I’m seeking advice from experts in instrumentation: must high/low selection control and override be present simultaneously?
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Similar to this, PIC and FIC are used for low-select mode, but there is also an HPO high-pressure override; both override and low-select modes operate simultaneously
Selective control is also known as override control. Whether they exist simultaneously or separately depends on the process control requirements.
Look at PIC11X011A and FIC11X011 in this PID: they have both a low-select symbol and an HPO high-voltage override symbol. How should this logic be interpreted?
1. The diagram above should be part of the COT control for the SW cracking furnace: COT temperature control – calorific value control (calorific value compensation) – fuel gas flow rate control (high/low pressure override). Override is a mechanism used to prevent excessive pressure or low pressure of the fuel gas, while the choice between high and low pressure is the method used to achieve this. 2. The P&ID is easier to understand when viewed together with the complex control description and control circuit diagrams.
Thank you so much; now I totally understand.