Deposition velocity, slurry

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 ... 

Gilles, R.G. Shook, C.A. A deposition velocity correlation for water slurries. Can. J. Chem. 

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. 

One of the best known correlations for the deposit velocity of sand and gravel-water slurries is that of Durand and Condolios (12) ... 

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. 

The flow rate in a transportation pipeline has to be larger than the deposition velocity of the mineral particles in order to avoid the segregation of solids. Typical flow rates range from 1 to 2 m s for coal and oil sand froth pipelining to about 4ms for some heavy metal concentrates [2]. For mineral slurries, abrasion becomes a signiflcant concern at velocities above about 2.5 m s and a major problem above about 4.5 m s [2]. 

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 ... 

A practical engineer sifting through the literature on slurry flows would be astonished by the number of different equations. Since the work of the French scientists Durand and Condolios in 1952, and British scientists Newitt et al. in 1955, engineers and scientists have continued to develop new equations for deposition velocity and friction losses. 

V3 is effectively the deposition velocity, often called in the past the Durand velocity for uniformly sized coarse particles. It is no longer recommended that it be called the Durand velocity, as tests in the last 20 years have led to new equations that include the effects of particle size and composition of the slurry. The magnitude of the velocity depends on the volumetric concentration (Figure 4-7). 

Gillies, R. G. J. Schaan, R. J. Sumner, M. J. McKibben, and C. A. Shook. 1999. Deposition velocities for Newtonian slurries in turbulent flows. Paper presented at the Engineering Foundation Conference, Oahu, HI. Submitted for publication in the Canadian J. Chem. Eng. Reference cited by Saskatchewan Research Council (2000). Slurry pipeline course handout. 

Wasp, E. J. et al. 1970. Deposition velocities, transition velocities, and spatial distribution of solids in slurry pipelines. Paper read at 1st International Conference on Hydraulic Transportation of Solids in Pipes, Cranfield, England. 

A slurry mixture of coarse particles of <45 = 12 mm with C = 40%, density of solids 4100 kg/m , and a specific gravity 4.1 is flowing in a circular pipe with an inner diameter of 438 mm (17.25 in). Assuming that the pipe is to be half full, determine the deposition velocity. 

Dominguez, B., R. Souyris, and A. Nazer. 1996. Deposit velocity of slurry flow in open channels. Paper read at the symposium, Slurry Handling and Pipeline Transport. Thirteenth annual International Conference of the British Hydromechanic Research Association, Johannesburg, South Africa. 

In a typical slurry pipeline design situation, the flowrates and solids concentrations are fixed by process material balances and equipment performance specifications. In these circumstances, a primary goal in design is selection of the optimum pipe diameter. For slurries in turbulent flow, the optimum transport condition almost invariably occurs when all the particles are suspended but moving at the lowest possible mean velocity. By operating the pipeline at the slurry deposition velocity, the frictional energy losses and wear are minimized and the whole of the pipe cross-section is available for flow. 

Because of its importance, the deposition velocity has been the subject of innumerable experimental investigations, some of which have had a theoretical component. Rather than attempt to summarize all of these, the present communication is intended to provide a guide to the designer. In addition to presenting correlations for use in estimating the deposition velocity, the limitations of these correlations are described so that experimental tests may be considered for particular slurries. 

Because viscosity is an important parameter for the deposition velocity of fine-particle slurries, and because the viscosity of water may increase substantially with the concentration of fine particles, it is important that the viscosity of the carrier mixture (water + fines) be measured. Unfortimately this has not always been done. An indication of the importance of measurement is given from the fact that the intrinsic viscosity [(p/pO - l]/Cfi,Ks for water containing fine particles may often be as high as 30 whereas the value for deflocculated spheres is only 2.5. Particles of diameter fine enough to flocculate are often present in the original slurry and large quantities of fines may be generated by repeated recirculation of a slurry in small diameter test loops. 

