Indian Journal of Engineering & Material s Sciences Vol. 9. December 2002. pp. 480-486

Effects of particle size and size distribution on estimating erosion wear of cast iron in sand-water slurries t

B K Gandhi" & S V BOrSCh

"DepartI11ent o f M echanica l & Industri al Engineering. Indian Institute of Technology. Roorkce 247667. India

hS.E.S. Poly technic Co llege. Dhule 424 00 I . India

Reeeil 'ed /3 DeeI'll/bel' 200/; accep/('(( 2 DC/ober 2002

Erosion wea r of cast iron has been eval uated in a pot tes ter for narrow-sized and I11ulti -s ized parti culate sand- water slurri es . The effects o f particl e size and size di stributi on on erosion wear have been analyzed and the appli cabil i ty of medi an di;1Il1cter or we ighted I11ean diameter as an effecti ve particle size for multi -s ized slurry for estimating erosion wear has been investigated. It is observed that the mean particle size of the narrow-sized particul ate slurry can be taken as the effecti ve pa rticl; size for evalualing the eros ion wear. However, the observed trend of va ri ati on of erosion wear with parti c le size is not followcd by I11edian or weighted I11ean diamcters o f multi -sized slurri es, showing large dev iat ions.

Erosion wea r is a very cru cial parameter for selection and des ign of slurry transportation systems as it aCrects direc tl y to the economy of hydraulic co nveyancc of so lids. The servicc life of equipment handling solid-liquid mi xtures are limited duc to erosion wear and therefore efforts have been made in past few decades to predict the eros ion loss of material s.

Erosion wear studi es reported in the literature 1-(,

have generall y been carri ed out using narrow-sized particulate slurri es. Bree ef 01. 1 have used a specially dcsigned pot tester to eva lu ate eros ion wear of steel grade 42 and C BR 3 15.3 in sa nd-water suspension. They conduc ted wea r tes ts at different impact angles usi ng two types of sands having med ian particle size as 0.26 mm and 0.99 mm. They observed hi gher \Veight loss due to eros ion with bigger size particles during the ex periment s at vari ous velocities and solid concentrations. The rati o of the weight loss due to the t"vo median diameters varied in the range of 2.9- 12.2 and they reported th at the rate of increase in eros ion wear with in crease in parti cle size decreases at hi gher ve loc it ics.

Elkho l/ proposed a correlati on for predicting the pump wear based on wear tests conducted on jet i mpingcment tester (J IT). He performed a seri es of tes ts with three target material s, namely cast iron, mild steel and aluminum . Wear tes ts were conducted

t presentecl al the 28'h National Conference on Fluid M echanics & Flu id Power, held al Punjab Eng ineering College, Chandigarh , during I :1- 15 December 200 I

for short durati on in the veloc ity range of 3-30 m/sec, impact angle range of 15-90° and solid co ncentrati on (by vo lume) range o f' 7-1 8%, using Semd particles lying in narrow size range between 0.41-0.5 111111. He reported that the erosion wear rate of cast iron increases with increase in particle size according to the power law relationship with th e exponent va lue as 0.616. Lynn ef al .] conducted experiments in a pot tester at a speed of 18.7 m/s for parti cle size varyi ng between 20-500 )1m. They used cy lindri cal stee l specimen in 1.2% weight suspens ion of SiC in oil. They found th at the eros ion rate decreases with decrease in particle size and impact veloc ity. They concluded that the eros ion rate for particle size greater than 100 )1.m is proportional to the ki neti c energy di ss ipated by the particles during impact and for smaller parti cle sizes, the materi al removal is in significant. Clark\ based on experiments in a pot tes ter, reported that the crater formed due to im pact of so lid parti cles increases with increase in parti cle sizc. The colli sion efficiency of a sol ie! particle with res pect to the spec imen surface was reported to decrease with decrease in nominal veloc ity, increase in carrier liquid vi scosity and decrease in particle size . He also reported that the kinetic energy of impacting particles di ssipate in plastic work, wh ich cause erosion. He observed that the erosio n wear was strongly affected by the fl ow field around a solid parti cle.

