- Email: [email protected]
An experimental investigation of a new method for protecting bends from erosion in gas-particle flows
Wear 240 Ž2000. 215–222 www.elsevier.comrlocaterwear
An experimental investigation of a new method for protecting bends from erosion in gas-particle flows Jun Yao a , Benzhao Zhang b, Jianren Fan a,) a
Department of Energy Engineering, Zhejiang UniÕersity, Hangzhou 310027, People’s Republic of China b Department of Mechanics, Zhejiang UniÕersity, Hangzhou 310027, People’s Republic of China Received 2 March 2000; received in revised form 2 March 2000; accepted 2 March 2000
Abstract This paper intends to introduce a new way of bend erosion protection: the ribbed bend erosion protection method. Experimental research has come out fixing ribs on the outer-wall of the inside bend as well as on the wall of the straight pipe extending behind the bend in order to decrease the erosion damage caused by the ‘‘bend effect’’. The experiment shows that ribbed bend technology is a simple and efficient erosion protection method, which can greatly enhance bend erosion protection ability. It also shows that the rib relative height and the rib gap between two adjacent ribs are two important parameters in the ribbed bend erosion protection ability. The same result can be reached on the straight pipe extending from the bend. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas-particle flow; Bend erosion; Experimental; Protection method
1. Introduction As a common phenomenon in industry and daily life, gas-particle flow has been widely used in many fields such as the conveyer pipe system. In spite of the convenience and benefits, many problems are also inevitable. For example, when solid particles are transported through a channel, fan or dust cleaner the particles in the gas flow will cause serious erosion on the surface by impinging. Under these conditions, facility life will be shortened. It is important, therefore, to find an effective erosion protection method to improve equipment life span. Furthermore, research w1x shows that erosion efficiency of bend is 50 times higher than that of straight pipe, therefore, improving bend erosion protection ability is an extremely urgent task.
havior. Tilly w5x illustrates the effect of impact angle on the erosion of different materials by sand particles sieved to 60–125 mm impinging at 340 ftrs. The glass Žbrittle. suffers little erosion at low impact angles and maximum erosion at an impact angle of 908. The aluminum alloy Žductile. shows excellent resistance to erosion at high impact angles, whilst the erosion is maximum at an angle of about 148. The 11% chromium steel exhibits both types of erosion, maximum erosion occurring at about 208 and moderate erosion at 908. This variation is because ductile materials suffer a volume loss by plastic deformation, the material being removed by the cutting action of the eroding particles; whilst the mechanism of brittle erosion is probably due to the Hertzian stress generated by the impinging particles resulting in fracture and removal of material during subsequent impacts.
1.1. The mechanism of erosion Earlier studies w2–4x have shown that ductile and brittle materials exhibit distinctly different types of erosion be-
) Corresponding author. Tel.: q86-0571-7951764; fax: q86-05717951358. E-mail address: [email protected] ŽJ. Fan..
1.2. Physical properties of the abrasiÕe Finnie w2x states that the influence of the eroding particle depends upon its shape, hardness and strength. However, the angle for maximum erosion is not particularly dependent upon shape, and if the particle is harder than the pipe surface then variation in abrasive hardness assumes little importance. The strength of the particles determines
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 5 9 - 8
216
J. Yao et al.r Wear 240 (2000) 215–222
the extent to which they cut as single entity or fracture internally while cutting and so influence the degree of erosion. 1.3. Methods of erosion protection At present, there are several methods that can be used to reduce erosion damage caused by gas-particle flow impingement. Ži. Try to eliminate the existence of particles. For example, to install dust remover or filter on an equipment inlet. The major shortcoming of this method is that it will increase equipment cost and require the regular exchange of dust remover or filter. Besides, it is difficult to eliminate all of the vent small particles. In some cases, for instance, where the particle is transported by pneumatic energy in pipe, the particle phase itself is an actuating medium and it should not be eliminated. Žii. Locate the major erosion place and weld high hardness alloy or coat erosion protection lining Žsuch as alumina cement, diabase, and middle manganese nodular cast iron. there. However, this greatly increases the cost of manufacture. Žiii. Investigate properties of gas-particle flows; analyze particle impinging velocity, impinging angle and place, particle number density at impact and properties of the carrier fluid; conduct researches on the relationship between different material impinging angle and erosion protection properties. Then design the equipment parts to keep the particles impinging angle out of the serious erosion range. This method is usually limited by flow conditions and might be against the work requirement. The present paper is a study of a new method that reduces bend erosion by changing particle motion pattern, decreasing the number of particles impinging on the wall
Table 1 Summary of prediction conditions Parameter
Prediction range
Average particle diameter d p Žmm. Particle diameter d p Žmm. Volume flow Q v Žm3 rh. Standard volume flow Qs Žm3 rh. Crack degree ´ Reynolds number of pipe fluid R e Section area of square pipe A s Žmm2 . Fluid average speed Vm Žmrs. Volume two-phase density C v Weight two-phase density C w Two-phase mixture density rm Žkgrm3 . Bend curvature R r D
50 20–80 30 59.2769 0.99673 8.826=10 4 20=20 41.2 0.327% 84.483% 7.74 3
and reducing particle impacting speed. In the test, a certain number of ribs with certain cross-section shape were fixed on the outer wall of the inside pipe and then intensive turbulent flow was produced in the vicinity of the ribs Žsee Fig. 4.. The turbulent layer reduced the momentum of particles tending to impact on the wall and decrease their speed and change their trajectories. As a result, wall erosion damage is greatly reduced. Previous work w6x has demonstrated that ribbed straight pipe would reduce erosion. Another paper w7x introduced a finned tube erosion protection method. The experiment result showed that the technology could be used to protect tube from erosion effectively. Inspired by the above works, along with the investigation of bend particulate two-phase flow character and pattern w8x and bend erosion factors, the present paper adds ribs on the outer-wall inside the bend in order to protect the erosion.
2. Experimental details 2.1. Apparatus and instruments Schematic drawing of overall experiment system is shown in Fig. 1. The experiments were carried out in the facility Žsee Fig. 1. at the Department of Mechanics of Zhejiang University. The erosion tunnel consists of a long duct into which the particles are fed at a controlled rate. The parti-
Fig. 1. Schematic drawing of overall experiment system. Ž1. Air compressor, Ž2. oil water separator, Ž3. stop valve, Ž4. gas reservoir, Ž5. stop valve, Ž6. compressed air filtering reduce pressure valve ŽQPJ-1 type., Ž7. pressure gauge, Ž8. stop valve, Ž9. glass rotor flow-meter ŽLZB-50 type., Ž10. stop valve, Ž11. stop valve, Ž12. pneumatic convey store pump, Ž13. stop valve, Ž14. exhaust valve, Ž15. revolving wind separator, Ž16. specimen sealed chamber, Ž17. power supply, Ž18. blower.
Fig. 2. Bend specimen part picture 1.
J. Yao et al.r Wear 240 (2000) 215–222
217
Table 2 Bend specimens comprehensive data table
Fig. 3. Bend specimen part picture 2. Ž1. Straight pipe specimen, Ž2. bend specimen.
cles entrained in the gas reach a steady velocity before impacting on the specimens at the test section. Each specimen was tested three times. During each test, a researcher regulated the glass rotor flow meter at all time in order to keep the flow’s speed constant. In order to reduce the measurement error, the specimens were weighed accurately using a precision balance. The specimens were washed in acetone before measurement. The abrasive particles used in the experiment are pulverized coal that is used as fuel in many power plants. Before the experiment, researchers dried and sifted the abrasive particles. The specimen material was plexiglass and its wear property was similar to medium carbon steel, which has been widely used as transport pipe material in China. In the test, 4000 kg abrasive particles were used and 50 pieces of specimen were made by machining and highly finished by hand. Experiment parameters are shown in Table 1. 2.2. Specimens design The test arranged ribs not only on the bend wall Žsee Figs. 2 and 3., but also on the extending straight pipe wall Ž LrD s 6.. It could help study the ribs erosion protection
Fig. 4. Experiment model schematic drawing. Ž1. Inlet pipe Ž f 30., Ž2. variable diameter nipple, Ž3. test model Ž f 20., Ž4. bend specimen, Ž5. straight pipe specimen. View I. A: 20 mm; B: 20 mm; T : 5 mm; W, H, S: refer to Table 2 and 3.
Specimens
H Žmm.
W Žmm.
Sr H
Hr D L
AR
Rib numbers
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4 F1 F2 F3 F4 O
5 5 5 5 4 4 4 4 3 3 3 3 5 5 5 5 4 4 4 4 3 3 3 3
5 5 5 5 4 4 4 4 3 3 3 3 2.5 2.5 2.5 2.5 2 2 2 2 2 2 2 2
1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6
0.25 0.25 0.25 0.25 0.2 0.2 0.2 0.2 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.2 0.2 0.2 0.2 0.15 0.15 0.15 0.15
1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 1.5 1.5 1.5 1.5
10 7 5 4 13 9 7 5 17 12 9 6 13 9 6 4 16 11 8 5 20 14 9 7 0
effect fully. Experiment model schematic drawing is shown in Fig. 4. The comprehensive data of ribs on bend wall are shown in Table 2. Table 3 Straight pipe specimens comprehensive data table Specimens
H Žmm.
W Žmm.
Sr H
Rib number
a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4 d1 d2 d3 d4 e1 e2 e3 e4 f1 f2 f3 f4 o
5 5 5 5 4 4 4 4 3 3 3 3 5 5 5 5 4 4 4 4 3 3 3 3
5 5 5 5 4 4 4 4 3 3 3 3 2.5 2.5 2.5 2.5 2 2 2 2 2 2 2 2
1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6
9 7 5 3 11 9 6 4 14 11 7 6 11 8 5 4 15 10 6 5 17 12 9 6 0
J. Yao et al.r Wear 240 (2000) 215–222
218
All data of ribs on the straight pipe wall behind bend are shown in Table 3.
3. Results and discussion 3.1. Definition At first, two concepts are to be defined: erosion rate Ee and erosion protection efficiency Ea . Ee s
D Ms Ma
,
Ea s 1 y
Ere Enre
.
Erosion rate Ee is defined as the ratio of the eroded mass of wall material D Ms to the mass of impacting abrasive particles Ma , which is measured by electron balance and ribs are still stuck on the specimen when they are measured. Erosion rate Ee describes the extent of erosion on a single specimen. It can be used to compare the extent of erosion on different specimen under the same experimental condition and then to find out the optimum specimen with the optimum arrangement and the optimum cross-section geometric figure. The lower the erosion rate, the better the erosion protection effect is. Erosion protection efficiency Ea is defined as the ratio of the difference of erosion rate between non-ribbed bend specimen and ribbed bend specimen Ž Enre y Ere . to the erosion rate of non-ribbed bend specimen Enre Ž Enre is the
Ee of non-ribbed specimen and Ere is the Ee of ribbed specimen.. One kind of the specimens’ erosion protection effect is reflected by the comparison among different erosion protection efficiency that can help to identify the optimum rib arrangement and the optimum cross-section geometric figure. The higher the erosion protection efficiency, the better the erosion protection effect is. So, the lower erosion rate, the better the erosion protection effect is and the higher erosion protection efficiency, the better the erosion protection effect is. It is clear that a way with lower erosion rate or higher erosion protection efficiency is an ideal way for the conveyer pipe system of gas-particle flow. Bend specimens comprehensive analysis table is shown in Table 4. 3.2. Comparison of ribbed and non-ribbed specimens 3.2.1. Comparing erosion of ribbed and non-ribbed specimens by data Non-ribbed bend erosion rate 1: 2.741 = 10y6 . The lowest ribbed bend specimen erosion rate 2: 0.123 = 10y6 . The average of ribbed bend specimens erosion rate 3: 0.2465 = 10y6 . Ribbed bend specimen erosion protection efficiency: 1 y 0.2465 = 10y6 r2.741 = 10y6 s 91.01%. Therefore, it is obvious that ribs on the inside of the pipe’s outer wall can achieve erosion protection efficiency of 91.01%. It means that the erosion rate of the ribbed
Table 4 Bend specimens comprehensive analysis table Specimens
Pre-test weight Žg.
Post-test weight Žg.
Erosion amount Žmg.
Used particles Žkg.
Erosion rate Žmass. Ž=10y6 .
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4 F1 F2 F3 F4 O
30.59230 29.73796 28.14233 26.45364 29.09200 26.92771 26.93958 26.56630 27.66235 27.08472 27.44541 24.58583 27.48184 27.98685 27.10020 27.40911 29.41499 26.68006 26.62105 25.32877 30.52161 26.68370 27.24983 25.94893 22.82421
30.55775 29.72329 28.13080 26.43042 29.07862 26.92031 26.93055 26.54982 27.64765 27.07176 27.43234 24.57666 27.47012 27.97744 27.08943 27.38301 29.40531 26.67210 26.61135 25.31558 30.51222 26.67713 27.23911 25.93914 22.52265
34.55 14.67 11.53 23.22 13.38 7.4 9.03 16.48 14.7 12.96 13.07 9.17 11.72 9.41 10.77 26.1 9.68 7.96 9.7 13.19 9.39 6.57 10.72 9.79 301.56
78 49 49.75 48.75 49.5 49.75 50 50.5 50.75 51.5 50.75 51 52 51 51.25 52.25 52.5 52.5 53 54 53.5 53.25 54 55 110
0.443 0.299 0.232 0.476 0.270 0.149 0.181 0.326 0.290 0.252 0.258 0.180 0.225 0.185 0.210 0.500 0.184 0.152 0.183 0.244 0.176 0.123 0.199 0.178 2.741
J. Yao et al.r Wear 240 (2000) 215–222
219
bend is decreased by 91.01% in comparison with the non-ribbed bend. It is evident that technology can largely improve bend erosion protection ability. 3.2.2. The erosion state of specimens From the specimen surface, the erosion state of nonribbed specimen is fairly serious. There are many obvious wavy strips engraved on the surface and the most serious erosion happened about 20–358 and 70–858 Žcoinciding with Ref. w8x.. On the other hand, there is no clear erosion scar on the surface of ribbed specimen and only some burrs appeared on the edge of ribs. The erosion of ribs specimen is so little that it is as new as the original ones. No hole or worn place appeared on all specimens by the end of the test. The test inferred that the installment of ribs altered the distribution of wear and turned the wear-concentrated wear to well-distributed faint erosion. 3.3. Effect of rib arrangement on erosion protection In the test, there are four kinds of arrangement of ribs, 1, 2, 3, and 4, corresponding to SrH s 1.5, 2.5, 4, and 6. Considering the relationship between rib arrangement and erosion rate, the present paper compares the erosion of ribs with different arrangement under the prerequisite condition that their cross-section geometric figures Ž H = W . are equal. In Fig. 5, we can see that A3, B2, C4, D2, E2, and F2 are the ribs, respectively, with the lowest erosion rate. ŽA2 and C2 are the second lowest erosion ribs in their corresponding group.. It indicates that the second arrangement type Ž SrH s 2.5. is the optimum arrangement type. In order to confirm this conclusion, the average erosion rates of the four arrangements are listed and compared. Fig. 6 shows that the average erosion rate of the second arrangement is the lowest.
Fig. 5. Effect of ribs arrangement on erosion rate Žunder the same ribs cross-section figure, comparing erosion rate of ribs with different arrangement..
Fig. 6. Average erosion rate of the four kinds of ribs with different arrangement.
With the same cross-section geometric figure, the second type of rib arrangement possesses the lowest erosion rate, that is the highest erosion protection efficiency. In the test, we can say that SrH s 2.5 is the optimum arrangement type for the bend to attain the optimal erosion protection effect. 3.4. Effect of rib cross-section shape on erosion protection In the test, there are six kinds of cross-section geometric figure of ribs, A, B, C, D, E, and F, corresponding to 5 = 5, 4 = 4, 3 = 3, 5 = 2.5, 4 = 2, and 3 = 2. Considering the relationship between ribs shape and erosion, the present paper compares the erosion of ribs with different cross-section geometric figures under the prerequisite condition that their arrangements are equal. From Fig. 7, we infer that F1, F2, B3, and F4 are the ribs, respectively, with the lowest erosion rate. It implies
Fig. 7. Effect of ribs cross-section figure on erosion rate Žunder the same ribs arrangement, comparing erosion rate of ribs with different cross-section figure..
J. Yao et al.r Wear 240 (2000) 215–222
220
same height condition, rectangle ribs possess higher erosion protection efficiency than square ribs do. 3.5. Study of the erosion of straight pipe extending behind bend
Fig. 8. Average erosion rate of the six kinds of ribs with different cross-section geometric figure.
that the F kind of ribs which possess cross-section geometric figure 3 = 2 Ž H = W . are more beneficial in terms of improving bend erosion protection ability. In order to confirm this conclusion, the average erosion rates of the six kinds of cross-section geometric figure ribs are listed and compared in Fig. 8, which shows that F type ribs possess the lowest average erosion rate, meaning the highest erosion protection efficiency. It says that with the same arrangement, ribs with 3 = 2 cross-section geometric figure can make bend attain optimum erosion protection ability. In the present test, the height of square A, B, and C is correspondingly equal to that of rectangle D, E, and F. Fig. 8 shows that A, B, and C erosion rate is correspondingly higher than D, E, and F erosion rate. Therefore, under the
Under the force of bend centrifugation and particle inertia strength, particles turn to flow along one side of the pipe and concentrate close to the wall, which cause the density of particle in particulate two-phase flow keep intensive in the closed-wall area. The tendency will keep a certain distance w9x Ž LrD s 5–6. of following bend and cause serious erosion on the wall. In the present test, ribs were added on the straight pipe wall extending from the bend in order to investigate its erosion protection result. For the test, ribs were arranged extending on the straight pipe wall behind the bend Žsee Figs. 3 and 4.. Using the same approach as above, comparing the erosion rate of ribbed straight pipe with that of non-ribbed straight pipe, we can get ribbed straight pipe erosion protection efficiency: 26.14%. It means that the erosion rate of the ribbed straight pipe is decreased by 26.14% in comparison with non-ribbed straight pipe. The study of the effect of rib arrangement on erosion protection shows that a rib arrangement of SrH s 1.5 is optimal. The study of the effect of rib cross-section figure on erosion protection shows that ribs with cross-section figure as 3 = 2 possess the optimum erosion protection efficiency.
Table 5 Straight pipe specimen comprehensive analysis table Specimens
Pre-test mass Žg.
Post-test mass Žg.
Erosion amount Žmg.
Used particles Žkg.
Erosion rate Ž=10y6 .
a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4 d1 d2 d3 d4 e1 e2 e3 e4 f1 f2 f3 f4 o
25.82724 26.93390 24.16488 22.32614 24.82717 23.87342 21.96819 23.09949 23.03308 22.59415 21.67349 22.23158 23.42307 23.61328 23.48660 25.60587 24.24126 23.60492 22.05960 21.87226 24.89429 23.81912 22.57637 22.17177 19.13203
25.81373 26.92562 24.15162 22.31545 24.81956 23.86421 21.96602 23.09340 23.02975 22.58704 21.66842 22.22825 23.41871 23.60815 23.47656 25.59089 24.24107 23.60095 22.05849 21.86628 24.89396 23.81739 22.57315 22.16975 19.11517
13.51 8.28 13.26 10.69 7.61 9.21 2.17 6.09 3.33 7.11 5.07 3.33 4.36 5.13 10.04 14.98 0.19 3.97 1.11 5.98 0.33 1.73 3.22 2.02 16.86
78 49 49.75 48.75 49.5 49.75 50 50.5 50.75 51.5 50.75 51 52 51 51.25 52.25 52.5 52.5 53 54 53.5 53.25 54 55 110
0.173 0.170 0.267 0.219 0.154 0.185 0.043 0.121 0.066 0.138 0.100 0.065 0.084 0.101 0.196 0.287 0.004 0.076 0.021 0.111 0.006 0.032 0.060 0.037 0.153
J. Yao et al.r Wear 240 (2000) 215–222
221
Comprehensive data of straight pipe specimen is shown in Table 5. 3.6. Comparison of bend erosion with straight pipe erosion Enre
2.741 s
Enrse
0.153
Ere
0.2465 s
Erse
0.113
s 17.915
s 2.181.
