DSC/PLC Flow Diagram Instrumentation Tools

A control system flow diagram represents the signal flow starting from the field transmitters to the final operator graphic display.

Here we are discussing about traditional 4-20mA analog input devices only for easy of understading the basic concept.

Signal Flow

We have thousands of field transmitters installed in a proccessplant. So it is practicaally difficult to laystraight individualcables from each field transmitter to control room for displaying the process variables on the workstation. As per design standars,particular number of field devices / transmitters are wired and terminated ina Field junction box. Cable used for connecting the field device to junction boxare called as Branch cables or field cables. Generally we use one pair cable for branches cables.

The below figure shows 5 field devices are conected to a junction box using individual branch cables. In this example, for carrying out these five signal to control room we need minimum 5 pair cable (means one cable have 5 pairs) and also we have to consider spare cable requirement for future purpose. So at least 6 pair cable or better a 12 pair cable best suits the below requirement. Say we chosose 12 pair cable and is nothing but a Main Cable or Home run cable as it interfaces the field junction box and marshallling cabinet in the control room. So finally one big main cable is required and serves our purpose.

In a process plant, we have so many junction boxes installed and connected with number of field devices.Say we have 100 no'sof junctions boxes installed that means we have 100 no's main cables are these which are coming from field to control room. So, practically it is not possible to directly wire these main cables to analog input/output card. To avoid these problems we use marshalling cabiner for terminating these 100 no's main cables.

Marshalling cabinet main purpose is to provide main cables termination and then, re-distribute the field devices to respective analog input/output card using internal wiring. Internal wiring will be used to connect from marshalling cabinet to system cabinet.

System cabinet consistsin a processor card (CPU), analog input cards,analog output cards, communication cards, etc. Once main cables are teminated in marshalling cabinet, we have to take thesefield devices to the respective analog input card channel. So, we use internal wiring to route these main cables/fielddevices from marshalling to system cabinet.

Once the main cables are connected to the analog input card via internal wiring and it converts the 4-20mA which is coming from field devices into equivalent digital signali.e.in binary codes and the same will be communicated to processorcard. the processor card performsasper the predefined or programmed instructions. the processor card may have inbuilt or separate ethernet communication link which is used to display the measured process variables on the workstation.

Summary

The above animation depicts a typical cabling scheme that collects and distributes signals to and from the field devices. An field junction box is used to concentrate the signals intomulticonductor home run cables or main cables. the home run cables then terminate in a remote or marshalling cabinet where the signals are marshalled (reorganized) as necessary to efficiently terminate at the I/O interface of the DCS or PLC system.

Source: http://instrumentationtools.com/dcsplc-flow-diagram/