W~A~ IN PUMI:~S AND PIPELINES

G.F. Truscott

BtII~A Fluid ~ngineering, U.Ko

1. INTI~ ODUCTION

Wear is a very important consideration in the design and operation of slurry systems, as it affectsboth the initial capitalcosts and the life of components. It may be defined as the progressive volume loss ofmaterial from a surface, due to all causes.

There are two main causes of wear:-(I) Erosion (in the most general sense), involving mechanical action by:-

(a) ’solid particles - abrasive wear;(b) cavitation.

(II) Corrosion, involving chemical.or electro-chemical action.Both effects can take place simultaneously - ’erosion- corrosion-’ - the relative proportions depend-

ing on the pa’rticular application. Hence the choice of construction materials is usually a compromise betweenabrasion resistance, corrosion resistance, mechanical strength and cost.

The information given below has been collected mainly from previous ’Hydrotransport’ SlurryPipelining Course Notes (l~efs. 1, 2 and 3) on wear topics, BHI~A literature surveys on abrasive wear inrotodynamic machines (l~ef. 4) and in pipelines (l~ef. 5), and a ’Pipe Protection’ l~eview (Bef. 6), carriedout jointly by BHI~A and the Paint 1~ .A.

The more general~ aspects of wear, applicable to both pumps and pipelines, are considered first,followed by a discussion of those aspects relating specifically to either pumps or pipes. There is, however,a disturbing though consistent theme running throughout: that is, in spite of a considerable amount ofliterature, the general lack of reliable quantitative data on many of the factors affecting wear, from whichwear rates for new systems might be predicted with reasonable certainty.

2. FACTOI~S AFFECTING WEAI~; TYPES OF WEAI~

In general, these apply to both pumps and pipes.

2.1 Basic factors affecting abrasive wear

There are 3 main components in the basic wear process: the abrasive particles, the materialagainst which they impinge, and the relative impact velocity. Thus one may divide the relevant factors into3 main groups, comprising the properties relating to:-(I) abrasive slurry- partiole hardness, size, shape (i.e. angularity or sharpness) and relative

density; solids concentration in mixture;(II) construction materials - composition, structure, hardness;

(III) flow - speed, direction (i.e. impact angle), pipe flow regime (i.e. type of particle motion).

2.2 Types of abrasive wear

It has been suggested (l~ef. 1) .that wear terminology tends to be rather loose - the terms ’wear’,’erosion’, ’abrasion’, ’abrasive wear’ are often applied to any wear situation~ The various types of ’abrasivewear’ are defined and discussed below.

Many references concerned with simulation wear tests on materials distinguish between varioustypes of wear. In general, wear in slurry systems can be classified under 3 main types of abrasion - (a)erosion, (b) gouging and (c) grinding.

2.2.1 Erosion abrasion

This arises from the impingement of smaller particles on the wearing surface, and probably pre-dominates in most systems. It is usually considered to have two mechanisms, as described by Bitter (l~ef. 7):(a) Cutting wear, associated with the particle velocity component parallel to the surface, where

stresses are dueto velocity rather than impact and material is removed by a cutting or scratching typeprocess.

(b) Deformation wear, due to the normal component of velocity, where the elastic limit of thematerial is exceeded and plastic deformation occurs, eventually destroying the surface layer afterinitial work hardening.

In practice, a wide variation of impact angle usually exists, and these t~¢o mechanisms.will occursimultaneously. Figs. l(a) and l(b) show the results of Bitter’S tests (l~ef. 7) on ductile" and brittle materials,giving the relative proportions of cutting and deformation wear for a range of impact angles from 0-90°.

2

2.2.2 Gouging abrasion

This occurs when large particles impinge with sufficient force that high impact stresses are imposed;

resulting in the tea.ring out of sizeable fragments from the wearing surface. This type of wear would besignificant in dredging and gravel-handling systems.

2°2°3 Grinding wear

This involves the introduction and crushing of fine particles between two surfaces in close proximity,e.g. pump internal leakage clearances and glands. Very high stresses are involved, and wear occurs bycutting, local plastic flow and micro-cracking.

2o 3 Wear theory

A number of simple expressions are given in the literature, based on wear test results, for wearrate as a functi0h of velocity, material hardness, grain size or solids concentration. The one most oftenquoted for both pump and pipe wear is:- -

wear ~ (vel)n

where index ’n’ may vary depending on the material and the other factors involved, For pump wear (l~ef.the most common value of n appears to be 3, but for pipe wear (l~ef. 5), there is much more variation in thevalue quoted, lying between 0.85 and 4.5. However, in practice it is doubtful if wear will vary with velocityas a simple power law.

Bitter’s fundamental study of erosion phenomena (l~ef. 7) - strictly for dry conditions - developsseparate expressions for cutting and deformation wear, based on particle mass, velocity and impact angle,and the various properties of the surface being eroded.

Other detailed analyses relating to wear in hydraulic machines consider wear as affected byforces and velocities acting on a particle in liquid flow. Bergeron (Bef. 8) attempts to predict wear ratesin similar centrifugal pumps handling solids with varying properties, with simplified assumptions such a~pure sliding of the particles over the surface, from the initial expression:

U3wear a ~ (Ps - P l) d3pK

where:-

PS =p =

1d =

U = characteristic velocity of liquid

density of particles

density of liquid

diameter of particles (assumed spherical)

D = characteristic dimension of machine

p = number of particles/unit surface area

K = experimental coefficient depending on particle abrasivity.

In a much more involved analysis in a later paper (l~ef. 9), but starting with the same basicassumptions, he develops a complicated expression based on the statement:-

wear a solid-liquid density difference x acceleration of main flow x coefficient of friction x thickness ofparticle layer x flow velocity

This takes account of the difference between solid and liquid velocities.Some authors, mostly from Bussia and E. Europe, also develop expressions for pump service life,

either in terms of pump total head and the other properties of slurry and construction materials which effectwear, or based on a statistical analysis of pump wear tests.

It is, perhaps, debatable’ whether these more complex theories can be used to predict absolutewear rates with any certainty; most involve coefficients which have to be determined experimentally in any .......case. However, a theoretical treatment may help in predicting trends when only one or two of the relevantfactors are altered, or in the correlation of results.

2o4 Corrosion

Corrosion may be defined as the destructive attack on metal by chemical or electro-chemicalreaction with its environment.

