Literature Survey of Abrasive Wear in Hydraulic Machinery

Wear - Elsevier Sequoii S.A., Lausanne - Printed in the Netherlands 29

A LITERATURE SURVEY ON ABRASIVE WEAR IN HYDRAULIC MACHINERY*

G. F. TRUSCOTT

The Brirish Hydromechanics Research Association, Cranfield, Bedford (Gt. Britain)

(Received September 29, 1971)

SUMMARY

The survey considers the factors affecting abrasive wear-the properties of the solid particles, the construction materials and the flow-and various types of wear. The main sources of information are from laboratory wear tests on materials and pumps, and from service experience on pumps and water-turbines. The effects of wear on performance and working life are also discussed. Finally, the main points emerging from the survey are listed.

1, INTRODUCTION

There is a growing demand for both pumps and water-turbines which have to deal with abrasive solids in suspension. This requirement may be either by design- as in pumps for sewage, dredging or any other solids transport application-or default, e.g. any scheme involving river, land-drainage or glacial waters. In either case, the resulting wear is an increasing problem, particularly with the trend to higher running speeds.

This survey is intended to provide a better understanding of abrasive wear phenomena, and as an aid to the selection of materials. It must be stressed, however, that the survey has been limited to abrasive wear only; other important factors affecting the final material choice for any given application, such as corrosion and cavitation erosion, are not covered, except where these properties happen to be mentioned for comparison in a particular report. Also, only those aspects of machine design which affect wear are considered, rather than the more general solids-handling capability, e.g. max. size of solid to be passed.

The amount of published information, covering the past 20 years or so, is not large-there are only 38 references-and nearly all the original work is from continental sources. The data may be conveniently divided into 3 main groups, together with the more comprehensive and useful references, as follows:

(a) Wear tests on materials-Wellinger, Stauffer (b) Wear tests on pumps-Zarzycki3 (c) Service experience on pumps-Bergeron4 on general solids-handling,

* This paper is based on TN.1079 of the same title which is available from The British Hydromechanics Research Association, Cranfield, Bedford, at f2.

Wear, 20 (1972)

30 G. F. TRUSCOl.7

Welte5 on dredging, Warman on sands and gravel, Bezinge on pumped-storage; and Bovet and Kermabong on water-turbines.

Some attempts at theoretical wear analysis have also been made, notably by Bergeroni~. Most of the service experience concerns pumps, but it seems likely that similar wear processes occur in both types of hydraulic machinery. Quantitative wear tests on pumps are few--only two Polish papers, and one Russian, have been discovered.

The survey considers the factors affecting and types of wear, and then deals with each of these in more detail. Finally, the effects of wear on performance and working life are discussed.

2. FACTORS AFFECTING AND TYPES OF WEAR

Most of the references deal with these topics in varying detail.

2.1. Basicfizctors affecting wear These are the various properties of: (1) Solid particles-hardness, size, form (i.e. sharpness), relative density,

concentration,2,4~5*10- 13. (2) Construction materials-composition. structure, hardness - 5,7 - , 2 - 14. (3) Flow-speed, impact angle~2~4-6.s.10.11.13.

Only the more detailed references are listed above.

2.2. Types of wear These are also discussed in many of the references. In the material tests,

Wellinger distinguishes between sliding, scouring and jet impact (sand-blasting) wear. Stauffer2 suggests grazing(i.e. 0 impact angle) scouring abrasion predominates in hydraulic machines. In papers on wear analysis (see Section 2.3), both Bergeron,l 1 and Bitter 5 also attempt to separate wear due to friction (or cutting) and impact (or deformation) ; Bergeron suggests how this wear mechanism may account for the typical pitting (or gouging) type of surface damage encountered in practice.

Service experience on pumps4,5 and water-turbines,, and pump wear tests3,3,6-1g, all show typical wear patterns of impellers, runners and casings for various running times. Warman discusses the differences in wear pattern between his design of pump and the conventional, also mentioned by Warring and Arnstein.

2.3. Wear theory Several authors,2,3*22-25 give simple expressions, based on wear test results,

for wear rate as a function of velocity, material hardness, grain size or solids concentration. The one most often quoted is:

wear u; (vel.)

where the index n may vary depending on the material and other factors involved; the most common value appears to be 3 2,13,24*25. It should be noted that Wellingers sand-blasing tests and Goodwins whirling-arm tests23 were carried out under dry conditions; however, although absolute wear rates presumably will be higher than in a liquid, the relative rates should be similar.

Wear, 20 (1972)

ABRASIVE WEAR IN HYDRAULIC MACHINERY 31

Some more detailed analysess~0,~15 consider wear as affected by the forces and velocities acting on a particle in a liquid flow. Bovet states that wear CC abrasive power, Pf, of a particle impinging on a surface, and

P f

= PVP,-PJC3

4 where p = coefficient of friction between particle and surface, I/ = volume of particle, ps = density of particle, p 1 = density of liquid, c = velocity of particle, R, = radius of curvature of surface.

