this post was submitted on 24 Jun 2024
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I've unfortunately been working on process control strategies for almost a year now on new and novel applications for my company, so I've been intimately familiar with this. If it isn't obvious, this isn't my favorite professional area of interest hahaha.
Designating fail open and fail closed valves is so intrinsic to what I've been doing that I can't imagine someone designing a car control system and not thinking about that at all.
I designed a quencher system that failed closed, no water flowing, during outages once. Granted I was an intern but still not my proudest moment.
It's weird now as my employer is slowing moving into motion control tech for waste. Seeing the changes like having to really think about hardwired limit switches and safety relays. Chemical world I feel is easier.
We all make mistakes. I once forgot to include gravity in a pressure drop calculation for a 100 ft vertical pipe as part of a steam drum system. I had to send an awkward email revising the design pressure I previously communicated out.
But hey, if we were perfect, we wouldn't need peer review.
I have a little bit of experience with limit switches, but that's really interesting. It certainly seems like an unusual system. I'm a lot more familiar with safety relays.
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.
It happens! The important part is review and learning from the mistakes.