Re: FF failsafe modes


> > An outfit in our part of the world is planning to run an additional AS-i bus > network next to a newly installed FF bus. The idea behind it is to force the > various control valve positioners (ABB) via the AS-i bus to specified > failsafe conditions should the controller (PLC) fail on the FF network. > > Surely this is wrong? IMHO you are now using AS-i to patch up a perceived FF > shortcoming and this is not an elegant engineering solution. Being not a FF > knowledgeable person I was under the impression that some function block > within FF will take care of the desired failure mode should the FF master > remain healthy but the controller fails. > > The website borders on uselessness in this regard - your learned > opinions will be appreciated.

It sure sounds like a wierd arrangement. Standard ASi is no more "failsafe" than FF... and loss of bus power will stuff them both.

The usual way to implement a "fail-safe" solution is to design/select the valve positioners to be *inherently* fail-safe under all known scenarios (eg. spring-return-to-zero) so that in the event of comms failure the positioners themselves can act appropriately.

Sometimes people try to keep the plant operating in some fashion on comms failure (eg. to reduce downtime) but it is usually cheaper, safer and simpler in the long run to shut the entire process down. IMHO, putting in an ASi bus simply to backup a Fieldbus is a dangerous waste of the client's money.

HTH, Cameron:-)

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Cameron Dorrough
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