In a much more involved analysis, but starting with the same basic assumption, Bergeron 1 l develops a complicated expression based on the statement :

wear oc solid/liquid density difference x acceleration of main flow x coefficient of friction x thickness of particle layer x flow velocity.

He thus takes account of the difference between the solid and liquid velocities. His previous paper attempts to predict wear rates in similar pumps handling solids with varying properties, with simplified assumptions such as pure sliding of the particles over the surface, from the initial expression

wear cc -g (P-p)d3p K

where U = characteristic velocity of liquid, P = density of particles, p =density of liquid. d = diam. of particles (assumed spherical), D = characteristic dimension of machine, p = no. of particles/unit surface area, K = experimental coefficient depending on abrasive nature of particles.

Bitter, in a fundamental study of erosion phenomena-but strictly for dry conditions-gives expressions for cutting and deformation wear, also based on energy considerations and the type of material eroded, i.e. whether brittle or ductile.

A few authors4*0*13*1Q also develop expressions for pump service life. Both Bak13 and Bergeron4* consider this in terms of pump total head for given conditions (see Section 6.2). Vasilievp gives a somewhat involved method, based on statistical analysis of pump wear tests, to predict life based on a specified maximum permitted wear.

It is perhaps debatable whether these more complex theories can be used to predict absolute wear rates with anycertainty; most involve empirical constants and other parameters difficult to determine for an actual machine. In fact, BergeronloT1 l admits that some of the assumptions made may be questionable. However, such theories are of some value in predicting likely trends in wear rates when only one or two of the relevant factors are altered,

3. EFFECTS OF ABRASIVE PARTICLE PROPERTIES

3.1. Hardness Both Wehingers and Stauffers laboratory tests show that, for metals in

general, wear increases rapidly once the particle hardness exceeds that of the metal

Wear, 20 (1972)

32 G. F. TRUSCOTT

1.50

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1.00

9, % I- 0.75 L

3 0.50

Fig. 1. Effect of grain hardness of abrasive media on steels and Vulkollan from scouring-wear tests. Water; solids mixture ratio by vol. 1 :l, velocity of test specimen 6.4 m/set; the steel hardness range is shown cross-hatched. (H,. = 110 kg/mm for St37; H,.=750 kg/mm* for C 60H). (From Wellinger and Uetz.)

Fig. 2. Effect of blasting abrasive hardness on direct impact wear from plate tests. Curves for steels, rubber and cast basalt. The hardness ranges for St37 (& = 125 kg/mm) and C 60H (Ifr = 830 kg/mm) are shown cross-hatched. (From WeLinger and Uetz.)

lo 20 30 50 70 100 2CO300 5007001000 2CCO3000

Vickers hardness of abrading media

Fig. 3. Effect of Vickers Hardness of abrading media on resistance factor.

50 m&/kg

40

30 al % L 20

k g 10

Vickers 0 hardness : 115 material: St37 C60H

(From Stauffer.)

Fig. 4. Effect of grain form of abrasive on direct impact wear. Plate tests with blast pressure of 2 atmos.: blank area for rounded shot, shaded area for angular shot with 1.6 mm grain size and Vickers Hardness H,,z 720 kg/mm (From Wellinger and Uetz.)

Wear. 20 (1972)


for both scourmg and impact abrasion. Beyond this, the wear rate may become fairly constant, or even reduce, with increasing abrasive hardness. These effects are shown in Figs. 1, 2 and 3; note that wear rates may be expressed in a variety of ways, both absolute and relative. Stauffer notes that the wear resistance of a 13% Cr cast steel was only slightly better than that of the unalloyed reference steel, whereas it is usually considerably better in practice; he suggests this might have been due to the excessive hardness of the test abrasive.

From tests with various grades of very fine sand (< 200 pm) under dry conditions, Goodwin et al. 23 found that erosion varied as (hardness)23, and depended on the amount of quartz present.

Rubber behaviour is more difficult to compare on a relative hardness basis; both Vulkollan and Perbunan synthetic rubbers showed fairly constant scouring wear rates (Fig. l), but Perbunan behaved like the reference steels in the sand-blasting tests (Fig. 2). For both scouring and direct-impact wear, Vulkollan gave much lower wear rates than the steels, except with the less hard abrasives; the other rubbers were also better under direct (i.e. 90) impact.

3.2. Grain size and form Many of the references2,4,5~1,13~17~18,25 state that, in general, the absolute

wear rate increases with grain size and sharpness. Other authors,24 state that wear cc size for sliding or grazing abrasion, but is independent of size for direct impact; Goodwins tests23 show that the erosion rate for impact abrasion becomes constant only above a certain grain size (about 50-100 pm depending on velocity). Stauffer2 also states that the relative wear (compared to the reference steel) of metals decreases with increasing size, but gives no results. Bergeron l1 found, from tests on Al. Br. that wear cc (size)0.75, but states that for general application, wear cc size x function of coefficient of friction, densities, and size/surface curvature ratio.