2.4.1 Basic mechanism

Corrosion of metals by liquids is n0rmal[y electro-chemical, and involves the two processesconcerned with basic galvanic cell type reactions. As stated in Ref. 2, the metal shows a tendency to go intosolution, forming positively charged ions. This departure of metal ions leaves the surface with an excess ofelectrons. A closed electrical circuit can then be set up in which the electrons travel from one area (anodic)of the surface through the metal to other areas (cathodic), where they are absorbed in a number of ways.As this process continues, corrosion results. The chemical reactions are:-a) Anodic - in which metal ions (M) and electrons (e) are produced:

b) Cathodic - where the electrons are absorbed by:(i) Conversion of dissolved oxygen to hydroxyl ions

O2 + 2H20 + 4e -+ 4OH

(ii) Discharge of hydrogen ions to give hydrogen gas

+2H + 2 -~H

e 2

(iii) Reaction with other ions in the electrolyte

YIx+ + xe -~ M (deposited).

The anodic and cathodic reactions occur simultaneously and at the same rate.The most important cathodic process is reaction (i) involving dissolved oxygen. Removal of oxygen

from solution greatly reduces the corrosion rate. Reaction (iii) is slow above a pH of 4. Some corrosion ofmetal can occur in absence of oxygen due to the combined effect of reactions (ii) and (iii).

Inhibitors or polarizing agents are organic or inorganic substances which are added to thecorrosive environment. If the anodic reaction, whereby metal passes into solution, is inhibited by an anodicinhibitor, corrosion ceases. If the cathodic reaction stops, the anodic one must also stop and attack is againprevented. Sodium chromate is an anodic inhibitor. The presence of anions of sulphate and chloride insolution increases the corrosion rate and renders protection by inhibitors more difficult.

2.4o 2 Types of corrosion

Probably the most common type of attack ~n slurry systems is !erosion-corrosion’, where the rateof corrosion is accelerated by scouring of the surface and removal of protective oxide or scale films by theimpacting solids. This is discussed fully in Ref. 3.

Apart from general corrosion, certain types of localised corrosion may also occur in pumps andpipelines, depending on the construction materials and the particular work-ing environment. These types couldinclude crevice corrosion, bimetallic corrosion (’electrolytic action’), intergranular corrosion, weld decayand pitting, as well as local erosion-corrosion. Localised attack linked to some mechanical factor may takethe form of fretting corrosion, impingement attack, cavitation, damage, stress corrosion cracking, hydrogencracking and corrosion fatigue.. These types are all discussed in some detail in the Pipe Protection Review(~ef. 6).

2.4.3 Evaluation of corrosion effect

It is often advisable to carry out field or laboratory tests to evaluate the corrosion effect forparticular operating conditions. Care is needed in the selection of a method to simulate actual conditions asclose as possible. Among the factors to be considered when testing corrosion resistance are temperature,composition of the solution, velocity, concentration, galvanic effect, type of corrosion and susceptibility tolocalised attack, equipment design and stress effects~ Ref. 1 gives the more common methods of evaluation;those observing changes in weight and physical properties are most often used.

Most data have been obtained from laboratory simulation tests, as it is very difficult to measurethe effects in isolation in practical systems, due to interaction between the various factors involved.

Particle hardness

Laboratory tests (e.g. Ref. 10) have shown that, for metals in general, wear increases rapidlyonce particle harndess exceeds that of the metal surface, for both scouring and impact abrasion (Fig. 2).However, little or no quantitative information is available on the direct effect of grain hardness on eitherpump or pipe wear.

Rubber behaviour is more difficult to compare on a relative ’hardness’ basis; tests on syntheticrubbers, e.g. ’Vulkollan’ polyurethane, showed fairly constant scouring wear rates (Fig. 2), and muchlower than for the steels, except with the less hard abrasives.

An abrasivity index, known as the ’Miller Number’, has been established for a wide range ofslurries, (Ref. 11). The Miller number has two values; the first relates to abrasivity and represents therate of weight loss of test specimen, and the second to attrition (or degradation) as measured by the changeof abrasivity due to particle breakdown. Although the Miller test machine was developed to investigatewear in reciprocating pumps, these results give some general indication of relative abrasiveness which mayalso apply to centrifugal pump and pipe wear. Coal and clays are usually regarded as having a lowabrasivity (index Nos. 5-50), whereas various metal ores and concentrates, silica sand and some mine’railings’ are highly abrasive (index Nos. 70-650).

3.2 Particle size and shape

It has been found that, in general, wear rate increases with grain size.and sharpness; angularparticles will cause more wear than rounded ones. Some authors suggest that wear a size for cutting wear,but is independent of size for direct impact. Large particles can also cause gouging of the surface onimp act.

Wear tests on a small dredge pump impeller (Ref. 12) shoWed differences in blade wear patternsdepending on grain size (L e. sand or gravel), but no quantitative results are given on wear rate.

There is a wide variation in the results from different pipe wear simulation rigs (e, g. rotatingof pipe, rotating pipe specimens, etc.). Fig. 3 shows the effect of grain size vs wear from a closed-

pipe wear test rig at the Colorado School of Mines Research Inst., U.S.A. (Refs. 2 and 13); .16particles caused over twice the wear due to those of 35 mesh. However, those below 250 mesh

insignificant wear.Size and shape effects are more critical for rubber linings of both pumps and pipes than for metals;

sharp particles are likely to. cause severe damage. There is some variation in the size limits quoted,on the types of abrasive and rubber, but approximately 5-6 mm (3/16"- ¼") is the most generally

3.2.1 Degradation (attrition)

Degradation or attrition of particles (i. e. reduction of size and/or sharpness) will occur as solidss along a pipeline, causing a reduction of wear along its length, or when passing through a pump. The

of degr.adation will depend on a number of factors, including the structure and hardness of particles,~city~ mixture concentration and pipeline length. It is a particular problem in recirculation type slurryrigs, affecting both pumps and pipe wear results.

A few authors (e.g. Ref. 14) give expressions for calculating degradation i’n pipelines, usingcoefficients based on test data. Only limited data is available on degradation in pumps, which is

an order or magnitude greater than that in pipes. There is some evidence (Ref. 15) thatpumps cause uniform attrition, unlike centrifugal pumps.

Degradation is particularly severe with coal slurries, the solids being relatively soft, but maya~ higher concentrations of about 50-60% by weight. As mentioned earlier (Sedtion 3.1), the secondL the ~iller number relates to attrition susceptibility.

