13 Conveying of Cement and Drilling Mud Powders
1
INTRODUCTION
Cement is another commodity that is manufactured on a large scale and, by the nature of the material, uses pneumatic conveying systems extensively for its conveying. Because of its use in the construction industry it is distributed internationally, nationally and locally. Large bulk carriers are used for international transport and a wide range of ship loading and off-loading systems have been used and developed over the years. Large and small scale storage depots are used for its distribution nationally, with inland locations generally supplied by rail wagons, and coastal locations, often based at existing ports, supplied by self off-loading ships. Local distribution is normally by specialized road vehicles, generally capable of being pressurized for self off-loading directly into storage silos. Most large countries around the world have at least one cement manufacturing plant, and there are close to thirty countries with a manufacturing capability in excess of 10,000,000 ton/year. Economy of scale is such that individual plants are rarely built to produce less than about one million ton per year. 1.1
Material Grade
Although the problem of material grade having an influence on the conveying capability of the material does exist with cement, it is not the major problem that it
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Chapter 13
is with most other materials. Grade can vary with cement and it can have a secondary effect, but is rarely to a degree that the material cannot be conveyed in dense phase. Cement is manufactured to standards and these are based on fineness. A convenient means of determining the degree of fineness is by measuring permeability, from which the specific surface of a material can be ascertained. The most common device used is that devised by Blaine, and as a consequence the Blaine number is generally used as the international reference for the fineness of cement. The finished product is produced by grinding and so the finer the cement the greater the cost. Cement is always manufactured to a fineness that will allow the material to be conveyed in dense phase and at low velocity in a conventional pneumatic conveying system. The influence of Blaine number on the conveying capability of cement, however, is not known. In Chapter 10 it was shown that the conveying capability of fine fly ash could vary by a significant degree for just small changes in mean particle size. It is suspected that cement will be similarly effected, but not necessarily in the same way, for the particle shape of cement is very different from that of fly ash. 1.2
Materials Considered
Two types of cement are considered; ordinary Portland and oil well cement. The oil and gas industry, in addition to using oil well cement also uses large quantities of barite and bentonite and so these materials are also included here. For drilling purposes they are produced as fine powders and so these materials also have very good air retention properties. As a consequence these materials can also be conveyed in dense phase and at low velocity in conventional pneumatic conveying systems, provided that the pressure gradient available for conveying is sufficiently high to convey at the value of solids loading ratio required. All four of these materials have been conveyed through the Figure 12.11 pipeline and so their conveying characteristics are presented together for reference and comparison in Figure 13.1. The pipeline was of two inch nominal bore, 230 feet long and incorporated nine 90° bends. The materials were fed into the pipeline by means of a high pressure top discharge blow tank. 200 ftVmin of free air was available for conveying. The barite had a mean particle size of about 12 micron, a bulk density of about 100 lb/ft3 and a particle density of approximately 265 lb/ft3. For the bentonite these figures were 24 micron, 50 and 145 lb/ft' respectively. It will be seen that there are only very slight differences between the ordinary Portland and the oil well cements, and the barite was also very similar in performance. Only with the bentonite is there any real difference in conveying performance. This material did not exhibit any pressure minimum effect, and so as the material flow rate increased continuously with reduction in air flow rate, there was a very significant difference in performance at low values of air flow rate.
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Cement and Drilling Mud Powders
Conveying Line .Pressure Drop Solids Loading - lbf/in 2 40 Ratio/ 100 80 60 /
o 230
Conveying Line _ Pressure Drop - lbf/in 2 40 100 80 120 160,
Solids Loading Ratio / 60
X
.c X)
I20 _o fc.
