In-depth analysis of accident cases: How to handle emergencies when a bottom-mounted quick shut-off valve suddenly closes

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As the cornerstone of crude oil processing, the stability of the operation of atmospheric and vacuum distillation units directly determines the safe and efficient functioning of the entire facility. As a key valve connecting the atmospheric pressure system and the reduced-pressure system, the normal-pressure rapid shut-off valve (HSVCx020) can, if it shuts down suddenly, lead to cascading risks such as a spike in furnace temperature and a disruption in the flow of materials; it thus represents a \"critical control point\" for emergency management in the facility. Based on accident cases, it breaks down the key practical aspects at the front line, covering phenomena, causes, handling procedures, and post-mortem analysis. I. Accident phenomena: Dual-warning for abnormal signals and parameters. When an accident occurs, there is a significant correlation between abnormal signals and changes in process parameters, which is key to quickly identifying risks: 1. Sudden changes in control system signals: In the DCS system, the valve status of the normal bottom fast shut-off valve HSVCx020 changes instantly from “OPEN” to “CLOSE”; the abnormal shift in the valve feedback signal is the first obvious indicator for identifying an accident. 2. Interconnected fluctuations in process parameters: The feed flow rates of the four branches connected to the reductor are reset to zero simultaneously; this interruption in material supply causes the outlet temperature of the vacuum reactor and the level of the liquid in the constant-bottom tank to rise sharply. These rapid fluctuations in these two parameters represent key signs of risks for the plant, and immediate action is required. II. Causes of the accident: Precise identification of the three key contributing factors. The main reasons for the closure of the quick shut-off valve lie in equipment failures, human error, and external power supply issues. On-site personnel need to quickly determine the cause by considering the actual situation: 1. External and equipment-related failures: Sudden interruption of instrument air (a problem with the air supply system), interruption of the power supply to the quick shut-off valve’s instruments, or malfunctioning circuits behind the control panel or the valve actuator are the primary types of equipment-related failures. 2. Human operational errors: Mistakes made by operators during DCS operations or when starting/stopping auxiliary control panels, as well as incorrect settings for valve parameters, constitute the main causes of human-related issues. 3. Other potential hazards: sticking of valve actuators, incorrect triggering of control system logic, etc., can also lead to abnormal closure of the valve; a thorough investigation on site is required to confirm these issues. III. Accident confirmation: Rapid determination of valve status. The first step in handling an accident is to accurately determine the actual status of the valves, in order to avoid misjudgments based solely on DCS signals. Through on-site inspections, verification of valve position feedback, or by utilizing real-time DCS monitoring data, it was confirmed that the normal bottom quick shut-off valve HVx020 was in the closed state, providing a crucial basis for subsequent actions. IV. Accident Handling: Precise Handling Procedures by Scenario. The handling process is centered around \"rapid risk control, accurate cause identification, and orderly maintenance of production.\" Depending on the reason for the valve closure, there are two main handling scenarios, with clear and actionable steps: Scenario 1: Valve closure caused by operational error (prioritize restoring production). 1. Immediate temperature control to stop further damage: The central control room promptly operates the temperature control valve TICx738 to reduce the temperature at the furnace outlet ； If the upward trend in temperature does not abate, press the F1002 emergency shutdown button on the auxiliary control panel to extinguish the flame in the heating furnace, thereby preventing further rises in temperature that could cause damage to the equipment. 2. Manual restoration of the valve: After identifying the cause of the error, manually open HVx020 in the DCS system, and change the display status of HSVC1020 from “CLOSE” to “OPEN” in order to restore the material flow pathway. 3. Gradual restoration of operating conditions: Organize adjustments to the operation status of the constant-bottom pump to restore it to the operating parameters prior to fluctuations in the shut-off valve ； In coordination with the internal operations, feed to various branches of the vacuum furnace is gradually restored, and the outlet temperature of the vacuum furnace is slowly increased until it returns to its stable level before the accident. Parameter fluctuations are monitored throughout the process to prevent secondary shocks. Scenario 2: Valve closure due to wind stoppage/power failure (handled as an emergency shutdown). 1. Reporting and coordination: Immediately inform the shift leader, and at the same time contact the dispatch team and the workshop; activate the emergency shutdown plan for the facility to prepare for a full shutdown of the entire plant. 2. Equipment and system processing: Contact instrumentation professionals to diagnose and repair the HVx020 ； If the air supply is interrupted and cannot be restored quickly, the instrumentation staff shall open the quick shut-off valve and the instrument air balance valve to relieve pressure, manually operate HVx020, and feed base oil into the furnace reduction feed line in order to prepare for resuming operations. 3. Control of the shutdown state: Strictly implement emergency shutdown procedures for the equipment, monitor changes in the parameters of various units, and ensure that there are no risks such as overheating, overpressure, or leaks during the shutdown process, thereby achieving a safe and stable shutdown state. V. Core Disposal Principles and Potential Risk Control 1. Disposal Priority: Safety first; control the temperature before attempting to restore normal operations. Whatever the reason, when the furnace temperature soars, cooling measures and shutdown procedures must be taken immediately to prevent safety accidents such as coking and damage to the heating furnace. 2. Key parameters for monitoring: During the recovery process, it is necessary to closely monitor three core parameters – the flow rate at the furnace reduction branch, the temperature at the furnace outlet, and the level of the constant-bottom liquid – to ensure a gradual return to stability and prevent new faults from arising due to sudden changes in these parameters. 3. Emergency shutdown boundary: If wind stoppage or power supply failures cannot be resolved in a short time, immediately initiate an emergency shutdown. Do not act recklessly to prevent the faults from worsening; this is the minimum requirement for the safe operation of the facility. VI. Post-event review: Optimization directions from response to prevention 1. Strengthening operational standards: Special training sessions are conducted for key valves such as quick-shut valves, to identify potential risks associated with incorrect operations; the procedures for DCS operation and button management on auxiliary control panels are standardized in order to reduce human errors. 2. Equipment and power supply assurance: Regularly inspect the instrument air system, valve power supply circuits, and actuators to identify potential issues in advance ； Establish emergency response plans for failures of critical valves, stockpile emergency repair materials, and reduce the time required to address such failures. 3. Regularization of emergency drills: Such emergency drills for sudden accidents should be organized on a regular basis to improve the speed of coordinated response between the control room and field operators, optimize the handling procedures, and ensure a seamless integration of theory and practical application. Emergency response to rapid cut-off valves in normal operating conditions tests the rapid judgment, precise operation skills, and emergency coordination abilities of frontline personnel. Only by thoroughly understanding the logic behind accidents, standardizing disposal procedures, and improving prevention mechanisms can we respond in a steady, accurate, and rapid manner in the face of sudden risks, thereby ensuring the safe and efficient operation of the facilities.