1 Question:
In a process, it's known that is better to not use fieldbus for safety reason (it is not analog). Considering this, the question is if the reduction of wiring in process control could be an advantage by itself or it could be an advantage only if coupled with a reduction of all the wirings (safety system too).

2 Question:
Why with Fieldbus it is not easy to insert a new device but you have to recalculate the length of the cable? The impedance has to be setted before?

3 Question:
Do you have any pictures or diagrams showing the logic of fieldbus? Also compared with DCS?

Thank you for your help on these questions.

 

Hello;

By "fieldbus", do you refer only to Fieldbus Foundation?

If you refer to fieldbuses in general, I would like to point out that As-i Safety, Profisafe (Profibus-DP safety profile) and CIP Safety (Safety comms over DeviceNet for GuardLogix processors) have been available for a number of years, and the number of safety applications on these buses is increasing at a great rate. In these cases cabling reduction is advantageous.

If you are referring to Foundation Fieldbus and are looking for diagrams and info, I can recommend you look at Emerson's Plantweb University website. Once you register, you can enroll in a very complete tutorial on FF (also on other subjects); it should cover most of your questions.

Hope this helps,
Daniel Chartier

 

Lorenzo,

1. Fieldbus can be used for safety. One reason it is not is that there are few products that support it for SIL 2 and higher. Foundation fieldbus will have numerous beta tests next year (Chevron, Shell, Saudi Aramco, and BP) and product (we think) by the end of 2008. "Profisafe" has been around a while but I have not heard of any applications like ours (SIL 2 or higher). I know of people who are using present-day Foundation fieldbus for SIL 1 and lower interlocks.

I like FF for safety because I get more information than 4-20 or HART. I think I am safer with intelligent instruments that are digitally
integrated with the host logic solver. Like going from a discrete switch to an analog transmitter, digital integration a la Foundation fieldbus will mean even fewer "non-self-revealing" or covert failures.

2. I add FF devices to existing segments all the time and rarely do any calculations. Spurs < 120 meters and overall < 1900 meters and all is well, under most circumstances. Constraints are different for IS.

3. I usually point people here:
http://relcominc.com/fieldbus/fbtechinfo.htm . The Emerson "Plantweb University" I hear is pretty good too,
http://plantweb.emersonprocess.com/University/engSch_Fieldbus_XML.asp

(be sure to get the whole link)

John Rezabek
ISP Lima LLC

 

Are we talking about Foundation Fieldbus or fielbuses in general? Some fieldbuses have been approved for safety applications (ASIsafe and ProfiSafe).

David

 

Answer to your first question regarding why a field bus cannot be used for safety.

The means of communication between two safety devices, such as a Safety PLC and a Safety remote IO can by all means be transmitted over field buses. The safety of a field bus is not dependant entirely on the medium used such as Ethernet, Modbus serial, etc., but it is dependant on the Protocol. How the end devices react to a failure, either a problem with the message or other failures such as early or missing messages, corrupt messages and so on. With the telemecanique safety systems you are able to use your standard Ethernet cable to transmit your safety related data, without the need for any additional devices. In addition, you are able on the same network send non-safety related data, by means of a non-safety related protocol.

 

Thank you all,

I'm talking about Foundation, that (at least for what I know) is the one used in process control (oil and gas).

By documentation appears that the spur cables are to be recalculate depending on device linked, why?

Another question, what about redundancy in Fieldbus? Is it critical?

Best regards,

 

Foundation fieldbus for safety ("FF-SIS") is coming and products will become available.

There is no re-calculation required when you change devices. I am trying to second guess where this doubt may have come from. It could possibly be related to intrinsic safety: that is, since devices may have different entity parameters you had to evaluate the "system" in view of the entity parameters of the new device if you changed it to make sure it is compatible with the barrier. However, with the FISCO concept this evaluation of fieldbus has become much simpler. You can replace one FISCO device with another FISCO device without having to re-evaluate entity parameters.

Fieldbus has very good support for mult-master so redundant interface cards is no problem. Redundant power suppliers/conditioners is also no problem. A fieldbus device has only one communication port so you can only connect one pair of wires to it. However, there is a ring-topology solution in the market that provides some measure of wire redundancy, although I have never tried it. One should keep in mind that a fieldbus is fundamentally different from the control-network in the sense that all the loops in the entire plant rely on the ONE and only control-network (which therefore MUST be redundant) while typically only three loops or so rely on each of the MANY fieldbuses around the plant. Therefore redundant wiring is not used for fieldbus, and sometimes not even redundant power or interface cards. If redundant instruments are used, they are located on different Fieldbuses.

To learn more about fieldbus design and redundancy take a look at chapters 3 and 10 respectively in the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" buy online: http://www.isa.org/fieldbuses

Jonas

 

You know there is also CANopen Safety. CANopen is more often used than DeviceNet in Europe. CANopen Safety can be used in SIL3 applications.

All newer Ethernet based "field busses", Powerlink, EtherCat, Profinet, Ethernet/IP... also have "Safety" additions.

Regards,
Heinz

 

You might enjoy following my Blog for updates on many of the more controversial Process Safety related topics, like Safety fieldbus, Integrated Control and Safety, etc... without all the FUD.

http://www2.sea.siemens.com/news/blogs.htm