Critical deposition velocity

The most widely used form for the correlation yielding an estimate of the critical deposition velocity is based on the early work of Durand and Condohos (1954). They found the following simple relationship well described the critical deposition velocity Vc.  

More recently. Gillies et al. (2000) have shown that F is best represented in terms of the particle Archimedes number Ar [Ar = x p (pg — Pf)g/iTf] 

It should be emphasized that the above relationships between F and the particle Archimedes number do not include the influence of variables such as particle shape or particle concentration so the correlation yields only an approximate value for the deposition velocity Vc- 

For Archimedes number less than 80, the correlation of Wilson and Judge (1976) should be used. 

While Wicks technique was originally derived for liquid-solid systems, it has been found to apply to fluids in general and is recommended herein for gas-solids systems in addition to liquid-solid systems. 

Since Eq. (12.2.1) is implicit in the unknown variable, V5, it must be solved by an iterative (trial-and-error) technique if we wish to obtain a rigorous solution. If one wishes to avoid trial-and-error calculations, the writers have found that the following, explicit equation 

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The pressure drop and pumping requirements are functions of the type of flow and of the rheological properties of the dispersion. If the flow rate in a pipeline falls below the critical deposit velocity then particles or emulsion droplets will either sediment or cream to form a layer on the bottom or top wall, respectively, of the pipe. Some correlations that have been developed for the prediction of critical deposit velocity are discussed by Nasr-El-Din [86] and Shook et al. [90]. 

The flow velocity in a pipe or stirred vessel that corresponds to a transition from laminar to turbulent flow conditions, or vice versa. See also Critical Deposition Velocity. 

To keep the particles in suspension, the flow should be at least 0.15m/sec faster than either 1) the critical deposition velocity of the coarsest particles, or 2) the laminar/turbulent flow transition velocity. The flow rate should also be kept below approximately 3 m/sec to minimize pipe wear. The critical deposition velocity is the fluid flow rate that will just keep the coarsest particles suspended, and is dependent on the particle diameter, the effective slurry density, and the slurry viscosity. It is best determined experimentally by slurry loop testing, and for typical slurries it will lie in the range from 1 m/s to 4.5 m/sec. Many empirical models exist for estimating the value of the deposition velocity, such as the following relations, which are valid over the ranges of slurry characteristics typical for coal slurries ... 

Velocity and concentration profiles are two important parameters often needed by the operator of slurry handling equipment. Several experimental techniques and mathematical models have been developed to predict these profiles. The aim of this chapter is to give the reader an overall picture of various experimental techniques and models used to measure and predict particle velocity and concentration distributions in slurry pipelines. I begin with a brief discussion of flow behavior in horizontal slurry pipelines, followed by a revision of the important correlations used to predict the critical deposit velocity. In the second part, I discuss various methods for measuring solids concentration in slurry pipelines. In the third part, I summarize methods for measuring bulk and local particle velocity. Finally, I review models for predicting solids concentration profiles in horizontal slurry pipelines. 

The Oroskar-Turian s correlation and previous ones were developed to determine the critical deposit velocity of Newtonian carrier fluids with various particle sizes and concentrations. Shah and Lord (7) generalized equation 2 to extend its capability to correlate the critical deposit velocity for non-Newtonian carrier fluids (power law). The parameter X was eliminated from equation 2 because of its insignificant contribution to the correlation results and because it would be undefined for the laminar flow regime of non-Newtonian fluids. The generalized form of equation 2, which can be applied to either critical deposit (VD) or resuspension velocity (Vs), is as follows ... 

Other correlations to determine the critical deposit velocity are given by Hanks (11), Sommerville (19), Roco and Shook (20), and Gillies and Shook (21). 

The slurry velocity at which a particle bed forms is defined as critical deposition velocity, VD, and represents the lower pump rate limit for minimum particle settling. A further decrease in slurry velocity leads to increased friction loss, as indicated by a characteristic hook upward of curve A, and may also lead to pipe plugging. After shutdown, if flow rate over the settled solids is gradually increased, a response similar to curve A of Figure 16 is once again obtained. With increasing nominal shear rate, wall shear stress decreases until a minimum is reached and then increases rapidly thereafter. The fluid velocity that corresponds to this minimum stress value is the critical resuspension velocity, Vs. 

Oroskar and Turian (48) developed a critical deposition velocity correlation based on balancing energy required to suspend particles with energy dissipated by an appropriate fraction, F, of turbulent eddies present in the flow. They found F to be usually very close to unity (>0.95) and therefore its inclusion, especially when raised to a fractional power, has essentially no influence on correlation predictions. Their equation appears in the following form ... 

The data were extracted from a number of experimental investigations reported in the literature for the development of the previous equation. The Oroskar-Turian correlation and others appearing in the literature were all developed to describe critical deposition velocity of Newtonian carrier fluids with various solid types, sizes, and concentrations. 

Figure 17. Effect of pipe diameter on critical deposition velocity (based on model predictions). (Reproduced with permission from reference 49. Copyright 1990 Society of Petroleum Engineers.)...

The minimum point on the hydraulic characteristic curve for a settling slurry corresponds to the critical deposition velocity. This is the flow velocity when particles begin to settle out. Good slurry transport design dictates that the pipe diameter and/or pump are selected so that the velocity in the pipeline over the... 

What is meant by the term critical deposition velocity in reference to a setting slurry ... 

Thomas A. D. 1979.Therole of laminar/turbulent transition in determining the critical deposit veloc ity and the operating pressttre gradient for long distance sltmy pipelines. Paper read at the 6th International Conference of the Hydraulic Transport of Solids in Pipes. Cranfield, UK BHRA Fluid Engineering, pp. 13-26. 

This chapter presents information about three diverse topics relevant to cyclone technology thus the title, Some special topics . Two of the topics are related to the gas velocity in the separator cyclone erosion and the critical deposition velocity. The last topic is the working of cyclones or swirl tubes under conditions of high vacuum. 

A Worked Example for Calculation of the Critical Deposition Velocity... 

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