There are many flammable, explosive, toxic and corrosive media involved in the production of chemical enterprises, and the working conditions are dangerous conditions such as high temperature and high pressure, so the accident of pressure vessels has become the focus of enterprise monitoring, while the safety management of atmospheric vessels has become a relatively weak link. Some enterprises almost ignored the safety management of atmospheric equipment, did not analyze the working characteristics of atmospheric equipment well, and formulated corresponding safety management measures, which led to potential safety hazards of atmospheric equipment.
1 accident case and analysis
1. 1 situation 1: overpressure of atmospheric pressure vessel leads to explosion of vessel.
On February 25th, 2006, when the 2,6-dinitro-p-cresol plant of a chemical enterprise in Zibo started, the operators added nitric acid into the reactor and began to drip o-nitrop-cresol into the reactor. During the falling process, the reactor suddenly exploded, causing damage to the roof and production facilities. The direct cause of the accident is that the operator of the enterprise did not completely open the vent line valve during the reaction, which led to the pressure increase in the atmospheric reaction kettle and the explosion accident. The nitrification reactor is equipped with automatic explosion-proof and pressure-relief facilities to prevent misoperation; Second, the enterprise has not compiled detailed safety operation regulations, and has not put forward clear requirements for the operation of vent pipeline valves.
1.2 Case 2: Explosion caused by atmospheric container with flammable and explosive medium mixed with air.
2015165438+1October 17 An explosion occurred in the polymerization inhibitor preparation tank of C5 separation unit of a petrochemical enterprise in Liaoning Province, which caused a fire accident, damaged some equipment in the unit area and leaked some materials in the unit. Fortunately, there were no casualties. The direct cause of the accident was that a large space was formed on the liquid surface of the polymerization inhibitor tank. During the actual operation, the nitrogen filling facilities were not used to replace and protect the air brought in by negative pressure suction and temporary pipelines, so that the explosive mixed gas of acetonitrile vapor and air was formed in the tank space (the explosion limit of acetonitrile was 3% ~ 16%). During the long-term use of the electric tracing belt around the equipment, the insulator is damaged, which causes the tank wall to discharge, resulting in local high temperature and accidents.
2. Analyze the working conditions and safety measures of atmospheric vessels.
Atmospheric pressure vessels are different from pressure vessels. Atmospheric pressure container generally refers to a container with no pressure in it, and the pipeline or opening in the container is connected with the atmosphere, such as water tank, low concentration acid tank, alkali tank, etc. In the chemical industry, because the materials involved are mostly flammable, explosive, toxic and corrosive media, the leakage or discharge of these media in the form of gas will lead to personnel poisoning, environmental pollution, fire and explosion and other accidents. Therefore, although many storage vessels are atmospheric vessels, they are all sealed, and the reaction equipment is mostly connected with other equipment. The interior of these containers is slightly positive pressure (
The typical safety hazards of several atmospheric vessels are analyzed as follows (the following analysis is mainly based on the analysis of working pressure, regardless of other process operations, reaction temperature changes, material properties changes and other factors).
2. 1 atmospheric reactor
General working conditions: normal pressure, normal temperature, low temperature or moderate temperature. The emptying pipeline is connected to the subsequent condenser or tail gas absorption device, or heated by heat transfer oil, or the circulating water (chilled water) cooling system. Possible problems and safety measures: (1) The valve of the reactor vent line is not opened or the line is blocked; Operation error, resulting in out-of-control reaction. The pressure in the container rises, the equipment seal is damaged, and leakage occurs; The pressure exceeded the endurance limit of the equipment, which led to the explosion of the container. Safety measures: the valve on the emptying pipeline of the reaction kettle is hung with a normally open sign; The reactor is equipped with safety valve or bursting disc; Check and maintain the equipment regularly to keep the pipeline unblocked; The selection and design of the reaction kettle should consider the highest possible pressure that the reaction kettle can bear during the reaction. (2) The working state of the reaction kettle involving flammable and explosive media is negative pressure, and air is sucked into the reaction system due to misoperation or poor sealing. Therefore, the air and the combustible medium in the reactor may form an explosive mixture, and fire and explosion may occur in case of sparks or high energy. Safety measures: check and maintain the equipment regularly, and keep the equipment and pipeline flanges sealed well; For containers using inflammable and explosive materials, warning signs such as "No Opening" and "Normally Closed" shall be hung on the vent and standby port according to the process requirements. (3) There is a reaction in the closed heat preservation stage. If the reaction is not complete, the emptying valve will be closed for heat preservation. As a result, the pressure in the container increases; The equipment seal is damaged and leaked; The pressure exceeded the endurance limit of the equipment, which led to the explosion of the container. Safety measures: strictly implement process regulations; The reactor is equipped with safety valve or bursting disc; Install pressure alarm and interlock pressure relief valve.
2.2 Atmospheric storage tank for inflammable and explosive articles
General working conditions: storage at normal temperature and pressure, installation of flame arrester in vent pipeline to communicate with the atmosphere, or sealed storage of nitrogen.