Wellinger shows the effects of particle shape on impact abrasion in Fig. 4 ; angular grains cause about twice the wear due to rounded ones. Goodwin23 also discusses erosiveness of particles, and defines a shape factor; he states that hardness and sharpness are interrelated.

Wiedenroths wear tests17*18 on a small dredge pump impeller, using a lacquer- removal technique, show differences in the blade wear pattern depending on grain size (i.e. sand or gravel).

For rubber linings, the size and shape effects are more critical than for metals. Most of the service experience papers on pumps mention some limitation; actual size limits, varying from l/16 in. (10 mesh) up to 2 in. are quoted in Refs. 6,24-27. Two Eastern European papers on pump wear tests state limits of 5-6 mm (about $ in.)12 and 4 mm (5/32 in.)13. Other references4*5*20*28 merely state that the solids should not be large or sharp. The size limit depends largely on the types of abrasive and rubber.

3.3 Mixture concentration and density There is surprisingly little quantitative information on the effect of solids

concentration. It is generally accepted that wear increases with concentra- tionl,4,11,13,19,22,24,25. Some authors3,XS consider this relationship to be direct. Bergeron , from tests on Al. Br., suggested this applies only to small amounts of

Wear, 20 (1972)

34 G. F. TRUSCOTT

solids, but for larger values wear increases more slowly; his theory states that wear x no. of grains/unit surface area, i.e. dependent on concentration and flow pattern. Kozirevs jet impact tests show wear x concentration, up to 10% solids, for pure abrasion, but this no longer applies for combined cavitation/abrasion. From the only pump test to consider this aspect, Vasiliev concludes that wear x (concn.).x2. independent of material or flow properties, for sand/water mixtures between 3 and 150/, by vol.

Wellinger gives sliding-wear results for water/sand ratios from 0 to I; 1 ; his scouring-wear tests were carried out with a constant l/l sand/water mixture by vol., whereas Stauffe? used a 2/l mixture. For the Polish pump tests, Bak mentions a l/3 sand/water ratio, but no figure is quoted by Zarzycki3.

Both Bovet8 and Bergeronr, give expressions (see Section 2.3) for wear depending on the density difference between solids and liquid, either varying directly-if other factors remain constant-or as a more complicated function. .

4. EFFECTS OF CONSTRUCTION MATERIAL PROPERTIES

4.1. Type : composition, structure 4.1.1. Metals Wellingers material tests show that a hardened steel (C60H) had the highest

resistance, followed by a hardened 13% Cr steel and an 18/8 stainless steel, to scouring wear (see Fig. 5). Hardened steel (St. 70H) and hard C.I. were better than the un- hardened reference steel (St. 37) for grazing abrasion, but worse for direct impact. under sand-blasting, as shown in Fig. 8.

Stauffer tested over 300 materials, and gives 9 tables of results, a selection of which are given in Table I, on a basis of resistance factor, R = (vol. wear of ref. steel)/ (vol. wear of test material). Of the forged steels, a 12.5% Cr oil-hardened steel was best (R =6.0), and of the cast steels, a 14% Cr, 1.5-2% Mn nitrided steel (R=2.5). followed by a 12% Mn hardened austenitic steel (R = 1.9) ; 18/8 austenitic steels were not very resistant (R about 1.5). Ni-hard gave the highest resistance (R = 6.0) of the cast irons, and the S.G. irons were better (R = 1.0-2.3) than ordinary C.I. (R = 0.5-0.8). Almost all the non-ferrous metals had a lower resistance than the reference case-hardening steel (C15) ; only a titanium alloy equalled it. Tin bronzes generally had the highest values (R =0.74.8) of the cast copper alloys-slightly better than the aluminium bronzes (R=0.554.7). A 30/; Ni/2.5% Al bronze gave the best result (R = 0.94) of the wrought alloys. The most wear-resistant materials of all were the sintered tungsten carbides (R values up to 170), followed by hard chromium plating (R= 11.&18.0) and the hard Co-Cr-W alloy weld materials (R=4.5-18.0).

Leith and McIlquham2 give tables of comparative cavitation and abrasive erosion test results, referring to Stauffers work. Al. Br. has relatively poor abrasion resistance, but is excellent against cavitation; a Mn stainless steel shows only fair abrasion resistance, but cavitation resistance is good. Hard Cr plating gives excellent resistance to both, provided surface preparation of the base metal is adequate.

Shchelkanovs report r4 on water-turbine steel tests states that microstructure and work-hardening ability affect wear resistance considerably, austenitic and martensitic steels being notably better than the ferritic. It recommends using low and medium (3.5-10.5%) Cr alloy hardening steels, though both these and hardened 11.5% Ni alloy and tool steels gave good abrasion and cavitation resistance. Kozirevs

Wear. 20 (1972)

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Wear, 20 (1972)

40 (;. F. TRUSCOTI

jet-impact tests, under both pure abrasion and combined cavitation/abrasion, showed an 18/S stainless steel to be more resistant than a case-hardening steel, cast iron and brass.