Several conclusions can be made from the data above. Ž1. With no ribs, the erosion rate of a bend is 17.915 times as high as that of the following straight pipe. It means that under the same condition, the erosion of bend is far more serious than that of the straight pipe behind it. Ž2. With ribs, the erosion rate of bend is only 2.181 times as high as that of the following straight sectional pipe. The difference between them decreases significantly. Ž3. Comparing the effect of adding ribs on bend wall with that of adding ribs on behind-extending straight pipe wall Enrse
0.153 s
Erse
0.113
Enre
2.714
Ere
s 0.2465
Fig. 9. Experimental results of erosion rate for ribbed bend and non-ribbed bend Ž1: non-ribbed bend erosion rate, 2: the lowest ribbed bend specimens erosion rate, 3: the average of ribbed bend specimens erosion rate..
ribbed specimen and ribbed specimen, which is caused by ribs. Because of centrifugal force and inertia strength, particles will flow along closed-wall surface in the bend and continually extend this trend in a certain length of straight pipe just behind the bend and then it will enlarge erosion field. Therefore adding ribs on this field will also improve the straight pipe erosion protection ability to some degree.
s 1.354, 4. Conclusions s 11.120.
It shows that adding ribs on the outer wall of the inside bend will reduce erosion to a greater extent. That is to say, adding ribs on the outer wall of inside pipe is more effective for bend than for straight pipe in improving their erosion protection ability. 3.7. Reasons for ribs erosion protection effect Based on some physical knowledge, we can know that gas-particle two-phase flow with certain speed will cause particle to impact on the wall by inertia strength and centrifugal force in the bend. Particles with high speed will not only cause impingement erosion on the wall but also bring rubbing erosion to the wall by some force because of the particles sliding and rolling on the wall surface. Adding ribs on the wall will alter this situation. On the one hand, ribs can enhance fluid turbulence and disrupt the usual motion of particles along the wall boundary fluid field, absorb particle solid energy used for impinging on the wall and decrease the number of particles impacting on the wall directly. On the other hand, ribs will prevent particles from sliding and rolling along the wall surface, change their motion direction, and lead particles jump out of the wall surface. In this way, ribs will greatly reduce particle erosion degree. From erosion protection degree, the latter reason exerts more important effect. From Fig. 9, one can get an obvious difference of erosion rate between non-
The following conclusions can be drawn from the tests above. Ž1. Adding ribs on the outer-wall of the inside bend can significantly improve bend’s erosion protection ability. The erosion protection efficiency in the test is as high as 91.01%. In other words, the erosion rate of the ribbed bend is decreased by 91.01% in comparison with the non-ribbed bend. Ž2. The investigation into bend property shows that the rib relative height and the rib gap between two adjacent ribs are two important parameters that affect ribbed bend erosion protection ability. Ribs height being equal, different ribs arrangement possesses different erosion protection efficiency. In the test, SrH s 2.5 is the optimum arrangement type for bend to attain the optimum erosion protection effect. On the other hand, ribs arrangement being equal, different rib cross-section geometric figure possesses different erosion protection efficiency. In the test, ribs with 3 = 2 cross-section shape can make bend attain the optimum erosion protection ability. In addition, rectangle ribs possess higher erosion protection efficiency than square ribs do when they have equal height. Ž3. The results of experiments on straight pipe extending behind a bend shows that adding ribs on the straight pipe wall is also an efficient erosion protection method. In the test, the erosion protection efficiency is about 26.14%. It means that the erosion rate of the ribbed straight pipe is decreased by 26.14% in comparison with the non-ribbed straight pipe. Besides, ribs arrangement and cross-section
J. Yao et al.r Wear 240 (2000) 215–222
222
geometric figure will also affect rib’s erosion protection efficiency. As a result, the test shows that ribs arrangement as SrH s 1.5 and rib’s cross-section figure as 3 = 2 is, respectively, the optimum arrangement and the optimum cross-section shape to reach the optimum erosion protection efficiency. Ž4. Comparing bend erosion with following straight pipe erosion, it is obvious that smooth bend erosion is as 17.915 times high as smooth straight pipe erosion. However, once ribs are added to the outer wall inside of pipe, ribbed bend erosion is decreased to only 2.181 times that of ribbed straight pipe erosion. It is concluded therefore that adding ribs to the outer wall is more beneficial to bends than straight pipe in terms of improving erosion protection ability. The results from the present experiment show that the method of fixing ribs on bend wall and its extending straight pipe wall is a simple and efficient erosion protection method. However, it still has some problems. As we all know, erosion experiments are sensitive to different conditions, equipment state and specimen type and number. The more kinds of specimen in the experiment, the more accurate the conclusions which can be drawn. The more experiment conditions there is, the more convincing the results are. The problem will be progressively solved in subsequent study. The present paper is only concerned with some basic properties of the ribbed bend found in the initial research. It is further noted that the ribbed bend with the lower erosion creates a turbulent ‘stagnant’ pool of air in the vicinity of the ribs that may affect the transport efficiency of the conveyer. The benefit between the material transfer efficiency and the wear rates will contribute to understanding in the design choices of real systems.
5. Nomenclature A AR As B Cv Cw
Width of duct Ž20 mm. HrW Section area of square pipe Žmm2 . Height of duct Ž20 mm. Volume two-phase density Weight two-phase density
D DL dp dp Ea Ee Enre Enrse Ere Erse H L Ma D Ms Qs Qv RrD Re S T Vm W rm ´
Pipe diameter Žmm. Duct hydraulic diameter Žmm. Average particle diameter Žmm. Particle diameter Žmm. Erosion protection efficiency Erosion rate Bend non-ribbed specimen erosion rate Non-ribbed straight pipe specimen erosion rate Bend ribbed specimen erosion rate Ribbed straight pipe specimen erosion rate Height of rib Žmm. Pipe lenth Žmm. Used abraisive particles mass Žkg. Specimen erosion mass Žmg. Standard volume flow Žm3rh. Volume flow Žm3rh. Bend curvature Reynolds number of pipe fluid Pith length Žmm. Thickness of pipe wall Ž5 mm. Fluid average speed Žmrs. Width of rib Žmm. Two-phase mixture density Žkgrm3 . Crack degre
Acknowledgements This project was supported by National Natural Science Foundation of China.
References w1x G.P. Tilly, Mater. Sci. Technol. 13 Ž1979. 287. w2x I. Finnie, Wear 3 Ž1960. 87. w3x I. Finnie, Symposium on erosion and cavitation, ASTM Spec. Tech. Publ. 307 Ž1962. 70. w4x I. Finnie, J. Wolak, Y. Kabil, J. Mater. 2 Ž1967. 682. w5x G.P. Tilly, Wear 14 Ž1969. 63. w6x X.Q. Song, J.Z. Lin, J.F. Zhao, T.Y. Shen, Wear 193 Ž1996. 1–7. w7x J. Fan, D. Zhou, K. Zeng, K. Cen, Wear 152 Ž1992. 1–19. w8x J.S. Mason, B.V. Smith, Powder Technol. 6 Ž1972. 323–335. w9x B. Chai, T.M. Xu, J. Xianjiaotong Univ. 28 Ž5. Ž1994. August.
