From the accident of Jiaxing thermal power plant: measures to prevent steam pipe burst in power plant
The steam pipe burst in the boiler operation room of Jiaxing thermal power plant. According to preliminary investigation, the main steam pipe of boiler 1 in Fuxin thermal power plant burst at the branch pipe of temperature and pressure reduction station. The accident happened in the boiler operation room. There was a lot of steam, no open smoke and no open fire.
Explosion causes of high pressure steam pipeline
Unreasonable pipeline design
Insufficient pipeline flexibility
Due to the unreasonable structure of the pipeline, the connection form of the pipe fitting and the valve or the inconsistent screw system, the pipeline will not be flexible enough. If this kind of situation is found, and the appropriate fixing method is not adopted, it is easy to cause the pipeline vibration due to the vibration of equipment and machine, air flow pulsation, so as to cause the weld crack, fatigue and deformation of fulcrum, and finally lead to the pipeline fracture.
Pipeline process design defects
This is a problem of pipeline process design, such as the connection of nitrogen and oxygen pipelines, closing the nitrogen filling valve by mistake during operation, resulting in oxygen entering into the synthetic water washing system, forming explosive mixture, which will lead to the explosion of the whole system (including the pipe network). In addition, in the design of the pipeline, the problem of uplift caused by thermal expansion of the pipeline is not considered, resulting in the collapse of the pipeline support due to the possibility of no free elongation when the pipeline support sinks or the temperature changes.
- (1) The pipeline shall be laid in a straight line as far as possible, and thermal expansion shall be considered for the connection of parallel pipes.
- (2) Two valves shall be installed for the pipeline of inert gas and combustible gas for replacement or process, and the vent valve shall be installed in the middle to vent the leaked oxygen and prevent the oxygen from entering the nitrogen pipeline. For nitrogen replacement of oxygen inlet pipe of nozzle, medium pressure steam can be used to replace and purge, so as to avoid the connection between oxygen and nitrogen pipe.
Material defects, misuse of substitute materials, etc
Due to defects of materials
If there are sand holes on the pipe wall, the method used in the processing of the bend does not match with the pipe material or the processing conditions are not suitable, so that the wall thickness of the pipe is too thin and uneven (for example, the wall thickness difference of the refining gas main pipe of φ 56 × 7 is 0.5-1.5 Mm; when the pipeline is cold processed, there are scratches on the inner and outer walls, which make the wall thickness thinner. Under the action of corrosive medium, it is easy to produce stress corrosion, accelerate the development of scars and even fracture), and the ovality exceeds the allowable range.
The selected substitute materials do not meet the requirements or are misused
The misuse of materials in design, material classification, processing and other aspects are likely to occur, such as the misuse of carbon steel pipe instead of the original design of alloy steel pipe, which will greatly reduce the mechanical strength and impact toughness of the whole pipe or part of the pipe, resulting in fracture and explosion accidents in the operation of the pipe, which has profound lessons at home and abroad.
Poor welding quality
The welding defects of pipeline mainly refer to weld crack, dislocation, burn through, incomplete penetration, weld beading and undercut, etc.
- (1) Non destructive inspection of material defects, especially castings, forgings and high-pressure pipes, shall be strictly carried out, and defective materials shall not be put into use. After installation, carry out hydraulic test, and the test pressure shall be 1.5 times of the working pressure.
- (2) The steel pipe form and material shall be selected correctly according to the process conditions of the pipe, and shall not be replaced or misused at will.
- (3) Conduct visual inspection and non-destructive inspection on the welds of pipes to ensure the welding quality.
Illegal operation and operation error
During shutdown, maintenance and start-up, the pipeline system is not replaced, or non inert gas is used for replacement, or the replacement is incomplete, air is mixed into the pipeline, and the oxygen content increases.
If the concentration does not reach the explosion limit, the oxygen mixed into the pipeline will react abnormally with the combustible gas in the pipeline, and the pressure generated after the reaction is far higher than the design pressure, the pipeline will be damaged together with the equipment.
