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Economics of Fault-Tolerant Fieldbus Wiring
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Автор:  san [ 23 серпня 2014, 00:50 ]
Тема повідомлення:  Economics of Fault-Tolerant Fieldbus Wiring

Economics of Fault-Tolerant Fieldbus Wiring
http://e-ditionsbyfry.com/Olive/ODE/CTL ... w=document
Careful and clever analysis of fieldbus segments can yield fault tolerance where it’s needed most without adding hardware costs over less effective strategies.
It’s ironic: Network cables in a safe and wellmanaged control room environment are almost always made redundant, whereas field cables exposed to the harsh and sometimes corrosive environment of a modern industrial plant have to fend for themselves. Of course, if those field cables carry simple pointto-point communications such as 4-20 mA, then redundancy isn’t really a concern in general, and specific devices can be duplicated as required. However, now that the lowest fieldbus physical layer carries data from up to 32 devices, the vulnerability of that cable can constitute a reliability issue, particularly if those devices are safety-related or process-critical. Conventional FOUNDATION Fieldbus (FF) segment design does not lend itself to any version of fault-tolerance except through complete and wholesale duplication. In a fieldbus context, that duplication brings with it special software requirements to implement one-outof-two and two-out-of-three voting schemes and special measures required for safe maintenance, device replacement, etc.
In late 2007, MooreHawke Fieldbus released a new fault-tolerant segment design that permits a far higher segment MTBF (mean time between failure) than conventional designs without any special software in the DCS and for only the additional cost of an extra trunk cable. Working with a major DCS provider and a large oil & gas end user, this package was installed on a set of platforms in the South China Sea simply to mitigate the huge financial risk associated with loss of control. The question is, does this increase in availability really make a significant difference to the economics of a general fieldbus installation?


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