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Particle size effects in bend erosion
Wear, 44 (1977) 311 - 328 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
PARTICLE
SIZE EFFECTS
311
IN BEND EROSION
D. MILLS and J. S. MASON Department
of Mechanical Engineering,
Thames Polytechnic,
London S. E. 18 (Gt. Britain)
(Received October 26, 1976)
Summary From work on the erosion of flat plates in test rigs of the sand blast type published in recent years it has generally been concluded that there is a critical particle size above which erosion is not influenced by size. Recent work on the erosion of pipe bends by sand in a pneumatic conveying line tends to confirm this theory. For pipe bends, however, it was observed that particle size has two additional effects of major significance: the appearances of the eroded surfaces are totally different and the depth of penetration by erosion is much greater for smaller particles; this means that pipe bends will fail much earlier if smaller particles are conveyed.
1. Introduction In the last year two major series of tests have been undertaken on the erosive wear of pipe bends. These have been carried out using a full scale pneumatic conveying rig in the Powder Handling Laboratory at Thames Polytechnic. The first was an investigation into the effect of phase density or particle concentration, and the second was on the effect of conveying velocity. In both programmes sand was pneumatically conveyed through the bends and about 40 bends were tested in each series of tests. They were all 90” bends, of 50 mm bore and 140 mm bend radius; they were tested in the horizontal plane. For the investigation into the effect of particle concentration, sand with a mean particle size of about 70 pm was used. Tests at phase densities ranging from 0.5 to 8.0 were carried out and the velocity was held constant at about 25 m s- ’ for each test. The results and analysis of this work are presented in ref. 1. In investigating the effect of velocity, sand with a mean particle size of about 230 pm was used. Tests were carried out at phase densities of 1.0, 2.0 and 4.0, and the conveying velocities ranged from about 15 to 32 m s- ‘. The results and analysis of this work are presented in ref. 2.
312
1.1. Wear patterns A characteristic feature of the erosion of the bends by the sand with a mean particle size of 70 pm was that the erosion of the outer bend wall surface material was very irregular. Beyond a bend angle of about 30” the surface was worn in a complex pattern of steps. Some of the steps were quite pronounced, presenting vertical faces of about 2 mm in height to the oncoming flow. In most of the bends the steps continued right round to a bend angle of about 90”. Between bend angles of about 35” and 45” there were often as many as 20 such small steps or ridges clearly visible. These were all concentrated over a small area of the outer perimeter of the bend. This wear pattern was observed on bends tested at every phase density from 0.5 to 8.0. Typical photographs of the bend wear caused by this sand are shown in Figs. 4(b) and 9, and typical bend wear profiles are shown in Figs. 5,6 and 11. The pattern of surface erosion on the bends caused by the 230 pm sand, however, was entirely different. No abrupt steps or ridges were produced at any stage in the erosion process even up to the point of bend failure. Even beyond a bend angle of 30” the erosion left a smooth surface and no steps or ridges were produced at all. This was a common feature of all the bends tested with this sand, regardless of both the phase density and the velocity of the suspension. A typical photograph of the bend wear caused by this sand is shown in Fig. 4(a) and typical bend wear profiles are shown in Figs. 7, 8 and 13. 1.2. Mass eroded Apart from the totally different appearance of the bend wall surfaces for the two particle sizes, there was also a marked difference in the mass of metal removed from the bends when failure occurred. With the 70 pm sand two of the bends tested at a phase density of 3.0 and velocity of about 25 m s-l failed. The masses of metal removed from these bends at the time of failure were about 42 g and 43 g. With the 230 pm sand another two bends failed, this time at a phase density of 1.0 and velocity of about 32 m s- ‘. In this case the masses of metal removed from these bends at the time of failure were about 104 g and 112 g. Figure 1 shows the variation of the mass of metal eroded with mass of sand conveyed for these four bends. There can be no direct comparison between these two sets of bends, as they refer to different conveying velocities and phase densities, as well as different particle sizes. However, with such a large difference in the mass eroded for failure and the fact that the erosion patterns appear to be characterised by the particle size, it was felt that particle size was the dominant factor in the process. It was therefore decided to examine some of the bends that were tested at the same phase density and velocity in order to see whether the rate of penetration by the finer sand was more rapid. A closer look was also taken at the erosive wear results to see whether particle size caused any difference in the rate of erosion, in terms of mass of metal eroded per unit mass of sand conveyed, in the case of pipe bends.
313 120 P
J
u
4
L
Mass conveyed
0
0
tonne
Fig. 1. The erosion history of bends that failed: -LI- and --cP- represent erosion by 230 ,um sand of phase density 1 at a velocity of 32 m s-l ; --o-- and -+-- represent erosion by 70 pm sand of phase density 3 at a velocity of 26 m s-l.
2. Experimental
rig
In order to carry out realistic tests appropriate to industrial pneumatic conveying situations, a full scale rig was built. The rig consists of a blow tank having a capacity of about 1 m3, a similar sized hopper mounted on three load cells vertically above and a Roots-type blower capable of delivering about 0.07 m3 s-l at a pressure of 2 bar absolute. A diagrammatic layout of the conveying plant and test loops is shown in Fig. 2. A full description of the rig, together with further sketches, has been given [l] . This included details of the test loops, plant operation, control and inst~mentation, and ch~ge/disch~ge facilities. Briefly it is a batch-type system with the powder being conveyed from the blow tank, through the test loops and into the receiving hopper. The test section consists of two horizontal loops in a rectangular configuration with a 90” bend at each corner. Each loop has two 8.2 m pipe runs, giving a total length of 27.2 m for each loop. A diverter valve in the test loops allows one loop to be bypassed and enables sampling of the powder to be carried out whilst it is actually being conveyed. The bends tested in the two programmes were all mild steel of 140 mm bend radius and 50 mm bore. The sand used in the programme of tests to
314 r----c /Filter
1
Fig. 2. The test loops and conveying
plant layout.
investigate the effect of phase density on erosion had a mean particle size of 70 pm, with approximately 10% greater than 180 pm and 10% less than 40 I.tm. For the investigation into the effect of velocity on erosion, sand with a mean particle size of 230 pm was used and this had approximately 10% greater than 360 pm and 10% less than 160 ,um. In the following section a review of previous work on the effects of particle size on erosion rate is presented. In all the cases cited this work has been carried out with bench-type test rigs and for the majority of these the velocity of the particles has been measured. In the work presented here on actual pipe bends, however, it is the velocity of the air that has been recorded. In all cases the pipe bends followed sufficiently long straight runs of pipe to ensure that even the largest particles were accelerated to their terminal velocity and it was felt that results and information based on these parameters would be of more practical use in industrial situations. 3. Erosion rate The erosive wear rate of materials is usually expressed in specific terms, be. the amount eroded per unit mass of particles conveyed. When comparing the performance of different surface materials the erosion is normally expressed in volumetric terms to give cm3 kg- ’ and when comparing similar materials it is generally expressed on a mass basis such as g kg- ‘. In erosive wear testing to investigate the effect of particle size there are two specific problem areas with regard to the particles themselves. One is the difficulty of specifying a characteristic particle size for a given mass of powder. The other is the fund~en~ problem of either measuring the velocity of the particles or ensuring that they have been accelerated to their terminal velocity before impact. The majority of work on the erosive wear of materials has been carried out with bench-type rigs in which abrasive particles have been impacted
315
upon target materials. Whether the work has been carried out for the purpose of investigating erosion problems with gas turbine blades, boiler tubes or pneumatic conveying plant and equipment, erosion rigs have provided valuable information, The number and range of the variables involved in the problem of the erosion of surface materials by gas-solid suspension flows are such that their use has been almost essential. A review of the various types of erosion rig used in these investigations and a summary of the variables involved in the erosion process were given in ref. 3. 3.1. Review of previous work Particle size is a variable that has been investigated by a number of researchers [ 4 - 81. The investigation of Goodwin et al. [4] was confined to crushed quartz, which was sieved into narrow size ranges from 25 pm to 210 grn and tested at normal impact against an 11% Cr steel at particle velocities of approximately 130, 240 and 305 m s-l. Their resulting erosion curves showed that there was a significant size effect which was itself dependent upon impact velocity. They showed that there was a critical particle size above which erosion was not influenced by size and that this value appeared to increase linearly with velocity, being about 50 pm at and 120 pm at 305 m T’. The maximum value of the erosion of 130ms-’ the metal at a particle impact velocity of 130 m s-l was 1.3 g kg-‘. Tilly [6] reported on similar tests which showed that different materials exhibited different types of size dependence. With engineering alloys and resilient plastics there was an initial increase in erosion with particle size until the onset of a saturation plateau where it was independent of size. Resins and composite materials such as fibreglass, however, suffered a continual though varying increase in erosion with particle size and did not exhibit the characteristic plateau for the range investigated. Sage and Tilly [5] also tested glass, which suffered a rapid increase in erosion with particle size, and found that the erosion could be represented by a power law over the size range tested. Wood and Espenschade [S] also used quartz dust but it was not sieved into narrow size ranges. They tested at an impact angle of 40” against a pearlitic steel at an air velocity of 260 m s- ‘. In comparison with the erosion of the 0 - 150 pm size range, the 0 - 74 pm was 75% of the value, the 0 - 10 pm was 35% and the 0 - 5 ,um was about 10%. Kleis [7] also used quartz sand particles and impacted them, at velocities ranging from 83 to 225 m k ‘, at 90” against steel specimens ranging in hardness from 130 to 850 kg mmw2. He showed that the erosion loss by a given mass of abrasive grains was independent of the particle size for particles greater than about 160 pm, but only for the lower velocities tested. At higher velocities an increasingly distinct maximum value of erosion was observed at the critical particle size, beyond which erosion decreased to a lower constant value. This critical particle size varied slightly with particle velocity, and both particle and surface material hardnesses. For a steel with a hardness of 130 kg mrnm2 impacted by 130 ,um particles at 83 m s-l the erosion rate was 0.25 g kg-l.
