Research on Compressor Pipe Vibration

1 Reasons for pipe vibration

Compressors and their pipelines, pipelines and their supports, these connected devices or devices constitute a complex mechanical structural system. The vibration generated by this system is a process affected by many factors. The main reasons are as follows:

1.1 The phenomenon that the pressure, velocity, density and other parameters of the pipe flow pulsation pipe change periodically with time is called pipe flow pulsation. Reciprocating compressors vary the parameters such as pressure, velocity, and density of the pipeline fluid due to suction, intermittent and periodic displacement. This change is manifested in both the change with position and the change with time. When the pulsating airflow encounters pipe components such as elbows, tees, valves, flanges, blind plates, and reducers during pipeline transportation, it will produce exciting vibrations with time. These exciting forces act on the pipelines and ancillary equipment. Generate vibration.

1.2 Vibration due to improper assembly of equipment or improper design of foundation

The inertial force generated by the unbalanced mass of the compressor moving mechanism or the improper design of the foundation can cause the unit and the pipeline to vibrate. The installation should conform to the installation specifications to ensure that the vibration is within the design range. The anchor support is unstable and cannot effectively absorb the vibration energy of the pipeline. Therefore, the vibration of the pipeline is often caused by improper design of the foundation. Compressor API Standard (API617-2002) Chapter 3 333 clearly defines the floor and pad assembly of the compressor. Installation according to the assembly drawing can effectively reduce the vibration of the pipe.

The anchor support is unstable, and the looseness of the foundation will cause severe vibration of the compressor itself and the pipeline, which will cause severe vibration of the unit even under no-load conditions. Therefore, in actual operation, the assembly generally conforms to the installation specification. If the vibration of the equipment and the pipeline is extremely severe, the first consideration is the anchor bolt, and whether the foundation is loose or not.

1.3 resonance

(1) Air column resonance: The gas contained in the piping system is called a gas column. The gas column itself is like a spring in a vibrating system. The gas column can be compressed and expanded with a certain quality. Therefore, the gas column itself is a vibration system, and vibration will occur under a certain excitation force. When the compressor piston moves, it periodically inhales into the pipeline, and the exhaust gas generates an exciting force to the gas in the pipeline, causing the gas column to vibrate. Air column resonance means that when the compressor excitation frequency is (08~12) times the gas column resonance frequency, the gas column in the pipeline is in a resonance state. At this time, the airflow pressure pulsation is abnormally serious, causing

Factors affecting the natural frequency of the gas column In addition to the composition of the medium (gas), there are also the size and setting position of the buffer, the diameter of the connecting tube, the branch of the piping system, the length of the straight pipe section, the end state of the pipe section, etc., and changing these conditions can be changed. The natural frequency of the gas column.

For simple piping, the design can be simplified by calculating and changing the length of the resonant tube to avoid resonance. The so-called resonance tube length refers to the length of the straight pipe section that causes the gas column to resonate when the excitation frequency is constant. The length of the resonance tube can be divided into the following cases for calculation.

(a) One end is open and one end is closed

In this case, the vessel connected to the pipeline has a smaller volume at one end and a larger volume at one end, such as a buffer to the compressor cylinder inlet and a conduit between the compressor cylinder outlet and the buffer tube.

(b) Both ends are closed, and both ends are open. In this case,

(2) Pipe mechanical resonance: The pipe consisting of pipes, pipe fittings and brackets is itself an elastic system. According to the piping condition, the type and position of the support will also have a series of natural frequencies. When the excitation frequency is equal or close to a certain natural frequency, the mechanical resonance of the pipeline occurs.

When the excitation frequency is (08~12) times the natural frequency of each order of the pipeline (especially the low order), the mechanical vibration of the pipeline can be avoided. Like air column resonance, mechanical resonance must be avoided in engineering design. When the natural frequency of the piping system cannot completely shift the excitation frequency, the piping structure should be adjusted to meet the requirements.

2 Vibration reduction measures For the above reasons, there are three points in the solution for the vibration of the reciprocating compressor:

(1) Control tube flow pulsation without resonance;

(2) Fasten anchor bolts, compressor foundations, etc., to eliminate the vibration of the body and pipeline caused by looseness;

(3) Adjust the natural frequency of the pipeline, adjust the vibration frequency of the gas column of the piping system so that it does not coincide with the excitation frequency, and eliminate or reduce the vibration of the pipeline caused by resonance.

