Poor valve control provides inconsistent feed rates that can lead to more serious difficulties.
The heavy bottoms from the crude distillation unit are the feed to a vacuum crude unit, and would not have undergone any processing other than the preliminary separation of lighter components.
Feed valves in this application perform flow control and are fail open, so that a valve failure would protect the furnace radiant section tubes. If a radiant tube either loses or has insufficient flow, the tube can become so hot that they melt and rupture, which would cause fuel to dump directly into the furnace firebox.
Poor valve control also provides inconsistent feed rates that can lead to difficulties controlling the outlet temperature of the furnaces, or allow layers of coke to build up on the inside of the radiant tubes. This typically shortens the furnace cycle time between decoking procedures, which normally requires the process unit downstream to shut down.