Material property influences
The properties of the materials to be conveyed feature prominently in the decisions that have to be made with regard to the selection of a pneumatic conveying system. As with ‘System Requirements’, considered above, some of the more common material properties can be identified and are detailed here for easy access and reference.
Problems may be experienced with cohesive materials in hopper discharge, pipeline feeding and conveying. If there is any difficulty in discharging a cohesive material from a rotary valve, a blow-through type should be used. If there is any difficulty in conveying a cohesive material in a conventional system, then an innovatory system should be considered. The pulse phase system, for example, was developed for the handling of such fine cohesive powders.
There is a wide range of materials which, in a finely divided state, dispersed in air, will propagate a flame through the suspension if ignited. These materials include food- stuffs such as sugar, flour and cocoa, synthetic materials such as plastics, chemical and pharmaceutical materials, metal powders, and fuels such as wood and coal. If a closed system is used the oxygen level of the conveying air can be controlled to an acceptable level, or nitrogen can be used. If an open system is to be used, then adequate safety devices must be put in place. One possibility is to use a suppressant system. Another is to employ pressure relief vents and other safety features.
Damp or wet
Materials containing a high level of moisture can generally be conveyed in conventional systems if they can be fed into the pipeline, and do not contain too many fines. Most of the handling problems with wet materials occur in trying to discharge them from hoppers. Fine materials may not discharge satisfactorily from a conventional rotary valve and so a blow-through type should be used.
Fine materials which are wet will tend to coat the pipeline and bends, and gradually block the line. Lump coal having a large proportion of fines is a particular problem in this respect. Single plug blow tank systems and some of the innovatory systems are capable of handling this type of material. If a conventional system must be used, the problem can be relieved by heating the conveying air, if the material is not too wet.
If the build-up of electrostatic charge is a problem when conveying a material, the air can be humidified. This process can be carried out on-line and does not usually require a closed system. In dense phase the quantity of air which needs to be conditioned is much less than in dilute phase systems, and so for materials capable of being conveyed in dense phase, the operating costs for air quality control will be lower. The entire system and pipework network should be earthed.
If the hardness of the particles to be conveyed is higher than that of the system components, such as feeders and pipeline bends, then erosive wear will occur at all surfaces against which the particles impact. Velocity is one of the major parameters and so the problem will be significantly reduced in a low velocity system. If a dilute phase system must be used, feeding devices with moving parts, such as rotary valves and screws, should be avoided, and all pipeline bends should be protected.
If degradation of the conveyed material is to be avoided, a system in which the material can be conveyed at low velocity should be used. The magnitude of particle impacts, particularly against bends in the pipeline, should be reduced as this is one of the major causes of the problem. Pipeline feeding devices that can cause particle breakage, such as screws, should also be avoided.
Granular materials can be conveyed with few problems in pneumatic conveying systems provided that they can be fed into the pipeline. Problems with feeding can occur with top discharge blow tanks and conventional rotary valves. Air will often permeate through granular materials in top discharge blow tanks and the materials will not convey, particu- larly if the blow tank does not have a discharge valve. Granular materials containing a large percentage of fines, and which are not capable of dense phase conveying, may block in a top discharge line. In rotary valves, shearing of granular materials should be avoided, and so a valve with an off-set inlet should be used.
If a material is hygroscopic the air used for conveying can be dried to reduce the mois- ture level to an acceptable level. This process can be carried out on-line and does not usually require a closed system. For a material which is only slightly hygroscopic, suc- cessful conveying may be achieved if the material is conveyed in dense phase, without the need for air drying equipment, since air quantities required for conveying can be significantly lower than those for dilute phase conveying.
Low melting point
The energy from the impact of particles against bends and pipe walls at high velocity in dilute phase conveying can result in high particle temperatures being generated. The effect is localized to the small area around the point of contact on the particle surface, but can result in that part of the particle melting. The problem is accentuated if the par- ticles slide on the pipe wall. Plastic pellets such as nylon, polyethylene and polyesters are prone to melting when conveyed in suspension flow.
Velocity is a major variable and so the problem will be eliminated for most materials in a low velocity, dense phase system. If such materials have to be conveyed in dilute phase, a roughened pipeline surface will reduce the problem considerably as this will prevent the particles from sliding.
Radioactive materials must be conveyed under conditions of absolute safety, and so it would be essential to employ a closed system so that strict control of the conveying environment could be obtained. A vacuum system would also be necessary to ensure that no conveying air could escape from the system, or material in the event of a bend eroding.
If toxic materials are to be handled, strict control of the working environment must be maintained. A vacuum system, therefore, would be essential to ensure that there could be no possibility of material leakage. If the conveying air, after filtration, could be vented safely to the atmosphere, an open system would be satisfactory. If not, a closed loop system would have to be used.
A problem of pipeline coating can occur with very fine powders in the low-micron and sub-micron range, such as carbon black and titanium dioxide. These materials tend to adhere to the pipe wall when conveyed in conventional systems. The coating gradually builds up and can cause a marked reduction in the pipe section area, and hence a reduction in conveying capacity. Many of the innovatory systems are capable of handling this type of material successfully.
If a conventional system is to be used the material should be conveyed through a flexible pipeline so that the material build-up can be shaken free on a regular basis. It is quite likely that the natural pulsations that occur within the system would be sufficient to vibrate the material free to enable it to be re-entrained in the conveying line.