Goodwins testsz3 with very tine, dry sand show that an il,c Cr steel and a Cu-Cr---Ni alloy gave the same erosion rate-appreciably lower than for titanium and aluminium alloys. Antunes and Youlden25 give a table of results for a limited number of materials from mechanical grinding tests.

The two Polish pump wear reports3Ti3 give generally similar results; of the 31 materials tested by Zarzycki3 (see Table II) the 14:; and 4//, Cr cast iron impellers had the highest wear resistance, followed by the 18-230.{ Cr and 12% Mn cast steels; S.G. cast iron was also quite good.

Pump service experience may be loosely divided into dredging, sand and gravel, and slurries generally. Two German authors5*28 recommend either Mn or Ni-Cr-Mo-V cast steel for impellers and casing liners, with impeller sealing-rings of 30% Cr steel, for dredge pumps. N&hard (Ni-Cr white C.I.) and high Cr cast irons-for better corrosion resistance2--appear to be the most commonly used for general solids-handling duties4.6,20,,h.?7,30~3, although Bergeron* states that, whilst Ni-hard is very resistant to sharp abrasives, it tends to be brittle and hence prone to shock damage, so is unsuitable for dredge pumps. He also says that the high Mn steels, being work-hardened by impact, give good resistance against large, rounded solids, but are not much better than unalloyed steels against sand; some of the Ni-Cr-Mo alloy steels are very resistant to friction wear, but not to saltating (bouncing) particles. A good stainless steel may be used for resistance to erosion and corrosion. However, both Allis-Chalmer? and Warman pumps use Ni-hard for impellers handling coarse abrasives (Simonacco-Warman catalogue claims up to 7.5 in. diam. for an 8 in. pump) ; both also use high Cr cast iron (27:/i Cr C.I. from Simonacco-Warman catalogue). Ref. 26 briefly mentions the use of hard-facing Cr or Ni alloys by welding, electrodeposition or metal spraying.

Several references 7-9, 32, 33 relate to experience with hydroelectric plant. Be- zinge mentions improvements in storage pump wear by replacing impellers and casings originally in 13 ::, Cr,/l y


4.1.2. Rubbers There is a large variation in the wear rate depending on both type of rubber

and abrasive, as shown by Wellingers tests. The synthetic Vulkollan E (72 Shore H.) was the most resistant-better than the steels for the harder abrasives- but Perbunan rubber (88-90 Shore) was much worse, in the scouring-wear tests. The sand-blast tests show, in Fig. 9, how wear depends on impact angle (see Section 5.2.1.), with least relative wear occurring at 90--opposite to that for steels; Vulkol- lan was better than the other rubbers, and all were better than the steels and C.I.s for direct impact. Stauffers tests gave very low resistance factors for rubbers, soft rubber (R = 0.08) being slightly better than hard (R = 0.05) ; he explains these results with reference to Wellingers. Bitters analysis ls also helps to explain this phenomenon.

The rubber-coated impellers gave fairly good resistance-slightly inferior to high Mn alloy steel-in the Polish pump wear tests3,13. Russian tests12 on rubber coatings claim a much-improved resistance for natural and methylstyrene rubbers over that for the butadiene-styrene rubber previously used; isoprene rubber was also very promising. The report states that wear reduces with hardness for particles < 1 mm (0.04 in.), and with increasing tensile strength and elasticity for particles < 556 mm (about a in.) (See also Section 3.2.).

Although most of the references4-6,20,21,24,26-28*30 on pump service experience mention the greater wear resistance of rubber over metal, within the limits of particle size and form discussed in Section 3.2-also provided that operating temperatures are below about 130-1600F2,26, and the bonding is good4,12,26,28,30- there is relatively little information on the types of rubber used. Welte recommends rubber of 50-65 Shore hardness for coating both impellers and casings, and 40-60 Shore hardness for impeller and shaft seals, for dredge pumps. Again, Bergeron4 suggests that rubber is unsuitable for such pumps, owing to the danger of impact from large solids. The article26 on slurry pumping states that natural and the softer synthetic rubbers are more wear-resistant than the semi-hard ones, but are not so corrosion-resistant. Eggers paper3 gives similar recommendations, soft rubber linings being most suitable for sand, quartz, kaolin and other abrasive slurries, whilst hard rubbers are for chemical applications ; Vulkollan impellers and sealing- rings gave the highest abrasion resistance-about twice that of a 16% Cr hard C.I. Kermabon and Mosseg briefly mention the satisfactory use of synthetic rubber in some water-turbine applications.

