and high-tech

Slurry pumps

Description of slurry pumps

Slurry pumps are hydraulic machines designated for moving water-thinned solid-containing mechanical mixtures (hydraulic fluids) via pressure pipelines. Slurry pumps are installed on dredgers (a floating machine for excavating and gathering up soils and sediments from underwater) or suction dredge units. The original home of slurry pumps is France. It is there in 1859 that the first slurry pump was installed on a dredger. It was a piston pump then, but 5 years later centrifugal pumps appeared which replaced piston pumps in no time. In imperial Russia the first dredge pumps were developed and manufactured in late 19th century. The term “dredge pump” widely used in hydraulic excavation literature was replaced by “slurry pump”. Today slurry pumps are usually single-stage (centrifugal) single-suction pumps. Slurry pumps may have a horizontal and vertical shaft design. However the second option is quite rare and designated for special conditions.

Besides, slurry pumps may have a single or double casing. Double casing slurry pumps are used for compound (abrasive, coarse) materials, so, their casings are equipped with a protective sleeve. The core operating element of a slurry pump is an impeller which is transferring energy to the hydraulic fluid flow with minimum losses, while internal casing (for double-casing pumps) protects the main outer casing from abrasive particles in handled fluid.

Application of slurry pumps

This type of pumps is widely used in various fields:

  • quarry operations and common excavation;
  • hydraulic construction works;
  • mining;
  • soil handling;
  • movement of minerals;
  • ash handling at co-generation plants;
  • pumping of abrasive mixtures with maximum density of 1,600 kg/m3 at concentration plants;
  • slag removal;
  • drilling mud cleaning;
  • application at cement plants.

Design features of slurry pumps

In terms of design a slurry pump is a unit consisting of a pump and a drive, which is normally an electric or diesel motor. Design solutions for the engineering of slurry pumps are quite special and are determined by a large quantity of solids in the handled media and their abrasive impact on the pump components. Thus, the target is to move large abrasive particles, such as rocks and pebbles entering the pump with soil by increasing the internal cross-section. However a larger cross-section reduces velocity and makes the pump slow-moving, which calls for a larger size and weight. For unimpeded movement of large particles the number of impeller blades should be from two to four and impeller width should be larger. Slurry pump efficiency is much lower than that of the pumps having the same capacity but designed for clean water handling.

The flow path of a slurry pump includes one or two casings (inner and outer), where a centrifugal impeller is located (closed type). Considering that abrasive inclusions in hydraulic fluids may result in early wear of casing cover, the space between the impeller and the cover is equipped with a protection disk preventing from wear. The running gear of a slurry pump is represented by a shaft installed in ball-bearing supports. The impeller is overhung on the shaft. The point of shaft exit from the pump casing is sealed.

The key element is an impeller consisting of two disks (closed type). Blades are arranged between the disks. When rotating the impeller creates negative pressure in its central area which causes suction of handled media. Hydraulic fluid enters the inlet pipe and is fed onto the impeller, where each slurry particle is impacted by centrifugal forces which push the slurry into the pressurized discharge line.

Purpose of slurry pumps

Depending on the handled media and operational conditions these pumps are divided into:

  • sand pumps;
  • soil pumps;
  • pulp pumps;
  • dredger pumps;
  • sludge pumps.

Slurry pumps are used successfully for such a challenging task as evacuation of pulp (mixture of soil and water) in mining operations and extraction of minerals. The difficulty of handling such media is that the equipment and tools contacting the fluid are exposed to excessive abrasive wear. To combat this issue slurry pumps are manufactured from hard wear-resistant alloys, and any failed components are easy to replace. Operating elements of the pump are protected with strong armor disks. Besides, slurry pumps may be equipped with flushing systems preventing them from clogging.

Wear of slurry pump components

However in spite of all these measures the parts of slurry pumps are exposed to excessive wear: hydroabrasive, cavitational and mechanical. Hydroabrasive impact is the main cause of the flow path wear in a slurry pump. Its intensity depends on many characteristics of the slurry or particles in handled materials, as well as on wear resistance of the flow path parts. Hydroabrasive wear may be general and local. General wear is demonstrated by a relatively uniform thinning of parts and is typical for the surfaces of armor disks, side and radial liners, and pipes. Local wear, which is much more intensive than the general one, attacks vortex and cavitation areas.

Cavitational wear usually disrupts the pump operation and builds up vacuum in the suction line. Uneven interfacing of parts, high spots from electric welding and worn-out surfaces contribute to the cavitation, which results in stronger vibration of the pump and wear of absolutely all parts: drive shaft, support bearings and even foundations. To reduce the cavitation effect ejectors are installed into drag heads.

Mechanical wear of a slurry pump is caused by friction and impacts of rocks contained in the fluid on the pump parts. This is typical for handling pebble-rich soils. Any type of wear has a negative impact on the pump characteristics, reduces output by 20 to 30%, after which the pump is subject to immediate repair. Each element of a slurry pump suffers from its own prevailing type of wear.

The parts of slurry pumps are made of different kinds of alloyed steel with subsequent heat treatment which imparts additional mechanical strength and wear resistance. Steels used for the manufacturing of slurry pump parts are usually alloyed with chromium, silicon, manganese, nickel, tungsten and vanadium. Iron is rarely used for the parts contacting sand, gravel and large solids, since it is practically non-resistant to impact load. Wear resistance of regular gray iron is 10 to 15 times less than that of special steel alloys. One of the upcoming trends in the improvement of slurry pump design is gumming, which is rubber coating of pump parts. However rubber is not much resistant to impacts of coarse particles, that is why rubber-lined slurry pumps should only be used for handling fluids containing small solids.

In general the following practices are recommended to reduce the wear of slurry pumps:

  • improvement of hydraulic flow conditions in the lines of slurry pumps;
  • protection of gaps in slurry pumps from penetration of solids;
  • improvement of wear-resistant properties of pump part materials;
  • use of protective liners;
  • improvement of design solutions in slurry pump engineering.

Selection of slurry pumps

The main parameters to consider when selecting a slurry pump are flow rate, pressure, output and efficiency. Besides, feed rate is of critical importance to slurry pumps, which, if optimal, significantly improves pump service life and energy efficiency. During design stage it is also necessary to consider the hydraulic fluid (slurry) to be moved, its properties and size of solids. Further, the durability of the flow path of a slurry pump is determined by the concentration of solids and their size. Slurry flows demonstrate different behavior depending on their characteristics, thus, the pattern of abrasive wear in the flow systems of slurry pumps will be different. A full process calculation considering all the factors is mostly quite challenging, so in order to select a slurry pump special regulatory documents are used which are universal for the pumping equipment handling the media heavily contaminated with solid inclusions.