316
3.2. Bend wear results Two major programmes
of work on bend wear have been carried out at Thames Polytechnic. One was with sand having a mean particle size of 70 pm and the other was with sand of 230 I.rm mean particle size. Both programmes were carried out with single batches of sand, these being about 740 kg for the 70 pm sand and 975 kg for the 230 pm sand. Degradation of the sand, however, was quite considerable in both cases. For the programmes carried out this did not present a problem as they were comparative studies into the effects of phase density and velocity and a sequential testing plan was adopted to overcome such difficulties. The rapid change in character of the particles as a result of degradation, however, made it very difficult to compare the two sizes of sand used in terms of specific erosion rates. This not only meant that tests carried out at the same velocity and phase density had to be compared but that the sand had also to be in the same condition. For this purpose the comparison was based additionally on tests carried out with the sand conveyed through the same number of bends. This assumed that the rate of degradation and its effects were comparable for the two particle sizes. The effects of particle degradation and wear for a number of the tests that were carried out are shown in Fig. 3, which is a plot of the specific erosion rate for each indjvidu~ batch conveyed against the total number of bends through which the sand had been previously conveyed.
Number
of bends
sand conveyed through
Fig. 3. The effect of particle degradation and wear on erosion: -Q-, -o-and -G-represent erosion by 230 pm sand of phase densities 1.0, 2.0 and 4.0, respectively; * represents erosion by 70 firn sand of phase density 1.0.
317 TABLE 1 Comparison of specific erosion rates for 70 pm and 230 pm sand Mean air velocity (m s-l)
Number of bends sand was conveyed through 70 ym
230 pm
1.0
26
170
2.0
27
4.0
25
Phase density
Specific erosion (mg kg-‘) 70/~m
230pm
170
6.0
5.1
50 122
30 110
11.0 9.4
12.8 5.6
218
218
3.0
2.2
One set of results which could be compared directly was that at a phase density of 1.0 and velocity of 25 m s-I. These were some of the first tests carried out with the fresh sand and the specific erosion with the 70 pm sand was 13.1 mg kg-’ and that for the 230 pm sand was 12.8 mg kg-r. Other specific erosion values for tests that could possibly be compared are given in Table 1. No firm conclusions can be drawn on the basis of this evidence but it appears that in specific erosion terms there is little difference between the two sizes of sand. To obtain a more accurate answer to this question it would be necessary to use a fresh batch of sand for each test at each velocity and phase density and at least four different particle sizes should be employed. 4. Bend erosion patterns The marked difference in appearance in the eroded surfaces of the bends for the two particle sizes prompted further investigation. After comparing erosion rates, a number of the bends were cut in half so that accurate wear profiles could be recorded and photographs could be taken of the eroded surfaces. To a certain extent bend erosion patterns can be explained in terms of impact angle effects. 4.1. Impact angle effects The variation of erosion with impact angle has been investigated by a number of researchers [ 7 - 111. In general, erosion plots against impact angle for different materials are of two different shapes, characterised by ductile and brittle materials. For brittle materials erosion tends to increase with increasing impact angle, with maximum erosion occurring at normal impact. For ductile materials erosion tends to increase very rapidly with increasing impact angle up to about 20”) beyond which the erosion decreases gradually until under normal impact it may be only 10% of the critical value at about 20”. Theories proposed to explain these phenomena consider that for ductile materials removal of material is predominantly due to plastic deformation
318
(e.g. as with the cutting edge of a machine tool) and that for brittle materials it is due to the propagation of fracture surfaces into the material. Bend wear is usually simulated in bench tests by imparting particles at a low angle onto flat target specimens. The bend initially presents a surface at a low impact angle to the particles issuing from the preceding straight pipe run and so with a ductile material rapid erosion takes place. Gradualiy the impact angle changes to almost 90” and little further erosion takes place in this area. Mason and Smith 1121 carried out work with square section Perspex bends so that they could observe the change in wear pattern with respect to time. Their findings agree with the theory postulated above for the erosion of ductile bend materials. The bend wear patterns that they presented, however, showed that the trajectory, and hence the erosion, of the particles beyond this impact point are also important. From the bend wear profiles caused by the erosion of the sand, it is difficult to imagine how bench-type erosion rigs can hope to represent bend wear situations. It is the removal of the particles after they have been impacted onto test materials that represents the main problem with erosion rigs. In bench tests there is bound to be an interaction between the oncoming particles and those leaving the surface, the magnitude largely depending on the impact angle. The continuous motion of the particles in flowing round a bend just cannot be simulated, and hence neither can the wear beyond the initial impact point. 4.2. Bend wear results A number of photo~aphs of the bend wear surfaces and sketches of the bend wear profiles are presented. Profiles are given both of the outer wall surface in the flow direction and of pipe wall cross sections at various bend angles. They are included to show how and where the erosion has taken place and for the comparison of the erosion patterns caused by the two sizes of sand used. In the photographs the flow direction is from the bottom to the top in each case. In the wear profiles the wall thickness, the bend and the pipe radii are not drawn to the same scale. The wall thickness is drawn to a slightly exaggerated scale in order to show the erosion effects in greater detail. Dimensions, however, are included on the sketches for reference purposes. Figure 4 shows two of the bends which failed and these correspond to the erosion plots in Fig. 1. Figure 4(a) shows a photograph of the outer inside surface of the bend eroded by the 230 pm sand at a-phase density of 1.0 and velocity of about 32 m s- ’ and from which 115 g was eroded. Figure 4(b) is for the bend eroded by the 70 pm sand at a phase density of 3.0 and velocity of about 26 m s- * and from which 44 g was eroded. Wear profiles for these two bends are shown in Figs. 5 - 8. Figutes 5 and 7 show the profiles for the two outer wall surfaces and Figs. 6 and 8 each show three cross-sectional profiles at bend angle intervals of 10”. These were taken to correspond with points (1) 10” beyond the hole in the bend, (2) at the downstream edge of the hole and (3) 10” in advance of this.
319
-
-1 e
(b)
(a)
Fig. 4. Bend wear photographs showing bends in which (a) 115 g was eroded by 230 pm sand at a phase density of 1.0 and a velocity of 32 m s-l and (b) 44 g was eroded by 70 pm sand at a phase density of 3.0 and a velocity of 26 m s-l.
Iso
L3mm
Mmm .-.
@ Fig. 5. The outer bend wall wear profile of a bend that was eroded phase density of 3.0 and a velocity of 26 m s-l. Fig. 6. Pipe section
profiles
at various
bend angles
by 70 pm sand at a
for the bend shown
in Fig. 5.
320
,Eendangle
5o”
LJmm
Ml-ml .-_
.-.