2.1 Compression of compressor flow pulsation

The control of the pressure pulsation of the compressor line is complicated. In addition to repeated calculations and reasonable adjustments, the buffer, orifice plate, branch pipe, components of the header, or some parts such as liquid flow must be properly arranged at the appropriate position of the system. Dampers, dampers, accumulators, etc., to reduce or suppress pressure pulsations.

There are several types of vibration damping equipment.

(1) Cylinder outlet setting buffer tube The buffer tube can effectively reduce the impact of pressure pulsation on the pipeline. The main principle is the energy storage effect of the buffer volume. The intake damper limits the radiation pressure waves from the upstream of the cylinder into the intake duct; the exhaust damper not only limits the returning reflected waves from entering the cylinder, but also limits the pressure waves from entering the exhaust duct.

The damping effect of the damper depends on the size and position of the damper volume being close enough to the cylinder. The installation of the damper close to the cylinder of the compressor is a simple and effective damping measure, and the buffer away from the cylinder often does not provide the desired cushioning effect.

(2) increase the orifice plate

The so-called orifice attenuator is to insert a steel plate with a circular hole and a thickness of several millimeters between the flanges of the connecting pipe. The orifice plate is capable of pulsing the gas flow as it is a resistance element. If we can place an appropriately sized orifice in the proper position of the pipe and the container, a partial pressure drop will occur as the airflow passes through it, rendering the originally reflective end point incapable of reflecting, forming an end condition without acoustic reflection.

When the compressor unit structure does not allow the buffer to abut against the cylinder, the buffering effect is not ideal, especially when the cylinder and the buffer connecting tube are long in the resonance tube, the inner tube can be installed by installing an appropriately sized orifice plate at the buffer flange. The airflow changes from a standing wave to a traveling wave, thereby reducing the unevenness of the airflow pressure to enhance the buffering effect and achieve the purpose of vibration reduction.

It is important to choose the orifice size and mounting position. The size of the orifice plate also has a great influence on the damping effect. According to the test, the ratio of the orifice diameter to the inner diameter of the pipe is: d/D=043~050, thickness b=3~5mm, when selected for low-sonic medium, d /D can be close to 05; for high-sonic medium, it can be close to 043. The material of the orifice plate is the same as that of the pipe material. The edge of the inner diameter of the orifice plate must retain sharp edges and corners, and must not be chamfered, otherwise the effect should be reduced. The orifice plate should be installed in a container that is large enough to enter and exit the flange. When the orifice plate is away from the container, no reflection-free condition is formed, but a simple local resistance element cannot provide a vibration-damping effect. Therefore, the use of an orifice plate without a buffer tank will not provide a vibration damping effect.

(3) increase the collector

When several compressors are used in parallel at the same time, the airflow meets at the gas collecting pipe, and the pulsation amounts are superimposed on each other. The result of the superposition: sometimes offset each other and sometimes strengthen each other. Generally, in order to reduce the forward pulsation superposition, the gas collecting pipe has a certain buffering effect, and the general principle is to increase the flow area of ​​the collecting pipe, and the size thereof should be three times the total flow area of ​​all the intake pipes. The collector vibration is damped by several compressor exhaust pipes operating in parallel and joined to a manifold (called a header).

In addition, the turbulent flow pulsation damper can be used to reduce the vibration caused by the pulsation of the air flow, and the vibration absorbing mechanism of the turbulent flow pulsation damper is vibration isolation type. The charged gas provides a very soft spring action, which isolates the liquid column in the subsequent tube from the vibration source. The filter tube plays a good role in suppressing the pulse flow, and the subsequent tube flow pulsation is a typical forced vibration.

2.2 Reduction measures caused by basic loosening

The anchor support is unstable, and the loose foundation will cause severe vibration of the reciprocating compressor and pipeline. When the reciprocating compressor starts, even if it is under no-load conditions, it will cause severe vibration of the unit and pipeline. The vibration caused by mechanical looseness is very obvious, and the damage to the unit is also relatively large.

