Stepped Pipeline Systems - Freenguyen.hong.hai.free.fr/EBOOKS/SCIENCE AND... · 272 Chapter 9 Flow ~* (f) di-2 Direction ^^ Figure 9.2 Stepped pipeline notation. In the design of

Stepped Pipeline Systems

1 INTRODUCTION

When either a high pressure or a high vacuum is used for pneumatic conveying, itis generally recommended that the pipeline should be stepped to a larger bore partway along the length of the line at least once. This is the case whether the materialis being conveyed in dilute or dense phase, and whether the pipeline is long orshort. Stepping of the pipeline is particularly recommended if the material beinghandled is either abrasive or friable. Problems of both erosive wear and particledegradation increase markedly with increase in velocity and so stepping the pipe-line can have a very significant effect on limiting conveying air velocity values,and hence in minimizing the magnitude of erosion and degradation.

For many materials it is possible that the lower velocity profile achieved in astepped pipeline will also bring benefits in terms of improved conveying perform-ance. A particular problem, however, is in the location of such steps, for if they areincorrectly located, pipeline blockage could result.

The capability of purging material from a stepped bore pipeline is anotherissue that might have to be taken into account. A situation in a continuous pipelinein which the pipeline may require to be reduced in diameter, rather than increasedwhich is generally the norm, is where the pipeline incorporates a long section ofvertically downward flow.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

270 Chapter 9

2 CONVEYING AIR VELOCITY

For the pneumatic conveying of bulk particulate materials, one of the critical pa-rameters is the minimum conveying air velocity necessary to convey a material.For dilute phase conveying this is typically about 3000 ft/mm, but it does dependvery much upon the size, shape and density of the particles of the bulk material.For dense phase conveying it can be as low as 600 ft/min, but this depends uponthe solids loading ratio at which the material is conveyed and the nature of theconveyed material. If the velocity drops below the minimum value the pipeline islikely to block. It is important, therefore, that the volumetric flow rate of air, speci-fied for any conveying system, is sufficient to maintain the required minimumvalue of velocity throughout the length of the conveying system.

2.1 Compressibility of Air

The following equations were presented in Chapter 5 and are presented below forfurther development. The first of these is from Equation 5.2 and relates volumetricflow rate with conveying air velocity:

TI d2 CV = ft3/min - - (1)

576

The second is the Ideal Gas Law from Equation 5.4:

144/7 V = ma R T - (2)

The third comes from Equations 5.5 and 5.6 and is the direct derivative fromthe Ideal Gas Law that equates any two points anywhere along the length of apipeline, and will also equate to free air conditions:

r, T2 TH

Using this group of equations the problem of compressibility with air in sin-gle bore pipelines was demonstrated with Figure 5.6 and this is presented here inFigure 9.1 for reference. A free air flow rate of 1500 ftVmin was selected and theinfluence of pipeline bore and pressure are clearly illustrated. The lines of constantpipeline bore represent the velocity profile through a pipeline in single bore pipe-lines. It will be seen that the slope of the lines of constant pipeline bore changeconstantly with pressure, and as the air pressure reduces the slope increases con-siderably. The problem of air expansion, therefore, is very marked in low pressuresystems and particularly so in negative pressure systems [1],


Stepped Pipelines 271

6000

30 40

Air Pressure - Ibfin2 gauge

Figure 9.1 The influence of air pressure and pipeline bore on conveying air velocity fora free air flow rate of 1500 ft3/min.

3 STEPPED PIPELINE SYSTEMS

Figure 9.1 shows quite clearly the nature of the problem of single bore pipelineconveying, with respect to air expansion and hence conveying air velocities, par-ticularly where high pressures or vacuums are employed. For both long distance,and dense phase conveying, it is generally necessary to have a fairly high air pres-sure at the start of the conveying line. As the pressure of the conveying air de-creases along the length of the line, its density decreases, with a correspondingincrease in velocity, as illustrated above.

A simple means of limiting the very high velocities that can occur towardsthe end of a pipeline is to step the pipeline to a larger bore once or twice along itslength. By this means it will also be possible to keep the conveying air velocitywithin reasonable limits [2].

The ultimate solution, of course, is to use a tapered pipeline, for in this theconveying air velocity could remain constant along the entire length of the pipe-line. This, however, is neither practical nor possible, but it does provide the basisfor a model of what is required. A stepped pipeline, therefore, should be designedto achieve a velocity profile that is as close as practically possible to a constantvalue.

3.1 Step Location

The critical parameter in the design of any pipeline is the minimum value of con-veying air velocity required for the given material and conveying conditions.


272 Chapter 9

Flow~* (f) di-2

Direction ^^

Figure 9.2 Stepped pipeline notation.

In the design of a stepped pipeline system it is essential to ensure that theconveying air velocity does not fall below the minimum value anywhere along thelength of the pipeline. In this respect it is the location of the steps to each largerbore section of the pipeline that are crucial. With the air expanding into a largerbore pipe the velocity will fall, approximately in proportion to the change in pipesection area, at the step. The location of the step, therefore, must be such that thepressure is low enough to ensure that the velocity in the larger bore section at thestep does not drop below the given minimum conveying air velocity.

A pipeline having two steps, and hence three sections of pipeline of differentbore, is shown diagrammatically in Figure 9.2. Reference numbers are assigned tothe start and end of each section, and provided that there is no leakage of air into or

out of the pipeline between the material feed point at © and the discharge point at

©, the air mass flow rate will remain constant and the continuity equation can beused to equate conditions at any point along the length of the stepped pipeline.

