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.
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.
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 is the vertical height difference from the surface of the slurry source to the discharge point.
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.
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.
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.
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.),
A major effect of cavitation is a marked drop in efficiency, caused by a drop-off in capacity and head.
slurry pump guide
For a detailed overview of the above content and much more,
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.