Gupta ef al .s have proposed two correlations for esti mating erosion wear of brass and mi Id steel based

GANDHI & BORSE: EFFECTS OF PARTICLE SIZE ON EROSION WEAR OF CAST IRON 48 1

on the ex perimental data generated in a pot tester with narrow-sized "opper tailings-water slurri es using cylindrical wear specimens. According to them, the power index value for particl e size is around 0.3 for both the materi als in the power law relationship with eros ion wear. They reported that the correlations proposed by them estimate the erosion wear in pipelines within ± 14% error band. Gandhi6 performed wear tes ts ina si Illi lar pot tes ter at various impact angles using spec!all y designed tes t fixtures. He observed that the erosion wear rate increases with increase in parti cle size at all impact angles irrespec ti ve of change In velocity or solid concentrati on. Ex periments by Gandhi et al. 7 were conducted in the same pot tes ter to evaluate the eros ion wear of brass for parallel flow of solid-liquid mi xtures. They selected three narrow-sized zinc tailings-water slurries namel y 890 ~lIn , 448 .5 ~m and 225 ~lIn to conduct the ex periments at various solid concentrati ons and velociti es . Based on the ex perimental data, they proposed a correlati on to es timate the erosion wear of brass for parall el fl ow conditi ons in which the value of power index for parti cle size is 0.85.

The rev iew of the literature shows that the eros ion wear increases with increase in particle si ze normall y according to the power law relati onship in which the power index fo r particl e size vari es over a wide range. However, based on these studi es, many correlations2

.5.7 have been deve loped and proposed to pred ict the erosion wear for equipment handling slurri es whi ch show large vari ati ons in the predicted wear. Further, the slurry transportati on systems generally convey commercial slurri es, which consist of multi -s ized parti cles normall y in the range of three orders of magn i t ude (~m to mm). I n absence of ex tensive experimental data on eros ion wear due to mu lti -s ized slurri es, the effec ti ve size for these slurries is generally taken as medi an diameter or weighted mea n di ameter whi ch are de fin ed as:

( i) Media l/ dialll eter Cd)o) This is the size (di ameter) of the screen through whi ch 50% so lids pass

( i i) Weigllfed lII eal/ dialll eter (d'"l) Thi s is defi ned as,

N

( 1 \\'1/ = "f·rI L..J . t I . .. ( I )

where N is number of size groups into which the total sample is divided, j i is fraction of solids i! 1 e:.I , I , ~j !.,

group, and d i is the average diameter of t\ovo successive sieve sizes one on which the solids are retained and the other through which the solids have passed completely .

These two particle sizes have been reported to estimate reasonably the additional hydraulic losses due to suspended solids for multi-sized slurries8

.

However the use of these diameters as effective size fo r estimating erosion wear due to multi-si zed slurri es has not been thoroughly in vesti gated. Thus, the difficulti es in predicting erosion wear in the field usi ng the avai lable correlations is further aggravated due to the assumption of effecti ve particle size for the commercial slurry.

Gupta9 made an attempt to study the effect of particle size di stribution on erosion wear due to multi­sized slurries . He evaluated the erosion wear of mild steel and brass using multi-sized copper tailings-water slurries prepared by mixing five narrow sized particles namely, 448.5 ~m, 223.5 ~m , 128 ~m, 90.5 ~lm, 37.5 ~m indifferent proportions. Based on the experiments in the pot tester, he reported that weighted mean di ameter of multi-sized slurry is the effective size for estimating the erosion wear. For weighted mean diameter, he observed that the two correlati ons proposed by Gupta et al.s for predi ct ing erosion wear of brass and mild steel estimates the uneven wear in pipeline due to the multi-sized slurries within an error of ± 13.5%. He, however, attri buted thi s error to many reasons including the assumption of weighted mean diameter as the effecti ve size for the multi-sized slurry.

Gandhi et al. lo have reported that the additi on of fi ner particles « 75 ~m) in narrow-sized coarse parti cul ate slurry reduces the erosion wear of brass which depends on the proportions of finer part ic les present in the mi xture. This shows that the role of fin er parti cles on eros ion wear is signi ficantly eli fferent co mpared to the bigger parti cles. A detai led study is, therefore, needed to es tabli sh the effect of particle size di stributi on on eros ion wear due to mult i­sized slurries by removing the finer particles. Also, th e wear reducti on due to presence of finer parti cles may be analysed separately.