O0
~ Fig. 7. The outer bend wall wear profile of a bend that was eroded phase density of 1.0 and a velocity of 32 m s-l. Fig. 8. Pipe section
wear profiles
at various
by 230 I_rrnsand at a
bend angles for the bend shown
in Fig. 7
The photo~aphs and wear profiles for these two bends show the difference in erosion caused by the two different sizes of sand used. Although conveying conditions in respect of phase density and velocity are different, these can be discounted as factors influencing the basic pattern of erosion. The stepped appearance of the eroded surface was characteristic of all the bends tested with the 70 pm sand from a phase density of 0.5 - 8.0. Photo~aphs of bends eroded by the 70 pm sand at phase densities of 1.0 and 8.0, shown in Fig. 9, confirm this. The smooth and rounded appearance of the eroded surface produced by the 230 pm sand was also characteristic of all the bends tested with this sand over a range of phase densities of 1.0 to 4.0 and velocities of 15 - 32 m s- ‘. When one compares the outer perimeter profiles for erosion by the two sand sizes in Figs. 5 and 7 it is difficult to see how 115 g of met& came to be removed from the bend by the 230 pm sand and only 44 g by the 70 I.rm sand, for, in terms of the cross-sectional area eroded, that for the 230 pm sand is only slightly greater. The answer lies with the pipe cross-sectional profiles presented in Figs. 6 and 8. These show how the finer sand has cut a much narrower path into the bend surface, affecting only a small area of the pipe wall, whereas the 230 iurn sand has worn a very much wider path. This is p~ic~~ly noticeable in the region where the bend was holed and in the region upstream from this. The profiles 10” downstream of the hole again show little difference in erosion. 4.2.2. Surface ripples The bend wear profiles for the 70 pm rand are similar to those presented by Mason and Smith [ 121. They used highly abrasive alumina powder having a mean particle diameter between 50 and 60 ym in their tests. Surface ripples are a characteristic feature of ductile erosion of materials and have been reported by several researchers [8, 11,131. However, these are not to
321
(a)
(b)
Fig. 9. Bend wear photographs showing bends in which (a) 36 g was eroded by 70 pm sand at a phase density of 1.0 and a velocity of 26 m s-l and (b) 19 g was eroded by 70 pm sand at a phase density of 8.0 and a velocity of 22 m s-l.
be confused with the stepped bend erosion pattern which appears to be characteristic of erosion by fine abrasive particles. The surface ripples are very much more regular in formation and are considerably finer in structure. They were noticed on the surface of the bends eroded by the 230 pm sand. Figure 10(a) shows a typical area from the bend shown in Figs. 4(a), 7 and 8, taken from an area at a bend angle of about 45”. Surface ripples were found on all the bends eroded by the 230 pm sand, over the whole range of velocities and phase densities. They were generally found over a wide area on the flanks of the main erosion centres. No such ripples were found on any of the bends eroded by the 70 pm sand, irrespective of phase density. Figure 10(b) shows a typical area of surface eroded by the 70 pm sand. Wood and Espenschade [8] observed a series of ripples, running at right angles to the direction of the air flow, on the scrolls and nozzle vanes of small gas turbines, which resulted from dust ingestion. Sheldon and Finnie [ll] presented a series of photographs of rippled surfaces obtained with 127 and 9 pm particles impacted at an angle of 30” onto
322
Fig. 10. Bend surface photographs of a surface eroded by (a) 230 pm sand and (b) 70 pm sand, where the direction of flow is from the top to the bottom of the photographs (magnification 5X).
surfaces at a velocity of 150 m s-i. Finnie and Kabil [ 131 developed theories which considered that the ripple wavelength was approximately multiple (about six) of the length of the cut taken by a single particle.
a
4.2.2. Position of maximum wear Bikbaev et al, [14] tested bends at a number of different air velocities and phase densities and found that the location of the zone of maximum wear was independent of both of these parameters. If two of the bends which failed are compared, as in Figs. 5 and 7, it is seen that there is a difference in the bend angle of about 6” between them. The centre of the hole in the bend eroded by the 70 pm sand is at about 32” and that for the 230 pm sand is at about 38”. A similar difference is also noted for the points of maximum wear for the two bends compared in Figs. 11 and 13. This is perhaps a little surprising as one would expect the larger and heavier particles to follow a straighter path from the preceding straight pipe than that of the smaller particles and to impact on the wall at a lower bend angle. A scale drawing of the approach section of one of these bends is given in Fig. 12. Bend angles are shown and the position of maximum wear for the 70 pm and 230 I.trn sand particles are indicated. The position for maximum wear with the 70 I.trn sand almost coincides with the extension of the centre line of the preceding straight pipe to the bend surface. In terms of impact angle, bench tests have shown that there is no change in the characteristic erosion curve with respect to particle size for ductile erosion [ll].With the formation of steps in the surface by the smaller particles, however, the impact angle rapidly changes.
323
Bend
II
angle
1Flow
dwactmn
Fig. 11. Bend wear profiles of a bend that was eroded by 70 pm sand at a phase density of 1.0 and velocity of 26 m 6-l. Fig. 12. The approach section to the bends tested: - - - the wear profile for 230 pm sand. The positions of maximum wear for 230 pm and 70 pm sand are indicated by arrows.
5. Depth of penetration
of particles
There appears to be little difference in the rate of erosion caused by different sized particles, in terms of mass eroded for a given mass of particles conveyed. However, it is obvious that for pipe bends different sized particles erode in completely different ways. There is a possibility therefore that the depth of penetration of the particles into the bend wall may vary with the particle size. For pipe bends it is really the depth of wear that is the important factor, as this determines the service life of a bend. The mass eroded will therefore be of secondary importance, if for a given mass eroded the depth of penetration by the particles is influenced to any degree by other factors. This analysis was prompted initially by the difference in the mass eroded from the bends at the time of failure for the two sizes of sand used, as recorded in Fig. 1. Although they were carried out additionally at different velocities and phase densities it was apparent from the wear profiles recorded that there must be some basic difference in the mechanism of the erosion process. As a result a number of the bends that had been eroded in the two programmes were examined in closer detail to see if any trends could be detected with regard to the depth of penetration by the particles. The bends were examined after being cut in half and the wear profiles were obtained by using a micrometer gauge.
324
5.1. Review of previous work Relatively little work has been carried out and reported on this aspect of erosive wear, whether with pipe bends or with flat plates tested in erosion rigs. Mason and coworkers [12, 151, Brauer and Kriegel [lo, 16 - 181 and Krasnov and coworkers [14, 19, 201 are probably the only research groups to have considered this in any detail. This is surprising because the depth of penetration by particles is obviously an important aspect of erosive wear. Apart from pipe bends in pneumatic conveying lines there must be many other engineering situations in which failure by erosion will be primarily dependent upon the depth of wear rather than upon the mass of material eroded. Mason and Smith [12] carried out work with square section Perspex bends. They expressed the erosive wear rate in terms of the mass of particles conveyed per unit depth eroded. They investigated the effects of velocity and phase density, but with just one particle size. Brauer and Kriegel have shown how depth of wear varies with time and impact angle for a number of situations. They have not, however, investigated the effect of particle size in this respect. Krasnov and coworkers have also carried out a wide range of experiments and in their case the wear rate was expressed in mm h-l. Their experiments were also confined to the use of a single powder: sand with a mean particle size of 295 pm. 5.2. Bend wear results In order to investigate the influence of particle size on the depth of wear for the bend surfaces, comparable bends from the two programmes of tests, eroded by sand with mean particle sizes of 70 pm and 230 pm, were inspected. Two of the requirements for comparability were that the tests should have been carried out at the same velocity and phase density. It was also necessary that the same mass of material should have been eroded from them. This was decided upon as the additional requirement for comparison, in view of the fact that little difference was detected in the specific erosion values for the two sizes of sand used. Once again this combination of requirements reduced the potential number of bends available to relatively few. Those used as a basis for comparison of the particle size effect are presented in Table 2. A direct comparison can be made at a phase density of 1.0 and velocity with each bend having lost 36 g by erosion. The depth of bend of26ms‘-’ wear by the 230 pm sand was 1.25 mm but that by the 70 pm sand was 1.84 mm, which represents an increase in depth of about 45% for the 70 ,um sand. If this rate of penetration were to continue, a bend conveying the 70 pm sand would fail long before a bend conveying the 230 pm sand. Surface wear profiles are presented for both of these bends in Figs. 11 and 13. As only 35 g was eroded only one pipe wall cross section is included with the outer wall surface profile and this corresponds approximately to the point of maximum wear.
325 TABLE 2 Comparison of depth of penetration by erosion for 70 /.fm and 230 pm sand Phase density
1.0
2.0 4.0
Mean air velocity (m s-l)
26 26 25
Depth of penetration (mm)
Mass eroded (g) 70pm
230 pm
70 pm
230 pm
36 20 20
36 36 19
1.84 1.33 1.40
1.25 1.43 1.02
Fig. 13. Bend wear profiles of a bend that was eroded by 230 pm sand at a phase density of 1.0 and velocity of 26 m s-l.
