Abstract: In power plants, pipes and valves are common equipment materials and have an important role. We must tangibly understand the vibration hazards of pipeline valves, analyze the vibration hazard comprehensively, find the root cause of vibration, and then protect the pipeline valves. This paper analyzes the hazards of pipeline vibration and proposes countermeasures to solve the harm of pipeline valves.
Keywords: thermal power plant; pipeline; valve; vibration; hazard DOI: 10.16640/j.cnki.37-1222/t.2018.07.158
In the thermal power plant, the stand-alone capacity has gradually increased. Therefore, new requirements have also been put forward for pipeline valves. Due to the influence of many factors, pipe valves often exhibit vibration. Not only shorten the service life, but also lead to the rupture of pipeline valves, which seriously threatens the safe production of thermal power plants.
1 The reason why pipeline vibration occurs in power plant pipeline vibration analysis is that the reason is complicated and needs in-depth analysis. Due to vibration, it will have a serious impact on the pipeline. The main hazards are the following:
(1) Due to material overload, the working period is severely shortened.
(2) The interface of the pipeline welding is damaged, the support and hanger will fail, and even the pipeline will burst, causing serious accidents.
(3) Pipeline valve damage. Because the valve head vibration speed is too large, the valve components are not firm, causing out of control, leakage and other phenomena, at this time, the system will also appear the corresponding failure, so the test unit to stop working.
(4) Damage to the equipment, such as measuring meters, meters, etc., prevent the control system from working properly.
2 Pipeline vibration classification (1) Pipeline resonance. In the pipeline, when steam and water flow, the pipeline will vibrate. The main frequency of vibration is often a low-frequency, continuous vibration. Once the amplitude is large, it will cause great harm to the pipeline and equipment.
(2) Forced vibration. In terms of the excitation band of the medium, if the width is wide or the intensity is too large, the pipe will be forced to vibrate. For this vibration, the main vibration frequency cannot be found. Not only that, but also the mixing of different frequency components occurs. Compared to the resonant vibration, the frequency is relatively high. For this kind of pipeline, when the vibration is controlled, there exists Very difficult.
(3) Valve self-excited vibration. Due to the action of the valve, the pipe is vibrated. If this vibration is effectively suppressed, the vibration will disappear. When the medium flows through the valve, not only the flow will change significantly, but also the local part will be subject to great pressure fluctuations, and then the exciting force will appear. When the valve core or the valve stem is vibrated, the medium will be excited. This kind of exciting vibration is transmitted in the form of waves. The propagation characteristics are high frequency and strong vibration. In the graph of the frequency spectrum, the dominant frequency is seen. For this kind of vibration, valves can be used for inspection and modification, and the control system can also be adjusted.
(4) Two-phase flow pipe vibration. Due to the rise of the pipeline, the pipeline pressure of the deaerator is reduced. If the saturation point is reached, part of the water will vaporize and form vapor and liquid phases, which will cause the pipeline to be compromised. Between the two phases of vapor and liquid, there will be a large density difference, the flow field is also relatively confusing, and the damage will be worse than the vibration of a medium.
(5) Pipe hammer impact vibration of water hammer. The sudden start of the valve is mainly due to the effect of condensed water in the steam pipe. The steam can accelerate the water instantaneously and reach the speed of the steam. At this time, the valve can be started instantaneously, Δt is small. In the formula of F=mV/Δt, F stands for impact force, m stands for mass, v stands for speed, and Δt stands for pole time. Therefore, the larger the F, the more vulnerable the pipeline valves are. When the water is not drained, it will also cause shock problems.
(6) Due to the low flow rate of the feed pump, the pipeline is vibrated. For a feed pump, if it is smaller than its own minimum flow rate, cavitation is likely to occur at this time, and through the effect of medium pressure, significant fluctuations will occur, and therefore, a strong pipeline vibration is generated. To protect the pump and piping system, low-flow work should be avoided.
3 Principles of Pipeline Vibration Treatment The main reason for studying the vibration of pipelines is to further clarify the vibration mechanism of different types of pipelines, so as to take effective measures to eliminate vibration and minimize the harm of vibration. If the piping is stiff, the inherent frequency will be greater. When adjusting the natural frequency of the pipe system, it is mainly to adjust the stiffness of the system, the number of elbows can be adjusted, the wall thickness of the pipe diameter can be increased, and the number of brackets can be increased. These methods can change the rigidity of the pipe system. In general, the first step is to analyze the natural frequency of the pipe system. The second step is to adjust the frequency to eliminate low-order excitation frequency. This can prevent resonance. Under the influence of the excitation frequency, the excitation frequency, and the stiffness of the pipeline, it will increase the difficulty of governance. When the pipeline vibration is to be treated, the stiffness of the piping system should be changed scientifically to maintain the reasonable stress of the pipeline.
