Hydraulics and Pneumatics

Optimizing existing systems In the majority of cases optimization of an existing system will be achieved by reducing the amount of air used for conveying the material. The problem with existing plants, however, is the potential disruption of production, particularly if a change in conditions results in a pipeline blockage. A large degree of control, therefore, is required so that changes can be made gradually and their effects can be carefully monitored. Control and instrumentation Although reducing the speed of the blower can produce the additional benefit of a slight increase in delivery pressure, it is not very convenient in terms of control and gradual adjustment. An off-take to atmosphere in the air supply line between the blower and the point at which the material is fed into the pipeline provides much more flexibility. This can easily be arranged by fitting a Tee-piece into the line, with a control valve on the off-take. If there is not already a pressure gauge on the air su...

Gate lock valves These are probably the least used of all devices for feeding pneumatic conveying sys- tem pipelines. They are variously known as double flap valves, double dump valves, and double door discharge gates. They basically consist of two doors or gates that alternately open and close to permit the passage of the material from the supply hop- per into the conveying line, as illustrated in Figure 3.20. These gates may be motor driven, cam or air cylinder operated, or may work under gravity. The air that passes the lower gate from the conveying pipeline is vented so that it does not interfere with the material about to flow through the upper gate, in positive pressure systems. As with rotary valves, the blower should be sized to allow for this leakage, although this is not as effective in this case, as there is an order of magnitude in difference in the operating frequency. Like the venturi feeder, care must be taken to ensure that the material is metred into the gat...

Suction nozzles A specific application of vacuum conveying systems is the pneumatic conveying of bulk particulate materials from open storage and stockpiles, where the top surface of the material is accessible. Vacuum systems can be used most effectively for the off- loading of ships and for the transfer of materials from open piles to storage hoppers. They are particularly useful for cleaning processes such as the removal of material spillages and dust accumulations. In this role they are very similar to the domestic vacuum cleaner. For industrial applications with powdered and granular materials, however, the suction nozzles are rather more complex. It is essential with suction nozzles to avoid filling the inlet tube solidly with material, and to maintain an adequate flow of air through the conveying line at all times. To avoid blocking the inlet pipe, sufficient air must be available at the material feed point, even if the suction nozzle is buried deep into the bulk solid materia...

Pressure drop Material flow rate through a pipeline is primarily dependent upon the pressure drop available across the pipeline. A basic requirement of any feeding device, therefore, is that the pressure loss across the device should be as low as possible in low pressure systems, and as small a proportion of the total as possible in high pressure systems. If the feeder takes an unnecessarily high proportion of the total pressure drop from the air source, less pressure will be available for conveying the material through the pipeline, and so the material flow rate will have to be reduced to compensate. Alterna- tively, if a higher air supply pressure is employed to compensate, more energy will be required, and hence the operating cost will be greater. Maintenance Maintenance of these items is another important factor. If air leakage has to be accepted with a particular feeding system, the rate of loss must not increase unduly with time, otherwise insufficient air may ultimately be...

Rotary valves The rotary valve is probably the most commonly used device for feeding material into pipelines. This type of feeder consists of a bladed rotor working in a fixed housing. In many applications in which it is used its primary function is as an airlock, and so is often referred to as a rotary air lock. This basic type of valve is generally suitable for free flowing materials. Drop-through valve The type of valve described above is usually referred to as a ‘drop-through’ feeder and is depicted in Figure 3.4. This type of feeder is generally suitable for free flowing mate- rials. Material from the supply hopper continuously fills the rotor pockets at the inlet port which is situated above the rotor. It is then transferred by the motor-driven rotor to the outlet where it is discharged and entrained into the conveying line. Valve wear By the nature of the feeding mechanism, rotary valves are more suited to relatively non- abrasive materials. This is particularly the case ...

Feeding requirements For a given conveying system the air mover can be positioned at either end, as will be seen with Figure 3.3. If the air is blown into the pipeline, therefore, the air at the feed point will be at a pressure close to that of the air supply. In this case the material has to be fed into the pipeline at pressure, and so consideration has to be given to the possibility of air leakage across the device. If the air mover is positioned downstream of the system, so that it acts as an exhauster to the separator/discharge hopper, the air at the material feed point will be close to atmospheric pressure. In this case the effect of a pressure gradient on the feeding device need not be taken into account. A further requirement of the feeding device is that it should feed the material into the conveying line at as uniform a rate as possible. This is particularly so in the case of dilute phase systems, for the material is conveyed in suspension and quite high values of minimum c...

