I'm looking for articles/publications/presentations on the topic of PCS technology selections (e.g. DCS vs PLC vs Hybrid). Can anyone provide links to information of this type? I'm interested in all perspectives.

thanks.

 

There is article in "Machine Design" archives entitled "The push behind PC control" May,2001. I can email if you want?

 

Hello,

I have a variety of articles that cover the DCS vs PLC Hybrid if you are interested email-me your email address I will forward you the articles, just write in the subject title something DCS vs PLC.

Thank you

Mahfoud
[email protected]

 

Please can you send the article to me too?
Thanks

Julio M. Paredes Libio
[email protected]

 

Instrument Society of America is excellent source on this. Our site went through this process. We ended up with a Hybrid called Delta V. They are all good some have different philosphies and idiosyncrasies. Of all the ones we evaluated, Foxboro IA, Honeywell Experion, Fisher Delta V, Seimens PCS7, and Yokogawa I personally like the I/O, bus highway, and connectivity of the Honeywell the most.

Hope this helps.
Mark

 

You can download a chapter 1 of the ISA book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" for free in softcopy form. This chapter is an overview of modern network based PCS architecture. It's
free, but you must register an account. If your email does not support this hyperlink feature correctly, please copy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 4585:
http://www.isa.org/Template.cfm?Sec...=/Ecommerce/ProductDisplay.cfm&ProductID=4585

You may want to buy the entire book (buy online in hardcopy or download immediately in softcopy):
http://www.isa.org/fieldbuses
Chapter 2 covers the benefits and savings. The rest of the book is dedicated to details Fieldbus and industrial Ethernet networking, which definitely should be part of your system.

If you can't buy the book now, you can download chapter 1 (overview)

Jonas Berge
SMAR
==================
[email protected]
http://www.smar.com
Learn Fieldbus at your own pace: http://www.isa.org/fieldbuses

 

Hello

If you can forward this email to me would be highly appreciated.

Jari
[email protected]

 

Hi:

I would like the articles too. My email is [email protected]

 

I used to work for a Steel Mill in Southeast Michigan. I left there due to industry instability. Prior to my departure, I was privy to an upgrade that incorported plant modernization (installation of an ethernet backbone) and process upgrade (HMI/PLC data acquisition). The methodology for going with the HMI/PLC was based on technical expertise and raw dollars. The mill had been run so lean that the Electronics Techs (of which I was one) had become accustomed to providing our own solutions to the Data Acquisition needs of our plant. Utilizing the "INTOUCH" products and our myriad of off-brand, non-standard PLCs worked best for us because the communication mediums were not compatible. The new ethernet backbone provided the only real "standard" data path that we had. The SQL arrangement (also through the INTOUCH software) that we developed was easily adapted to an HTML format that could be viewed throughout the company's intranet.

I think that one of the prime factors that should be considered is the knowledge base of the people that have to maintain the product on a day to day basis. By choosing to develop ourselves, we provided our own "lab" to learn in. This made all of the technical people stronger in the long run. Just food for thought.

If I were building from the ground up, with compatible equipment, I would definately lean on the DCS world.

*** Note: I read most of the other comments and thought that you might want a perspective that went the other direction.

 

Hello I have been recently "know" and operating the siemens PCS7 and a few years a go been a part of a team that use a DCS in our work yes indeed they both look similiar and as my own opinion as former user of both system the difference for DCS and PLC's nowadays are just about the reliability of both system including if you can mention about the communication between field equipment and the system.

The DCS are more reliable since it can read, configure, execute commands to "anything that hooked into". It seem a lot of problem that occur during the operation specially the instrument signal and some of the process that have they own PLC's and controlled by the PCS.

First I asume that because the compabilty of the PLC's from other "brand" some times not to support with the PCS comparising with DCS even some of the equipment we use are from different vendors it seem more compact with it and the communication between field and control room are dependable since its come {mostly they're} in one package when we purchased it. But still thats are my personal opinion I haven't got some real "justified" argument that can explain it well let see this are my own experience with those two.

