Vibration analysis of pressure pipeline
In industrial production, the pressure pipeline used is affected by the environment itself and the external environment, causing the pipeline to cause vibration. If the pipeline is affected by vibration for a long time, the fatigue will be generated at the location where the stress is concentrated, and the pipeline will be broken. Causes more serious safety accidents, which causes the leakage of the medium. Therefore, the vibration of the pipeline should be minimized during production to avoid unnecessary safety accidents.
1 Analysis of the vibration source of the pressure pipeline
The pipes, supports and connected equipment of the pressure pipe form a structural system that vibrates in the presence of exciting forces. The vibration sources of pressure pipes are roughly divided into two categories: from the system itself and from outside the system.
- From the system itself, mainly the vibration of the machine connected to the pipeline and the vibration caused by the unstable flow of the fluid in the pipe;
- There are earthquakes and winds from outside the system. Vibration is a kind of alternating load on the pressure pipeline, and the degree of damage depends on the magnitude of the exciting force and the vibration resistance of the pipeline itself.
The main influencing factors are as follows:
- Gas column natural frequency. The fluid filled in the pipe is a flexible gas column. When the compressor or the cylinder of the pump draws or exhausts from the pipe, the gas column in the pipe is disturbed and vibrates.
- Mechanical natural frequency. The pipe system is a continuous elastic body, and there is a natural frequency of the structure. If the vibration frequency of the machine is equal to the natural frequency of the pipe structure, resonance will also aggravate the vibration of the pipe system.
- Vibration caused by tube flow pulsation. The pipeline fluid is pulsating under the action of the compressor or the pump. When flowing through the pipeline components such as the elbow head, the reducer, the tee, the valve, etc., the force on the pipeline is also constantly changing, resulting in a certain change with time. Exciting force, generating vibration.
- Vibration caused by two-phase flow. In the pipeline system, the two-phase flow of gas and liquid is mainly used, and the compressibility of the gas should be larger than that of the liquid. When the two-phase flow forms a large bubble flow pattern in the pipeline, the bubble is compressed in different regions. Different, so that the pipe vibrates.
- Liquid shock vibration. When the valve in the pipeline suddenly closes or opens, the fluid speed suddenly changes, causing a great impact on the piping system and generating vibration. Liquid hammer is a very important source of vibration in pipeline systems. Sometimes it causes a large change in pressure inside the pipeline. In severe cases, the pipe can burst.
- The fluid flow rate in the pipeline is too fast, and the turbulence caused by the separation of the fluid boundary layer causes the pipeline to vibrate.
- Earthquakes and winds from outside the system pose a threat to the pipeline. Once a strong earthquake occurs, there is a danger of damage to the buried pipeline. Although the overhead pipeline is not directly affected by the earthquake, the pipeline support points are mostly based on the ground. If it is damaged, the pipe will collapse and cause damage. The influence of the wind is untimed as long as the pipe is fixed firmly, although the pipe will vibrate due to interference, but the impact is not great.
2 Measurement of pipeline vibration
For a pipe system with vibration, to reduce or eliminate vibration, it is necessary to fully grasp the relevant parameters and take appropriate anti-vibration measures.
- Pipe structure vibration measurement: Vibration measurement is to measure the displacement of vibration. The measuring device consists of three parts: sensor, amplifier and recorder. The sensor is a speed sensor, and the output voltage is proportional to the measured vibration speed. The obtained signal is converted into a displacement signal after one integration, and amplified by the amplifier to the recorder.
- Pipeline pressure pulsation measurement: The most basic part of the airflow pressure pulsation measuring device in the pipeline is the sensor, amplifier and recorder. The function of the sensor is to convert the signal of pressure change into a signal of electric quantity change. The amplifier accurately linearly amplifies the electric signal obtained by the sensor and pushes the recorder to record the pressure pulsation curve.
- Pipeline system natural frequency measurement: Generally, the pipe system is used to excite the pipe system. The point or point of the pipe is tapped to record the attenuation vibration signal, and the signal is quickly analyzed to obtain the natural frequencies of each order. This method is very simple and commonly used.
3 Measures to reduce and eliminate pressure pipe vibration
3.1 Change the natural frequency of the pipeline
According to the vibration theory: the multi-degree-of-freedom vibration equation of a mechanical system can be expressed by a matrix differential equation:
Where: M is the mass matrix, X is the node displacement vector, C is the damping matrix, K stiffness matrix, and F is the interference force and the excitation force vector.
It can be seen from the above formula that to change the vibration characteristics of the pipeline system, the following points can be considered:
- Add a balancing block to the piping system and change the mass matrix to change the natural frequency of the system to avoid pipeline resonance.
- Change the damping matrix of the pipeline system, and place flexible isolation objects such as metal springs and cork in the fixed support part of the pipeline to achieve the purpose of vibration isolation and vibration elimination.
