Stephen Mitschke
August 12th, 2003, 10:44 AM
Dear Ian,I have recently been included in the circuit of Field Bus endusers.Thanks to you and my good friend Mr. Pai. I am a newcomer to thisfield asmy installation on Field Bus application is presently underway at myammonia unloading terminal in Goa, India. I am using FisherRosemount'sFB-Plant Web and the secondary instrumentation is their Delta V. Iam yetto assess the performance vis a vis the DCS of Yokogawa which I havein mymain plant 12 km away which manufactures ammonia and from which wemanufacture phospate fertilizers.Of course I have been regularly going through the problemsencounterd byvarious end users on the same, but I would be much obliged if Icould getproblems faced by the end users on their FB systems on aninstallation likemine.ie ammonia unloading wherin the environment is extremely harshdue tothe proximity of the sea and RH at @ 95-98%.Regards,Alex Pacheco-Sr Manager-(Head)-Instrumentation-Zuari IndustriesLtd-Goa.[Verhappen, Ian] Since Fieldbus does not change the sensors you areusing on your plant, I assume you have your past experience from similarinstallations to guide you in selection of equipment in that regard.Hopefully others on this list can provide assistance for the Fieldbus partof your installation. (NO commercials folks) - - - - - My experience is that Foundation Fieldbus works as advertised. However, youmay want to emphasize termination quality and any measures taken to sealterminations against the environment (seals, gaskets, conduit drains)because these can now become a common mode failure point for manyinstruments.For example, we had a Micromotion flow meter whose conduit entries werefitted with standard electrical conduit seals. Between commissioning and thetime seals were poured, water got into the flow transmitter's terminationarea. This caused not only the Micromotion to fail, but also otherinstruments on the same segment.In another case, a loose screw in a positioner shorted a segment and causedit to fail.We are now using short-circuit protectors on all new spurs. These areavailable from Relcom (http://www.relcominc.com) and interlinkBT(http://www.interlinkbt.com/index.htm). John Rezabek------ In addition to the remarks by John Rezabek:In one of the projects (offshore platform with high RH and salt) wesupported with our consultancy on FF implementation the following isplanned:running extra home wire (multipair cable), feeding the segment back to thebenign environment and installing the termination in this benignenvironment. Note 1: this works only if the distances stay within thespecified limit (no problem on offshore platforms).Note 2: the disadvantage is that the double amount of multipairs is neededin the home run cable; the higher cost needs to be traded off against othersolutions.- - - -Alex,I think the most important feature is that you segregate any instrumentsconcerned with safety away from any instruments to do with anything else.Safety Instrumented Systems should be designed and installed the way youhavealways done them (or in line with IEC61508). Do not execute them inFieldBus.Tom NobesBritish Nuclear Fuels Ltd. (tom.s.nobes@bnfl.com)- - - - - TomWhilst I agree with your sentiment, I do not think it is as simple as that! The extra information available via Fieldbus can make a majorcontribution to enhancing the reliability of a trip loop as failures arereported immediately rather than waiting for the next trip test. If all'safety related' alarm & trip systems cannot be implemented via Fieldbus,then the use of Fieldbus on many plants will be severly restricted. Whatare the thoughts of others in this area ?Laurence Thring- - - - Laurence:I also am of the opinion that we need to validate FF for use with SafetySystems.I am already hearing yet another reason not to use FF at all is "Let's waituntil somebody uses FF in a really critical application".In the offshore oil and gas arena, safety systems have as many or more I/Othan basic process contol systems, so having to use conventional analog for this large segment of the field devices really dilutes themotivation for using FF.I have been following with interest the work being done by our FF EUCcolleagues in Britian and I was just as surprised as you to see the commentfrom Tom Nobes.If any of the FF End Users in the Americas have a severe interest in thissubject I would like to promote a get together to see if we can make someprogess on this subject in our venue.Regards,Charles Hill- - - - Now I feel motivated to respond to this thread.Any fool can design a safety system to reliably shut down a process on theedge of disaster. The benefit of using a Programmable Safety System (thesubject of ISA S84) is to prevent unnecessary shutdowns resulting from falsealarms. A further benefit of using a Process Control System in coordinationwith a Safety System is to prevent unsafe process conditions from occurringin the first place. In my opinion, if the consequences of shutdown areeither economically a disaster, or even a greater threat than continuousoperation (and I can give examples of both), then why is the Process ControlSystem NOT part of the safety system.During the development of S50, I gave several papers showing that Fieldbusenabled the configuration of redundant measurements, actuators, and controlsin addition to redundant communications paths. In ISA TR50 available fromISA, there are several features to those function blocks which were tailoredto the configuration of redundant measurements and controls. In some cases,these have appeared in Foundation Function Blocks, but not the full set.I feel that Foundation Fieldbus is the best answer yet to configuration ofgenuinely safe systems that do not shut down good working processes.Dick Caro- - - - - IanThe slides from the presentations given at the recent seminar " Fieldbus &Functional Safety: Is there a match ? " are available on the WIB website,www.wib.nl. There is no requirement to be a member of the WIB to view theslides.Laurence Thring