Pipeline Material, Orientation, and Bendsnguyen.hong.hai.free.fr/EBOOKS/SCIENCE AND ENGINEERING/ENGI… · (fly ash) in the two inch bore pipeline are presented in Figure 8.4 [2,

8Pipeline Material, Orientation, andBends

1 INTRODUCTION

A major advantage that pneumatic conveying systems have over alternative me-chanical conveying systems is in flexibility in continuous pipeline routing. Pipe-lines can run horizontally, and with bends in the pipeline, flows can go verticallyup or vertically down, with little restriction on numbers of bends or distances.Pipelines inclined upwards are not generally recommended and so flow in inclinedpipelines is examined.

Up to now pressure gradient has been discussed in global terms of pressuredrop available and distance over which a material must be conveyed, with highpressure gradients being required for dense phase conveying. Data is included inthis chapter to show how pressure gradient varies with conveying parameters forhorizontal and vertical conveying in both dilute and dense phase flows.

Conveying parameters were introduced in the previous chapter for pipelinebore and conveying distance. In this chapter scaling parameters are presented forother pipeline features including vertical flow. The influence of conveying pa-rameters on pressure drop across bends is considered, for both dilute and densephase flow, and losses are presented in terms of both a pressure drop and anequivalent length. Pipeline material is also considered, with particular reference tothe use of flexible rubber hose.

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

242 Chapter 8

2 PRESSURE GRADIENT DATA

All the conveying data presented so far has been for total pipeline systems. This isusually obtained from a test facility comprising a pipeline test loop that generallyincludes horizontal pipeline lengths, a number of bends and possibly an element ofvertical lift. The pressure drop data in the conveying characteristics presented hasbeen for the entire pipeline.

In order to isolate the effect of any individual element of pipeline, such as astraight section of horizontal or vertical pipeline, or a bend in the pipeline, pres-sure tappings must be fitted into the pipeline. Some of these issues are consideredin this chapter but are considered in more detail in Chapter 23.

2.1 Horizontal Conveying

Typical conveying data for flow in a horizontal section of pipeline is presented inFigure 8.1. The data is for barite, which is often used as a drilling mud powder.This material has a particle density of about 260 lb/ft3 but despite this it will beseen that the material could be conveyed at solids loading ratios in excess of 100and at low velocity. For drilling purposes it is used as a very fine powder and sohas very good air retention properties in this form. As a consequence of the airretention properties the material will convey in dense phase flow.

60 h

40

o

a

20

0

Solids Loading Ratio 120 JOO

40

20

'ressure GradientIb t7 in 2 per l00f t

0 50 100 150

Free Air Flow Rate - ftVmin

Figure 8.1 Pressure gradient in horizontal flow for barite in 2 inch bore line.


Material, Orientation, and Bends 243

The data in Figure 8.1 is presented in exactly the same form as the convey-ing characteristics, with material flow rate in Ib/h plotted against free air flow ratein fVVmin. The family of curves plotted is now of pressure gradient in lbf/in2 per100 ft length of pipeline rather than a pressure drop for the total pipeline. Lines ofconstant solids loading ratio are also included as these are simply straight linesthrough the origin as before. The juxtaposition of these two sets of curves on theone plot is particularly useful for illustrating once again the problem of maintain-ing flow in dense phase with increase in conveying distance.

2.1.1 Long Distance Conveying

As expected, it will be seen that as the solids loading ratio increases, the pressuregradient increases. At a solid loading ratio of about 100 the pressure gradient isapproximately 10 lbf/in2 per 100 ft length of pipeline. With a limit on air supplypressure because of air expansion problems, and the consequent need to step thepipeline, the scope for long distance dense phase flow is strictly limited. This doesnot take account of the additional pressure drop due to bends and sections of verti-cally upward pipeline that might need to be included either.

For longer distance conveying there must be a compromise and this is toconvey at a lower value of solids loading ratio where the pressure gradient islower. In Figure 7.2Id, magnesium sulfate conveyed over a distance of 2500 ft ispresented and the maximum value of solids loading ratio, with a conveying linepressure drop of 30 lbf/in2, is only about 1 '/2.

3 VERTICAL CONVEYING

Apart from the difficulty of finding a suitable wall or structure on which to mounta vertical pipeline for testing purposes, a test loop needs to be used, unless twoconveying systems are available, one conveying to the other. The former was usedfor the test work reported here [1]. An advantage of this method is that the pipelinemust go down as well as up and so data can be obtained for both sections of pipe-line in every test run.

A sketch of the test pipeline is given in Figure 8.2 together with dimensionaldetails. A high pressure top discharge blow tank was used to feed material into thepipeline. The layout of the test facility was such that the material was conveyedvertically down first and then vertically up. The fall and rise elements of the pipe-line were both 53 ft long. The total pipeline length was about 185 ft. Two pipe-lines were available; one of two inch and another of three inch nominal bore, bothfollowing an identical routing.

Typical conveying characteristics for the total pipeline system are presentedin Figure 8.3 [2]. These are for barite conveyed through the three inch bore pipe-line. Barite can be conveyed in dense phase, as was illustrated in Figure 8.1 and soconveying with air supply pressures up to 30 lbf/in2 was possible and solids load-ing ratios of well over 100 were achieved.


244 Chapter 8

ooo

120

80

BendNumber

Figure 8.2 Details of test pipelineused for vertical conveying.

E 40

0

X3 0

'onveyingLine PressureDropIbf/in2

20

10

SolidsLoadingRatio

0 100 200 300 400

Free Air Flow Rate - ft3/min

Figure 8.3 Conveying characteristics forbarite in figure 8.2 pipeline of 3 inch bore.

Once again this illustrates the conveying potential of relatively small borepipelines in that material flow rates of over 100,000 Ib/h were achieved. For thetotal pipeline the form of the conveying characteristics is little different from thatfor other pipeline systems presented in earlier chapters.

In order to obtain pressure gradient data for the two test sections there were15 pressure tappings (seven along the down section and eight along the up sec-tion). The first and last tappings at each section were placed about five feet fromthe bends in order to ensure that any upstream or downstream effects would haveminimum influence on the pressure readings. At each location a ring of four tap-pings was used and all four were coupled to a common point.

