Stepped Pipeline Systems
1
INTRODUCTION
When either a high pressure or a high vacuum is used for pneumatic conveying, it is generally recommended that the pipeline should be stepped to a larger bore part way along the length of the line at least once. This is the case whether the material is being conveyed in dilute or dense phase, and whether the pipeline is long or short. Stepping of the pipeline is particularly recommended if the material being handled is either abrasive or friable. Problems of both erosive wear and particle degradation increase markedly with increase in velocity and so stepping the pipeline 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 a stepped pipeline will also bring benefits in terms of improved conveying performance. A particular problem, however, is in the location of such steps, for if they are incorrectly located, pipeline blockage could result. The capability of purging material from a stepped bore pipeline is another issue that might have to be taken into account. A situation in a continuous pipeline in which the pipeline may require to be reduced in diameter, rather than increased which is generally the norm, is where the pipeline incorporates a long section of vertically downward flow.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
270
2
Chapter 9
CONVEYING AIR VELOCITY
For the pneumatic conveying of bulk particulate materials, one of the critical parameters 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 depend very 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 upon the solids loading ratio at which the material is conveyed and the nature of the conveyed material. If the velocity drops below the minimum value the pipeline is likely to block. It is important, therefore, that the volumetric flow rate of air, specified for any conveying system, is sufficient to maintain the required minimum value 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 for further development. The first of these is from Equation 5.2 and relates volumetric flow rate with conveying air velocity:
TI d2 C V =
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 from the Ideal Gas Law that equates any two points anywhere along the length of a pipeline, and will also equate to free air conditions:
r,
T2
TH
Using this group of equations the problem of compressibility with air in single bore pipelines was demonstrated with Figure 5.6 and this is presented here in Figure 9.1 for reference. A free air flow rate of 1500 ftVmin was selected and the influence of pipeline bore and pressure are clearly illustrated. The lines of constant pipeline bore represent the velocity profile through a pipeline in single bore pipelines. It will be seen that the slope of the lines of constant pipeline bore change constantly with pressure, and as the air pressure reduces the slope increases considerably. The problem of air expansion, therefore, is very marked in low pressure systems and particularly so in negative pressure systems [1],
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
271
Stepped Pipelines 6000
30
40
2
Air Pressure - Ibfin gauge Figure 9.1 The influence of air pressure and pipeline bore on conveying air velocity for a 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 pipeline conveying, with respect to air expansion and hence conveying air velocities, particularly 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 pressure at the start of the conveying line. As the pressure of the conveying air decreases along the length of the line, its density decreases, with a corresponding increase in velocity, as illustrated above. A simple means of limiting the very high velocities that can occur towards the end of a pipeline is to step the pipeline to a larger bore once or twice along its length. By this means it will also be possible to keep the conveying air velocity within reasonable limits [2]. The ultimate solution, of course, is to use a tapered pipeline, for in this the conveying air velocity could remain constant along the entire length of the pipeline. This, however, is neither practical nor possible, but it does provide the basis for a model of what is required. A stepped pipeline, therefore, should be designed to achieve a velocity profile that is as close as practically possible to a constant value. 3.1
Step Location
The critical parameter in the design of any pipeline is the minimum value of conveying air velocity required for the given material and conveying conditions.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
272
Chapter 9
Flow ~* Direction
di-2
(f) ^^
Figure 9.2
Stepped pipeline notation.