By combining Equations 1 and 2 and substituting V from Equation 3 gives:

576 Po V0 TC3 = - — f t / m i n - - - - - ( 4 )

n J _ /> T0

and substituting values for/?,, and T0 gives:

= 5-19 ~2 - — ft/mind3-4 Pi

This will give the conveying air velocity at the start of the second section ofthe stepped pipeline. By equating to the free air conditions in this way, the velocityat any section of the pipeline can be evaluated.

If it is the pressure at a step in the pipeline that is required Equation 4 can berearranged to give:



P3

576 Po V0 T3

n 3—4 0 3(5)

= 5-19 Ibf/in absolute

It should be noted that since the end of one section of pipeline terminates atthe point where the next section of pipeline starts, the pressure difference betweenthese two points can be disregarded, and so in the above case: p2 = p} and/?./ = ps.It would generally be recommended that a tapered expansion section should beused to join any two sections of pipeline at a step. As a first approximation, theposition of the steps can be judged in terms of the ratio of the pressure drop valuesevaluated for the individual sections of pipeline, equating these in proportion tothe equivalent lengths of the pipeline, with due allowance for bends.

3.2 Dilute Phase Conveying

Figure 9.3 illustrates the case of a dilute phase conveying system. The minimumconveying air velocity that must be maintained for the material is about 3000ft/min, and 2000 ftVmin of free air is available to convey the material. The con-veying line inlet air pressure is 45 Ibf/in2 gauge.

12,000

10,000_g

^ 8000

.4

.3 6000u

I 4000

'&g 20003

o

14,660

10 20 30Air Pressure - Ibf/in2 gauge

40 50

Figure 9.3 Stepped pipeline velocity profile for high pressure dilute phase system using2000 ft3/min of air at free air conditions.


274 Chapter 9

From Figure 9.3 it will be seen that a 5 in bore pipeline will be required forthese conditions, and the resulting conveying line inlet air velocity will be about3610 ft/min. If a single bore pipeline was to be used for the entire length of the linethe conveying line exit air velocity would be about 14,660 ft/min. The inlet airpressure is 45 Ibf/in gauge, which is approximately 60 lbf/in2 absolute, and so ifthe discharge is to atmospheric pressure, a near four fold increase in air velocitycan be expected.

If the material being conveyed is only slightly abrasive, severe wear will oc-cur at any bend towards the end of the pipeline, because of the excessive velocity,and significant degradation of the conveyed material will also occur, even if thematerial is not particularly friable.

If the velocity was allowed to rise to 7000 ft/min in this 5 in bore pipe achange to a 6 in bore pipe would only reduce the velocity to 5000 ft/min. The ve-locity in an 8 in bore pipe would be about 2800 ft/min, however, and this isunlikely to be acceptable. A 7 in bore pipe would probably be satisfactory, butcare must be taken that standard pipe sizes are selected. Even in a 7 in bore pipe-line the velocity at exit would be almost 7500 ft/min and so it is clear that twosteps and three different pipe sizes would be required.

The velocity profile for a possible combination of 5, 6 and 8 in bore pipes isshown superimposed on Figure 9.3, but even with this the exit velocity is about5725 ft/min, and the velocity at the end of the second pipe section reaches 6315ft/min. A plot similar to that shown in Figure 9.3, however, will give a clear indi-cation of what is possible. The velocities at the six reference points along the pipe-line are also presented on Figure 9.3 and these can be evaluated by using Equa-tions 4 and 5. It would always be recommended that a graph similar to that in-cluded in Figure 9.3 be drawn for any proposed stepped pipeline system.

3.3 Dense Phase Conveying

Figure 9.4 illustrates the case of a dense phase conveying system. The minimumconveying air velocity that must be maintained for the material is about 1200ft/min, and 350 ft /min of free air is available to convey the material. The convey-ing line inlet air pressure is 45 lbf/in gauge. From Figure 9.4 it will be seen that a3 in bore pipeline will be required for these conditions, and the resulting convey-ing line inlet air velocity will be about 1755 ft/min.

If a single bore pipeline is used the conveying line exit air velocity will beabout 7125 ft/min. Although this might be accepted in a dilute phase conveyingsystem it is quite unnecessary in a dense phase system. Apart from reducing prob-lems of erosive wear and particle degradation, by reducing conveying air veloci-ties, a stepped pipeline is also likely to achieve an improved conveying perform-ance, compared with a single bore pipeline, for the same air flow conditions. Thevelocity profile for a combination of 3, 4 and 5 in bore pipes is shown superim-posed on Figure 9.4. This has resulted in the conveying air velocity being confinedto a relatively narrow band, with the maximum value being limited to 2640 ft/min.



6000

10 20 30


40 50

Figure 9.4 Stepped pipeline velocity profile for high pressure dense phase system us-ing 350 ftVmin of air at free air conditions.

3.4 Vacuum Conveying

Although negative pressure systems are naturally limited to a maximum convey-ing line pressure drop of less than 14-7 lbf/in2, stepping of the pipeline with vac-uum conveying systems is just as important as it is with high positive pressureconveying systems.

A typical vacuum conveying system is shown in Figure 9.5. It is drawn for adilute phase system, where a minimum conveying air velocity of 3000 ft/min mustbe maintained, using 500 ftVmin of free air at a temperature of 59°F and exhaust-ing to -9 lbf/in2 gauge (14-7 -9 = 5-7 lbf/in2 absolute). It must be rememberedthat absolute values of temperature and pressure must be used in all the equationsrelating to the evaluation of both velocity and pressure along the length of a pipe-line.