Results from two tests carried out with a fine grade of pulverized fuel ash(fly ash) in the two inch bore pipeline are presented in Figure 8.4 [2, 3]. The hori-zontal axis represents the length of pipeline (see Figure 8.2) from the bend inwhich the flow is horizontal to vertically down (bend number 4), to the bend inwhich the flow is vertically up to horizontal (bend number 7).

The first section, therefore, represents the flow vertically down, along whichthere were seven pressure tapping locations, and the second section represents theflow vertically up, along which there were eight pressure tapping locations. Thevertical axis represents the pressure of the conveying air.

The solid lines drawn represent the linearized dependence, from the meas-ured values of pressure, while the dotted lines represent an approximate develop-ment of the pressure in the region where the pressure was not measured.



Ofl3taeo

20 \-

16

12

•OC

3 4

-a T3c cD U

CQ CQ 8 9 10 11 12 13 14

I

40 60 80 100 120Distance - feet

140 160

Figure 8.4 Typical pressure gradient results obtained with pulverized fuel ash.

It will be noticed that in one case the pressure gradient in the verticallydown section was negative, while in the other it was positive. In the vertically upflow the pressure gradient was negative in each case, but there was a significantdifference between the two tests. The influence that conveying conditions canhave on the values of pressure gradient are considered below.

3.1 Flow Vertically Up

Pressure gradient data obtained in this way for the vertically upward flow of baritein the three inch bore pipeline is presented in Figure 8.5. The barite was conveyedover a wide range of both air and material flow rates, and some forty to fifty indi-vidual tests had to be carried out in order to provide the necessary pressure gradi-ent data to obtain the plot or performance map shown in Figure 8.5. Once againsolids loading ratios well in excess of 100 were achieved with this material.

The data is plotted in terms of a pressure gradient in lbf/in2 per 100 feet ofvertically up pipeline. If the data is compared with that for the horizontal pipelinein Figure 8.1, which also relates to the conveying of barite, it will be seen that thepressure gradient values are significantly higher.


246 Chapter 8

140 120 100

120

80

E 40

"c3

Solids Loading Ratio

PressureGradient - '"'

M/in 2 per lOOft

24

32

40

20

100 200


300

Figure 8.5 Pressure gradient data for barite conveyed vertically up in 3 inch bore pipe-line.

Material flow rates are also very much higher but this is because the data isfor a larger bore pipeline. It is by comparing sets of data such as this that scalingparameters can be determined, but ideally they need to be of the same bore pipe-line, and so this is considered later in this chapter.

The data can also be compared with the conveying characteristics presentedin Figure 8.3. Figure 8.3 was generated from exactly the same test program as thatfor the data in Figure 8.5. One was plotted from the total pipeline pressure dropdata and the other from the pressure gradient data derived from the pressure tap-ping readings.

A comparison of the two will show that the slope of the pressure gradientlines on Figure 8.5 is very different from the slope of the lines of constant pressuredrop on Figure 8.3. Figure 8.5 is for the vertically upward section of pipeline inisolation, while Figure 8.3 is for the total pipeline system, including nine bends.The influence of bends is also considered later in this chapter.

Two additional sets of data are included to reinforce the nature of the curves.These are for cement and fly ash, both conveyed vertically up through a two inchbore pipeline. There are clearly differences between the two sets of data but fol-lowing the comparative data presented in Chapter 4 this will not come as a sur-prise.

The unknown factor is how the different elements of the pipeline contributeto the overall differences observed. Data for the cement is presented in Figure 8.6aand that for the fly ash in Figure 8.6b.



60

50

£ 400)a

oE

1 20o

10


Pressure 140Gradient -

lbf/in2 per 100ft


60

o:o

3*30_oE320

10

Pressure• Gradient -Ibf/m1 per 100ft

\

16

12

20

(a)

0 50 100 150

Free Air Flow Rate - fVVmin (b)

50 100 150


Figure 8.6 Pressure gradient data for flow vertically up in a two inch nominal borepipeline, (a) Cement and (b) a fine grade of fly ash.

3.1.1 Scaling Parameter

In the majority of pneumatic conveying system pipelines the proportion of hori-zontal conveying is very much greater than that of vertical conveying. A scalingparameter, therefore, is required in terms of an equivalent length of straight hori-

zontal pipeline.In order to provide a comparison between the data for conveying vertically

up and conveying horizontally, and hence to obtain the necessary scaling parame-ter, a rectangular grid was placed on the various sets of pressure gradient data. Thegrid was set at corresponding values of air and material flow rates, and the ratio ofthe pressure gradient values obtained from the vertical and horizontal data were

determined.The results of this process, carried out for barite in two inch bore pipeline,

are presented in Figure 8.7a. They are presented on the same axes, together withthe solids loading ratio lines, so that any pattern in the values with respect to con-veying conditions could be determined. From this it will be seen that the ratio ofthe pressure gradient for vertically upward flow to that for horizontal flow variesfrom a minimum of about 1-9 to a maximum of about 2-4 and that the predominant

value is about two.


248 Chapter 8

50

o8-

3(

I20ca'Blio

(a)

40 80 120 160Free Air Flow Rate - ftVmin

(b)

40 80 120 160


Figure 8.7 Ratio of vertical to horizontal conveying line pressure drop data for flow intwo inch nominal bore pipeline, (a) Barite and (b) fly ash.

It can be seen that the relationship obtained covers a very wide range ofconveying conditions. A similar analysis, carried out with fly ash in a two inchbore pipeline is presented in Figure 8.7b. It will be noticed that there is very littlevariation in this ratio from minimum to maximum values of conveying air velocityand from minimum to maximum values of solids loading ratio. The only deviationfrom a mean value of about two would appear to be at the two extreme limits ofthe pressure gradient curves, where the data is least reliable. This, therefore, showsthat the pressure drop in conveying vertically up is approximately double that inhorizontal conveying, for given conveying conditions, over the entire range ofconveying conditions.

