Computational and experimental study of frequencies and natural mode of welded shell of coriolis flowmeter

The article is dedicated to computational and experimental determination of frequencies and natural modes of welded shell of Coriolis flowmeter. Coriolis flowmeter is intended for measuring mass flow of fluids and gases. A shell of Corilis flowmeter is welded out of 12Х18Н10Т thin steel plates. Forms and natural oscillation modes of the shell are determined by calculation using the finite elements method and experimentally, using the technique of experimental modal analysis. During the experimental determination of modal characteristics of a freely hanging shell, oscillations were excited with the use of an impact hammer and a modal oscillation table.
In order to evaluate precision of computational and experimental forms, Modal Assurance Criterion (MAC) is used. It is shown that the difference of frequencies and natural modes of the shell between calculation and experiment exceeds 30 %, and the difference between frequencies and natural modes of separate elements of the shell which do not contain welded joints does not exceed 3 %. Therefore, the most probable cause of differences of computational and experimental frequencies and natural modes of the shell are welded joints which are not considered in its finite element model.
An assumption that such significant difference can be explained by the occurring after welding residual stress is made. In order to test this hypothesis, a tempering of the welded shell is carried out. It is determined that after the tempering the difference between computational and experimental forms and frequencies of natural modes of the shell decreased to 6 %. The obtained result allowed explaining the cause of inconsistency of computational and experimental frequencies and forms of natural modes of the flowmeter's shell.