PLC or DCS

M

Thread Starter

Mehmet Bicer

I have to decide on the control system of a new medium sized plant. I am really in confusion which one to choose. Shoud it be a PLC or a DCS control. The plant has about 50 AI, 16 AO, 100 DO, 300 DI and 16 PID loops.

Thank you very much for your comments.

M. BICER
 
DCS and PLC control are two different methods for plant control. It is not really possible to decide between them based on I/O. You need to look at all the futers of DCS and PLC. Also you need to look at the support nessesary for the plant. Where does the support come from. Take also a good look at the prices for Support on PLC or DSC. Take a look at the needed equipment for PLC or DCS beside the main controls. Look at network equipment the amount of needed computer systems and the needed base software like Windows or Unix.
I can give you a personal opinion about this but is not in perspective with not knowing information about what is good for the plant.

I suggest you let the possible providers of PLC or DCS systems in the neighborhood of the plant decide what is good and let them give you information where you can choose from. Like price, performance and support. Ask for reference plants using these solutions and go there for their opinion.

Good luck, Sisko Bos
Application engineer
[email protected]
 
E

Edgar Hilton

You probably need something that is highly networked, can be monitored remotely, can be programmed remotely, has hard real time control capabilities (<50microsecond); has soft and no real time capabilities; has secure shells to avoid external hackers from affecting your system, and can set up secure web services for each machine to simplify monitoring. The idea would be to be able to monitor the entire plant from either a central locale or from various stations across the plant. Most importantly, you probably need flexibility and to have total control of its development and future development. Most importantly, you should be able to monitor all of your machinery and change computational parameters without ever having to get your machinery off-line.

I would avoid using any full fledged operating system such as Windows on the floor itself. I would personally look for simplicity and thus increased reliability. You can use any operating system that you prefer at your command center, but I strongly suggest to avoid those types of OSs on the floor.

If this is roughly what you are looking for, I recently wrote a non-technical white paper which may help you in this area. Please email me if you are interested for me to send it to you. I would be more than happy to email it.
 
The amount of I/O you are give is not much an PLC solution can be chosen with an SCADA system running on a windows NT based solution.
This is the most common method for your solution.
I suggest a SCADA solution like Citect (www.citect.com) in combination with any kind of PLC you would like.
This will also be good priced solution with a lot of people who can service the solution.

Good luck, Sisko Bos
Application engineer
[email protected]
 
Hi,
Thank you very much for comments,

Please send your paper, I will appreciate.

best regards
Mehmet Bicer
 
A
As previous comments, it depends upon the functional requirement fo the system rather than the I/O. At Silvertech, we deliver all kinds of control and safety solutions from small relay panels through PLC/SCADA to full-blown DCS.

Our experience shows a PLC is better for sequential control and safety related systems, where a DCS may be better for process control. That said, a PLC can be made to do anything a DCS can do.

One thing worth considering is the probability of change once the system is installed. Ease of re-programming and re-configutation, as well as adding to the system should be a decisive factor.

let me know if you want case studies/comparison of products etc. we have done for existing customers.
 
Others have discussed the hardware/software requirements when determining the choice between PLC and DCS to control a process. None have looked at the process itself to determine the level of control that must be provided.

Ask the following questions as you make your decision:

1. How hazardous are the products that your are processing, and what will be the results of a loss of control of one or more circuits during the process? (Is redundancy of the system, including I/O, required for safety/health reasons?)

2. What is the cost of an unplanned process shutdown? (Is the cost of redundancy
justified?)

3. Is this a batch or continuous process? (Is on-the-fly change capability required?)

4. What will happen to the rest of the control system when one component of the system fails?

5. What provisions does the proposed control system have for changing of any component while the system is "On Control"? How many outputs does each card/device support. If it is more than four per card, I recommend that you look very closely at the method of maintaining the signal to the final control element when replacing a failed output card, either analog or digital. Also is there a method of matching the control systems output to the manually maintained signal,
allowing return of automated control without unnecessary process upsets.