Mixture concentr.~tion and density

turn depends on the concentration. However, at higher concentrations, mutual interference betweenparticles will tend to reduce the frequency of impacts, (l~ef. 16). It has been suggested that wear is approx-imately a concentration for the lower concentrations, but the rate of increase reduces at the higher values,(l~efs. 2 and 4).

Most of the published data relates to pipe wear. Fig. 4 shows results of a pipe wear test at theColorado School of Mines on a copper concentrate slurry, (l~efs. 2 and 13); only about 10% increase in wearoccurred for an increase in concentration from 30 to 60% by weight. In general, the rate of increase isfound to reduce at concentrations over 20% by weight.

Some theoretical expressions ~how wear depending on the solid/liquid density difference, varyingeither directly - if other factors remain constant - or as a more complicated function.

3.4 Corrosive slurries; corrosion control

Most available information on corrosion is concerned with pipelines." This aspect is particularly.important in unlined steel pipelines, which comprise the majority in slurry systems, and where botherosion-corrosion and mechanical erosion will exist simultaneously. Corrosion may be more of a problemthan erosion, depending on the type of slurry; it is said to be the only wear problem with the Savage l~iver ironore line in Tasmania,~ where erosion was claimed to be negligible, l~ipe wear tests at the Colorado School ofMines ~efs. 2 and 13), where corrosion and erosion effects could be separated, showed that coal and copperconcentrate slurries caused wear mainly by corrosion, whereas phosphate lines tend to wear by erosion,as shown in Table 1A. However, alkalinity will also have an effect; corrosion may be negligible with acoa! slurry if the pH value is sufficiently high. Combined corrosion and erosion effects caused fairly rapidfailure of the original unlined steel pipe handling a micaceous residue slurry in a china clay process (Ref.17), with pH values in the range 2. 5-5.5.

Corrosion may be controlled in long slurry lines either by ’solution conditioning’ involving oxygenremoval e.g. by de-aeration or the addition of sodium sulphite and pH control, or by adding oxidisinginhibitord, such as high concentrations of nitrites or chromates, (l~ef. 3). Corrosion is found to be moresevere at the beginning of some c0al and iron ore pipelines. Some authors suggest that chromates,particularly ff used at too low a concentration, will promote !ocal pitting, which m.ay be avoided by theaddition of hexametaphosphate (’Calgon’); however, for the Ohio coal pipeline, it was found that sodiumdichromate alone could be used without causing pitting. (l~ef. 3). It is recommended that laboratory testsshould be carried out to determine the most effective inhibitor for a particular slurry (l~efs. 1, 2 and 3),(See Section 2.4.3).

Other possible methods for controlling or preventing corrosion, but which may be economic onlyfor the shorter lines, are cathodic protection and the use of coatings or linings. The main advantage of rubberand polyurethane linings is that they are also abrasion-resistant (See Section 4.1), whereas the thinner coat-ings and points tend to give corrosion protection only.

EFFECTS OF CONSTt~UCTION MATEI~IAL PI~OPEI~TIES FOI~ PUMPS AND PIPES

As noted in the previous section, most of the quantitative data comes from laboratory simulationtests; only very limited data are available from actual pump or pipe tests, though the choice of pipematerials in commercial use is limited in any case. Otherwise, information from service experience isusually either qualitative or related to component life in fairly general terms.

4.1 Type: composition, structure

This section considers mainl.y abrasion resistan’ce aspects.

4.1.1 Metals

Chemical composition, mierostruoture and work-hardening ability all play an important part indetermining the wear resistance of metals~ .austenitio and martensitic steels being notably better than.ferritio.

The most comprehensive set of data applicable to rotodynamic pump wear comes from Stauffer’smaterial tests (l~ef. 18) in Switzerland. (Esther Wyss), a selection of which is given in Table 2, comparedon a basis of ’resisthnce factorv, 1~ (= volume wear of ref. steal/volume wear of test, material), sometiolish pump Wear tests (l~ef. 19) also give useful comparative data on wear resistance. In general, vNi-hard’(4~ Ni, 2% Cr) white iron and high chrome (12-26%) cast irons and cast steel alloys were found to’have thehighest.resistance of the cast materials, followed by 12% Mn austenitic cast steel. However, 18/8 austeniticstainless steel gave only moderate resistance. S.G. irons were somewhat better than ordinary grey C.Is.

Almost all the non-ferrous metals in Stauffer’s tests were less resistant than mild steel aud cast iron; tinbronzes tended to be slightly better than aluminium bronzes for the cast alloys, although 30% Ni 2.5% Albronze gave the best result for the wrought alloys. The most wear-resistant materials of all were sinteredtungsten carbide, followed by hard chrome plating and the hard CT-Co-W alloy weld overlays.

Although having not very high abrasion resistance, alumlniuln bronze, 18/8 and some Mn stain-

less steels all have good cavitation resistance. Hard Cr plating can give excellent resistance to both, providedsurface preparation of the base metal is adequate. 13% Cr stainless steel is also reported as giving goodabrasion and cavitation resistance, from water-turbine experience.

Service experience with centrifugal pumps (Refs. i, 4 and 20) suggests that ’Ni-hard’ and highCr cast irons are most commonly used for general abrasive sollds-handling duties with moderate-sizedparticles. However, ’ for gravel and dredging applications, where the solids are relatively large, high Mnsteels are often preferred, being work-hardened by impact; ’Ni-hard’ and high Cr C.I. tend to be brittleand hence prone to shock damage, although still offered by reputable manufacturers for impellers handlingcoarse abrasives.

Information relating to pipe materials is limited to some German jet-impact simulation tests(Bef. 21), claimed to be relevant to bend wear (See Section 7.2), and some Hungarian pipe wear tests wherespecific wear rates for C.I. and aluminium pipes were about 35% and 55% respectively than that for steel.However, the Pipe Protection Review (Ref. 6) suggests that cast and spun iron pipes are more resistantthan mild steel, although brittle and prone to damage during handling. There is no published data on relativewear rates between various grades of steel pipe. Weld overlays, e.g. high chrome carbide, are sometimesused as a lining for pipes (Ref. 6) for very abrasive duties.

4.1.2 Rubbers

The ability of rubber, when vulcanised, to deform elastically under impact makes it ideally suitableto resist erosion abrasion, provided that it is of sufficient thickness t~ avoid crushing, that bonding to thebase metal is good, and that particles are not large or sharp (See Section 3.2); there is also a temperaturelimitation of about 80°C. However, simulation tests (Ref. 10) have shown that there can be a large variationin Wear rate depending on both the types of rubber and abrasive.

