Hey can anyone tell me why PLC are used for controlling critical loop in plant and not DCS..?

Dear Sunil,

The major difference in PLC & DCS is the Response Time and Geographical Location in the Plant.

PLC response time is higher than DCS.
DCS is Distributed Control System which gets geographically distributed over the entire plant / multiple plants. The system remains centrally located at particular place and from there cabling & wiring is fed to different different locations inside the plant system wise.
Being DCS, it has lot IOs & other functionality along with Process Control.

For a particular loop, obviously such a big system is not necessary.
So PLC can cater to such application in the plant which indeed becomes specially dedicated for control of that particular aspect only. This enable faster response to system disturbances & loop IOs.
Hence less interrupts are present in this case.

However for other functionality of the same, PLC Can in turn communicate to Plant DCS and fed/fetch data/commands to/from it.

Again it is commercially thought which becomes cost benefit to go with PLC.
Likewise, it becomes Techno-Commercial Solution.

Hope your query is cleared now.

Mr Amol has very nicely and aptly described the difference and advantages of both PLCand DCS. very brief and comprehensive

Very brief and comprehensive

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thanks for your fruitful information

thanks Mr. Amol, you have said it all. i initially thought other wise.

Thanks a lot!!! Mr. Anmol..,,

If we will speak about loop criticality control there is not important which "kind" of device controls loop, but:
- solution availability (working without interruption, because critical loops have impact directly on plant shut down)
- solution ability for "quick" loop troubleshooting (i.e. support HART, Namur etc.) to quickly see problem to fix it, why = previous point
- impact on "safety" - if the loop is critical because of impact on human safety there has to be taken into account other factors based on HAZOP (SIL)

Generally there is not important which solution will control critical loop (whether DCS or PLC), more important is fulfil requests from topics above. There is not so much correct that PLC "response" time is higher (also DCS controllers scan times starts on 10ms or there is available/possible to have solution with complete loop execution time within 25ms). Criticality doesn´t mean always high-scan loop (can be one of the factors).

The gap between a PLC and a DCS grows smaller by the day. In the old days, PLC's were not only slow, but very limited mathematically. If you were controlling a lot of PID loops, older PLCs were not only limited by math, but also program memory, communication capability and speed. Because of this, most DCSs had a higher level processor or a super computer that did all of the complex math for controlling loops etc. These days, PLCs have plenty of power, communications and memory to do the job. I have replaced many legacy DCS systems with networked PLC/Drive systems. You could actually deploy PLCs to work in a DCS capacity. There are many packages like DeltaV that will work with almost any hardware.

What it really comes down to is this..... What ever the engineer/integrator/support staff is familiar with will determine the upgrade path they choose.

Typically DCS hardware costs are much more than PLC's whereas programming and configuration costs for PLC's are much more than DCS. A lot of end users just look at hardware without taking into consideration programming and engineering costs. PLC's are getting much better in this area, however with the advent of floating point math and process control functionality.

nowadays there are not so much difference between PLC and DCS. Previously there was. The main differences are: 1. PLC was not able to work with analog signals but DCS was. 2. PLC did not have a SCADA but DCS was. In the old times DCS's computing time was so fast but the response was slow. Nowadays there are no extremely differences between DCS and PLC. But we can say that DCS is better for analog signals, PIDs etc. They have their own loops which work really good. You have to write your software if you insist on using DCS. But also remember that PLC is also growing and it is getting these features also. As a result, both can be used instead of the other. If i am building a system, i will check surely the cost and then the signal types, loops etc. If i need so many signals for flow, pressure, PID etc. then i would prefer DCS.