Slurry is a mixture of suspended solid particles and liquids. Its physical characteristics depend on a variety of factors, such as the size and distribution of particles, the level of turbulence, the temperature, the concentration of solids in the liquid phase, the size of the conduit, and the viscosity of the carrier. The most common carrying fluids used for slurry are water, crude oils with milled coal, and air in pneumatic conveying. Following is outlines of this article:
- Introduction to Slurry Piping and Pipeline System Design
- Examples of Slurry Piping System
- Factors Affecting Slurry Behaviour
- Line Sizing and Pressure Drop
- Slurry Piping and Pipeline Design Considerations
Slurry piping or pipeline systems are made up of pipelines, valves, and slurry pumps. The flow of slurry inside a pipe is different than that of a liquid or gas phase because slurry is non-homogeneous. To flow inside a pipeline, the slurry has to overcome a deposition critical velocity. If the flow velocity of the slurry is not sufficient, the suspension of solid particles will not be maintained. Slurry flow can be grouped into two categories: homogeneous slurry flow, where solids are distributed uniformly in the liquid carrier; and heterogeneous slurry flow, where solids are not uniformly mixed.
Examples of Slurry Piping System
Slurry piping is used for a variety of industrial purposes, including coal washeries, wax manufacturing processes, feed for common filtration equipment, pulp suspension in papermaking, and sludge in effluent treatment processes.
Factors Affecting Slurry Behaviour
The design of a slurry piping system is governed by several considerations, such as avoiding solid deposition in the system, ensuring that there is no change in slurry composition from the inlet to the outlet of the system, and minimizing wear and tear or erosion. The main design parameters that affect the behavior of slurry in slurry piping are the solid concentration (expressed in wt. % of solid), the size and nature of solid particles (soft, hard, abrasive), the density, viscosity, and chemical nature of the liquid, and the degree of turbulence.
Line Sizing And Pressure Drop
The basic design steps for a slurry system involve identifying the slurry characteristics (homogeneous, heterogeneous, or mixed behavior), selecting an appropriate slurry concentration, selecting a trial pipe size, calculating the design velocity and critical velocity, and calculating the design friction loss and pump discharge pressure.
The critical velocity for a specified slurry is dependent on various parameters, such as the size and specific gravity of solids, solids concentration, viscosity of the liquid, and degree of turbulence. For homogeneous slurry, the critical velocity is calculated by determining the critical Reynolds Number from a chart. The design velocity (Vd) should be greater than the critical velocity (Vc) by more than 0.3 m/s, i.e. (Vd-Vc)>0.3 m/s. If this criterion is not met, the trial pipe size selection must be changed.
Slurry Piping and Pipeline Design Considerations
When designing a slurry piping system, the following considerations should be taken into account: the use of short and direct routes to minimize pressure drop, elbows of 5D or greater, Y connections in place of tee branch connections, a minimum of directional changes to avoid wear, removal of all dead spots where solid accumulation is possible, valves installed horizontally, eccentric reducers flat on bottom type, provision for cleaning to clear solid build-up, and steam and cleaning oil bleed connections from the top of the slurry lines.
Steel pipes lined with abrasion-resistant lining can improve the service life of slurry pipes, and rotating the pipe periodically can reduce wear at the bottom of the slurry pipes.
The ASME B 31.11 code provides guidelines for the design of Slurry transportation piping systems.
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