It is generally agreed, from both wear tests and service experience (Refs. 1, 4, 12 and 20), thatsoft natural rubber (Ref. 1 suggests about 35-38° Shore hardness) gives best results, and is often equal oreven superior to hard metal for both pump and pipe applications. Russian tests claim a much improvedresistance for both natural and methylstyrene rubbers over butadiene styrene rubber; isoprene rubber w~tsal~o very resistant. Polyurethane synthetic rubber appears to be another promising and relatively new

material, particularly for pipe lining where improvements in life over natural rubber havebeen reported(l~ef. 17), (See Section 7.1.2). However, seports on its application to centrifugal pumps have been more

(Ref. 20); it has been suggested that it may be more suitable for static components, such as casings,than impellers, but this could well depend on the grade of.polyurethane and the quality of bonding.

4. I. 3 Plastics and plastics coatings

There appears to be little published information so far on the use and behaviour of plastics for.ds-handling pumps. Simulation jet-impact tests (e.g. Ref. 21) indicate that most, if not all, wereresistant than mild steel, with certain grades 9f polyethylene best, followed by nylon and ’Teflon’.

claim to have had some success in service with epoxy resins either filled with emery or graniteor with glassLfibre reinforcement. However, epoxy resin impellers failed rapidly in the Polish

wear tests (l~ef. 19).Other types of simulation test, intended to relate to pipe wear, appear to give conflicting results,

in terms of wear rates and order of resistance for apparently similar types of plastics. The Germannoted in the literature surveys on pump and pipe wear (Refs. 4 and 5) showed that most plastizs were

~ior to mild steel, except for the softest grades of PVC, in the sand-blasting tests. However, a list of~rials iven in the Pie Protection Review (Ref. 6), based in part on results from more recent BHRA......... g P~testS (Ref. 22), suggests thatGRP (’fibre-glass’), uPVC, ABS, (Aorylonitrile-Butadiene-Styrene),~ density polyethylene and polybutylene (a relatively new material) - in increasing order of abrasioni~ance - ma all be superior to mild steel. Hence, again some doubt exists as to whether this

Y

evidence is due to’ differences in test methods or in grades of similar materials; hardly any)erience is available for comparison.. Plastics would probably be considered only for relatively

pipelines, from both pressure limitation and economic considerations. Although somecoatings and paints have better abrasion resistance than others (Ref. 23), their main advantage is as

~tion against corrosion (Befs. 5 and 6).

7

4.1.4 Ceramics and related materials

Once again, there is very little information, and probably only limited application, regarding pumpand pipeline use.. Although very abrasion resistant, the use of ceramics in pumps has been limited so far’by their brittlefiess and susceptibility to thermal shock damage; there may also be particle size limitations,as for rubber linings. Some makes of centrifugal pump use ceramic impeller sealing rings and ceramic coat-ings on shaft sleeves in. way of the gland. However, new developments in small puffip applications may showimprovements.

Cast basalt and fused alumina, which are also normally regarded as being very wear resistant -although basalt gave poor results in sand blasting tests (l~ef. 10) - are used as insert linings for mild steeland, more recently, GRP pipe (l~ef. 6), particularly for those sections of pipeline subjected to excessivewear, such as bends (See Section 7.2). These materials are also likely to be economic only for short-distance lines handling very abrasive slurries.

4.1.5 Concrete and cement based materials (for pipes)

Russian work on concrete pipe linings claims improved abrasion resistance by ’vibro-activation’,and also by the addition of a large proportion of C.I. or steel particles, or plastics resin binders(’plastic Concrete’), to the mix. However, all types have a much lower resistance than mild steel.

There is little data on asbestos cement pipes; test results suggest a performance inferior tosteel (l~ef. 23) and even concrete (l~ef. 6), but one report of site experience on an Australian heavy mineralconcentrate pipeline indicates a wear resistance equivalent to steel.

4.2 Material hardness

In very general terms, abrasion resistance for ferrous metals tends to increase with hardness.It must be emphasised that metal hardness is not an absolute criterion of wear. Stauffer’s results (l~ef. 18)given in Table -2 show this general trend, but there is a large scatter, wi~h some harder materials givingmuch lower resistanc~ factors than the softer ones; the l~olish pump wear tests (l~ef. 19), also showedsimilar effects. Even a trend is not apparent for the copper alloys, the resistance of tin bronzes beingvirtually constant, independent of hardness. It may be noted, however, that the most wear-resistantmaterials, such as tungsten car.bide, hard chrome plating and weld overlays were also the hardest.

These regults suggest that a reasonable abrasion resistance is achieved above about 300 BrinellHardness No. (HB). Typica~ hardness values for high Cr alloy C.I.’s.and ’Ni-hard’ as used in abrasiveduty-centrifugal pumps would be in the range 550-700 HB. (1).

Hard materials generally are not suitable for pipe construction on economic grounds, apart frombasalt and ceramic linings having hardness values equivalent to about 1000 HB, and weld overlay cladding,for very abrasive dhties ~efo 6). !t has been noted from German steel pipe wear tests (l~ef. 12) thathardness may vary across the pipe wall - a similar property" could also apply to pump impeller and casingsections - and hence life will depend on the average rather than just surface hardness.

Soft rubbers are usually accepted as being more abrasion-resistant than hard ones, but there islittle or no information on the effect of hardness of plastics.

4.3 Corrosion and chemical resistance

Most slurry systems are concerned with suspensions of solids in water, the main exceptions beingthose involved with particular chemical processes.

4.3.1 Corrosion resistance of metals

The corrosion resistance of mild steel is low compared with mosf other metals, whereas thevarious types of cast iron are not particularly prone to corrosive attack.~ l~egarding pump materials, ingeneral, the alloys of iron and steel containing Cr, Ni, Co or Mn give improved corrosion performance overnon-alloyed types. The 18/8 and 13/4 Cr/Ni stainless steels, bronzes and ’Hastelloy’ alloys all have goodcorrosion resistance, although only limited abrasion resistance.

Of the more abrasion-resistant materials, 28% Cr C.I. is reported to have better corrosionresistance than ’Ni-hard’ or 15% Cr C.I. (l~ef. 1). The very hard materials, such as metallic carbides,weld overlays and chrome plating, are also generally corrosion-resistant.

Surface finish also affects the corrosion rate, smooth surfaces being less prone to attack thanrougher ones.