An experimental investigation of a new method for protecting bends from erosion in gas-particle flows Jun Yao a , Benzhao Zhang b, Jianren Fan a,) a
Department of Energy Engineering, Zhejiang UniÕersity, Hangzhou 310027, People’s Republic of China b Department of Mechanics, Zhejiang UniÕersity, Hangzhou 310027, People’s Republic of China Received 2 March 2000; received in revised form 2 March 2000; accepted 2 March 2000
Abstract This paper intends to introduce a new way of bend erosion protection: the ribbed bend erosion protection method. Experimental research has come out fixing ribs on the outer-wall of the inside bend as well as on the wall of the straight pipe extending behind the bend in order to decrease the erosion damage caused by the ‘‘bend effect’’. The experiment shows that ribbed bend technology is a simple and efficient erosion protection method, which can greatly enhance bend erosion protection ability. It also shows that the rib relative height and the rib gap between two adjacent ribs are two important parameters in the ribbed bend erosion protection ability. The same result can be reached on the straight pipe extending from the bend. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas-particle flow; Bend erosion; Experimental; Protection method
1. Introduction As a common phenomenon in industry and daily life, gas-particle flow has been widely used in many fields such as the conveyer pipe system. In spite of the convenience and benefits, many problems are also inevitable. For example, when solid particles are transported through a channel, fan or dust cleaner the particles in the gas flow will cause serious erosion on the surface by impinging. Under these conditions, facility life will be shortened. It is important, therefore, to find an effective erosion protection method to improve equipment life span. Furthermore, research w1x shows that erosion efficiency of bend is 50 times higher than that of straight pipe, therefore, improving bend erosion protection ability is an extremely urgent task.
havior. Tilly w5x illustrates the effect of impact angle on the erosion of different materials by sand particles sieved to 60–125 mm impinging at 340 ftrs. The glass Žbrittle. suffers little erosion at low impact angles and maximum erosion at an impact angle of 908. The aluminum alloy Žductile. shows excellent resistance to erosion at high impact angles, whilst the erosion is maximum at an angle of about 148. The 11% chromium steel exhibits both types of erosion, maximum erosion occurring at about 208 and moderate erosion at 908. This variation is because ductile materials suffer a volume loss by plastic deformation, the material being removed by the cutting action of the eroding particles; whilst the mechanism of brittle erosion is probably due to the Hertzian stress generated by the impinging particles resulting in fracture and removal of material during subsequent impacts.
1.1. The mechanism of erosion Earlier studies w2–4x have shown that ductile and brittle materials exhibit distinctly different types of erosion be-
) Corresponding author. Tel.: q86-0571-7951764; fax: q86-05717951358. E-mail address: [email protected] ŽJ. Fan..
1.2. Physical properties of the abrasiÕe Finnie w2x states that the influence of the eroding particle depends upon its shape, hardness and strength. However, the angle for maximum erosion is not particularly dependent upon shape, and if the particle is harder than the pipe surface then variation in abrasive hardness assumes little importance. The strength of the particles determines
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 5 9 - 8
216
J. Yao et al.r Wear 240 (2000) 215–222
the extent to which they cut as single entity or fracture internally while cutting and so influence the degree of erosion. 1.3. Methods of erosion protection At present, there are several methods that can be used to reduce erosion damage caused by gas-particle flow impingement. Ži. Try to eliminate the existence of particles. For example, to install dust remover or filter on an equipment inlet. The major shortcoming of this method is that it will increase equipment cost and require the regular exchange of dust remover or filter. Besides, it is difficult to eliminate all of the vent small particles. In some cases, for instance, where the particle is transported by pneumatic energy in pipe, the particle phase itself is an actuating medium and it should not be eliminated. Žii. Locate the major erosion place and weld high hardness alloy or coat erosion protection lining Žsuch as alumina cement, diabase, and middle manganese nodular cast iron. there. However, this greatly increases the cost of manufacture. Žiii. Investigate properties of gas-particle flows; analyze particle impinging velocity, impinging angle and place, particle number density at impact and properties of the carrier fluid; conduct researches on the relationship between different material impinging angle and erosion protection properties. Then design the equipment parts to keep the particles impinging angle out of the serious erosion range. This method is usually limited by flow conditions and might be against the work requirement. The present paper is a study of a new method that reduces bend erosion by changing particle motion pattern, decreasing the number of particles impinging on the wall
Table 1 Summary of prediction conditions Parameter
Prediction range
Average particle diameter d p Žmm. Particle diameter d p Žmm. Volume flow Q v Žm3 rh. Standard volume flow Qs Žm3 rh. Crack degree ´ Reynolds number of pipe fluid R e Section area of square pipe A s Žmm2 . Fluid average speed Vm Žmrs. Volume two-phase density C v Weight two-phase density C w Two-phase mixture density rm Žkgrm3 . Bend curvature R r D
50 20–80 30 59.2769 0.99673 8.826=10 4 20=20 41.2 0.327% 84.483% 7.74 3
and reducing particle impacting speed. In the test, a certain number of ribs with certain cross-section shape were fixed on the outer wall of the inside pipe and then intensive turbulent flow was produced in the vicinity of the ribs Žsee Fig. 4.. The turbulent layer reduced the momentum of particles tending to impact on the wall and decrease their speed and change their trajectories. As a result, wall erosion damage is greatly reduced. Previous work w6x has demonstrated that ribbed straight pipe would reduce erosion. Another paper w7x introduced a finned tube erosion protection method. The experiment result showed that the technology could be used to protect tube from erosion effectively. Inspired by the above works, along with the investigation of bend particulate two-phase flow character and pattern w8x and bend erosion factors, the present paper adds ribs on the outer-wall inside the bend in order to protect the erosion.
2. Experimental details 2.1. Apparatus and instruments Schematic drawing of overall experiment system is shown in Fig. 1. The experiments were carried out in the facility Žsee Fig. 1. at the Department of Mechanics of Zhejiang University. The erosion tunnel consists of a long duct into which the particles are fed at a controlled rate. The parti-
Fig. 1. Schematic drawing of overall experiment system. Ž1. Air compressor, Ž2. oil water separator, Ž3. stop valve, Ž4. gas reservoir, Ž5. stop valve, Ž6. compressed air filtering reduce pressure valve ŽQPJ-1 type., Ž7. pressure gauge, Ž8. stop valve, Ž9. glass rotor flow-meter ŽLZB-50 type., Ž10. stop valve, Ž11. stop valve, Ž12. pneumatic convey store pump, Ž13. stop valve, Ž14. exhaust valve, Ž15. revolving wind separator, Ž16. specimen sealed chamber, Ž17. power supply, Ž18. blower.
Fig. 2. Bend specimen part picture 1.