4.1.3. Plastics There appears to be little published information so far on the use and behaviour

of plastics. Wellingerl tested 3 plastics for scouring wear; Lupolen H (stabilized polyethylene) was best-better than Perbunan rubber, or about the same as 18/8 stainless steel-but Vinidur (vinyl type) and Polystyrol EH (polystyrene) were much less resistant. Stauffer2 also tested several types, but all were much worse than steel ; polyethylene was again the best (R = 0.32), followed by Nylon and Teflon (R =0.28). Perspex, Bakelite and other synthetic resins were poor (R = 0.04-0.07). However, two Russian papers34v3s report encouraging results from abrasion and cavitation tests on polyether and epoxy resins, and elastomers, but give few details, apart from stating that specimens were undamaged after 30 h; the resin materials included 2&40x by wt. of fillers (e.g. emery or granite powders, steel tilings) or

Wear, 20 (1972)


Vickers hardness Hv

80, , , I

I LVickers l-wdn Material St37 !C6ohadened

tempere

%&A&/ mm2 ess Hv : ; . I I and !d k

Fig. 5. Effect of steel hardness on scouring wear with quartz sand. Water/solids mixture ratio by vol. 1 : 1; velocity of test specimen 6.4 m/set. (From Wellinger and Uetz.)

Fig. 6. Effect 01 material hardness on direct impact wear from plate tests. Blast pressure 3 atmos. (a) Curve for blasting with quartz sand (grain size 0.2-1.5 mm Vickers Hardness, H, = 1290 kg/mm) ; (b) curve for blasting with cast shot no. 1 (l-l.5 mm, H,= 395-550 kg/mm*); (c) curve for blasting with cast shot no. 7 (1.6 mm H, = 69G750 kg/mm). Hardness ranges of cast shots 1 and 7 shown cross-hatched. (From Wellinger and Uetz.)

whilst the next best (low Cr C.I.) had the much higher value of 516 HB, as shown in Table II. Baks results3 were generally similar, though the most resistant materials (Ni-hard and high Cr C.I.) were, in fact, the hardest-about 800 HB.

Such hardness values as are quoted for production pump materials vary from 400 to 650 HB (special alloy steels) for American dredge pump liners38, 34CL450 HB (Cr C.I.) for Allis-Chalmers pumps27, 550 HB (N&hard) for Warman pumps (from sales literature), and 25&700 HB for various European solids-handling pumps 3. The change in storage pump materials, which Bezinge7 mentions gave improved life, meant an increase in hardness range from 180 to 200 HB (13% Cr/l% Ni stainless steel) to 23&300 HB (13% Cr/4% Ni stainless steel); new labyrinth seal materials, either specially treated steel of 50&550 HB or hard-chrome deposition of 650-700 HB are also being used.

5. EFFECTS OF FLOW PROPERTIES

5.1. Speed ; speed and head limits The more straightforward wear theoriess9r0 suggest that wear cc (ve1.)3, or cc

(total head) 32 if all other factors are constant (see Section 2.3.) ; even Bergerons more complex expressionl, taking account of the difference in velocity between fluid and particle, gives a similar result, provided that the particle velocity is considered. Bitters theory, however, considers total wear as the sum of deformation and cutting erosion, both involving the material properties as well as speed, so that wear cannot be stated as a simple function of velocity.

Material tests show some variation in the velocity index. Wellingers sand- blast tests, shown in Fig. 10, indicate that it depends on the material-for steel (St. 37), the index is 1.4, and for rubber, 4.6. Stauffer found that wear approx. cc

Wear, 20 (1972)

44 G. F. TKUSCOT~I

(vel.)3, as mentioned by Worsterz4, and Bergeron suggests that the index is >i. Kozirevs jet impact tests ** showed that, for constant mixture concentration and without cavitation, wear =c (vel.)2,2. GoodwinZ3 found that wear T/ (vcl.). for all materials tested (both metals and plastics) and for particle sizes > 125 /Lrn, under dry conditions and at relatively high speed (up to 1800 ft/sec). Antunes and Youldens conclude from wear literature that for ductile materials, wear approx. #x (vcl.) if vel. < 100 ft/sec, or x (ve1.)2 if vel. > 100 ftisec; for brittle materials. the index may bc higher.

Baks pump wear tests I3 also indicate that wear x (vel.)3; the other pump tests do not investigate this aspect.

Many of the service experience references5*6.20.2.25-28.38 on pumps give speed and/or head limitations. For dredge pumps, maximum impeller tip speeds vary from 70 to 150 ft/sec5.28,38 and maximum heads from 80 ft to nearly 300 ft5.28; the type of lining to which these limits apply is not stated specifically in Refs. 5 and 28 but probably the lower limits refer to rubber. For metal-lined sands and slurry pumps, maximum heads quoted range from 160 to 200 ft/stage in genera120.2, and with Ni-hard linings up to 260 ft for Warman Series A pumps (from selection chart)6, or 320 ft for Allis-Chalmers pumps2. Rubber-lined pumps have much lower limits, e.g. 70 ft/sec maximum tip speed* , and 90-I 50 ft maximum 12.13.20,21,25 head (120 ft

for Wilkinson Linatex pumps*). Ceramic linings are also said to be unsuitable

01 1 I i I I I 0 30 60 900

Implngement angle cx

High-speeki sieei

Tooi steel &7()

Impingement angle a I1 1 1 6 I I I

6660 1100 900 785 583 624 y/h

Wear rate V, for St 37

Fig. 7. Blasting-wear rate for steel St37 plates. Sand-blasting tests by M. Gary. Blasting material: quartz sand of grain size 0.2-1.5 mm. V,, measured blasting-wear rate; Vi= V,./sinx, specific blasting-wear rate. (From Wellinger and Uetz.)