If its concentration reaches the explosion limit, the explosive gas mixture is in danger of explosion.
During the maintenance, no blind plate is installed on the pipeline (especially the high-pressure pipeline), which causes the air and combustible gas to mix, forming explosive gas mixture, and explosion occurs during the maintenance and hot work; or after the maintenance, the blind plate on the pipeline is forgotten to be removed, and explosion occurs due to cutting off the path of gas or water vapor, resulting in pressure holding.
After the water is discharged from the maintenance tower, the air enters the pipeline and mixes with the hydrogen overflowing from the washing water to form an explosive gas mixture. When the blind plate is blocked with iron tools, sparks will occur and explode.
When purging the pipeline with steam, it is caused by forgetting to close or not to close the steam valve; during emergency shutdown and maintenance, it is also caused by forgetting to open the cover plate and vent valve of the gas producer in time, and not to conduct purging treatment, etc., as well as the causes such as water seal being blocked, check valve failure, sudden power failure, blower stopping operation, etc., resulting in combustible gas (such as gas) pipeline and water steam pipeline, gas pipeline and air There is a pressure difference between the gas pipeline, the gas or heavy oil pipeline and the oxygen pipeline, which causes the combustible gas (such as gas), the heavy oil backflow into the water vapor pipeline under maintenance, the air main pipeline and the oxygen pipeline under normal pressure, forming an explosive mixed gas, which leads to the pipeline explosion.
Because the oxygen content exceeds the standard (the oxygen content is up to 3%), the pressure of chemical reaction (transformation reaction) is too high to make the pipeline overpressure, or when the medium fracturing gas leads to the low-pressure water pipeline, it will break or explode in case of fire when it exceeds the strength limit of the pipeline.
Illegal operation and hot work during maintenance. In order to make comprehensive use of energy, a part of hydrogen produced by water electrolysis was mistakenly used for mixed combustion with gas. In the mixed combustion, air was mixed in the mixed hydrogen, which exploded in case of open fire at the nozzle of cyclohexanone dehydrogenation furnace. During the maintenance, the maintenance was carried out without fire analysis, resulting in explosion.
(1) During shutdown, maintenance and start-up, the replacement and purging of the pipeline system shall be carried out according to the regulations, and the hot work or start-up can be started only after the inspection and confirmation are qualified.
(2) Before and after the maintenance, the blind plate of the pipeline shall be pulled and blocked according to the regulations. The correct method of pulling and blocking shall be adopted. Metal tools shall not be used to avoid sparks.
(3) If it is found that combustible gas (such as gas) flows back into the steam (or air, oxygen) pipeline, the steam pressure shall be increased immediately or the flange on the steam pipeline shall be removed for sectional purging.
In case of shutdown due to sudden power failure, the furnace cover and vent valve shall be opened in time according to the regulations, and the main air valve shall be cut off to prevent the gas from flowing back into the main air pipe. It is suggested that an emergency stop interlock device and an explosion-proof membrane for the main air pipe should be added in case of emergency.
(4) Strictly control the oxygen content. When the oxygen content of the semi water gas in the ammonia plant is more than 1%, the oxygen must be cut off to prevent the high-pressure gas from entering the low-pressure pipeline. Emergency measures shall be taken in case of high pressure.
(5) It is strictly prohibited to burn the hydrogen gas which is easy to form explosive gas mixture with gas. If this method is necessary for the process, extremely strict safety measures shall be taken.
The relevant regulations on hot work shall be strictly implemented. Before hot work, the hot work analysis must be conducted. After the confirmation is qualified, the hot work permit shall be obtained, and the hot work can only be carried out in the non prohibited fire area.
The pipeline has been corroded by mother liquor and sea water for a long time, or buried underground for a long time, or laid in the trench to connect with the drainage ditch, soaked by water, which causes serious corrosion and fracture, resulting in a large number of combustible gas leakage, forming explosive mixed gas.