In the design of a stepped pipeline system it is essential to ensure that the conveying air velocity does not fall below the minimum value anywhere along the length of the pipeline. In this respect it is the location of the steps to each larger bore section of the pipeline that are crucial. With the air expanding into a larger bore pipe the velocity will fall, approximately in proportion to the change in pipe section area, at the step. The location of the step, therefore, must be such that the pressure is low enough to ensure that the velocity in the larger bore section at the step does not drop below the given minimum conveying air velocity. A pipeline having two steps, and hence three sections of pipeline of different bore, is shown diagrammatically in Figure 9.2. Reference numbers are assigned to the 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 be used 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 C3
=
Po
V0 T
-—f t / m i n- - - - -
n J _ /> T0 and substituting values for/?,, and T0 gives:
= 5-19 ~2-— ft/min d
3-4 Pi
This will give the conveying air velocity at the start of the second section of the stepped pipeline. By equating to the free air conditions in this way, the velocity at 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 be rearranged to give:
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
( 4 )
273
Stepped Pipelines
576
Po
V0 T3 (5)
P3 n
3—4 0 3
= 5-19
Ibf/in absolute
It should be noted that since the end of one section of pipeline terminates at the point where the next section of pipeline starts, the pressure difference between these 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 be used to join any two sections of pipeline at a step. As a first approximation, the position of the steps can be judged in terms of the ratio of the pressure drop values evaluated for the individual sections of pipeline, equating these in proportion to the 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 minimum conveying air velocity that must be maintained for the material is about 3000 ft/min, and 2000 ftVmin of free air is available to convey the material. The conveying line inlet air pressure is 45 Ibf/in 2 gauge. 12,000
14,660
10,000 _g
^ 8000 .4
.3 6000 u I 4000
'& g 2000
3 o
10
20 30 Air Pressure - Ibf/in 2 gauge
40
50
Figure 9.3 Stepped pipeline velocity profile for high pressure dilute phase system using 2000 ft3/min of air at free air conditions.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
274
Chapter 9
From Figure 9.3 it will be seen that a 5 in bore pipeline will be required for these conditions, and the resulting conveying line inlet air velocity will be about 3610 ft/min. If a single bore pipeline was to be used for the entire length of the line the conveying line exit air velocity would be about 14,660 ft/min. The inlet air pressure is 45 Ibf/in gauge, which is approximately 60 lbf/in 2 absolute, and so if the discharge is to atmospheric pressure, a near four fold increase in air velocity can be expected. If the material being conveyed is only slightly abrasive, severe wear will occur 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 the material is not particularly friable. If the velocity was allowed to rise to 7000 ft/min in this 5 in bore pipe a change to a 6 in bore pipe would only reduce the velocity to 5000 ft/min. The velocity in an 8 in bore pipe would be about 2800 ft/min, however, and this is unlikely to be acceptable. A 7 in bore pipe would probably be satisfactory, but care must be taken that standard pipe sizes are selected. Even in a 7 in bore pipeline the velocity at exit would be almost 7500 ft/min and so it is clear that two steps and three different pipe sizes would be required. The velocity profile for a possible combination of 5, 6 and 8 in bore pipes is shown superimposed on Figure 9.3, but even with this the exit velocity is about 5725 ft/min, and the velocity at the end of the second pipe section reaches 6315 ft/min. A plot similar to that shown in Figure 9.3, however, will give a clear indication of what is possible. The velocities at the six reference points along the pipeline are also presented on Figure 9.3 and these can be evaluated by using Equations 4 and 5. It would always be recommended that a graph similar to that included 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 minimum conveying air velocity that must be maintained for the material is about 1200 ft/min, and 350 ft /min of free air is available to convey the material. The conveying line inlet air pressure is 45 lbf/in gauge. From Figure 9.4 it will be seen that a 3 in bore pipeline will be required for these conditions, and the resulting conveying line inlet air velocity will be about 1755 ft/min. If a single bore pipeline is used the conveying line exit air velocity will be about 7125 ft/min. Although this might be accepted in a dilute phase conveying system it is quite unnecessary in a dense phase system. Apart from reducing problems of erosive wear and particle degradation, by reducing conveying air velocities, a stepped pipeline is also likely to achieve an improved conveying performance, compared with a single bore pipeline, for the same air flow conditions. The velocity profile for a combination of 3, 4 and 5 in bore pipes is shown superimposed on Figure 9.4. This has resulted in the conveying air velocity being confined to a relatively narrow band, with the maximum value being limited to 2640 ft/min.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Stepped Pipelines