Fig. 8. Blasting-wear/jet impingement angle diagram. Wear curves using quartz sand (grain size 0.2 --I .S mm). (From Wellinger and Uetz.)

Wrar. 20 (1972)


for high heads 5 ; the first of the new Gould range is designed for 60 ft head (140 ft at shut-valve) 36 Warman also states that a lower specific speed design for a given . duty results in reduced wear, although heavier and more costly, since lower peripheral speeds are involved compared to the higher N, alternative.

5.2. Direction (impact angle) ; hydraulic design 5.2.1. Impact angle Bovets theory8 results in wear depending on the tangential component of

particle velocity, so that as the impact angle is increased wear is reduced. Bergerons simpler theory lo directly applies only to pure sliding (friction) wear, but the more complex onelr deals with the more general case of oblique impact (see Section 2.3). Bitters expressionsr5 for cutting and deformation wear imply that total wear depends on both normal and tangential velocity components.

103

102

1

0

z L

3 lo

IO0

Impingement angle tx Air veloctty C z3v m/s

Fig. 9. Blasting-wear/jet impingement angle diagram. Wear range of different material groups using quartz sand (grain size 0.2-1.5 mm). (From Wellinger and Uetz.)

Fig. 10. Effect of air velocity on direct impact wear. Plate tests with quartz sand (grain size 0.2-1.5 mm). m, curve slope. (From Wellinger and Uetz.)

Wear, 20 (1972)

Wellingers sand-blasting tests show, in Figs. 7, 8 and 9, how the effect of impact angle depends on the type of material; for steels and CLs, both absolute and relative wear rates tend to increase with angle, reaching a maximum between 60 and 90, whilst for rubbers the reverse is true (see Sections 4.1.1. and 4.1.2). Stauffer, Wiedenroth18 and Welte all note Wellingers results; Antunes and Youlden also mention the impact angle effect.

5.2.2. Hydraulic design The Polish pump wear tests3*i3 investigated different types of impellers. Both

report slightly higher wear rates for a conventional bladed design than for an un- chokable channel type; Zarzycki3 gives results for both types in all materials, as well as for 2-bladed propeller designs-see Table II. Wiedenroths visual studies gl 8 from his lacquer-wear tests showed wear only on the suction side of the impeller blades when pumping sand, but extending to the pressure side with line gravel ; wear at the outlet tips increased with flow. Herbichs reporti on dredge pump design mentions that least wear occurred for a blade outlet angle of 22.5, over the range 225-35 ; the exit angle of the solid particles then corresponded closely to the blade angle.

Several authors4.5.3.18~25,28 stress the importance of maintaining good hydraulic design, as far as solids-handling considerations will allow, to minimize wear, and particularly avoiding rapid changes of direction4,5.8.25. There also seems to be a general preference for shrouded pump impellers, notably for dredging4,, 6.28 though it has been suggested* that the choice between shrouded or open type depends on the solids being pumped. Welte , discussing wear patterns in dredge pumps, states that wear is greatest at the impeller blade inlet and outlet edges, and on the outer shroud walls on the suction side; casing wear is usually greatest near the cut- water. Generally similar tendencies are noted by other authors3~4,3~7- 19. Both German dredge pump papers 5.28 show designs having a relatively small volute side clearance. However, Bergeron4, in discussing the effects of primary and secondary flow patterns on pump wear, recommends a large side clearance-except where scraper-vanes are used-as well as shrouded impellers and large radii of curvature.

Regarding the less conventional pump types, Warman compares casing wear patterns using the conventional and his own design, and claims that wear is reduced with the latters special impeller shape. References 20 and 21 also mention this aspect. A few references20,30.31 discuss wear in torque-flow (or free-flow) pumps; Eggers

paper 3o also gives constructional details of TURO designs. Wear is stated to bc less of a problem in this type than the conventional*, but the only comparison reported31 involves a different construction material for each type.

6. EFFECTS ON HYDRAULIC PERFORMANCE, WORKING LIFE AND SEALING

6.1. Performance There is very little quantitative information available, and only on hydro-

electric plant. Bezinge shows the effect on storage pump performance of worn labyrinth seal clearances. Ferry et al.33 discuss the reduction in efficiency due to increased clearances in Francis turbines, and worn nozzles and runners in Pelton machines.

