transducer 42b converts the electronic 4-20mA signal into a pneumatic 3-15 PSI air pressure signal. Each instrument  ''bubble''  in a loop diagram represents an individual device, with its own terminals for connecting wires.

The dashed lines represent individual copper wires instead of whole cables. Terminal blocks where these wires connect to are represented by squares with numbers in them. Cable numbers, wire colors, junction block numbers, panel identification and even grounding points are all shown in the loop diagrams.  You can also notice from this loop diagram, the action on each instrument. You will see a box and arrow (pointing either up or down) next to each instrument bubble. An ” up” arrow (↑) represents a direct-acting instrument: one whose output signal increases as the input stimulus increases. A down arrow (↓) represents a reverse-acting instrument, one whose output signal decreases as the input stimulus increases.

All the instruments in this loop are direct-acting with the exception of the pressure differential transmitter PDT-42. Here the down arrow tell us that the transmitter will output a full-range signal (20 mA) when it senses zero differential pressure, and a 0% signal (4 mA) when it sensing a full 200 PSI differential. Excessive pressure drop across the compressor is considered dangerous because it may lead to the compressor surging. For this reason, the controller will naturally take action to prevent surge by commanding the anti-surge control valve to open, because it, “thinks” the compressor is about to surge i.e. the transmitter is intentionally calibrated to be reverse-acting such that any break in the signal wiring will naturally bring the system to its safest condition.

You can also read: 

• Process and Instrument Diagrams
• Basics of Instrumentation and Control Systems

The only diagram that can be more detailed than a loop diagram is the electronic schematic diagram for an individual instrument but then it shows details for that particular instrument alone, thus the loop diagram is the most detailed form of diagram for any control system as a whole and it must contain details omitted by PFDs and P & IDs.