I have worked with both DCS and PLC systems in a refinery enviroment for the past 19 years. If you have further question, feel free to contact me directly.

John Beck
[email protected]
 
M
If a PLC system can do anything a DCS system can do, then why do you say that a DCS system is better for process control?
 
H

Heavner, Lou [FRS/AUS]

Mike,

I'll bite on this. Being able to do anything does not mean being able to do it better. Depends on the meaning of better. Faster? Easier to design/implement? Easier to operate? Easier to integrate with other business and control systems? Easier to upgrade? more inexpensive to buy? more inexpensive to maintain? more inexpensive to operate? Easier to document?

I won't debate whether PLCs can do everything DCSs can do or vice versa. Clearly, each has evolved by adding functionality that used to be the sole domain of the other creating substantial overlap. You have to look at each application on its own merits. However, it is a reasonable rule of thumb that PLCs are going to be more cost effective in factory (discrete control)
applications and DCS will be moreso in process (regulatory control) applications. Why? Because the focus of those products is on where their
primary market opportunity lies. It's where their inherent design advantages are found.

I'll even throw out some examples to consider.

Batch Control
Complex, multi-product, multi-stream, S88, validated pharmaceutical process
where the value of one lost batch is worth more than the entire control
system cost vs. water deionizer regen sequence control system or air dryer
regen sequence control system.

Hazardous Control
Boiler control including combustion and drum level with multiple boilers and fuels vs. burner management system.

Regulatory Control
Control of an FCCU or an ethylene furnace vs control of a cooling tower.

I guess it also depends on the definition of process control. Or more correctly, it depends on all of the objectives and requirements for the
application in question.

Merry Christmas to all...

Lou Heavner
 
M
Lou,
In your response, you say "However, it is a reasonable rule of thumb that PLCs are going to be more cost effective in factory (discrete control) applications and DCS will be moreso in process (regulatory control) applications. Why? Because the focus of those products is on where their primary market opportunity lies. It's where their inherent design advantages are found.". And your reason is due to the inherent design of each system respectively. I hear this same argument around here ( meaning my place of work ). There point is that DCS systems are easier to program PID loops with and with all the pretty accessories ( like limit blocks square root function for analog inputs, etc.) that are usually not in PLCs, regulatory control in a DCS is superior to a PLC. To me, that seems like a
weak argument for using a DCS over a PLC. Anyway, what makes regulatory control so unique that DCS systems are best suited for this type of
control? Is it the speed of data processing? Are all DCS systems use a form of parallel processing of data? Do they have a superior means of signal
processing ( digital filtering ) over PLC systems? For me, the part of the answer is that there are those that love DCS systems over PLC systems and there are those that love PLC systems over DCS systems.
I guess where I am coming from is that if I look at the Boiler control system example in the SAMA functional diagramming of instrument and control systems standard dated September 1981, it seems to that both the combustion controls and the three element feedwater control can easily be implemented in a PLC. I would even say that a multi-Boiler system could be implemented on multi-processor PLC system ( like using a few ControlLogix processors in a single rack and having a bunch of remote IO for all the instruments ). I would even gamble that a multi-fuel multi-Boiler systems could be implemented using a multi-processor PLC system. Now unless DCS systems have this incredible processor and IO speeds, I do not see the reason outside of what the designing engineer likes, what the consumer likes, what the maintenance people can handle, and most importantly the dynamics of the process ( heat and mass balancing, chemical kinetics, etc.). I don't know, maybe I should just sit down and reread chapter 7 on DCS systems in the book, "Instrument Engineers' Handbook 3rd Edition" by Bela G. Liptak, in order to seem what I am missing here.
Would you use a DCS system or a PLC system for a distillation process ( like the separation of petroleum products from crude oil )?
 