Wear, 20 (1972)


6.2. Life Expressions for predicting pump life are given in Refs. 4, 13 and 19. Both

Bak13 and Bergeron4 state that, for pumps,

lfe Oc (total lead)/ (ie K weaf rate)

Bak then gives a formula which includes the other factors affecting wear :

life in h., T = A K Q" H312 WX s

where A = constant factor, Q = solids concentration in mixture, /& K = impeller shape factor (1.0 for multi-bladed impellers, 1.4 for channel impellers), H = total head/stage, m.H,O, W,=coefficient of abrasive wear for impeller material, e.g. from

Table II vol. wear of test material

= = vol. wear of ref. material (C.I.)

, X = coefficient of abrasiveness of

solids. (Factor A is probably based on some known life figure, e.g. for coal pumps, AQ= 25,500 approx.). Bergeron4 also develops expressions for determining service lives of geometrically similar pumps of different size, in terms of head and flow variations. Vasiliev analyzes the statistical probability of a pump achieving a certain length of trouble-free service, defined by a specified maximum wear, based on erosion tests.

6.2.1. Metal vs. rubber lining ; impeller seals Many authors mention the longer life of rubber over metal linings, within the

limitations previously discussed (see Sections 3.2. and 4.1.2.). Improvements by factors of 610 have been reported 28 for German dredge pumps, and 2.5-5 x life with special C.I. (or 2&30 x life with grey C.I.) for Russian solids-handling pumps l2 ; this Russian report also notes that the newer grades of rubber were 5-10 times more resistant than the old. Welte states that the life of some dredgepump metal parts may be only 4&60 h, but improvements by factors of 3-10 have resulted from wear research, particularly on impeller seals-various designs are shown, all using rubber, with a clean water supply 5,28. The economic choice of materials depends on the ratio of (total cost)/(wear resistance). Bergeron4 also discussed possible impeller seal designs.

A few references6*13,30,31 give life figures for specific solids-handling pump applications. Baki3 quotes some service lives from Continental experience, varying from 84 h for 25-30x Cr steel parts pumping sand, to 20,000 h for a similar steel with coal slurry. Warman also gives some life figures for casings and impellers when handling different abrasives. For torque-flow pumps, Egger3 shows the variation of TURO-pump life with type of abrasive, for various construction materials. Rubber lining may reduce wear down to f of that for metals; Vulkollan had the highest resistance, and gave about twice the life for 16% Cr hard C.I. Grabow31 compares casing and impeller wear of a conventional Cr cast steel-lined pump with that of a torque-flow pump in Ni-hard 4, and notes about 50-80x improvement in life for the latter.

Wear, 20 (1972)

48 c;. F. TRUSCOTI

Regarding hydroelectric machinery, Bezinge7 gives case histories of a number of pumped-storage schemes, with improvements in repair and maintenance schedules resulting from changes in materials and sand settling. Bovet and Kermabon and Masse both show wear patterns for different water-turbine materials after various running periods.

6.2.2. Shuft sealing Many of the papers4-6~20~21~26~28 on solids-handling pumps make some

reference to gland-sealing; for soft-packed glands, nearly all recommended either a grease or clean water supply, with or without scraper-vanes on the impeller, or the separate centrifugal seal suggested by Warman 6. The review article by Warring gives a list of manufacturers using different seal types.

There is not much information on the use of mechanical seals. Koch37 discusses their application for abrasive duties, investigates possible materials-including metallic carbides and oxides-design and cooling problems, and gives typical examples. Welte5 and Ernst* show dredge p ump designs involving lip-seals, with clear water and/or grease supply. The slurry pump reviewz6 also mentioned the Trist seal as suitable, without separate flushing.

7. MAIN POINTS EMERGING FROM THE SURVEY

Owing to the large number of factors affecting abrasive wear, it does not appear possible to make just a few overall hard-and-fast rules as to the best way of reducing it ; each case will still have to be treated on its merits, not least of which must be economic. However, it is worth noting some general trends derived from the literature for the designers consideration.

(1) Wear increases rapidly when the particle hardness exceeds that of the metal surface being abraded.

(2) Wear increases generally with grain size, sharpness and solids concentration. Rubber lining is particularly vulnerable to large, sharp particles.

(3) Metal hardness is not an absolute criteria of wear, although for ferrous metals, the expected trend for wear resistance to increase with hardness applies very generally. A reasonable resistance appears to be achieved above about 300 HB. The very hard alloys (e.g. tungsten carbide) and surface treatments are extremely resistant.

(4) Chemical composition, microstructure and work-hardening ability all play an important part in wear resistance of metals. Austenitic Cr-Ni (12-14% Cr) and Mn alloy steels are good, as is Ni-hard (Ni-Cr) cast iron. 18/8 stainless steel (though resistant to cavitation) and most non-ferrous metals, except cupro-aluminium, have rather poor abrasion resistance.

(5) Soft rubber appears generally more resistant than hard. (6) Plastics coatings do not appear very promising so far, except possibly in

particular applications; bonding can also be a problem. Ceramics are very wear- resistant, but their use to date has been limited by brittleness and susceptibility to thermal shock. New developments in small pump applications may show improvements.

