Slurry critical deposit 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 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 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 ... 

To design a successful hydraulic fracturing treatment for horizontal wells, accurate information on the transport properties of slurry in horizontal pipe is required. One must know the critical deposition and resuspension velocities of various fluids in horizontal pipe flow. 

Thomas (1962) defined a critical friction velocity at which the slurry starts to deposit for a given concentration as... 

To continuously separate FT wax products from ultrafine iron catalyst particles in an SBCR employed for FTS, a modified cross-flow filtration technique can be developed using the cross-flow filter element placed in a down-comer slurry recirculation line of the SBCR. Counter to the traditional cross-flow filtration technique described earlier, this system would use a bulk slurry flow rate below the critical velocity, thereby forcing a filter cake of solids to form between the filter media and the bulk slurry flow, as depicted in Figure 15.2b. In this mode, multiple layers of catalyst particles that deposit upon the filter medium would act as a prefilter layer.10 Both the inertial and filter cake mechanisms can be effective however, the latter can be unstable if the filter cake depth is allowed to grow indefinitely. In the context of the SBCR operation, the filter cake could potentially occlude the slurry recirculation flow path if allowed to grow uncontrollably. 

Critical velocity of a slurry in horizontal pipes is the fluid velocity below which the solids begin to deposit at the bottom of the pipe. In vertical pipes, solids that would be deposited if the pipe were horizontal are easily transported because their settling velocity is usually much lower than normal flow velocities. In general ... 

The head versus flow characteristic for a centrifugal slurry pump is relatively flat. Therefore, if the flow resistance of the system increases and the flow rate drops below the critical velocity, a fixed bed of deposited solids, potentially developing into a plugged pipeline, can result. To prevent this situation from occurring, most centrifugal pumps have variable speed drives to maintain the flow rate. 

The flow of slurry in a pipeline is much different from the flow of a single-phase liquid. Theoretically, a single-phase liquid of low absolute (or dynamic) viscosity can be allowed to flow at slow speeds from a laminar flow to a turbulent flow. However, a two-phase mixture, such as slurry, must overcome a deposition critical velocity or a viscous transition critical velocity. The analogy can be made here in terms of an airplane if the speed drops excessively, the airplane stalls and stops flying. If the slurry s speed of flow is not sufficiently high, the particles will not be maintained in suspension. On the other hand, in the case of highly viscous mixtures, if the shear rate in the pipeline is excessively low, the mixture will be too viscous and will resist flow. 

In Chapters 3, 4, and 5 the mechanics of solid suspensions are described in detail. An im portant parameter to introduce in this chapter is the critical velocity of a slurry flow. Figure 1-8 plots the pressure loss per unit length on the y-axis, versus the velocity Kof a slurry flow on the x-axis. Five points are shown for flow at a constant volume concentration. For this slurry of moderate viscosity, the flow is stationary and the solids clog the pipeline below point 1. There is insufficient speed to move the particles. As the flow is accelerated, the speed reaches point 1, which is called the deposition critical velocity Vp, or minimum speed to start the flow. Between points 1 and 2, the bed builds up, dunes form, and the different phases are well separated. Between points 2 and 3 the flow is streaking but... 

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