4o 3.2 Chemical resistance of non-metals

The Pipe Protection Reivew (Ref. 6) gives a good indication of the resistance of most rubbersand plastics to chemical attack. Natural rubber has generally good chemical resistance, except when incontact with mineral oils and solvents, or strong oxidizing acids. ’Neoprene’ and nitrile rubbers give betterresistance to oils; butyl rubbers (e.g. ’Hypalon’) are also more resistant to oxidising agents, and to highertemperatures. Polyurethane also has good overall resistance. Hard rubbers tend t.ohave better chemicalresistance - but less abrasion resistance - than the softer grades.

High-density polyethylene, ABS and uPVC pipe and many plastics coatings all withstand chemicalattack well, as do other non-metals, such as ceramics and asbestos cement.

5. EFFECTS OF FLOW PROPERTIES IN PUMI~S AND PI~ES

5.1 Velocity

It is generally agreed that abrasive wear increases rapidly with flow - or particle - velocity, and,as indicated in Section 2.3, many authors suggest a simple power law relationship for~both pump and pipewear, based either on laboratory wear tests or theoretical considerations. For pump wear, it is most oftenquoted that wear approximately a (velocity)3, or a . (pump head)3/2, although the German sand-blastsimulation tests (lqef. 10) indicated that it may depend on the material; e.g. for steel, the index was 1.4 andfor rubber, 4.6. Other authors suggest that there may be differences between ductile and brittle materials(4), or tt~at there may be some critical value of velocity, below which wear increases, say, linearly, butabove which the increase is much more rapid (1). Besults from the only pump wear test to investigate theeffect of speed, noted in l~ef. 4, agreed with the cubic exponent of velocity relationship for wear.

Although there is more Variation in the values qudted for the velocity index in pipe wear, dependingon the type of simulation test, the actual pipe wear tests give values mainly between. 2 and 3; tests at theColorado School of Mines with a phosphate slurry gave an exponent of 2.1,as shown in F~g. 5. The relation-.ship is further complicated in practice by the effect of velocity on flow regime (See.. Section 5.3).

Thus, it it likely that the actual relationship between wear and velocity for both pumps and pipesprobably depends on most of the other factors involved in the overall wear process.

Direction (impact angle)

The type of material is very important in determining the effect of impa~t angle, as previously]iscussed in Section 2.2,1 and shown in Figs. l(a), l(b), and 1(c). Wellinger’s sand-blast tests (Bef. 10~

the wear rate tending to increase with angle for the steels and cast irons, reaching a maximum60° and 90°, whereas for rubbers, the opposite effect occurs, with cutting wear predominating at

angles~ deformation wear being negligible.In solids-handling centrifugal pumps, impact angle will vary as the solids pass through the

let and casing passages, and will depend on the operating point on the pump head/flow characteristic.. hydraulic design will also have a considerable effect both on the actual wear rate and where maximumoccurs, as .discussed in Section 6.1.

Regarding pipelines, presumably the effect of impact angle is significant only for flow roundSection 7.2) and over discontinuities, e.g. pipe joints, or for saltating particles.

Flow regime (pipe wear)

The type of flow regime e~isting in a pipeline will have an important effect on pipe wear, sinceby the particle dispersion in the slurry and affects the type of particle motion (sliding,

or saltating) along the pipe .wall. It is possible that pump wear could also be affected indirectly,distribution across the suction pipe is changed significantly. However, the .effect of flow

is.difficult to isolate, since the type will be determined by flow velocity, mixture concentration,

iize and density, eac.h of which will also affect the wear rate, and hence little test data is available.systems handling settling slurries operate in the heterogeneous regime with turbulent flow,

: in a solids concentration gradient across a horizontal’ pipe, with the highest concentration at themore wear will occur in this region, due to the sliding or saltating abrasion of the larger

a stationary or slow-moving bed can protect the bct~om of the pipe from the faster-if the increased risk of blockage with such a regime can be accepted.

in the homogeneousregime can occur with non-settling suspensions of very fine particles,etc., when the slurry behaves..as a Bingham plastic and laminar flow. occurs at lowwill tend to be more uniformly distributed round a horizontal pipe wall, and generally less

than with a heterogeneous regime, due to the lower velocities and less turbulence.Vertical pipelines also wear evenly, due to uniform distribution of solids over the pipe cross-

section.

6. SOLIDS-HANDLING PUMP WEAI~

Most published information.relates to centrifugal slurry pumps, although some - mainly from serviexperience - is also, available for positive-displacement reciprocating pu~nps.

6.1 Design aspects ": high wear-rate regions, wear patterns

6.1.1 Centrifugal pumps

Several leading authorities on solids-handling pump design mentioned in the pump wear literaturesurvey (l~ef. 4) stress the importance of maintaining good hydraulic design features, .as far as solids-handlinflconsiderations (e. g. unchokeability, increased section thickness) will allow, to minimise wear; in particularrapid changes in flow direction should be avoided. Fully - shrouded impellers seem to be generally preferre~particularly for dredge pumps, though the choice between a closed or semi-open type may depend on the solid~being pumped.

Fig. 6 shows where high wear-rate regions would occur in a conventional centrifugal pump forclean water duty if required to pump solids; Fig. 7 shows the design changes necessary to produce asatisfactory solids-handling pump and so reduce or accommodate wear (l~ef. 1). However, even for thelatter design, the ’critical wear points~ will still tend to occur in the same regions - at least for all-metalwetted parts - as follows :-l~oint i Casing suction branch.

2 Impeller shrouds near eye, especially hub side.3 Impeller blade inlet edges. Blades much thicker for solids-handling design.4 Impeller bla~e outlet edges. Blades much thicker for solids-handling design.5 Casing near ’cfit-water’. It is suggested that a concentric or semi-concentric casing for the

solids-handling design is more tolerant of ’off design’ operation compared with a conventionalvolute, though with some sacrifice of peak efficiency; it also gives lower velocities near the cut-water, hence less wear.

6 Casing side walls, and impeller outer shroud walls. Worst wear usually occurs on the suctionside. Solids-handling impellers are usually fitted with ~scraper’ vanes on both shrouds to reducethe flow of solids down the side spaces.

7 Impeller/casing sealing rings. Flat-faced (i.e. axial clearance) rings used in the solids-handlingdesign are less prone to wear than the cylindrical type.

8 Casing cut-water. Concentric type casing results in a larger radius cut-water further from theimpeller, giving lower velocities and less turbulence at the cut-water, hence less wear.