The fault caused by mechanical loosening is obvious and easy to judge. When the unit is started, it can be judged and checked whether the vibration is caused by the looseness of the unit and the pipeline. The basic vibration reduction should replace the nut, re-manufacture the anchor bolt; strengthen the base; partially or redo the cement concrete foundation; adjust the coupling radial and axial coaxiality. This situation is easy to rule out, and the routine maintenance and inspection of the unit can eliminate the damage to the unit. Here we do not focus on discussion.

2.3 Resonance reduction measures

(1) Controlling the natural frequency Because the frequency process of the power system needs to be difficult to change, consideration should be given to changing the natural frequency of the pipe and the length of the gas column resonance to avoid resonance. For the air column resonance, the natural frequency resonance of the gas column should be changed by adjusting the gas column resonance length of the pipeline; when the excitation frequency is equal to the natural frequency of the pipeline, the direction of the adjustment pipeline, the support position, the support structure, and the pipeline are changed. The structure size and other methods can adjust the natural frequency of the pipeline to effectively eliminate the resonance phenomenon.

(2) Reduce the number of pipe bends and increase the pipe turning angle

In the operation of the reciprocating compressor pipeline, the exciting force is mainly generated at the joint of the elbow and the reducer. Therefore, the use of the elbow should be minimized in the installation of the pipeline so that the pipeline runs straight to reduce the excitation. The number of forces, and the exciting force at the elbow is related to the turning angle. Increasing the turning angle can enhance the damping effect, and at the same time, fixing, that is, avoiding space turning. This is to reduce the occurrence of exciting vibrations, thereby mitigating mechanical vibration.

(3) Adjusting the support position and support stiffness

Properly adjust the bearing position and support stiffness so that the natural frequency of the pipe avoids the excitation frequency to avoid the occurrence of mechanical resonance. In actual pipelines, the effect of the number of bearings, the spacing and the stiffness of the bearings on the overall stiffness of the pipe is significant and relatively easy to implement.

Different types of brackets have different stiffness, and the degree of influence on the natural frequency of the piping system will be different. Generally, the spans of the brackets should be substantially the same (the adjacent brackets are preferably not completely equal), because as long as one of the spans of each bracket is larger, the mechanical natural frequency of the pipeline is significantly reduced.

3 identification method of vibration causes

Analyze the causes of the vibration of the reciprocating compressor pipeline, summarize, identify the vibration causes of the pipeline, find the vibration source that causes the vibration of the reciprocating compressor pipeline, and formulate reasonable vibration reduction measures.

If the vibration of the unit and the pipeline is extremely severe when the unit is turned on, even if the vibration is extremely severe under no-load conditions, it can be judged first whether the vibration is caused by improper assembly of the unit or the foundation is unstable; if the vibration of the pipeline is very severe after the pressure operation It is possible to judge whether or not vibration due to pulsation of the airflow. The pulsation of the airflow can be transmitted to a place far away from the pipeline. When the pulsation and resonance of the airflow occur, the violent vibration of the pipeline is generally caused. At this time, the vibration amplitude of the pipeline and the unit and the vibration of the unit caused by the unstable foundation are significantly different.

Distinguishing the pulsation of the airflow or the violent vibration of the pipeline caused by the resonance can be judged by the vibration test method. The pulsation is caused by the encounter of the pipeline component during the transmission of the airflow in the pipeline, and the resonance is caused when the excitation frequency of the powertrain of the unit falls within the natural frequency (0.8~1.2) octave interval of the air column or pipeline. The pipeline vibrates violently.

4 summary

The arrangement and anti-vibration of the reciprocating compressor is a very complicated problem, and it is also a problem often encountered in the operation of the unit. Pipeline anti-vibration involves pipeline design and manufacturing, equipment layout, equipment selection, bracket form and equipment installation. The precision of the reciprocating compressor manufacturing process, the high standards of the installation process, the rationality of equipment and piping arrangements, etc., will reduce the vibration of the entire piping system from the source. On-site vibration reduction, the pipeline that has already vibrated must first determine the cause of the vibration. It is determined by calculation that it is because the unit foundation is unstable, the pressure unevenness is too large, or the pipeline generates resonance, and then the corresponding Vibration reduction measures. This will eliminate abnormal vibrations in the pipeline and ensure safe production.

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