How To Succeed At Slurry Pumping


Optimal transfer of two-phase solid-liquid flow (slurry flow) has long been a

major industrial challenge.

Slurry pumps are among the most common types of centrifugal pumps used to deal

with this transfer issue. The approach of improving slurry pumps and consequently

increasing the efficiency of a flow transmission system requires overcoming the

effects of slurry flow such as the reduction in head, efficiency, and wear. This study

attempts to investigate the changes in the pump head by modifying the slip factor

distribution in the impeller channel. For this purpose, the effect of splitter blades

on slip factor distribution to improve the pump head was investigated using numerical

simulation tools and validated based on experimental test data. Next, an optimization

process was used to determine the characteristics of the splitter (i.e., length,

number, and environmental position of the splitter) based on a combination of

experimental design methods, surface response, and genetic algorithm. The optimization

results indicate that the splitters were in a relative circumferential position of

67.2% to the suction surface of the main blade. Also, the optimal number and length of

splitter blades were 6 and 62.8% of the length of the main blades, respectively.

Because of adding splitter blades and the reduction in the flow passage, the best

efficiency point (BEP) of the slurry pump moved toward lower flow rates. The result of

splitter optimization was the increase in pump head from 29.7?m to 31.7?m and the

upkeep of efficiency in the initial values.

The effect of solids on a centrifugal



horizontal slurry pump performance is a major concern to the design of slurry

transportation system. In the present study, the multiphase modeling of centrifugal

slurry pump is performed using two models, Mixture and Eulerian-Eulerian multiphase.

Sliding mesh approach is employed for unsteady simulation of the pump. The accuracy of

the simulations is ascertained by comparing the performance characteristics of the

pump obtained numerically and experimentally. Experimental results are obtained by

measurements in a pilot plant test rig with three different mean size sand particulate

slurries. The Eulerian-Eulerian multiphase model predicted the effect of the solids on

pump performance close to the experimental results as compared to Mixture model. The

obtained accuracy with Eulerian-Eulerian model for predicting the effect of solids on

head and efficiency is around ±2% and ±3%, respectively. The predicted results using

Eulerian-Eulerian model confirm that the head and efficiency of the pump decrease with

the increase in particle size and concentration. The particles of high specific

gravity show less reduction in head and efficiency of the pump. Further, the effect of

variation in particle size and concentration on the flow field in the impeller and

casing has also been analyzed at best efficiency point operation. Non-homogeneous

suspension of particles inside the blade channels and casing passages is examined. The

particulate concentration is observed higher near the impeller back shroud, pressure

side of the blades, and non-suction side of the casing as compared to other locations.

Plants often must handle slurries in applications ranging from processing to

wastewater treatment. Dealing with such mixtures of liquid and solids is challenging

and difficult. Some key elements in slurry pumping are the size and nature of the

solids in the liquid and the kind of the abrasive wear they cause. Another is the

corrosiveness of the liquid or the mixture.

Sites frequently rely on centrifugal pumps for slurry services. These pumps (and

their associated piping systems) need special provisions that call for a detailed

knowledge of the solid and slurry properties to prevent wear, corrosion, erosion and

other adverse effects such as settling of the solids. Specifying the optimum

combination of speed, geometry and materials requires properly balancing often

conflicting pump priorities; this demands consideration of stable operation, maximum

wear life, operational flexibility and minimal energy consumption.

In this article, we’ll cover practical guidelines and rules for centrifugal pumps

for slurries. We’ll also discuss key operational features, material selection and

other considerations.

Tailored Pumps

Horizontal centrifugal pumps usually are used for slurry services, although

vertical and other types of pumps are favored for some specific applications.

Centrifugal pumps for handling slurries have features tailored to the particular

service that reflect the corrosive or abrasive nature of the slurry and the solids

concentration. These may include choice of materials, use of liners and even different

driver sizing.

The first major requirement of a

mining

slurry pump is to provide adequate service life. Erosion and corrosion effects of

slurries, such as the impingement of high velocity flow of liquid/solid mixtures, are

really challenging. In many applications, some solids in the mixture are larger than

usually specified particles; so, the pump should be able to pass them without any

damage or operational problems.