3.2 Flow Vertically DownIn the majority of pneumatic conveying systems, flow vertically down usuallyoccurs only when the pipeline is routed over some obstruction such as a road orrailway line. In these cases the influence of the vertically downward section isgenerally disregarded. It is essential, however, that the additional bends requiredare taken into account.

It is with mining that long vertical pipelines come into their own, both forconveying vertically down as well as vertically up. The removal of muck from theboring of vertical mine shafts is often undertaken pneumatically. Much of the coal



mined around the world is obtained from deep mines. With mechanization of coalface operations in the 1970's the mining capability exceeded the hoisting capabil-ity of winding gear and so additional means had to be found for extracting theadditional capacity. Pneumatic conveying was widely used for this purpose andthe conveying of coal 1650 feet vertically upwards at 110,000 Ib/h was quitecommon in the 1970's [4].

Back-filling of mined out areas is generally a requirement and cement andfly ash are widely used for this purpose. These materials, therefore, are often con-veyed vertically down mine shafts. Because of the vast quantities of fly ash beingproduced around the world from power generation with coal, and the environ-mental problems associated with the material, the disposal of fly ash in this way isbeing considered more widely [3]. The longest pipelines conveying material verti-cally down are probably in South Africa. Ice is used in many deep gold mines as aheat transfer medium for cooling ventilation air. Ice making plant is located at thesurface level and the ice produced is pneumatically conveyed over distances up tothree miles, with vertically down distances up to about 7900 feet [5].

Pressure gradient data for the pneumatic conveying of cement verticallydown in the Figure 8.2 pipeline of two inch nominal bore is presented in Figure8.8. Although the form of the data is similar to that for the other pressure gradientdata presented, it will be seen that signs have been added to the values.

60

40

O

20


PressureGradient -

Ibf7in2perl00

\

20

50 100Free Air Flow Rate - ftVmin

150

Figure 8.8 Pressure gradient in vertically down flow for cement in two inch nominalbore pipeline.


250 Chapter 8

At high values of solids loading ratio the pressure gradient is negative whichmeans that there is a rise in pressure along the length of the pipeline, rather than apressure drop. Where the pressure gradient is 16 lbf/in2 per 100 ft of pipeline, forexample, it means that at the bottom of the 53 ft vertical fall in the test facility thepressure will have risen by about 8'A lbf/in2. It will also be seen that some of thepressure gradients are positive which means that there is a pressure drop along thelength of the vertical fall.

The magnitude of the pressure gradient varies with solids loading ratio, andpressure rise for the flow vertically down increases with increase in solids loadingratio. At a solids loading ratio just below about forty the pressure gradient is zero,which means that the material is conveyed with no pressure drop whatsoever un-der these conditions. At lower values of solids loading ratio there is a pressuredrop and this covers the entire range of dilute phase conveying.

Two additional sets of data are included to reinforce the nature of the curves.These are for barite and fly ash, both conveyed vertically down through the sametwo inch bore pipeline. Data for the cement is presented in Figure 8.9a and that forthe fly ash in Figure 8.9b. It will be seen that all three materials follow a very simi-lar pattern, although material flow rates differ, as might be expected. The zeropressure gradient curve is also consistent in occurring at a solids loading ratio ofabout 35 in each case.

60

50

1 40

wa* 30oE

1 20

10

(a)

Solids Loading Ratio 60

Pressui- Gradientlbf/in2 per 100 ft

140 120,100 80oo .o •


^ .̂ 140Pressure

Gradient -Ibf/in2 per 100ft

120 100

20

0 50 100 150Free Air Flow Rate - ft3 / min

£40

_oE

120

10

(b)

80

\

-2+2

0 50 100 150Free Air Flow Rate - ft3 / min

Figure 8.9 Pressure gradient data for flow vertically down in a two inch nominal borepipe, (a) Barite and (b) fly ash.



For conveying vertically down, therefore, materials capable of dense phaseconveying could be conveyed very long distances with a relatively low air supplypressure. A particular advantage is that in conveying materials such as cement andfly ash down a mine shaft, the pressure generated at the bottom could be highenough to automatically convey the material to underground mine workings an-other 5000 ft distant.

A problem with this, however, is in the sizing of the pipelines, for the veloc-ity of the material at the start of the horizontal run may be too low as a result of thehigh pressure generated. This point is considered further in the next chapter onStepped Pipeline Systems.

3.3 Minimum Conveying Air Velocity

Much has been said about minimum conveying air velocities and conveying lineinlet air velocities. The data presented so far has essentially been for total pipelinesystems comprising horizontal and vertical sections of pipeline, and bends. Mini-mum conveying air velocities in vertically upward flow are lower than those forhorizontal conveying but in a mix of orientations in the one pipeline it is usuallydifficult to take the benefit of this into account and so the worst case of velocityrequirements for horizontal conveying are usually specified.

In horizontal pipelines particles that drop out of suspension, or saltate, willcome to rest on the bottom of the pipeline. With an increase in the thickness of thesaltated layer the cross sectional area will reduce and there will be a correspondingincrease in conveying air velocity. Depending upon the nature of the material thismay result in a steady equilibrium situation. More often than not, however, thesaltated layer will be formed into dunes and these will be swept up and block thepipeline, often at a bend in the pipeline.

In vertically upward flow this process is referred to as choking. When parti-cles drop out of suspension, usually in the boundary layer at first, where the veloc-ity is lowest, they will enter into free fall. At velocities at which particles will set-tle on the bottom of the pipeline in horizontal flow, particles are likely to be re-entrained in flow vertically up because of impact with other particles moving upand the general turbulence. As a consequence minimum conveying air velocitiescan be lower for vertically upward flow. In mining situations, as discussed earlier,this can be used to advantage where there will be very long runs of vertical pipe-line. Where the majority of a pipeline runs horizontal it is more difficult to takeadvantage of this fact.

4 INCLINED PIPELINES

There is little published information on the advisability of using inclined pipelines.Much of it is anecdotal, but as it is generally experiential it would generally bewise to avoid pipelines that incline upwards. An inclined section of pipeline maywell reduce the overall length of a pipeline but their use is not recommended.


