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Particle concentration effects in bend erosion
Powder Technology. 17 (1977) 3i - 53 0 Elsevier Sequoia S-A., Lausanne - Printed in the Netherlands
Particle Concentration
D. MILLS
in Bend Erosion
and .I_ S. MASON
Department (Received
Effects
of Mechanical
Engineering.
Thames Polytechnic.
Wellington Street, London
SEIS
iiPF (Gt. Britain)
August 23, 1976)
SUMMARY From
37
ranging from 0.5 to S-0 were carried out and work
on the erosion
of flat
plates
in
sand blast type test rigs it has generally been concluded that the effects of particle concentration are very small. The erosion of surfaces, in terms of the mass of material eroded from the surface per unit mass of abrasive particles impacted upon the surface, has been shown to decrease slightly with increase in particle concentration_ Recent work on the erosion of pipe bends by sand in a pneumatic conveying line would tend to confirm this theory. For pipe bends, however, it was observed that particle concentration over the range investigated has two additional effects of major significance; the appearance of the eroded surfaces differ, and the depth of penetration of the particles into the bend surface material increases quite considerably with increase in particle concentration_ The implications of this in terms of the potential conveying capacity and life of bends in pneumatic conveying systems is considered.
1. INTRODUCTION During 1975 a major series of tests was carried out on the erosive wear of pipe bends. The investigation was into the effect of phase density, or particle concentration, on the erosive wear of the bends. The tests were carried out on a full scale pneumatic conveying rig in the Powder Handling Laboratory at Thames Polytechnic_ Sand with a mean particle size of about 70 pm was conveyed pneumatically through the bends, and over forty bends were tested iu the series of tests. They were all 90” bends, of 50 mm bore,
140 mm bend radius and were tested in the horizontal plane. Tests at phase densities
the velocity was held constant at about 25 m/s for each test. The results and analysis of this work were presented in [l] _ Early in 1976 another major series of tests was carried out on the erosive wear of pipe bends. This was primarily an investigation into tests
the
effect
of panicle
velocity,
although
were carried out over a small range of
phase densities_ The bends tested were identical to those in the first programme, and they were eroded once again by a pneumatically conveyed suspension of sand. The results and analysis of this work were presented in [2] _ Apart from the first test series being an investigation into the effects of phase density and the second into effects of velocity, sands of different mean particle sizes were used in the two programmes. For the first test series the sand had a mean particle size of about 70 pm, and for the second test series it was about 230 pm. X comparison of the results of the two test series in terms of the particle size effects was presented in [3] _ One part of this analysis was a comparison on the basis of depth of wear, and this produced a rather surprising result_ Although on the basis of mass eroded there was very little difference in the erosion of the bends by the two sizes of sand used, in terms of depth of wear the erosion by the 70 pm sand was over 40% greater than that of the 230 pm sand. It was the difference in appearance between the bend surfaces, eroded by the different sizes of send, that prompted the authors to investigate that aspect further. A difference in the appearance of the eroded surfaces was also noted with the bends tested to investigate the effect of phase density in the first major series of tests. Although on a basis of mass eroded there was a small
38
decrease in erosion of the bends eroded by the 70 pm sand as the phase density increased, this trend could easily be over-ridden if there was any marked change in the rate of penetration of the particles into the bend wall surface with respect to phase density. For pipe bends, it is really the depth of penetration of the particles into the bend wall which is the important factor, as this determines the service life of a bend. Mass eroded could, therefore, be of secondary importance if, for a given mass eroded, the depth of penetration by the particle is influenced to any degree by ether factors In a previous paper [ 33 the authors showed that mass eroded and depth of wear were not related when the variable considered was particle size. In this paper the variable under investigation is phase density, and a similar analysis is presented here to show whether this has any influence on the depth of wear. Industrial data has been presented [4] which would indicate that phase density has a considerable effect in this respect. Little data and few results, however, have been presented to support this and so this paper will, in part, attempt to rectify the situationAt the time of writing this paper a third major series of tests on the erosion of pipe bends was commencing. This was an investigation into the effect of particle size on the erosive wear of the bends. Sand was the powder being pneumatically conveyed once again, and the bends being eroded were identical to those of the previous test progmmmes. Sand with a mean particle size of 70 pm was first to be used, and as this was identical to that used in the fit programme of tests, the results obtained with the 70 pm sand from this third programme are incorporated in this paper. For the purpose of investigating the effect of particle size, tests were to be carried out at a phase density of 2.0 and a velocity of about 25 m/s_ In order to provide more information for this present paper, tests were carried out additionally at phase densities of 4.0 and 6-O with this particular sand_
2. REVIEW
OF PREVIOUS
WORK
As mentioned earlier, the first major programme of work carried out by the authors was an investigation into the effect
of
phase density on the erosive wear of pipe bends. The work, however, was concerned only with the effect of the mass eroded from the bends. The results and analysis were given in [1] _ This also included a review of much of the previous research work in this aspect of erosive wear and so it will not be considered any further here, as the main concern of this paper is depth of wear rather than mass or volume eroded. Relatively little work, however, has been reported on the effect of phase density on the depth of penetration of particles into surface materials, whether in relation to pipe bends or with flat plates tested in erosion rigs. Mason and co-workers [4, 53, Brauer and Kriegel [6,9] and Krasnov and coworkers [lo - 121 are probably the only research groups to have considered the effect of depth of wear in any detail, whether in respect of phase density or any other variable_ From the work of Brauer and Kriegel [6] and Krasnov and Zhilinskii 1121, it has been shown that the critical impact angle for maximum wear, in terms of depth of penetration, is about 55”_ The equivalent angle for maximum wear, in terms of volume removed, is about 20”. The vast majority of the work on the erosive wear of surface materials by abrasive particles, however, has been carried out on a basis of mass or volume removed, and little account has been taken of depth of wear. This is rather surprising, as 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 on the mass of material eroded. Mason and Smith [S] carried out work with square section perspex bends so that they could observe the change in particle flow and wear patterns with respect to time. They expressed the erosive wear rate in terms of the mass of particles conveyed per unit depth eroded (lb./in. in their case). Their ranges of velocities and phase densities were rather limited, but their analysis did show that phase density should be considered, in ad&ion to velocity, when evaluating bend erosion- They also developed equations based on their results, and from these predicted that a critical minimum should exist with respect to phase density_ IndustriaI data on bend wear was
39
included in the paper by hlason ef al_ 141, in which the wear rate was espressed in terms of
depth of wear per unit mass conveyed. This included a plot of erosive wear against phase density, which indicated a distinct maximum value of erosion. As the units of erosive wear rate are proportional to the reciprocal of those employed by hlason and Smith 153 T this would tend to confirm their predictions. Brauer and Kriegel have carried out a considerable amount of work on the erosive wear of materials, with much of it being theoretically based. They have worked on pipe bends [‘i - 9] as well as using erosion rigs [S] , and have considered both hydraulic [7 - 91 and pneumatic transport situations 16, 8, 9]_ In a blast nozzle erosion rig they impacted steel grit at a velocity of 77 m/s against a range of plastics, metals and glass. They reported on the formation of wear troughs in the materials and presented data which showed how the depth of wear varied with time and impact angle. For work on bend wear, the variation in wall thickness per unit time was employed as a measure of the wear. Most of their work, however, has been concerned with the evaluation of track curves in bends of different curvature and for various velocities so that impact angles could be evaluated and wear points predicted_ Krasnov and co-workers carried out tests on bends in a pneumatic conveying loop [ 10,111 and on test plates with an erosion rig [12]_ The bends were tested downstream of a 5.2 m long vertical pipe of 50 mm bore, and the erosion rate was expressed in mm/h. Bikbaev et al. [lo] investigated the effect of bend radius on erosive wear, with air at a velocity of 50 m/s and the suspension at a phase density of 2.8. Bikbaev et al_ [ll] investigated the effect of phase density on erosive wear, for bends with a radius of 380 mm and at an air velocity of about 55 m/s. Their tests were carried out at phase densities of 0.57,1.39, 2.10 and 2.33 and their results showed a considerable increase in depth of wear with phase density. With erosion rate on a time basis, however, this is what one would expect with varying phase density, for the mass conveyed is not constant_ This is not a suitable basis for comparing actual particle effects in the erosion process, and makes comparisons with other work very difficult. They also tested at a ncmber of different air velocities and
they found that the location of the zone of maximum wear was independent of both phase density and velocity.
3. EXPERIMENTXL
RIG
In order to carry out realistic tests, appropriate to industrial pneumatic conveying situations, a full scale rig was built which consists of a blow tank, having a capacity of about 1 m3; a similar sized hopper mounted on load cells vertically above; and a Roots type blower capable of delivering about 0.07 ms/s at a pressure of 2 bar absolute. -4 diagrammatic layout of the ccjnveying plant is shown in Fig. 1. X full description of the rig, together with further sketches, was given in a previous paper by the authors [l] . This also included details of the test loops, plant operation, control and instrumentation, and charge/discharge 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_ Fine control of both phase density and velocity is pcssible, and start-up and shutdown transient effects are limited to a matter of seconds_ The test section consists of two horizontal loops in a rectangular configuration with a 90” bend at each corner_ Each loop has two S.2 m pipe runs and two 5.4 m pipe runs, giving a total length of 27.2 m for each loop. X diverter valve in the test loops allows one loop to be by-passed, which makes it possible to convey at much higher phase densities. It also enables sampling of the powder to be carried out while it is actually being conveyedThe bends tested in the two pro,mmmes reported here were all mild steel, of 140 mm bend radius and 50 mm bore. For tests at phase densities up to 4.0 both test loops were used, and a total of six bends were incorporated in the two loops for each test. For phase densities above 4.0 and up to 8.0 just one loop was used- with three bends in the loop for testing. The sand used in both the first programme of tests to investigate the effect of phase density on erosion. and in the first phase of the third programme to investigate the effect of particle size, had a mean particle size of 70 pm. The sand had a fairly wide size distribution, having approximately 10% greater than 180 pm and 10% less than 40 cun_
Fig. 1. Research rig plant layout.