M
> Lou,
> In your response, you say "However, it is a reasonable rule of thumb
> that PLCs are going to be more cost effective in factory
> (discrete control)
> applications and DCS will be moreso in process (regulatory control)
> applications. Why? Because the focus of those products is
> on where their
> primary market opportunity lies. It's where their inherent design
> advantages are found.". And your reason is due to the
> inherent design of each system respectively. I hear this same argument
> around here ( meaning my place of work ). There point is that DCS
> systems are easier to program PID loops with and with all the pretty
> accessories ( like limit blocks square root function for analog inputs,
> etc.) that are usually not in PLCs, regulatory control in a DCS is
> superior to a PLC. To me, that seems like a weak argument for using
> a DCS over a PLC.

It seems like a strong arguement to me. At least until PLCs / PC MMIs catch up (and they are). The time it take to program/maintain control system is a significant cost.

> Anyway, what makes regulatory control so unique that DCS systems are
> best suited for this type of control? Is it the speed of data
> processing? Are all DCS systems use a form of parallel processing of
> data? Do they have a superior means of signal
> processing ( digital filtering ) over PLC systems?

No, DCS's are not fast, they are usually pretty slow. but then scan times of .1 to .5 seconds are much faster than what is typically needed in most processes.

> For me, the part of the answer is that there are those that love
> DCS systems over PLC systems and there are those that love PLC systems
> over DCS systems.
> I guess where I am coming from is that if I look at the
> Boiler control system example in the SAMA functional diagramming of
> instrument and control systems standard dated September 1981, it seems
> to that both the combustion controls and the three element feedwater
> control can easily be implemented in a PLC. I would even say that a
> multi-Boiler system could be implemented on multi-processor PLC system
> ( like using a few ControlLogix processors in a single rack and
> having a bunch of remote IO for all the instruments ). I would even
> gamble that a multi-fuel multi-Boiler systems could be implemented
> using a multi-processor PLC system. Now unless DCS systems have this
> incredible processor and IO speeds, I do not see the reason outside of
> what the designing engineer likes, what the consumer likes, what the
> maintenance people can handle, and most importantly the dynamics of
> the process ( heat and mass balancing, chemical kinetics, etc.).

The designing engineer likes the DCS over the PLC for the boiler control because it is easier to configure/maintain/ensure 100% control system uptime, not because it can control better.

> I don't know, maybe I should just sit down and reread chapter 7 on
> DCS systems in the book, "Instrument Engineers' Handbook 3rd Edition"
> by Bela G. Liptak, in order to seem what I am missing here.
> Would you use a DCS system or a PLC system for a distillation
> process like the separation of petroleum products from crude oil )?

It depends on how you want I needed to run the system. If it is a big system that needs to run 24/7/365, I'd probably lean toward the DCS. If is was a small system, I'd consider a PLC, or a
Fieldbus system.

Mark Blunier
Any opinions expressed in this message are not necessarily those of the
company.
 
S

Stockler Paulo AD BR

Pay attention that new generation PLC's, like Controllogix, has capabilities before found only in DCS's systems, mainly TAG programming facility, so you may have your hardware wherever you want, you can make hot changing of cards, you'll not worry about address and if you really want a powerful tool look what ProcessLogix can do. It's really a DCS in PLC platform + PC - HMI.
 
Take a look at Moore's Procidia. We considered it for a smaller application, but finally went with single loop controller.

Terry Dixon
IP
 
M

Mike Johnson

Have you ever used the ProcessLogix system, a buddy of mine said that during a Rockwell demo the reps had a hell of a time with the software. What are your experience with it?
 
Hi everybody and happy new years for you all, We are searching control systems for more than 3 months. Finally we decided to use neither a PLC nor a DCS instead a hybrid system. Altough there are many hybrid providers , only the following are widespread (as far as I know) in Turkey. They are, * PCS 7 * Delta V * PlantScape The cost of these three is very close to each other. I mean the price is not the dominant factor to decide. Would you please tell me the advantages and disadvantages of these systems fairly? I hope we will end up with the most suitable system for our plant. Thank you very much for your comments. M. Bicer [email protected]
 
