PLC and DCS
PLC and DCS
(OP)
In control system of small plants (power or water ....etc)
we will use PLC to control and monitoring Data, while in big plants we will use DCS so the control will be direct through the DCS in the main control room.
I read that we can divide plant to many parts each control and monitoring with one PLC in the site, the sum of PLCs will connect to the control room it may call DCS. what is the differece between PLC and DCS and can we connect many PLCs together in control room how and through what can we say master PLC and sleeve PLCs and can we call master and sleeve PLCs a DCS system or DCS is another type of techniq.
we will use PLC to control and monitoring Data, while in big plants we will use DCS so the control will be direct through the DCS in the main control room.
I read that we can divide plant to many parts each control and monitoring with one PLC in the site, the sum of PLCs will connect to the control room it may call DCS. what is the differece between PLC and DCS and can we connect many PLCs together in control room how and through what can we say master PLC and sleeve PLCs and can we call master and sleeve PLCs a DCS system or DCS is another type of techniq.





RE: PLC and DCS
Each system has it's advantages, for eaxmaple Siemens make the S7/WinCC combination or the PCS7 system. The first for mainly digital systems the second for mainly analogue systems. ABB have moved away from their Freelance platform to the Industrial IT range.
Traditional DCS have advantages still but it is now very difficult to see a sharp division between the two types of system. The last time we had to decide was on an oil platform which needed a system to run the power management with four GT's and a diesel black start. The decision was to install an Allen Bradley ControlLogix PLc system with an RSView SCADA. This would traditionally have been a DCS application and would tradiationally have cost at least five times as much
Hope this helps
RE: PLC and DCS
When you say "Over the past three or four years PLc's communication systems have become available...."
I was using networked PLC's in the early 1980's
RE: PLC and DCS
In response to some of your direct questions, the DCS can be used to coordinate a large number of PLCs. This can be done as strictly a monitoring function or the PLCs can be slaved to the DCS. The combinations are quite flexible depending upon your needs.
The most important aspect I believe is to perform a good needs analysis early on to determine what you really need or want to accomplish. Then start moving towards a system that will meet those needs.
RE: PLC and DCS
But it is true, new DCS architecture are very similar to PLC's network. This is now possible (but not recommended)because of the increased capacity of the ethernet.
As far as I know PLC's network applications are very limited for oil and gas plants.
RE: PLC and DCS
PLC's are moving towards having configuration tools that treat the networked group of PLC's as one system e.g. Siemens SIMATIC MANAGER but not every PLC manufacturer is equal e.g. you cannot simultaneoulsy "open" two PLC configurations at the one time using Schneider Quantum.
RE: PLC and DCS
My office is located at one of our 200 mmscfd gas processing facilites. The primary PLC has over 200 analog alarm blocks, 70 PID loops, 1,100 lines of ladder logic, 21 Special Function programs (language similar to structured text) including one that calculates gas orifice flow using an AGA7 routine for 15 orifice plates, and 6 remote IO racks and 3 remote Profibus IO racks. There are 13 other PLC's that monitor and control fired heaters and natural gas engine driven compressors and pumps. The backbone is 100 Mb/s ethernet. I've got the system linked to our WAN and can make programming changes and troubleshoot the system from anywhere that I can connect to the WAN with my laptop. We use Wonderware for our HMI. Actually there are 5 separate WW nodes (no single point of failure). We set them up on NAD and use one machine (separate from the control room) for a historian / configuration machine. The system has been very reliable and very easy to maintain and expand.
Most people that see a block diagram of our system or even the graphics assume that the system is a DCS. We have implemented numerous advanced control strategies in the system. While it would admittedly have been easier to do in a DCS because of the blocks that come prepackaged, not having the overhead and cost associated with a DCS installation and maintenance out weighed the slight benefit that a DCS may have given us. After all, we implemented the strategies and they work flawlessly and that is the desired result.
RE: PLC and DCS