Single batches of sand of about 725 kg were used in both cases. It should be noted that, in the work presented here, it is the velocity of the air that has been recorded and not that of the particlesIn bench type rigs it is usual. and indeed almost certainly necessary, to record particle velocity if meaningful results are to be obtained. For this work on pipe bends, however, it was decided to use air velocity, for it was felt that this is probably more appropriate to industry, as considerable difficulty would be esperienced in measuring particle velocity in plant pipeline situations_ Provided that, for a given air velocity, the particles are accelerated to their terminal velocity, air velocity should be satisfactory as a substitute parameter for the purpose of analysis. In aI1 cases here the pipe bends followed sufficiently long straight runs of pipe to ensure that eve’n the largest particles were accelerated to their terminal velocity [I] _ It might, perhaps, be appropriate at this point to explain the difference between the two terms used for expressing the relative quantities of solids and air. Particle concentration is basically the ratio of the mass of solids to the quantity of air in a given volume of the suspension. This is particuIarIy useful for specifying dust clouds, and is the term generahy used in tests relating to dust ingestion of aircraft engines. Phase density is the ratio of the mass flow rate of the powder being conveyed to the mass flew rate of the air being used. This is more appropriate to pneumatic conveying situations, for it is a dimensionless quantity and its value remains
constant at any section in a pipeline, unlike particle concentration which varies with density, and hence the air pressure-t_hl_4SS ERODED
FROM
BENDS
In this section a brief summary is given of the results of the first two test programmes on the erosion of pipe bends that were carried out at Thames Polytechnic. These are included so that a comparison can be made between erosion plots based on mass eroded and depth of wear, as this is a basic feature of this paper. The information on specific erosion will aIso be required in Section 7. It also aIIows different methods of presenting the data to be demonstrated and the magnitude of the scatter in the results to be shown, and this is an aspect that is discussed further in Section 8-
CLI. Specific erosion results Figure 2 is a log plot of the erosion results from the first test programme with 70 pm mean particle size sand. The erosion here is expressed in specific terms, that is, mass eroded from the bend per unit mass of sand conveyed through the bend. The points plotted represent the mean value of erosion for ail the bends tested at a gken phase density, normahsed to an air velocity of 25 m/s [1]. The slope of the line drawn through these points is -0.37, which would indicate that with this sand flowing through these bends the erosion can be expressed by: erosion = constant X (phase density)-“=
41
8
Fig. 2. Variation 70 ,um sand.
of erosion
with
Tat
SC
3
45
phase
density
for
phase
density
for
I
2
t
!=hCW
Fig. 3. Variation 230 pm sand.
o,aulty
of erosion
with
This means that the specific erosion will be appro_ximately haIved for a six-fold increase in the phase density of the suspension conveyed. Figure 3 is a leg plot of the erosion results from the second test programme with 230 pm mean particle size sand_ The four lines on this graph correspond to the test results for the four sets of tests carried out- The results plotted are accumulative values, and the gradual reduction in specific erosion can be attributed to the progressive degradation and wear of the conveyed product- A single batch of sand of 975 kg was used, and each test set consisted of eight separate test runs at different combinations of velocity and phase density. An interesting result here was the change in the slope of the line drawn through
the points for each test set. Tests were carried out at only three different phase densities, and so any analysis is obviously very limited. The results, however, showed a definite trend, with the slope changing from about -0.16 with the fresh sand in the first test set to about -0.38 with the degraded and worn sand in the fourth test set. A slope of -0.16 means that the specific erosion will be approsimately halved for an eighty-fold increase in the phase density- For fresh sand, therefore, phase density would appear to have little effect on specific erosion. The slope of the line through the last test set results for the 230 pm sand was very close to that presented in Fig. 2 for the result obtained in the previous programme with 70 pm sand. In both of these cases, the sand had become worn and degraded. In the case of the 70 pm sand it was not possible to plot intermediate results. From this it would appear that the condition of the conveyed product has a significant effect on the variation of erosion with phase density. The actual magnitude, of course, will be very much greater than that indicated here, for it is accumuiative results that are presented and not individual test results. The inclusion of a graph of individual test results to isolate the effect of wear and degradation could not be justified, owing to the small number of tests carried out at different phase densities and the individual accuracy of the results in the latter stages of testing [ 23 . Erosion results obtained from bench tests have generally shown that specific erosion decreases only very gradually with increase in phase density. This would tend to confirm the result obtained with the least worn and degraded 230 I_tm sand, for in erosion rigs fresh abrasive powders are normaily employed_ Particle size has been shown to have little effect on the variation of specific erosion over this range of phase densities [S] , and so some other property of the powder must be responsible for this rapid decrease in erosion with respect to time, particularly at the higher phase densities. Particle shape, or sharpness, is most likely the important property here, and so it would provide an interesting exercise to test a number of other powders with different shape characteristics over a range of phase densities to determine the validity of this assumption.
4.2
Presentation of results The erosion results presented in Figs. 2 and 3 are log plots of specific erosion against phase density_ This method of presentation is used to determine possible functional relationships between the variables_ It is difficult, however, to visualise actual changes with these graphs and so, for this purpose, Fig_ 2 is redrawn as a linear scale plot in Fig. 4. The curve on Fig. 1 represents the values from the straight line in the log plot_ AIso plotted on this,graph axe the results for each bend tested, six at each phase density from 0.5 to -I.0 and three at phase densities of
Fig_ -1. Variation of specific density for ‘70 urn sand.
Fig. 6. Variation individual bends
erosion
with
phase
of erosion with mass conveyed tested at a phase density of 3.
for
6-O and SO. This shows at a glance the magnitude of the problem with regard to repeatability and, hence, to recommendations for in-plant use_ At a phase density of three, the scatter of the results was particularly marked, varying between values of about l-5 and 10.6 mg/kg, for identical bends tested at the same time in the same pipeline_ X plot of the erosion history of all the bends tested at a phase density of 3.0 is given in Fig. 5. The bend numbers refer to the position in the loops [I ] _ In this graph the velocities have not been normahsed to 25 m/s, and so a slight increase in erosion from bend 1 to bend 7 can be expected, for the velocity increased slightly as the air expanded in flowing through the pipeline_ The results, however, do not follow any pattern in this respect_ The magnitude of the potential difference in erosion from one bend to another in the same line is clearly illustrated by the third bend in the test loops_ This failed while the sisth batch of sand was being conveyed, and a replacement was substituted. After two further batches had been conveyed, the erosion of the replacement bend exceeded that of two of the original bends in the line. In papers on pipe bend erosion which reIate to industrial situations, the erosion plots are often represented on a time basis. This results in a graph with an entirely different shape, and so care must be taken when interpreting data presented in this way. Erosion loss in
Fig. 6. Variation for 70 ym sand.
of erosion
rate with
phase
density
43
terms of grams of metal removed from a bend per hour is commonly employed_ Figure 6 is a graph of all the informarion in Fig. 4 replotted in terms of erosion rate in g/h against phase density. Individual results are plotted once again to show the degree of scatter in relation to the mean curve for this graph. Both graphs are very useful in their own way; specific erosion provides a direct comparison in terms of unit mass of conveyed product, and erosion rate gives an indication of the potential sen-ice fife of a bend for given conveying conditions.
6. SURFACE
EROSION
PATTERS
X feature of the erosion of bends by the ‘70 pm mean particle size sand was the formation of steps or ridges on the outer inside surfaces of the bends. Photographs of bends eroded by this sand are presented in Figs. 7 and 17, and these show typical surface erosion patterns_ For the erosion by the 70 ~.lm sand there was a noticeable difference
in the initial formation of the stepped erosion surface with respect to phase density. At a phase density of 0.5 it was not until an average of some 16 g had been eroded from the bends that the first steps were noticed, and then they were well downstream, being formed at a bend angle of about 60c. At a phase density of 2.0 the first steps appeared at a bend angle of about 15” with 12 g eroded. At a phase density of S-0 only S g of metal was eroded from the bends before steps appeared, and these formed at a bend angle of about 30”. In Fig. ‘7 the photographs of LWO bends are included which illustrate this difference. Figure ‘i(a) is of a bend eroded at a phase density of 0.5 and from wbicb 33 g had been eroded_ but only 19 g had been eroded from the bend in Fig_ 7(b), at a phase density of 6.0. It was these differences in surface erosion patterns that prompted the authors to investigate this aspect further. On completion of the first and second test pro,~mmes, a number of the bends were cut in half so that accurate wear profiles could be taken. Differences in surface wear patterns
Fig. ‘7. Photographs of bends eroded by 50 pm sand at a velocity of 25 m/s. (a) 33 g eroded from bend at a phase density of 0.5. Minimum bend wall thickness is 2.6 mm. (b) 19 g eroded from bend at a phase density of 6.0. Minimum bend wall thickness is 1.6 mm.
resulting from erosion by the 70 pm and 230 pm mean panicle size sand batches were discussed in a previous paper by the authors [ 33 _ The present analysis concerns the erosion of bends by the 70 pm sand only, as a much wider range of phase densities was investigated and all the tests were carried out at an essentially constant air velocity_ The surface erosion patterns on the bend surfaces, as influenced by the mass eroded and the phase density, are presented in Figs- S, 9 and lo_ These are all “straightened” plots of pipe wall thickness against bend and pipe angle. Figure S shows the effect on the wear profile of different masses eroded from the bends, all at a constant phase density of 3.0_ Figures 9 and 10 show the effect of phase density on the wear profiles, and the common feature of these plots is that approsimately 20 g had been eroded from each bendAt a phase density of 3-O there was a considerable variation in the masses eroded from the bends, as can be seen from Fig. 5. .A number of these bends were cut in half and the wear profiles are shown in Fig_ 8. The actual bend wall thickness varied around the perimeter of the circular cross-section, as can be seen in Fig- 10, and so minimum values of the bend wall thickness were used in plotting the outer bend wall perimeter profiles_ In
Fig_ 9 density
Variation of bend wear profile with phase for a constant
mass eroded
of 20 g.
Fig. 10. Variation of bend wear profile with phase density for a constant mass eroded of 20 g at a bend angle of 33’.
Fig. 6 Variation of bend wear profile with mass eroded at a phase density of 3.0.
Fig. S the gradual wearing process of the bends is depicted, and this shows that for an unreinforced bend it is only between bend angles of about 15” and 50” that the wear is of any significance. The position of maximum wear is at a bend angle of about 33O, and this
would abpear to remain fairly constant during the erosion process. The increase in depth of erosion with respect to mass eroded, however, does not appear to be linear, and this point is considered further in the next section. Figure 9 is a similar plot of minimum bend wall thickness around the outer bend wall perimeter. These profiles show slight variations in pipe wall thickness, as shown by the plots beyond the bend angles of O” and 90° on the straight pipe sections. These are within the vge of manufacturing tolerances and are unlikely to have any significant effect on the results presented. The effect of phase density on the erosion of the bends is shown very clearly in Fig. 9. For a constant mass eroded from each bend of about 20 g, the depth of wear increased quite considerably over the
range of phase densities investigated_ The position of maximum wear was at a bend angle of about 33” for all the phase densities tested, and once again it was only between bend angles of 15O and 50° that the wear was of any significance_ The variation in the depth of wear with phase density is obviously of considerable importance in relation to bend wear, and this point will also be considered further in the nest section. If the four curves on Fig. 9 are compared, one might question how it is possible that 20 g was eroded from each bend, for the mass eroded should be proportional to the area above each curve, and it is quite obvious that this area increases with increasing phase density. The answer to this lies in the next graph. Figure 10 is another plot of the four wear profiles from Fig. 9 and shows the variation of the pipe wall thickness around the outer perimeter of the circular crosssection. These are all taken at a common bend angle of 33”. which approximates to the angle of maximum depth of wear at each phase density in this plane. This is an extremely interesting graph, and it helps considerably in esplaining how the depth of wear increases with increasing phase density. At a phase density of 2.0 a wide trough is shown which indicates that the erosion is taking place fairly uniformly over quite a wide range of pipe angle_ At a phase density of 4.0 the erosion has caused more of a shallow valley to form. At phase densities of 6.0 and 8.0 this valley gets progressively steeper and the erosion takes place over a progressively narrower range of pipe angle and erodes to greater depths in the process. Another feature of Fig. 10 is that the pipe angle at which maximum erosion occurs is not constant with phase density, as is the bend angle at which it occurs. It would appear that as the phase density increases, the pipe angle at which maximum erosion occurs progressively increases above the horizontal centre line of the pipe. Insufficient dater are available at this stage to say whether this is a common feature in erosion. It is also not possible, at this point in time, to say how the curves in Fig. 10 would appear for other masses eroded_ If they were replotted with 10 g eroded, would the higher phase densities show shallower valleys and, if they were drawn with 30 g eroded, would the plot at a phase density of
2-O show a shallow valley instead of a trough, and that at a phase density of SO have worn a hole through the bend? These are some of the questions that it is hoped will be answered in the test programmes to be carried out at Thames Polytechnic in the near future.
