The design of a horizontal centrifugal slurry pump is a balance of design considerations to best meet the requirements of a particular slurry duty. These requirements may include one or more of the following:
The ability to pump high density abrasive slurries with adequate wear life.
The ability to pass large diameter solids.
The ability to handle air entrained and/or viscous fluids with reliability and minimal performance corrections.
When compared with clear liquid pumps, the above requirements often result in the slurry pump being larger than its clear liquid counterpart and sacrificing maximum efficiency in exchange for the ability to achieve the above goals.
Within a slurry pump it is expected that the components which come into contact with the abrasive slurry will wear. It will be shown that minimizing wear is done through appropriate pump design, proper material selection and proper pump application.
There are unlined horizontal centrifugal slurry pump and fully lined version of the same pump. The corresponding wear components for each have been appropriately marked. To maximize wear life, thick casting sections are provided on the impeller, the casing of the unlined pump, and the wear liners of the fully lined pump. Often, the unlined casing casting thickness is greater than fully lined version casing liners. This additional thickness requirement is due to a greater need for the unsupported, unlined casing to safely handle the internal pressure of the pump with an adequate factor of safety to account for wear. The lined pump also allows for the use of a wider variety of materials, such as elastomer liners, which often outperform metal in fine particle and corrosive applications. Therefore, while the unlined pump may offer the lower initial capital cost, the lined version allows for a greater number of material choices, which may have longer wear life and lower replacement spares cost. The lined design is also inherently safer from a pressure containment standpoint.
Large clearances are provided within the impeller and casing to allow for the passage of large diameter solids, while also reducing internal velocities and corresponding wear.
The selection of pumps for primary mill circuit applications is an extremely important part in the design or renovation of a mill. The pumps and associated equipment are large capital and relatively high-maintenance cost items. By using some basic selection criteria, both the initial capital cost and cost per ton of material being pumped can be reduced.
By careful analysis of the pump application data, the proper size pump can be selected. This can reduce the initial cost Proper sizing will also increase the life of the pump wear parts.
Factors Affecting Pump Sizing and Selection
The sizing and selection of a pump requires learning the most you can about the system and about the slurry you will be pumping Information required:
Top size of solids
Slime content of slurry
Concentration of solids
Characteristics of solids
Properties of carrying liquid
Diameter of pipe
Total dynamic head (TDH)
Net positive suction head (NPSH)
Top Size of Solids
The top size particles are critical in the selection of the pump to be used as it will determine whether the pump wet end is to be all metal, all elastomer, or a combination The industry has set a rule of thumb which suggests that hard metal pumps be used where the material size is ¼ inch or above. This is only a guideline and is also dependent on the size distribution of the solids. If the top size particles are coarse, but there are a lot of fines or slimes in the slurry, the selection may be altered.
Slime Content of Slurry
The slimes are generally considered to be minus 200 mesh material. This very fine material forms a homogeneous mixture with the carrying liquid. This increases the density of the carrying fluid and tends to retard the settling of larger particles.
Concentration of Solids
As the percent solids in a slurry is increased the critical settling velocity and corresponding critical carrying velocity are reduced This is caused by the increased interference of smaller particles traveling at high velocity tending to support the larger particles and preventing their settling
Characteristics of Solids
The settling rate of particles which are crushed and are of irregular shape will settle at slower rates than particles which are rounded. Also, as the specific gravity of the solids increases the settling velocity will also increase.
Properties of Carrying Liquid
It is important to know about the liquid in the slurry makeup and the pH to determine if there may be a corrosion or material compatibility problem in addition to the abrasion. Such things as chlorides, sulfides, etc can affect the selection of materials for the wet end of the pump. The makeup of the carrying liquid, as well as the temperature, is also important in determining the vapor pressure.
The carrying velocity in larger diameter pipes must be higher due to the tendency of slurry to classify in larger cross sections.
The rate of flow will determine the size of the pump to be used. The rate of flow for a given size pump will determine the internal velocities in the pump, the velocity of flow through the pipe of a given size as well as the friction loss in that pipe
Total Dynamic Head (TDH)
Total dynamic head, simply stated, is the difference between the level of the sump and the point of discharge of the slurry plus the friction head on the suction and discharge side of the pump. In the case where the pump is feeding another piece of equipment, such as a cyclone, then the pressure drop in feet of liquid must be added to obtain the TDH.
The friction head or head loss through the piping is a function of the pipe diameter, length, number of valves, fittings, etc. It also varies with the type or condition of the pipe.
The method most frequently used is the Williams and Hazen formula for friction This information is available in tabular or nomograph form in hydraulic handbooks such as Cameron or The Hydraulic Institute Handbook. This data is usually sufficient for in-plant pump installations. In such service as long distance tailings lines actual tests are recommended.
Net Positive Suction Head (NPSH)
Net positive suction head is the pressure required to cause the slurry to flow into the impeller of the pump. Contrary to the belief of some people, PUMPS DON’T SUCK! The NPSH available in the system must be greater than the NPSH required by the pump or the pump simply will not pump. The net positive suction head available is the energy in the liquid at the suction inlet of the pump over and above the energy in the liquid due to its vapor pressure.
In most applications where the suction of the pump is flooded, the temperature of the slurry is ambient and the carrying liquid is water, the NPSH will not be a problem providing the suction line is short