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what is a Slurry pump 

A centrifugal slurry pump utilises the centrifugal forces that are generated by its rotating impeller to impart kinetic energy to the slurry in the same manner as clear liquid type centrifugal pumps. However, this is where the similarities end. Slurry pumps are heavy-duty versions of centrifugal pumps that because of the extreme working environments that they operate in require special considerations regarding materials, impellers, and seal selection. It is also worth noting that a slurry pump will ever be as efficient as a water pump even when both are pumping water. Also because of the many, unknown variable that slurry pumps must overcome much of the design is based on accumulated knowledge and experience. Therefore, a good deal of compromise is always necessary between what is ideal and what is possible in terms of wear life and all-up operating costs. (for more information contact us to receive a copy of our slurry pump guide)

Warman 6/4 AH slurry pump

Slurry pumps work in extremely harsh environments and careful consideration needs to be given to the selection of the materials that will be used to protect the components of the pump that are continuously exposed to slurry. The material selection procedure must take many variables in to account, the correct selection of impellers and casings is just as important as the pump selection itself.

Three primary conditions create wear within a Slurry Pump.

Abrasion
Erosion
Corrosion


Erosion is the primary cause of wear in slurry pumps. The reason is that particles in the slurry hit the pump’s internal surfaces at different angles resulting in the loss of surface material. Unlike abrasion, erosion transfers kinetic energy from the particles in the slurry on to the surfaces of the pump that are in contact with the slurry. Even though the overall contact pressure at each individual impact zone is small, the specific contact pressure is high because of the irregular shape and impact of the particles. This results in high contact stress. (for more information contact us to receive a copy of our slurry pump guide)

Pump selection

It is important to know the steps for sizing slurry pumps and the relationship between them, to ensure that these procedures are correctly understood. The following only covers the basic parameters that must be considered.  A detailed procedure is available in our Slurry Pump Guide.

Installed in a piping system a Slurry Pump must be rated against the static head, any delivery pressure requirement and all friction losses to be able to provide the required flow rate. The duty point is where the pump performance curve crosses the system curve.

Duty Point Curve

When sizing a pump it is vitally important to never overestimate the system resistance as a multitude of problems can arise. Always use the best estimate of system head. Add safety margins to the calculated power only.

The performance of a centrifugal pump, pumping slurry differs from the performance with clean water depending on the amount of solid particles in the slurry. This difference depends on the characteristics of the slurry (particle size, density, and shape). This is described in our Slurry pump Guide. The factors that are affected are the power (P), head (H), and efficiency (h). The differences between slurry and water are shown schematically in the curves below.

Water vs Slurry curve

A correctly dimensioned slurry pump must be capable of overcoming the losses caused by friction in the piping and any valves. It is also important that the flow velocity does not fall below the critical velocity otherwise sedimentation will result. It is important that all the parameters for the slurry and pipe system, are specified as accurately as possible. In cases when assumptions must be madein the calculations, it is important that the customer is made aware of them.(for more information contact us to receive a copy of our slurry pump guide)

STATIC HEAD
Static head is the vertical height difference from the surface of the slurry source to the discharge point.

FRICTION LOSSES
When the liquid starts to flow through the discharge line and valves, friction will happen. When pumping slurry, friction losses that are caused by the pipe roughness, bends and valves, are different compared to the corresponding losses when pumping water. We provide an example calculation for later in the manual.

TOTAL DISCHARGE HEAD
This value is used for pump calculations and comprises the static head plus friction losses caused by pipes and valves, converted to meters of water.
Total Dynamic head

CRITICAL VELOCITY
In general, the flow velocity in the pipes must be kept above a certain minimum value. If the flow velocity is too high, friction losses will increase. This may also increase the wear in the pipe system. Flow velocities that are too low will result in sedimentation in the pipes and, therefore create, increased losses. This is illustrated in the diagram below, in which the critical velocity (Vc) indicates the optimum velocity where losses are kept to a minimum.

When making calculations for a slurry pump for a certain flow, the desired flow velocity (V) must be compared to the critical velocity (Vc) for the slurry and the pipe system in question. As the figure below shows, the ideal velocity (marked green) is immediately above the critical velocity but with a margin for the extreme cases that can arise.

To determine the critical velocity, the pipe diameter and the particle size (d50) must be known. The value is then corrected with a factor, which depends on the specific gravity of the solids.

Critical velocity

NPSH
Whenever centrifugal pumps are used, it is important that the pump’s inlet pressure exceeds the vapour pressure of the liquid inside the pump. The necessary inlet pressure that is stated for the pump, NPSHreq must not be less than the available value in the pump system, NPSHa.

The available value depends on the ambient air pressure (height above sea level), the vapour pressure of the liquid, the density of the slurry, and the level in the at which the pump is working.

CAVITATION
We need to check the hydraulic conditions at the inlet (suction) side of the pump to prevent cavitationfrom taking place.

To ensure that a Slurry Pump performs satisfactorily, the liquid must at all times be above the vapour pressure inside the pump. This is achieved by having sufficient pressure on the inlet side of the pump. This pressure is called: Net Positive Suction Head, referred to as NPSH, and is usually expressed in meter (feet) of liquid column absolute. Should NPSH be too low, the pressure in the impeller eye would decrease down to the lowest possible pressure of the pumped liquid, the vapour pressure. When the pressure in the impeller eye, near the vane edge, drops down to or below the liquid vapour pressure, vapour bubbles start to form. These are carried by the liquid to locations under higher pressure, where they collapse (implode) creating extremely high local pressures (up to 10,000 bar), which can erode the pump surfaces. These mini implosions are called cavitation.

Cavitation is not, as is sometimes believed, due to air in the liquid, but is the liquid boiling at ambient temperature, due to the reduction in pressure. At sea level, atmospheric pressure is 1 bar (14.5 psi.), and water boils at 100oC (212oF). At an altitude of 2800 m (9180 ft.), atmospheric pressure reduces to 0,72 bar (10.44 psi.) and water boils at 92oC (198oF).

A major effect of cavitation is a marked drop in efficiency, caused by a drop-off in capacity and head. Vibrations and mechanical damage can also occur.

slurry pump guide

For a detailed overview of the above content and much more, please contact us to get a copy of our Slurry Pump Guide. Please ensure that you provide all your contact information, including company details when requesting a copy of he Guide.  enquiries@totalpumpgroup.com

Slurry pump Guide

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