At a phase density of 4.0 another direct comparison can be made. There is a slight difference in the mass eroded, it being 20 g for the 70 I.tm sand and 19 g for the 230 pm sand, but this difference is relatively small. In this case the increase in depth of erosion by the 70 pm sand is about 40%. No direct comparison could be made at a phase density of 2.0 but instead the results of two bends are included for which the depth of wear was about the same. In this case the masses eroded can be compared and this shows that some 60 or 70% more sand of 230 pm mean particle size can be conveyed before the depth of wear reaches that for the 70 km sand. These results are by no means conclusive, as there are relatively few available for analysis, but they all show the same basic trend, i.e. a considerable increase in depth of wear per unit mass of sand conveyed for the smaller particle size. Apart from the fact that the comparison was based on a single conveying air velocity and over a small phase density range, only two particle sizes were considered. This is obviously an area that warrants further consideration and comparative tests using at least four different particle sizes would seem an obvious starting point.
326
5.3. Impact angle effects With reference to Fig. 12 and the effect of impact angle on the rate of erosive wear, the analysis presented in Section 4.1 was with respect to the variation of specific erosion rate with impact angle. This is the usual form of presentation of this information from such work with erosion rigs. It does not, however, explain the differences in terms of the rate of penetration. For this purpose bench tests carried out by Brauer and Kriegel [ 10 J and by Krasnov and Zhilinskii 1201, in which test plates were eroded at varying angles of attack, are useful. Brauer and Kriegel recorded the depth of wear for a given time and Krasnov and Zhilinskii recorded the time required to hole the plate. When they were plotted against impact angle these showed that the mild steel which Brauer and Kriegel tested wore at a maximum rate when the impact angle was between 45” and 55” and the equivalent angle for that tested by Krasnov and Zhilinskii was about 60”. These are very much different from the impact angle of about 20” for maximum wear in terms of volume eroded. In terms of bend wear both of these features might have to be taken into account. For the erosion of a new bend the surface is obviously at a low impact angle in relation to the particles and the wear, in terms of mass eroded, will be relatively high. This could be referred to as primary wear, as it will take place over a wide bend angle range. The impact angle will change gradually in the region of maximum wear and will at some time reach that for maximum penetration. This will take place over a relatively narrow range of bend angle and could be referred to as secondary wear. Primary wear will continue over the rest of the bend but it will probably be at a decreasing rate as the effective impact angle is reduced by the erosion. With a normal bend secondary wear will probably cause rapid failure once established. For a reinforced bend, or a bend with a very thick wall, secondary wear will phase itself out, for the impact angle will increase beyond that for maximum penetration to something of the order of 80” or 90”, at which both the volumetric erosion rate and penetration rate are considerably reduced. This is the tertiary stage and little further erosion will take place once established. It will change the flow path of the particles in the bend and may lead to erosion at other sections of the bend, as pointed out by Mason and Smith ]I21 * This theory, in conjunction with the wear profiles observed, could help to explain why the rate of penetration is lower for the larger particles. With the small particles steps in the surface were formed and these were particularly pronounced immediately beyond the point of maximum wear. For the larger particles there was no abrupt transition and with respect to the particles impacting there is little change in impact angle. This can be seen in Fig. 12 by comparing the relative angles of the original pipe wall surface and the final surface profile for the bend which failed. For the larger particles therefore the stage of secondary erosion is probably not reached at all in these bends and hence the rate of penetration is much slower. In order to check this theory it would be interesting to test bends either with large wall
327
thicknesses or to introduce an artificial step and to observe the effect with large abrasive particles. The wider path cut by the larger particles can also be explained in terms of the absence of steps. The steps formed in the bend surface with the 70 pm sand tended to be quite narrow, being about 2 or 3 mm wide on average. This would result in the secondary erosion’s being confined to narrow paths and hence the formation of deep grooves rather than the more uniform but shallower wear with the 230 pm sand. The formation of steps in the bend wall surface with the 70 vrn sand probably accounts also for the position of maximum wear’s being at a smaller bend angle. 6. Discussion
and conclusions
In terms of the mass eroded per unit mass conveyed, or the specific erosion, the particle size above a given minimum value may not be a particularly significant parameter in the erosion process. For the erosive wear of bends, however, it is obviously a variable that will have to be taken into account. Apart from the totally different appearances of the bend wall surfaces that were observed with the two particle sizes tested, the depth of the penetration was found to be very much greater with the 70 pm sand than that with the 230 pm sand. A basic industrial requirement with regard to bends in pneumatic conveying systems is a knowledge of life expectancy. This is dependent upon the time taken for the conveyed product to penetrate the bend wall. In this respect the rate of penetration of particles into surfaces should be the dependent variable in problems of this nature rather than the mass eroded or the specific erosion. An explanation is offered for the differing rates and patterns of penetration in terms of three basic stages of erosion which are dependent upon impact angle. These relate to the maximum for volume removal at about 20”, the maximum for depth of wear at about 55” and a minimum for both that occurs with normal impact. These relate to ductile materials. The observations, however, are based upon only two programmes of tests, one with sand having a mean particle size of about 70 pm and the other with sand of 230 pm mean particle size. Apart from the difference in penetration rate, the striking difference between the eroded surfaces makes further investigation essential and the next programme of tests will be specifically devoted to an examination of the effect of particle size. One of the interesting facts is that no steps were formed on any of the bends eroded by the 230 pm sand and that no ripples were observed on any of the bends eroded by the 70 pm sand. There must be an overlap or transition from one to the other at some intermediate particle size. From results of bench tests specific erosion has been shown to decrease with particles below 70 pm and to remain constant for particles above 230 pm. An entirely different curve for erosion with respect to particle size can confidently be expected for the erosion of pipe bends, with erosion expressed in terms of depth of penetration.
328
Acknowledgments The authors gratefully acknowledge the assistance given by Mr. R. B. Stacey, Manager, Pneumatic Conveying Division, Mucon Engineering Company Limited, and by Mr. W. S. Churchill, Chief Laboratory Technician, and his staff at Thames Polytechnic on a number of aspects relating to both the experimental rig and the programmes of work.
References 1 D. Mills and J. S. Mason, Pneumotransport 3, Paper A7, BHRA Conference, Bath, April 1976. 2 D. Mills and J. S. Mason, Int. Powder and Bulk Solids Handling and Processing Conf., Chicago, III., May 19’76. 3 D. Mills and J. S. Mason, 1st Int. Conf. on Protection of Pipes, Paper Gl, BHRA Conference, Durham, Sept. 1975. 4 J. E. Goodwin, W. Sage and G. P. Tilly, Proc. Inst. Mech. Eng., London, 184 (1969 70) 279 - 292. 5 W. Sage and G. P. Tilly, Aeronaut. J., 79 (1969) 427 - 428. 6 G. P. Tilly, Wear, 14 (1969) 241 - 248. 7 I. Kleis, Wear, 13 (1969) 199 - 215. 8 C. D. Wood and P. W. Espenschade, Sot. Auto. Engrs., Summer Mtg., Chicago, Ill., Preprint 880a, 1964. 9 W. J. Head and M. E. Harr, Wear, 15 (1970) 1 - 46. 10 H. Brauer and E. Kriegel, Chem. Ing. Tech., 35 (1963) 697 - 707. 11 G. L. Sheldon and I. Finnie, Trans. ASME., Ser. B, 88 (1966) 387 - 392. 12 J. S. Mason and B. V. Smith, Powder Technol., 6 (1972) 323 - 335. 13 I. Finnie and Y. H. Kabil, Wear, 8 (1965) 60 - 69. 14 F. A. Bikbaev, V. I. Krasnov, M. Z. Maksimenko, V. L. Berezin, I. B. Zhilinskii and N. 1‘. Otroshko, Chem. Pet. Eng. (USSR), 9 (1973) 73 - 75. 15 J. S. Mason, P. A. Arundel, I. A. Taylor, W. Dean and T. E. Doran, Pneumotransport 2, BHRA Conference, Paper El, Guildford, Sept. 1973. 16 H. Brauer and E. Kriegel, Stahl Eisen, 84 (1964) 1313 - 1322. 17 H. Brauer and E. Kriegel, Maschinenmarkt (Wiirzburg, Ger.), 71 (1965) 20 - 31. 18 H. Brauer and E. Kriegel, Chem. Ing. Tech., 37 (1965) 265 - 276. 19 F. A. Bikbaev, M. Z. Maksimenko, V. L. Berezin, V. I. Krasnov and I. B. Zhilinskii, Chem. Pet. Eng. (USSR), 8 (1972) 465 - 466. 20 V. I. Krasnov and I. B. Zhilinskii, Chem. Pet. Eng. (USSR), 9 (1973) 1029 - 1032.