If the vacuum were a little higher than 9 lbf/in2, a step to a third section ofpipeline of 8 in bore would be required. Even with a conveying line exit air pres-sure of-7 lbf/in2 gauge, a step could be usefully incorporated in the case presentedin Figure 9.5.

Because the slope of the constant pipe bore curves increase at an increasingrate with decrease in pressure, steps are required more frequently at low air pres-sures. From Equation 9.4 it will be seen that pressure is on the bottom line and sowhen values get very low, as they will in high vacuum systems, a small change inpressure will result in a large change in conveying air velocity.


276 Chapter 9

8000k

Air Pressure - Ibfin gauge

Figure 9.5 Stepped pipeline velocity profile for high vacuum system using 500 ft'/minof air at free air conditions.

3.4.1 Step Position

A practical problem that arises from this is the actual positioning of the varioussteps along the length of the pipeline. As a first approximation, in the absence ofany other information, pipeline lengths can be sized in proportion to the conveyingline pressure drop for each section, provided that a reasonably uniform value ofconveying air velocity is maintained along the length of the pipeline. It can be seenfrom Figures 9.3 to 9.5 that if there is a risk of the velocity being too low at thestart of the next section, and the pipeline blocking, then the transition to the largerpipe size should be moved a little further downstream, where the pressure will beslightly lower.

4 PIPELINE STAGING

With reference to Figure 9.1 and Equation 4 it will be seen that with increase inpressure the slope of the curves decrease. If a stepped pipeline system was to bedesigned on the basis of a doubling in conveying air velocity, for each section ofpipeline, the working pressure for each section of pipeline would increase signifi-cantly with increase in pressure, as shown in Table 9.1. If it were required to con-vey a material over a distance of the order of 100 miles, it would only be eco-nomical if an air supply pressure very much higher than 100 lbf/in2 was to be used.It would also be necessary to divide the system into stages, such that the materialwas discharged from one system, when the pressure had fallen to a given value,and be fed into the next system with high pressure air.



Table 9.1 Typical Working Pressures Relating to a 2:1 Conveying Line AirVelocity Expansion Ratio

Air Inlet Pressure Air Outlet Pressure Pressure Difference

lbf/in2 absolute lbf/in2 gauge Ibf/in" gauge lbf/in2

14-7

294

58-8

117-6

235-2

470-4

0

14-7

44-1

102-9

220-5

455-7

-7-35

0

14-7

44-1

102-9

220-5

7-35

14-7

29-4

58-8

117-6

235-2

With a conveying line inlet air pressure of 455-7 lbf/in2 gauge, for example,the first step would not be necessary until the pressure had fallen to 220-5 lbf/in2

gauge, which gives a working pressure difference of 235-2 lbf/in2. If the systemdischarged to atmospheric pressure, the pressure at entry to the last section ofpipeline would be 14-7 lbf/in2 gauge and the working pressure difference wouldonly be 14-7 lbf/in2. This effect is shown in Figure 9.6, which illustrates the veloc-ity profile for the latter sections of a very high pressure stepped pipeline system inwhich the material is conveyed in dilute phase.

7000

6000

g 5000

I^ 4000M

3000

I2000

0 400100 200 300


Figure 9.6 Velocity profile for very high pressure stepped pipeline system.

500


278 Chapter 9

It would be recommended, therefore, that for a very long distance conveyingsystem, at the end of each stage along the pipeline, and at the very end of the pipe-line, the material should be discharged at a pressure of at least 44 Ibfin2 gauge. Bydischarging at a high pressure, rather than atmospheric, the last two or three sec-tions of the largest bore pipeline can be dispensed with. The reduction in workingpressure drop would be very small in comparison and it would make for a verymuch simpler pipeline design and layout.

5 PIPELINE PURGING

In many applications it is necessary to purge the pipeline clear of material at theend of a conveying run, particularly with perishable commodities and time-limitedproducts. In single bore pipelines this is rarely a problem, even if the material isconveyed in dense phase, because the velocity at the end of the pipeline is usuallysufficiently high. There can, however, be problems with stepped pipelines. Acomparison of the velocity profiles for flow in single and stepped bore pipelines ispresented in Figure 9.7.

5.1 Dense Phase Conveying

Figure 9.7 is drawn for an air flow rate of 1000 ft3/min at free air conditions. Itrelates to the dense phase conveying of a material for which the minimum convey-ing air velocity is about 1000 ft/min. This is similar to the plot shown in Figure9.4, except that the flow of air is from left to right with the new figure.

PipelinbBore *• in

Air Flow5Q90f

2860

6000

5000

4000

3000

50 40 30 20Air Pressure - Ibf/in2 gauge

10

I

2000

Ia

Figure 9.7 Comparison of velocity profiles in single and stepped bore pipelines.



Although this may be more conventional in terms of system sketching, itdoes mean that the air pressure axis is reversed, and is offered simply as an alter-native means of presentation.

Figure 9.7 is developed further in Figure 9.8 with empty line velocity pro-files added. This also provides a comparison between single bore and stepped borepipelines, with respect to purging, and clearly illustrates the problem towards theend of a stepped pipeline. At the end of a conveying run, with no material to con-vey, the pressure at the material feed point, at the start of the pipeline, will drop tothe air only pressure drop value.

For low velocity dense phase conveying the empty line pressure drop willonly be a fraction of the pressure drop required for conveying. Thus the velocity ofthe air through a single bore empty pipeline will be very high throughout itslength. At the end of the pipeline the air velocity will be exactly the same as in theconveying case, because the pressure here is always atmospheric. At the materialfeed point, however, the air velocity will only be slightly lower than that at the exitsince the air pressure at the feed point is so much lower when material is not beingconveyed.

With the stepped bore pipeline this same volumetric flow rate of air has toexpand into the larger bore section of pipeline, and so its velocity will reduce, asshown in Figure 9.9. At the end of the pipeline the situation is exactly the same asin the single bore pipeline case. The velocity for both conveying and purging willbe the same, because the pressure here is always atmospheric. Since the purgingvelocity will not be constant throughout the pipeline the potential for clearing ma-terial from the latter sections of stepped pipelines by purging, therefore, will beseverely limited.

Purging Mode

PipelineBore - in

40 30 20


10

Figure 9.8 Comparison of velocity profiles in single and stepped bore pipelines inconveying and purging modes.

both


280 Chapter 9

5.2 Material Deposition

To illustrate the problem of material deposition in pipelines with low velocityconveying, data from a program of conveying trials carried out with a fine gradeof fly ash is presented in Figure 9.9 [3]. The fly ash was being conveyed through a425 ft long pipeline of 2'A inch nominal bore that incorporated nine 90° bends.

5.2.7 Fly Ash

In tests conducted with low air flow rates, and hence at low conveying air veloci-ties, it was observed that not all the batch of material in the blow tank was dis-charged into the receiving hopper. The material was, in fact, being deposited in thepipeline and remaining there at the end of the conveying run, when the conveyingair velocity used was too low to purge the pipeline clear.

The fly ash left in the pipeline did not represent a problem because it wasswept up with the next batch of fly ash conveyed. As a result the pipeline was onlypurged for a short time before starting the next test run. To give some indication ofthe potential problem of material deposition in a pipeline when conveying at lowvelocity, the data for every test carried out was analyzed to provide a figure for thepercentage of the batch conveyed that was discharged into the receiving hopper.

100% data points simply mean that the entire batch of 1000 Ib was con-veyed. For the very high velocity tests the data points have not been included. If80% of the batch was conveyed, then 20% of the batch remained in the pipeline atthe end of the test run, which amounted to 200 Ib of fly ash. The results and analy-sis are presented in Figure 9.9.

Percentage ofBatch Conveyed

'—! 1 1—i 1 1 1

40 80 120 160

Free Air Flow Rate - ftVmin

Figure 9.9 Analysis of pipeline purging data for fine fly ash.



5.2.2 Cement

In an earlier program of work ordinary portland cement was similarly tested [4]. Itwas conveyed through a 310 ft long pipeline of 4 in nominal bore having nine 90°bends (Figure 7.13 pipeline). For this shorter pipeline of larger bore the batch sizeof the cement was 2750 Ib, since tests with material flow rates up to about 100,000Ib/h were undertaken. Testing was carried out with air supply pressures up to 40lbf/in2 gauge. An analysis of the test data obtained with the cement is presented inFigure 9.10.

The normal conveying characteristics for the material are presented in Fig-ure 9.10, together with conveying air velocity data. This is in terms of a full set ofcurves for the conveying line inlet air velocity and a parallel axis in terms of theconveying line exit air velocity.

From Figure 9.10 it will be seen that the cement could be conveyed withconveying line inlet air velocities down to about 500 ft/min and at solids loadingratios of over 100. Lines showing the percentage of the batch that was conveyedare also superimposed on Figure 9.10. In this case, when only 70% of the batchwas conveyed, 825 Ib of cement was left in the pipeline. As with the fly ash, thiscement was swept up by the next batch that was conveyed.

Conveying Line InletAir Velocity - ft/min

Solids Loading

14n i_ Ratio """-*. I^ Conveying m }QQ J

120 ~ x 10°

Conveying LinePressure Drop - lbf/in2

«c 100X>

i 80pi

I 60

I 40

20

0

80

NO GOAREA

Percentage ofBatch Conveyed

1600

2000

2400

100 200 300

Free Air Flow Rate - ftVmin400

1000 2000 3000 4000

Conveying Line Exit Air Velocity - ft/min

Figure 9.10 Conveying characteristics for cement in 4 inch bore pipeline.


282 Chapter 9

6 DIVERSE MATERIAL CONVEYING

Not all pneumatic conveying systems are dedicated to the conveying of a singlematerial. There is often a need for a system to transport a number of different ma-terials. In many industries, such as food and glass, a wide variety of materials haveto be conveyed by a common system, since there is a requirement to deliver agiven 'menu' for a particular process [5]. In the case of packet soups, for example,it could involve more than twenty different materials. One of the authors cameacross a total of 78 different materials, ranging from iron powder to vermiculite, ina plant manufacturing welding rods.

Some of the materials to be transported may be capable of being conveyedin dense phase, and hence at low velocity, while others may have no natural densephase conveying capability and will have to be conveyed in dilute phase with ahigh conveying air velocity. The air requirements for the various materials, there-fore, could differ widely.

This is illustrated with the case of floury and sandy grades of alumina, con-veyed through the same pipeline, with conveying line inlet air pressures up to 45lbf/in2 gauge. The pipeline used was 155 ft long, of two inch nominal bore andincorporated six 90° bends. Conveying characteristics for these two materials arepresented in Figure 9.11.

