In an earlier post, Considerations for Wired versus Wireless Instrumentation, we provided some guidance on when to consider digital communications-enabled technologies such as Foundation fieldbus and when to consider wireless technologies such as WirelessHART. Emerson’s Jonas Berge provided a great comment and links to additional information that I wanted to make more visible in a separate post.
Jonas Berge Director, Applied Technology
FOUNDATION fieldbus takes the place not only of 4-20 mA but also of the various types of on-off I/O signals such as dry contact, sink and source for DC, as well as relay and Triac [triode for alternating current] for AC. That is, you can mix transmitters, gas chromatographs, control valves, intelligent on-off valves, and two-wire tank gauging systems on the same bus.
I agree that a bus is typically designed for up to 12 devices but would also like to highlight this is roughly equivalent to 36 conventional I/O as many devices have multiple signals each such as 3 signals on a control or on-off valve, up to 16 signals each for an electric actuator / motor operated valve (MOV), a.s.o.: Foundation fieldbus overview
If more fieldbus devices have to be added later wires only have to run up to the junction boxes. Additional signals in fieldbus devices can be enabled at a click of a button.
A fieldbus segment can be, and usually is, far longer than 120 m. The distance from the control system to the junction box can be more than 1000 m. 120 m is the distance from junction box to the device.
Plants now use couplers with short circuit protection as standard thus ensuring a short at one device does not affect the others.
Most of the time the fieldbus interface card sits in the DCS backplane such that HSE is not required.
Redundant interface cards and power supplies is common, but redundant cable is extremely rare.
Using repeaters, fieldbus can reach several kilometers but since a bus segment can reach more than 1000 m without repeater, a repeater is extremely rare.
When upgrading 4-20 mA to 4-20 mA/HART make sure the cable is shielded twisted pair grounded in one end, meeting capacitance and separation requirements: Wire Length & Capacitance
I personally agree that wireless is an excellent way to add instrumentation in existing plants, and that plant-wide WirelessHART infrastructure is an excellent way to modernize existing plant to improve reliability, maintenance and operation productivity, energy efficiency, and to reduce HS&E [health, safety & environmental] incidents and response time. New plants should build in this WirelessHART infrastructure from the very beginning so that it need not be added later. This requires an early involvement of the run & maintain organization in the project. Problem is, the run & maintain organization may not have been established yet when the project is in FEED [front end engineering design].
Plants need to figure out how to make sure the automation, data collection, and data processing needs of the maintenance and reliability engineers, energy managers, and HS&E officers for a new plant are taken care of at the start of the project. Anyway, even if you have missed this step in FEED in your current project, you can still add WirelessHART in the detail design stage because the infrastructure can be installed plant-wide with minimal wiring.
WirelessHART is the most cost effective way to add instruments when there is no junction box nearby.
I personally agree that fieldbus and wireless complement each other and modern plant automation should be designed with both. Learn more about WirelessHART for operational excellence from these articles:
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