275
6000
10
20 30 Air Pressure - Ibfin2 gauge
40
50
Figure 9.4 Stepped pipeline velocity profile for high pressure dense phase system using 350 ftVmin of air at free air conditions. 3.4
Vacuum Conveying
Although negative pressure systems are naturally limited to a maximum conveying line pressure drop of less than 14-7 lbf/in 2 , stepping of the pipeline with vacuum conveying systems is just as important as it is with high positive pressure conveying systems. A typical vacuum conveying system is shown in Figure 9.5. It is drawn for a dilute phase system, where a minimum conveying air velocity of 3000 ft/min must be maintained, using 500 ftVmin of free air at a temperature of 59°F and exhausting to -9 lbf/in2 gauge (14-7 -9 = 5-7 lbf/in2 absolute). It must be remembered that absolute values of temperature and pressure must be used in all the equations relating to the evaluation of both velocity and pressure along the length of a pipeline. If the vacuum were a little higher than 9 lbf/in 2 , a step to a third section of pipeline of 8 in bore would be required. Even with a conveying line exit air pressure of-7 lbf/in 2 gauge, a step could be usefully incorporated in the case presented in Figure 9.5. Because the slope of the constant pipe bore curves increase at an increasing rate with decrease in pressure, steps are required more frequently at low air pressures. From Equation 9.4 it will be seen that pressure is on the bottom line and so when values get very low, as they will in high vacuum systems, a small change in pressure will result in a large change in conveying air velocity.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
276
Chapter 9
8000k
Air Pressure - Ibfin gauge Figure 9.5 Stepped pipeline velocity profile for high vacuum system using 500 ft'/min of air at free air conditions.
3.4.1 Step Position A practical problem that arises from this is the actual positioning of the various steps along the length of the pipeline. As a first approximation, in the absence of any other information, pipeline lengths can be sized in proportion to the conveying line pressure drop for each section, provided that a reasonably uniform value of conveying air velocity is maintained along the length of the pipeline. It can be seen from Figures 9.3 to 9.5 that if there is a risk of the velocity being too low at the start of the next section, and the pipeline blocking, then the transition to the larger pipe size should be moved a little further downstream, where the pressure will be slightly lower. 4
PIPELINE STAGING
With reference to Figure 9.1 and Equation 4 it will be seen that with increase in pressure the slope of the curves decrease. If a stepped pipeline system was to be designed on the basis of a doubling in conveying air velocity, for each section of pipeline, the working pressure for each section of pipeline would increase significantly with increase in pressure, as shown in Table 9.1. If it were required to convey a material over a distance of the order of 100 miles, it would only be economical if an air supply pressure very much higher than 100 lbf/in 2 was to be used. It would also be necessary to divide the system into stages, such that the material was discharged from one system, when the pressure had fallen to a given value, and be fed into the next system with high pressure air.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
277
Stepped Pipelines
Table 9.1 Typical Working Pressures Relating to a 2:1 Conveying Line Air Velocity Expansion Ratio Air Outlet Pressure
Air Inlet Pressure lbf/in 2 absolute
lbf/in 2 gauge
14-7
0
294
14-7
Pressure Difference lbf/in 2
Ibf/in" gauge -7-35
7-35
0
14-7
58-8
44-1
14-7
29-4
117-6
102-9
44-1
58-8
235-2
220-5
102-9
117-6
455-7
220-5
235-2
470-4
With a conveying line inlet air pressure of 455-7 lbf/in 2 gauge, for example, the first step would not be necessary until the pressure had fallen to 220-5 lbf/in 2 gauge, which gives a working pressure difference of 235-2 lbf/in 2 . If the system discharged to atmospheric pressure, the pressure at entry to the last section of pipeline would be 14-7 lbf/in 2 gauge and the working pressure difference would only be 14-7 lbf/in2. This effect is shown in Figure 9.6, which illustrates the velocity profile for the latter sections of a very high pressure stepped pipeline system in which the material is conveyed in dilute phase.