6. DEPTH
OF WEAR
Two aspects of the problem of the depth of wear by particles into bend wall surfaces were considered in the previous section. From Fig_ S it was shown that the volume of material removed was not directly proportional to the depth of wear, and from Figs. 9 and 10 it was shown that the depth of wear increased with increasing phase density for a constant mass eroded_ It would appear, therefore, that depth of wear is, as suggested earlier, a more important parameter than mass of metal eroded in assessing the effects of erosion on pipe bends. The effect of mass eroded and the influence of phase density are, therefore, considered in a little more detail in this section. Once again the results relate only to the 70 pm sand, and to a conveying air velocity of about 25 m/s. Several additional graphs on the effects of mass eroded and phase density on the depth of wear are presented in this section. Two of them display a series of curves and include considerably more test results than the graphs in the previous section_ Additional results can. in fact, be added to these graphs at any time and can thereby build up into a valuable source of information_ One is a plot cf minimum bend wall thickness against phase density with lines of constant mass eroded represented. Another is a plot of minimum bend wall thickness against mass eroded with lines of constant phase density superimposed_ It is hoped that Figs_ 8, 9 and 10 might provide the means by which a better understanding of the influence of the variables involved might be obtained, and that some of these other figures might provide a basis on which the life expectancy of pipe bends might in future be evaluated_ 6.1. Effect of mass eroded Figure 11 is a graph of minimum bend wall thickness against phase density, with lines of constant mass eroded plotted. This includes
46
Fig_ 1 I_ Variation of depth of we&r with phase density for a constant mass eroded.
Fig. 1% Variation of depth of wear with mass eroded for various phase densities.
results in Fig_ 9 and shows how the depth of wear increases with phase density. It also shows how this is influenced by mass eroded_ In addition to the 20 g line, two estre,mes are also included_ With only 6 g eroded, phase density would appear to have no effect at all on depth of wear, and this may account for the fact that so Iittie work appears to have been presented on this aspect of erosive wear. With 36 g eroded, however, the magnitude of the problem is clearly shown. Perhaps a more useful representation of the data is shown in Fig_ 12_ This is a graph of minimum bend wall thickness against mass eroded with lines of constant phase density plotted_ This provides confirmation of the fact that depth of wear is not proportional to the mass eroded_ It shows that the rate of change of depth of wear increases with mass eroded and that it also increases with increasing phase density_ Nearly all the esperimental results obtained are plotted on this graph end, as can be seen, there is surprisingly little scatter of the points from the lines drawn. For work of this nature this is really quite a remarkable feature, and one which makes this type of representation of the results particularly useful. the
Tie da’ta available for plotting the curves at phase densities of 1.0, 2.0, 3-0, 4.0, 6-O and SO was rather limited as this was obtained only at the end of the first programme of tests_ When the effect of perticle size on the depth of wear was recognised, after the second programme of tests, a special diel gauge caliper was made so that wear profile readings could be taken of each bend after each batch of sand had been conveyed_ This provided a wealth of data from the third programme of tests, and results at phase densities of 2_0.4_0 and 6-O obtained by this means are also included. Although the date at the other phase densities were limited, all the curves produced showed the same trends. 6-2. Effect of phase density The point at which the curves reach the horizontal axis in Fig. 12 gives the value of the mess of metal eroded from the bend for it to be holed, and hence fail, at any given phase density. As this is a particularly useful piece of information, the results, though limited, are plotted out separately in Fig. 13. This graph shows the ultimate effect of phase density on the erosive wear of pipe bends, end provides the means by which bend life can be determined and the true effect of phase density can be assessed_ It shows quite clearly that as the phase density increases, the mass of metal eroded from the bend at the time of failure decreases. In order to determine the actual effect of phase density on the service life of a bend, this information has to be
Fig. 13. Variation of mass eroded to cause bend failure with phase density_
47 incorporated
with
that
presented
in Section
4_
The effect of phase density on the specific erosion results for the bends, as presented in Section 4, was a little inconclusive, as there was so much scatter in the results, but the general trend was to a decrease in mass eroded with increase in phase density_ The condition of the conveyed product also appeared to have an influence. For the purpose of this analysis, therefore, it was decided to consider the case of fresh abrasive sand and to use the relationship: specific
erosion
= (phase density)-‘-16
This equation also gives reasonable agreement with results obtained from bench tests, as mentioned earlier. X log plot of the erosion results in Fig. i3 is shown in Fig. 14_ That a log plot of these results should have worked is really quite amazing, but they produced a reasonably straight line with a slope of -0-74, which would indicate that over this range of phase density the erosive wear of the bends with this sand can be represented by: mass eroded at failure c (phase density)-‘-” 103
the procedure for determining bend life is given, and for an example the results obtained with the 70 pm sand and the mild steel bends are used. It must be emphasised that, although the method presented can be applied to any situation, the resulting data relate only to this sand, conveyed at this velocity, through these bends, in the horizontal plane. In the first phase of the third programme of tests carried out, a set of six bends was eroded by a fresh batch of 70 pm sand. The bends were tested at a phase density of 2.0 and velocity of about 25 m/s. Five consecutive batches of sand of 700 kg were conveyed through the bends, and so with this large mass and small number of batches the effects of de,wdation should be negligible_ The total mass eroded from the sis bends was 232 g, and so for any one bend this gives a mean specific erosion value of 11.0 g of metal eroded per tonne of sand conveyed_ section
7-l. CalcuIaationprocedure The variation of specific erosion phase density (0) is given by: specific
95
t i
63
:
erosion
(E) with
= constant X (phase density)-‘-16
i.e_ E =k,
X o-O-l6 g/tonne
Results from the above test give
.
11.0
2
= iZl x 2.0-“~16
for which
-1i \
k, = 12.3 thus \
Fig_ l-S_ Variation of ITESSeroded failure with phase density.
E = 12.3 Q-"'~
for this sand in these bends. The variation of the mass eroded from the bend at the time of failure (Ebf) with phase density (@) is given by: mass eroded
7_ DETEFWINATION
OF BEND LIFE
Neither the information on specific erosion rates nor the data on depth of wear are suffi-
cient in themselves for the determination of bend life_ To do this, the two have to be incorporated as mentioned earlier. In this
g/tonne
to failure = constant X (phase density )-oe7a
ie. Ebf
= k2
X
o-~-= g
‘Itvo bends failed in the above test at a phase density of 2.0 and the average mass eroded was 58 g. Substituting, this gives
k, x 2_O-"-i4
58=
from which k., = 96.9 thus I&=
96.9
o-e.‘4
g
for this sand in these bends. The mass of sand that can be conveyed through these bends before failure occurs (~12~)is given by:
E 111, =
bf E
p-“=
= 96.9
Fig_ 16. Influence of bends.
l2.3o-0-1s = 7.88
of phase
A graphical representation given in Fig_ 15_
of this equation
is
7.88 -1ibf
=
0.071x
= 30-S o-‘.=
0
;
t FYSZ
Fig.
6
a
;zcl:)r
15. Infiuence of bends.
of phase
on the service
life
an air velocity of 25 m/s in the 50 mm bore pipeline. Thus
o-o_“8 tonne
0’
density
density
on the conveying
capacity
If the life of the bend is required, conveying time to bend failure (4rh,) by:
the is given
In the tests reported here, the mass fIow rate of the air was O-071 kg/s in order to achieve
o-O-s8
3600x
lo-'x
0
h
A graphical representation of this equation is given in Fig. 16. From this analysis, Fig_ 15 provides the real basis on which the performance of bends, in respect of phase density, should be compared. This shows that as the phase density is increased, over the range considered, there is a definite decrease in the amount of sand that can be conveyed through the bends before failure occurs. In Fig. 16 the influence on the service life of the bends is represented, which is particularly useful information for the pneumatic conveying engineer. Presentation of results was discussed earlier in Section 4.2, and Fig. 16 is another type of time rate graph. It is proportional to the reciprocal of the plot on Fig. 6, and so one would expect this graph to show a decrease with increasing phase density, but the magnitude of the change here is reahy quite considerable. 7.2. Comparison with previous work As mentioned earlier, Bikbaev et al. [ll] investigated the effect of phase density on the erosive wear of pipe bends. Their maximum phase density, however, was only 2.33 and their results were plotted in terms of an erosion rate in mm/h_ It has been shown in thispaper,ofcourse,thatdepth ofwearis
49
not proportional to time. Insufficient data
were given in their papers to relate their results to this work, but if the erosion rate values are divided by the respective phase densities the results should be proportional to one another in terms of depth of erosion per unit mass conveyed_ This, however. produced no recognisable trend in the results_ Industrial data on bend wear, included in the paper by Mason et al_ 141, expressed the erosion rate in terms of depth of wear per unit mass conveyed_ These two quantities have also been shown to be not proportional in this paper. Their results, however, agree with those presented here. They showed that the relative wear rate (proportional to mm/tonne) increased with increasing phase density. Their results, however, showed a critical maximum at a phase density of about 24, beyond which the wear rate reduced to a much lower value. Above a phase density of about 50 to the maximum recorded of about 140, the wear rate remained constant at this lower value. Mason and Smith [ 51, as mentioned in Section 2, worked in terms of mass conveyed per unit depth eroded, and obtained a very similar result to that presented here_ They also predicted that a critical minimum should exist. To date, the general consensus of opinion has been that it is better, in terms of erosion, to convey at a high phase density. This false impression is probably based on the large number of test results which have shown that mass eroded decreases with increased phase density. Although this is true, the increase in depth of wear by particles at increasing phase densities has an over-riding effect. The general idea has been that a change in phase density will give rise to a corresponding change in the
average time interval between successive impacts and that an increase in phase density will result in fe!yer impacts due to the interference of:other particles_ It is obvious that very much more work requires to be done, particularly at phase densities above ten. This is required in order to establish whether a critical minimum does exist, as this obviously has serious implications for the dense phase pneumatic conveying of abrasive materials. 7.3. The influence of other variables Velocity and particle size are two of the major variables which must be considered. Once again, in terms of mass eroded, the
influence of both of these variables has been well established [13] _ Recent work by the authors, however, on the effect of particle size on depth of wear [ 3 ] has shown that it also has a considerable effect, totally unpredicted from measurements of mass eroded. The ranges of both particle size and phase density were rather limited, but the work did show that the effect of particle size on the depth of wear was of fundamental importance and that it was most important that more work be carried out on this subject. To the best knowledge of the authors, no such work has ever been carried out on the effect of velocity on penetration rate. All the bends tested and reported here have been in the horizontal plane. Very little work has been carried out on bends in other planes, and in many reports no mention is even made of the bend orientation and flow direction_ Once again a generally held belief is that it has no significant effect. This might be a reasonable assumption if the wear pattern was symmetrical about the horizontal centre line of the bend surface- From Fig. 10 it can be seen that this is just not the case at the higher phase densities_ This raises the possibility of the gravitational field having an influence at high phase densities, which might lead to different wear patterns in other orientations. This, therefore, is another aspect worthy of further consideration. Two recommendations often made for reducing the effects of erosion on bends are to use bends with a high bend radius to pipe bore ratio and to use backing plates. Care has to be esercised in using large radius bends, as Mason and Smith have pointed out [5], for several wear points are likely to be established with the conveyed product traversing the bend. The use of thicker bends poses an interesting question, for it has been shown in Fig. 12 that the rate of change of depth of wear increases as the depth of wear increases. One could extrapolate the data to a certain extent and predict that bends with a slightly thicker wall would last only marginally longer. This rate of change, however, must at some point reduce and ultimately, with a thick wear plate, the depth of wear should increase only very slowly [l, 3]_
S. THE
EFFECT
OF SECOKD-ART
FLO’A-S
it must he emphasised that the results presented in the previous section on bend life are also influenced by the scatter in the results as discussed in Section -1-2 and illustrated in Figs_ 4 - 6. Although the plots of minimum bend wall thickness in Figs_ 11 and 12 and those of mass eroded at point of failure in Figs_ 13 and 1-Z were remarkably consistent, bend life ultim-ately depends on the value of specific erosion- From Fig_ 16, the mean life expectancy of a mild steel bend conveying '10pm sand at an air velocity of 25 mk and phase density of 6.0 is about two hours. In one test under these conditions, a bend failed after only 1'5 hours_ and thesand was not even Desh, as it had been previously used in another series of tests. The point, however. is that while one bend failed_ the bend immediately before it in the same pipeline lost only 2 g by erosion, and that immediately after it lost only 5 g_ X similar effect is sho\in in Fig. 5 for bends tested at a phase density of 3.0.
