The feed valve is challenging as it can cause swings in the amount of conversion through the unit.
If the swings are wide enough, this will actually limit unit throughput and lead to increased coke laydown on the catalyst, potentially shortening reactor life.
Feed valves are usually set up as flow-control loops and are configured to fail open so that a valve failure will protect the furnace’s radiant section tubes. If a radiant tube loses or has insufficient flow, it can become so hot that it melts and ruptures, causing fuel to dump directly into the furnace firebox.
Poor valve control provides inconsistent feed rates and can lead to difficulties controlling the outlet temperature of the furnaces, or allow layers of the coke to build on the inside of the radiant tubes. This shortens the furnace cycle time between decoking procedures, which causes increased and unplanned unit shut downs.