Possible problems and safety measures: (1) The flame arrester is not installed in the storage tank communicating with the atmosphere or damaged. Therefore, air and combustible medium in the storage tank may form an explosive mixture, and open flames or sparks may enter the storage tank, causing fire and explosion. Safety measures: install flame arresters on the vent pipes of storage tanks, and check and maintain them regularly; The spare port should be blind and sealed. (2) The nitrogen seal tank leaks and the nitrogen seal fails. Therefore, the air and the combustible medium in the reaction kettle may form an explosive mixture, leading to fire and explosion. Safety measures: the pressure monitoring and alarm device is installed at the top of the storage tank; Nitrogen replenishment system shall be an automatic replenishment system; Check and maintain the equipment regularly, and keep the equipment and pipeline flanges sealed well. (3) The nitrogen pressure relief valve of the nitrogen sealed storage tank is damaged and the nitrogen pressure is out of control; The pressure in the container increases (this usually happens in small nitrogen sealed storage tanks, and large nitrogen sealed storage tanks are equipped with breathing valves). Lead to equipment seal damage and leakage; The pressure exceeded the endurance limit of the equipment, which led to the explosion of the container. Safety measures: strictly implement process regulations and control the pressure of nitrogen system; Nitrogen sealed storage tank is equipped with safety valve; The nitrogen buffer tank is equipped with a safety valve; Check and maintain the equipment and safety valve regularly, and keep the equipment and pipeline flanges sealed well.
2.3 Atmospheric tank operated by pressure for a short time
General working conditions: The normal process operating conditions are atmospheric pressure, and it bears certain pressure for a short time due to specific process operations such as gas pressure transfer, material transfer and connecting equipment conversion.
Possible problems and safety measures: (1) Short-term pressure rise is not considered in equipment selection and design. The result is seal leakage; The storage tank is deformed or broken; Container explosion occurs; Flammable and explosive materials can cause fire and explosion. Safety measures: select and design storage tanks according to process operating conditions; Strengthen equipment management, regularly detect equipment corrosion, and prohibit the use of containers with wall thickness less than the design wall thickness. (2) The pressure of pressurized gas exceeds the design value; Pressurized gas pressure control failed. The result is seal leakage; The storage tank is deformed or broken; Container explosion occurs; Flammable and explosive materials can cause fire and explosion. Safety measures: the selection of pressurized gas source should meet the design requirements; Before the pressurized gas enters the equipment, a buffer tank should be set up and a safety valve should be installed. The set pressure of the safety valve shall not exceed the design pressure of the atmospheric container using the gas. (3) For inflammable and explosive articles, after pressurized gas is used to pressurize and stop the gas, the equipment is under negative pressure after the container is opened or cooled, and air may enter the container; Using vacuum, the equipment is not tightly sealed, and air may enter the container; After vacuumizing, air may enter the container due to operational errors. Therefore, the air and the combustible medium in the container may form an explosive mixture, leading to fire and explosion. Safety measures: make perfect safety operation rules according to process conditions and strictly implement them; For inflammable and explosive articles, inert gas such as nitrogen is used as pressurized gas, and it is forbidden to mix with air. For vacuum pumping equipment, inert gas should be used to break the vacuum; Regular inspection and maintenance to keep the equipment and pipeline flanges sealed well; For containers using inflammable and explosive materials, warning signs such as "No Opening" and "Normally Closed" shall be hung on the vent and standby port according to the process requirements. The above analysis is a typical hidden danger analysis of a certain equipment. In the actual production process, several typical operating conditions may exist in a container at the same time. Enterprises should analyze the equipment types and uses, process conditions and material properties.
Measures for the administration of atmospheric vessels in chemical enterprises
Atmospheric pressure vessel is an indispensable part of chemical production system, which is widely distributed in all parts of the device. Relaxing the safety management of atmospheric equipment will directly lead to the reduction of system safety. Atmospheric pressure vessels belong to atmospheric pressure under normal working conditions, but if there is misoperation or failure of devices and control systems, certain pressure may be generated, and the pressure bearing capacity of the equipment itself is low. Once overpressure occurs, it is easy to cause explosion accidents. People generally focus on the operation and management of pressure vessels, so the probability of misoperation of atmospheric vessels is relatively high. To ensure the safe production of enterprises, only by strengthening management, eliminating all kinds of unsafe factors and strictly regulating the management of atmospheric vessels. It is suggested that enterprises should pay attention to the following aspects in the management of atmospheric equipment: (1) The design and selection of atmospheric vessels in the factory system should consider the possible maximum working pressure of the vessels to ensure that the vessels can withstand short-term pressure operation. (2) If misoperation or failure of related devices may lead to overpressure of atmospheric vessels, atmospheric vessels shall be equipped with pressure relief devices such as safety valves or bursting discs. (3) Atmospheric vessels with chemical reaction process should be equipped with reaction speed control system, cooling system and pressure release system according to process conditions, and equipped with safety interlock system according to process characteristics. (4) Atmospheric vessels protected by inert gas such as nitrogen seal shall be equipped with pressure detection and control system to ensure the nitrogen seal is effective, and there are automatic control measures for nitrogen pressure.
To sum up, only by strengthening the safety management of atmospheric vessels can we avoid the illegal operation of atmospheric vessels, eliminate the hidden dangers of vessels, avoid the occurrence of explosion accidents and ensure the safe production of chemical enterprises.
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