The photographs of two bends eroded by about 3.5 tonnes of 70 pm sand in 6% hours at a phase density of 2.0 are shown in Fig. 1’7. From one, 60 g had been eroded and it had failed, but from the other only 35 g had been eroded. The difference between the two can be seen quite clearly. The surface of the bend which failed shows pronounced tracking in the erosion pattern on the surface from one side of the bend to the other, while that on the other bend does not. This tracking has been a characteristic feature on the surface of every bend that has failed prematurely. They show clear evidence of swirling in the flow, and it can only be concluded that if swirling flow occurs in a bend, then it will wear out very muchfasterthanabendinwhichitdoesnotoccur Another observation, as already reported in introducing Section 5, was the change in formation of the initial steps and ridges on the surface at different phase densities_ In Fig. 7 another two photographs of bends are shown. For the bend eroded at a phase density of 0.5, 33 g had been eroded, but only a fex steps were visibie well downstream. and the
Photographs of bends eroded by '70pm sand at a velocity of 25 m/s and phase density fdure after conveying 3.5 tonne in 6:: hours. 60 g eroded from bend_ (b) Bend service identical
Fig. li_
35 g eroded
from bend.
of 3_0_ (a) Bend to (a). hut only
minimum bend mall thickness was still about 2.5 mm. Only 19 g had been eroded from the other bend tested at a phase density of 6.0. The surface, however, was marked by a large number of small steps which had formed very much further upstream, and the minimum bend wall thickness was only about 1-S mm. The difference in mass eroded at the time of failure, of course, has already been mentioned in relation to Fig_ 13. It would appear, therefore_ that the mechanics of the erosion process VW in some fundamental way with the phase density of the suspension. In an earlier paper [ 1) _ the authors discussed at length the flow pattern of particles in traversing a bend. It was pointed out that the essentially uniform phase density of the suspension in the preceding straight pipe run was just not representative of the phase density of the suspension in contact with the bend wall. The variation in erosion with phase density must in some way be a result of the combined interaction between the particles and the bend wall, and between the particles themselves_ At low phase densities there is probably little interference between particles, and the suspension appears to be swept smoothly round the bend_ At high phase densities interparticulate reaction is quite severe and considerable turbulence must be generated upon initial impact with the bend Wall. Two observations are worth recalliCg at this point. From Fig. 4 it can be seen that at low phase densities the scatter of the results is very much less than that at high phase densities, and the minimum value of specific erosion recorded decreases quite considerably with increase in phase density_ From this it would appear that conveying at high phase density can potentially result in very much lower specific erosion values, and this can be attributed to an increase in the interaction between particles, and hence a reduction in that between the particles and the bend wal!. The increase in turbulence on impact with the bend wall at high phase densities, however, will still cauw the bend to fail after only a small mass of metal has been eroded from it, and so althocgh there is potential for a considerable increase in life there is unlikely to be any marked change in the characteristic shape of the curves in Figs. 15 and 16. Finally, to return to the problem of the scatter in the results obtained for specific
erosion_ It is concluded that this is due to the degree of swirling flow in the bends. Bends which have failed prematurely have shown pronounced tracking in the surface erosion patterns, and those that have worn least have shown no trace of tracking at all. That the degree of this scatter increases with increase in phase density must be due to a combination and interaction of the swirling flow and the turbulence generated upon initial impact with the bend wall. If the swirling flow can be reduced it is likely that much longer bend lives can be guaranteed, particularly at high
phase densities. Only a radical change in the design of the bend, however, is likely to reduce the effects of impact turbulence before any further improvements can be obtained_ It might_ of course, also be a function of the bend radius, which bri,lgs us back to the point about the need for very much more work being carried out on the subject_
9.
COSCLUSION
9.1. Presentation of data X major feature of this paper has been the presentation of data in a number of different graphical forms, many of which have not been employed before. While they are all by no means necessary, it is hoped that certain groups might find general use in work of this nature. _A number of the graphs provide a useful basis on which the performance of bends under different conveying conditions can be compared; several might form the basis on which design data for bends might be compiled, for additional results can easily be added, and whole families of curves can be built up to cover other variables; and some of the plots might provide the lead to a better understanding of the erosion process in respect of phase density.
9.2
Depth of wear The most important contribution
of this paper is probably in demonstrating the interrelating effects between the mass eroded from a bend, the depth of wear in a bend and the phase density of the conveyed product. As the specific erosion of the bends does not appear to decrease very significantly with increase in phase density, the marked decrease in mass eroded from a bend to cause failure, with
52
increase in phase density, is of major significance in bend wear. For the conveying of this sand at an air velocity of 25 m/s through these bends, an increase in phase density will, on average, result not only in a reduction in bend life, but in a reduction in the conveying capacity as melI_ This, of course, only applies over the range investigated, which was for phase densities up to eight, but the implications for dense phase pneumatic conveying of abrasive materials are quite serious. It is obvious that very much more work requires to be done, particularly at phase densities above ten, in order to obtain a complete picture with regard to phase density, and to establish whether a critical maximum, in terms of depth of wear for a given mass conveyed, does exist, and at what phase density_ 9.3. Future work Apart from czrrying out more work at very much higher phase densities, it is obvious that a wide range of other variables needs to be considered also_ The over-riding influence of erosion in terms of depth of wear, over that of mass eroded, has been demonstrated here in respect of the phase density of the conveyed product_ A similar effect has aheady been shown with respect to particle size in the erosion of pipe bends. With phase density having such a pronounced effect, and with so many other potential variables associated with both the bends and the product being conveyed, this will be a considerable task, for it is obvious that phase density will have to be considered as an inter-relating variable with each one. Apart from tests to investigate the effects of particle type, size, velocity; and bend material, radius, bore, orientation, and so on, further work is clearly required in order to obtain a better understanding of the mechanics of the erosion process_ The scatter of the results is such that in some cases it should be possible to get a ten-fold improvement in conveying capacity and bend life by reducing the cause of the premature failure. This is attributed to the swirling of the flow around a bend, and so work aimed specifically at investigating this complex phenomena would also be well worthwhile_ It is further suggested that the decrease in mass eroded with increase in phase density is due to the increased
turbulence of the suspension on its first impact with the pipe wall and that this, in conjunction with swirling flow, will cause rapid failure of a bend_
ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge the assistance given by Mr. R. B. Stacey, Manager, Pneumatic Conveying Division, Mucon Engineering Company Limited, on a number of aspects relating to both the plant and the test programme. Sincere gratitude is also extended to Mr_ W. S. Churchill, Chief Laboratory Technician, and his staff at Thames Polytechnic, for their help in maintaining and developing the plant and in supporting the test programme_
LIST E
0 .
m, ma I
"rbf
a- ’
bf
OF SYMBOLS
specific erosion, g/tonne = mass of material eroded from bend per unit mass of product conveyed phase density = riz,/&, = solids to air mass flow ratio mass flow rate of solids, kg/s mass flow rate of air, kg/s mass eroded from bend at time of failure, g mass of sand conveyed through bend before failure, tonne conveying time to bend failure, h
REFERENCES
D. Mills and J. S. BIason, The effect of particle concentration on the erosion of pipe bends in pneumatic conveying systems, Pneumot-port 3, BHRA Conference. Bath, April 1976. Paper A7_ D. Mills and J. S. Mason, The interaction of particle concentration and conveying velocity on the erosive wear of pipe bends in pneumatic conveying lines. Preserxted at the International Powder and Bulk Solids Handling and Processing Conference, Chicago, May 1976. Sponsored by the Powder Adwisorj Centre and the IIT Research Institute_ D. Mills and J. S. Mason, Particle size effects in bend erosion, to be published_ J. S. Mason, P. A_ Anmdel, L A_ Taylor, W_ Dean and T_ E_ Doran, The rapid erosion of various pipe-wall materials by a stream of abrasive alumina particles, Pneumotransport 2, BHRA
53
5
6
5 8
9
Conference, Guildford, September 19’73, Paper El. J. S. Mason and B. V. Smith, The erosion of bends by pneumatically conveyed suspensions of abrasive particles, Powder Technol, 6 (1972) 323. H. Brauer and E. Kriegel, Untersuchungen iiber der Verschleiss von Kunststoffen und Metallen, Chem. Ing. Tech., 35 (1963) 697. H. Brauer and E. Kriegel, Verschleiss an Rohrleitungen bei Hydraulischer Fordenmg von Feststoffen, Stahl Eisen, 84 (1964) i313. H. Brauer and E. Kriegel. Die Probleme des Verschleisses von Rohrlwitungen beim Pneumatischen und Hydraulischen Feststofftransport. hlaschinenmarkt, 71 (1965) 20. H_ Brauer and E. Kriegel, Verschleiss von Rohrkrummern beim Pneumatischen und Hydrauli-
schen Feststofftransport, Chem. Ing. Tech., 37 (1965) 265. 10 F. A. Bikbaev, hf. Z. hlaksimenko, V. L. Berezin, V. I. Krasnov and I. B. Zhilinskii, Wear on branches in pneumatic conveying ducting, Chem_ Pet. Eng., 8 (1972) 165. 11 F. A_ Bikbaev, V. L Krasnov. hi. Z_ Maksimenko. V. L. Berezin, I. B. Zhilinskii and N. T. Otroshko. hlain factors affecting gas abrasive u-ear of elbows in pneumatic conveying pipes, Chem. Pet. Egg., 9 (1973) 73. 12 V. I. Krasnov and I. B. Zhilinskii, Investigations of material reliabilities under gaseous abrasive wearing conditions, Chem. Fet. Eng., 9 (1973) 1029. 13 D. Mills and J. S. Mason, Learning to live with erosion of bends. First International Conference on Protection of Pipes, BHRA Conference, Durham, September 1975, Paper Gl.
