New approaches for remote I/O installations

03/11/2013



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Jonas , , 08/10/13 02:10 AM:

In principle I personally think we should move beyond conventional hardwired signals like 4-20 mA and different kinds of on/off signals, also for the “first meter” between the sensor/actuator and junction box.. The system shall be networked with digital communication all the way down to sensors and actuators like transmitters and positioners. That is, the first meter from junction box to individual instruments shall also be real-time digital communication. In other words, digital closed loop control from sensor to actuator, from field to control room. Instead of running multiple conventional signal wires per device (2 or 3 signals per control valve or on/off valve, maybe 6-12 signals per electric actuator, 2 or 3 per level and flow transmitter) you instead just branch out a fieldbus trunk cable from the junction box to each device, handling all signals in that device. 5,000 conventional I/O points becomes 1,700 fieldbus devices

By fieldbussing the I/O, you don’t need enlarged I/O enclosures in the field and need not put any I/O cards in the harsh field environment. Instead of system cards in large field cabinets, you retain small simple passive field junction boxes suitable for zone 2 or 1. There is no need to send system technicians to the field for service on I/O cards.
Each bus is designed for 10 devices but can be expanded up to 12, 14, or even 16 devices. Additional signals for each device such as feedback can be added in devices without using I/O channels
It doesn’t matter if a device is a sensor or actuator, regulator or discrete in nature, because the fieldbus interface card and safety barrier is the same, so device types can change late in the project without requiring redesign. Cross-wiring is eliminated
http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf
Fieldbus also lends itself well to this concept. See for instance this FPSO:
http://www2.emersonprocess.com/siteadmincenter/PM%20Articles/worldoil0709_fpso.pdf
Fieldbus on mammoth modular construction on-shore projects is also done. For instance this Alumina plant:
http://www2.emersonprocess.com/siteadmincenter/PM%20Central%20Web%20Documents/QBRAlcanGove13dec.pdf
http://www.jumpaboard.org/JA07info/SLIDES/MP-G3s3f.pdf
http://my.alacd.com/tms/2006/papers/115.pdf
Fieldbus is also ideal for those "nice to have features". You can access all those 12-16 signals in an electric actuator / motor operated valve (MOV) or other devices.
I personally believe signals should be digital all the way; no 4-20 mA or on/off signals, like many plants already do with fieldbus. Transmitters are digital, controllers are digital, and positioners are digital, why should they have an analog signal between them? Some vibrating fork level switches and on/off valves now have intelligence, why should there be an on/off signal to isolate them from the rest of the digital nervous system that cover the plant. Only with fieldbus can we say we have real-time “communication with field instrumentation and devices” and a truly “digital plant”. Cameras are digital, phones are digital, mail is digital, music is digital, video is digital, and TV is digital etc. Process control should also be digital such that more plants can reap the benefits of digital.
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