Recommendations and practical issues
The results from the various programmes of work reported here have produced some very interesting relationships with respect to many of the variables investigated, and should provide useful guidance to the design engineer who has to ensure that material degradation is reduced to a minimum in pneumatic conveying system pipelines.
Particle velocity has been a major consideration in this presentation and it has been shown quite clearly that there is a threshold value of particle velocity below which no degradation occurs. The value of this particle velocity for the aluminium oxide was about 10 m/s and was influenced only slightly by particle size, target material, and par- ticle impact angle above about 15°.
Dense phase conveying
At velocities only slightly lower than this, however, the mode of conveying changes from dilute phase, suspension flow, to dense phase, non-suspension flow, for many of those materials capable of being conveyed in dense phase. In dense phase conveying little impact occurs in horizontal pipelines and the mode of conveying mostly involves sliding of the particles through the pipeline. With materials having good permeability, conveying is in plugs and slugs, and for materials having good air retention, it is as a moving bed along the bottom of the pipeline.
When particles slide through a pipeline the interaction results in attrition rather than degradation of the material. In dilute phase there may be little particle to pipe wall interaction, and it is suspected that most of the damage results from impact with pipeline bends. In dense phase, although the velocity is low, there is a significant
amount of particle to pipe wall interaction and this is likely to cause more damage to the particles than the bends. As a consequence it is possible for some materials to suf- fer a greater amount of damage in low velocity dense phase flow than they would in higher velocity dilute phase flow. It is important, therefore, to examine the relative effects of degradation and attrition on the conveyed material before deciding upon the type of pneumatic conveying system to be employed.
Dilute phase conveying
For many materials dense phase conveying is not an option, for the majority of materials cannot be conveyed at low velocity in a conventional conveying system. For these materials conveying has to be in suspension flow and so if the material is friable, degradation must be limited. To this end the material should be conveyed at as low a velocity as possible, consistent with reliable conveying, and the pipeline should be stepped to a larger bore part way along its length to reduce the high conveying air velocities that result at the end of the pipeline.
With a 1 bar pressure drop in a positive pressure system, discharging to atmos- pheric pressure, the conveying air velocity will approximately double from the mater- ial feed point to discharge. For the situation presented in Figure 24.7 it will be seen that at 10 m/s no damage occurs, but at 20 m/s 80 per cent of the particles are broken. As the air expands through the pipeline, therefore, it is the bends at the end of the pipeline, in a single bore line, that are likely to cause the majority of the damage. By stepping the pipeline the maximum velocity in the pipeline could possibly be limited to 15 or 16 m/s, at which the degradation would be limited to only 30 per cent.
Particle impact angle
For given conveying conditions, particle impact angle is probably the most important variable with respect to pneumatic conveying system pipelines. Particle impact angles against pipeline walls will generally be very low since particles will only suffer a glancing impact. From the data presented here it would appear that little degradation will occur in straight pipeline, even for long pipelines and repeated impacts.
It is clearly major changes in flow direction, and in particular bends, that are likely to result in the majority of degradation occurring. In this respect, particle impact angle can be related approximately to the radius of curvature of a bend. In a short radius bend the particles will impact at a high value of angle, but in a long radius bend the impact angle will be much lower, as illustrated in Figure 23.15. Since degradation reduces sig- nificantly with reduction in impact angle, the use of long radius bends would be rec- ommended in any system where particle degradation needs to be minimized.
The choice of material for the pipeline, and in particular the bends, provides another means by which particle degradation can be minimized. Although there is little change in the value of the lower threshold velocity, below which no degradation occurs, with respect to target material, there is a very significant effect on the upper threshold value. Thus, for a given particle impact velocity, very much less damage will result to particles if they impact against a surface such as Plexiglas or aluminium, than will occur if they impact against steel or glass. If it is possible to use a more resilient mater- ial, such as rubber or polyurethane, an even more significant reduction in particle degradation may be achieved.