252 Chapter 8

4.1 Upward Incline

The general consensus of opinion is that pipelines inclined upwards should beavoided and that for any vertical rise, a combination of horizontal and verticalsections only should be used. The problem relates essentially to low velocity con-veying and the influence that an inclined line might have on the minimum veloc-ity. There is, of course, the additional issue of pressure drop

When saltation occurs in a horizontal pipeline, particles will be deposited onthe bottom of the pipeline, as mentioned above. In a pipeline inclined upwards,however, particles dropping out of suspension will be more mobile and will tendto roll backwards. The saltated layer will readily form dunes and these will resultin pipeline blockage

Although the minimum conveying air velocity for vertically upward flow islower than that for horizontal flow, the minimum conveying air velocity for pipe-lines inclined upward is higher than that for horizontal pipeline. If it is known thatthe velocity in an inclined section of pipeline will be high there should be no riskof blockage.

It is also understood, however, that the pressure drop in a pipeline inclinedupwards is much greater and so on this basis it would be better to keep to horizon-tal and vertical sections for any vertical rise required. The scaling parameter is onefor horizontal flow and two for vertically upward flow. At an angle of inclinationof about 60° the scaling parameter is a maximum and is slightly greater than thatfor vertically up flow at 90° [6].

4.2 Downward Incline

The mechanism of flow in downward inclined pipeline is somewhat different andso there should be little difference in minimum conveying air velocity from that inhorizontal pipelines. Saltated particles will tend to roll in the direction of flow andbe re-entrained in the gas flow, rather than form dunes, at velocities just above theminimum conveying air velocity for horizontal flow.

5 PIPELINE BENDS

Although pipeline bends provide pneumatic conveying system pipelines with theirflexibility in routing, they do have an impact on the performance of a conveyingsystem. Determining the pressure drop due to bends in a pipeline, however, is nota simple matter.

Apart from the influence of the conveyed material, the location of the bendalong the length of the pipeline and the geometry of the bend are also likely tohave an influence on the pressure drop across the bend. Data on the influence ofbends may be required as an equivalent length rather than a pressure drop value.These issues are considered as well as the general influence of bends on conveyingperformance.



5.1 Classical Analysis

The difficulties of pressure measurement in pneumatic conveying system pipelinesare highlighted most effectively with the problem of measuring the pressure dropacross a bend in a pipeline. It is not just a matter of recording the pressure at inletto and outlet from the bend and subtracting the two readings. This will give a to-tally false recording, being significantly lower than the actual value. It is necessaryto record the pressure at regular intervals along the sections of pipeline both beforeand after the bend [3].

Part of the problem lies in the complexity of the flow in the region of abend. The conveyed particles approaching a bend, if fully accelerated, will have avelocity that is about 80% of that of the conveying air. This velocity, of course,depends upon the particle size, shape and density, and the pipeline orientation. Atoutlet from a bend the velocity of the particles will be reduced and so they willhave to be re-accelerated back to their terminal velocity in the straight length ofpipeline following the bend. The situation is depicted in Figure 8.10.

The pressure drop associated with this re-acceleration of the particles, there-fore, is not registered in the bend, but occurs in the pipeline following, and so itmust be taken into account as illustrated in Figure 8.10. The method by which thetotal pressure drop associated with a bend is determined is to instrument the pipe-line before and after the bend with pressure transducers. Typical data for a whitewheat flour is shown in Figure 8.11 [7].

IApproach | Bend

I

¥Ap in Bend *

Ap in Line f IFollowing Bend1 AP Total

^-^ \ 1

Following Straight

Distance

Figure 8.10 Pressure drop elements and evaluation for bends.


254 Chapter 8

^c<£HX>

a,k*

<

17

16

15

14

13

Conveyed Material - Flour

Slope =2-7 lbf/in2

per 10ft

I Pressure DropDue to Bend = 2-0 lbf/in2

I = 75ft

Locationof Bend

>-20 -10 0 10 20 30

Distance From Bend - ft

40 50

Figure 8.1 1 Pressure profile in straight pipeline either side of a steel bend.

The bend was tested in a two inch nominal bore steel pipeline and had abend diameter, D, to pipe bore, d, ratio of 5:1. The bend was tested in the horizon-tal plane, with the flour conveyed at a solids loading ratio of about 32. The meanparticle size of the flour was 78 micron, and the particle and poured bulk densitieswere 87 and 30 Ib/ft3 respectively.

From Figure 8.1 1 it will be seen that the total pressure drop across the bendwas about 2-0 lbf/in2. Since the pressure gradient in the straight pipeline, both be-fore and after the bend was also available, the equivalent length of the bend couldbe determined. In the case presented this equivalent length was evaluated at about75 feet. Re-acceleration of the particles may require a significant distance down-stream of the bend, particularly if the particles have a large mass and density, andsomething of the order of a dozen pressure transducers would be required, asshown.

In practical terms this pressure drop is a little high. With eight such bends ina pipeline it would require the output of a positive displacement blower just tonegotiate the bends. The conveying air velocity at the bend was about 3500 ft/minand this is almost double that necessary to convey the flour at a solids loading ratioof 32.

The data was obtained from a test on an instrumented pipeline in a labora-tory facility and is used for illustration purposes. Systems, however, are frequentlyover designed, particularly if the designers are not certain of the minimum convey-ing air velocity value, and so there is often scope for improving the performance ofexisting conveying systems as a consequence.



5.2 Comparative Analysis

An alternative, and potentially quicker, means of determining the energy loss as-sociated with bends is to compare the conveying performance of two pipelines inwhich the same material has been conveyed. Ideally both pipelines should be ofthe same bore and preferably of a similar length and contain a different number ofbends of the same geometry. By comparing the performance data of materialsconveyed in the two pipelines it is possible to determine the influence of the addi-tional bends.

