In your pits do you have any air or n2 sweep? Are your vapors routed directly to the incinerator?
With no pit instrumentation (Temp, Pressure, level) you are a bit in the dark. If you are running your pits dry, that can trigger fire by sulfur dropping down in the pit and causing a static charge.
Your jacketing steam? What pressure do you run at? 50#? 70#? If you had a TI, that would be helpful in checking if your pumps are seizing from running your jacketingsteam at the wrong temperature. Without a TI, take a heat gun and perform a temperature survey?
When you take your pumps out of service for maintenance, do you follow a warm up procedure so it doesn’t seize up? I would work with your sulfur pump vendor and double check if you are following the correct reliability practices.
With a fire in the pit, it would make sense that you would have high S02 numbers at your incinerator(burning sulfur = S02) and then after introducing the snuffing steam, your numbers go back down. Your not alone though, a lot of sulfur folks have had experiences like this.
Here is a link to what I think is a pretty good website: