Many of the problems encountered in pneumatic conveying systems are associated with the various components that go to comprise the system itself. The problems generally result from either incorrect specification, or a failure to take account of the properties of the material to be conveyed. Not all types of system components are mentioned individually. Most of the problems associated with screw feeders, for example, are common to rotary valves, and so simple representative components are considered.
The rotary lobes in blowers are machined to close tolerances, as are moving parts in many other air compressors. Any ingress of dust or material into the machine will have a serious effect on the performance of the blower. Downstream of the blower, or any other air mover, non-return valves should be fitted into the air supply lines to prevent the possibility of backflushing of materials. This is always a possibility if the pipeline blocks.
Some materials that have very poor permeability are capable of holding back air pressures of 6 bar gauge with just a short plug of material in the pipeline. If the pipeline blocks and the air mover is switched off while the pipeline is being cleared, the material in the pipeline could easily be backflushed to the compressor if it was not protected with non-return valves.
If a blower, or any other positive displacement air mover, is operating in a dusty environment a filter should be fitted to the air inlet. This filter should be cleaned or changed periodically, for if it becomes choked with dust, the added resistance will have an adverse effect on the blower performance. A source of air away from the plant or out- side the building is generally recommended in these circumstances.
In negative pressure, closed loop and combined systems, blowers have to operate with air that has been used for conveying material. In these cases it is essential that the air is effectively filtered. Unless the filtration unit is 100 per cent reliable, it is generally advisable to add a back-up filter in order to provide a measure of protection for the blower in the event of a rupture of one of the filter elements. If a gradual change in per- formance of a conveying system is observed over a period of time, it could be due to wear of the blower. Ingress of dusty air into the blower will cause a gradual change in its operating characteristics.
Of all systems components, the operation and control of blow tanks is probably least understood. The transient nature of their operation must be taken into account in speci- fying material flow rate (see Figure 2.7) and air requirements. A variety of blow tank designs and configurations exist and these were considered in both Chapters 3 and 4. A particular advantage of blow tanks is that they have no moving parts, which makes them ideal for the feeding of abrasive materials, but the means by which material feed rate is controlled is by no means obvious.
The discharge rate of a blow tank is controlled by means of proportioning the air sup- ply between the fluidizing and supplementary air lines as considered with Figure 3.28. A control system fitted to a blow tank was presented in Figure 4.16. For complete system control the blow tank characteristics need to be considered in conjunction with the pipeline conveying characteristics and Figure 22.1 is included to illustrate the interaction between the two.
Figure 22.1 combines the discharge characteristics of the blow tank as a feeder and the potential of a given pipeline for conveying material. The blow tank was a top dis- charge type having a fluidizing membrane and the material conveyed was cement. The conveying system relates to pipeline no 7 shown in Figure 13.4. This was 101 m long, of 53 mm bore and incorporated seventeen 90° bends. By combining the blow tank and conveying line characteristics in this way it can be seen how the total conveying system can be controlled to achieve a given material delivery rate. It is important that the required conveying duty can be achieved by both the blow tank and the pipeline.
The upper discharge limit of a blow tank will be reached when all the air is directed to the blow tank. If a further increase in material flow rate is required, this can be achieved by increasing the volumetric flow rate of air, although this may have an adverse effect on the conveying of the material in the pipeline. The alternative is to increase the diam- eter of the blow tank discharge pipe. The diameter of the discharge pipe within the blow tank does not have to be the same as that of the pipeline.
If an attempt is made to convey a material at a low flow rate from a top discharge blow tank with only a small proportion of the air flow rate directed to the blow tank, the blow tank could ‘stall’ and cease to discharge material into the conveying line. This is because the air velocity in the blow tank discharge line will be very much lower than that at the material pick-up point. For a material having poor permeability and air reten- tion properties this could result in blockage of the discharge pipe. If this occurs a smaller diameter discharge pipe should be used.
Change of distance or material
If a blow tank is to be used to convey a material over a range of distances it will be neces- sary to change the proportion of the air according to the distance conveyed. If this is not done the pipeline will be under-utilized for shorter distances, and may block on longer distances. Feeder control, with respect to a change of distance, is an issue that must be considered with regard to any type of feeder. The same applies to a change of material but is particularly critical with regard to blow tanks as illustrated with Figure
4.15. An automatic control system, as mentioned above, would be recommended in both of these cases.
If the conveyed material is abrasive, any valve in the conveying line will be subject to wear. With top discharge blow tanks, discharge valves are not necessary. They will, however, enable a blow tank to be pressurized quickly and so give an overall increase in the conveying efficiency and material flow rate (see Section 4.4.2). In bottom dis- charge blow tanks the discharge valve may be necessary to prevent flooding of free flowing materials into the conveying line, and hence overload the conveying system on start-up.
Moisture in air
When air is compressed, its capacity for supporting water vapour decreases. Even relatively dry air may reach its saturation point and condensation may occur as the pres- sure is increased. With moist air the quantity of water precipitated can be very high, particularly with respect to a change in temperature (see Chapter 25). Unless positive measures are taken to remove this water, drops of water will be transported through the air supply lines with the conveying air. If a fluidizing membrane is used in a blow tank, this water can cause blinding of the membrane with certain materials and this can affect system performance.
Since most blow tanks are used for batch conveying, it is possible for water to accu- mulate in the supply lines as a result of the intermittent operation. On start up with the next batch, a small pool of water could be blown into the blow tank. With materials such as cement and fly ash this could cause the material to set in the discharge area and cause a major restriction to the flow. Most problems associated with moisture can be overcome by drying the air. If the material is hygroscopic it will probably be necessary to incorporate a desiccant type dryer. If moisture and condensation are to be avoided, then a refrigerant dryer should be satisfactory for most applications.
Both the blinding of a fluidizing membrane and a restriction in the discharge pipe will add to the pressure drop across a blow tank. If the pressure drop across the material feeder increases, the pressure drop available for the material in the pipeline will decrease, and result in a decrease in conveying capacity, if it is taken into account, and pipeline blockage if not.
Part of the blow tank pressure drop occurs in discharging the material from the blow tank. This is particularly a problem in top discharge blow tanks where a long length of discharge pipe may be required. The conveying air should be introduced as close to the
blow tank as possible in order to minimize this pressure drop. In a tall blow tank it may be necessary to bring the discharge line out through the side of the blow tank in order to reduce its length.
The performance of a blow tank can be monitored quite easily by means of pressure gauges. If a pressure gauge is installed in the supplementary air supply line this will effectively give a measure of the conveying line pressure drop, and hence the utilization of the pipeline in conveying the material. A pressure gauge in the blow tank will then give an indication of the pressure drop across the blow tank discharge line. If the blow tank has a fluidizing membrane, a further pressure gauge in the air supply line to the blow tank will help to monitor the state of the membrane. A sketch of a top discharge blow tank, with a discharge valve, arranged with pressure gauges for performance monitoring, is shown in Figure 22.2.
Difficulty may be experienced in discharging granular materials from a top discharge blow tank. Air permeates very easily through these materials and it is possible that insufficient resistance will be built up to discharge the material. Bottom discharge blow tanks are generally recommended for granular materials. Granular materials with a high percentage of fines are very much less permeable. These materials are not generally capable of dense phase conveying in conventional systems. They will require very little air for their discharge from a blow tank, and so if the discharge line is unnecessarily long or has a long horizontal section, the discharge line is likely to block.