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Accident phenomenon: The temperature of the process gas at the inlet of the 109-D pre-conversion furnace dropped sharply, and consequently the temperature of the catalyst bed also decreased. Accident cause: While heating tower E66401, the control room instructed the site to confirm that the valve leading the steam from the effluent of the stripping tower to Section 1 of the furnace was closed. The site reported that the valve was closed, but in reality it was not fully closed; as a result, during the process of increasing temperature and pressure in the stripping tower, steam containing water entered the pre-conversion furnace via FRCA-2. Accident handling: Immediately notify the site to close the valve at the steam outlet of the stripping tower and leading to the first-stage furnace. After it was closed, the inlet temperature of the pre-conversion furnace began to rise gradually, returning to normal after 5 minutes. Thoughts on the accident: The issue of water in the catalyst in the pre-conversion furnace has occurred several times already; it is necessary to strictly follow the operation procedures and carry out inspections and verifications in accordance with the requirements outlined in those procedures. Previously, it was because the valve for water removal in the stripping tower was not opened, which caused the stripping tower to become filled with liquid; this liquid then made its way into the pre-conversion furnace. Such situations must be avoided in the future.