7000 6000 g 5000 I
^ 4000 M
3000 I
2000 0
100
200
300
400
Air Pressure - Ibfin2 gauge Figure 9.6
Velocity profile for very high pressure stepped pipeline system.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
500
Chapter 9
278
It would be recommended, therefore, that for a very long distance conveying system, at the end of each stage along the pipeline, and at the very end of the pipeline, the material should be discharged at a pressure of at least 44 Ibfin 2 gauge. By discharging at a high pressure, rather than atmospheric, the last two or three sections of the largest bore pipeline can be dispensed with. The reduction in working pressure drop would be very small in comparison and it would make for a very much simpler pipeline design and layout. 5
PIPELINE PURGING
In many applications it is necessary to purge the pipeline clear of material at the end of a conveying run, particularly with perishable commodities and time-limited products. In single bore pipelines this is rarely a problem, even if the material is conveyed in dense phase, because the velocity at the end of the pipeline is usually sufficiently high. There can, however, be problems with stepped pipelines. A comparison of the velocity profiles for flow in single and stepped bore pipelines is presented 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. It relates to the dense phase conveying of a material for which the minimum conveying air velocity is about 1000 ft/min. This is similar to the plot shown in Figure 9.4, except that the flow of air is from left to right with the new figure.
6000 5Q90f
5000
Air Flow
4000
I
2860
Pipelinb Bore *• in
3000 2000
I
a 50
Figure 9.7
40
30 20 Air Pressure - Ibf/in2 gauge
10
Comparison of velocity profiles in single and stepped bore pipelines.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
279
Stepped Pipelines
Although this may be more conventional in terms of system sketching, it does mean that the air pressure axis is reversed, and is offered simply as an alternative means of presentation. Figure 9.7 is developed further in Figure 9.8 with empty line velocity profiles added. This also provides a comparison between single bore and stepped bore pipelines, with respect to purging, and clearly illustrates the problem towards the end of a stepped pipeline. At the end of a conveying run, with no material to convey, the pressure at the material feed point, at the start of the pipeline, will drop to the air only pressure drop value. For low velocity dense phase conveying the empty line pressure drop will only be a fraction of the pressure drop required for conveying. Thus the velocity of the air through a single bore empty pipeline will be very high throughout its length. At the end of the pipeline the air velocity will be exactly the same as in the conveying case, because the pressure here is always atmospheric. At the material feed point, however, the air velocity will only be slightly lower than that at the exit since the air pressure at the feed point is so much lower when material is not being conveyed. With the stepped bore pipeline this same volumetric flow rate of air has to expand into the larger bore section of pipeline, and so its velocity will reduce, as shown in Figure 9.9. At the end of the pipeline the situation is exactly the same as in the single bore pipeline case. The velocity for both conveying and purging will be the same, because the pressure here is always atmospheric. Since the purging velocity will not be constant throughout the pipeline the potential for clearing material from the latter sections of stepped pipelines by purging, therefore, will be severely limited. Purging Mode
Pipeline Bore - in
40
30
20
10
2
Air Pressure - Ibfin gauge Figure 9.8 Comparison of velocity profiles in single and stepped bore pipelines in both conveying and purging modes.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
280 5.2
Chapter 9 Material Deposition
To illustrate the problem of material deposition in pipelines with low velocity conveying, data from a program of conveying trials carried out with a fine grade of fly ash is presented in Figure 9.9 [3]. The fly ash was being conveyed through a 425 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 velocities, it was observed that not all the batch of material in the blow tank was discharged into the receiving hopper. The material was, in fact, being deposited in the pipeline and remaining there at the end of the conveying run, when the conveying air velocity used was too low to purge the pipeline clear. The fly ash left in the pipeline did not represent a problem because it was swept up with the next batch of fly ash conveyed. As a result the pipeline was only purged for a short time before starting the next test run. To give some indication of the potential problem of material deposition in a pipeline when conveying at low velocity, the data for every test carried out was analyzed to provide a figure for the percentage of the batch conveyed that was discharged into the receiving hopper. 100% data points simply mean that the entire batch of 1000 Ib was conveyed. For the very high velocity tests the data points have not been included. If 80% of the batch was conveyed, then 20% of the batch remained in the pipeline at the end of the test run, which amounted to 200 Ib of fly ash. The results and analysis are presented in Figure 9.9. Percentage of Batch Conveyed
'—!