Conveying data obtained with barite conveyed through two such pipelines ispresented in Figures 8.12a and b. Both pipelines tested were two inch nominalbore and all the bends in the two pipelines had a bend diameter, D, to pipe bore, d,ratio of 24:1. One pipeline was 340 feet long and incorporated nine 90° bends andthe other was 330 feet long and incorporated seventeen 90° bends.

The bends were uniformly positioned along the length of the pipelines, andthere was sufficient length of straight pipeline before every bend to ensure that thematerial was fully accelerated to its terminal velocity [8].

50

40

30

Solids LoadingRatio

„ Conveyin- Line- PressureI Drop

- Ibf7in2

20

0

|'°

0

(a)

0 50 100 150


Solids LoadingRatio

Conveying-Line

Pressure

0

(b)

.60

0 50 100 150


Figure 8.12 Conveying data for barite in two inch bore pipelines of approximately thesame length. Pipeline with (a) 9 bends and (b) with 17 bends.


256 Chapter 8

Conveying characteristics for the barite in the pipeline with nine bends arepresented in Figure 8.12a and for the pipeline with seventeen bends in Figure8.12b. Barite was chosen so that a very wide range of conveying conditions couldbe examined, from low velocity dense phase to high velocity dilute phase. Theconveying data has been presented on the same axes for both pipelines, with mate-rial flow rates up to 50,000 Ib/h considered for each, and it will be seen that for thepipeline with 9 bends, conveying line inlet air pressures up to 50 lbf/in2 were em-ployed, but this had to be increased to 60 lbf/in2 for the pipeline with 17 bends.The only essential difference between the two pipelines is eight bends and so thedifference between the two sets of data can reasonably be attributed to eight bends.

With complete sets of conveying characteristics obtained for the same mate-rial conveyed through two pipelines of approximately the same length, but withdifferent numbers of bends, it should be possible to compare the results and de-termine the influence that the bends have, since the influence of pipe bore andconveying distance have been isolated. The comparison is based on the mass flowrates of the barite achieved for given values of air flow rate and conveying linepressure drop.

A grid was drawn on each set of conveying characteristics and the ratio ofthe barite flow rates was determined for every grid point. In order to determinewhether there is any pattern in the value of this ratio, with respect to conveyingconditions, the values corresponding to the grid points have been plotted on theconveying characteristics for the pipeline with 17 bends. These are shown in Fig-ure 8.13.

0-85 0-80 0-7540

oo2 30

I 20?_o

fan

"310

a

Material - barite

mp in 330 ft x 17 bends

mp in 340 ft x 9 bends

0-70

0-65

0-69067

064

064

50 100 150Free Air Flow Rate - ftVmin

200

Figure 8.13 Ratio of material flow rates in pipeline with 17 bends, to pipeline withnine bends.



It will be seen that the material mass flow rates for the line with 17 bendsare lower in every case, varying from 88% to 64% of the value obtained with thepipeline with only nine bends. Figure 8.13 shows very clearly that bends haverelatively little influence when conveying at very high solids loading ratios withlow air flow rates, but have a very significant effect when conveying at low solidsloading ratios with high air flow rates. Solids loading ratios have not been shownon Figure 8.13 but this information is available from Figures 8.12a and b.

This also shows that no single value can be applied to allow for the influ-ence of bends in a pipeline. An allowance will quite clearly depend on the convey-ing conditions. Figure 8.13, however, shows that the influence of conveying con-ditions on the effect of the bends is very uniform and consistent, and so it shouldbe possible to determine a simple relationship between the allowance to be madeand some parameter that defines the conveying conditions.

5. 2. 1 Equivalent Length

The next stage in the analysis is to assign an order of magnitude, or value, to theallowance to be made for the bends. For this purpose an equivalent length isprobably the best way of allowing for the added resistance. An equivalent lengthof straight horizontal pipeline in feet is therefore required, so that this can beadded to the existing pipeline length to give the total equivalent length of the pipe-line [8].

As the equivalent length will vary with conveying conditions it is necessaryto superimpose regular grids on the two sets of conveying characteristics, as pre-sented earlier and to evaluate the value at every grid point established. The equiva-lent length of the bends can be determined with a model that relates material flowrate and equivalent conveying distance for a pneumatic conveying pipeline. Such amodel was presented in Chapter 7 with Equation 7.10:

...... - - 0)

where rhp = mass flow rate of material - Ib/h

Le = equivalent length of pipeline - ft

and subscripts 1 and 2 refer to differentpipelines of the same bore

The equivalent lengths of the two pipelines will be:

Le, = (340+ 9b) ftand Le2 = (330+176) ft .......... (2)

where b = equivalent length of straighthorizontal pipeline per bend - ft


258 Chapter 8

Substituting Equation 8.2 into 8.1 and re-arranging gives:

mn, 330 + lib

m. 340 + 9 b(3)

It is this ratio that is plotted on Figure 8.13. The only unknown in this equa-tion, therefore, is b. The equivalent length, therefore, will increase in a patternsimilar to that shown for the ratios on Figure 8.13. An analysis of the data pro-duced the relationship shown in Figure 8.14. The entire program of test work andanalysis was repeated with cement, in place of the barite, and a very similar set ofresults was obtained.

The correlation is in terms of a single parameter, which is conveying lineinlet air velocity, which makes its application very convenient. This would indi-cate that the location of the bend along the length of the pipeline is only of secon-dary importance, despite the fact that the conveying air velocity will increasealong the length of the pipeline. It might, however, be that the difference in parti-cle velocities across the bends do not vary significantly with their position alongthe length of the pipeline.

From Figure 8.14 it will be seen that equivalent lengths of bends can be aslow as 5 ft per bend for low velocity dense phase conveying, with conveying lineinlet air velocities of 600 ft/min. For dilute phase conveying, however, with a con-veying line inlet air velocity of 4000 ft/min, for example, the equivalent length isabout 80 ft per bend. The curve continues to rise to higher values of equivalentlength with further increase in velocity.

1000 2000 3000

Conveying Line Inlet Air Velocity - ft/min

4000

Figure 8.14 Influence of conveying line inlet air velocity on equivalent length of longradius 90° steel bends for conveying barite.