Particle Concentration
D. MILLS
in Bend Erosion
and .I_ S. MASON
Department (Received
Effects
of Mechanical
Engineering.
Thames Polytechnic.
Wellington Street, London
SEIS
iiPF (Gt. Britain)
August 23, 1976)
SUMMARY From
37
ranging from 0.5 to S-0 were carried out and work
on the erosion
of flat
plates
in
sand blast type test rigs it has generally been concluded that the effects of particle concentration are very small. The erosion of surfaces, in terms of the mass of material eroded from the surface per unit mass of abrasive particles impacted upon the surface, has been shown to decrease slightly with increase in particle concentration_ Recent work on the erosion of pipe bends by sand in a pneumatic conveying line would tend to confirm this theory. For pipe bends, however, it was observed that particle concentration over the range investigated has two additional effects of major significance; the appearance of the eroded surfaces differ, and the depth of penetration of the particles into the bend surface material increases quite considerably with increase in particle concentration_ The implications of this in terms of the potential conveying capacity and life of bends in pneumatic conveying systems is considered.
1. INTRODUCTION During 1975 a major series of tests was carried out on the erosive wear of pipe bends. The investigation was into the effect of phase density, or particle concentration, on the erosive wear of the bends. The tests were carried out on a full scale pneumatic conveying rig in the Powder Handling Laboratory at Thames Polytechnic_ Sand with a mean particle size of about 70 pm was conveyed pneumatically through the bends, and over forty bends were tested iu the series of tests. They were all 90” bends, of 50 mm bore,
140 mm bend radius and were tested in the horizontal plane. Tests at phase densities
the velocity was held constant at about 25 m/s for each test. The results and analysis of this work were presented in [l] _ Early in 1976 another major series of tests was carried out on the erosive wear of pipe bends. This was primarily an investigation into tests
the
effect
of panicle
velocity,
although
were carried out over a small range of
phase densities_ The bends tested were identical to those in the first programme, and they were eroded once again by a pneumatically conveyed suspension of sand. The results and analysis of this work were presented in [2] _ Apart from the first test series being an investigation into the effects of phase density and the second into effects of velocity, sands of different mean particle sizes were used in the two programmes. For the first test series the sand had a mean particle size of about 70 pm, and for the second test series it was about 230 pm. X comparison of the results of the two test series in terms of the particle size effects was presented in [3] _ One part of this analysis was a comparison on the basis of depth of wear, and this produced a rather surprising result_ Although on the basis of mass eroded there was very little difference in the erosion of the bends by the two sizes of sand used, in terms of depth of wear the erosion by the 70 pm sand was over 40% greater than that of the 230 pm sand. It was the difference in appearance between the bend surfaces, eroded by the different sizes of send, that prompted the authors to investigate that aspect further. A difference in the appearance of the eroded surfaces was also noted with the bends tested to investigate the effect of phase density in the first major series of tests. Although on a basis of mass eroded there was a small
38
decrease in erosion of the bends eroded by the 70 pm sand as the phase density increased, this trend could easily be over-ridden if there was any marked change in the rate of penetration of the particles into the bend wall surface with respect to phase density. For pipe bends, it is really the depth of penetration of the particles into the bend wall which is the important factor, as this determines the service life of a bend. Mass eroded could, therefore, be of secondary importance if, for a given mass eroded, the depth of penetration by the particle is influenced to any degree by ether factors In a previous paper [ 33 the authors showed that mass eroded and depth of wear were not related when the variable considered was particle size. In this paper the variable under investigation is phase density, and a similar analysis is presented here to show whether this has any influence on the depth of wear. Industrial data has been presented [4] which would indicate that phase density has a considerable effect in this respect. Little data and few results, however, have been presented to support this and so this paper will, in part, attempt to rectify the situationAt the time of writing this paper a third major series of tests on the erosion of pipe bends was commencing. This was an investigation into the effect of particle size on the erosive wear of the bends. Sand was the powder being pneumatically conveyed once again, and the bends being eroded were identical to those of the previous test progmmmes. Sand with a mean particle size of 70 pm was first to be used, and as this was identical to that used in the fit programme of tests, the results obtained with the 70 pm sand from this third programme are incorporated in this paper. For the purpose of investigating the effect of particle size, tests were to be carried out at a phase density of 2.0 and a velocity of about 25 m/s_ In order to provide more information for this present paper, tests were carried out additionally at phase densities of 4.0 and 6-O with this particular sand_
2. REVIEW
OF PREVIOUS
WORK
As mentioned earlier, the first major programme of work carried out by the authors was an investigation into the effect
of
phase density on the erosive wear of pipe bends. The work, however, was concerned only with the effect of the mass eroded from the bends. The results and analysis were given in [1] _ This also included a review of much of the previous research work in this aspect of erosive wear and so it will not be considered any further here, as the main concern of this paper is depth of wear rather than mass or volume eroded. Relatively little work, however, has been reported on the effect of phase density on the depth of penetration of particles into surface materials, whether in relation to pipe bends or with flat plates tested in erosion rigs. Mason and co-workers [4, 53, Brauer and Kriegel [6,9] and Krasnov and coworkers [lo - 121 are probably the only research groups to have considered the effect of depth of wear in any detail, whether in respect of phase density or any other variable_ From the work of Brauer and Kriegel [6] and Krasnov and Zhilinskii 1121, it has been shown that the critical impact angle for maximum wear, in terms of depth of penetration, is about 55”_ The equivalent angle for maximum wear, in terms of volume removed, is about 20”. The vast majority of the work on the erosive wear of surface materials by abrasive particles, however, has been carried out on a basis of mass or volume removed, and little account has been taken of depth of wear. This is rather surprising, as 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 on the mass of material eroded. Mason and Smith [S] carried out work with square section perspex bends so that they could observe the change in particle flow and wear patterns with respect to time. They expressed the erosive wear rate in terms of the mass of particles conveyed per unit depth eroded (lb./in. in their case). Their ranges of velocities and phase densities were rather limited, but their analysis did show that phase density should be considered, in ad&ion to velocity, when evaluating bend erosion- They also developed equations based on their results, and from these predicted that a critical minimum should exist with respect to phase density_ IndustriaI data on bend wear was
39
included in the paper by hlason ef al_ 141, in which the wear rate was espressed in terms of
depth of wear per unit mass conveyed. This included a plot of erosive wear against phase density, which indicated a distinct maximum value of erosion. As the units of erosive wear rate are proportional to the reciprocal of those employed by hlason and Smith 153 T this would tend to confirm their predictions. Brauer and Kriegel have carried out a considerable amount of work on the erosive wear of materials, with much of it being theoretically based. They have worked on pipe bends [‘i - 9] as well as using erosion rigs [S] , and have considered both hydraulic [7 - 91 and pneumatic transport situations 16, 8, 9]_ In a blast nozzle erosion rig they impacted steel grit at a velocity of 77 m/s against a range of plastics, metals and glass. They reported on the formation of wear troughs in the materials and presented data which showed how the depth of wear varied with time and impact angle. For work on bend wear, the variation in wall thickness per unit time was employed as a measure of the wear. Most of their work, however, has been concerned with the evaluation of track curves in bends of different curvature and for various velocities so that impact angles could be evaluated and wear points predicted_ Krasnov and co-workers carried out tests on bends in a pneumatic conveying loop [ 10,111 and on test plates with an erosion rig [12]_ The bends were tested downstream of a 5.2 m long vertical pipe of 50 mm bore, and the erosion rate was expressed in mm/h. Bikbaev et al. [lo] investigated the effect of bend radius on erosive wear, with air at a velocity of 50 m/s and the suspension at a phase density of 2.8. Bikbaev et al_ [ll] investigated the effect of phase density on erosive wear, for bends with a radius of 380 mm and at an air velocity of about 55 m/s. Their tests were carried out at phase densities of 0.57,1.39, 2.10 and 2.33 and their results showed a considerable increase in depth of wear with phase density. With erosion rate on a time basis, however, this is what one would expect with varying phase density, for the mass conveyed is not constant_ This is not a suitable basis for comparing actual particle effects in the erosion process, and makes comparisons with other work very difficult. They also tested at a ncmber of different air velocities and
they found that the location of the zone of maximum wear was independent of both phase density and velocity.
3. EXPERIMENTXL
RIG
In order to carry out realistic tests, appropriate to industrial pneumatic conveying situations, a full scale rig was built which consists of a blow tank, having a capacity of about 1 m3; a similar sized hopper mounted on load cells vertically above; and a Roots type blower capable of delivering about 0.07 ms/s at a pressure of 2 bar absolute. -4 diagrammatic layout of the ccjnveying plant is shown in Fig. 1. X full description of the rig, together with further sketches, was given in a previous paper by the authors [l] . This also included details of the test loops, plant operation, control and instrumentation, and charge/discharge 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_ Fine control of both phase density and velocity is pcssible, and start-up and shutdown transient effects are limited to a matter of seconds_ The test section consists of two horizontal loops in a rectangular configuration with a 90” bend at each corner_ Each loop has two S.2 m pipe runs and two 5.4 m pipe runs, giving a total length of 27.2 m for each loop. X diverter valve in the test loops allows one loop to be by-passed, which makes it possible to convey at much higher phase densities. It also enables sampling of the powder to be carried out while it is actually being conveyedThe bends tested in the two pro,mmmes reported here were all mild steel, of 140 mm bend radius and 50 mm bore. For tests at phase densities up to 4.0 both test loops were used, and a total of six bends were incorporated in the two loops for each test. For phase densities above 4.0 and up to 8.0 just one loop was used- with three bends in the loop for testing. The sand used in both the first programme of tests to investigate the effect of phase density on erosion. and in the first phase of the third programme to investigate the effect of particle size, had a mean particle size of 70 pm. The sand had a fairly wide size distribution, having approximately 10% greater than 180 pm and 10% less than 40 cun_
Fig. 1. Research rig plant layout.