$10 a
Free Air Flow Rate - ft /min
(a)
Conveying Line Pressure Drop 40 i - Ibf7in2 120
40 80 120 160 200 Free Air Flow Rate - ftVmin
(b)
Conveying Line .Pressure Drop 2 40 _ \ -!bf/in 120100
Solids Loading Ratio
160^ /
/80
Solids Loading / Ratio 60 50
o o
o o
230
230 30
I20
20
- 20
o
io
o
10 a\
15
10 0
(c)
40
80
120 160
0
200
Free Air Flow Rate - ft3 / min
(d)
40
80
120
160
200
Free Air Flow Rate - ft / min
Figure 13.1 Conveying characteristics for (a) ordinary portland cement, (b) oil well cement, (c) barite, and (d) bentonite conveyed through the pipeline shown in figure 12.11.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
382 2
Chapter 13 CEMENT
Conveying data for both ordinary portland and oil well cement, conveyed through the Figure 12.11 pipeline were presented above in Figure 13.1. Data on ordinary Portland cement was presented earlier in Chapter 4 where the material was used to illustrate the conveying capability of materials capable of being convey in dense phase in a sliding bed mode of flow. For this purpose data obtained with the material conveyed through the Figure 4.2 pipeline of two inch bore and 165 ft long was used. This same data was analyzed further in Chapter 7 to illustrate the influence of conveying line inlet air velocity. Ordinary portland cement was also used in Chapter 9 to illustrate the influence of conveying air velocities on pipeline purging with Figure 9.10. In this case the Figure 7.13 pipeline of four inch nominal bore and 310 ft long was used. Data for oil well cement conveyed through the Figure 10.20 pipeline of two inch bore and 140 ft long was presented earlier in Chapter 8. The oil well cement was used to illustrate the influence of pipeline material on conveying performance. Identical pipelines of steel pipe and rubber hose were tested and the data was presented in Figure 8.21 and 8.22. Barite was similarly conveyed through these pipelines and this data is presented in a section on rubber hose later in this chapter. 2.1
Ordinary Portland Cement
Conveying characteristics for ordinary portland cement conveyed through the relatively short Figure 10.20 pipeline of two inch nominal bore are presented in Figure 13.2. Since the pipeline was only 140 feet long and included six bends, conveying at solids loading ratios in excess of 200 was possible with conveying air pressures of 25 psig. Even with a conveying line pressure drop of 10 lbf/in 2 the cement could be conveyed at a solids loading ratio of 100 and with a conveying line inlet air velocity down to about 600 ft/min. With air supply pressures less than about 10 psig the pressure gradient was not sufficiently high to maintain such high solids loading ratios and so the minimum value of conveying line inlet air velocity had to increase as a consequence. Hence the increase in the volumetric flow rate of free air required at lower pressures, as shown on Figure 13.2. The conveying limit for the material is dictated by the relationship between the solids loading ratio and the minimum conveying air velocity. This was discussed in Chapter 4, where the cement was used for illustrating purposes for materials capable of dense phase conveying in a sliding bed mode of flow. The relationship for ordinary portland cement is presented again in Figure 13.3 for reference. From this it will be seen that if the cement is conveyed in dilute phase suspension flow a minimum conveying air velocity of about 2000 ft/min will have to be maintained.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Cement and Drilling Mud Powders
60
383
Pressure Drop - Ibf/irv
Solids Loading Ratio
o50
J:40
20 SlO
50
100 Free Air Flow Rate - ftVmin
200
150
Figure 13.2 Conveying characteristics for ordinary portland cement conveyed through the pipeline shown in figure 10.20. 2400
20
40
60
80
100
Solids loading ratio Figure 13.3 Influence of solids loading ratio on minimum conveying air velocity for ordinary portland cement.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
384
2. /. /
Chapter 13 Dilute to Dense Phase Transition
Situations in which it may only be possible to convey cement in dilute phase will occur if the pressure available for conveying is very low or if the pipeline is very long. The critical parameter is the pressure gradient available, and for cement this needs to be above about 2'/2 Ibf/in 2 per 100 ft of horizontal pipeline. This is an 'equivalent' length and so an allowance must be made for bends and any vertical lift in the pipeline. To convey at a solids loading ratio of about 100 typically requires a pressure gradient of about 10 Ibf/in 2 per 100 ft. The full transition from dense phase conveying capability, with a conveying line inlet air velocity of 600 ft/min, to dilute phase suspension flow, with a minimum inlet air velocity of 2000 ft/min for the cement, does not occur in Figure 13.2. This is because the pipeline was too short. The effect, however, will be highlighted with data for a longer pipeline, and this is presented below. 2.1.2 High Pressure Conveying A sketch of a very much longer pipeline through which the ordinary portland cement has been conveyed is shown in Figure 13.4. This pipeline was 535 ft long and incorporated seventeen 90° bends. The pipeline was two inch nominal bore and so the air only pressure drop was consequently very high. One would not normally build a pipeline of such a geometry, being of a relatively small bore for the length, and incorporating so many bends, but it is very useful for illustrating the influence of the various parameters. Pipeline: length = bore = bends = D/d =
Figure 13.4 land cement.