5.2.1.1 Coefficient of RestitutionAlthough the data in Figure 8.14 relates to barite, the entire process was also car-ried out with cement, as mentioned above. There were obviously differences in theconveying characteristics between the barite and cement conveyed through thetwo pipelines but the analysis carried out produced an almost identical result interms of equivalent length [8]. It is suspected that many other materials will followthis same pattern in terms of equivalent length. However, it is believed that thevalue of the coefficient of restitution between the particles and the bend wall mightwell be an additional influencing parameter.

If materials having a high value of coefficient of restitution impact against abend the velocity of the particles on leaving the bend will not be as low as thosefor materials such as flour, barite and cement. As a consequence the energy lossacross the bend will not be as high, particularly for higher velocity flows. Thispoint is considered further, later in this chapter, when the analogous situation ofconveying materials through rubber hose is investigated.

5.2.2 Pressure Drop

An alternative presentation of the data in the form of conveying characteristics ispresented in Figure 8.15. The bend loss here is expressed in terms of lbf/in2 perbend. It will be seen that the most significant parameter is air flow rate, and henceconveying air velocity, with losses varying from about '/2 lbf/in2 per bend in lowvelocity dense phase flow to 2/4 lbf/in2 per bend in high velocity dilute phase flowover the range of air flow rates considered.

50

oo2 40

j5

i 30

I

S3a

10

Solids LoadinRatio x ' ^ , 1 0 0 ,80

60

PipelineBore

20

Bend Loss

= 2 inch -*•• ^ ""lbf/in /bend

0 40 80 120 160

Free Air Flow Rate - fWmin

Figure 8.15 Influence of conveying conditions on pressure drop for barite conveyedthrough long radius 90° steel bends.


260 Chapter 8

From the pipeline and conveying parameters for the flour, presented in Fig-ure 8.11, the air flow rate was about 155 ftVmin and the solids loading ratio wasgiven as 32. If this data is plotted on Figure 8.15 for the barite it will be seen thatthe pressure drop would be about 2 lbf/in2 which is the same value as that reportedon Figure 8.11. This is despite the difference in bend geometry.

5.2.2.1 Comparative ValuesIt will be noted that in terms of equivalent lengths the spread of values over therange of conveying conditions considered is of the order of 20:1 from Figure 8.14,but in terms of pressure drop values from Figure 9.15 it is only about 5:1. Thevalues in terms of pressure drop are much closer because pressure gradient valuesin dense phase are very much higher than those for dilute phase. The use of scalingparameters for evaluating pneumatic conveying system performance and capabil-ity is very different from that of summing pressure drop values for individual ele-ments of the pipeline.

5.3 Bend Geometry

The majority of the work reported in this chapter has been undertaken with longradius bends having a D/d ratio of about 24:1. Tests with the wheat flour related toa bend with a D/d ratio of 5:1 but the data agreed quite closely with that for baritein the long radius bends. The influence of bend geometry on the air only pressuredrop for bends was considered in Chapter 6 with Figure 6.6 and this is reproducedhere in Figure 8.16 for reference. From this it will be seen that it is only with veryshort radius bends that pressure drops will be high for air only.

Rough Pipesf = 0-0075

Smooth Pipes/i= 0-0045

10 20

Ratio of D/d

Figure 8.16 Head loss for 90° radiused bends.

30 40



Bends having a wide range of geometries are employed in pneumatic con-veying system pipelines. Short radius bends and tight elbows are cheaper and eas-ier to install than long radius bends. Blind tees are often used in pipelines in whichabrasive materials are conveyed. In order to determine the influence of bend radiuson pressure drop and conveying performance a program of tests was carried outwith a range of bend geometries [9].

A pipeline was specially built with a double loop in the horizontal plane, inwhich the bends at the corners could be replaced. The pipeline included eleven 90°bends and seven of these could be conveniently changed. The pipeline was 165 ftlong and of two inch nominal bore. A fine grade of fly ash was used as the con-veyed material to ensure that tests could be carried out over as wide a range ofconveying conditions as possible. A sketch of the pipeline is given in Figure 8.17for reference.

The central group of seven bends, positioned in the corners of the doubleloop were arranged so that bends of different geometry could be conveniently in-corporated. The location of the bends is indicated on Figure 8.17 and were chosensince there was a reasonable length of straight pipeline before the bend to ensurethat the fly ash was accelerated to its terminal velocity before meeting the nextbend.

The group of seven bends, all having the same geometry, were all changedfor each test program. Tests were carried out with sets of long radius bends havinga bend diameter, D, to pipe bore, d, ratio of 24:1; with short radius bends (D/d =6); elbows (D/d = 2); and with blind tees. A proportioned sketch of the differentbends tested is given in Figure 8.22.

Return toHopper

SupplementaryAir t

Discharge fromBlow Tank

Figure 8.17 Pipeline used for bend geometry tests.


262 Chapter 8

Elbow Shortradius

Longradius

Figure 8.18 Sketch of bends tested.

A complete set of conveying characteristics was obtained with the fly ashconveyed through the pipeline for each of the four different sets of bends. Theconveying characteristics obtained with the long radius bends and the blind tees

are presented in Figure 8.19.

Solids Loading. Ratio ^~-^

-Conveying Line• Pressure Drop'- lbf/in2" Conveying Li

_ Pressure Drop

10

\32

24

(a)

0 50 100 150Free Air Flow Rate - ftVmin (b)


Figure 8.19 Conveying characteristics for fly ash conveyed through the pipelineshown in figure 8.21 having, (a) Long radius bends and (b) blind tees.



If these two sets of conveying characteristics are compared it will be seenthat over a large area of conveying conditions an increase of about 50% in pres-sure drop is required in the pipeline with blind tees to achieve the same materialflow rate in the pipeline with long radius bends. If material flow rates are com-pared for a given value of conveying line pressure drop it will be seen that theflow rate achieved in the pipeline with blind tees is approximately half of thatachieved in the pipeline with long radius bends, particularly at high air flow rates.