Single batches of sand of about 725 kg were used in both cases. It should be noted that, in the work presented here, it is the velocity of the air that has been recorded and not that of the particlesIn bench type rigs it is usual. and indeed almost certainly necessary, to record particle velocity if meaningful results are to be obtained. For this work on pipe bends, however, it was decided to use air velocity, for it was felt that this is probably more appropriate to industry, as considerable difficulty would be esperienced in measuring particle velocity in plant pipeline situations_ Provided that, for a given air velocity, the particles are accelerated to their terminal velocity, air velocity should be satisfactory as a substitute parameter for the purpose of analysis. In aI1 cases here the pipe bends followed sufficiently long straight runs of pipe to ensure that eve’n the largest particles were accelerated to their terminal velocity [I] _ It might, perhaps, be appropriate at this point to explain the difference between the two terms used for expressing the relative quantities of solids and air. Particle concentration is basically the ratio of the mass of solids to the quantity of air in a given volume of the suspension. This is particuIarIy useful for specifying dust clouds, and is the term generahy used in tests relating to dust ingestion of aircraft engines. Phase density is the ratio of the mass flow rate of the powder being conveyed to the mass flew rate of the air being used. This is more appropriate to pneumatic conveying situations, for it is a dimensionless quantity and its value remains
constant at any section in a pipeline, unlike particle concentration which varies with density, and hence the air pressure-t_hl_4SS ERODED
FROM
BENDS
In this section a brief summary is given of the results of the first two test programmes on the erosion of pipe bends that were carried out at Thames Polytechnic. These are included so that a comparison can be made between erosion plots based on mass eroded and depth of wear, as this is a basic feature of this paper. The information on specific erosion will aIso be required in Section 7. It also aIIows different methods of presenting the data to be demonstrated and the magnitude of the scatter in the results to be shown, and this is an aspect that is discussed further in Section 8-
CLI. Specific erosion results Figure 2 is a log plot of the erosion results from the first test programme with 70 pm mean particle size sand. The erosion here is expressed in specific terms, that is, mass eroded from the bend per unit mass of sand conveyed through the bend. The points plotted represent the mean value of erosion for ail the bends tested at a gken phase density, normahsed to an air velocity of 25 m/s [1]. The slope of the line drawn through these points is -0.37, which would indicate that with this sand flowing through these bends the erosion can be expressed by: erosion = constant X (phase density)-“=
41
8
Fig. 2. Variation 70 ,um sand.
of erosion
with
Tat
SC
3
45
phase
density
for
phase
density
for
I
2
t
!=hCW
Fig. 3. Variation 230 pm sand.
o,aulty
of erosion
with
This means that the specific erosion will be appro_ximately haIved for a six-fold increase in the phase density of the suspension conveyed. Figure 3 is a leg plot of the erosion results from the second test programme with 230 pm mean particle size sand_ The four lines on this graph correspond to the test results for the four sets of tests carried out- The results plotted are accumulative values, and the gradual reduction in specific erosion can be attributed to the progressive degradation and wear of the conveyed product- A single batch of sand of 975 kg was used, and each test set consisted of eight separate test runs at different combinations of velocity and phase density. An interesting result here was the change in the slope of the line drawn through
the points for each test set. Tests were carried out at only three different phase densities, and so any analysis is obviously very limited. The results, however, showed a definite trend, with the slope changing from about -0.16 with the fresh sand in the first test set to about -0.38 with the degraded and worn sand in the fourth test set. A slope of -0.16 means that the specific erosion will be approsimately halved for an eighty-fold increase in the phase density- For fresh sand, therefore, phase density would appear to have little effect on specific erosion. The slope of the line through the last test set results for the 230 pm sand was very close to that presented in Fig. 2 for the result obtained in the previous programme with 70 pm sand. In both of these cases, the sand had become worn and degraded. In the case of the 70 pm sand it was not possible to plot intermediate results. From this it would appear that the condition of the conveyed product has a significant effect on the variation of erosion with phase density. The actual magnitude, of course, will be very much greater than that indicated here, for it is accumuiative results that are presented and not individual test results. The inclusion of a graph of individual test results to isolate the effect of wear and degradation could not be justified, owing to the small number of tests carried out at different phase densities and the individual accuracy of the results in the latter stages of testing [ 23 . Erosion results obtained from bench tests have generally shown that specific erosion decreases only very gradually with increase in phase density. This would tend to confirm the result obtained with the least worn and degraded 230 I_tm sand, for in erosion rigs fresh abrasive powders are normaily employed_ Particle size has been shown to have little effect on the variation of specific erosion over this range of phase densities [S] , and so some other property of the powder must be responsible for this rapid decrease in erosion with respect to time, particularly at the higher phase densities. Particle shape, or sharpness, is most likely the important property here, and so it would provide an interesting exercise to test a number of other powders with different shape characteristics over a range of phase densities to determine the validity of this assumption.
4.2
Presentation of results The erosion results presented in Figs. 2 and 3 are log plots of specific erosion against phase density_ This method of presentation is used to determine possible functional relationships between the variables_ It is difficult, however, to visualise actual changes with these graphs and so, for this purpose, Fig_ 2 is redrawn as a linear scale plot in Fig. 4. The curve on Fig. 1 represents the values from the straight line in the log plot_ AIso plotted on this,graph axe the results for each bend tested, six at each phase density from 0.5 to -I.0 and three at phase densities of
Fig_ -1. Variation of specific density for ‘70 urn sand.
Fig. 6. Variation individual bends
erosion
with
phase
of erosion with mass conveyed tested at a phase density of 3.
for
6-O and SO. This shows at a glance the magnitude of the problem with regard to repeatability and, hence, to recommendations for in-plant use_ At a phase density of three, the scatter of the results was particularly marked, varying between values of about l-5 and 10.6 mg/kg, for identical bends tested at the same time in the same pipeline_ X plot of the erosion history of all the bends tested at a phase density of 3.0 is given in Fig. 5. The bend numbers refer to the position in the loops [I ] _ In this graph the velocities have not been normahsed to 25 m/s, and so a slight increase in erosion from bend 1 to bend 7 can be expected, for the velocity increased slightly as the air expanded in flowing through the pipeline_ The results, however, do not follow any pattern in this respect_ The magnitude of the potential difference in erosion from one bend to another in the same line is clearly illustrated by the third bend in the test loops_ This failed while the sisth batch of sand was being conveyed, and a replacement was substituted. After two further batches had been conveyed, the erosion of the replacement bend exceeded that of two of the original bends in the line. In papers on pipe bend erosion which reIate to industrial situations, the erosion plots are often represented on a time basis. This results in a graph with an entirely different shape, and so care must be taken when interpreting data presented in this way. Erosion loss in
Fig. 6. Variation for 70 ym sand.
of erosion
rate with
phase
density
43
terms of grams of metal removed from a bend per hour is commonly employed_ Figure 6 is a graph of all the informarion in Fig. 4 replotted in terms of erosion rate in g/h against phase density. Individual results are plotted once again to show the degree of scatter in relation to the mean curve for this graph. Both graphs are very useful in their own way; specific erosion provides a direct comparison in terms of unit mass of conveyed product, and erosion rate gives an indication of the potential sen-ice fife of a bend for given conveying conditions.
6. SURFACE
EROSION
PATTERS
X feature of the erosion of bends by the ‘70 pm mean particle size sand was the formation of steps or ridges on the outer inside surfaces of the bends. Photographs of bends eroded by this sand are presented in Figs. 7 and 17, and these show typical surface erosion patterns_ For the erosion by the 70 ~.lm sand there was a noticeable difference
in the initial formation of the stepped erosion surface with respect to phase density. At a phase density of 0.5 it was not until an average of some 16 g had been eroded from the bends that the first steps were noticed, and then they were well downstream, being formed at a bend angle of about 60c. At a phase density of 2.0 the first steps appeared at a bend angle of about 15” with 12 g eroded. At a phase density of S-0 only S g of metal was eroded from the bends before steps appeared, and these formed at a bend angle of about 30”. In Fig. ‘7 the photographs of LWO bends are included which illustrate this difference. Figure ‘i(a) is of a bend eroded at a phase density of 0.5 and from wbicb 33 g had been eroded_ but only 19 g had been eroded from the bend in Fig_ 7(b), at a phase density of 6.0. It was these differences in surface erosion patterns that prompted the authors to investigate this aspect further. On completion of the first and second test pro,~mmes, a number of the bends were cut in half so that accurate wear profiles could be taken. Differences in surface wear patterns
Fig. ‘7. Photographs of bends eroded by 50 pm sand at a velocity of 25 m/s. (a) 33 g eroded from bend at a phase density of 0.5. Minimum bend wall thickness is 2.6 mm. (b) 19 g eroded from bend at a phase density of 6.0. Minimum bend wall thickness is 1.6 mm.
resulting from erosion by the 70 pm and 230 pm mean panicle size sand batches were discussed in a previous paper by the authors [ 33 _ The present analysis concerns the erosion of bends by the 70 pm sand only, as a much wider range of phase densities was investigated and all the tests were carried out at an essentially constant air velocity_ The surface erosion patterns on the bend surfaces, as influenced by the mass eroded and the phase density, are presented in Figs- S, 9 and lo_ These are all “straightened” plots of pipe wall thickness against bend and pipe angle. Figure S shows the effect on the wear profile of different masses eroded from the bends, all at a constant phase density of 3.0_ Figures 9 and 10 show the effect of phase density on the wear profiles, and the common feature of these plots is that approsimately 20 g had been eroded from each bendAt a phase density of 3-O there was a considerable variation in the masses eroded from the bends, as can be seen from Fig. 5. .A number of these bends were cut in half and the wear profiles are shown in Fig_ 8. The actual bend wall thickness varied around the perimeter of the circular cross-section, as can be seen in Fig- 10, and so minimum values of the bend wall thickness were used in plotting the outer bend wall perimeter profiles_ In
Fig_ 9 density
Variation of bend wear profile with phase for a constant
mass eroded
of 20 g.
Fig. 10. Variation of bend wear profile with phase density for a constant mass eroded of 20 g at a bend angle of 33’.
Fig. 6 Variation of bend wear profile with mass eroded at a phase density of 3.0.
Fig. S the gradual wearing process of the bends is depicted, and this shows that for an unreinforced bend it is only between bend angles of about 15” and 50” that the wear is of any significance. The position of maximum wear is at a bend angle of about 33O, and this
would abpear to remain fairly constant during the erosion process. The increase in depth of erosion with respect to mass eroded, however, does not appear to be linear, and this point is considered further in the next section. Figure 9 is a similar plot of minimum bend wall thickness around the outer bend wall perimeter. These profiles show slight variations in pipe wall thickness, as shown by the plots beyond the bend angles of O” and 90° on the straight pipe sections. These are within the vge of manufacturing tolerances and are unlikely to have any significant effect on the results presented. The effect of phase density on the erosion of the bends is shown very clearly in Fig. 9. For a constant mass eroded from each bend of about 20 g, the depth of wear increased quite considerably over the
range of phase densities investigated_ The position of maximum wear was at a bend angle of about 33” for all the phase densities tested, and once again it was only between bend angles of 15O and 50° that the wear was of any significance_ The variation in the depth of wear with phase density is obviously of considerable importance in relation to bend wear, and this point will also be considered further in the nest section. If the four curves on Fig. 9 are compared, one might question how it is possible that 20 g was eroded from each bend, for the mass eroded should be proportional to the area above each curve, and it is quite obvious that this area increases with increasing phase density. The answer to this lies in the next graph. Figure 10 is another plot of the four wear profiles from Fig. 9 and shows the variation of the pipe wall thickness around the outer perimeter of the circular crosssection. These are all taken at a common bend angle of 33”. which approximates to the angle of maximum depth of wear at each phase density in this plane. This is an extremely interesting graph, and it helps considerably in esplaining how the depth of wear increases with increasing phase density. At a phase density of 2.0 a wide trough is shown which indicates that the erosion is taking place fairly uniformly over quite a wide range of pipe angle_ At a phase density of 4.0 the erosion has caused more of a shallow valley to form. At phase densities of 6.0 and 8.0 this valley gets progressively steeper and the erosion takes place over a progressively narrower range of pipe angle and erodes to greater depths in the process. Another feature of Fig. 10 is that the pipe angle at which maximum erosion occurs is not constant with phase density, as is the bend angle at which it occurs. It would appear that as the phase density increases, the pipe angle at which maximum erosion occurs progressively increases above the horizontal centre line of the pipe. Insufficient dater are available at this stage to say whether this is a common feature in erosion. It is also not possible, at this point in time, to say how the curves in Fig. 10 would appear for other masses eroded_ If they were replotted with 10 g eroded, would the higher phase densities show shallower valleys and, if they were drawn with 30 g eroded, would the plot at a phase density of
2-O show a shallow valley instead of a trough, and that at a phase density of SO have worn a hole through the bend? These are some of the questions that it is hoped will be answered in the test programmes to be carried out at Thames Polytechnic in the near future.