535ft 2 in 17x90° 24
Details of pipeline used for the high pressure conveying of ordinary port-
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Cement and Drilling Mud Powders
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Once again a high pressure top discharge blow tank was used to feed the material into the Figure 13.4 pipeline. 200 fVYmin of free air at a pressure of 100 lbf/in 2 was available for conveying. Since the pipeline was long and high pressure air was available the opportunity was taken to carry out test work on the cement with conveying line inlet air pressures of up to 75 lbf/in 2 gauge. The maximum discharge capability of the blow tank used was about 55,000 Ib/h, but as the pipeline was very long, and hence of high resistance, it would be possible to carry out tests with very much higher air supply pressures. Air flow rate and control was made possible at these pressures by using convergentdivergent choked flow nozzles, as discussed in Chapter 6. Conveying data for the cement in this pipeline is shown in Figure 13.5. Lines of constant conveying line inlet air velocity have been superimposed in addition, in order to illustrate the nature of the dilute to dense phase conveying transition. Conveying line exit air velocity has also been added as an additional axis. As this is a constant bore pipeline, the magnitude of the air expansion through the pipeline, with such high air supply pressures, can be clearly seen. With a conveying line inlet air pressure of 75 lbf/in~, for example, the expansion in conveying air velocity is approximately 6:1. For materials that can be conveyed at low velocity, high pressure air can be utilized quite conveniently, for the slope of the constant conveying line inlet air velocity curves is very steep. From Figure 13.5 it will be seen that a very significant increase in material flow rate can be obtained through a pipeline by using a higher air supply pressure, and the corresponding increase in air flow rate required is not proportionately large. For dilute phase flow, however, where the conveying line inlet air velocity may be 2000 or 3000 ft/min, the slope of the constant inlet velocity curves is not as steep and so considerably more air must be used if a higher air supply pressure is to be utilized. The conveying facility had 200 ftVmin of free air available and so if the conveying line inlet air velocity had to be 3000 ft/min the maximum pressure that could be utilized would only be about 25 psig. With an inlet air velocity of 600 ft/min only 90 ft3/min of air was required at 75 psig. Since the pipeline was of small bore and relatively long it will be seen from Figure 13.5 that a pressure drop of about 30 lbf/in 2 was required before the cement could be conveyed in dense phase. As a consequence a marked transition is shown between the minimum conveying limits for the dense and dilute phase areas on the conveying characteristics in this region. The transition from dilute to dense phase conveying is very smooth. Indeed, with such high air supply pressures, for many operating points on Figure 13.5 the material is quite likely to be in dense phase at the start of the pipeline and in dilute phase at the end of the pipeline. An operating problem that does arise with this dilute to dense phase transition relates to proximity to minimum conveying conditions. It is essential to avoid operating a conveying system in dense phase in the region where the air supply pressure is marginal for dense phase conveying, as illustrated in Figure 13.5 [1].
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Chapter 13
386
40
Conveying Line Inlet Air Velocity - ft/min
750 80 / 70
1000 60 /
Solids Loading Ratio
50
1250 40
J500 30 .30
u ta
oi
o
20
E "2
10
3000
Conveying Line Pressure Drop - Ibf7in 2
40
80
120
160
200
Free Air Flow Rate - ft /min
2500 5000 Conveying Line Exit Air Velocity - ft/min
7500
Figure 13.5 Conveying data for ordinary portland cement conveyed through the pipeline shown in figure 13.4.
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2.1.3 Influence of Pipeline Bore The data reported so far has been for two inch nominal bore pipelines. Despite this, material flow rates have been typically up to about 40,000 Ib/h in the data presented. Higher flow rates can be achieved by utilizing higher air supply pressures, as illustrated with Figure 13.5 but there is a limit to this. An increase in pipeline bore will allow a significant increase in material flow rate, being approximately in proportion to the increase in pipe section area. Air flow rates also have to increase in proportion to the increase in cross sectional area, in order to maintain the necessary conveying air velocities, and so this means that the conveying characteristics are almost geometrically similar. To illustrate the influence of pipeline bore, three sets of conveying data are presented for ordinary portland cement conveyed through the Figure 7.13 pipeline. This pipeline was 310 feet long and included nine 90° bends. Data for the cement conveyed through the Figure 7.13 pipeline of two inch nominal bore is presented in Figure 13.6. The maximum value of material flow rate here was just over 50,000 Ib/h because the pressure gradient available (air supply pressure divided by equivalent length of pipeline) was higher than that in the previous two inch bore pipelines that have been used to present data for the cement. Data for this same cement conveyed through the same pipeline, but of three inch nominal bore, is presented in Figure 13.7.