As a result of such requirements, a

vertical

slurry pump often is larger than its clear liquid counterpart. Moreover, it

generally sacrifices efficiency, both maximum efficiency and efficiencies over the

whole operating range, in exchange for the ability to achieve good operation in these

challenging services.

Because wear is a function of velocity, a slurry pump’s speed should be as low as

possible; units usually operate at 1,200 rpm or slower. Often, direct coupling between

the pump and a low-speed electric motor or other driver makes most sense. On the other

hand, many other applications favor gearboxes to meet the desired speed and duty

point. In services requiring variable flow, variable frequency drives are used to

provide the necessary continual speed changes.

Although the emphasis on a ceramic slurry pump tends to be on the size and

percentage of solids to be pumped, corrosion resistance is also an important factor

for material selection in many applications. In such cases, the material chosen must

provide an adequate combination of both erosion and corrosion resistance.

For slurry services, a pump operating on the left of the pump performance curve or

at the best efficiency point (BEP) is usually preferred; as an indication, the rated

point should lie somewhere between 85% and 100% of the BEP point.

The performance curves of many centrifugal pumps are based on handling water. So,

to obtain the performance characteristics for these slurry centrifugal pumps, you must

modify the results for the presence of solids. Many correlations and correction

methods exist for predicting the performance of centrifugal pumps when handling

slurries; these account for factors such as individual effects of particle size,

particle size distribution, specific gravity and concentration of solids. They usually

provide a head reduction factor and efficiency reduction factor for slurries compared

to clear water. However, every pump has unique service-specific factors for a given

application. You should verify these by experiments.

For slurry services, a pump operating on the left of the pump performance curve or

at the best efficiency point (BEP) is usually preferred; as an indication, the rated

point should lie somewhere between 85% and 100% of the BEP point.

The performance curves of many centrifugal pumps are based on handling water. So,

to obtain the performance characteristics for these slurry centrifugal pumps, you must

modify the results for the presence of solids. Many correlations and correction

methods exist for predicting the performance of centrifugal pumps when handling

slurries; these account for factors such as individual effects of particle size,

particle size distribution, specific gravity and concentration of solids. They usually

provide a head reduction factor and efficiency reduction factor for slurries compared

to clear water. However, every pump has unique service-specific factors for a given

application. You should verify these by experiments.
Wear, Erosion And Corrosion
Major factors that influence wear include the following:
? details of erosive particles (material, size, shape, etc.);
? concentration of solids;
? fluid velocity and particle velocity; and
? hydrodynamic properties of the flow (Reynolds number, etc.).
Turbulent flow analysis usually isn’t applicable for slurry pumps because the

presence of solid materials will directly influence the turbulence parameters. The

mechanisms of turbulence become a complex problem particularly for dense slurries.

This, combined with the nature of the flow inside a slurry pump, which is

characterized by unsteadiness as well as deformed velocity distribution patterns,

cause a very chaotic situation. However, some simple rules have been verified both in

theory and experiments. For instance, the erosive wear rate is proportional to the

flow velocity. It also depends on the solids concentration; as that concentration

increases, so, too, does the wear rate. In addition, the sizing and specific gravity

of solids in the slurry affect wear.

As a very rough indication, in medium and low concentrations, the mechanism of

erosion resembles cavitation erosion. Here are some observations about a slurry pump

that operated for a short period with low concentration slurry and then with a

moderate concentration one. With the slurry at low concentration, the suction side of

the blade showed very limited erosion at the leading edge while the rest of the

surface was nearly unscathed. Most of the erosion of the back shroud was concentrated

in the eye of the impeller and around the leading edge of the back-shroud corner. The

maximum erosion appeared on the leading edge and towards the back shroud of the

impeller. This pump, operated with a relatively higher concertation slurry, showed a

similar erosion pattern — with the only notable difference being that the region

close to the back shroud was more heavily eroded. Unfortunately, here and more

generally, theoretical studies don’t properly predict the level of erosion.

To cope with wear, pump designers generally rely on two options: use of thicker

components or liners.

In the first approach, designers apply thickness allowances depending on estimates

of erosion and corrosion of each part or component. Such pumps feature thick sections

for all parts and components in contact with slurry including impeller and casing;

those elements subject to high speed flow of slurry or solid impingements receive a

greater thickness allowance.