J. Yao et al.r Wear 240 (2000) 215–222
217
Table 2 Bend specimens comprehensive data table
Fig. 3. Bend specimen part picture 2. Ž1. Straight pipe specimen, Ž2. bend specimen.
cles entrained in the gas reach a steady velocity before impacting on the specimens at the test section. Each specimen was tested three times. During each test, a researcher regulated the glass rotor flow meter at all time in order to keep the flow’s speed constant. In order to reduce the measurement error, the specimens were weighed accurately using a precision balance. The specimens were washed in acetone before measurement. The abrasive particles used in the experiment are pulverized coal that is used as fuel in many power plants. Before the experiment, researchers dried and sifted the abrasive particles. The specimen material was plexiglass and its wear property was similar to medium carbon steel, which has been widely used as transport pipe material in China. In the test, 4000 kg abrasive particles were used and 50 pieces of specimen were made by machining and highly finished by hand. Experiment parameters are shown in Table 1. 2.2. Specimens design The test arranged ribs not only on the bend wall Žsee Figs. 2 and 3., but also on the extending straight pipe wall Ž LrD s 6.. It could help study the ribs erosion protection
Fig. 4. Experiment model schematic drawing. Ž1. Inlet pipe Ž f 30., Ž2. variable diameter nipple, Ž3. test model Ž f 20., Ž4. bend specimen, Ž5. straight pipe specimen. View I. A: 20 mm; B: 20 mm; T : 5 mm; W, H, S: refer to Table 2 and 3.
Specimens
H Žmm.
W Žmm.
Sr H
Hr D L
AR
Rib numbers
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4 F1 F2 F3 F4 O
5 5 5 5 4 4 4 4 3 3 3 3 5 5 5 5 4 4 4 4 3 3 3 3
5 5 5 5 4 4 4 4 3 3 3 3 2.5 2.5 2.5 2.5 2 2 2 2 2 2 2 2
1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6
0.25 0.25 0.25 0.25 0.2 0.2 0.2 0.2 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.2 0.2 0.2 0.2 0.15 0.15 0.15 0.15
1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 1.5 1.5 1.5 1.5
10 7 5 4 13 9 7 5 17 12 9 6 13 9 6 4 16 11 8 5 20 14 9 7 0
effect fully. Experiment model schematic drawing is shown in Fig. 4. The comprehensive data of ribs on bend wall are shown in Table 2. Table 3 Straight pipe specimens comprehensive data table Specimens
H Žmm.
W Žmm.
Sr H
Rib number
a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4 d1 d2 d3 d4 e1 e2 e3 e4 f1 f2 f3 f4 o
5 5 5 5 4 4 4 4 3 3 3 3 5 5 5 5 4 4 4 4 3 3 3 3
5 5 5 5 4 4 4 4 3 3 3 3 2.5 2.5 2.5 2.5 2 2 2 2 2 2 2 2
1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6 1.5 2.5 4 6
9 7 5 3 11 9 6 4 14 11 7 6 11 8 5 4 15 10 6 5 17 12 9 6 0
J. Yao et al.r Wear 240 (2000) 215–222
218
All data of ribs on the straight pipe wall behind bend are shown in Table 3.
3. Results and discussion 3.1. Definition At first, two concepts are to be defined: erosion rate Ee and erosion protection efficiency Ea . Ee s
D Ms Ma
,
Ea s 1 y
Ere Enre
.
Erosion rate Ee is defined as the ratio of the eroded mass of wall material D Ms to the mass of impacting abrasive particles Ma , which is measured by electron balance and ribs are still stuck on the specimen when they are measured. Erosion rate Ee describes the extent of erosion on a single specimen. It can be used to compare the extent of erosion on different specimen under the same experimental condition and then to find out the optimum specimen with the optimum arrangement and the optimum cross-section geometric figure. The lower the erosion rate, the better the erosion protection effect is. Erosion protection efficiency Ea is defined as the ratio of the difference of erosion rate between non-ribbed bend specimen and ribbed bend specimen Ž Enre y Ere . to the erosion rate of non-ribbed bend specimen Enre Ž Enre is the
Ee of non-ribbed specimen and Ere is the Ee of ribbed specimen.. One kind of the specimens’ erosion protection effect is reflected by the comparison among different erosion protection efficiency that can help to identify the optimum rib arrangement and the optimum cross-section geometric figure. The higher the erosion protection efficiency, the better the erosion protection effect is. So, the lower erosion rate, the better the erosion protection effect is and the higher erosion protection efficiency, the better the erosion protection effect is. It is clear that a way with lower erosion rate or higher erosion protection efficiency is an ideal way for the conveyer pipe system of gas-particle flow. Bend specimens comprehensive analysis table is shown in Table 4. 3.2. Comparison of ribbed and non-ribbed specimens 3.2.1. Comparing erosion of ribbed and non-ribbed specimens by data Non-ribbed bend erosion rate 1: 2.741 = 10y6 . The lowest ribbed bend specimen erosion rate 2: 0.123 = 10y6 . The average of ribbed bend specimens erosion rate 3: 0.2465 = 10y6 . Ribbed bend specimen erosion protection efficiency: 1 y 0.2465 = 10y6 r2.741 = 10y6 s 91.01%. Therefore, it is obvious that ribs on the inside of the pipe’s outer wall can achieve erosion protection efficiency of 91.01%. It means that the erosion rate of the ribbed
Table 4 Bend specimens comprehensive analysis table Specimens
Pre-test weight Žg.
Post-test weight Žg.
Erosion amount Žmg.
Used particles Žkg.
Erosion rate Žmass. Ž=10y6 .
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4 F1 F2 F3 F4 O
30.59230 29.73796 28.14233 26.45364 29.09200 26.92771 26.93958 26.56630 27.66235 27.08472 27.44541 24.58583 27.48184 27.98685 27.10020 27.40911 29.41499 26.68006 26.62105 25.32877 30.52161 26.68370 27.24983 25.94893 22.82421
30.55775 29.72329 28.13080 26.43042 29.07862 26.92031 26.93055 26.54982 27.64765 27.07176 27.43234 24.57666 27.47012 27.97744 27.08943 27.38301 29.40531 26.67210 26.61135 25.31558 30.51222 26.67713 27.23911 25.93914 22.52265
34.55 14.67 11.53 23.22 13.38 7.4 9.03 16.48 14.7 12.96 13.07 9.17 11.72 9.41 10.77 26.1 9.68 7.96 9.7 13.19 9.39 6.57 10.72 9.79 301.56
78 49 49.75 48.75 49.5 49.75 50 50.5 50.75 51.5 50.75 51 52 51 51.25 52.25 52.5 52.5 53 54 53.5 53.25 54 55 110
0.443 0.299 0.232 0.476 0.270 0.149 0.181 0.326 0.290 0.252 0.258 0.180 0.225 0.185 0.210 0.500 0.184 0.152 0.183 0.244 0.176 0.123 0.199 0.178 2.741
J. Yao et al.r Wear 240 (2000) 215–222
219
bend is decreased by 91.01% in comparison with the non-ribbed bend. It is evident that technology can largely improve bend erosion protection ability. 3.2.2. The erosion state of specimens From the specimen surface, the erosion state of nonribbed specimen is fairly serious. There are many obvious wavy strips engraved on the surface and the most serious erosion happened about 20–358 and 70–858 Žcoinciding with Ref. w8x.. On the other hand, there is no clear erosion scar on the surface of ribbed specimen and only some burrs appeared on the edge of ribs. The erosion of ribs specimen is so little that it is as new as the original ones. No hole or worn place appeared on all specimens by the end of the test. The test inferred that the installment of ribs altered the distribution of wear and turned the wear-concentrated wear to well-distributed faint erosion. 3.3. Effect of rib arrangement on erosion protection In the test, there are four kinds of arrangement of ribs, 1, 2, 3, and 4, corresponding to SrH s 1.5, 2.5, 4, and 6. Considering the relationship between rib arrangement and erosion rate, the present paper compares the erosion of ribs with different arrangement under the prerequisite condition that their cross-section geometric figures Ž H = W . are equal. In Fig. 5, we can see that A3, B2, C4, D2, E2, and F2 are the ribs, respectively, with the lowest erosion rate. ŽA2 and C2 are the second lowest erosion ribs in their corresponding group.. It indicates that the second arrangement type Ž SrH s 2.5. is the optimum arrangement type. In order to confirm this conclusion, the average erosion rates of the four arrangements are listed and compared. Fig. 6 shows that the average erosion rate of the second arrangement is the lowest.