60
Solids Loading Ratio
o o
250 .c jB 740
:3o
" Conveying Line pressure Drop - Ibt7in2
CS
'£20 10
100
150
200
Free Air Flow Rate - frYmin Figure 13.6 Conveying data for ordinary portland cement conveyed through the pipeline shown in figure 7.13 of two inch nominal bore.
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Chapter 13
388
Solids Loading Ratio
120
Conveying Line Pressure Drop - Ibt7in2
_g
I
40
20 I
40
80
120
I
I
I
160
I
L_
200
j
Free Air Flow Rate - ft /min Figure 13.7 Conveying data for ordinary portland cement conveyed through the pipeline shown in figure 7.13 of three inch nominal bore. It will be noted that the same range of air supply pressures was used in conveying the cement through the three inch bore pipeline and so a direct comparison with the two inch bore pipeline data is possible. The material flow rate axis has been doubled and it will be seen that the flow rate of cement has more than doubled, as would be expected, for the increase in cross sectional area from two to three inch bore pipe is more than double. It will be seen that the two sets of conveying characteristics are approximately geometrically similar, as mentioned above. This is because the air flow rate axis has been scaled in proportion to pipe section area and the resulting increase in cement flow rate is approximately in proportion to this ratio also. Solids loading ratios achieved are slightly higher with the three inch bore pipeline and this is partly due to the fact that the air only pressure drop for the three inch bore pipeline is much lower than that for the two inch bore pipeline. With a lower air only pressure drop, more of the pressure is available for conveying material. Data for this same cement conveyed through the same Figure 7.13 pipeline of four inch nominal bore is presented in Figure 13.8. A further increase in air flow rate is required, as will be seen, but the same axis for material flow rate has been maintained. As a consequence a lower maximum value of conveying line pressure drop has been employed. It will be seen, however, that for a given value of conveying line pressure drop, the material flow rate achieved in the four inch bore pipeline is significantly greater than that achieved in the three inch bore line.
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Cement and Drilling Mud Powders
389
Solids Loading Ratio o o o
Conveying Line Pressure Drop
80 a
o E 40
20
U
"S
0
100
200
300
400
Free Air Flow Rate - ftVmin Figure 13.8 Conveying data for ordinary portland cement conveyed through the pipeline shown in figure 7.13 of four inch nominal bore.
2.1.4 Industrial Installations Because of the empirical nature of pneumatic conveying, little practical information on industrial plant finds its way into the literature. System design is generally based on the scaling of data for the particular material to be conveyed. If previous experience with a material is not available, the material will be conveyed through a test facility in order to generate the necessary data. The building, maintaining and operating of test facilities is an expensive support item for a company, and so any data that they obtain on materials has too much commercial value to publish. Most reputable companies that manufacture pneumatic conveying systems have such test facilities. Companies will request a representative sample of the material to be conveyed and undertake tests, even if they have previous experience, simply because of the problems of different grades of the same material behaving very differently. Sometimes data does get published, particularly for advertising purposes, but very often it is incomplete so that it is not possible to extract useful design information. Some interesting examples for cement are presented below. 2.1.4.1 Positive Pressure Conveying Schaberg and Mehring [2] reported on a pneumatic conveying facility for cement in England that was commissioned in 1986. The distance to the furthest mill was about 2800 feet and a material flow rate of 264,000 Ib/h was required. The conveying route included twelve bends and incorporated five diverters. A single blow
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Chapter 13
