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/

Four aspects of good control panel design

"Good control panel design should feature strong layout and component placement, labeling, panel sizing and component spacing, and wireway design".

Most of us have seen a clean, well-organized control panel out on the factory floor—you know it when you see it. Conversely, it's also easy to spot a poorly designed panel—not well laid out, crowded, messy, wires hanging all over the place, etc.

Whereas neatness is probably the first thing that jumps out about a well-designed panel, neatness is really the by-product several other well-executed aspects of panel design. Four other aspects of good panel design include: 

• Layout and component placement
• Labeling
• Panel sizing and component spacing
• Wireway design.

Layout and component placement 

In any control panel, components or "component groups" must be laid out in a logical-functional manner. Since most panels have a main incoming power disconnect switch, most commonly located in the upper right of the panel, it makes both logical and functional sense to locate the components with the highest voltage rating at the top of the panel. From there, the power distribution down to the power components at the lowest voltage level (most commonly 24Vdc) should generally follow a left-right and top-bottom hierarchy.

Each group of power distribution components should start with a main breaker for that power level at the left, followed by distribution breakers, fuses, and terminals. This keeps each power distribution group consistent and functionally sound, and facilitates easy troubleshooting in conjunction with a good design package that reflects this hierarchy in the schematics. Enough space should be left between these groups to allow for ease of expansion. This is done when the control panel is sized appropriately for the application wherein it will be used.

The programmable logic controller (PLC) racks and input/output (I/O) terminals are typically placed below the power distribution components. This is good practice from a couple of standpoints. For one, since heat rises, it makes sense to place sensitive electronic equipment (such as a PLC) below the hotter power components at the top. A well-designed panel will incorporate the means for expelling the excess heat at the top of the enclosure. Secondly, field instrument I/O wiring is usually brought in from the bottom of the panel. When I/O terminals are located at the bottom of the panel, this makes it very easy to land the field wiring.

Labeling

Not only should every component in the panel be labeled, but the syntax on the labels must make sense, and the labels should be placed such that each one is clearly visible.

For wiring, labels must be applied at each end of the wire. For power distribution wiring that connects to a power distribution terminal, the wire is labeled per the terminal number. For all other power distribution wiring and for "general" wiring, the wire is labeled per the corresponding line number in the schematics. For PLC I/O wiring, each wire should be labeled per syntax that corresponds with its PLC address.

For panel components such as power supplies and breakers, the component is typically labeled with a standard abbreviated prefix identifying the type of component, followed by the corresponding line number for that component in the schematics. When wiring and components are labeled in this manner, it makes it easy for troubleshooting, because it's easy to identify them in the schematics as well.

Panel sizing and component spacing

For a well-designed control panel, the panel is sized to allow for "generous" component placement. Adequate horizontal room will give space for addition/expansion for components such as power distribution breakers and terminals, PLC racks, I/O terminals, etc., and will also allow for proper heat dissipation for power components. Adequate vertical room will give much needed space to land wiring into terminals neatly - and thus avoid crowding. Adequate vertical space will also allow power components to dissipate heat properly. In addition, plenty of room should be left at the bottom of the control panel to allow for coiling up spare field wiring.

Wireway design

A good control panel design incorporates the right type and the right amount of wireway. The whole purpose here is to give plenty of room for both internal panel wiring and for field I/O wiring to be routed to the I/O terminals.

Wireway must be designed to allow ease of termination of internal wiring to internal panel components. As mentioned earlier, enough space should be given so that the wiring can be brought neatly to each panel component, and such that the wire labels are clearly legible. Also, the wireway must be sized properly to allow for future wiring additions when components are added to the panel.

Wireway that conveys field I/O wiring to its respective I/O terminals must be generously sized to meet the assumption that field wiring will be brought in and terminated into every existing I/O terminal. In addition, the sizing should account for additional potential I/O wiring in the future if components are added to the panel.

Adhering to these guidelines will not only produce a great looking and organized panel, but it will also produce a highly versatile panel that is capable of expansion and is also easy to troubleshoot when problems arise.

Source: http://innovativecontrols.com/blog/what-comprises-good-control-panel-design