SLURRY AND SLUDGE PIPING
Ramesh L. Gandhi
Principal Engineer Bechtel Corporation San Francisco, CA
DEFINITION AND BACKGROUND
Slurry is a mixture of solids and liquid. A sludge denotes a mud or a concentrated slurry having a considerable amount of ﬁne material that imparts high viscosity. Typical examples of slurries are the solid-liquid mixtures encountered in mineralprocessing plants and dredged material from waterways and dams. Most of the slurries are made up with water. However, industrial paints, rocket fuel, coal-oil mixture, and coal-methanol slurries are made up with liquids other than water. River sediment in the form of slurry appears to have been handled since ancient times.1 All ancient civilizations arose on river banks. Maintenance of waterways requiresperiodic dredging which results in a sand and silt water slurry. Today dredging represents the largest volume of solids handled in slurry form. Slurry transport is also used for dam construction. Blatch2 reported the ﬁrst hydraulic test results for a sand-water slurry ﬂowing through NPS 1 (DN 25) pipe. Gregory,3 O’Brien and Folsom,4 and Howard5 reported results of tests of clay, sand, and gravelslurries. The ﬂow of muds and sludges through pipes was ﬁrst examined by Caldwell and Babbit.6 The ﬁrst large-scale experimental program on the ﬂow of slurries through pipes was reported by Durand.7 The correlations proposed by Durand and his coworkers serve as a basis for the present-day design methods. Design of a slurry piping system involves
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Selection of pipe diameter Estimate offriction loss and pumping requirements Selection of pipe material, valves, and ﬁttings Selection of pumps Selection of instruments and control system for safe and reliable operation
Pipelines transporting liquids such as oil and water can be operated at any velocity up to their design limits. In most slurry applications, a certain minimum velocity needs to be maintained, to keep solids fromsettling out in horizontal
sections of the pipe. The velocity below which particles tend to settle out and form a deposit in the pipe is called the deposition velocity. The pipe diameter should be selected such that the velocity in the pipeline is maintained above the deposition velocity over the operating range of ﬂow rates. The operating ﬂow rate range isdetermined by the expected range of solids throughput and slurry concentration. Solids throughput is deﬁned as the weight of solids to be transported per unit time. It is normally expressed in tons per hour (tons/h). The slurry concentration is expressed as the weight of solids per unit weight of slurry, or volume of solids per unit volume of slurry. The slurry concentration may be established by therequirements of the upstream or downstream processing facilities. This is normally the case with inplant piping. In the case of long-distance pipelines, it becomes advantageous to adjust the slurry characteristics and concentration to reduce the cost of the pipeline system. An economic study is performed to select parameters acceptable to upstream and downstream plants while offering economies inpipeline construction and operation. The deposition velocity and friction loss in a given size pipe at a given concentration depend upon the slurry ﬂow behavior. The selection of pipe material, valves, ﬁttings, and pumps depends upon the velocity of ﬂow, abrasivity of the slurry, and pumping pressures which are in turn governed by the slurry ﬂow behavior.
SLURRY FLOW BEHAVIOR
Flow of slurry inpipes depends upon the interaction between the solids and liquid as well as between the slurry and the pipe. Depending upon the velocity of ﬂow, pipe diameter, solids size distribution, ﬂuid properties, and solids characteristics, four different ﬂow conditions can be encountered in a horizontal or nearly horizontal pipeline.8 These are homogeneous ﬂow, heterogeneous ﬂow, intermediate regime,...