High-density solids pumps are hydrostatically operating machines which displace the medium being pumped and thus create a flow.
High-density solids and their transport High-density solids are mixtures of liquid and solid constituents; examples include farm grain, pulled pork, etc. Each have its own typical physical and chemical characteristics such as specific weight, solids content, and maximum particle size as well as how they behave, for example thixotropically, abrasively or adhesively. Typical examples of high-density solids are concrete, sludges and slurries. Within industrial plants many substances with a high proportion of solids - such as mechanically dewatered sewage sludges, filter cakes, bituminous coal sludges, waste and process sludges - have to be transported over relatively long distances to landfills or incineration plants. In principle, such long distances can be bridged with mechanical conveying equipment or high-density solids pumps. Types of mechanical conveyors include belts, screws (worm) and scrapers. They are suitable for nearly all types of high-density solids. High-density solids pumps move sludges and slurries within an enclosed pipeline. However, not every sludge or slurry is pumpable, depending on:
Ratio: The mixture of solid and liquid components must be such that it results in a plastically deformable mass. Saturation: The volume of pore space in the sludges must be filled with a sufficient quantity of liquid matter for a particle to be able to rest on the particle via a plastically viscous liquid and for the interstices to be filled. Gas component: By injection of gasses, sludges which do not naturally have a gas component can be brought to a pumpable consistency.
Types of high-density solids pumps Depending on how the displacement principle is implemented, a distinction can be drawn between rotary and reciprocating pumps. The rotary circulation pumps includes eccentric screw pumps, centrifugal pumps, and squeezed tube (peristaltic) pumps. Reciprocating pumps include plunger, diaphragm, and piston pumps. The high-density solids pumps which can be used in the widest range of situations are piston (or reciprocating) pumps. They can be realized as single-cylinder or two-cylinder pumps. In the latter case, power is transferred to the material via delivery pistons working in push-pull-mode. While one delivery piston sucks material from the feed hopper into the cylinder, the second piston simultaneously pushes the material in the other delivery cylinder and into the delivery line.
Types of piston pumps To make a pump out of a reciprocating piston, some mechanism enforce unidirectional flow from the inlet to the cylinder to the outlet. Thinner fluids can use simple check valves, but solids pumps require more elaborate mechanisms.
Type: Piston pump with transfer tube (two-cylinder pump)
Construction details The most characteristic element of the transfer tube pump is S-shaped "transfer tube" installed inside the feed hopper, which connects the advancing piston to the outlet. In other words, the pump cylinder which is currently pushing the material is connected to the delivery line by the transfer tube, while the retracting (pulling) cylinder is open to the feed hopper. At the end of each piston stroke, the transfer tube is swung over to the other piston with the aid of two switch cylinders. A hydraulic circuit is used for synchronizing the position of the transfer tube and the movements of the two delivery pistons. The S tube should swing over rapidly and not suffer undue wear. This is effected with two large-sized plunger cylinders and a so-called spectacle plate (so named from its shape) including seal rings. In cross section the S-tube is circular with a diameter which tapers in the direction of flow. This keeps the risk of clogging down. The sealing faces of the spectacle plate and of the seal ring lie parallel with the swivelling movement of the S-tube, so that they cannot be damaged when foreign bodies are cut through or get stuck.
Operating features Piston pumps with S-tube are suitable for concrete and mortar, for sludges with a solids content of up to 50% by weight and a fluctuating particle size distribution but also for conveying fly ash, coal or minerals in a suspension. The large cross section of the holes in the spectacle plate means that with a delivery cylinder diameter of e.g. 200 mm (7.874 in) it is possible to pump high-density solids with a mean grain size of as much as 80 mm (3.150 in). The maximum diameter of individual foreign bodies may be as high as 60% of the diameter of the delivery line - in this example this would be 120 mm (4.724 in).
Type: Piston pump with seat valves (two-cylinder pump)
Construction details In lieu of check valves operated by the fluid being pumped, the steel housing of a seat valve pump contains four hydraulically operated seat valves, one suction and one delivery valve per piston. The suction and discharge valves are synchronized with the hydraulics of the delivery pistons - this ensures that the contents of the delivery cylinder is equal at all times to the volume being pumped. Once the "sucking" piston reaches its end position, the corresponding suction or delivery valve is simultaneously closed or opened. If there is overpressure in the delivery line, the suction valve will close first. This prevents the pumped medium from being pushed out of the pressure line back into the hopper.
Operating features Seat valve pumps are suitable for an especially even pumping of media with a solids content of up to 50% and for the high-pressure pumping of paste-like industrial media such as sludges. Since valves are used in this two-cylinder pump. it is limited to grain sizes of 8 mm at most. Which type of valve is selected will depend on the high-density solids to be conveyed. For materials containing grains with a solids content up to 50% a sharp-edged metal valve seat is required. With low-viscosity, rather watery fine sludges the choice would be a large-area elastomeric valve seal. In one hydraulic control variant the valve is opened passively by the thrust of the medium. In this case the discharge valve has a non-return function which prevents flowback from the pressure line. At the same time the high-density solids are precompacted to close to the line pressure before the discharge valve opens. This means that pipe knocking resulting from pressure pulsations can be avoided.
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