PARTICLE
SIZE EFFECTS
311
IN BEND EROSION
D. MILLS and J. S. MASON Department
of Mechanical Engineering,
Thames Polytechnic,
London S. E. 18 (Gt. Britain)
(Received October 26, 1976)
Summary From work on the erosion of flat plates in test rigs of the sand blast type published in recent years it has generally been concluded that there is a critical particle size above which erosion is not influenced by size. Recent work on the erosion of pipe bends by sand in a pneumatic conveying line tends to confirm this theory. For pipe bends, however, it was observed that particle size has two additional effects of major significance: the appearances of the eroded surfaces are totally different and the depth of penetration by erosion is much greater for smaller particles; this means that pipe bends will fail much earlier if smaller particles are conveyed.
1. Introduction In the last year two major series of tests have been undertaken on the erosive wear of pipe bends. These have been carried out using a full scale pneumatic conveying rig in the Powder Handling Laboratory at Thames Polytechnic. The first was an investigation into the effect of phase density or particle concentration, and the second was on the effect of conveying velocity. In both programmes sand was pneumatically conveyed through the bends and about 40 bends were tested in each series of tests. They were all 90” bends, of 50 mm bore and 140 mm bend radius; they were tested in the horizontal plane. For the investigation into the effect of particle concentration, sand with a mean particle size of about 70 pm was used. Tests at phase densities ranging from 0.5 to 8.0 were carried out and the velocity was held constant at about 25 m s- ’ for each test. The results and analysis of this work are presented in ref. 1. In investigating the effect of velocity, sand with a mean particle size of about 230 pm was used. Tests were carried out at phase densities of 1.0, 2.0 and 4.0, and the conveying velocities ranged from about 15 to 32 m s- ‘. The results and analysis of this work are presented in ref. 2.
312
1.1. Wear patterns A characteristic feature of the erosion of the bends by the sand with a mean particle size of 70 pm was that the erosion of the outer bend wall surface material was very irregular. Beyond a bend angle of about 30” the surface was worn in a complex pattern of steps. Some of the steps were quite pronounced, presenting vertical faces of about 2 mm in height to the oncoming flow. In most of the bends the steps continued right round to a bend angle of about 90”. Between bend angles of about 35” and 45” there were often as many as 20 such small steps or ridges clearly visible. These were all concentrated over a small area of the outer perimeter of the bend. This wear pattern was observed on bends tested at every phase density from 0.5 to 8.0. Typical photographs of the bend wear caused by this sand are shown in Figs. 4(b) and 9, and typical bend wear profiles are shown in Figs. 5,6 and 11. The pattern of surface erosion on the bends caused by the 230 pm sand, however, was entirely different. No abrupt steps or ridges were produced at any stage in the erosion process even up to the point of bend failure. Even beyond a bend angle of 30” the erosion left a smooth surface and no steps or ridges were produced at all. This was a common feature of all the bends tested with this sand, regardless of both the phase density and the velocity of the suspension. A typical photograph of the bend wear caused by this sand is shown in Fig. 4(a) and typical bend wear profiles are shown in Figs. 7, 8 and 13. 1.2. Mass eroded Apart from the totally different appearance of the bend wall surfaces for the two particle sizes, there was also a marked difference in the mass of metal removed from the bends when failure occurred. With the 70 pm sand two of the bends tested at a phase density of 3.0 and velocity of about 25 m s-l failed. The masses of metal removed from these bends at the time of failure were about 42 g and 43 g. With the 230 pm sand another two bends failed, this time at a phase density of 1.0 and velocity of about 32 m s- ‘. In this case the masses of metal removed from these bends at the time of failure were about 104 g and 112 g. Figure 1 shows the variation of the mass of metal eroded with mass of sand conveyed for these four bends. There can be no direct comparison between these two sets of bends, as they refer to different conveying velocities and phase densities, as well as different particle sizes. However, with such a large difference in the mass eroded for failure and the fact that the erosion patterns appear to be characterised by the particle size, it was felt that particle size was the dominant factor in the process. It was therefore decided to examine some of the bends that were tested at the same phase density and velocity in order to see whether the rate of penetration by the finer sand was more rapid. A closer look was also taken at the erosive wear results to see whether particle size caused any difference in the rate of erosion, in terms of mass of metal eroded per unit mass of sand conveyed, in the case of pipe bends.
313 120 P
J
u
4
L
Mass conveyed
0
0
tonne
Fig. 1. The erosion history of bends that failed: -LI- and --cP- represent erosion by 230 ,um sand of phase density 1 at a velocity of 32 m s-l ; --o-- and -+-- represent erosion by 70 pm sand of phase density 3 at a velocity of 26 m s-l.
2. Experimental
rig
In order to carry out realistic tests appropriate to industrial pneumatic conveying situations, a full scale rig was built. The rig consists of a blow tank having a capacity of about 1 m3, a similar sized hopper mounted on three load cells vertically above and a Roots-type blower capable of delivering about 0.07 m3 s-l at a pressure of 2 bar absolute. A diagrammatic layout of the conveying plant and test loops is shown in Fig. 2. A full description of the rig, together with further sketches, has been given [l] . This included details of the test loops, plant operation, control and inst~mentation, and ch~ge/disch~ge facilities. Briefly it is a batch-type system with the powder being conveyed from the blow tank, through the test loops and into the receiving hopper. The test section consists of two horizontal loops in a rectangular configuration with a 90” bend at each corner. Each loop has two 8.2 m pipe runs, giving a total length of 27.2 m for each loop. A diverter valve in the test loops allows one loop to be bypassed and enables sampling of the powder to be carried out whilst it is actually being conveyed. The bends tested in the two programmes were all mild steel of 140 mm bend radius and 50 mm bore. The sand used in the programme of tests to
314 r----c /Filter
1
Fig. 2. The test loops and conveying
plant layout.
investigate the effect of phase density on erosion had a mean particle size of 70 pm, with approximately 10% greater than 180 pm and 10% less than 40 I.tm. For the investigation into the effect of velocity on erosion, sand with a mean particle size of 230 pm was used and this had approximately 10% greater than 360 pm and 10% less than 160 ,um. In the following section a review of previous work on the effects of particle size on erosion rate is presented. In all the cases cited this work has been carried out with bench-type test rigs and for the majority of these the velocity of the particles has been measured. In the work presented here on actual pipe bends, however, it is the velocity of the air that has been recorded. In all cases the pipe bends followed sufficiently long straight runs of pipe to ensure that even the largest particles were accelerated to their terminal velocity and it was felt that results and information based on these parameters would be of more practical use in industrial situations. 3. Erosion rate The erosive wear rate of materials is usually expressed in specific terms, be. the amount eroded per unit mass of particles conveyed. When comparing the performance of different surface materials the erosion is normally expressed in volumetric terms to give cm3 kg- ’ and when comparing similar materials it is generally expressed on a mass basis such as g kg- ‘. In erosive wear testing to investigate the effect of particle size there are two specific problem areas with regard to the particles themselves. One is the difficulty of specifying a characteristic particle size for a given mass of powder. The other is the fund~en~ problem of either measuring the velocity of the particles or ensuring that they have been accelerated to their terminal velocity before impact. The majority of work on the erosive wear of materials has been carried out with bench-type rigs in which abrasive particles have been impacted
315
upon target materials. Whether the work has been carried out for the purpose of investigating erosion problems with gas turbine blades, boiler tubes or pneumatic conveying plant and equipment, erosion rigs have provided valuable information, The number and range of the variables involved in the problem of the erosion of surface materials by gas-solid suspension flows are such that their use has been almost essential. A review of the various types of erosion rig used in these investigations and a summary of the variables involved in the erosion process were given in ref. 3. 3.1. Review of previous work Particle size is a variable that has been investigated by a number of researchers [ 4 - 81. The investigation of Goodwin et al. [4] was confined to crushed quartz, which was sieved into narrow size ranges from 25 pm to 210 grn and tested at normal impact against an 11% Cr steel at particle velocities of approximately 130, 240 and 305 m s-l. Their resulting erosion curves showed that there was a significant size effect which was itself dependent upon impact velocity. They showed that there was a critical particle size above which erosion was not influenced by size and that this value appeared to increase linearly with velocity, being about 50 pm at and 120 pm at 305 m T’. The maximum value of the erosion of 130ms-’ the metal at a particle impact velocity of 130 m s-l was 1.3 g kg-‘. Tilly [6] reported on similar tests which showed that different materials exhibited different types of size dependence. With engineering alloys and resilient plastics there was an initial increase in erosion with particle size until the onset of a saturation plateau where it was independent of size. Resins and composite materials such as fibreglass, however, suffered a continual though varying increase in erosion with particle size and did not exhibit the characteristic plateau for the range investigated. Sage and Tilly [5] also tested glass, which suffered a rapid increase in erosion with particle size, and found that the erosion could be represented by a power law over the size range tested. Wood and Espenschade [S] also used quartz dust but it was not sieved into narrow size ranges. They tested at an impact angle of 40” against a pearlitic steel at an air velocity of 260 m s- ‘. In comparison with the erosion of the 0 - 150 pm size range, the 0 - 74 pm was 75% of the value, the 0 - 10 pm was 35% and the 0 - 5 ,um was about 10%. Kleis [7] also used quartz sand particles and impacted them, at velocities ranging from 83 to 225 m k ‘, at 90” against steel specimens ranging in hardness from 130 to 850 kg mmw2. He showed that the erosion loss by a given mass of abrasive grains was independent of the particle size for particles greater than about 160 pm, but only for the lower velocities tested. At higher velocities an increasingly distinct maximum value of erosion was observed at the critical particle size, beyond which erosion decreased to a lower constant value. This critical particle size varied slightly with particle velocity, and both particle and surface material hardnesses. For a steel with a hardness of 130 kg mrnm2 impacted by 130 ,um particles at 83 m s-l the erosion rate was 0.25 g kg-l.