tank with a 66,000 Ib batch capability was used. The total cycle time was fifteen minutes, giving the 264,000 Ib/h. The conveying time was twelve minutes, which is equivalent to 330,000 Ib/h during the conveying phase of the cycle. The pipeline bore was 10 inch (257 mm) and conveying line inlet and exit air velocities were quoted as being 732 ft/min (3-72 m/s) and 4468 ft/min (22-7 m/s), with a conveying line pressure drop of 75 lbf/in 2 (5-1 bar). The cement was conveyed at a solids loading ratio of 32 and the compressor delivered 3425 ftVmin (97 nrVmin) of free air at a pressure of 100 lbf/in 2 gauge (7 bar gauge), giving a specific power consumption of 5-6 hp-h/ton (4-6 kWh/tonne). From the above conveying air velocity data it will be seen that a single bore pipeline was used; for 732 x [(75 + !4-7)/14-7] = 4468 ft/min. Using the outlet velocity at atmospheric pressure, the free air flow rate used for conveying will be approximately 4467 x n x 102/576 = 2436 frYmin which means that only about 70% of the air available is used for conveying. Using this lower air flow rate and an air density of 0-0765 lb/ft3, a check on the solids loading ratio gives 330,000/(2436 x 0-0765 x 60) = 30, which is close enough for a check. From Figure 13.3 it will be seen that at a solids loading ratio of 32 the minimum conveying air velocity suggested is approximately 850 ft/min and so for the cement in question this is obviously a conservative value. 2.1.4.2 Ship Off-loading Ligthart [3] reported in 1991 on a pneumatic conveying system for off-loading cement from bulk carriers at 1,764,000 Ib/h (800 tonne/h), and its onward conveying to silos 1640 feet (500 m) distant through twin pipelines. The company had a need to import up to one million tonne/yr of cement at a terminal 15 miles east of London on the River Thames. Because the river is tidal (23 feet) it was necessary to build a jetty in the river against which the ships could berth, and hence the long conveying distance. A single vacuum nozzle was employed to off-load at 800 tonne/h, but it was decided to use two pipelines at 400 tonne/h each for the transfer to silos over 1640 feet, as it was considered that a single bore pipeline would be more expensive to build. Four concrete silos of 10,000 tonne capacity were available for storage. The single unloader is mounted on rails on the jetty to service the entire ship. For onward conveying to the silos it is connected to the pipelines by flexible hoses, through manifolds provided every 50 feet along the length of the ship docking section. Air is blown into the pipelines at their start, at the end of the jetty, and this dilutes the flow and transports the cement the 1640 feet to the silos. The unloader has a single filter receiver vessel with four 20 tonne capacity blow tanks beneath, arranged in two pairs. It also has eight vacuum pumps, and two compressors to provide air to the start of the two pipelines. All air movers are screw type with an 85% vacuum capability for suction, and deliver oil free air, without cooling, at 44 lbf/in 2 (3 bar) gauge for blowing. Although the total installed power is 4290 hp (3200 kW), only 75% of this is required for conveying at 800 tonne/h over the 1640 feet.
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A pneumatic system was chosen in preference to alternative mechanical systems for the duty for cost, maintenance and environmental reasons. Although the cost of the actual pneumatic ship off-loading system was higher than the price for a mechanical un-loader, the cost for the overall system was lower for the pneumatic system. This is because the pneumatic system required only two pipelines to convey the cement the 1640 ft to the silos. A mechanical un-loader would have required long conveying belts, and vertical screw conveyors in addition, to bring the cement to the top of the silos. 3
BARITE
As mentioned above, barite is a relatively dense material, having a bulk density of about 100 lb/ft 3 and a particle density of about 265 lb/ft3. In drilling mud applications, however, the material is typically ground down to a mean particle size of about 12 micron and at this value the material has very good air retention properties. As a consequence it is capable of being conveyed in dense phase at low velocity in a conventional conveying system, despite the high values of density. The material will, of course, convey in dilute phase and a minimum conveying air velocity of about 2400 ft/min is required. 3.1
Low Pressure Conveying
Low pressure, dilute phase data for barite conveyed through the Figure 10.16 is shown in Figure 13.9. Although the conveying air pressure available was relatively low, the conveying distance was short, and so the start of the transition from dilute to dense phase conveying is seen to occur with conveying line pressure drop values above about 6 lbf/in~. This is the point, on Figure 13.9, where the solids loading ratio is already up to a value of 14 under minimum conveying conditions. Further increase in pressure results in an increase in pressure gradient to allow the material to be conveyed at a higher solids loading ratio. This, in turn means that the material can be conveyed at a lower velocity, and hence with a slightly lower air flow rate. It will be recalled that the influence of pressure gradient was illustrated in Chapter 4 on Gas-Solid Flows with Figure 4.23 for low pressure conveying, and this included both positive pressure and vacuum systems. It must be emphasized that the conveying distance on Figure 4.23 is an equivalent distance that takes account of vertical lift and the number of bends in the pipeline, as well as the horizontal conveying distance. Although the pipeline for which the data relates was only 110 ft long, it did contain seven 90° bends and it will be seen from Figure 8.16a that the equivalent length of the bends, for which the conveying line inlet air velocity is 2400 ft/min, is about 40 ft each. Since the length of vertical lift in the pipeline was negligible, the equivalent length would be about 390 ft.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 13
