What are the key points for shutting down a horizontal pressure vessel?

Shutting down a horizontal pressure vessel requires a strict pressure-temperature gradient control process to ensure safety. During the depressurization phase, first reduce the pressure to 30% of the design value using regulating valves, maintain this state for at least 30 minutes to allow stress redistribution, and then continue depressurizing to atmospheric pressure at a rate not exceeding 0.1 MPa/min. Temperature control must be carried out simultaneously, with the cooling rate controlled at 25-30℃/h to avoid thermal stress cracking caused by excessively rapid cooling. For flammable or toxic media, purging with inert gas (nitrogen) at least three times is required until the flammable gas concentration is below 10% of the lower explosive limit. The material discharge sequence should prioritize the gas phase, followed by the liquid phase drained through the bottom drain valve. High-viscosity or easily solidified media require auxiliary discharge using steam-traced pipelines.

Safety isolation is a critical step in shutdown; all process piping must be physically disconnected (with blind flanges), and the electrical system must be locked and tagged (LOTO). Before opening the manhole, ensure the pressure gauge is zero, the oxygen content is within the acceptable range (19.5%-23.5%), and that a forced ventilation system is in place. Special handling procedures include catalyst protection (maintaining a slightly positive pressure nitrogen atmosphere), anti-frost measures for cryogenic containers (gradually raising the temperature above the dew point), and injection of polymerization inhibitors into polymerization reaction vessels. Shutdown records must fully document the pressure-temperature curve, media discharge volume, isolation measures, and any abnormal conditions. This data is crucial for subsequent start-up and equipment evaluation. Afterward, verify the safety systems, checking that the pressure relief valve has reseated and the interlock devices have reset, creating safe conditions for future maintenance.