Will PID/OUT_LO_LIM be ignored or overwritten in Local Override mode?

Commissioning & FAT
1

I’ve searched extensively for a clear answer to this but haven’t found one.

We have a control valve (0–100% open) that regulates steam flow to the reboiler on our main distillation tower. Under normal operating conditions, this valve should never fully close, so the PID block has OUT_LO_LIM = 5%.

However, during an emergency shutdown, the valve must close completely per our HAZOP. The PID already has a tracking condition configured for this scenario.

I’ve read in another discussion that TRK_VAL does not override OUT_LO_LIM, but instead bypasses all output limits entirely. Is that correct?

Put another way:

If the valve is forced into local override during an emergency shutdown, and TRK_VAL = 0%, will the valve actually drive fully closed, even though OUT_LO_LIM is set to 5%?

2

I tested the PID tracking behavior on a test system and confirmed that tracking does not override the OUT_LO_LIM parameter. When TRACK_IN_D = TRUE, the PID block output is driven to OUT_LO_LIM, not to TRACK_VAL.

I agree that the ESD system should ideally be designed so that no workaround is required.

I considered an alternative approach using a CALC block between the PID and AO blocks, where the CALC would drive the AO’s CASIN to zero whenever the actual mode is LO. However, this introduces potential side effects. For example, the DeltaV Tuner forces the actual mode to LO during process dynamics testing, which would unintentionally trigger the CALC logic. Other unintended interactions would also need to bee evaluated.