## Deposition velocity

Deposition velocity is defined as the velocity below which particles begin to deposit to form a moving bed of particles on the bottom of a straight horizontal pipe during slurry-transport operations. This causes the flow to become unstable, and the pipe will eventually clog. The deposition velocity depends on the system geometry and the physical properties of the particles and fluid. Non-Newtonian slurries may have a higher deposition velocity than Newtonian slurries.

Limit deposit velocity

When the flow velocity decreases, there will be a moment where sedimentation of the particles starts to occur. The corresponding line speed is called the Limit Deposit Velocity. Other terms, such as critical velocity, critical deposition velocity, deposit velocity, deposition velocity, settling velocity, minimum velocity or suspending velocity are frequently used. This Limit Deposit Velocity can be thought of as the transition between two different flow regimes, however, for most sands, it will be somewhere in the heterogeneous regime.

For a specified volumetric throughput, critical deposit velocity is used for the selection of pipe diameter. To minimize energy consumption, slurry pipelines are usually designed to operate at velocities that are as low as possible, and therefore, near Vc.

Wilson's (1979) nomogram provides a convenient method for estimating the upper limit of Vc. The effects of viscosity are not included, so the nomogram is best for aqueous slurries with low fines content. The nomogram predicts that Vc is highest when the particle diameter is approximately 0.5 mm. Where:

• C is the solids concentration
• vss is the particle settling velocity
• D is the diameter of the pipe
• f is the friction factor indicating flow resistance
• K is the correlation parameter

Durand (9) provided an equation for the lower limit of Vc for heterogeneous flow, defining the critical deposit velocity of sand mixtures: Where,

• FL is the modified Froude number
• ss is the relative density of the slurry

Dominguez et al. (1996) published an equation for deposition velocity estimation: Where,

• RH is the hydraulic radius
• ρs is the density of solid
• ρm is the density of slurry
• d85 is the solid particle size

The critical deposit velocity is an important design criterion both for safe operation and for system economics. So, it must be properly understood before designing a slurry transport system.