Literature reveals th at normall y pot testers have been used to generate accelerated wear data despite the fact that the parti cul ate motion in the pot tester is very complex due to ex istence of highly turbulent

482 INDI AN J. ENG. M ATER . SCI.. DECEMBER 2002

fl ow conditi ons. During these tes ts, it is difficult to exactl y quantify the particle impact angl e and veloc ity, and the so lid concentration over the tes t spec imen. However it has been observed that these devices have been successfully used to charac teri ze eros ion resistance and to prov ide data for selection of material s I I. Over the years, efforts have been made by few in ves ti gatorsl.7 to ori ent the wear piece at spec ified impac t ang les and so me of the in ves ti gators.7

to stir the slurry using a propeller and break the vortex motion by providi ng baffles. Fu rther, the eros ion wear studies on brittl e type material s like cast iron are scanty .

The present study has been undertaken to evaluate eros ion wear of cast iron using a pot tes ter. Sand water mi xture has been used to in ves tigate the effect of parti cle size di stributi on on the weight loss due to mu lti -s ized particulate slurri es by remov ing fin er parti cles «75 pm). An effort has been made to in ves ti ga te the deviat ions of the eros ion wear represented by median diameter and weighted mean diameter of the multi-sized slurry compared to that by the mean particle size of narrow sized slurry.

Experimental Test apparatus

A pot tes ter of 6.6 L capacity as shown in Fig. I, sim ilar in des ign to the one used by Gandhi ef 0 /.

7, has

been used in the present study. Slurry is kept in suspension by rotati ng a mi xer propell er attached at the bottom of a 12 mm steel shaft. A brass sleeve of 84 mm length and inner diameter 12 mm is provided wit h prov isions for fi xing four horizo ntal arms to hold two tes t fi xtures at diametri cally oppos ite ends. The inner diameter and height of the cy lindri cal aluminium pot are 245 mm and 144 mm, respectively. Four U-shaped baffles are prov ided at the inner wall of the cy lindri cal pot at equal di stances to break the vortex mot ion produced (if any) due to rotat ion of the prope ll er. A 12 mm thi ck transparent acrylic sheet is used to cover the pot, whi ch allows visual observati ons. The shaft is held in pos iti on by two bearings placed suffi cientl y apa rt over the cover of the cy I i ndrical pot. The shaft is coupl ed to the chuck of a drilling machine using fl exibl e coupling. The drill chuck is driven by a 1.5 kW D.C. motor through V-belt pu ll ey dri ve.

Test fixture and we ... " specimens

Special tes t fixtures, as shown in Fi g. 2, have been used to fi x 2 mm thick cast iron wear pieces (specifi c

grav ity=8.8) of overall size 40 mm x 5 mm with semi circular shape at the ends. A slot is provided at the center of 4 mm thick test fixture to fix the wear piece inside it leav ing approximately 2 mm plane surface at all around the wear piece surface. The arrangement facilitates to minimize the random impact of so lid parti cles at the wear surface and the later can be ori ented at des ired angle of impact with so lid particles. Two such fixtures have been fitted at 180° apart to minimi ze the wake interference effects. Provis ions have been made to vary the impact angle in the range of 0-90°, in the steps of 15°. The fixtures are made of high-chromium hi gh-carbon steel and heat-treated properly to achi eve a hardness of approximately RC 65 (Rockwell scale-C).

Properties of IIsed material

Sand coll ected fro m Narmada ri verbank has been used in the present study. The spec ifi c gra vity of sand

DlnECTION OF ROTATION

BRASS SLEEVE

TEST FIXTURE

x L

PROPELLER

Top View (Section at X4 X) I

0245

Sectional Front View

10

BAFFLES ("NOS.)

"'- MOUNTING HOLES

Fi g. I- Sc hema tic layo ut o f pot tes te r used for co nducting wcar test wi th so lid- liqu id mix lUrcs

GANDHI & BORS E: EFFECTS OF PARTI CLE SIZE ON EROSION WEAR or CAST IRO N 483

I b ~I

10

n r- - f--

~

-~

~ ~

~

I :: "1-" " ~

" " -

, , , , , , ,

Fig. 2- Detail s of test fi xture used for lix ing wear piece

I S measured as 2.68. The final static settl ed concentrati on of sand-water slurry is observed as 5J.7JC'/r (by weight ).