_, . Conveying Line Pressure ^ ,. ,Conveying Drop. M/in2 SolidsLimit \ / Loading

Ratio

(a)

0

0 40 80 120 160 200Free Air Flow Rate - ftVmin

Conveying LinePressure Drop

- Ibffitf

ConveyingLimit

SolidsLoadingRatio \

\

(b)

0 40 80 120 160 200Free Air Flow Rate - ftVmin

Figure 9.11 Conveying characteristics for two grades of alumina conveyed through 155ft long pipeline of 2 in bore incorporating six 90° bends, (a) Floury and (b) sandy.



6.1 Pipeline Selection

There is often a requirement for these two grades of alumina to be conveyedthrough a common pipeline. From Figures 9.1 la and b, however, it will be seenthat there are considerable differences in the conveying capabilities of these twomaterials. The floury alumina can be conveyed in dense phase and with conveyingair velocities down to about 600 ft/min, and with a conveying line pressure drop of40 lbf/in2 a material flow rate of 52,000 Ib/h can be achieved with a free air flowrate of approximately 55 ftVmin. The sandy alumina, however, can only be con-veyed in dilute phase and requires a minimum conveying air velocity of about2000 ft/min, and with the same pressure drop of 40 lbf/in2 a material flow rate ofonly 32,000 Ib/h can be achieved and this requires a free air flow rate of approxi-mately 170ft3/min.

If a 20% margin is allowed on minimum conveying air velocity, in order tospecify a conveying line inlet air velocity for design purposes, the value for thesandy alumina will be 2400 ft/min and for the floury alumina it will be 720 ft/min.To show how a common conveying system might be able to convey both materi-als, a graph is plotted of conveying air pressure and a series of curves for differentpipeline bore is superimposed in Figure 9.12. Onto this are drawn possible veloc-ity profiles for the two materials. Because of the extremely wide difference inconveying air velocities a single bore line is suggested for the floury alumina, andthree steps are required in the pipeline for the sandy alumina, but it will be seenthat the pipeline system meets the requirements of both materials.

PipelineBore - in

6000 h

e 5000

's^4000

1^ 3000

.5 2000

c

6 1000

20 30

Air Pressure - lbf/in2 gauge

Figure 9.12 Velocity profiles for sandy and floury alumina in a common positive pres-sure conveying system for a free air flow rate of 1000 ftVmin.


284 Chapter 9

At entry to the reception hopper a common pipeline is possible in this case,as shown, but this is not necessarily a requirement. The use of two completelydifferent pipelines is not likely to be a problem. The pipeline used for the flouryalumina in Figure 9.12, therefore, could well be stepped part way along its lengthto 10 in bore, which could not possibly be used with the sandy alumina.

Consideration would have to be given in this case, however, to purging ofthe pipeline, since the maximum value of conveying air velocity in the pipelinewould be only 2000 ft/min. There are many alternative solutions to the problem ofconveying diverse materials, but the one illustrated is probably the simplest as itutilizes exactly the same air supply in terms of both pressure and volumetric flowrate. Material flow rates will clearly be different, but an extremely complex systemwould be needed to achieve this equality as will be seen from Figures 9.1 la and b.A sketch of a system relating to the data given in Figure 9.12 is presented in Fig-ure 9.13.

6.2 Low Pressure Systems

Although Figure 9.13 is drawn with a common pipeline feeding both materialsinto the reception silo, this is not a requirement, as mentioned above. Indeed, witha low pressure system this may not be a possibility. Two different pipelines, how-ever, could be utilized in exactly the same way. There would probably be no needto step any of the pipelines either.

8 in Bore

\

=0-1VCompressor

Common 8 in Bore

Hoppers forFloury Alumina

. vv/•

X

/

^ \ly"

Xif /C9 6 in Bore /| Hoppers for1 Sandy Alumina

RAA/ 1

r ReceptionSilo

\/vy

5 in Bore

^4 in Bore

Figure 9.13 Typical layout of a high positive pressure conveying system for conveyingdiverse materials.



6.2.1 Vacuum Conveying Systems

Exactly the same principles apply to vacuum conveying systems. It may well bepossible to have a common pipeline delivering all materials into the reception siloand for materials with no dense phase conveying capability a stepped pipelinecould be utilized to provide the necessary pick-up velocity for the given air flowrate.

7 MATERIAL FLOW RATE

The influence that a stepped pipeline might have on material flow rate is not im-mediately obvious. For the flow of air only through a pipeline models are wellestablished. That for pressure drop takes the form:

LoC1

Apa °c — Ibf/in2 (6)d

where Apa = air only pressure drop - Ibfin2

L = pipeline length - ftp = density of air - lb/ft3

C = conveying air velocity - ft/minand d = pipeline bore - in

As pressure drop increases with increase in (velocity) , and decreases withincrease in pipeline bore, the pressure drop for a stepped pipeline will be signifi-cantly lower than that for a single bore pipeline of the same length, the same initialdiameter and for the same volumetric flow rate of air.

7.1 Fine Fly Ash

Comparative data for the performance of single bore and stepped pipelines israther limited but such work has been carried out with a fine grade of fly ash [6]. A380 ft long pipeline of 2 inch nominal bore and incorporating ten 90° bends wasbuilt for the purpose. A fine grade of fly ash was used, since it is capable of beingconveyed over a very wide range of flow conditions. A sketch of the pipeline ispresented in Figure 9.14 for reference. This also indicates where the steps in thepipeline were made to larger bore sections of pipe.