Fig. 5. Effect of ribs arrangement on erosion rate Žunder the same ribs cross-section figure, comparing erosion rate of ribs with different arrangement..
Fig. 6. Average erosion rate of the four kinds of ribs with different arrangement.
With the same cross-section geometric figure, the second type of rib arrangement possesses the lowest erosion rate, that is the highest erosion protection efficiency. In the test, we can say that SrH s 2.5 is the optimum arrangement type for the bend to attain the optimal erosion protection effect. 3.4. Effect of rib cross-section shape on erosion protection In the test, there are six kinds of cross-section geometric figure of ribs, A, B, C, D, E, and F, corresponding to 5 = 5, 4 = 4, 3 = 3, 5 = 2.5, 4 = 2, and 3 = 2. Considering the relationship between ribs shape and erosion, the present paper compares the erosion of ribs with different cross-section geometric figures under the prerequisite condition that their arrangements are equal. From Fig. 7, we infer that F1, F2, B3, and F4 are the ribs, respectively, with the lowest erosion rate. It implies
Fig. 7. Effect of ribs cross-section figure on erosion rate Žunder the same ribs arrangement, comparing erosion rate of ribs with different cross-section figure..
J. Yao et al.r Wear 240 (2000) 215–222
220
same height condition, rectangle ribs possess higher erosion protection efficiency than square ribs do. 3.5. Study of the erosion of straight pipe extending behind bend
Fig. 8. Average erosion rate of the six kinds of ribs with different cross-section geometric figure.
that the F kind of ribs which possess cross-section geometric figure 3 = 2 Ž H = W . are more beneficial in terms of improving bend erosion protection ability. In order to confirm this conclusion, the average erosion rates of the six kinds of cross-section geometric figure ribs are listed and compared in Fig. 8, which shows that F type ribs possess the lowest average erosion rate, meaning the highest erosion protection efficiency. It says that with the same arrangement, ribs with 3 = 2 cross-section geometric figure can make bend attain optimum erosion protection ability. In the present test, the height of square A, B, and C is correspondingly equal to that of rectangle D, E, and F. Fig. 8 shows that A, B, and C erosion rate is correspondingly higher than D, E, and F erosion rate. Therefore, under the
Under the force of bend centrifugation and particle inertia strength, particles turn to flow along one side of the pipe and concentrate close to the wall, which cause the density of particle in particulate two-phase flow keep intensive in the closed-wall area. The tendency will keep a certain distance w9x Ž LrD s 5–6. of following bend and cause serious erosion on the wall. In the present test, ribs were added on the straight pipe wall extending from the bend in order to investigate its erosion protection result. For the test, ribs were arranged extending on the straight pipe wall behind the bend Žsee Figs. 3 and 4.. Using the same approach as above, comparing the erosion rate of ribbed straight pipe with that of non-ribbed straight pipe, we can get ribbed straight pipe erosion protection efficiency: 26.14%. It means that the erosion rate of the ribbed straight pipe is decreased by 26.14% in comparison with non-ribbed straight pipe. The study of the effect of rib arrangement on erosion protection shows that a rib arrangement of SrH s 1.5 is optimal. The study of the effect of rib cross-section figure on erosion protection shows that ribs with cross-section figure as 3 = 2 possess the optimum erosion protection efficiency.
Table 5 Straight pipe specimen comprehensive analysis table Specimens
Pre-test mass Žg.
Post-test mass Žg.
Erosion amount Žmg.
Used particles Žkg.
Erosion rate Ž=10y6 .
a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4 d1 d2 d3 d4 e1 e2 e3 e4 f1 f2 f3 f4 o
25.82724 26.93390 24.16488 22.32614 24.82717 23.87342 21.96819 23.09949 23.03308 22.59415 21.67349 22.23158 23.42307 23.61328 23.48660 25.60587 24.24126 23.60492 22.05960 21.87226 24.89429 23.81912 22.57637 22.17177 19.13203
25.81373 26.92562 24.15162 22.31545 24.81956 23.86421 21.96602 23.09340 23.02975 22.58704 21.66842 22.22825 23.41871 23.60815 23.47656 25.59089 24.24107 23.60095 22.05849 21.86628 24.89396 23.81739 22.57315 22.16975 19.11517
13.51 8.28 13.26 10.69 7.61 9.21 2.17 6.09 3.33 7.11 5.07 3.33 4.36 5.13 10.04 14.98 0.19 3.97 1.11 5.98 0.33 1.73 3.22 2.02 16.86
78 49 49.75 48.75 49.5 49.75 50 50.5 50.75 51.5 50.75 51 52 51 51.25 52.25 52.5 52.5 53 54 53.5 53.25 54 55 110
0.173 0.170 0.267 0.219 0.154 0.185 0.043 0.121 0.066 0.138 0.100 0.065 0.084 0.101 0.196 0.287 0.004 0.076 0.021 0.111 0.006 0.032 0.060 0.037 0.153
J. Yao et al.r Wear 240 (2000) 215–222
221
Comprehensive data of straight pipe specimen is shown in Table 5. 3.6. Comparison of bend erosion with straight pipe erosion Enre
2.741 s
Enrse
0.153
Ere
0.2465 s
Erse
0.113
s 17.915
s 2.181.