Sand was wet sieved over 75 ~lm size sieve to rcmove th e ri ner parti cles and then dri ed to grade in rour narrow size ranges namely, - 1000+710 (855 ~lm ), - 710+300 (50S ~lm ), - 300+150 (225 ~m) and - 150+75 ( 11 2.5 ~m). The numbers indicate the sieve sizes in mi crons between which samples were co ll ected and mean particle size is given in the parenthes is. For narrow panicle size of I 12.5 ~m .

wear tes ts are co nducted at two impact angles namel y, 30° and 75° for 20% so lid concentrati on (by weight) at a veloc ity or 3.62 m/s. Si x multi -s ized particulate sl urri es as show n in Table I have been prepared by mi xing the above four narrow-s ized parti cles in different proporti ons to obtai ned median diameter in the rangc of 11 2.5-HS5 ~lm and weighted mean diameter in the range or 186-716 ~m. Wear tes ts for these multi-sized parti culate slurri es are conducted at the two impact angles of 30° and 75° at 3.62 m/s veloc it y and 20% solid co nce ntrati on (by we ight).

Procedure Initial cxperimen ts havc show n th ,tt the e f'kc ts of

attri tion and consequent rounding-o lT of th e parti cle

Table I- Particle size di stribution for multi -sized slurries

Weighted Med ian

Sample Mean panicle size. pm mean di ameter.

No. diameter, 11 2 225 50S 855 ~tlll

~trn

75 IS 5 5 186 112.5 2 40 30 20 10 299 225 3 25 25 25 25 424 3 15 4 10 20 30 40 549 505 5 5 15 30 50 6 18 680 6 5 5 20 70 7 16 855

edges on eros ion wear are not significant for in iti al two hours durati on at 3.62 m/s velocity. Thus, the average weight loss of the two cast iron pi eces was determined after running them for a total time period of two hours in the slurry pot. Each wear piece was poli shed before conducting the experiment to ensure identical initi al conditions for each set of data. Proper care was taken to cl ean and dry the wear pi eces after the test, which involves washing in tap water, rinsing with acetone and drying with hot air blower. Weight loss of the wear pieces was measured accurately in an electronic balance (LIBROR AEL-200, Shimadzu Corporation , KYOTO, JAPAN ) having a least count of 0.1 mg. The variation in the weight loss of two wear pi eces at any given operating condition was observed within ±5%.

Results and Discussion The aim of the present study is to inves ti gate rhe

effects of particle size and particle size di stri butions on erosion wear of cast iron in sand-water slurry. Oesale l 2 carried out experiments in the same pot tester to study the erosion wear characteri sti cs of cast iron using Narmada sand . He measured eros ion weight loss of the same size wear pi eces using sand-\vater slurry of three narrow-sized parti cles namely, 855 /-1m , 50S /-1m and 225 /-1m . The data presented by him at 3.62 m/s ve loc ity and 20% so lid concentrati on (by weight ) for a ll the three parti cle sizes at the two impact angles of 30° and 75° are used in the present anal ys is. Similar data with the narrow-sized particulate sa nd-water mixtures of 11 2.5 Jlm fo r the two impact angles at 3.62 mls veloc ity and 20% solid co ncentration (by weight) is al so generated in the present work to enhance th e range of in ves ti gati on on th e effect of parti cle size on eros ion wear. All these data ha ve been used to in vest igate the parti cle size effec ts on eros ion wear of cast iron due to narro\\'-

484 INDIA N J. ENG. MATER. SCI., DECEMB ER 2002

sized particu late slurries. The wear data obtained for mul ti-sized slurries have been compared with these data to investigate the app licability of using median di ameter or weighted mea n di ameter for mult i-sized slurri es. The error involved using these diameters as effect ive parti cle size has al so been evaluated and di scussed.

The average weight loss of the two wear pi eces of cast i ron meas ured before and after the wear tes ts are prese nted graphi call y in Figs 3 and 4 along with the data reported by Desale 12

. The tes t res ults are analysed and di scussed to highli ght the effects of parti cle size and size distribution on erosion wear of cast iron.