6. DEPTH
OF WEAR
Two aspects of the problem of the depth of wear by particles into bend wall surfaces were considered in the previous section. From Fig_ S it was shown that the volume of material removed was not directly proportional to the depth of wear, and from Figs. 9 and 10 it was shown that the depth of wear increased with increasing phase density for a constant mass eroded_ It would appear, therefore, that depth of wear is, as suggested earlier, a more important parameter than mass of metal eroded in assessing the effects of erosion on pipe bends. The effect of mass eroded and the influence of phase density are, therefore, considered in a little more detail in this section. Once again the results relate only to the 70 pm sand, and to a conveying air velocity of about 25 m/s. Several additional graphs on the effects of mass eroded and phase density on the depth of wear are presented in this section. Two of them display a series of curves and include considerably more test results than the graphs in the previous section_ Additional results can. in fact, be added to these graphs at any time and can thereby build up into a valuable source of information_ One is a plot cf minimum bend wall thickness against phase density with lines of constant mass eroded represented. Another is a plot of minimum bend wall thickness against mass eroded with lines of constant phase density superimposed_ It is hoped that Figs_ 8, 9 and 10 might provide the means by which a better understanding of the influence of the variables involved might be obtained, and that some of these other figures might provide a basis on which the life expectancy of pipe bends might in future be evaluated_ 6.1. Effect of mass eroded Figure 11 is a graph of minimum bend wall thickness against phase density, with lines of constant mass eroded plotted. This includes
46
Fig_ 1 I_ Variation of depth of we&r with phase density for a constant mass eroded.
Fig. 1% Variation of depth of wear with mass eroded for various phase densities.
results in Fig_ 9 and shows how the depth of wear increases with phase density. It also shows how this is influenced by mass eroded_ In addition to the 20 g line, two estre,mes are also included_ With only 6 g eroded, phase density would appear to have no effect at all on depth of wear, and this may account for the fact that so Iittie work appears to have been presented on this aspect of erosive wear. With 36 g eroded, however, the magnitude of the problem is clearly shown. Perhaps a more useful representation of the data is shown in Fig_ 12_ This is a graph of minimum bend wall thickness against mass eroded with lines of constant phase density plotted_ This provides confirmation of the fact that depth of wear is not proportional to the mass eroded_ It shows that the rate of change of depth of wear increases with mass eroded and that it also increases with increasing phase density_ Nearly all the esperimental results obtained are plotted on this graph end, as can be seen, there is surprisingly little scatter of the points from the lines drawn. For work of this nature this is really quite a remarkable feature, and one which makes this type of representation of the results particularly useful. the
Tie da’ta available for plotting the curves at phase densities of 1.0, 2.0, 3-0, 4.0, 6-O and SO was rather limited as this was obtained only at the end of the first programme of tests_ When the effect of perticle size on the depth of wear was recognised, after the second programme of tests, a special diel gauge caliper was made so that wear profile readings could be taken of each bend after each batch of sand had been conveyed_ This provided a wealth of data from the third programme of tests, and results at phase densities of 2_0.4_0 and 6-O obtained by this means are also included. Although the date at the other phase densities were limited, all the curves produced showed the same trends. 6-2. Effect of phase density The point at which the curves reach the horizontal axis in Fig. 12 gives the value of the mess of metal eroded from the bend for it to be holed, and hence fail, at any given phase density. As this is a particularly useful piece of information, the results, though limited, are plotted out separately in Fig. 13. This graph shows the ultimate effect of phase density on the erosive wear of pipe bends, end provides the means by which bend life can be determined and the true effect of phase density can be assessed_ It shows quite clearly that as the phase density increases, the mass of metal eroded from the bend at the time of failure decreases. In order to determine the actual effect of phase density on the service life of a bend, this information has to be
Fig. 13. Variation of mass eroded to cause bend failure with phase density_
47 incorporated
with
that
presented
in Section
4_
The effect of phase density on the specific erosion results for the bends, as presented in Section 4, was a little inconclusive, as there was so much scatter in the results, but the general trend was to a decrease in mass eroded with increase in phase density_ The condition of the conveyed product also appeared to have an influence. For the purpose of this analysis, therefore, it was decided to consider the case of fresh abrasive sand and to use the relationship: specific
erosion
= (phase density)-‘-16
This equation also gives reasonable agreement with results obtained from bench tests, as mentioned earlier. X log plot of the erosion results in Fig. i3 is shown in Fig. 14_ That a log plot of these results should have worked is really quite amazing, but they produced a reasonably straight line with a slope of -0-74, which would indicate that over this range of phase density the erosive wear of the bends with this sand can be represented by: mass eroded at failure c (phase density)-‘-” 103
the procedure for determining bend life is given, and for an example the results obtained with the 70 pm sand and the mild steel bends are used. It must be emphasised that, although the method presented can be applied to any situation, the resulting data relate only to this sand, conveyed at this velocity, through these bends, in the horizontal plane. In the first phase of the third programme of tests carried out, a set of six bends was eroded by a fresh batch of 70 pm sand. The bends were tested at a phase density of 2.0 and velocity of about 25 m/s. Five consecutive batches of sand of 700 kg were conveyed through the bends, and so with this large mass and small number of batches the effects of de,wdation should be negligible_ The total mass eroded from the sis bends was 232 g, and so for any one bend this gives a mean specific erosion value of 11.0 g of metal eroded per tonne of sand conveyed_ section
7-l. CalcuIaationprocedure The variation of specific erosion phase density (0) is given by: specific
95
t i
63
:
erosion
(E) with
= constant X (phase density)-‘-16
i.e_ E =k,
X o-O-l6 g/tonne
Results from the above test give
.
11.0
2
= iZl x 2.0-“~16
for which
-1i \
k, = 12.3 thus \
Fig_ l-S_ Variation of ITESSeroded failure with phase density.
E = 12.3 Q-"'~
for this sand in these bends. The variation of the mass eroded from the bend at the time of failure (Ebf) with phase density (@) is given by: mass eroded
7_ DETEFWINATION
OF BEND LIFE
Neither the information on specific erosion rates nor the data on depth of wear are suffi-
cient in themselves for the determination of bend life_ To do this, the two have to be incorporated as mentioned earlier. In this
g/tonne
to failure = constant X (phase density )-oe7a
ie. Ebf
= k2
X
o-~-= g
‘Itvo bends failed in the above test at a phase density of 2.0 and the average mass eroded was 58 g. Substituting, this gives
k, x 2_O-"-i4
58=
from which k., = 96.9 thus I&=
96.9
o-e.‘4
g
for this sand in these bends. The mass of sand that can be conveyed through these bends before failure occurs (~12~)is given by:
E 111, =
bf E
p-“=
= 96.9
Fig_ 16. Influence of bends.
l2.3o-0-1s = 7.88
of phase
A graphical representation given in Fig_ 15_
of this equation
is
7.88 -1ibf
=
0.071x
= 30-S o-‘.=
0
;
t FYSZ
Fig.
6
a
;zcl:)r
15. Infiuence of bends.
of phase
on the service
life
an air velocity of 25 m/s in the 50 mm bore pipeline. Thus
o-o_“8 tonne
0’
density
density
on the conveying
capacity
If the life of the bend is required, conveying time to bend failure (4rh,) by:
the is given
In the tests reported here, the mass fIow rate of the air was O-071 kg/s in order to achieve
o-O-s8
3600x
lo-'x
0
h
A graphical representation of this equation is given in Fig. 16. From this analysis, Fig_ 15 provides the real basis on which the performance of bends, in respect of phase density, should be compared. This shows that as the phase density is increased, over the range considered, there is a definite decrease in the amount of sand that can be conveyed through the bends before failure occurs. In Fig. 16 the influence on the service life of the bends is represented, which is particularly useful information for the pneumatic conveying engineer. Presentation of results was discussed earlier in Section 4.2, and Fig. 16 is another type of time rate graph. It is proportional to the reciprocal of the plot on Fig. 6, and so one would expect this graph to show a decrease with increasing phase density, but the magnitude of the change here is reahy quite considerable. 7.2. Comparison with previous work As mentioned earlier, Bikbaev et al. [ll] investigated the effect of phase density on the erosive wear of pipe bends. Their maximum phase density, however, was only 2.33 and their results were plotted in terms of an erosion rate in mm/h_ It has been shown in thispaper,ofcourse,thatdepth ofwearis
49
not proportional to time. Insufficient data
were given in their papers to relate their results to this work, but if the erosion rate values are divided by the respective phase densities the results should be proportional to one another in terms of depth of erosion per unit mass conveyed_ This, however. produced no recognisable trend in the results_ Industrial data on bend wear, included in the paper by Mason et al_ 141, expressed the erosion rate in terms of depth of wear per unit mass conveyed_ These two quantities have also been shown to be not proportional in this paper. Their results, however, agree with those presented here. They showed that the relative wear rate (proportional to mm/tonne) increased with increasing phase density. Their results, however, showed a critical maximum at a phase density of about 24, beyond which the wear rate reduced to a much lower value. Above a phase density of about 50 to the maximum recorded of about 140, the wear rate remained constant at this lower value. Mason and Smith [ 51, as mentioned in Section 2, worked in terms of mass conveyed per unit depth eroded, and obtained a very similar result to that presented here_ They also predicted that a critical minimum should exist. To date, the general consensus of opinion has been that it is better, in terms of erosion, to convey at a high phase density. This false impression is probably based on the large number of test results which have shown that mass eroded decreases with increased phase density. Although this is true, the increase in depth of wear by particles at increasing phase densities has an over-riding effect. The general idea has been that a change in phase density will give rise to a corresponding change in the
average time interval between successive impacts and that an increase in phase density will result in fe!yer impacts due to the interference of:other particles_ It is obvious that very much more work requires to be done, particularly at phase densities above ten. This is required in order to establish whether a critical minimum does exist, as this obviously has serious implications for the dense phase pneumatic conveying of abrasive materials. 7.3. The influence of other variables Velocity and particle size are two of the major variables which must be considered. Once again, in terms of mass eroded, the
influence of both of these variables has been well established [13] _ Recent work by the authors, however, on the effect of particle size on depth of wear [ 3 ] has shown that it also has a considerable effect, totally unpredicted from measurements of mass eroded. The ranges of both particle size and phase density were rather limited, but the work did show that the effect of particle size on the depth of wear was of fundamental importance and that it was most important that more work be carried out on this subject. To the best knowledge of the authors, no such work has ever been carried out on the effect of velocity on penetration rate. All the bends tested and reported here have been in the horizontal plane. Very little work has been carried out on bends in other planes, and in many reports no mention is even made of the bend orientation and flow direction_ Once again a generally held belief is that it has no significant effect. This might be a reasonable assumption if the wear pattern was symmetrical about the horizontal centre line of the bend surface- From Fig. 10 it can be seen that this is just not the case at the higher phase densities_ This raises the possibility of the gravitational field having an influence at high phase densities, which might lead to different wear patterns in other orientations. This, therefore, is another aspect worthy of further consideration. Two recommendations often made for reducing the effects of erosion on bends are to use bends with a high bend radius to pipe bore ratio and to use backing plates. Care has to be esercised in using large radius bends, as Mason and Smith have pointed out [5], for several wear points are likely to be established with the conveyed product traversing the bend. The use of thicker bends poses an interesting question, for it has been shown in Fig. 12 that the rate of change of depth of wear increases as the depth of wear increases. One could extrapolate the data to a certain extent and predict that bends with a slightly thicker wall would last only marginally longer. This rate of change, however, must at some point reduce and ultimately, with a thick wear plate, the depth of wear should increase only very slowly [l, 3]_
S. THE
EFFECT
OF SECOKD-ART
FLO’A-S
it must he emphasised that the results presented in the previous section on bend life are also influenced by the scatter in the results as discussed in Section -1-2 and illustrated in Figs_ 4 - 6. Although the plots of minimum bend wall thickness in Figs_ 11 and 12 and those of mass eroded at point of failure in Figs_ 13 and 1-Z were remarkably consistent, bend life ultim-ately depends on the value of specific erosion- From Fig_ 16, the mean life expectancy of a mild steel bend conveying '10pm sand at an air velocity of 25 mk and phase density of 6.0 is about two hours. In one test under these conditions, a bend failed after only 1'5 hours_ and thesand was not even Desh, as it had been previously used in another series of tests. The point, however. is that while one bend failed_ the bend immediately before it in the same pipeline lost only 2 g by erosion, and that immediately after it lost only 5 g_ X similar effect is sho\in in Fig. 5 for bends tested at a phase density of 3.0.
