DCS - Basic concept

In FERTILIZERS AND CHEMICAL TRAVANCORE LTD (FACT) there is a plant called RAP and its product is ammonia, there a lots of chain reactions taking place in the background.  And some of them in fact is very dangerous too…

Take a small example, the constituents of ammonia, nitrogen and hydrogen are compressed to liquid we use big compressors for this process.

Extreme care is required when dealing with these compressors, one should be very vigilant in monitoring the reading of pressure gauge, which shows the pressure of the inlet gas, the flow meter which indicates the flow if gas into the compressor, temperature of the steam which is used for running the compressor turbines, the pressure gauge at the outlet of the compressor.

If any one among them shows an abnormal reading then there is some trouble with the process.

In olden days the operators have to go into the field and manually carry out remedy measures, or if things the not coming in the right way , as a last , a safe shutdown.

Now using DCS everything is automatic, the transmitters are used now instead of gauges for transmitting the pressure, temperature, flow, etc from the field to the control room.

The DCS system consists of a

w Power supply

w Processor, (of course a spare one also will be there, for taking up the control if the first one fails.),

w Digital and Analog inputs and outputs.

w Other cards.

w Operator terminals

w Data Highway

All are inserted as cards into a backplane. Just like we insert internal modems or LAN cards into a system mother boards.

All are arranged inside a panel as shown in the figure.

Now there are operator terminals for viewing the data coming in from the field transmitter, (which we discussed a little before.) We can also send data into the field for altering the current status of the outputs.

Operator terminals are touch screen just like the one which we see in railway stations. The Data highway is used for connecting the operator terminals, processors and the data coming in from the field. Through this highway, we can receive data from the field, sent the data to the field etc.

 In the above picture,

1. represents the field devices like, Diff. pressure transmitters, temp transmitter,

Flow transmitter, thermocouple etc.

Separate junctions boxes (see in the pic) are there in the field itself for taking up wires coming in from each category of the transmitter, even they can be distinguished by the colour of their cables.

ii. Marshelling box is the boundary of DCS. From this point onward the control room components of the DCS starts. In the Marshelling box, the wires drawn from all the Junctions will be plugged in.

iii. The signals coming from the field will be current signals or voltage signals in the range of

4-20mA or 0 -10V. These signals are to be converted into binary signals before they are fed into the controllers. The I/O interface will do this job.  

The inputs and the outputs are processed inside the DCS program which is loaded in the procesors. The DCS program is made using the custom software issued by the manufacturer like Honeywell, yokogawa, emerson etc. and are loaded into the Processors. It will be mainly control loops.

Eg. If an incident A happens, then C has to be the output following A or of Another incident B happens, D has to follow….like that

If the reading of a pressure transmitter is above a particular value. there will be certain measures to bring it back to old values……DCS will initiate those measures.

iv.) This is an operator terminals where we can see the inputs of the transmitter as well as the outputs automatically given out by the DCS processors.