In the second method, the designers provide wear liners on pump internals; many

such pumps are fully lined internally. A lined pump, if properly done, can allow use

of a wider variety of materials and also cost-effective application of exotic

(expensive) materials. An unlined pump may offer lower initial capital cost. However,

a properly lined pump usually affords a greater number of material choices, which may

result in longer wear life and lower replacement spares cost. The lined pump also is

inherently safer from a pressure containment standpoint because, in pumps relying on

added thickness for components, corrosion and erosion over time might reduce the

thickness to below the original value. Large clearances within the impeller and casing

allow for the passage of large diameter solids, while also reducing internal

velocities and corresponding wear. On the other hand, proper design and installation

of liners can pose challenges — many pumps with liners have failed because of such

inadequacies.

The materials used for wear components are hard metals, elastomers and, to a

lesser extent, ceramics. Hard metals and ceramics combat erosion due to their high

hardness values. Elastomers combat erosion by absorbing the energy of an impacting

particle due to their resilience and tear resistance. Elastomers often can provide

better erosion resistance in some applications. As a very rough indication, these are

services where particle size is smaller than 250 microns, impeller tip speed is within

the limits of the application of elastomers, and there’s no risk of large particle

damage. Elastomer-lined pumps have been successfully used in numerous slurry services.

Many metals have been used for slurry pumps. The ASTM A532 standard for abrasion-

resistant cast irons covers the three basic types — “Martensitic White Irons”

(Class I), “Chromium-Molybdenum White Irons” (Class II) and “High Chrome Irons”

(Class III). These materials, which are made by melting processes, have been alloyed

to achieve high resistance to abrasive wear. The bulk hardness of these alloys depends

on many factors and parameters — for instance, not only upon the carbide and matrix

type but also upon the volume of the carbides within the matrix. For slurry

applications with small particles, microstructure, with smaller inter-carbide spacing,

is particularly important to minimize erosion of the softer matrix. For slurry

services with medium to large particles, the bulk (combined) material hardness is key.

For applications with very large particles, fracture toughness of the matrix is

important.

Impellers

These tend to be larger than their clear liquid counterparts. Achieving a given

head and providing more material for wear purposes requires a lower impeller speed.

Minimizing speed and maximizing wear life of both the impeller and suction side

depends upon a proper configuration.

High wear applications usually call for closed-type impellers. In applications

with coarse particles, expelling vanes on the face of the front shroud make sense.

These vanes prevent large particles from becoming trapped between the impeller and

suction side liner and minimize recirculation. The benefit is reduced gouging and

recirculation wear at the expense of a 2–3% drop in efficiency. In addition,

expelling vanes often are used on the back shroud of the impeller in coarse particle

applications to prevent the trapping of large particles between the impeller and back

liner. In this location, they also serve to decrease the forward axial load by

lowering the pressure acting on the back shroud and beneficially reducing the pressure

at the hub. The decreased axial load improves bearing life. All these effects also cut

the pressure differential at the shaft seal and reduce the tendency for slurry leakage

from the pump. As with expelling vanes on the front shroud, back vanes usually absorb

2–3% of efficiency.

To combat wear and allow for passing large diameter solids, many slurry pump

impellers feature fewer but thicker main pumping vanes. Both of these factors further

contribute to reduced efficiency compared with clear liquid impellers. While a clear

liquid impeller usually has five to nine vanes, most slurry pump impellers have two to

five. Applications requiring large particle passing often employ pumps with two or

three vanes. Slurry pumps use short blocky vanes in contrast to the thin long-length,

long-wrap vanes found on high efficiency pumps for clear liquids.

Sumps And Storage Tanks

Slurry pumps usually require sumps or suction tanks to act as suction source or

intermediate storage for slurries. However, lack of detailed knowledge about the

slurry pump’s behavior and sump hydraulics often leads to oversizing. The larger the

sump or storage tank, the more likely it may become a settling tank for solids. For

some services, the accumulation of solids leads to other problems, for instance, the

build-up of harmful gases, and requires periodic desludging of the sump or tank, which

increases operating costs and reduces overall safety and reliability. The optimal

volume of suction storage, in this context, should prevent the settling of solids

while avoiding problems for pump operation.