(7) Wear increases rapidly with flow velocity, and is often reported as being approx. yc (velocity)3, or cc (pump head) 3/2 from both theoretical considerations and ,

Weur. 20 (1972)


test results. The actual value of the index, for any given conditions, probably depends on at least some, if not all, of the other factors involved in the overall wear process.

Head limits quoted are up to about 300 ft/stage for all-metal pumps, and 150 ft/stage for rubber-lined.

(8) Impact angle has a marked effect on wear; metals and rubbers behave in opposite ways.

(9) Good hydraulic design, particularly by avoiding rapid changes in flow direction, decreases wear, and should be compromised as little as possible by solids- handling considerations. Shrouded impellers are generally favoured.

(10) Rubber lining can give a much-increased life compared to that for metal, provided that the solids are not large or sharp, bonding is good, and heads and temperatures relatively low.

(11) Soft-packed shaft glands require a grease or clean water supply; scraper- vanes on the impeller, or separate centrifugal seals, are also used to protect the glands. Mechanical seals with special materials, and usually with a flushing supply, are sometimes fitted.

(12) No outstanding new construction materials, suitable for commercial application to a wide range of machine sizes, have been reported to date.

REFERENCES

1 K. Wellinger and H. Uetz, Sliding scouring and blasting wear under the influence of granular solids, VDI-Forschungsheft, 21B (1955) 449. Also shorter versions in Wear, I (1957) 3 and Schweizer Arch& 24 (1958) 1.

2 W. A. Stauffer, The abrasion of hydraulic plant by sandy water, Schweizer Archiu. Angew. Wiss. Technik., 24 (7/8) (1958) 3-30. Translation by C.E.G.B. No. 1799, 1958. Also shorter version in Metal Pro+, January 1956.

3 M. Zarzycki, Influence of the pump material on service life of the impellers of rotodynamic pumps in transport of mechanically impure fluids, Proc. 3rd. Conf on Fluid Mechanics and Fhtid Machinery, Budapest, 1969.

4 P. Bergeron and J. Dollfus, The influence of the nature of the pumped mixture and hydraulic characteristics on the design and installation of liquid/solid mixture pumps, Proc. 5th Conf on Hydraulics, Turbines et Pompes Hydrau~i~ues. 2 (1958) 597-605.

5 A. Welte, Wear phenomena in dredging pumps, VDI-Ber., 75 ~1964~ 11 I-127. Translation by Lehigh University, Fritz Eng. Lab. Report No. 310.17, 1966.

6 C. H. Warman, The pumping of abrasive slurries, Proc. Ist Pumping Exhibition and Conf, London, 1965. K. Solymos, Some aspects of designing and operating the up-to-date slurry pumps manufactured at the Tatabanya Mining Corp., Proc. 3rd Conf. on Fluid Mechanics and Fluid Machinery, Budapest, 1969.

7 A. Bezinge and F. Schafer, Storage pumps and glacial waters, Bull. Tech. Suisse Romande, 49 (20) (1968) 282.-290. B.H.R.A. translation T 1019, 1969.

8 T. Bovet, Contribution to the study of the phenomenon of abrasive erosion in the realm of hydraulic turbines, Bull. Tech. Suisse Romande, 84 (3) (1958) 37-49.

9 R. Kermabon and G. Mosse, Operational behaviour of alloys and lining materials in hydraulic turbines, Proc. 5th Hyd. Conf Hyd. Turbines and Pumps, I (1958) 328-337.

10 P. Bergeron, Similarity conditions for erosion caused by liquids carrying solids in suspension. Applica- tion to centrifugal pump impellers, La Ho&k B&r&e, 5 (Spec. No. 2) (1950) 716-729. B.H.R.A. translation T 408, 1950.

I 1 P. Bergeron, Consideration of the factors influencing wear due to hydraulic transport of solid materials, Proc. 2nd. Conf Hyd. Transport and Separation of Solid Materials, 1952.

12 N. T. Tsybaev, Use of wear-resistant rubber linings in pumps carrying abrasive fluid mixtures, Tsvet. Metally, 38 (2) (1965) 8-13. Translation in Son. J. Non-Ferrous Metals, 6 (2) (1965) 8-l 1.

Wear, 20 (1972)

50 c;. F. TRUSCOTI-

13 E. Bak, Construction materials and testing results of the wear of pumps for transporting solid media, Biuletyn Gtownego Ins&y&u&a Gornictwa, (12) (1966). B.H.R.A. translation available.

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11 PP. 36 Anon., Pumps of ceramic and epoxy withstand abusive fluids, Prod. Eng., 4 (1970) 4. 37 R. Koch, Mechanical seals working in abrasive media, Pumps, 38 (1969). 38 0. P. Erickson. Latest dredging practice, Proc. A.S.C.E., 87 (WWI) (1961) 15-28.

Wear, 20 (1972)

Documents

Literature Survey of Abrasive Wear in Hydraulic Machinery