Without you have to came out from the control room in the middle of rainy day go to the field and adjust some local controller that the PLC Hybrid can only monitored them. but I have heard that it also have been solved by some third party software unfortunately I don't know exactly about it. now if there's some one who have some great explanation for this issue will reply to my E-mail well I hope the vendors that may produce both the DCS's and Hybrid PLC's themself that give some good explanation as well thank you

 

PLC is used in the Situations were the SPEED Of OPERATION is an important factor.

DCS is used to control the HUGE PLANT with certain Speed but it can handle more complex loops and handle large Inputs and Outputs. As the Pictorial Reprentation of the Entire plant is provided it is considered as the Advanced version of the Industrial Control System.

so the user has to decide weather to use DCS or the PLC Can do the Job.

It's not really clear what you're describing, but, if you are saying that your company's management wants to use a PLC for gas turbine control, there are many reasons not to do so.

DCS vs. PLC for turbine control? DCS. Why? More, better hardware for inputs and outputs, which leads to less complicated control systems, fewer imaginative and creative work-arounds, and a more robust and reliable control system. Many DCSs are used in petro-chemical and refinery applications for plant-wide control--PLCs aren't that commonly used in such plant-wide control applications, probably because of the I/O requirements (in addition to some redundancy limitations, as well).

Gas turbines generally have very "unique" inputs and outputs and protective requirements. Passive and/or active speed pick-ups; electro-hydraulic servo-valves; fuel valve and inlet guide vane position feedback (usually the OEMs use LVDTs, not 4-20 mA rotary devices); these make up some of the list of inputs which must be sampled at higher intervals than many PLCs are capable of, or require a bipolar milliamp output instead of unipolar milliamp- or voltage output, or require high-frequency (3 kHz) AC (7 VAC RMS) excitation and differential AC feedback comparators/converters which aren't typical for the majority of PLC applications.

This author has seen some "interesting" work-arounds done by some very intelligent PLC application engineers and programmers--which make the PLC program very difficult to understand by most technicians and contribute to varying degress of control and stability based on the operating mode of the unit (start-up, acceleration, rated speed, and cooldown operation).

The protective schemes used by most turbine-generator OEMs don't adapt well to most PLCs and also require some "creative" wiring and interconnection--which, again, leads to difficulties understanding what the intent of the scheme is and how to troubleshoot it.

And, then there's the implementation of the control schemes and logic in the PLC. This author has also seen some incredibly obtuse PLC programming, which is barely functional at best, and which is unintelligible to everyone but the person(s) who wrote the code (and it was obvious that more than two or three people worked on trying to get the functions to work!). PLC programmers, while they may be very familiar with the commands and blocks and functions available in the PLC, don't GENERALLY have the turbine-generator operating and functional experience necessary to integrate algorithms and states seamlessly in the PLC to control and protect a turbine adequately.

For example, this author has recently been asked by a municipal utility which purchased some refurbished heavy-duty gas turbine-generators equipped with GE-Fanuc 90-70 PLC controls to assist with reliability and control issues. They have had several PLC programmers through their facility, all of whom have commented on the complexity of the application and have been unable to resolve all but one of the nine major issues plaguing the units. (There are many more minor issues, most of which appear while trying to understand one function or another!) They have sent their personnel to PLC classes, and they still have difficulty understanding what's happening and why.

Now, they're hiring an experienced turbine professional and are willing to pay that person to learn PLC programming in order to solve their control issues. (The units have been in peaking service for some time, but have always been unreliable and problematic. A change in their operating status and contract requires a high degree of availability and reliability, and the units are being operated more than they have been in the past. The control system integrator who supplied the packages hasn't been able to provide much help over the years, and seems to have had a high personnel turnover....)

Because the speed measuring cards used in the PLC can't see passive frequency inputs below a certain level, the system is programmed to switch between active and passive pick-ups at a certain speed. (The active pick-ups require an excitation source, and mean there must be two types of spare pick-ups in the warehouse.) This causes problems during acceleration and deceleration at times.

The LVDTs are excited and have their feedback converted to a 4-20 mA signal by separate modules, which must be powered seperately from the PLC and which require the LVDTs to be re-wired because of the low excitation voltage of the modules. So, the feedback from the LVDTs is low quality, and that signal is converted to a 4-20 mA signal which isn't of very high quality either.