316
3.2. Bend wear results Two major programmes
of work on bend wear have been carried out at Thames Polytechnic. One was with sand having a mean particle size of 70 pm and the other was with sand of 230 I.rm mean particle size. Both programmes were carried out with single batches of sand, these being about 740 kg for the 70 pm sand and 975 kg for the 230 pm sand. Degradation of the sand, however, was quite considerable in both cases. For the programmes carried out this did not present a problem as they were comparative studies into the effects of phase density and velocity and a sequential testing plan was adopted to overcome such difficulties. The rapid change in character of the particles as a result of degradation, however, made it very difficult to compare the two sizes of sand used in terms of specific erosion rates. This not only meant that tests carried out at the same velocity and phase density had to be compared but that the sand had also to be in the same condition. For this purpose the comparison was based additionally on tests carried out with the sand conveyed through the same number of bends. This assumed that the rate of degradation and its effects were comparable for the two particle sizes. The effects of particle degradation and wear for a number of the tests that were carried out are shown in Fig. 3, which is a plot of the specific erosion rate for each indjvidu~ batch conveyed against the total number of bends through which the sand had been previously conveyed.
Number
of bends
sand conveyed through
Fig. 3. The effect of particle degradation and wear on erosion: -Q-, -o-and -G-represent erosion by 230 pm sand of phase densities 1.0, 2.0 and 4.0, respectively; * represents erosion by 70 firn sand of phase density 1.0.
317 TABLE 1 Comparison of specific erosion rates for 70 pm and 230 pm sand Mean air velocity (m s-l)
Number of bends sand was conveyed through 70 ym
230 pm
1.0
26
170
2.0
27
4.0
25
Phase density
Specific erosion (mg kg-‘) 70/~m
230pm
170
6.0
5.1
50 122
30 110
11.0 9.4
12.8 5.6
218
218
3.0
2.2
One set of results which could be compared directly was that at a phase density of 1.0 and velocity of 25 m s-I. These were some of the first tests carried out with the fresh sand and the specific erosion with the 70 pm sand was 13.1 mg kg-’ and that for the 230 pm sand was 12.8 mg kg-r. Other specific erosion values for tests that could possibly be compared are given in Table 1. No firm conclusions can be drawn on the basis of this evidence but it appears that in specific erosion terms there is little difference between the two sizes of sand. To obtain a more accurate answer to this question it would be necessary to use a fresh batch of sand for each test at each velocity and phase density and at least four different particle sizes should be employed. 4. Bend erosion patterns The marked difference in appearance in the eroded surfaces of the bends for the two particle sizes prompted further investigation. After comparing erosion rates, a number of the bends were cut in half so that accurate wear profiles could be recorded and photographs could be taken of the eroded surfaces. To a certain extent bend erosion patterns can be explained in terms of impact angle effects. 4.1. Impact angle effects The variation of erosion with impact angle has been investigated by a number of researchers [ 7 - 111. In general, erosion plots against impact angle for different materials are of two different shapes, characterised by ductile and brittle materials. For brittle materials erosion tends to increase with increasing impact angle, with maximum erosion occurring at normal impact. For ductile materials erosion tends to increase very rapidly with increasing impact angle up to about 20”) beyond which the erosion decreases gradually until under normal impact it may be only 10% of the critical value at about 20”. Theories proposed to explain these phenomena consider that for ductile materials removal of material is predominantly due to plastic deformation
318
(e.g. as with the cutting edge of a machine tool) and that for brittle materials it is due to the propagation of fracture surfaces into the material. Bend wear is usually simulated in bench tests by imparting particles at a low angle onto flat target specimens. The bend initially presents a surface at a low impact angle to the particles issuing from the preceding straight pipe run and so with a ductile material rapid erosion takes place. Gradualiy the impact angle changes to almost 90” and little further erosion takes place in this area. Mason and Smith 1121 carried out work with square section Perspex bends so that they could observe the change in wear pattern with respect to time. Their findings agree with the theory postulated above for the erosion of ductile bend materials. The bend wear patterns that they presented, however, showed that the trajectory, and hence the erosion, of the particles beyond this impact point are also important. From the bend wear profiles caused by the erosion of the sand, it is difficult to imagine how bench-type erosion rigs can hope to represent bend wear situations. It is the removal of the particles after they have been impacted onto test materials that represents the main problem with erosion rigs. In bench tests there is bound to be an interaction between the oncoming particles and those leaving the surface, the magnitude largely depending on the impact angle. The continuous motion of the particles in flowing round a bend just cannot be simulated, and hence neither can the wear beyond the initial impact point. 4.2. Bend wear results A number of photo~aphs of the bend wear surfaces and sketches of the bend wear profiles are presented. Profiles are given both of the outer wall surface in the flow direction and of pipe wall cross sections at various bend angles. They are included to show how and where the erosion has taken place and for the comparison of the erosion patterns caused by the two sizes of sand used. In the photographs the flow direction is from the bottom to the top in each case. In the wear profiles the wall thickness, the bend and the pipe radii are not drawn to the same scale. The wall thickness is drawn to a slightly exaggerated scale in order to show the erosion effects in greater detail. Dimensions, however, are included on the sketches for reference purposes. Figure 4 shows two of the bends which failed and these correspond to the erosion plots in Fig. 1. Figure 4(a) shows a photograph of the outer inside surface of the bend eroded by the 230 pm sand at a-phase density of 1.0 and velocity of about 32 m s- ’ and from which 115 g was eroded. Figure 4(b) is for the bend eroded by the 70 pm sand at a phase density of 3.0 and velocity of about 26 m s- * and from which 44 g was eroded. Wear profiles for these two bends are shown in Figs. 5 - 8. Figutes 5 and 7 show the profiles for the two outer wall surfaces and Figs. 6 and 8 each show three cross-sectional profiles at bend angle intervals of 10”. These were taken to correspond with points (1) 10” beyond the hole in the bend, (2) at the downstream edge of the hole and (3) 10” in advance of this.
319
-
-1 e
(b)
(a)
Fig. 4. Bend wear photographs showing bends in which (a) 115 g was eroded by 230 pm sand at a phase density of 1.0 and a velocity of 32 m s-l and (b) 44 g was eroded by 70 pm sand at a phase density of 3.0 and a velocity of 26 m s-l.
Iso
L3mm
Mmm .-.
@ Fig. 5. The outer bend wall wear profile of a bend that was eroded phase density of 3.0 and a velocity of 26 m s-l. Fig. 6. Pipe section
profiles
at various
bend angles
by 70 pm sand at a
for the bend shown
in Fig. 5.
320
,Eendangle
5o”
LJmm
Ml-ml .-_
.-.
