I have a 4 pump program that needs a second wet well level transmitter to fall back on in the event that the primary wet well level transmitter fails.

I am using a Siemens s7-300 PLC combined with a Maple system HMI.

Can someone please break down the logic for me or provide examples? i.e. fail-overs?

Any advice or assistance would be greatly appreciated.

 

with only two indications fail-over is not beneficial as you do not know which has failed, so you need a minimum of three transmitters. In really critical cases even more transmitters are required often of different manufacture to avoid all transmitters having the same failure modes.

so with two as you have described, the second is for back up alarming/tripping

simple redundancy where either sensor can trip/alarm is usually the rule.

 

We measured temperature using 3 probes and averaged the values. At the same time, we monitored each input for "out of range" condition (usually configured at the analog input card). We also looked for an excess difference of reading with respect to the others. If one of the probes was different than the others (outside the range of tolerance), or if it was out of range as declared by the input card, then that probe was removed from the average calculation and an alarm was declared.

The same principal could be applied to redundant level measurement.

Cheers

 

It's not bulletproof, but you can do a redundant-ish system with just two transmitters. When they read the same within a tolerance all is good. When they diverge, trust the one trending more steadily (and also flag an alarm). The only way that approach will fail you is if the process variable slews at the same moment one transmitter fails, and in such a way as to keep sending the previous value range, thus causing the working one and not the broken one to change value; not very likely.

If both are trending with similar steadiness and just drift apart beyond the tolerance (neither one kneed over), THEN you have no idea which one (or both) is bad, so declare a fault and shut down.

Also, this scheme catches transmitter failures dramatic enough to raise a broken transmitter alarm in the analog input while still leaving you operational.

 

Cyanogen ... can you provide additional installation details to determne if an inferential-method is possible?

Regards, Phil Corso

 

Thanks for all the help guys I figured this one out over the weekend.

I ended up using a watchdog timer in conjunction with a TON block with a set and reset bit.