40
80
1
120
Free Air Flow Rate - ftVmin Figure 9.9
Analysis of pipeline purging data for fine fly ash.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
1—i
160
1
1
1
281
Stepped Pipelines 5.2.2
Cement
In an earlier program of work ordinary portland cement was similarly tested [4]. It was 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 size of the cement was 2750 Ib, since tests with material flow rates up to about 100,000 Ib/h were undertaken. Testing was carried out with air supply pressures up to 40 lbf/in2 gauge. An analysis of the test data obtained with the cement is presented in Figure 9.10. The normal conveying characteristics for the material are presented in Figure 9.10, together with conveying air velocity data. This is in terms of a full set of curves for the conveying line inlet air velocity and a parallel axis in terms of the conveying line exit air velocity. From Figure 9.10 it will be seen that the cement could be conveyed with conveying line inlet air velocities down to about 500 ft/min and at solids loading ratios of over 100. Lines showing the percentage of the batch that was conveyed are also superimposed on Figure 9.10. In this case, when only 70% of the batch was conveyed, 825 Ib of cement was left in the pipeline. As with the fly ash, this cement was swept up by the next batch that was conveyed.
Solids Loading 14n i_ ^ Conveying 120
~
Ratio
Conveying Line Inlet Air Velocity - ft/min
"""-*. m
x
}QQ
I J
Conveying Line Pressure Drop - lbf/in 2
80
10
°
«c 100 X>
i 80 pi
I 60
1600
NO GO AREA
2000
I 40 20
2400 Percentage of Batch Conveyed
0 100
1000
400
200 300 Free Air Flow Rate - ftVmin
2000
3000
4000
Conveying Line Exit Air Velocity - ft/min Figure 9.10
Conveying characteristics for cement in 4 inch bore pipeline.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 9
282 6
DIVERSE MATERIAL CONVEYING
Not all pneumatic conveying systems are dedicated to the conveying of a single material. There is often a need for a system to transport a number of different materials. In many industries, such as food and glass, a wide variety of materials have to be conveyed by a common system, since there is a requirement to deliver a given '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 came across a total of 78 different materials, ranging from iron powder to vermiculite, in a plant manufacturing welding rods. Some of the materials to be transported may be capable of being conveyed in dense phase, and hence at low velocity, while others may have no natural dense phase conveying capability and will have to be conveyed in dilute phase with a high conveying air velocity. The air requirements for the various materials, therefore, could differ widely. This is illustrated with the case of floury and sandy grades of alumina, conveyed through the same pipeline, with conveying line inlet air pressures up to 45 lbf/in 2 gauge. The pipeline used was 155 ft long, of two inch nominal bore and incorporated six 90° bends. Conveying characteristics for these two materials are presented in Figure 9.11. _, . Conveying Limit \
Conveying Line Pressure Drop . M/in2 /
^ ,. , Solids Loading Ratio
Solids Loading Ratio \ Conveying Line Pressure Drop - Ibffitf
\
Conveying Limit
0
0
(a)
40 80 120 160 200 Free Air Flow Rate - ftVmin
(b)
0 40 80 120 160 200 Free Air Flow Rate - ftVmin
Figure 9.11 Conveying characteristics for two grades of alumina conveyed through 155 ft long pipeline of 2 in bore incorporating six 90° bends, (a) Floury and (b) sandy.