392
Solids Loading Ratio
10 o o o
a
24
Conveying Line Pressure Drop - lbf/in 2
erf
Conveyin Limit
50 Free Air Flow Rate - ftVmin
100
150
13.9 Conveying characteristics for barite conveyed through the pipeline shown in figure 10.16. 3.2
High Pressure Conveying
High pressure conveying data for barite conveyed through the 230 ft long, two inch bore Figure 12.11 pipeline was presented earlier in Figure 13.Ic. A similar transition from dilute to dense phase conveying occurred, as shown in Figure 13.9 above, but as conveying data was undertaken at pressures of up to 30 lbf/in gauge, the vast majority of the data presented related to dense phase conveying. Figure 13.9, in effect, provides a magnification of the very low pressure and material flow rate section of Figure 13.1c. 3.2.1 Influence of Pipeline Bore As with the cement, reported above, barite has also been conveyed through two, three and four inch bore pipelines. In this case the pipeline was the 165 ft long Figure 4.2 pipeline. Data for the barite in the two inch nominal bore pipeline is presented in Figure 13.10. With conveying line inlet air pressures of up to 30 lbf/in 2 gauge the vast majority of the data relates to dense phase conveying, with solids loading ratios up to 200. Similar data for the barite conveyed through the Figure 4.2 pipeline of three inch nominal bore is presented in Figure 13.11. This shows an unusual anomaly. At low values of conveying line pressure drop, and low values of air flow rate, the material flow rate through the three inch bore pipeline is less than that through the two inch bore pipeline of identical geometry.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Cement and Drilling Mud Powders
60 o o o
-O
50
Conveying Line Pressure Drop - Ibf/in 2
393
Solids Loading Ratio
200
40
I
30 20 20 10 1
0
'
50 100 Free Air Flow Rate - ft3/min
150
Figure 13.10 Conveying characteristics for barite conveyed through the pipeline shown in figure 4.2 of two inch nominal bore. This is a rare occurrence, but is not unknown, and is yet another problem with which to contend in pneumatic conveying. Solids Loading Ratio
120
^ ,200 150
o o o
80
Conveying Line Pressure Drop - lbf/in 2
oi
o
40
80
120
160
200
Free Air Flow Rate - ft 3 /min Figure 13.11 Conveying characteristics for barite conveyed through the pipeline shown in figure 4.2 of three inch nominal bore.
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Chapter 13
394
The reason for this is not fully understood at the present time and it is doubtful whether any computer aided design program currently available would correctly predict this reduction in performance with this particular material. The fact that such data is recorded throws into doubt the use of small bore pipelines to derive conveying data for system design purposes. In recent years, however, most companies that manufacture pneumatic conveying systems have been installing larger bore test facilities. A number of companies now have six inch bore pipelines and four inch is typically the smallest bore of pipeline that is currently used. Conveying data for the barite conveyed through the four inch bore pipeline is presented in Figure 13.12. Material flow rates here are significantly greater than those for the three inch bore pipeline, as would normally be expected. 3.2.2 Influence of Pipeline Material Cement and drilling mud powders, as mentioned earlier, are regularly transported by small ships to storage facilities at ports, particularly those ports that are used to service off-shore drilling operations. Drilling mud powders are then loaded onto service boats to supply the off-shore drilling rigs. These vessels are generally self off-loading, usually by means of single or twin blow tanks, and flexible hoses are widely used. At ports, flexibility is needed to overcome problems of tidal movement. For the loading of materials onto off-shore platforms flexibility is required to accommodate movement of the vessel on the open sea in bad weather, and the fact that the vessel must stand some distance off from the oil or gas rig.
200 x
Solids Loadin; Ratio
160
1
kC3 120
ei
Conveying Line Pressure Drop - Min2
I 80 40
200
300
400
Free Air Flow Rate - fWmin Figure 13.12 Conveying characteristics for barite conveyed through the pipeline shown in figure 4.2 of four inch nominal bore.