The conveying characteristics for the fly ash in the 380 ft length of singlebore pipeline are presented in Figure 9.15a. These are the reference set of convey-ing characteristics for the basis of comparison with the stepped pipelines exam-ined. From this it will be seen that the material could be conveyed at solids loadingratios up to almost 200, with conveying line pressure drop values up to 45 Ibf/in2,and over a very wide range of air flow rates.


286 Chapter 9

Return to Hopper

First Step

Second Step

Figure 9.14 Pipeline used for stepped pipeline conveying tests.

In order to provide a comparison with the single bore pipeline, the secondhalf of the pipeline was changed from 2 inch to 21/2 inch bore pipe. At the transi-tion section the 2 in bore pipe was simply sleeved inside the 21/2 in bore pipe andwelded. The resulting conveying characteristics are presented in Figure 9.15b.

80

70

60

50

I 40

30

20

10

0


- Ibf7in2

Solids Loading^—-"" Ratio

200 160 120

80


lbf/in2

o 70

°60

200 160 120

SolidsLoading

Ratio

(a)

0 40 80 120 160 200

Free Air Flow Rate - ftVmin(b)

0 40 80 120 160 200


Figure 9.15 Conveying characteristics for fine fly ash in 380 ft long pipeline of 2 inchinitial bore, (a) Single bore pipeline and (b) single step pipeline.



By comparing Figures 9.15 a and b it will be seen that there is a very sig-nificant improvement in performance over the entire range of conveying condi-tions considered as a consequence of this single step. Much higher values of flyash flow rate were achieved, and with lower values of conveying line pressuredrop.

To illustrate the magnitude of the improvement a comparison of the singlestep and single bore pipelines is given in Figure 9.16a. For this purpose a grid wasdrawn on each set of conveying characteristics at regular increments of conveyingline pressure drop and air flow rate, and the value of the fly ash flow rate wasnoted at every grid point. The data points given on Figure 9.16a represent the ratioof the fly ash flow rates and this shows that the material flow rate achievedthrough the pipeline with the single step was about 1-9 times or 90% greater thanthat for the single bore pipeline for exactly the same inlet air conditions and hencepower required.

It is interesting to note that there is little change in the value of this ratioover the entire range of conveying conditions examined. The improvement appliesequally to low velocity dense phase conveying, and to high velocity dilute phaseconveying. Since there is no change in the air flow rate required to convey thematerial it is unlikely that there would be any need to change the filtration re-quirements for the conveying system either.

80

70

60

50

I 40

1 30

| 20<L>

1 10

0

(a)

Conveying LinePressure Drop- lbf/in2

-20

oaoi

_0

CL<

80

70

60

50

40

30

20

10

0

ConveyingLine PressureDrop

- lhf/in2

Solids Loading

200 160 120

0 40 80 120 160 200

Free Air Flow Rate - ftVmin(b)

0 40 80 120 160 200

Free Air Flow Rate - frVmin

Figure 9.16 Further conveying data and analysis for fly ash in 380 ft long pipeline of 2inch initial bore, (a) Single step comparison and (b) double step pipeline data.


288 Chapter 9

For the second comparison the last quarter of the pipeline was changed from2'/2 inch to 3 inch bore. Thus the first 190 ft was of 2 inch, the next 95 ft was 2'/2

inch and the last 95 ft was of 3 inch nominal bore pipeline. It should be noted thatthese are by no means the ideal proportions. They were selected to illustrate thepotential improvement that might be achieved over a very wide range of convey-ing conditions. The optimum position of the pipeline steps will depend very muchupon the air supply pressure and pipeline bores available.

The resulting conveying characteristics for this pipeline with two steps arepresented in Figure 9.16b. It will be seen from this that a further improvementover the single step pipeline has been obtained. A similar analysis to that presentedin Figure 9.16a showed that the ratio of material flow rates between the doublestep and the single bore pipelines was about 2-2:1 [6].

7.2 Existing Systems

Since the diameter of the first section of the pipeline remains the same, the airflow rate also remains the same. This, therefore, has direct application to existingsystems, for if a single bore pipeline is used with a high pressure system, the onlychange may be in terms of stepping the pipeline. It is also unlikely that changesneed be made to either the compressor or to the filtration plant.

7.3 Other Materials

It is suspected that the very significant improvement in material flow ratesachieved with the fine fly ash are mainly due to the nature of the conveying char-acteristics for this type of material and are unlikely to be repeated to such amarked extent for materials having different conveying characteristics. A compari-son of constant pressures drop lines for a wide range of materials was presented inChapter 4 with Figure 4.18 and fly ash was clearly the steepest of the fifteen mate-rial included.

The lines of constant conveying line pressure drop on the conveying charac-teristics presented in Figure 9.15b, for example, have a steep negative slope overthe entire range of air flow rates and conveying capability. This means that as theair flow rate, and hence conveying air velocity, is reduced, there is always an in-crease in material flow rate, even at very low values of air flow rate.

This tends not to be the case for the low velocity conveying of polymericpowders and pelletized materials. These materials often exhibit a pressure mini-mum point in the conveying characteristics and at air flow rates below the pressureminimum point the lines of constant pressure drop have a marked positive slope.This was illustrated earlier with polyethylene pellets in Figures 4.12b and 7.3.