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283
Stepped Pipelines 6.1
Pipeline Selection
There is often a requirement for these two grades of alumina to be conveyed through a common pipeline. From Figures 9.1 la and b, however, it will be seen that there are considerable differences in the conveying capabilities of these two materials. The floury alumina can be conveyed in dense phase and with conveying air velocities down to about 600 ft/min, and with a conveying line pressure drop of 40 lbf/in 2 a material flow rate of 52,000 Ib/h can be achieved with a free air flow rate of approximately 55 ftVmin. The sandy alumina, however, can only be conveyed in dilute phase and requires a minimum conveying air velocity of about 2000 ft/min, and with the same pressure drop of 40 lbf/in 2 a material flow rate of only 32,000 Ib/h can be achieved and this requires a free air flow rate of approximately 170ft 3 /min. If a 20% margin is allowed on minimum conveying air velocity, in order to specify a conveying line inlet air velocity for design purposes, the value for the sandy 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 materials, a graph is plotted of conveying air pressure and a series of curves for different pipeline bore is superimposed in Figure 9.12. Onto this are drawn possible velocity profiles for the two materials. Because of the extremely wide difference in conveying air velocities a single bore line is suggested for the floury alumina, and three steps are required in the pipeline for the sandy alumina, but it will be seen that the pipeline system meets the requirements of both materials. 6000 h
e
Pipeline Bore - in
5000
's ^4000 1
^ 3000
.5 2000 c
6 1000 20
30 2
Air Pressure - lbf/in gauge Figure 9.12 Velocity profiles for sandy and floury alumina in a common positive pressure conveying system for a free air flow rate of 1000 ftVmin.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 9
284
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 completely different pipelines is not likely to be a problem. The pipeline used for the floury alumina in Figure 9.12, therefore, could well be stepped part way along its length to 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 of the pipeline, since the maximum value of conveying air velocity in the pipeline would be only 2000 ft/min. There are many alternative solutions to the problem of conveying diverse materials, but the one illustrated is probably the simplest as it utilizes exactly the same air supply in terms of both pressure and volumetric flow rate. Material flow rates will clearly be different, but an extremely complex system would 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 Figure 9.13. 6.2
Low Pressure Systems
Although Figure 9.13 is drawn with a common pipeline feeding both materials into the reception silo, this is not a requirement, as mentioned above. Indeed, with a low pressure system this may not be a possibility. Two different pipelines, however, could be utilized in exactly the same way. There would probably be no need to step any of the pipelines either.
Common 8 in Bore
X
^ \l y"
8 in Bore
Hoppers for Floury Alumina
. vv
\ /•
//
if
=0-1
C9 | 1
Hoppers for Sandy Alumina
6 in Bore
VRAA/
Compressor
r
\vy/
X
/
1
^
Reception Silo
5 in Bore
4 in Bore Figure 9.13 Typical layout of a high positive pressure conveying system for conveying diverse materials.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Stepped Pipelines
285
6.2.1 Vacuum Conveying Systems Exactly the same principles apply to vacuum conveying systems. It may well be possible to have a common pipeline delivering all materials into the reception silo and for materials with no dense phase conveying capability a stepped pipeline could be utilized to provide the necessary pick-up velocity for the given air flow rate. 7
MATERIAL FLOW RATE
The influence that a stepped pipeline might have on material flow rate is not immediately obvious. For the flow of air only through a pipeline models are well established. That for pressure drop takes the form:
Apa
°c
LoC1
—
d
where Apa L p C and d
= = = = =
Ibf/in 2
air only pressure drop pipeline length density of air conveying air velocity pipeline bore
(6) -
Ibfin 2 ft lb/ft3 ft/min in