The photographs of two bends eroded by about 3.5 tonnes of 70 pm sand in 6% hours at a phase density of 2.0 are shown in Fig. 1’7. From one, 60 g had been eroded and it had failed, but from the other only 35 g had been eroded. The difference between the two can be seen quite clearly. The surface of the bend which failed shows pronounced tracking in the erosion pattern on the surface from one side of the bend to the other, while that on the other bend does not. This tracking has been a characteristic feature on the surface of every bend that has failed prematurely. They show clear evidence of swirling in the flow, and it can only be concluded that if swirling flow occurs in a bend, then it will wear out very muchfasterthanabendinwhichitdoesnotoccur Another observation, as already reported in introducing Section 5, was the change in formation of the initial steps and ridges on the surface at different phase densities_ In Fig. 7 another two photographs of bends are shown. For the bend eroded at a phase density of 0.5, 33 g had been eroded, but only a fex steps were visibie well downstream. and the
Photographs of bends eroded by '70pm sand at a velocity of 25 m/s and phase density fdure after conveying 3.5 tonne in 6:: hours. 60 g eroded from bend_ (b) Bend service identical
Fig. li_
35 g eroded
from bend.
of 3_0_ (a) Bend to (a). hut only
minimum bend mall thickness was still about 2.5 mm. Only 19 g had been eroded from the other bend tested at a phase density of 6.0. The surface, however, was marked by a large number of small steps which had formed very much further upstream, and the minimum bend wall thickness was only about 1-S mm. The difference in mass eroded at the time of failure, of course, has already been mentioned in relation to Fig_ 13. It would appear, therefore_ that the mechanics of the erosion process VW in some fundamental way with the phase density of the suspension. In an earlier paper [ 1) _ the authors discussed at length the flow pattern of particles in traversing a bend. It was pointed out that the essentially uniform phase density of the suspension in the preceding straight pipe run was just not representative of the phase density of the suspension in contact with the bend wall. The variation in erosion with phase density must in some way be a result of the combined interaction between the particles and the bend wall, and between the particles themselves_ At low phase densities there is probably little interference between particles, and the suspension appears to be swept smoothly round the bend_ At high phase densities interparticulate reaction is quite severe and considerable turbulence must be generated upon initial impact with the bend Wall. Two observations are worth recalliCg at this point. From Fig. 4 it can be seen that at low phase densities the scatter of the results is very much less than that at high phase densities, and the minimum value of specific erosion recorded decreases quite considerably with increase in phase density_ From this it would appear that conveying at high phase density can potentially result in very much lower specific erosion values, and this can be attributed to an increase in the interaction between particles, and hence a reduction in that between the particles and the bend wal!. The increase in turbulence on impact with the bend wall at high phase densities, however, will still cauw the bend to fail after only a small mass of metal has been eroded from it, and so althocgh there is potential for a considerable increase in life there is unlikely to be any marked change in the characteristic shape of the curves in Figs. 15 and 16. Finally, to return to the problem of the scatter in the results obtained for specific
erosion_ It is concluded that this is due to the degree of swirling flow in the bends. Bends which have failed prematurely have shown pronounced tracking in the surface erosion patterns, and those that have worn least have shown no trace of tracking at all. That the degree of this scatter increases with increase in phase density must be due to a combination and interaction of the swirling flow and the turbulence generated upon initial impact with the bend wall. If the swirling flow can be reduced it is likely that much longer bend lives can be guaranteed, particularly at high
phase densities. Only a radical change in the design of the bend, however, is likely to reduce the effects of impact turbulence before any further improvements can be obtained_ It might_ of course, also be a function of the bend radius, which bri,lgs us back to the point about the need for very much more work being carried out on the subject_
9.
COSCLUSION
9.1. Presentation of data X major feature of this paper has been the presentation of data in a number of different graphical forms, many of which have not been employed before. While they are all by no means necessary, it is hoped that certain groups might find general use in work of this nature. _A number of the graphs provide a useful basis on which the performance of bends under different conveying conditions can be compared; several might form the basis on which design data for bends might be compiled, for additional results can easily be added, and whole families of curves can be built up to cover other variables; and some of the plots might provide the lead to a better understanding of the erosion process in respect of phase density.
9.2
Depth of wear The most important contribution
of this paper is probably in demonstrating the interrelating effects between the mass eroded from a bend, the depth of wear in a bend and the phase density of the conveyed product. As the specific erosion of the bends does not appear to decrease very significantly with increase in phase density, the marked decrease in mass eroded from a bend to cause failure, with
52
increase in phase density, is of major significance in bend wear. For the conveying of this sand at an air velocity of 25 m/s through these bends, an increase in phase density will, on average, result not only in a reduction in bend life, but in a reduction in the conveying capacity as melI_ This, of course, only applies over the range investigated, which was for phase densities up to eight, but the implications for dense phase pneumatic conveying of abrasive materials are quite serious. It is obvious that very much more work requires to be done, particularly at phase densities above ten, in order to obtain a complete picture with regard to phase density, and to establish whether a critical maximum, in terms of depth of wear for a given mass conveyed, does exist, and at what phase density_ 9.3. Future work Apart from czrrying out more work at very much higher phase densities, it is obvious that a wide range of other variables needs to be considered also_ The over-riding influence of erosion in terms of depth of wear, over that of mass eroded, has been demonstrated here in respect of the phase density of the conveyed product_ A similar effect has aheady been shown with respect to particle size in the erosion of pipe bends. With phase density having such a pronounced effect, and with so many other potential variables associated with both the bends and the product being conveyed, this will be a considerable task, for it is obvious that phase density will have to be considered as an inter-relating variable with each one. Apart from tests to investigate the effects of particle type, size, velocity; and bend material, radius, bore, orientation, and so on, further work is clearly required in order to obtain a better understanding of the mechanics of the erosion process_ The scatter of the results is such that in some cases it should be possible to get a ten-fold improvement in conveying capacity and bend life by reducing the cause of the premature failure. This is attributed to the swirling of the flow around a bend, and so work aimed specifically at investigating this complex phenomena would also be well worthwhile_ It is further suggested that the decrease in mass eroded with increase in phase density is due to the increased
turbulence of the suspension on its first impact with the pipe wall and that this, in conjunction with swirling flow, will cause rapid failure of a bend_
ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge the assistance given by Mr. R. B. Stacey, Manager, Pneumatic Conveying Division, Mucon Engineering Company Limited, on a number of aspects relating to both the plant and the test programme. Sincere gratitude is also extended to Mr_ W. S. Churchill, Chief Laboratory Technician, and his staff at Thames Polytechnic, for their help in maintaining and developing the plant and in supporting the test programme_
LIST E
0 .
m, ma I
"rbf
a- ’
bf
OF SYMBOLS
specific erosion, g/tonne = mass of material eroded from bend per unit mass of product conveyed phase density = riz,/&, = solids to air mass flow ratio mass flow rate of solids, kg/s mass flow rate of air, kg/s mass eroded from bend at time of failure, g mass of sand conveyed through bend before failure, tonne conveying time to bend failure, h
REFERENCES
D. Mills and J. S. BIason, The effect of particle concentration on the erosion of pipe bends in pneumatic conveying systems, Pneumot-port 3, BHRA Conference. Bath, April 1976. Paper A7_ D. Mills and J. S. Mason, The interaction of particle concentration and conveying velocity on the erosive wear of pipe bends in pneumatic conveying lines. Preserxted at the International Powder and Bulk Solids Handling and Processing Conference, Chicago, May 1976. Sponsored by the Powder Adwisorj Centre and the IIT Research Institute_ D. Mills and J. S. Mason, Particle size effects in bend erosion, to be published_ J. S. Mason, P. A_ Anmdel, L A_ Taylor, W_ Dean and T_ E_ Doran, The rapid erosion of various pipe-wall materials by a stream of abrasive alumina particles, Pneumotransport 2, BHRA
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5
6
5 8
9
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schen Feststofftransport, Chem. Ing. Tech., 37 (1965) 265. 10 F. A. Bikbaev, hf. Z. hlaksimenko, V. L. Berezin, V. I. Krasnov and I. B. Zhilinskii, Wear on branches in pneumatic conveying ducting, Chem_ Pet. Eng., 8 (1972) 165. 11 F. A_ Bikbaev, V. L Krasnov. hi. Z_ Maksimenko. V. L. Berezin, I. B. Zhilinskii and N. T. Otroshko. hlain factors affecting gas abrasive u-ear of elbows in pneumatic conveying pipes, Chem. Pet. Egg., 9 (1973) 73. 12 V. I. Krasnov and I. B. Zhilinskii, Investigations of material reliabilities under gaseous abrasive wearing conditions, Chem. Fet. Eng., 9 (1973) 1029. 13 D. Mills and J. S. Mason, Learning to live with erosion of bends. First International Conference on Protection of Pipes, BHRA Conference, Durham, September 1975, Paper Gl.