- By increasing the stiffness matrix of the pipeline system, by increasing the support, adjusting the support position or changing the support properties, the natural frequency of the pipeline is increased, and the elastic support is a rigid support tube, which increases the natural frequency to achieve the purpose of vibration reduction.
3.2 Descending airflow pulse vibration
- In order to adjust the natural frequency of the gas column and avoid the vibration caused by the excitation frequency, it is necessary to avoid the gas resonance frequency. This depends on the length of the piping method, the diameter of the pipe, the size of the gas, and the type and temperature of the gas. When designing the piping, it is necessary to calculate the natural frequency of the piping column and adjust the frequency according to the design of the process.
- For pulsating piping systems, measures need to be taken to reduce pressure pulsations:
- 1) Select a compressor with a small pressure pulsation (the more the number of times the compressor cylinder is inhaled and the outward venting in one cycle, the smaller the pressure pulsation in the pipeline);
- 2) A buffer is provided on the compressor (the damper is a container whose volume is more than 10 times larger than the cylinder volume, and the pressure pulsation of the compressor intermittently discharged into the buffer will be significantly decreased);
- 3) Setting the orifice plate is one of the most effective ways to eliminate the vibration of the pipeline. Therefore, the function of the orifice plate is to reduce the pressure inside the pipe and effectively control the pulsation of the gas flow to eliminate the vibration.
3.3 Measures to mitigate the vibration of two-phase flow In order to avoid or reduce the vibration of the pipeline caused by two-phase flow, measures should be taken from the following two aspects:
- Reduce the excitation force, improve the heat insulation design requirements for the two-phase pipeline conveying the saturated state, minimize the length of the pipeline, reduce the liquefied gas by using the appropriate flow rate, and the process of disguising is not intense.
- Strengthen the rigidity of the bracket. Due to the particularity of the two-phase flow, the exciting force to the pipeline is inevitable. When the bracket is designed, the two-phase flow pipeline should be considered to have the necessary bracket stiffness to resist its exciting force.
- Avoid resonance, the exciting force of the two-phase flow occurs randomly, and the actual vibration frequency and displacement of the pipeline should be detected so as not to be close to the natural vibration frequency of the pipeline. Since the strong rigidity of the reinforcing bracket changes the natural frequency of the pipeline, the natural frequency is calculated after the rigidity of the pipeline bracket is strengthened.
3.4 Reduction of liquid shock vibration The main methods of eliminating or weakening liquid shock in the pipeline system are as follows:
- Slow down the valve, the longer the valve closes, the smaller the hydraulic pressure.
- Shorten the length of the pipe, the shorter the length of the pipeline, the smaller the hydraulic pressure.
- It is safe to release and absorb the energy of the liquid hit by installing safety valves, accumulators, etc. in the place where the pipeline is next.
- Use a valve with a liquid-proof function. Using a disc type check valve, the closing process is completed in two stages, which can adjust the fast and slow corner stroke according to the specific system requirements, and the slow closing time. This valve can prevent liquid shock when the pump is suddenly stopped. Effect. There is also an electro-hydraulic servo-adjusting ball valve disposed in the piping system, which controls the lower limit of the pressure of the inlet pump (station) and the upper limit of the pressure of the outlet (station), so that the pressure of the piping system is maintained within the set range. In the pipe system, a liquid blow is generated, and the liquid pressure pressure wave is transmitted to the regulating valve, and the regulating valve acts quickly. When the liquid pressure pressure wave is in an ascending state, the valve is quickly opened to release the pressure, and the liquid pressure pressure wave is rapidly closed when the pressure wave is negative. According to the actual test on site, the liquid pressure can be controlled within 3-10 seconds so that it does not exceed 10% of the operating pressure.
3.5 Improving the seismic capacity of pipeline systems.
The main measures taken by earthquakes on pipelines are: steel components are used for pipeline components. Flexible pipe fittings are used, and waveform compensators are the most important. The frame type bracket structure is adopted, and the length of the beam is appropriately lengthened, and the end of the frame is welded to prevent the pipe from slipping off the frame. For the long-distance and pipeline buried routes, it is necessary to choose appropriate to avoid liquefaction in the seismic instability zone when the power is acting, which requires a series of safety measures such as reinforcement of the poor land area; It should be laid on the platform made after cutting the soil, and set up as a soil wall, and leave an operating platform.
Due to the complicated structure of the pipeline, the vibration caused by the pipeline system is inevitable. Therefore, it is necessary to understand the causes of the vibration of the pipeline, the measurement method of the vibration, and the measures for reducing and eliminating the vibration. In the process of designing, installing and using the pipeline, it is necessary to fully consider the influence and harm of the vibration of the pipeline system, reduce or eliminate potential hidden dangers caused by vibration in the pipeline system, and ensure the safe and normal operation of the pressure pipeline.
Source: China Pressure Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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