It must be recalled that these two sets of conveying characteristics relate tothe same pipeline, for the 165 ft length of pipeline and four of the bends are ex-actly the same in each case. These differences, therefore, are due entirely to thechange in geometry of only seven of the bends in the pipeline.

To provide a full comparison of the different sets of bends the conveyingcharacteristics were compared over the entire range of conveying conditions. Thecomparison was based on the pressure drop required to achieve a specified mate-rial flow rate for a given air flow rate. To do this a grid was drawn on each set ofconveying characteristics at regular increments of both air and material flow rates,and the conveying line pressure drop at every grid point was noted [9].

The results of this analysis are presented in Figure 8.20 with both the blindtees and the short radius bends compared with the long radius bends. The longradius bends have been taken as the datum for reference.

10

(a)

Solids LoadingRatio

Solids Loading Ratiof APshoit rad

,00%p APlong radius | i QQ 60

0 50 100 150

Free Air Flow Rate - ftVmin (b)

0 50 100 150


Figure 8.20 Comparison of performance of long radius bends, (a) With blind tees and(b) short radius bends.


264 Chapter 8

The numbers on these plots are essentially the ratios of corresponding pres-sure drops, in terms of a percentage increase, or decrease where there is a negativesign. It will be seen that the pressure drop with the blind tees was about 40%greater than that for the line with the long radius bends, whether for the low veloc-ity dense phase or the high velocity dilute phase conveying of the material. Interms of energy considerations, therefore, blind tees could not be recommendedfor pneumatic conveying system pipelines with this type of material.

A comparison of the short radius with the long radius bends is given in Fig-ure 8.20b and from this it will be seen that there is little difference between thetwo, although at low values of both air and material flow rates the short radiusbends performed better. In terms of bend selection, therefore, long radius bendswould only be recommended if there were particular needs for such bends in termsof erosive wear resistance and the minimizing of material degradation.

A comparison of the elbows with the long radius bends showed an overallincrease in percentage ratios of about 15% over those shown on Figure 8.20b forthe short radius bends [9]. The data overall, therefore, shows a very close correla-tion with the data for air only in Figure 8.16, with respect to the influence of bendgeometry on pressure drop.

5.3.1 Pocketed Bends

Bradley [10] undertook a program of tests with a 90° pocketed bend and reportedthat the pressure drop was only marginally better than that for a blind tee. Thepocketed bend was of the vortice variety and was tested in a similar manner to thatdiscussed in relation to Figure 8.11.

5.4 Bend Location

The general recommendation is that a reasonable length of straight pipeline shouldproceed a bend in a pipeline, particularly the first bend in a pipeline followingmaterial feed into the pipeline.

This area in a pipeline is particularly critical because the conveying air ve-locity is at its lowest and the material is generally fed into the pipeline at zero ve-locity. Ideally the particles should be accelerated to their terminal velocity. Withlarge and high density particles this requires a relatively long distance. If this is notpossible, and particularly if the first bend is a blind tee or pocketed bend, it may benecessary to increase the conveying air velocity to compensate and this, of course,will increase the energy requirements for the system.

By similar reasoning no two bends in the pipeline should be spaced tooclosely together, particular in the low velocity area in a pipeline.

Long radius bends are also to be avoided if the first bend must feed verti-cally up. The problem here is analogous to inclined pipelines. A long radius bendin a horizontal to vertically up orientation will have a significant section of pipe-line on an incline, and a higher conveying air velocity may be required for materialto negotiate such a geometry.



6 PIPELINE MATERIAL

Although all the data so far has related to the pneumatic conveying of materialsthrough steel pipelines, not all pipelines are made of steel. Rubber hose is widelyused in pneumatic conveying systems, both for pipeline and bends, and in systemswhere a degree of natural flexibility is required, such as in vacuum off-loading andmobile systems.

By virtue of its natural resilience rubber hose can often be used to particulareffect in reducing erosive wear with abrasive materials and in minimizing degra-dation with friable materials. As a pipeline material it is particularly suited to theconveying of certain sticky and cohesive materials.

For the off-loading of ships, that have self-discharging facilities, high pres-sures are generally employed in order to keep the discharge time to a minimum.With materials such as cement, conveying air pressures up to 100 psig can be util-ized, and hose is available that will meet this requirement.

A particular application is the transfer of drilling mud powders, such as bar-ite, bentonite and oil well cement, from supply boats onto off-shore drilling plat-forms. As materials have to be off-loaded from boats in rough seas, a long lengthof hose is used to connect the discharge system on the boat with the fixed pipelineon the drilling rig.

Road trucks and rail tankers are most conveniently off-loaded throughlengths of flexible rubber hose, whether the vehicles are self off-loading or not. Inthese applications it would be impractical to use rigid metal pipelines because ofthe time required to achieve the necessary alignment. An unknown quantity, how-ever, is whether the pressure drop for rubber hose will be any different from thatof steel pipeline.

6.1 Pipeline Pressure Drop

In order to determine whether there is any difference in conveying performancebetween steel and rubber hose a program of tests was specifically undertaken. A140 ft long pipeline of two inch bore steel pipeline that incorporated five 90°bends was used.

Oil well cement was conveyed through this pipeline and its conveying char-acteristics were obtained. A 140 ft length of two inch bore rubber hose line wasthen strapped to the steel pipeline. By this means exactly the same routing andbend geometries were replicated. The oil well cement was then conveyed throughthis pipeline and its conveying characteristics were obtained.

The two sets of conveying characteristics are presented in Figure 8.21. Oilwell cement, like ordinary portland cement, is capable of being conveyed in densephase and at low velocity and so the two sets of data cover a very wide range ofconveying conditions. Tests were carried out with air supply pressures up to about28 lbf/in2 gauge and so, as the pipeline was relative short, solids loading ratios upto about 200 were achieved.


266 Chapter 8

Solids LoadingRatio -*• 200 160 130

50

40

3 30oi

1320

S 10

0

100 8024

50

40

Conveying Line SoUds

Pressure Drop 18Q150]20 Loa(jjng

r .bt/in

- 16

(a)


S 10

0

(b)

\00 Ratio

20

40

0 50 100 150

Free Air Flow Rate - ftVrnin

Figure 8.21 Conveying characteristics for oil well cement conveyed through 140 ftlong pipeline of two inch bore of different materials, (a) Steel pipe and (b) rubber hose line.