Hello, I do not want to give a bad advice but I have to tell this as an experence PCS7 programmer. I was one of four programmers using PCS7 version 5 of Siemens. We used a complete package hardware and software picked by siemens people. We programmed a plant with about 250 valves and 50 pumps. We had courses for programming and with our good PLC S7 knowledge we went straight to hell. The plant is running with to PLC's and two Tag servers and two client station. The amount of programming time went about 5 times needed for a PLC S7 project, the configuration of clients, servers etc needed more then 100 ours of siemens experties. And now the the servers need to be switch of ever week once or the become to slow. The application was a complete disappointment it was not even large..... My advice on this moment. If you choice siemens then choice S7 and a third party SCADA like Citect or intouch but not PCS7, maybe in some years siemens will solve these problems..... Greetings, S. Bos
 
P
Hello, I would like to disagree with Mr Bos. We have been involved with PCS7 since its inception and first release versions. We also come from a PLC/SCADA background and I am strongly convinced that PCS7 from Siemens is the way of the future. I would like to firstly state that we have successfully implemented PCS7 in 2 respective plants with systems that have up to 6 servers, 14 clients with two monitors each, and up to now 20 running AS's (Automation Systems or PLC's as they are commonly termed by those who don't understand the difference). There is definitely a saving in terms of engineering as all the neccessary blocks are pre developed and available in the libraries that are supplied. Even if you need to write your own blocks, it is possible using conventional languages that EXPERIENCED S7 programmers are familiar with. The actual saving is in the way tags are configured, as there is no need to create tags in the visualisation portion of the package. These tags are automatically created when a block is used for example, tags for a valve such as open an closed feedback, driving signals etc. Furthermore, should you wnat to add aditional tags to modules, you only have to add them to the base module and the rest is done automatically. These tag references are also kept up to date automatically and therefore there is no engineering to be done other than connecting the tags to graphics and archives. Furthermore I believe that custom applciations were inserted into the system at the abovementioned site and these applications are the cause of server resource hogging. Then lastly, I would like to comment on the magical ability that only a visualisation systems such as WinCC can deliver and that is the way the tags are updated. Instead of hogging all system resources for communications to update the WHOLE database all the time as in conventional SCADA systems, WinCC only updates those tags that are in use, i.e. those on the screen and those being archived. This is a huge advantage and saves valuable system resources and network bandwidth. I would suggest you not only take my word for these facts, but also contact other very happy customers and users and should you choose PCS7 I asure you, you will have only happy days ahead. (bearing in mind of course that you choose integrators that have done it before and not ones that are using client funds to go to school). Below I include the names of the projects that we and others like us have been involved with for other references: - Palabora Underground Mining Project - South Africa ++2715 780 2449 Piet Oosthuizen or Mike McCairns - Palabora Mining Company - Pebble Crusher - South Africa ++2715 780 2911 Willem Verhoef or Willem Joubert - Sappi Usutu - Swaziland ++268 602 0298 Aubrey Johnson Any of these customers will be more than willing to share with ou their success stories and should this not be enugh, you can contact Siemens for a list of more VERY happy customers I am sure. Paul Mulder System Integration Consultant Systems Management & Services South Africa ++2713 697 1884
 
It is always good to hear that someone has an other experience with PCS7 then I did and made some good provid on it! Using PCS7 needs probably very experienced people like your company can provide. Thats needed, but should it??? It is good for your and our company!!!! especially when every hour of work is paid!!!! This week I have seen a demo of PCS7 version 6 (it is still in development) and can conclude that it will be heaven for us and others that will use it (it will solve almost every problem we encountered and we will probably have the possibility to have some more inexperienced people using PCS7 and let them make the most usable good working systems ever). The new version 5.1 will solve half the problems we had. Till then I hope the customers of PCS7 have very experienced people for implementation. I agree that PCS7 will be the way of the future and running on a Microsoft product is something that gives a good feeling about using PC systems in industrial applications were a lot of DCS people are not shure about. I think the Year of version 6 will be the PCS7 year in industrial applications for me and the engineers I work with! I wish I had the time to go and see your running applications of PCS7 and take a look in the way you build them! Good luck, S. Bos
 
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