In the pipeline with orifice flowmeter, it is broken because of severe fluid erosion and serious wall thinning.
The vibration of the connected chemical machinery and equipment is disturbed by the air pulse, which causes the pipeline to vibrate violently and fatigue fracture.
Serious pipeline leakage, causing fire
The pipeline of compressor with oil lubrication causes combustion and explosion due to self ignition of carbon deposit under high temperature.
The pipeline bears too much external load, such as the distance between the buried pipeline and the ground surface is too shallow, the pipeline is damaged due to the rolling of heavy load of vehicles (such as multiple fractures of 140m long pipeline), or the backfill pressure is too large, resulting in the pipeline fracture.
The failure of pressure gauge and safety valve (such as the blockage of pressure gauge and safety valve pipeline) makes the pipeline and equipment unable to accurately reflect the pressure fluctuation when overpressure occurs, and the load cannot be discharged in time under overpressure.
- (1) Regularly check the corrosion of the pipeline, especially the pipeline buried underground, and repair or replace it according to the relevant regulations or the actual situation.
- (2) Control the flow rate of orifice plate and check its wear condition regularly.
- (3) Reasonable pipeline layout and proper reinforcement measures shall be taken. Buffer devices shall be set near the chemical machinery and equipment with large vibration to reduce the interference to the pipeline. When serious vibration is found, try to eliminate it in time.
- (4) Regularly check the leakage of the pipeline, find out the causes and take effective measures in time.
- (5) Select the cylinder lubricating oil reasonably, ensure the quality of the oil, fill the oil according to the requirements of the instructions, and the oil quantity is appropriate and timely.
- Adopt advanced water quality treatment process, regularly clean up the dirt and strictly control the exhaust temperature.
- Oil water separator shall be installed to discharge oil and water in intercooler, cylinder and pipeline in time. A filter shall be installed at the suction of the compressor, and the air storage tank shall be placed in a cool place.
- (6) The underground pipeline shall be laid according to the specified requirements to avoid frequent traffic, heavy traffic trunk lines or other areas with excessive external load, and the backfill shall be appropriate.
- (7) Check the pressure gauge regularly, readjust the opening pressure of the safety valve, and repair or replace the pressure gauge and safety valve in case of failure.
It can be seen from the above analysis that the main causes of the major accidents of pressure pipeline rupture and explosion are the overload inside and outside the pipeline, the mixture of combustible gas in the pipeline into the air or the backflow of combustible gas into the air system to form explosive mixture, which is caused by the explosion in case of open fire.
Of course, although there are many reasons for such accidents, most of them are operation errors, illegal operations and inadequate maintenance, followed by unreasonable design, manufacturing, installation and maintenance.
Risk points of high pressure steam pipeline
Selection of design parameters
There are many high-pressure steam pipes in the power plant, and many of them are still very hot. During the design, it is necessary to determine the design parameters of the pipeline accurately according to the operation of the unit, and then select the appropriate materials, pipe diameter and wall thickness according to the parameters of the pipeline.
Quality of pipes and pipe fittings
With the continuous popularization of large capacity and high parameter units, the parameters of the steam pipeline of the new construction project are constantly improving. The main steam pressure is usually above 30MPa (equivalent to 3000m water column height), and the temperature is above 620 ℃, so the requirements for pipes and pipe fittings are relatively high. Controlling the quality of pipes and fittings is the basic work to prevent the explosion of high pressure steam pipes.
Welds of pipes, pipe fittings, valves, etc
Valve, elbow, tee and other pipe fittings and pipe joints usually have relatively concentrated stress, and many of them are welded on site. If the welding process is improper and the quality is not good, such as improper selection of welding rod and welding wire, welding rod, welding flux is not dried and preserved as required, improper welding heating, cooling and heat preservation will cause welding cracks; if the post welding heat treatment process is improper, residual stress will be caused Cracking is caused by force; local stress is too concentrated due to undercut, staggered edge, incomplete penetration, etc. during welding, long-term operation results in aging of pipeline and cracking of welded junction. If the welding quality is not well controlled, it is easy to crack after long-term stress.