Introduction All pneumatic conveying systems, whether they are of the positive or negative pressure type, conveying continuously or in a batch-wise mode, can be considered to consist of the basic elements depicted in Figure 3.1. In terms of pneumatic conveying system components, a considerable number of devices have been specifically developed to feed materials into pipelines. The material feeding device is particularly critical to the successful operation of the system and so with a large number of devices from which to select, two chapters are devoted to this topic. A number of devices have also been developed to disengage materials from the conveying air at the reception point and these are considered separately in Chapter 7. Air movers are equally important components and these are considered in Chapter 6. With air movers, however, it is more a matter of selection from existing machines, but an early choice for pneumatic conveying systems was made with the positive dis- placem...

Venturi feeders Since the basic problem with feeding positive pressure systems is that the air leakage arising from the adverse pressure gradient can interfere with the flow of the material into the pipeline, this situation can be improved, to a certain extent, by using venturi feeders. These work on the principle of reducing the pipeline cross-sectional area in the region where the material is fed from the supply hopper, as shown in Figure 3.16. It will be seen that there are no moving parts with this type of feeding device, which has certain advantages with regard to wear problems. There are, however, no inherent means of flow control either, and so this has to be provided additionally. A consequence of the reduction in flow area is an increase in the entraining air velocity and a corresponding decrease in pressure in this region. With a correctly designed venturi the static pressure at the throat should be the same as that in the sup- ply hopper which, for the majority of appli...

Innovatory systems The systems illustrated above have all been conventional systems in which the material is simply fed into a pipeline and either blown or sucked to its destination. Unless the material to be conveyed has natural bulk characteristics such as good air retention or permeability, however, it is unlikely that it will be possible to convey the material at low velocity, and in dense phase, in a conventional conveying system such as those described above. Even if a high pressure system is employed it is unlikely that such a material will convey in dense phase, since dense phase conveying capability is dic- tated by the properties of the material. For materials that are either friable or abrasive, alternatives to conventional systems may have to be considered, particularly if the materials are not capable of being con- veyed in the dense phase mode, and hence at low velocities. For friable materials con- siderable particle degradation can occur in a high velocity suspension...

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. Cohesive 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. Combustible 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 mate...

Nomenclature The notation used throughout the book is given here for general reference as it presents the style adopted and the form of SI units used in one place. Where equations are developed in the book an abbreviated notation will also be given for reference at the point of use. Note (a) In a negative pressure system, p 1 will be slightly below atmospheric pressure if an artificial resistance is added to the air supply pipeline inlet for the purpose of assisting the feed of material into the pipeline; p 2 and T 2 will generally be equal to p 3 and T 3; but the mass flow rate of air at 3 might be higher than that at 2 if there is a leakage of air across the material outlet valve on the discharge hopper. (b) In a positive pressure system; p 1 will generally be equal to p 4 unless there is a pressure drop across the feeding device; p 2 and p 3 will generally be equal to the local atmospheric pressure; and the mass flow rate of air at 1 will be lower than that at 4 if there is ...

R e vi e w of pneumatic conveying systems Int r oduction A wide range of different pneumatic conveying systems are available to cater for an equally wide range of different applications. The majority of systems are generally conventional, continuously operating, open systems in a fixed location. To suit the material being conveyed, the application, or the process, however, innovatory, batch operating and closed systems are commonly used, as well as mobile systems. To add to the complexity of selection, systems can be either positive or negative pressure in operation, or a combination of the two. The combined system is effectively achieved by means of staging, but this is a further possibility in its own right. In this brief review some of the more common systems are presented, and an explanation is provided of the different types to help in the selection process. Numerous requirements of the conveying system, and conditions imposed by the material to be conveyed, also have to b...

Industries and materials A wide variety of materials are handled in powdered and granular form, and a large number of different industries have processes which involve their transfer and storage. Some of the industries in which bulk materials are conveyed include agriculture, mining, chemical, pharmaceuticals, paint manufacture, and metal refining and processing. In agriculture very large tonnages of harvested materials such as grain and rice are handled, as well as processed materials such as animal feed pellets. Fertilizers represent a large allied industry with a wide variety of materials. A vast range of food products from flour to sugar and tea to coffee are conveyed pneumatically in numerous manufacturing processes. Confectionery is a particular industry in which many of these materials are handled. In the oil industry fine powders such as barytes, cement and bentonite are used for drilling purposes. In mining and quarrying, lump coal and crushed ores, and minerals are conve...