DB Pipe Vibration Service Brochure

???????????????? ????? ?????The vibration is easily detected by the noise or the touch, even sometimes at the sight. The matter is to be able to evaluate the severity of the vibration. Several cases should be considered : - For the environment, concerning the instruments as well as the human beings, there are some standard and contractual vibratory criteria ; you only have to check them by making measures on pipings. - As far as the measuring devices are concerned, it’s essential to check the measures or the efficiency of the process to have some indication of failures. The problem is more intricate concerning the mechanical behaviour of the piping. The vibration amplitude is no more the only one important parameter but there is also the generated stress. This stress depends not only on the vibration amplitude but also on: • the vibratory deflection shape including angular misalignments. • the natural features of the piping (diameter, thickness, material and so on, … • some geometrical irregularities generating stress raiser such as tapping, flange, valves and so on, … Moreover, the aspect of the material fatigue with the type of oscillation, the frequency and the duration must be taken into consideration. ??????????The aims of the study about the piping vibrations are the following : ? To avoid the piping cracks that can lead to a break. ? To avoid the resultant leakage. ? To prevent. the measuring instruments mounted on the pipes from being disturbed. ? To see that the vibrations of the piping don’t excite the frame. ? To prevent the employees, who work close by, from coming under these vibrations. ????????????????????????????????????????????????????????? ???????? ??? ? ????????? ???? ?? ??? ?With resonance The resonance is a vibratory amplification of the structure, when the frequency at which this structure is excited coincide with one or several natural frequencies. This particular case is very usual, specially on the pipes connected to reciprocating machines, generating an excitation spectrum with high frequency lines. The determination of the changes to be done can be : • Empiric, by making tests as soon as the deflection shape is defined. The taken chance is : ? To generate other resonance frequencies which coincide with an other excitation. ? Not to know the effect of the solution in the piping and joints stresses. • By calculation of finite elements, by modelling the piping and its joints. For that, it is essential : ? To measure the joints impedances in order to model them, ? To force the maximum measured displacements when the operational deflection shape occurs. ? We can then calculate the generated stresses according to the known criteria. No resonance Any stiffening action is at risk ; it neither.cancel the reason nor the amplification ; it will generate mechanical stresses that could lead to cracking risks. It is so better to work the excitation sources by the implementation of dampers.and so on …… Remark : This fundamental distinction between the two above mentioned cases doesn’t prevent the following actions : - For an old installation, it’s better to check the supports (fastener, stiffness …) - The vibratory motion can also be reduced by installing viscous or active dampers such as shock absorbers. ?Preliminary diagnosis The first thing we do facing with a problem of piping vibration is to stiffen. If only this solution could be relevant ; it could ever be dangerous . The merit of a such action on the structure (for example, the stiffening) depends essentially on its type of vibratory behaviour, i.e. is the structure in resonance or not ? ???????? ?????? ???????? ?????? ?? ??? ????? ?????? ?? ??????????? ??????? ?????? ??????????? ??????????? Excitations finding Measure of the operational deflection shape – Operating installation Search of natural frequencies – Installation at shutdown ?? ? ? ?Methodological approach Measures Frequencies and natural modes The more efficient method, in order to define the natural frequencies and modes is to give hammer-strokes on the pipe. The vibratory response is measured on several points, with the installation at shutdown. We get then : - The transfer functions, i.e. the natural frequencies with their amplification and damping. - The natural forms associated to the natural excited frequencies. Operational Deflection Shape (ODS) Measure of the ODS at different frequencies while the installation operates (see technical data). Measure of the excitations In some cases (rare), it’s possible to measure the mechanical excitation with the force sensors : we get then the excitation time waveform that can be applied to the calculation method. For that, it’s essential to know the origin of force to make the measures. The dynamic excitations of the flux flow are analyzed by the dynamic pressure sensors. These measures particularly allow : - A quantization of the max. levels at sensible places, to check the flow features. - To make the signal spectral analysis in order to bring out the exciting frequency range. However, these measures don’t allow to define the amplitude and the origin of the fluid stresses on the pipes. Stresses They are measured by adding strain gauges at the required points. The time waveform helps to define the maximum instantaneous value of the deflection or stress. The Fourier Transformation of this signal allows to know the frequencies for which there are more stresses. ? Simulation Modelling The aim is to create a theoretic model representing the studied installation. This model will be used : - For estimated calculation, before realization - For change simulation, in order to evaluate the incidence on the installation behaviour. The most used method is the method by finite elements. The modelling know-how consists of : 1. the choice of finite elements : volume type, plate, beam. 2. the different modelling forms : model control into space, consideration of some elements deflections. 3. and especially the choice of boundary conditions, simulating the Model/Outer joint. Calculations The dynamic calculations are divided in 2 parts : Calculation of natural frequencies and associated natural modes : The aim is to avoid the proximity between a natural frequency and an excitation frequency. Response calculation : This one is expressed in stresses. It is a matter of calculating the standard mechanical aspect for which the “fatigue” concept can be considered. The generated stresses being unknown, we apply the prescribed displacements defined by the ODS to the pipe. Adjustment The choices and modelling hypothesis by finite elements lead to approximations and doubts. That’s why it’s more advisable to adjust the finite elements model, comparing the calculation results with the measure (for example, natural frequency measure). Modifications The aim is to remove the natural frequencies to the excitations. It’s essential to see that : - the lower natural frequencies don’t raise. - The other natural frequencies aren’t in frequential coincidence with the excitation harmonics. ? ??? ??????? ???????"????????????????? ????#???$?????%???%????????????? ????&???%? ??%'????????????????????????? ??????$???????(???)???*??? ??+????????? 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