Imagine there is a process that makes a gas that is too hot. The solution is to spray the gas outlet with water. That's a quencher. The PLC controls the amount the water valve is open or rather how much to close it. If the PLC dies the valve should open up as much as possible and blast water. It is better to waste water instead of risking hot gas going through ducting systems that can't handle it.

My mistake was putting failed closed valves in the system. If there was a power outage or a dead PLC no water would have cooled the gas. And presumably the ducting would have melted and there would have been fires.

Like I said my most embarrassing mistake. At least we caught it before shipment.