Narrow-sized particu la te slurries

The mean size of the parti cles collected between two successi ve sieve sizes has been taken as the effecti ve parti cle size for narrow-sized part icul ate slurri es . The wear tes t results show that the erosion wear of cast iron increases with increase in mean particle size of the slurry under sim il ar experimental conditi ons. The vari ations of erosion wear with mean panicle size at 3.62 m/s ve locity and 20% solid concentrati on (by weight) are shown in Figs 3 and 4 at the impact angles of 30° and 75°, respectively . It is seen that the erosion wear at each impact angle increases with increase in particle si ze, according to a pol ynomial relationship as:

, .. (2)

where EIV is the average weight loss of the wear pieces in t\VO hours in mg, d is the mean particles size In mm, and ai, ([2 and (I] are polynomial coefficients.

16,-==~--~~--~-------------------, IIJDCD Mean size of narrow-sized s lurry A/:J.t'J.tJ.l:r Median d iomcttH of mult i-sized slurry xxx x x We ighted menn diometer o f mult i-sized slur ry

E' 12

~ le °

2 +r~~~TITTnTnTn~nTnT~TITTnTnornTrrl o 2e0 400 600 800 1000

parLic le size, }..1m

Fig. 3- Vari at ion of weight loss of cast iron in sand-water slurry with pa ni cle size at 300 impact angle (veloc ity=3.62 m/s and so lid cO llcentr:lti on by weight=20%)

The value of polynomial coefficients are observed as 2.043, 0.0073 and 5.88x I0-6 and 4.614, 0.0251 and -4.7263x I 0-6 for 30° and 75° impact angle, res pectively. Si nce the value of the coeffici ents for d~ is very small , the assumpt ion of linear var iation of eros ion wear with part icle size will result in insignificants errors, partic ul arly fo r sma ll parti cle sizes. Desale l ~ has reported linear variati on of eros ion wear with particle size based on hi s experimen tal data for cast iron in narrow-sized sand-water slu rry. Many . . 2 ~ 7 h d I I' . Investigators" ave reporte a most tnear Increase in erosion wear with increase in parti cle size based on their ex periments with narrow-s ized particulate slurri es. Researchers)·7 have also shown that the mean size of the narrow-sized particles is the representative particle size for es timati ng erosion wear in solid­liquid flows. The present observation is in line with these investigations. It can, therefore, be concluded that the mean particle size of the narrow-sized slurry can be taken as the effective parti cle size for es timat ing the eros ion wear in so lid-liquid fl ows .

M ulti -sized particu la te slurries

The wear tes t res ults are conducted with six mu lti ­sized sand-water slu rri es having different parti cle size di stributi ons (Table I) at 3.62 m/s veloc ity and 20% solid concentration (by weight) for two impact angl es of 30° and 75°. The particle size distributions of the mult i-sized slurri es show that they are prepared by mixing different fractions of four narrow sizes namely, 855 ) . .lITI , 505 1 .. 1ITI, 225 )1.m and 11 2.5 )1.m . Thus, the mean particle size of all the mulri -sized slurries is same being the average of the bigges t and smallest sizes of the particles present in the mi xture.

28,-------~------------------------~ a:IIIl Mean size of narrow- sized s lurry ~ Median diameter o f m ulti-sized slurry xx xxx Weighted mean diameter of m ulti- sized slurry

24

bJl

E 20 " vi Xl> VI

" 0 - 16 .,.,

" .c: bJl

~ 12 "

8

4 0 200 400 600 8 00 102121

pa rLi c le size, }Jrn

Fig. 4- Variation of weight loss of cast iron in sand -water slurry with panicle size at 75 0 impact angle (velocity=3.62 m/s :Inc! so lid concentration by weight=20%)

GANDHI & GORSE: EFFECTS OF PARTICLE SIZE ON EROSION WEAR OF CAST IRON 485

Tab le 2- Erro r in estimat ion of e rosion wear cau sed by multi-s izcd s lurries

Impact an glc. Erosion wcar mcasure as avcrage we ight loss deg ree o f the two cast iro n pi eces

30 Measured weight loss, mg Using median d iamctcr Predicted, mg

'70 error Using wcighted mcan Pred ic ted, mg diamcter % error

75 Measured weight loss. mg Using media n diamcter Prcdictcd . mg

Using weight ed mean diamcter

'7c error Predi c ted. mg % erro r

However, the va ri at ion of weight loss data listed in Table 2 shows that the average of the measured weight loss of the wear pieces vari es from 5.3-11.2 mg at 30° impact angle and from 12.2-20.8 mg at 75° impact angle for these multi-sized slurri es. Hence, the mean particle size does not represent the effec ti ve parti cle size of the multi-sized particulate slurri es as far as erosion wear is concerned.