.9 Casing discharge branch near throat.10 Shaft seal. A solids-handling design normally has a separate clean water flush fitted to the gland,

sometimes (e.g. Simon-Warman design) with the addition of an ’expeller’ behind the main ifnpellerto prevent ingress of solids.Wear rates and patterns are likely to differ in detail from one design to another, e.g. bladed vs

~channel’ type impellers, l~egarding special designs, Warman claims that his impeller shape (Figs. 8(a) and8(b)) reverses the secondary flow patterns in the casing compared with a conventional design (Figs. 9(a) and9(b)), resulting in reduced casing and im.peller wear (l~ef. 24). A ’torque-flow’ pump (e.g. Egger design)with recessed impeller is also said to suffer relatively less wear than a normal radial-flow type, though itmust be remembered that it has to run faster to produce the same head and flow as the latter.

Other factors affecting wear rates and patterns are the operating point on the H-Q characteristic,and the part’icle size and density. As might be expected, least wear occurs at or near the best efficiencypoints, when flow angles should match blade and cut-water angles, giving correspondingly low impactangles - provided that the solids trajectories follow the liquid flow paths - and also [east turbulence due toflow separation. Hence wear will tend to increase both at higher and lower flow rates. Local wear pattern~are also likely to vary for any given pump design, particularly for the impeller blades, depending on the sizeand density of particles passing through the passages.

6o 1o 2 Heciprocating pumps

For piston and plunger pumps where the whole of the ~’fluid end’ is in contact with the slurry, partssubjected to the most severe wear are valves and seats, piston and piston rod or. plunger, cylinder liner orplunger sleeve, and piston rod or plunger packings and bushings. Plunger pumps are normally used for the

l0

~e

more abrasive duties, having only One sealing point, with a clean water flush to the packing. Some form ofstem-guided ’mushroom’ valve with rubber or polyurethane disc insert, and with a hardened metal seat, ’appears to give best results, although rubber or PU-coated ball valves have also been used successfully forlarger particle slurries (3-6 mm (1/8’~-¼’~) size) at lower pressures (150 psi maximum) (l~ef. 20). Normallythe particle size limit for reciprocating pumps is about 2 mm.

Special designs, e.g. the Japanese ’Mars’ pump with an oil buffer ~piston~, have been developedto isolate the fluid end parts, with the exception of the valves, from direct contact with the slurry.

6.2 Effects of wear on performance, component life and sealing

6.2.1 Performance

It is evident that pump performance will be reduced by wear, first due to increased internal andexternal leakage and eventually, .in the case of centrifugal pumps, by the reduction of impeller diameter andgeneral passage roughening. There is, .however, very little quantitative data available from service experienceon the effect of wear on performance.

6.2.2 Component life: metal vs rubber lining

As noted in Section 2.3, some theoretical expressions have been developed for estimating pumplife, but all involve empirical coefficients based on previous test data or service experience.

Although the survey of solids-handling pumps (l~ef. 20) gives limited component life data forvarious applications, it must be emphasised that these refer strictly to the particular slurry conditionsinvolved. Hence, no life figures can be reliably regarded as ~typical~, and no attempt has been made tocorrelate them. In fact, some pump manufacturers suggest that it is impossible to make a realisticcorrelation of data, even if more extensive, due to the wide variation in site conditions experienced, and.thus tend to be reluctant to give information on life, in case of misinterpretation. A Polish paper (l~ef. 25)quotes service lives of high Cr steel parts varying from 84 hours pumping sand to 20, 000 hours with coal.In general, pump impellers tend to wear faster than casings, only about ½ to ½ of the life being obtained in

~ome cases. For reciprocating p.d. pumps, valve wear tends to be the limiting factor.There are many reports of the longer life of rubber over metal parts, provided that the particles

are not large or sharp, bonding is good, and heads and temperatures are relatively low - about 45 m (150 ft)and 80°C (180oF) respectively. Improvements by factors of 2 to 20 over various grades of steel and

.’;I. and for different duties have been recorded.

6.2.3 Shatt sealin~

This is generally regarded as a major problem area for virtually all types of pump, and through-range of applications (l~ef. 20). Soft-packed glands are normally preferred for both rotating and

g shafts, with clean water flushing; hard-surfaced shafts or sleeves also assist in reducingi~r. Impeller ’scraper’ vanes and centrifugal s~als (e.g.S. Warman ’expeller’ seal) can effectively

the pressure at the gland, needing only grease lubrication of the packing.

User experience suggests that mechanical seals tend to be unsatisfactory, particularly for theapplications. However, where leakage cannot be tolerated, such as in some chemical

5ceases,. double mechanical seals with tungsten carbide, ’stellite’ or ceramic faces and clean liquid:~hing may be the only solution. Lip seals with either clean wate]: or grease supply have been used~Sfully in certain dredging applications.

It has been suggested by different authors that slurry pipelines should be designed for a mean lifeof 10 to 30 y~ars, if the pipe material is well chosen to suit the particular application.

ght pipes: general and localised wear

Metal (unlined) pipes

number of references in the pipe wear iiterature survey (l~ef. 5) dealing with pipe wear tests and~ ~e~:ience give mean wear rates when transporting different slur.ries under various conditions. Aon ~f these data is given in Tables 1A and B; unfortunately, there is no consistency in the units used,::~mpt has been made to correlate results. P~obably the most useful group is .from wear tests at the

of Mines (Refs. 2 and 13), referre~] to earlier, with wear due to erosion and corrosion shown

separately. Slurries containing iron ores and concentrates appear to cause a wide range of erosion rates,but there is little doubt that corrosion can be a significant part of the total wear, for both these and someother slurries. It also confirms the general view (See Section 3.4) that erosion with limestone and coalslurries tends to be relatively low, with corrosion predominating. Nearly all the above literature relates tosteel pipes.

~ As mentioned in Section 5.3, the Worst erosion occurs at the bottom of a horizontal pipe whentransporting a settling slurry. Hence an accessible short-distance pipeline may be turned through 90°~180°

periodically to give more even wear and thus prolong its life; buried or long-distance pipelines would requirean extra thickness allowance.

It is important toremember that severe local wear will occur at any discontinuity or obstructionin the pipe bore, e.g. mis-matched joints or flexible couplings (Viking-Johnson type), due to increased turb-ulence, especially with fine particles. It has also been reported that high local wear rates can occur at the’4 and 8 o’clock’ positions round the bore of a horizontal pipe, possibly due to local turbulence with a station.ary bed of solids at the bottom.