Installation of pipes, supports and hangers
Most of the high-pressure pipelines in the power plant are accompanied by high temperature. The stress caused by thermal expansion needs to be solved by reasonable pipeline layout and support and hanger settings. But in the process of construction, it is possible that the installation of supports and hangers has deviation, or the pipeline is hindered by some steel beams in the process of thermal expansion, which causes the stress change of the whole pipeline system. After long-term operation, some weak points in the pipeline system may burst.
Safety operation knowledge of pressure pipeline
High pressure steam pipeline may explode due to unstable welding joints or aging. The water vapor in the steam pipeline is generally 5600 degrees Celsius. “Once it explodes, it will fill a large area instantly, and the equipment and personnel in it are dangerous.”.
The steam pipeline is not the core of the power plant, but it is the “lifeline” connecting the boiler and the turbine. It is the power source of the generator, so it is very important.
The daily maintenance of pressure pipeline is an important basis to ensure and extend the service life and avoid accidents. The operators of the pressure pipeline must do a good job in the daily maintenance of the pressure pipeline:
- 1. Check the protective measures of pressure pipes regularly to ensure that they are intact and to reduce the corrosion on the pipe surface;
- 2. The operating mechanism of the valve shall be regularly derusted and oiled and operated regularly to ensure its flexible operation;
- 3. The safety valve and pressure gauge shall be wiped frequently to ensure their sensitivity and accuracy, and shall be calibrated on time;
- 4. Regularly check the integrity of fastening bolts to ensure that they are complete, not rusted, the screw thread is complete and the connection is reliable;
- 5. Pay attention to the vibration of the pipeline. In case of abnormal vibration, take vibration reduction measures such as isolating the vibration source, strengthening the support, etc., and take measures in time in case of friction;
- 6. The static bridging and grounding devices shall be kept in good condition and complete, and shall be repaired in time in case of damage;
- 7. The pressure pipe out of service shall be drained of internal medium, replaced, cleaned and dried, and protected by inert gas if necessary. The external surface shall be painted for protection, and the pipeline with insulation shall be in good condition;
- 8. Check the parts that are prone to corrosion and wear, such as the contact between the pipe and the support, and take measures in time in case of any problem;
- 9. Eliminate the phenomenon of running, emitting, dripping and leaking in the pipeline system in time;
- 10. For high-temperature pipeline, it is necessary to tighten the connecting bolts of the pipeline flange during the start-up temperature rise process; for low-temperature pipeline, it is necessary to tighten the bolts during the cooling process;
- 11. It is forbidden to use pipes and supports as anchor points for welding zero line and other tools, and as supporting points for prying and lifting heavy objects;
- 12. Cooperate with the pressure pipeline inspector to conduct regular inspection on the pipeline;
- 13. The maintenance and inspection shall be focused on the pressure pipes at the important parts of the production process, the pressure pipes crossing roads, bridges, railways, rivers and residential areas, the pressure pipes conveying natural, explosive, toxic and corrosive media, the pipes with severe working conditions and the pipes with alternating loads;
- 14. In case of the following situations during operation, emergency measures shall be taken immediately and relevant management departments and personnel shall be reported in time:
- (1) The pressure and temperature of the medium exceed the allowable range and are not effective after taking measures;
- (2) Cracks, bulges, deformation and leakage of pipes and components;
- (3) The pressure pipeline is frozen and blocked;
- (4) Abnormal vibration and noise of pressure pipeline endangers safe operation;
- (5) Failure of safety protection device;
- (6) Fire accident occurs and directly threatens normal and safe operation;
- (7) The valve and monitoring device of the pressure pipeline fail, endangering the safe operation.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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