It is unfortunate that systematic investigations of deposition velocities for large-particle slurries have not been conducted with a range of pipe sizes so that the insensitivity to pipe diameter suggested for large particles can be assessed. Figure 3 compares the velocity of a labelled 85 mm particle alone and in slurries of similar size (dso approximately 60 mm ) particles in a pipe of diameter 264 mm. 

The ordinate is the ratio of the particle velocity to the bulk velocity of the flow. The single particle velocity ratios are close to the values measured in the slurries. The deposition velocity in all three experiments was probably near 2.5 m/s which is consistent with the results shown in Figure 2. 

As the velocity of a slurry is reduced, the presence of a stationary deposit indicates that the deposition velocity has been passed. Normally the first observable stationary layer is... 

Fine mineral beneficiation plant tailings and concentrates are often transported in pipelines and low values of (d/D Co) occur frequently with these slurries because the median particle size is fine. Although it is now accepted that deposition velocities should be measured for these slurries, it is frequently not possible to obtain the relatively large quantities of slurry required for a full scale test. It is therefore necessary to scale-up the deposition velocities meeisured in smaller pipelines. In the next section some typical experimental results will be presented to illustrate the behaviour which may occur. 

Although mineral tailings and concentrates usually have fairly broad particle size distributions, it is useful to first examine experimental results obtained with narrow size distributions. Schaan et al. [5] have reported deposition velocities in pipes with inside diameters of 0.053 m and 0.159 m as functions of solids concentration. The median particle diameter for this sand was 90 microns. Figure 4 presents the deposition velocities for these slurries, expressed as Fm values, in terms of the solids concentration. 

The effect of solids concentration on deposition Froude number in Figure 4 was also reported by Thomas [7]. The increase in deposition velocity with solids concentration contrasts with that which be shown later to occur for finer particles with broad size distributions. As the slurry concentration increases the slurry viscosity begins to increase rapidly at concentrations above 30% by volume. This causes the Reynolds number of the mixture flow to decrease at a given velocity, which according to Gillies and Shook [6], tends to increase the contact load. 

Figures 6 and 7 present some deposition velocity data for two fine particle slurries (Tailings 1 and 2) with broad size distributions which are often found in mineral tailings and concentrates. The size distributions are presented in Figure 5. In contrast with Figure 4, Figure 6 shows a decrease in Fm with increasing solids concentration while Figure 7 shows little change in Fm.

An important characteristic of slurries of very fine particles is that the deposition velocity in turbulent flow gradually becomes insensitive to pipe diameter. Figure 8 illustrates this by presenting experimental deposition velocities for slurries of a fine iron ore concentrate as a function of solids concentration. The size distribution of these particles was slightly finer than those shown in Figure 5. For the particles of Figure 8 the deposition velocity decreases substantially at low and moderate solids concentrations. There is an increase in deposition velocity with pipe diameter at low concentrations but above 30% solids by volume this effect disappears. 

This insensitivity of deposition velocity to pipe diameter for fine particles has been discussed by Thomas [7] who derived a predictive equation for low and moderate solids concentrations. However the results presented in Figure 6 and 7 show that without experimental verification, it would be imprudent to assume that a slurry would behave in the maimer of Figure 8. Gillies et al. [8] assumed that for extremely fine particles, the contact load concentration Cc would be zero except in the deposit and derived a scale-up equation. 

Tests conducted with the iron ore concentrate in the 0.105 m pipe showed that at concentrations above 35% by volume, the deposition velocity began to increase. This was probably associated with the approach of laminar flow since a 40% slurry had a significant yield stress. Deposition for slurries in laminar flow will be discussed later. 

Fig. 8. Effect of solids concentration on deposition velocity for slurries of iron ore concentrate in various pipes.

Correlations are available for estimating deposition velocities for slurries of particles larger than about 90 pm in turbulent flow. 

Deposition Velocities for Pseudohomogeneous Slurries, Proc. ASME Fluid Eng. Summer Meeting Vancouver BC, June 22-26,1997, pp.7. 

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