7.1.2 Lined pipes

As listed in Section 4.1, rubber or plastics linings offer the advantage of protection against botherosion and corrosion, provided that the right grades are chosen and bonding is good. Numerous reportsmention the reduction of wear with rubber lining compared with unlined steel pipes and fittings,, particularlyif impact wear is present and.the solids noi large and sharp. Also there are some claims for the successfuluse’of polyurethane linings; English China Clay’s service exPerience (l~ef. 17) reports improvements of 8to 20 times the life for a mild steel pipe, and about twice the life of rubber lining. No other data appearsto have been published on service experience with other types of plastics piPeS or linings.

A few reports from W. Germany and l~ussia note considerable improvements in life with basaltlining, factors of 15 to 20 being claimed compared with steel pipe.

7.2 l~ipe bends

Pipe bends will wear much more rapidly than straight pipe, .particularly at the outer radius, dueto the increased impingement angle of the solids and more turbulence. Bend geometry also affects the wearrate; although some authors merely suggest that wear will increase as the bend radius/pipe diameter ratioreduces, Brauer’s comprehensive series of bend wear tests (Ref. 21) shows that wear increases significantl}for bends of 2 - 3.5 B/D ratio, rising to a maximum at B/D = 2.8. Hence.bends in this I~/D range shouldbe avoided if possible. Bend angle, as well as B/D ratio, also affects the position and depth of the wearcavity.

7 o 3 Units of wear rate

Pipe wear rates have been expressed in many different units, both absolute and derived (or’specific’). It would be desirable to have a more generally agreed type of unit, to enable results fromdifferent sources to be more readily correlated.

8. BEFERENCES

1. Wilson, G., and Vocadlo, J.J.

2. Faddick,

3. .Postlethwaite, Jo

4. Truscott~ G.F.

5o Truscot~ G~Fo

%Vear, attrition and corrosion". Lecture G, Hydraulic Trans-port Course Notes, BI-LRA, (September, 1972).

"Pipeline wear and particle attrition". Lecture F, SlurryPipelining Course Notes, BHI~A, (May, 1974).

"Pipeline wear: erosion-corrosion and erosion". SolidsPipelining Course Notes, BHI~A, (May, 1976).

"A literature survey in abrasive wear on hydraulic machinery".BHBA. Publn. TN 1079, (October, 1970).

"A literature survey on wear in pipelines". BHI~A, Publn. TN1295, (May, 1975).

6. Hassan, U o, Jewsbury, C.E.,and Yates, A.I~.J.

"Pipe protection : a review of current practice". Joint rePort byBHl~A/Paint I~.A. Publ. by BHRA, (1978).

7. Bitter, J.G.A.

8. Bergeron, P.

9. Bergeron~ P o

10. Wellinger, K., and Uetz, H.

11. Prudhomme, t{.J., l~izzone, M.L.,Schiemann, Toll., and Miller, J.E.

12. Wiedenroth, W.

13. Link, J.M., and Tuason, C.O.

14. Trainis, Vo V.

15. Schriek, Wo, Smith, L.G.,Haas, D.B., and Husband, W.H.W.

Bain, A.G., and Bonnington, S.T.

Sambells, D.F.

"A study of erosion phenomena, Parts 1 and 2". Wear, 6,(January/February and May/June, 1963).

"Similarity conditions for erosion caused by liquids carryingsolids in suspension. Application to centrifugal pump impellers".La Houille Blanche, 5, Spec. No. 2, pp. 716-729, (November1950), (In French),’BHRA, Transln. T 408, (1950).

"Consideration of the factors influencing wear due to hydraulictransport of solid materials". Proc. 2nd Hyd. Conf. ’~yd.Transport and Separatio.n of Solid Materials". Soc. Hydr0tech.de France (June, 1952), (In French).

"Sliding, scouring and blasting wear under the influence ofgranular solids". VDI- Forschungshef~, 21, 449, B, 40pp,85 Figs, (1955), (In German).

"Reciprocating pumps for long-distance slurry pipelines". Proc.’Hydrotransport 1’ Conf., Paper E4, BHRA, Cranfield, (September,1970).

"The influence of sand and gravel on the characteristics ofcentrifugal pumps; Some aspects of wear in hydraulic transport-ation installations". Proc. ’Hydrotransport 1’ Conf., Paper El.,BHRA, Cranfield, (September, 1970).

"Pipe wear in hydraulic transport of solids". Mining CongpessJnl., pp. 38-44, (July, 1972).

"A method for computing coal comminution in a pipeline duringhydraulic transport. " Ugol, No. 9, pp. 37-41, (1963), (InRussian).

"Experimental studies on the hydraulic transport of coal". ReportV, Saskatchewan Res. Council, (October, 1973)~

"The hydraulic transport of solids by pipeline". (Book), pp.131-136, ist Edn. Pergamon Press, (1970).

"A practical solution to pumping.an abrasive slurry". Proc.’Hydrotransport 3’, Conf., Paper J1, BHRA, Cranfield, (May,1974).

"The abrasion of hydraulic plant by sandy water". SchweizerArchiv fur Angewandte Wiss. u. Tec~anik, 24, 7/8, pp. 3-30,(1958), (In German), Transln. by C.E.G.B. No. 1799, 13 pp,20 Figs. (1958).

"Influence of the pump materi~l on service life of the impellersof rotodynamic pumps in transport of mechanically impurefluids". Proc. 3rd Conf. on Fluid Mechs. and Fluid Machy.,Budapest, (1969).

"A survey of solids-handling pumps and systems, Part II - Mainsurvey". BHRA, Publn. TN 1463, (May, 1976).

"Investigations on wear of plastics and metals". Chem. ing.Tech., 35, 10, pp, 697-707, (October, 1963), (In German).

"Pipe wear testing, 1976-1977", BHRA Publn, PR 1448, (D’ec.,1977).

"23. Johns,

24. Warman, C.H.

25. Bsk~ E.

"Erosion studies of pipe lining materials - Third ProgressReport", U.S. Dept. of Intr., Bureau of l~eclamn., Chem. Eng.Lab. Beport No. P-93, 27 pp., D.D.C. No. AD 428514 (unclass(June, 1963).

’~rhe pumping of abrasive slurries". Proc. 1st PumpingExhibn. and Conf., Earls Court, London, 15 pp, includ. 12 Figs(June, 1965).

"Construction materials and testing,results of the wear of pumpsfor transporting solid media". Biuletyn Glownego Instytuta Gorn-ictwa, 12 (December, 1966), (In Polish), BHRA transln.available.