The conveying characteristics for terephthalic acid, which is a powder hav-ing a mean particle size of about 60 micron, and nylon pellets, which are monosized having a mean particle size of about 0-15 inch, are presented in Figures 9.17aand b to illustrate this point. The terephthalic acid was conveyed through a 165 ftlong pipeline of 2 inch nominal bore incorporating eight 90° bends.



60

50

40

I30

o

10

0

c

(a)

Conveying LinePressure Drop- lbf/in2

Solids LoadingRatio

- 30

-25

50 100 150 200

Free Air Flow Rate - ft /min

Conveying LinePressure Drop Solids Loading

50

o 40oX

£30

I 20

I 10

- lbf/in2Ratio

15

(b)

0 100 200 300 400

Free Air Flow Rate - ft3/min

Figure 9.17 Conveying characteristics of materials exhibiting pressure minimumpoints, (a) Terephthalic acid and (b) nylon pellets.

The nylon pellets were conveyed through a 160 ft long pipeline of 3 inchnominal bore incorporating six 90° bends [7], It is suspected that the benefits ofstepping the pipeline will be very limited for the low velocity dense phase flow ofthese materials, but should be well worthwhile for dilute phase suspension flow.

8 EXPANDED BENDS

Since the magnitude of both erosive wear and particle degradation are influencedso markedly by conveying velocity, and that bends are the major cause or influ-ence, it has been suggested that the bends themselves could be stepped to a largerdiameter. The idea is to install bends into the pipeline that have a much larger borethan that of the pipeline [8].

The bends are fitted into the pipeline with tapered sections at inlet and out-let. There is no change in diameter of the pipeline either leading to the bend orfollowing the bend. A sketch of such a bend is given in Figure 9.18. From this itwill be seen that the step is only to and from the bend.


290 Chapter 9

Solids Loading Ratio

PressureGradient' -Ibf/m2 14°-per 100ft

20

10

20

50 100 150


Figure 9.18 Sketch of expanded bend Figure 9.19 Pressure gradient in verticallydown flow for fly ash.

In the expanding section prior to the bend the air velocity will fall and theparticles will be retarded. As a consequence the particles will impact against thebend wall at a lower velocity and so any erosive wear or particle degradation willbe reduced. Although the air velocity may fall below the minimum value for con-veying, the particles are decelerating and the turbulence in the region is so greatthat pipeline blockage does not appear to be a problem.

9 CONVEYING VERTICALLY DOWN

For the flow of bulk particulate materials through horizontal pipelines there is anassociated pressure drop. The situation is the same for flow vertically up, exceptthat the pressure gradient values are approximately double those for horizontalflow, as was discussed in the previous chapter. For flow vertically down, however,the situation is very different.

For materials that can be conveyed in dense phase there can be an increasein pressure as the material is conveyed vertically down the pipeline, if the materialis conveyed in dense phase. Data for cement, barite and a fine grade of pulverized



fuel ash, conveyed vertically down through a two inch nominal bore pipeline waspresented in the previous chapter [9]. That for the fly ash is reproduced here inFigure 9.19 for reference.

Figure 9.19 is essentially the conveying characteristics for the material forflow vertically down, with the pressure drop being in terms of a pressure gradient,in lbf/in2 per 100 ft of pipeline, rather than the pressure drop for the entire pipelinesystem, as shown in Figures 9.16b to 17b, for example. From Figure 9.19 it will beseen that if the material is conveyed at a solids loading ratio of about 35 there willbe no pressure drop associated with the conveying.

At increasingly higher values of solids loading ratio there is a pressure re-covery, and hence the negative values on Figure 9.19, increasing with further in-crease in solids loading ratio, and hence material flow rate. At solids loading ratiosbelow about 35, and hence for the entire dilute phase region of conveying, therewill be a pressure drop associated with the flow of the material.

9.1 Underground Stowing

In situations where materials need to be conveyed long distances vertically down,very high pressures can be generated if the conveying conditions are carefullyselected. The transfer of fly ash and cement down mine shafts for undergroundstowing and roof support are particular examples.

In these cases it is possible for the materials to be conveyed over a distanceof several thousand feet horizontally from the bottom of the mine shaft by virtueof the pressure generated from the downward conveying of the materials. Providedthat the distance conveyed horizontally, prior to the vertical drop down the mineshaft, is kept relatively short, this could theoretically be achieved with a very lowair supply pressure.

A particular problem here, however, is that the pressure generated could beso high that the conveying air velocity in the following horizontal section of pipe-line could be too low to support conveying and the pipeline could block. In thiscase the pipeline would need to be reduced in diameter, rather than increased, inorder to increase the conveying air velocity.

The horizontal section of pipeline would need to be expanded to a larger di-ameter along its length in the usual way, as it would be discharging material atatmospheric pressure. A sketch of a pipeline for such an application is given inFigure 9.20. A sketch of a velocity profile, for a free air flow rate of 900 ft3/min,for the Figure 9.20 pipeline is presented in Figure 9.21.

A minimum conveying air velocity of about 800 ft/min for the verticallydown flow and approximately 1200 ft/min for the horizontal flow has been as-sumed. The dotted lines represent the flow vertically down and the dashed linesthe horizontal flow. It will be seen that the conveying line inlet air pressure isabout 10 lbf/in2 gauge and so a positive displacement blower is all that would berequired for the air supply, despite the fact that pressures of up to about 90 lbf/in2

are generated within the pipeline system.


292 Chapter 9

MaterialFeed _Point

/-. -J 1 1

VerticallyDown

4 5 6

, 1 1 I

PipelineBore - in

8

I^ Material

Point

Figure 9.20 Proposal for a pipeline system for delivery of materials for undergroundstowing.