O0
~ Fig. 7. The outer bend wall wear profile of a bend that was eroded phase density of 1.0 and a velocity of 32 m s-l. Fig. 8. Pipe section
wear profiles
at various
by 230 I_rrnsand at a
bend angles for the bend shown
in Fig. 7
The photo~aphs and wear profiles for these two bends show the difference in erosion caused by the two different sizes of sand used. Although conveying conditions in respect of phase density and velocity are different, these can be discounted as factors influencing the basic pattern of erosion. The stepped appearance of the eroded surface was characteristic of all the bends tested with the 70 pm sand from a phase density of 0.5 - 8.0. Photo~aphs of bends eroded by the 70 pm sand at phase densities of 1.0 and 8.0, shown in Fig. 9, confirm this. The smooth and rounded appearance of the eroded surface produced by the 230 pm sand was also characteristic of all the bends tested with this sand over a range of phase densities of 1.0 to 4.0 and velocities of 15 - 32 m s- ‘. When one compares the outer perimeter profiles for erosion by the two sand sizes in Figs. 5 and 7 it is difficult to see how 115 g of met& came to be removed from the bend by the 230 pm sand and only 44 g by the 70 I.rm sand, for, in terms of the cross-sectional area eroded, that for the 230 pm sand is only slightly greater. The answer lies with the pipe cross-sectional profiles presented in Figs. 6 and 8. These show how the finer sand has cut a much narrower path into the bend surface, affecting only a small area of the pipe wall, whereas the 230 iurn sand has worn a very much wider path. This is p~ic~~ly noticeable in the region where the bend was holed and in the region upstream from this. The profiles 10” downstream of the hole again show little difference in erosion. 4.2.2. Surface ripples The bend wear profiles for the 70 pm rand are similar to those presented by Mason and Smith [ 121. They used highly abrasive alumina powder having a mean particle diameter between 50 and 60 ym in their tests. Surface ripples are a characteristic feature of ductile erosion of materials and have been reported by several researchers [8, 11,131. However, these are not to
321
(a)
(b)
Fig. 9. Bend wear photographs showing bends in which (a) 36 g was eroded by 70 pm sand at a phase density of 1.0 and a velocity of 26 m s-l and (b) 19 g was eroded by 70 pm sand at a phase density of 8.0 and a velocity of 22 m s-l.
be confused with the stepped bend erosion pattern which appears to be characteristic of erosion by fine abrasive particles. The surface ripples are very much more regular in formation and are considerably finer in structure. They were noticed on the surface of the bends eroded by the 230 pm sand. Figure 10(a) shows a typical area from the bend shown in Figs. 4(a), 7 and 8, taken from an area at a bend angle of about 45”. Surface ripples were found on all the bends eroded by the 230 pm sand, over the whole range of velocities and phase densities. They were generally found over a wide area on the flanks of the main erosion centres. No such ripples were found on any of the bends eroded by the 70 pm sand, irrespective of phase density. Figure 10(b) shows a typical area of surface eroded by the 70 pm sand. Wood and Espenschade [8] observed a series of ripples, running at right angles to the direction of the air flow, on the scrolls and nozzle vanes of small gas turbines, which resulted from dust ingestion. Sheldon and Finnie [ll] presented a series of photographs of rippled surfaces obtained with 127 and 9 pm particles impacted at an angle of 30” onto
322
Fig. 10. Bend surface photographs of a surface eroded by (a) 230 pm sand and (b) 70 pm sand, where the direction of flow is from the top to the bottom of the photographs (magnification 5X).
surfaces at a velocity of 150 m s-i. Finnie and Kabil [ 131 developed theories which considered that the ripple wavelength was approximately multiple (about six) of the length of the cut taken by a single particle.
a
4.2.2. Position of maximum wear Bikbaev et al, [14] tested bends at a number of different air velocities and phase densities and found that the location of the zone of maximum wear was independent of both of these parameters. If two of the bends which failed are compared, as in Figs. 5 and 7, it is seen that there is a difference in the bend angle of about 6” between them. The centre of the hole in the bend eroded by the 70 pm sand is at about 32” and that for the 230 pm sand is at about 38”. A similar difference is also noted for the points of maximum wear for the two bends compared in Figs. 11 and 13. This is perhaps a little surprising as one would expect the larger and heavier particles to follow a straighter path from the preceding straight pipe than that of the smaller particles and to impact on the wall at a lower bend angle. A scale drawing of the approach section of one of these bends is given in Fig. 12. Bend angles are shown and the position of maximum wear for the 70 pm and 230 I.trn sand particles are indicated. The position for maximum wear with the 70 I.trn sand almost coincides with the extension of the centre line of the preceding straight pipe to the bend surface. In terms of impact angle, bench tests have shown that there is no change in the characteristic erosion curve with respect to particle size for ductile erosion [ll].With the formation of steps in the surface by the smaller particles, however, the impact angle rapidly changes.
323
Bend
II
angle
1Flow
dwactmn
Fig. 11. Bend wear profiles of a bend that was eroded by 70 pm sand at a phase density of 1.0 and velocity of 26 m 6-l. Fig. 12. The approach section to the bends tested: - - - the wear profile for 230 pm sand. The positions of maximum wear for 230 pm and 70 pm sand are indicated by arrows.
5. Depth of penetration
of particles
There appears to be little difference in the rate of erosion caused by different sized particles, in terms of mass eroded for a given mass of particles conveyed. However, it is obvious that for pipe bends different sized particles erode in completely different ways. There is a possibility therefore that the depth of penetration of the particles into the bend wall may vary with the particle size. For pipe bends it is really the depth of wear that is the important factor, as this determines the service life of a bend. The mass eroded will therefore be of secondary importance, if for a given mass eroded the depth of penetration by the particles is influenced to any degree by other factors. This analysis was prompted initially by the difference in the mass eroded from the bends at the time of failure for the two sizes of sand used, as recorded in Fig. 1. Although they were carried out additionally at different velocities and phase densities it was apparent from the wear profiles recorded that there must be some basic difference in the mechanism of the erosion process. As a result a number of the bends that had been eroded in the two programmes were examined in closer detail to see if any trends could be detected with regard to the depth of penetration by the particles. The bends were examined after being cut in half and the wear profiles were obtained by using a micrometer gauge.
324
5.1. Review of previous work Relatively little work has been carried out and reported on this aspect of erosive wear, whether with pipe bends or with flat plates tested in erosion rigs. Mason and coworkers [12, 151, Brauer and Kriegel [lo, 16 - 181 and Krasnov and coworkers [14, 19, 201 are probably the only research groups to have considered this in any detail. This is surprising because the depth of penetration by particles is obviously an important aspect of erosive wear. Apart from pipe bends in pneumatic conveying lines there must be many other engineering situations in which failure by erosion will be primarily dependent upon the depth of wear rather than upon the mass of material eroded. Mason and Smith [12] carried out work with square section Perspex bends. They expressed the erosive wear rate in terms of the mass of particles conveyed per unit depth eroded. They investigated the effects of velocity and phase density, but with just one particle size. Brauer and Kriegel have shown how depth of wear varies with time and impact angle for a number of situations. They have not, however, investigated the effect of particle size in this respect. Krasnov and coworkers have also carried out a wide range of experiments and in their case the wear rate was expressed in mm h-l. Their experiments were also confined to the use of a single powder: sand with a mean particle size of 295 pm. 5.2. Bend wear results In order to investigate the influence of particle size on the depth of wear for the bend surfaces, comparable bends from the two programmes of tests, eroded by sand with mean particle sizes of 70 pm and 230 pm, were inspected. Two of the requirements for comparability were that the tests should have been carried out at the same velocity and phase density. It was also necessary that the same mass of material should have been eroded from them. This was decided upon as the additional requirement for comparison, in view of the fact that little difference was detected in the specific erosion values for the two sizes of sand used. Once again this combination of requirements reduced the potential number of bends available to relatively few. Those used as a basis for comparison of the particle size effect are presented in Table 2. A direct comparison can be made at a phase density of 1.0 and velocity with each bend having lost 36 g by erosion. The depth of bend of26ms‘-’ wear by the 230 pm sand was 1.25 mm but that by the 70 pm sand was 1.84 mm, which represents an increase in depth of about 45% for the 70 ,um sand. If this rate of penetration were to continue, a bend conveying the 70 pm sand would fail long before a bend conveying the 230 pm sand. Surface wear profiles are presented for both of these bends in Figs. 11 and 13. As only 35 g was eroded only one pipe wall cross section is included with the outer wall surface profile and this corresponds approximately to the point of maximum wear.
325 TABLE 2 Comparison of depth of penetration by erosion for 70 /.fm and 230 pm sand Phase density
1.0
2.0 4.0
Mean air velocity (m s-l)
26 26 25
Depth of penetration (mm)
Mass eroded (g) 70pm
230 pm
70 pm
230 pm
36 20 20
36 36 19
1.84 1.33 1.40
1.25 1.43 1.02
Fig. 13. Bend wear profiles of a bend that was eroded by 230 pm sand at a phase density of 1.0 and velocity of 26 m s-l.
