BLACK MESA PIPELINE
Slurry Transport Technology FAQ
Are all slurry pipelines operated alike?
NO. There are basically two types of slurry systems.
Brute Force - These are usually short, high velocity systems which operate in the heterogeneous flow regime at relatively low solids concentrations and usually carry relatively large particles. Most dredging and tailings pipelines are in this category.
Conventional - These are generally well designed systems in which particle size distribution and solids concentration are closely controlled in order to maintain an economical yet stable operating velocity slightly above critical. These systems usually have a high solids concentration and operate in the homogeneous or hetero-homogeneous flow regime. The Black Mesa coal pipeline in Arizona and the Samarco iron ore pipeline in Brazil are examples of conventional slurry systems.Is it true that almost any solid material can be transported by pipeline in a fluid?
From a technical standpoint, yes, as long as the carrier fluid and the solids being transported are compatible. Other constraints on feasibility include compatibility of required pipeline particle size with use.
In long-distance slurry pipelines, such as Black Mesa, does particle attrition take place?
NO. Contrary to rumors, particle attrition does not take place in a well-designed conventional slurry system.
What determines the minimum transport velocity in a slurry system?
Particle size distribution, particle shape, concentration of solids, and the resulting slurry rheological properties.
What are typical operating velocities?
Conventional long distance slurry pipeline systems generally operate at 5 to 7 feet per second. If the slurry concentration is properly matched to the particle size distribution, velocities in this range will result in a stable operating system which may be easily restarted after shutdown and in which pipe wall abrasion is minimized. Brute force systems , on the other hand, may require operating velocities as high as 18 to 20 feet per second.
Is pipe wall abrasion something that needs to be considered in slurry system design?
Abrasion may be thought of as the rubbing together of the slurried solids and the pipe. This only needs to be considered in the design of brute force pipelines such as dredging lines. In these systems, pipe wear is a major consideration. Conventional slurry systems are designed to operate at velocities low enough that abrasion is minimal and pipeline wall can be designed for long term operation.
Is corrosion a problem in conventional slurry pipelines?
Not really, if properly monitored and controlled. Corrosion can usually be controlled with inhibitors. The major cause is usually entrained oxygen; the solids in a slurry tend to erode the corrosion film which is established on the pipe wall. It is imperative to select an inhibitor which will resist this action.
Is there a special type of pipe used in slurry systems?
Conventional slurry systems use standard steel line pipe (API 5L Specification), the same as for oil and gas pipelines. Brute force lines, depending upon the system and anticipated pipe wear, may utilize various pipe linings or even different types of pipe such as hardened steel, concrete, etc. Often regular line pipe is used, then rotated and/or replaced as needed.
Are conventional slurry systems always designed for optimum operability-stability?
NO. Usually such things as product end use and/or solid-liquid separation at the terminal require a certain amount of compromise on the pipeline designer’s part.
What type of mainline pumps are used in slurry systems?
Positive displacement, either plunger or piston type, depending upon the application, are the most common for use in conventional slurry systems today because of their high pressure capability. In order to obtain equivalent pressures using single stage centrifugal pumps, each station would require eight to ten of them installed in series. The resulting multiple passes through so many centrifugal pumps would alter the slurry properties because of particle attrition by the action of the impellers. Centrifugal pumps have a much lower efficiency than PD pumps and are also limited by case pressure. Positive displacement pumps are limited by the maximum particle size which will pass the valves, approximately 2.4 mm. In brute force systems, due to their relatively short length and/or large particle size, positive displacement pumps are not normally used. Centrifugal pumps in series or connected in chamber pump configuration are used. Chamber pumps usually consist of two or more large chambers which are alternately filled with the product by a low pressure single-stage centrifugal pump and emptied by use of a high pressure multi-stage centrifugal which forces the product into the mainline utilizing clean carrier fluid. This is accomplished by careful sequencing of valves located at each end of the chambers.
How much water do slurry pipelines require?
This varies with the kind of slurry as well as the type of transport - brute force or conventional. Conventional coal slurry uses a relatively small amount of water for transport when compared to mine mouth utilization of the coal. For example, here are some generalized utilization ratios of coal versus water for three processes.
Gasification Plant - 1 Ton Coal to 2 Tons Water
Generating Plant - 1 Ton Coal to 7 Tons Water
Slurry Pipeline - 1 Ton Coal to 1 Ton Water
However, what is most important is that the water used for coal pipeline transport is not wasted. It is recovered and will provide approximately 12 to 14 percent of the water required for cooling at the generating plant. If the generating plant were located at the mine site, the resulting requirement for water from the mine would be 7 to 8 times higher than necessary for transportation of the coal by pipeline.
Are grinding costs a substantial part of slurry transport costs?
Not in most cases, if the project is analyzed from mining through processing, transportation, and end use. Most minerals are ground and slurried during the ore refining process, so a slurry pipeline may be a little more than an extension of the process. Modern pulverized fuel coal fired generating plants require finely ground coal as a burner feed, regardless of the size delivered to the plant or the method by which it is delivered. The grinding required for transport by slurry pipeline, therefore, represents a partial relocation of a process which must be performed prior to end use in any case.
What about dewatering costs?
For many minerals other than coal the slurrying in the beneficiation process mandates a dewatering and/or drying stage regardless of whether transportation of the refined product is by pipeline, truck, or rail. (Limestone used as feed for wet process cement plants is generally transported by pipeline at the concentration required as kiln feed, so no separation is required) Conventional coal slurry delivered to generating plants must be separated prior to final grinding, and this cost must be taken into account when comparing transportation charges because the alternatives do not require dewatering. Recent advances in CWM technology (coal/water mixtures featuring fine particles and high concentrations) show that these may be burned without separation.