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395
Cement and Drilling Mud Powders
In these applications very long lengths of flexible hose, usually ot natural or synthetic rubber, are used to connect the supply boat with the fixed pipeline on the platform In these situations the hose naturally forms a catenary and so bends are of exceptionally long radius and the additional pressure drop is minimal. Pressure drop for the hose, compared with that of a steel pipeline, however, must be taken into consideration. ., The influence of pipeline material on conveying performance was considered in detail in Chapter 8 with Figures 8.21 and 22. Oil well cement was the material considered and this was conveyed through identical pipelines of steel pipe and rubber hose (shown in Figure 10.20). Conveying characteristics for the cement conveyed through the two pipelines were given and an analysis of the comparative performance was presented. Barite has also been conveyed through these same 140 ft long Figure 10.20 pipelines and the two sets of data are presented in xac the same axes have been used for the two sets of barite data in Figure 13 13 as for the two sets of oil well cement data in Figure 8.21 so that direct comparisons can be made for both the different materials and the different pipelines. Conveying Line Pressure Drop - Ibf/in2 , Solids Loading \ 200 Ratio * 50
50
8 o
Conveying Line Pressure Drop - Ibf/in2 I Solids loading / ratio 1 200 / 160 / 24 130 100
:40
_0
I 20 I 10
0
(a)
50
100
50
150
Free Air Flow Rate - ftVmin
(b)
100
150
Free Air Flow Rate - frVmin
Figure 13.13 Conveying characteristics for barite conveyed through the pipeline shown in figure 10.20 pipeline made of (a) steel pipeline and (b) rubber hose.
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Chapter 13
396
The difference in conveying performance for the barite in Figures 13.13a and 13b follows a very similar pattern to that reported for the oil well cement and presented in Figure 8.22. At very low values of conveying air velocity there is little difference between the two sets of data and material flow rates for a given conveying line pressure drop are very similar. As velocity increases there is a constant reduction in material flow rate for the barite conveyed through the rubber hose. This is attributed to the fact that the coefficient of restitution for the material impacting against rubber pipe walls is much lower than that for material against steel pipe walls. As a consequence the material, after impact with the rubber pipe wall, will be at a lower velocity, compared with the steel pipe, and so additional energy will be lost in re-accelerating the material back to its terminal velocity. 4
BENTONITE
Both bentonite and barite have been conveyed through the 185 ft long Figure 8.2 pipeline of two inch nominal bore. For comparison purposes the conveying characteristics for both materials are included in Figure 13.14. Solids Loading Ratio ~
60
Conveying Line Pressure Drop ,- Ibf7in
2
iO 120
60
Conveying Line Pressure Drop - Ibf'/in"
Solids Loading Ratio
20 50
50 o o o
o o o
40
oi
o E 20
o
o E20 .3
"s 'C
u
I 10
0
(a)
50 100 150 Free Air Flow Rate - tf/min
50
(b)
100
150
Free Air Flow Rate - ft /min
Figure 13.14 Conveying characteristics for (a) bentonite and (b) barite conveyed through the pipeline shown in figure 8.2.
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Cement and Drilling Mud Powders
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Figure 13.14 illustrates quite clearly the differences in conveying capability between the barite and bentonite, particularly for low velocity dense phase conveying. While the conveying characteristics of barite are similar to those of cement, the conveying characteristics of bentonite are similar to those of a fine grade of fly ash. At low air flow rates, therefore, the differences in material flow rates between these different powdered materials can be quite considerable. Conveying data for bentonite was included in Figure 13.1 along with similar data for ordinary portland cement, barite and oil well cement, for comparison. All four materials were conveyed through the 230 ft long Figure 12.11 pipeline. Of the four materials included, bentonite was the only one that showed a significant difference in conveying capability in comparison with the other materials.
REFERENCES 1.
D. Mills. An investigation of the unstable region for dense phase conveying in sliding bed flow. Proc 4th Int Conf for Conveying and Handling of Paniculate Solids. Budapest. May 2003. 2. F. Schaberg and B.F. Mehring. Dense phase conveying. Large outputs/long distances. Proc Pneumatech 4. pp 281-299. Jersey, UK. March 1987. 3. A. Ligthart. World's largest cement unloader. Bulk Solids Handling. Vol 11, No 3. pp 671-676. August 1991.
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