The electro-hydraulic servo-valves used on the fuel valve actuators and the inlet guide vane actuator require a bipolar milliamp source, and the PLC isn't capable of providing this, so a special card which has a bipolar DC voltage output had to be designed (non-standard, proprietary design).

And, the logic in the PLC...to print it requires roughly 500 sheets of paper! Droop Speed Control doesn't really work properly, and if the ambient is below approximately 70 deg F the unit will not reach CPD-biased exhaust temperature control (it will hit the Droop speed setpoint of 5% which is just below the exhaust temp limit) and so it's power output is artificially limited. The units could make more power, but the control scheme doesn't allow for gas turbine operational characteristics!

And the loading rate adjustment..., because of the way the ladder diagram blocks are sequenced, well, a 2 MW/minute loading rate is more like 5.5 MW/minute. And to achieve any kind of known, verifiable loading rate requires adjustment and testing, more adjustment and more testing, and more adjustment and still more testing, and on, and on, and on.... We got close to what they wanted, and then the scale's maximum limit prevented any further adjustment.

We had to change the block and signal type, compile the program, download, and reboot the PLC--none of which could be done until the unit was cool enough to take off hydraulic ratchet operation because when the PLC is cycled all the pumps stop for about one minute, and then nearly every pump and fan starts and runs for about 20 seconds once the PLC's outputs are re-enabled until the program determines what state the turbine is in (or the operator selects Cooldown ON or initiates a STOP/OFF command sequence).

But the BIGGEST problem with the whole scheme is: Nobody can understand the PLC program, portions of which closely resemble the relay ladder diagram logic used by the OEM, and portions of which are just completely baffling and take hours to sort out how they work or why they don't work. And, while the original control company shall remain nameless in this post, they have been around for many years and have done a number of heavy-duty turbine control system retrofits in that time using PLCs (they have been named in other turbine control system-related threads on control.com). It just seems at that time (or at this time) that they lacked (or lack) the turbine knowledge and experience necessary to make these common heavy-duty gas turbine-generators operate reliably and stably using the PLC.

Now, your company may get "lucky" and the control company proposing the PLC may have a very turbine-savvy individual (or individuals) who can duplicate logic and control schemes so that the unit operates almost exactly like it did with the OEM control system. But, the chances of that are not great, since there just aren't that many (heavy-duty) gas turbines controlled by PLCs. And, the limitations of most PLCs with regard to hardware will also contribute to some "creative" and "interesting" schemes, which, unless they are will documented, will be challenging for most to understand and troubleshoot.

This author isn't even a great fan of using DCSs to control turbine-generators, but there are a couple of manufacturers who have the hardware and the application expertise to build a control system that is well-suited to turbine control applications and easy to understand and troubleshoot (Triconex comes immediately to mind).

Unfortunately, one of the most well-known heavy-duty turbine-generator manufacturers is having some difficulty in performing turbine control system retrofits in a timely and cost-effective manner, and they've "locked up" the sale of another governor control system manufacturer's panels which could be applied to their turbines. So, that leaves many owners with little choice if they don't want to use the OEM turbine control systems. There is a company in Florida, USA, Turbine Diagnostic Systems (www.turbinedoctor.com), which has designed and produced a dual-redundant turbine control system (the TurboNet DASH1), and has at least two of them in service as of this writing. So, there are other options which are coming out slowly.

Another reason for opting for a non PLC-based control system is that most every one of the PLC-based turbine control systems this author has ever seen have been "one-off" implementations, meaning that the turbine-generator owner/operator has to spend a good deal of money getting someone trained on the system, and they're not easily replaceable. While the same could be said for a DCS applied to a turbine control application (there aren't a lot of them), there are more people familiar with DCS applications of all types (usually power- or petro-chemical plants or refineries) than there are PLC people who are familiar with turbine-generator control.

In the final analysis, one wouldn't use a turbine control system in a cardboard box manufacturing plant, or a carpet-making factory, or a waste-water treatment plant, or a paint manufacturing facility, or an automobile manufacturing plant; why would one use a PLC for a turbine-generator control system?

 

Just so it's clear, the first three paragraphs of this response to a four year-old thread were written by "no one".

The rest came from a response on July 1, 2006, to a different thread:
http://www.control.com/thread/1026223561#1026223660.

"no one" should note the source of the majority of a response he/she is posting as his/her own.