4 Countermeasures for the treatment of pipeline vibrations (1) Check and audit the design drawings of the pipeline.
(2) Site investigation of pipeline vibrations, recording of vibration phenomena and analysis.
(3) Conduct a comprehensive inspection on the site of pipe hangers. Under normal circumstances, the following points should be examined:
1 Check the original damping limit and the corresponding constraint points to see if the installation work is normative.
2 Check the operation of the piping system and check for signs of swelling.
3 Check each hanger.
(4) Measure the vibration of the pipeline, process the corresponding data, and then obtain the time domain and frequency spectrum of the pipeline.
(5) Through the pipeline analysis software, analyze the pipeline modality, we must do a good job in two aspects, one is to calculate the natural frequency of the pipeline; the second is to analyze the vibration shape of the pipeline.
(6) Initially determine the treatment measures for pipeline vibrations and formulate a scientific governance plan.
(7) Analyze and calculate the stress of the pipeline, and the pipeline must have strong flexibility.
(8) Submit the corresponding material list and vibration reduction plan to implement the vibration management plan.
5 Discussion and Suggestions on Vibration Treatment of Pipeline (1) The looseness caused by pipe support and hangers and the vibration caused by cracks shall be adjusted to support and hanger, and the welding and hanger shall be reinforced.
(2) For the insufficient or weak strength of the pipe hangers, additional hangers are required.
(3) If the vibration excitation source cannot be found or the vibration of the excitation source cannot be controlled, a vibration damper should be added to reduce the vibration.
(4) Renovate the orifice plate.
(5) Adjust the operating mode and change the flow rate.
6 Conclusion In the production of power plants, in addition to having pipes and valves, vibration occurs in other equipment. Relevant personnel should master basic knowledge of vibration, properly handle this vibration, minimize equipment hazards, and extend the working life of various equipment. Therefore, we should conduct a thorough analysis of the operation of pipeline valves on site, formulate effective treatment plans, avoid unplanned downtime, and reduce unnecessary accidents, so as to increase the economic efficiency of thermal power plants.
references:
[1] Xi'an Thermal Engineering Research Institute of power generation equipment special detection and life management [M]. China Electric Power Press.
[2] Large-scale thermal power plant maintenance and practical technology series [M]. China Electric Power Press.
Keywords: thermal power plant; pipeline; valve; vibration; hazard DOI: 10.16640/j.cnki.37-1222/t.2018.07.158
In the thermal power plant, the stand-alone capacity has gradually increased. Therefore, new requirements have also been put forward for pipeline valves. Due to the influence of many factors, pipe valves often exhibit vibration. Not only shorten the service life, but also lead to the rupture of pipeline valves, which seriously threatens the safe production of thermal power plants.
1 The reason why pipeline vibration occurs in power plant pipeline vibration analysis is that the reason is complicated and needs in-depth analysis. Due to vibration, it will have a serious impact on the pipeline. The main hazards are the following:
(1) Due to material overload, the working period is severely shortened.
(2) The interface of the pipeline welding is damaged, the support and hanger will fail, and even the pipeline will burst, causing serious accidents.
(3) Pipeline valve damage. Because the valve head vibration speed is too large, the valve components are not firm, causing out of control, leakage and other phenomena, at this time, the system will also appear the corresponding failure, so the test unit to stop working.
(4) Damage to the equipment, such as measuring meters, meters, etc., prevent the control system from working properly.
2 Pipeline vibration classification (1) Pipeline resonance. In the pipeline, when steam and water flow, the pipeline will vibrate. The main frequency of vibration is often a low-frequency, continuous vibration. Once the amplitude is large, it will cause great harm to the pipeline and equipment.
(2) Forced vibration. In terms of the excitation band of the medium, if the width is wide or the intensity is too large, the pipe will be forced to vibrate. For this vibration, the main vibration frequency cannot be found. Not only that, but also the mixing of different frequency components occurs. Compared to the resonant vibration, the frequency is relatively high. For this kind of pipeline, when the vibration is controlled, there exists Very difficult.
(3) Valve self-excited vibration. Due to the action of the valve, the pipe is vibrated. If this vibration is effectively suppressed, the vibration will disappear. When the medium flows through the valve, not only the flow will change significantly, but also the local part will be subject to great pressure fluctuations, and then the exciting force will appear. When the valve core or the valve stem is vibrated, the medium will be excited. This kind of exciting vibration is transmitted in the form of waves. The propagation characteristics are high frequency and strong vibration. In the graph of the frequency spectrum, the dominant frequency is seen. For this kind of vibration, valves can be used for inspection and modification, and the control system can also be adjusted.
