As production takes place in the petroleum industry, impurities are produced along with oil and gas. Sand is a common impurity associated with production. If sand is present, it is desirable to have continuous flow of even small concentrations of sand through the pipeline to avoid multiple issues like accumulation of particles and under-deposit corrosion. Experiments are performed at extremely low sand concentrations for both liquid-sand (single phase) and gas-liquid-sand (multiphase) flows to determine the critical flow rates necessary to keep sand moving in horizontal lines. In the case of gas-liquid-sand (multiphase) flow, the flow regime is stratified. The critical velocity is defined as the fluid velocity required to keep the solid particles continuously moving. Generally, the critical velocity is considered to have a direct relationship with the solid concentration for relatively low concentrations. That is for low concentrations, as the sand concentration increases the critical velocity needed for transport increases and vice versa. However, it can be reasoned that as the concentration approaches zero, the critical velocity should not be a function of concentration, since the particle spacing becomes large. In this work, this behavior has been shown experimentally, and the concentration at which the critical velocity is no longer a function of concentration is referred to as the threshold concentration. In this paper, the threshold concentration and the minimum critical velocity are examined for liquid-sand and gas-liquid-sand flows.

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