At a phase density of 4.0 another direct comparison can be made. There is a slight difference in the mass eroded, it being 20 g for the 70 I.tm sand and 19 g for the 230 pm sand, but this difference is relatively small. In this case the increase in depth of erosion by the 70 pm sand is about 40%. No direct comparison could be made at a phase density of 2.0 but instead the results of two bends are included for which the depth of wear was about the same. In this case the masses eroded can be compared and this shows that some 60 or 70% more sand of 230 pm mean particle size can be conveyed before the depth of wear reaches that for the 70 km sand. These results are by no means conclusive, as there are relatively few available for analysis, but they all show the same basic trend, i.e. a considerable increase in depth of wear per unit mass of sand conveyed for the smaller particle size. Apart from the fact that the comparison was based on a single conveying air velocity and over a small phase density range, only two particle sizes were considered. This is obviously an area that warrants further consideration and comparative tests using at least four different particle sizes would seem an obvious starting point.
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5.3. Impact angle effects With reference to Fig. 12 and the effect of impact angle on the rate of erosive wear, the analysis presented in Section 4.1 was with respect to the variation of specific erosion rate with impact angle. This is the usual form of presentation of this information from such work with erosion rigs. It does not, however, explain the differences in terms of the rate of penetration. For this purpose bench tests carried out by Brauer and Kriegel [ 10 J and by Krasnov and Zhilinskii 1201, in which test plates were eroded at varying angles of attack, are useful. Brauer and Kriegel recorded the depth of wear for a given time and Krasnov and Zhilinskii recorded the time required to hole the plate. When they were plotted against impact angle these showed that the mild steel which Brauer and Kriegel tested wore at a maximum rate when the impact angle was between 45” and 55” and the equivalent angle for that tested by Krasnov and Zhilinskii was about 60”. These are very much different from the impact angle of about 20” for maximum wear in terms of volume eroded. In terms of bend wear both of these features might have to be taken into account. For the erosion of a new bend the surface is obviously at a low impact angle in relation to the particles and the wear, in terms of mass eroded, will be relatively high. This could be referred to as primary wear, as it will take place over a wide bend angle range. The impact angle will change gradually in the region of maximum wear and will at some time reach that for maximum penetration. This will take place over a relatively narrow range of bend angle and could be referred to as secondary wear. Primary wear will continue over the rest of the bend but it will probably be at a decreasing rate as the effective impact angle is reduced by the erosion. With a normal bend secondary wear will probably cause rapid failure once established. For a reinforced bend, or a bend with a very thick wall, secondary wear will phase itself out, for the impact angle will increase beyond that for maximum penetration to something of the order of 80” or 90”, at which both the volumetric erosion rate and penetration rate are considerably reduced. This is the tertiary stage and little further erosion will take place once established. It will change the flow path of the particles in the bend and may lead to erosion at other sections of the bend, as pointed out by Mason and Smith ]I21 * This theory, in conjunction with the wear profiles observed, could help to explain why the rate of penetration is lower for the larger particles. With the small particles steps in the surface were formed and these were particularly pronounced immediately beyond the point of maximum wear. For the larger particles there was no abrupt transition and with respect to the particles impacting there is little change in impact angle. This can be seen in Fig. 12 by comparing the relative angles of the original pipe wall surface and the final surface profile for the bend which failed. For the larger particles therefore the stage of secondary erosion is probably not reached at all in these bends and hence the rate of penetration is much slower. In order to check this theory it would be interesting to test bends either with large wall
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thicknesses or to introduce an artificial step and to observe the effect with large abrasive particles. The wider path cut by the larger particles can also be explained in terms of the absence of steps. The steps formed in the bend surface with the 70 pm sand tended to be quite narrow, being about 2 or 3 mm wide on average. This would result in the secondary erosion’s being confined to narrow paths and hence the formation of deep grooves rather than the more uniform but shallower wear with the 230 pm sand. The formation of steps in the bend wall surface with the 70 vrn sand probably accounts also for the position of maximum wear’s being at a smaller bend angle. 6. Discussion
and conclusions
In terms of the mass eroded per unit mass conveyed, or the specific erosion, the particle size above a given minimum value may not be a particularly significant parameter in the erosion process. For the erosive wear of bends, however, it is obviously a variable that will have to be taken into account. Apart from the totally different appearances of the bend wall surfaces that were observed with the two particle sizes tested, the depth of the penetration was found to be very much greater with the 70 pm sand than that with the 230 pm sand. A basic industrial requirement with regard to bends in pneumatic conveying systems is a knowledge of life expectancy. This is dependent upon the time taken for the conveyed product to penetrate the bend wall. In this respect the rate of penetration of particles into surfaces should be the dependent variable in problems of this nature rather than the mass eroded or the specific erosion. An explanation is offered for the differing rates and patterns of penetration in terms of three basic stages of erosion which are dependent upon impact angle. These relate to the maximum for volume removal at about 20”, the maximum for depth of wear at about 55” and a minimum for both that occurs with normal impact. These relate to ductile materials. The observations, however, are based upon only two programmes of tests, one with sand having a mean particle size of about 70 pm and the other with sand of 230 pm mean particle size. Apart from the difference in penetration rate, the striking difference between the eroded surfaces makes further investigation essential and the next programme of tests will be specifically devoted to an examination of the effect of particle size. One of the interesting facts is that no steps were formed on any of the bends eroded by the 230 pm sand and that no ripples were observed on any of the bends eroded by the 70 pm sand. There must be an overlap or transition from one to the other at some intermediate particle size. From results of bench tests specific erosion has been shown to decrease with particles below 70 pm and to remain constant for particles above 230 pm. An entirely different curve for erosion with respect to particle size can confidently be expected for the erosion of pipe bends, with erosion expressed in terms of depth of penetration.
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Acknowledgments The authors gratefully acknowledge the assistance given by Mr. R. B. Stacey, Manager, Pneumatic Conveying Division, Mucon Engineering Company Limited, and by Mr. W. S. Churchill, Chief Laboratory Technician, and his staff at Thames Polytechnic on a number of aspects relating to both the experimental rig and the programmes of work.
References 1 D. Mills and J. S. Mason, Pneumotransport 3, Paper A7, BHRA Conference, Bath, April 1976. 2 D. Mills and J. S. Mason, Int. Powder and Bulk Solids Handling and Processing Conf., Chicago, III., May 19’76. 3 D. Mills and J. S. Mason, 1st Int. Conf. on Protection of Pipes, Paper Gl, BHRA Conference, Durham, Sept. 1975. 4 J. E. Goodwin, W. Sage and G. P. Tilly, Proc. Inst. Mech. Eng., London, 184 (1969 70) 279 - 292. 5 W. Sage and G. P. Tilly, Aeronaut. J., 79 (1969) 427 - 428. 6 G. P. Tilly, Wear, 14 (1969) 241 - 248. 7 I. Kleis, Wear, 13 (1969) 199 - 215. 8 C. D. Wood and P. W. Espenschade, Sot. Auto. Engrs., Summer Mtg., Chicago, Ill., Preprint 880a, 1964. 9 W. J. Head and M. E. Harr, Wear, 15 (1970) 1 - 46. 10 H. Brauer and E. Kriegel, Chem. Ing. Tech., 35 (1963) 697 - 707. 11 G. L. Sheldon and I. Finnie, Trans. ASME., Ser. B, 88 (1966) 387 - 392. 12 J. S. Mason and B. V. Smith, Powder Technol., 6 (1972) 323 - 335. 13 I. Finnie and Y. H. Kabil, Wear, 8 (1965) 60 - 69. 14 F. A. Bikbaev, V. I. Krasnov, M. Z. Maksimenko, V. L. Berezin, I. B. Zhilinskii and N. 1‘. Otroshko, Chem. Pet. Eng. (USSR), 9 (1973) 73 - 75. 15 J. S. Mason, P. A. Arundel, I. A. Taylor, W. Dean and T. E. Doran, Pneumotransport 2, BHRA Conference, Paper El, Guildford, Sept. 1973. 16 H. Brauer and E. Kriegel, Stahl Eisen, 84 (1964) 1313 - 1322. 17 H. Brauer and E. Kriegel, Maschinenmarkt (Wiirzburg, Ger.), 71 (1965) 20 - 31. 18 H. Brauer and E. Kriegel, Chem. Ing. Tech., 37 (1965) 265 - 276. 19 F. A. Bikbaev, M. Z. Maksimenko, V. L. Berezin, V. I. Krasnov and I. B. Zhilinskii, Chem. Pet. Eng. (USSR), 8 (1972) 465 - 466. 20 V. I. Krasnov and I. B. Zhilinskii, Chem. Pet. Eng. (USSR), 9 (1973) 1029 - 1032.