(4) Two-phase flow pipe vibration. Due to the rise of the pipeline, the pipeline pressure of the deaerator is reduced. If the saturation point is reached, part of the water will vaporize and form vapor and liquid phases, which will cause the pipeline to be compromised. Between the two phases of vapor and liquid, there will be a large density difference, the flow field is also relatively confusing, and the damage will be worse than the vibration of a medium.
(5) Pipe hammer impact vibration of water hammer. The sudden start of the valve is mainly due to the effect of condensed water in the steam pipe. The steam can accelerate the water instantaneously and reach the speed of the steam. At this time, the valve can be started instantaneously, Δt is small. In the formula of F=mV/Δt, F stands for impact force, m stands for mass, v stands for speed, and Δt stands for pole time. Therefore, the larger the F, the more vulnerable the pipeline valves are. When the water is not drained, it will also cause shock problems.
(6) Due to the low flow rate of the feed pump, the pipeline is vibrated. For a feed pump, if it is smaller than its own minimum flow rate, cavitation is likely to occur at this time, and through the effect of medium pressure, significant fluctuations will occur, and therefore, a strong pipeline vibration is generated. To protect the pump and piping system, low-flow work should be avoided.
3 Principles of Pipeline Vibration Treatment The main reason for studying the vibration of pipelines is to further clarify the vibration mechanism of different types of pipelines, so as to take effective measures to eliminate vibration and minimize the harm of vibration. If the piping is stiff, the inherent frequency will be greater. When adjusting the natural frequency of the pipe system, it is mainly to adjust the stiffness of the system, the number of elbows can be adjusted, the wall thickness of the pipe diameter can be increased, and the number of brackets can be increased. These methods can change the rigidity of the pipe system. In general, the first step is to analyze the natural frequency of the pipe system. The second step is to adjust the frequency to eliminate low-order excitation frequency. This can prevent resonance. Under the influence of the excitation frequency, the excitation frequency, and the stiffness of the pipeline, it will increase the difficulty of governance. When the pipeline vibration is to be treated, the stiffness of the piping system should be changed scientifically to maintain the reasonable stress of the pipeline.
4 Countermeasures for the treatment of pipeline vibrations (1) Check and audit the design drawings of the pipeline.
(2) Site investigation of pipeline vibrations, recording of vibration phenomena and analysis.
(3) Conduct a comprehensive inspection on the site of pipe hangers. Under normal circumstances, the following points should be examined:
1 Check the original damping limit and the corresponding constraint points to see if the installation work is normative.
2 Check the operation of the piping system and check for signs of swelling.
3 Check each hanger.
(4) Measure the vibration of the pipeline, process the corresponding data, and then obtain the time domain and frequency spectrum of the pipeline.
(5) Through the pipeline analysis software, analyze the pipeline modality, we must do a good job in two aspects, one is to calculate the natural frequency of the pipeline; the second is to analyze the vibration shape of the pipeline.
(6) Initially determine the treatment measures for pipeline vibrations and formulate a scientific governance plan.
(7) Analyze and calculate the stress of the pipeline, and the pipeline must have strong flexibility.
(8) Submit the corresponding material list and vibration reduction plan to implement the vibration management plan.
5 Discussion and Suggestions on Vibration Treatment of Pipeline (1) The looseness caused by pipe support and hangers and the vibration caused by cracks shall be adjusted to support and hanger, and the welding and hanger shall be reinforced.
(2) For the insufficient or weak strength of the pipe hangers, additional hangers are required.
(3) If the vibration excitation source cannot be found or the vibration of the excitation source cannot be controlled, a vibration damper should be added to reduce the vibration.
(4) Renovate the orifice plate.
(5) Adjust the operating mode and change the flow rate.
6 Conclusion In the production of power plants, in addition to having pipes and valves, vibration occurs in other equipment. Relevant personnel should master basic knowledge of vibration, properly handle this vibration, minimize equipment hazards, and extend the working life of various equipment. Therefore, we should conduct a thorough analysis of the operation of pipeline valves on site, formulate effective treatment plans, avoid unplanned downtime, and reduce unnecessary accidents, so as to increase the economic efficiency of thermal power plants.
references:
[1] Xi'an Thermal Engineering Research Institute of power generation equipment special detection and life management [M]. China Electric Power Press.
[2] Large-scale thermal power plant maintenance and practical technology series [M]. China Electric Power Press.
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