Gupta') has reported that weighted mean diameter can be taken as the effecti ve particle size for es timating crosion wear due to multi-sized particulate slurries hav ing continuous parti cle size distribution. According to him, the slurries having different particle size distributions but identi cal weighted mean diameter show approximately iden tical wear.

To in vesti gate the effecti ve particle size for multi­sized slurry, the variat ion of erosion wear data with particle size fo r narrow-sized and multi-sized particulate slurri es has been presented graphicall y in Figs 3 and 4 for 30° and 75° impact angles, respectively. ft can be seen that the eros ion wear of multi-sized slurries represented by med ian or weighted mean diameter lies around the line (Eq. 2) representing the relationship between the part icle size and erosion wear. To anal yze the deviations , Eq . (2) is used to estimate the erosion wcar due to multi-sized slurri es using med ian and weighted mean di ameter and the cs timated va lues are li sted in Tab le 2 along with the probable errors. Compari son of the eros ion wear data depicted by the two diameters namely, medi an diameter and we ighted mean diameter of multi-sized slurri es and the eros ion wear by mean particle sizc of narrow-sized slu rries shows a large difference. It is seen that if Eq. (2) is used to estimate the erosion due to mu lti-sized particulate slurry

Mult i-s ized particulate sand-water mi xture sampl e nu mber

2 3 4 5 6

5.3 6.3 7.8 9.0 9.6 11 .2 2.9 3.6 4.8 8 .0 10.2 12.2

45. 3 42.9 38.5 11 . 1 - 6.2 -8 .9 3.6 4.8 6.2 7.8 8.8 \CD

32. 1 23.8 20.5 13.3 8.3 8.0

12.2 14.8 17.2 18.4 19.8 20 11 7 .3 9.1 11.7 17.2 20.1 22 .2

40.2 38.5 32.0 6.5 - 1.5 - 6 .7 9. 1 11 .7 14.4 17.0 18.3 20.2

25.4 20.9 16.3 7.6 7.5 2.9

represented by median diameter or we ighted mean diameter, the es timated eros ion wear is always lower for mu lti-sized slurry. Hence, the assumption of any of these diameters as an effect ive particle size for the multi-sized slurry may results in a large error in es timati on of the erosion wear for slurry hand ling equipment. It can be seen that the errors are as hi gh as 45 % for median diameter and 32% for weighted mean di ameter. ft is however observed that the assumption of these diameters results in less error i.e. wi thin ±20%, if the multi-sized slurry contains large amou nt of bigger particles (multi-sized samples 4-6 in Tab les I and 2).

Many in vesti gators U.~. 1:1 have reported that Ihe eros ion wear is proportional to loss in kinetic energy of the solid particle impacting at the target mate ri al surface . The kinetic energy of the so lid particle is directl y proportional to the particle mass. The median diameter or weighted mean diameter of the multi­sized slurry does not represent the average mass of the so lid parti cles and this may possibly be one of the reasons fo r large errors in es timating eros ion wear usi ng these diameters.

Conclusions Based on the present experimental inves tigati on on

eros ion wear of cast iron in sand-water mixtures, fo ll owing conclusions can be drawn: (i) Erosion wear increases almost linearly with increase in parti cle size; and, (ii ) For es timat ing erosion wear, the effecti ve particle size for narrow-s ized particulate slurri es can be taken as the mean particle size whereas the assumption of median diameter or weighted mean diamcter as effect ive parti cle size of mu lti-sized slurri es results in large error in estimation of erosion

4H6 INDIA J. ENG. MATER. SCI.. DECEMBER 2002

wear. The erosion loss of material (caused by multi­s ized slurries) estimated using med ian diameter or weighted mean diame te r is always on lower s ide compared to the actual loss .

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