At 9:03 pm, the control room operator responsible for the catalyst section of a refinery’s fluid catalytic cracking (FCC) unit entered an erroneous opening value for the atmospheric relief valve at the discharge of a compressor blowing the air needed to suspend a catalyst inside the regenerator. This valve deviated some air flow to the compressor discharge in order to protect the compressor from pumping phenomena. The operator on duty had input, then validated, an erroneous valve opening control value (less than 10%), when he actually wanted to lower the value from 20% to 19.5%. This instruction wound up increasing air flow to the regenerator and subsequently tripping the safety mechanism for the compressor and then for the entire unit. The 15-minute unit decompression caused flare emissions, followed by a gradual shutdown. The facility operator brought the unit back online incrementally between 11 pm and 5 am, resulting in new flare emissions. The updated guideline requested the panel operator to no longer enter a value, but instead solely use the «up» or «down» arrow commands to increment the initial value by 0.5% or max 1%.