As pressure drop increases with increase in (velocity) , and decreases with increase in pipeline bore, the pressure drop for a stepped pipeline will be significantly lower than that for a single bore pipeline of the same length, the same initial diameter 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 is rather limited but such work has been carried out with a fine grade of fly ash [6]. A 380 ft long pipeline of 2 inch nominal bore and incorporating ten 90° bends was built for the purpose. A fine grade of fly ash was used, since it is capable of being conveyed over a very wide range of flow conditions. A sketch of the pipeline is presented in Figure 9.14 for reference. This also indicates where the steps in the pipeline were made to larger bore sections of pipe. The conveying characteristics for the fly ash in the 380 ft length of single bore pipeline are presented in Figure 9.15a. These are the reference set of conveying characteristics for the basis of comparison with the stepped pipelines examined. From this it will be seen that the material could be conveyed at solids loading ratios 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.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 9
286
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 second half of the pipeline was changed from 2 inch to 21/2 inch bore pipe. At the transition section the 2 in bore pipe was simply sleeved inside the 21/2 in bore pipe and welded. The resulting conveying characteristics are presented in Figure 9.15b. Conveying Line Pressure Drop - Ibf7in 2
Solids Loading ^—-"" Ratio 200 160 120
80 70
Conveying Line Solids Pressure Drop Loading 200 160 120 lbf/in 2 Ratio o 70
80
°60
60
50 I 40 30 20 10
0
0
(a)
40
80
0
120 160 200
Free Air Flow Rate - ftVmin
(b)
40
80
120
160 200
Free Air Flow Rate - ftVmin
Figure 9.15 Conveying characteristics for fine fly ash in 380 ft long pipeline of 2 inch initial bore, (a) Single bore pipeline and (b) single step pipeline.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
287
Stepped Pipelines
By comparing Figures 9.15 a and b it will be seen that there is a very significant improvement in performance over the entire range of conveying conditions considered as a consequence of this single step. Much higher values of fly ash flow rate were achieved, and with lower values of conveying line pressure drop. To illustrate the magnitude of the improvement a comparison of the single step and single bore pipelines is given in Figure 9.16a. For this purpose a grid was drawn on each set of conveying characteristics at regular increments of conveying line pressure drop and air flow rate, and the value of the fly ash flow rate was noted at every grid point. The data points given on Figure 9.16a represent the ratio of the fly ash flow rates and this shows that the material flow rate achieved through the pipeline with the single step was about 1-9 times or 90% greater than that for the single bore pipeline for exactly the same inlet air conditions and hence power required. It is interesting to note that there is little change in the value of this ratio over the entire range of conveying conditions examined. The improvement applies equally to low velocity dense phase conveying, and to high velocity dilute phase conveying. Since there is no change in the air flow rate required to convey the material it is unlikely that there would be any need to change the filtration requirements for the conveying system either. Conveying Line Pressure 200 160 Drop 2 80 - lhf/in
Conveying Line Pressure Drop - lbf/in 2
80 70
70
60
60
50 o
Solids Loading 120
50
a
I
oi 40
40
_0 CL
oolOOO
I
• _
Vertically Down Minimum Velocity I
0
I
I
1
15
1
1
1
30
1
1
1
45
J
l_J—L
60
75
-I
L
90
Air Pressure - lbf/in 2 gauge Figure 9.21 Velocity profile for conveying system delivering materials for underground stowing.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Stepped Pipelines 10
293
AIR ONLY PRESSURE DROP
Stepped pipelines were discussed earlier to illustrate the problems of air expansion and velocity control along a pneumatic conveying system pipeline. The models necessary to evaluate conveying air velocities and air only pressure drop were also developed earlier, particularly in Chapter 6, and so it is now possible to consider stepped pipelines further. A sketch of a two section stepped pipeline is given in Figure 9.22. From Equation 6.12, for a single bore pipeline, the following expression was developed:
m RT (7)
_
r-
