As an important parameter in slurry pipeline transportation, there are many definitions of critical velocity. The definition in this paper is the velocity when there is a stationary bed in the pipeline. The test sediment is divided according to the size distribution. Through the tests of slurry pipeline transportation with different concentrations and particle size distribution, it can be found that with the increase of concentration, the critical velocity has a maximum; at the same time, the existing critical velocity formula is modified, which is suitable for different particle size distribution.


Horizontal pipeline transportation is a key process in dredging engineering and ocean mining, in which the safety and efficiency of transportation are the key points. In order to avoid pipe blockage, the mixture velocity far greater than the critical velocity is usually used in engineering, resulting in higher energy consumption. The critical velocity is therefore important for the safety and economy of slurry transportation pipeline. However, the definition of critical flow rate by researchers is not consistent. There are four main definitions of critical velocity: the first is the velocity of all particles without deposition with the increase of mixture velocity ( Azamathulla et al., 2013; Mehmet et al., 2001); Secondly, with the decrease of mixture velocity, the mixture velocity at which particles begin to deposit at the bottom of the pipeline (R. P. king, 2002 ; Wilson et al., 2006); The third is the mixture velocity when the stationary bed appears at the bottom of the pipe (Durand et al., 1952 and matousek et al., 2017). The fourth is the mixture velocity with the minimum hydraulic gradient when multiple particle size distributions are present in a slurry transportation. (WASP et al., 1977). Among them, the third definition is more suitable for transportation engineering, which can better avoid pipe blockage, so the critical velocity used in this paper is the third definition. Due to different definitions of critical velocity and different experimental conditions, the formulas of critical velocity obtained by each researcher are quite different, and the calculation results are also inconsistent (Miedema, 2016). Meanwhile, most of the critical velocity formulas are derived on the basis of a certain particle size, which is difficult to be applied to complex sediment.

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