TABLE 2

Selected results from sand erosion tests on materials. (Stauffer - Ref. 18)

ResistanceHardness factor

Materlaltype Condition Chemlcal composition, Kg/mm2 R

Rolled or forged steel c si Mn Ni Cr OthersAustenltlc NSP 2 quenched & ,07 .25 .3 6¯0 17.0 .TA1, 3.0 Cu. 342/152 87/1.34

annealed

Caee-hardening C15 ndrmalised .16 .3 .4 116 1.00(reference for a11 tests)

Mild, medium hard tempered ¯ 25 ,3 .4 205 1:21Austeniflc etainiees 63 quenched .03 ¯’43 10.0 17.7 ¯ 45 Nb/Ta 189 1,43 ~Martensitic stainless AK5 annealed/ .5 ,35 .6 15.5 191/507 I. 37/2.28

tempered

Iligh- speed (tool) annealed/ .7 ¯ L2 ,3 5.0 18.0 W, 5,0Co 319/857 i. 85/4.5hardened 1.0V, .6Mo

Chrome 2002 oil-hardened 2.0 .38 .6 12.5 847 6, 02

Cast steel

Unalloyed 23/45 normalised .22 .35 .5 142 1.01Auetenitic Cr 30 quenched .06 .6 .5 9.0 18,0 ? 1.48Martensitic stainless tempered .46 .36 ¯ 35 12.8 464 1.7671024

Abrasion-reslstant HH quenched 1.2 .3 12.0 2OO 1, 86Abrasion-resistant MG tempered i. 07 .41 1.49 ¯ 0814.0 0.6 P, .0385 625 2.52

Cast iron

No. 15 as cast 3.2 2.04 ¯ 62 ¯ 41 P, . 09 S 160 ,48Pearlitic GS/GSA 3.1 1.5 .8 .I P, .12S 230 64/1.14S.-G. austenitic 3.3 2.0 1.6 1.5 .05 P,.006S 175 1.24S.-G¯ I 3.6 2.6 .43 ¯ 09 ¯ 12 P, .004S 378 2.33Chilled 47-283-5

¯ 046 Mg3.03 1.57 .97 1.3 522 2.81

Special HC SI-143-2C hardened 2.86 .41 I, 04 .07 26.4 787 5, 43Ni .chilled NIB as cast ,Ni-hard~ 6O5 4.84]6.05

_Cas.t copper alloys Zn Sn Ve A1Special cast brass ae cast 5’6.9 ,2 .6: .68 .26 Pb, . 93 154 .42

.99 NlSpecial gun-metal 5 87.0 7.0 1.0Ni 53 ,54Ni AI bronze GTB 80.0 5.0 10.0 5.0Nl 179 .64A1 bronze Am22 81.0 4.5 14. 0 5 Ni 331 ,72Tin bronze No. 4 86.0 14.0 98 .81

Other non-ferrous metals

~Raffinalt (pure AC. ) wreught 99.99 22 .11~Avlonal’, forged untempered/ 4.0 .3 1.0 94.7 50/I05 26/. 5

hardened

Titanium a11oy Ti 15A rolled 1.3 2.8 02 N.~, bal. Tl 378 1.0

15

Sinter metals & carbides

Ti carbide W212b No. 42,beat ~sistant

Tungsten carbidesBG 3YV/TG 100

TH1

BH 31S

Weld overlays

18/10/2~Haste~oy C

Hastelloy B

Hard alloy Cc~ 6(~Stellite 6~)

Surface treatments

Metal spray, stainless I

hardening steel C15

Metal Spray M20 14%Cr steel

Nitride steel I

Hard Cr. plating onsteel . 01/. 05 mmo

autogenous

nitrided

TABLE _2 continued

Titanium carbide basis

Tungsten carbide basis

"¯ o 6.0 Co, 94.0W

Ni Cr Mo Co ’~ Others

10.0 18.0 2.0 .7

52.5 16.0 16.5 2,5 5.5 Fe, 1.0Si1.0 Mn, .11 C,4.25W

61.0 1.0 28.0 2.5 5. 5 Fe, 1.0Si1.0 Mn, .08 C,

28.0 67.0 1.0C, 4.0W

8.0 18.0 ¯ 05 Si

diffusion of S and N2

14.0

1.5 .25

919 1.92

1090/13007.56/22.4

1600 49.3

2450 169.9

229 1o23

253 1.46

274 1.53

605/4 4. s/16.0

227

219 .96

319 1,23

955 3.53

s49/8 11.2/12.4

15

OF MA.YIMUM.

2O

100

RATE AS A

OI: MA~(IMIJM.

2O

/.~Ikrn

o/IIIII

I

I

EROStON V’,’E A R

~TE AS A

PERCENTAG[0

oF PIAXIMUM

2O

FIG. I E’R05ION WEAR i~I~PIr4diEMENT ANGLE.

IVickerls Hardness l

scouring |I i ,medium: limestone g~ass flint quartz corundum

Fig. 2. Effect of grain hardness of abrasivemedia on steels and-Vulkollan from scouring-wear tests.

Water/solids mixture ratio by volume = 1:1,velocity of test specimen 6.4 m/sec; the steelhardness range is shown cross-hatched. (H =110 Kg/mm2 for St37, H = 750 Kg/mm2 fo~

VC 60H).

o~rC

RELATIVE WEAR RATE

.p_~0.0

/

/

//

FIGURE 4-

WEAR VS SLURRY CONCENTRATION

Cu CONCENTRATE SG 5.01V=18.5 FPS 97%-200 MESH

........ l ............ t .... i .... ~ .... ~ ........ ~ ........ ’ .......

.... ! ....t .................t .........~ .........I .....................t ...................,.t ....t ....t ...................t .........:2:.:; t ~ :,:-:’: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::’ ’ ": ........ :’~ ........ : F-"’::--’Z: ........................ t

10% 20% ~0°/~ 40% 5~% 60% 70CONCENTRATION,WEI6HT PERCENT SOLIDS

FIGURE .5.

WEAR VS VELOCITYPHOSPHATE CONCENTRATE AT 25.5% SOLIDS BY WEIGHT

2O

FIG. 6 CRITICAL WEAR POINTS IN CONVENTIONAL CENTRIFUGAL PUMPS

~OAT

Fig. 8a. Closed impeller. Fig. 8b. Flow in volute-Warman design.

Fig. 9a. Flow pattern conventionai design. Fig. 9b. Flow pattern conventional design,

22