From the two sets of conveying characteristics it will be seen that the natureof the curves is very different. With the steel pipeline there is a distinct pressureminimum point in the pressure drop curves. Conveying performance appears to besimilar at low values of air flow rate but are widely different at high values of airflow rate.

In order to compare the performance of the oil well cement in the two pipe-lines a grid was drawn on each of the sets of conveying characteristics, in muchthe same way as reported above for the program undertaken with bends of differ-ent geometry. The ratio of pressure drops for corresponding air and material flowrates were evaluated. The results of this exercise are presented in Figure 8.22.

From Figure 8.22 it will be seen that there is a gradual increase in pressuredrop for the rubber hose line, compared with that for the steel pipeline, with in-crease in air flow rate. The lines of constant percentage increase drawn on Figure8.22 slope in the same way as the lines of constant velocity on the conveying char-acteristics, as illustrated on Figure 7.4, and so it is clearly a conveying air velocityeffect. In dense phase flow at very low velocities there is little or no differencebetween the two pipeline materials, but with higher velocity dilute phase flow thepressure drop for flow through the rubber hose line is 50% greater than thatthrough the steel pipeline.



50

40

£ 30

.

20

10

+10 -1 x 100%

-10

Material - CementPipe Bore - 2 inchLength- 130 feet

1 I i 1 1 I L

0 40 80 120 160


Figure 8.22 Comparison of pressure drop data for steel and rubber hose lines.

The program of tests was repeated with another drilling mud powder (barite)and a similar set of results was obtained [11].

6.2 Coefficient of Restitution

It is suspected that the coefficient of restitution between the particles and the pipe-line wall plays an important part. Rubber, being resilient, will have a lower coeffi-cient of restitution for impacting particles than steel. If the rubber absorbs more ofthe energy of impact of the particles than the steel, a greater pressure drop willresult with the rubber pipeline, due to having to re-accelerate the particles from alower velocity. This is why the pressure drop for flow through the rubber hose isgreater than that through the steel pipeline, and since pressure drop increases with(velocity)2, this is why it increases with increase in conveying air velocity [11].

7 EQUIVALENT LENGTH

Scaling, whether for system design or for undertaking a review of alternative con-veying systems for a given duty, is generally undertaken in two stages. The firststage is to scale to the length and routing required and the second is to scale withrespect to pipeline bore. Scaling with respect to length and pipeline routing is usu-ally in terms of an equivalent length of the pipeline. The equivalent length incor-porates vertical lift and bends, as well as horizontal pipeline, and is expressed in


268 Chapter 8

terms of horizontal length. A factor of two is suggested for a scaling parameter forvertically upward sections of pipeline. Equivalent lengths for bends were pre-sented in Figure 8.14.

For non radiused bends and tight elbows an additional allowance will haveto be made. An additional allowance will also have to be made for rubber hose,but the data given here can be used in estimating appropriate values. Although theinformation presented relates to particular conveyed materials it must be appreci-ated that at this point in time there is no universal solution to the problem of de-signing pneumatic conveying systems and for determining the conveying capabil-ity of a pipeline. Different materials will behave differently, as was illustrated withtotal pipeline systems in Chapter 4.

REFERENCES

1. P. Marjanovic. An investigation of the behavior of gas-solid mixture flow propertiesfor vertical pneumatic conveying in pipelines. PhD Thesis. Thames Polytechnic (nowThe University of Greenwich) London. 1984.

2. D. Mills, J.S. Mason, and P. Marjanovic. The influence of product type on densephase pneumatic conveying in vertical pipelines. Proc Pneumatech 2, pp 193-210.Canterbury. Sept 1984.

3. D. Mills. Measuring pressure on pneumatic conveying systems. Chem Eng, Vol 108,No 10,pp 84-89. Sept 2001.

4. J. Firstbrook. Operation and development of the pneumatic coal transportation system.Proc Pneumotransport 5. BHR Group Conf. London. April 1980.

5. T.J. Sheer, R. Ramsden, and M. Butterworth. The design of pipeline systems for trans-porting ice into deep mines. Proc 3rd Israeli Conf for Conveying and Handling ofPaniculate Solids, pp 10.75-80. Dead Sea. May/June 2000.

6. D. Mills. A review of the research work of Professor Predrag Marjanovic. Proc 4th IntConf for Conveying and Handling of Paniculate Solids. Budapest. May 2003.

7. M.S.A. Bradley and D. Mills. Approaches to dealing with the problem of energylosses due to bends. Proc 13lh Powder and Bulk Solids Conf. pp 705-715. Chicago.May 1988.

8. P. Marjanovic, D. Mills, and J.S. Mason. The influence of bends on the performanceof a pneumatic conveying system. Proc 15th Powder and Bulk Solids Conf. pp 391-399. Chicago. June 1990.

9. D. Mills and J.S. Mason. The influence of bend geometry on pressure drop in pneu-matic conveying system pipelines. Proc 10th Powder and Bulk Solids Conf. pp 203-214. Chicago. May 1985.

10. M.S.A. Bradley. Pressure losses caused by bends in pneumatic conveying pipelines:effects of bend geometry and fittings. Proc 14th Powder and Bulk Solids Conf. pp 681-694. Chicago. May 1989.

11. P. Marjanovic, D. Mills, and J.S. Mason. The influence of pipeline material on theperformance of pneumatic conveying systems. Proc Pneumatech 4. pp 453-464. Glas-gow. June 1990.

12. D. Mills. Using rubber hose to enhance your pneumatic conveying process. Powderand Bulk Engineering, pp 79-87. March 2000.


Documents

Pipeline Material, Orientation, and Bendsnguyen.hong.hai.free.fr/EBOOKS/SCIENCE AND ENGINEERING/ENGI… · (fly ash) in the two inch bore pipeline are presented in Figure 8.4 [2,