EXPERIMENTAL METHOD FOR PIPELINE DYNAMIC STRESS ESTIMATION BY VIBRATION MEASUREMENT

摘要

For structure's stress estimation there are numerous cases where the classical strain gage method is very difficult to be used. One of this case is the strain measurement on pipeline structures in the chemical and power plants and petroleum industry. The strain gage technology requires the machinery to be put off, which in the above mentioned industries is quite impossible to be realized. I view to be established if the level of vibration motion on a given pipeline network, excited by inside flow disturbances, is dangerous for the integrity of its structure, in the literature there are some empirical forms. By these forms are calculate the approximate level admissible values depending of structure's material and geometrical configuration. The forms are valid only for the resonant vibration motions and are affected for a lot of empirical constants. The paper presents a new method for determination of pipeline dynamical stress distribution by vibration measurement. Using only two accelerometers, one placed fix in a reference point of pipeline and other in different placed point by point along the pipe axis, axed by them local reference frame. From FFT transforms of the two vibration signals it is obtained, by division between, the dimensionless complex functions for every points selected on pipe structure for vibration measurement. Then by a polynomial approximations are obtained, using a leas square algorithm, the continuous functions of dimensionless complex amplitudes along the pipeline. Applying the two time derivative operation to the polynomial functions are obtained the strain dimensionless functions which are multiplying with FFT spectrum of vibration motion in the reference point which lead to the FFT strain spectrum distribution function along the pipeline. One finale IFFT lead to dynamical strain/stress distribution law distribution. The method was verified on a pipeline sector with two elbow bend deserving a compressor plan in petroleum industry. The level of vibration induced by pressure fluctuation was high 1.5 mm peak to peak. Using the measurement technique and the method developed resented in the paper it was found a stress of level 20 MPa, which is not dangerous for the integrity of pipeline.