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Will OUT_LO_LIM be ignored or overwritten in Local Override mode?

I have searched around a lot for this but the answers seem unclear.

I have a control valve (0-100% open) which controls steam output to the heat exchanger of our main distillation tower. During normal operation, this valve should never close entirely, and so the OUT_LO_LIM in the PID is set to 5%

In case of an emergency shutdown however, the valve MUST be closed entirely, as according to our HAZOP. There is already a tracking condition on the PID for this. I read in a different topic that while TRK_VAL will not override the OUT_LO_LIM parameter, it will instead ignore any set limits entirely. Is this true?

In other words, if my control valve is forced into local override mode by an emergency shutdown, and TRK_VAL is set  to 0%, will it close entirely, even if OUT_LO_LIM is set to 5%?

Sander Hassing
  • JDWheelis

    In reply to Douglas Crowder:

    I tested the operation of the PID tracking on a test system and confirmed that the PID tracking does not override the OUT_LO_LIM; with TRACK_IN_D set to TRUE the PID block OUT value is driven to OUT_LO_LIM, not TRACK_VAL.

    I agree that the ESD system should be designed so that an alternate solution is not necessary.

    I considered an alternate solution involving a CALC block between the PID and AO blocks to drive the CAS_IN of the AO block to zero based on actual mode of LO, but realized the "side effects" could be a problem. For example, the DeltaV Tuner drives the actual mode to LO during the test for the process dynamics.

    Other possible side effects could also be investigated.

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