The arrows on the dotted and dashed velocity profiles indicate the actualflow direction through the various bore of pipeline utilized.

I 200°g'(3_o

>

oolOOO

I

me

• Vertically_ Down

Minimum Velocity

I I I 1 1 1 1 1 1 1 J l_J—L -I L

0 15 30 45 60

Air Pressure - lbf/in2 gauge

75 90

Figure 9.21 Velocity profile for conveying system delivering materials for under-ground stowing.



10 AIR ONLY PRESSURE DROP

Stepped pipelines were discussed earlier to illustrate the problems of air expansionand velocity control along a pneumatic conveying system pipeline. The modelsnecessary to evaluate conveying air velocities and air only pressure drop were alsodeveloped earlier, particularly in Chapter 6, and so it is now possible to considerstepped pipelines further. A sketch of a two section stepped pipeline is given inFigure 9.22.

From Equation 6.12, for a single bore pipeline, the following expression wasdeveloped:

m R T(7)

_ r-<(8)

which gives:

4? = Pi ~P4 = dp 1-2+ AP3--I

Figure 9.22 Velocity and pressure profiles and notation for a stepped pipeline.


294 Chapter 9

( Y'5either Apa = p - \ p2 - T lbf/in2 (9)

which is an expression in terms of the inlet pressure,/)/

or Apa = (pi + F) - p2 lbf/in2 (10)

which is an expression in terms of the outlet pressure, p2

For a stepped pipeline the total pressure drop will be equal to the sum of theindividual pressure drops for each section. For a two section pipeline the unknownpressure at the step can be eliminated by using both of the above expressions, andnoting that:

P2 = P3and

dpa = Pi ~ P4 = Ap,_2 + Ap3-4

For the first section:

\o - s

= P\ ~ \Pl ~ ri-2

and for the second section:

\o -52Jps-4 = (p, + r3_4\ i

adding these two expressions gives:

/ \0 -5 / \0 -5

PI - p4 = PI - P4 - \P\ - r ,_ 2 j + \p\ + r3_4

which reduces to:

P\-PI = r\.i + rw ( i i )This equation is of the same form as Equation 9.8 and so the solution can ei-

ther be in terms of the inlet pressure, plt as in Equation 9.9, or in terms of the exitpressure, p4, as in Equation 9.10. The choice will depend upon which value isknown, and whether the stepped pipeline is for a positive pressure or a vacuumsystem.



It should be noted that if the pipeline comprises more than one step, addi-tional equations will be needed to solve the additional unknown pressures at thesteps.

10.1 Position of Steps

The position of the transition to a larger bore line must be such that the conveyingair velocity does not drop below that of the conveying line inlet air velocity em-ployed at the start of the pipeline. As the pressure drops along the length of thepipeline the velocity will increase, but a change in pipeline bore will significantlyalter the situation, as illustrated in Figure 9.22, and with the earlier examplesshown in Figures 9.3 to 9.8.

It was also mentioned earlier that as a first approximation, pipeline lengthscan be sized in proportion to the conveying line pressure drop for each section,provided that a reasonably uniform value of conveying air velocity is maintainedalong the length of the pipeline. With reference to Figure 9.22, the length of thefirst section of pipeline, Lt.2, would be:

P\ - PiLi.2 = x L ft (12)

P^ - PA

The process would be similar for other pipeline sections. The pressure at thesteps can be evaluated from Equation 5, developed earlier, and the velocity at theend of each section and along the length of the pipeline can be determined fromEquation 4.

10.2 Transition Sections

A tapered transition from one section to another would be recommended, in orderto recover as much of the energy as possible in the preceding high velocity flow.The included angle of the transition would need to be about 5 to 10 degrees, asshown in Figure 6.9.

REFERENCES

1. D. Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13. pp74-80. Dec 2000.

2. D. Mills. Pneumatic Conveying Design Guide. Butterworth-Heinemann. 1990.3. D. Mills and V.K. Agarwal. Pneumatic conveying systems - design, selection, opera-

tion and troubleshooting with particular reference to pulverized fuel ash. 386 pp. TransTech Publications. 2001.


296 Chapter 9

4. D. Mills, V.K. Agarwal, and M.D. Bharathi. The pneumatic conveying of fly ash andcement at low velocity. Proc 24'1' Powder and Bulk Solids Conf. pp 147-163. ChicagoMay 1999.

5. D. Mills. The use of stepped pipelines to enable different materials to be conveyedpneumatically by a common system. Proc 7lh Int Conf on Bulk Materials Storage,Handling and Transportation. The University of Newcastle, Australia. October 2001.

6. D. Mills and J.S. Mason. An analysis of stepped pipelines for pneumatic conveyingsystems. Proc 12th Powder and Bulk Solids Conf. pp 696-704. Chicago. May 1987.

7. M.G. Jones and D. Mills. Performance characteristics for the pneumatic conveying ofplastic pellets. Proc 21st Powder and Bulk Solids Conf. Chicago. May 1996.

8. V.K. Agarwal, N. Kulkarni, and D. Mills. The influence of expanded bends on wearand particle degradation in pneumatic conveying system pipelines. Proc IMechE Confon Powder and Bulk Solids Handling, pp 307-317. London June 2000.

9. D. Mills, J.S. Mason and P. Marjanovic. Pneumatic conveying - vertically down. Proc8th Powder and Bulk Solids Conf. pp 546-557. Atlanta. May 1983.


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