Analysis of chemical pipeline types and fire preve

2022-08-14
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Type analysis and prevention of fire and explosion accidents (chemical pipelines) -- type analysis of fire and explosion accidents (2)

Author: Huang Zhenghua, Li Jianhua, Sheng Hongjie

1 Leakage causes fire and explosion

most petrochemical pipelines convey flammable and explosive media. When the pipeline breaks and leaks, it is very easy to cause fire and explosion accidents. This is because the leaked combustible medium can burn or explode when it meets the ignition source. The parts where the pipeline often breaks and leaks mainly include: the weld joint connected with the equipment; Valve sealing gasket; Reducer and elbow of pipe section; Pipeline valves, flanges, and pipe sections in long-term contact with corrosive media; Conveying machinery, etc

leakage of pipeline quality factors, such as unreasonable design, unreasonable connection form of pipeline structure, pipe fittings and valves, or inconsistent thread system, without considering the thermal expansion of pipeline; The material itself is defective, the pipe wall is too thin, there are sand holes, and the substitute material does not meet the requirements; Poor processing, internal and external walls are scratched during cold processing; Poor welding quality, welding cracks, dislocation, burn through, incomplete penetration, overlap and undercut, etc; Sealing failure of valves, flanges, etc. Dielectric strength>30 kv/mm

leakage of pipeline process factors, such as impact and wear of high-speed flowing medium in the pipeline; The effect of repeated stress; Corrosion of corrosive medium; Creep occurs when working at high temperature for a long time; Cold brittle fracture of Materials operated at low temperature; Aging and deterioration; High pressure materials enter the low-pressure pipeline and break

damage by external factors, such as foreign flying objects, strong winds and other external forces; Vibration and swing caused by equipment and machine vibration and air flow pulsation; Damage caused by construction; Earthquake, foundation subsidence, etc

leakage caused by misoperation, such as the leakage of combustible materials due to misoperation of the valve; Overtemperature, overpressure, overspeed and overload operation; Poor maintenance, untimely maintenance, overdue and faulty operation, etc

2. Explosive mixture formed in the pipeline

during shutdown, overhaul and start-up, the pipeline was not replaced, or non inert gas was used for replacement, or the replacement was not complete, and the air mixed into the pipeline to form explosive mixture; During maintenance, the blind plate is not blocked on the pipeline (especially the high-pressure pipeline), resulting in the mixing of air and combustible gas; Negative pressure pipe sucks air; The wrong operation of the valve causes air to leak into the pipeline, or the mixture of combustible gas and combustion supporting gas, which will explode in case of ignition source

3. Overpressure explosion in the pipeline

overpressure explosion of the pipeline is related to the operation error or abnormal reaction of the reaction vessel. The failure of the cooling medium transmission pipeline leads to insufficient supply or interruption of the cooling medium, which leads to a vicious cycle of overheating and overpressure in the production system, and finally leads to overpressure explosion accidents of equipment and pipelines

abnormal pressure will be caused due to polymerization or decomposition reaction in the pipeline. If the temperature in the pipeline of ethylene and peroxide catalyst is too high, and exceeds the catalyst initiation temperature, ethylene will polymerize or decompose in the pipeline, producing high heat, raising the pressure, causing the pipeline to expand or explode

the pipeline that continuously discharges fluid, especially the process pipeline that discharges gaseous materials, will cause the materials in the equipment to not be discharged in time due to factors such as the reduction of transmission speed, so that the equipment will have overpressure explosion accidents. In January, the average import price reached 10783.4 yuan/ton

the materials of the high-pressure system flowed back into the low-pressure pipeline, causing an increase in pressure

4. Explosion due to blockage in the pipeline

if the pipeline is blocked, the system pressure will increase sharply, resulting in explosion and fracture accidents

pipelines transporting low-temperature liquids or aqueous media are prone to "freeze plugging" under low-temperature environmental conditions, especially pipelines used intermittently. The "freeze plugging" is prone to occur at the reducing part where the flow rate slows down, the part where retention can occur, and the low position

pipes conveying powdery and granular materials with high viscosity or humidity are easy to adhere to the pipe wall at the feeding point and the turning point, resulting in blockage. Unreasonable pipeline design or installation, such as long-distance transportation with large pipe diameter or sudden increase of pipe diameter, inconsistent pipeline connections, and easy blockage at obstacles; It is easy to cause blockage when the material is mixed with too large fragments; The material has the property of adhesion. If it is not cleaned in time, retention and deposition may occur, which may cause pipeline blockage

if the valve in front of the pipeline is not opened or the valve is damaged and stuck due to improper operation, or the container receiving materials is full load, or the flow rate is too slow, sudden shutdown, etc., the materials will be deposited and blocked

5. In case of spontaneous combustion fire

coking and carbon deposition in the pipeline are easy to spontaneous combustion under high temperature and high pressure, causing combustion or explosion. In the high-pressure pipeline of the refinery processing sulfur-containing raw oil, ferrous sulfide is a very common substance. It is the product of the reaction between rust and hydrogen sulfide. When the equipment is opened after shutdown and in contact with air before maintenance, spontaneous combustion will occur rapidly

the medium temperature in the pipeline is a substance that exceeds the spontaneous combustion point. If it leaks out and contacts with air, it will spontaneously ignite

6. There are many ignition sources

when materials are transported in pipelines, there are many ignition sources. When opening and closing pipeline valves, the impact and extrusion of valve clack and valve seat can become the impact ignition source. When the valve is suddenly opened between the high and low pressure sections, the gas in the low pressure section is compressed sharply, and the local temperature rises, forming an adiabatic compression ignition source. In the process of high-speed flow of materials, between powder and pipe wall, between powder particles, between liquid and solid, between liquid and gas, between liquid and another immiscible liquid, and between gas and a small amount of solid or liquid impurities. Moreover, the equipment can also do experiments such as stretching and bending of metal materials, resulting in collision and friction, which is very easy to carry static electricity and spark

there are many external ignition sources around the transmission pipeline of hazardous materials, such as friction impact, open fire, high-temperature heat, electric spark, lightning strike and so on. Static electricity will be generated when combustible materials are ejected at a high speed from the broken or poorly sealed place of the pipeline, which will become the ignition source of leaked combustible materials or surrounding combustible materials

7. As the pipeline is connected with various equipment, a fire in the pipeline will not only affect the normal operation of the pipeline system, but also cause a chain reaction in the whole production system. The accident will spread and expand rapidly, especially when the medium in the pipeline is toxic, it will pose a greater threat to human life. The explosion spread in the pipeline will change from deflagration to detonation under certain conditions, causing serious damage to production equipment, workshops and other buildings. Kefel provides a complete platform for the production of excellent bottled products

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