Several conclusions can be made from the data above. Ž1. With no ribs, the erosion rate of a bend is 17.915 times as high as that of the following straight pipe. It means that under the same condition, the erosion of bend is far more serious than that of the straight pipe behind it. Ž2. With ribs, the erosion rate of bend is only 2.181 times as high as that of the following straight sectional pipe. The difference between them decreases significantly. Ž3. Comparing the effect of adding ribs on bend wall with that of adding ribs on behind-extending straight pipe wall Enrse
0.153 s
Erse
0.113
Enre
2.714
Ere
s 0.2465
Fig. 9. Experimental results of erosion rate for ribbed bend and non-ribbed bend Ž1: non-ribbed bend erosion rate, 2: the lowest ribbed bend specimens erosion rate, 3: the average of ribbed bend specimens erosion rate..
ribbed specimen and ribbed specimen, which is caused by ribs. Because of centrifugal force and inertia strength, particles will flow along closed-wall surface in the bend and continually extend this trend in a certain length of straight pipe just behind the bend and then it will enlarge erosion field. Therefore adding ribs on this field will also improve the straight pipe erosion protection ability to some degree.
s 1.354, 4. Conclusions s 11.120.
It shows that adding ribs on the outer wall of the inside bend will reduce erosion to a greater extent. That is to say, adding ribs on the outer wall of inside pipe is more effective for bend than for straight pipe in improving their erosion protection ability. 3.7. Reasons for ribs erosion protection effect Based on some physical knowledge, we can know that gas-particle two-phase flow with certain speed will cause particle to impact on the wall by inertia strength and centrifugal force in the bend. Particles with high speed will not only cause impingement erosion on the wall but also bring rubbing erosion to the wall by some force because of the particles sliding and rolling on the wall surface. Adding ribs on the wall will alter this situation. On the one hand, ribs can enhance fluid turbulence and disrupt the usual motion of particles along the wall boundary fluid field, absorb particle solid energy used for impinging on the wall and decrease the number of particles impacting on the wall directly. On the other hand, ribs will prevent particles from sliding and rolling along the wall surface, change their motion direction, and lead particles jump out of the wall surface. In this way, ribs will greatly reduce particle erosion degree. From erosion protection degree, the latter reason exerts more important effect. From Fig. 9, one can get an obvious difference of erosion rate between non-
The following conclusions can be drawn from the tests above. Ž1. Adding ribs on the outer-wall of the inside bend can significantly improve bend’s erosion protection ability. The erosion protection efficiency in the test is as high as 91.01%. In other words, the erosion rate of the ribbed bend is decreased by 91.01% in comparison with the non-ribbed bend. Ž2. The investigation into bend property shows that the rib relative height and the rib gap between two adjacent ribs are two important parameters that affect ribbed bend erosion protection ability. Ribs height being equal, different ribs arrangement possesses different erosion protection efficiency. In the test, SrH s 2.5 is the optimum arrangement type for bend to attain the optimum erosion protection effect. On the other hand, ribs arrangement being equal, different rib cross-section geometric figure possesses different erosion protection efficiency. In the test, ribs with 3 = 2 cross-section shape can make bend attain the optimum erosion protection ability. In addition, rectangle ribs possess higher erosion protection efficiency than square ribs do when they have equal height. Ž3. The results of experiments on straight pipe extending behind a bend shows that adding ribs on the straight pipe wall is also an efficient erosion protection method. In the test, the erosion protection efficiency is about 26.14%. It means that the erosion rate of the ribbed straight pipe is decreased by 26.14% in comparison with the non-ribbed straight pipe. Besides, ribs arrangement and cross-section
J. Yao et al.r Wear 240 (2000) 215–222
222
geometric figure will also affect rib’s erosion protection efficiency. As a result, the test shows that ribs arrangement as SrH s 1.5 and rib’s cross-section figure as 3 = 2 is, respectively, the optimum arrangement and the optimum cross-section shape to reach the optimum erosion protection efficiency. Ž4. Comparing bend erosion with following straight pipe erosion, it is obvious that smooth bend erosion is as 17.915 times high as smooth straight pipe erosion. However, once ribs are added to the outer wall inside of pipe, ribbed bend erosion is decreased to only 2.181 times that of ribbed straight pipe erosion. It is concluded therefore that adding ribs to the outer wall is more beneficial to bends than straight pipe in terms of improving erosion protection ability. The results from the present experiment show that the method of fixing ribs on bend wall and its extending straight pipe wall is a simple and efficient erosion protection method. However, it still has some problems. As we all know, erosion experiments are sensitive to different conditions, equipment state and specimen type and number. The more kinds of specimen in the experiment, the more accurate the conclusions which can be drawn. The more experiment conditions there is, the more convincing the results are. The problem will be progressively solved in subsequent study. The present paper is only concerned with some basic properties of the ribbed bend found in the initial research. It is further noted that the ribbed bend with the lower erosion creates a turbulent ‘stagnant’ pool of air in the vicinity of the ribs that may affect the transport efficiency of the conveyer. The benefit between the material transfer efficiency and the wear rates will contribute to understanding in the design choices of real systems.
5. Nomenclature A AR As B Cv Cw
Width of duct Ž20 mm. HrW Section area of square pipe Žmm2 . Height of duct Ž20 mm. Volume two-phase density Weight two-phase density
D DL dp dp Ea Ee Enre Enrse Ere Erse H L Ma D Ms Qs Qv RrD Re S T Vm W rm ´
Pipe diameter Žmm. Duct hydraulic diameter Žmm. Average particle diameter Žmm. Particle diameter Žmm. Erosion protection efficiency Erosion rate Bend non-ribbed specimen erosion rate Non-ribbed straight pipe specimen erosion rate Bend ribbed specimen erosion rate Ribbed straight pipe specimen erosion rate Height of rib Žmm. Pipe lenth Žmm. Used abraisive particles mass Žkg. Specimen erosion mass Žmg. Standard volume flow Žm3rh. Volume flow Žm3rh. Bend curvature Reynolds number of pipe fluid Pith length Žmm. Thickness of pipe wall Ž5 mm. Fluid average speed Žmrs. Width of rib Žmm. Two-phase mixture density Žkgrm3 . Crack degre
Acknowledgements This project was supported by National Natural Science Foundation of China.
References w1x G.P. Tilly, Mater. Sci. Technol. 13 Ž1979. 287. w2x I. Finnie, Wear 3 Ž1960. 87. w3x I. Finnie, Symposium on erosion and cavitation, ASTM Spec. Tech. Publ. 307 Ž1962. 70. w4x I. Finnie, J. Wolak, Y. Kabil, J. Mater. 2 Ž1967. 682. w5x G.P. Tilly, Wear 14 Ž1969. 63. w6x X.Q. Song, J.Z. Lin, J.F. Zhao, T.Y. Shen, Wear 193 Ž1996. 1–7. w7x J. Fan, D. Zhou, K. Zeng, K. Cen, Wear 152 Ž1992. 1–19. w8x J.S. Mason, B.V. Smith, Powder Technol. 6 Ž1972. 323–335. w9x B. Chai, T.M. Xu, J. Xianjiaotong Univ. 28 Ž5. Ž1994. August.