Consideration must be given to some conveying operations and the conveying of certain materials with regard to safety provisions. Mention has already been made of start-up and shut-down transients, for example. In most dense phase conveying situations, the concentration of the material will be well above the value at which an explosion would be possible. During transient operation, however, and plant shut-down in particular, the concentration of the material in the air cannot be guaranteed to be above the required value while the system is being purged. Regardless of the conveying sys- tem and the mode of conveying, however, the material will generally be discharged into a receiving vessel, where there is every possibility of the material being dispersed in a low concentration cloud.
Pneumatic conveying is an extremely aggressive means of conveying materials, and particularly so in dilute phase conveying where high gas velocities are required. As a result abrasive particles can cause severe wear of the conveying plant and friable particles can suffer considerable degradation. The consequences of these influences must be given every consideration.
High conveying air velocities also mean that tramp materials can be conveyed through the pipeline with the material being conveyed. It is possible for nuts, bolts and washers to find their way into the conveyed material, somewhere in the system, and these will be conveyed quite successfully through the pipeline, with the potential of generating showers of sparks, as they will inevitably make numerous contacts with the bends and pipeline walls in their passage through the pipeline.
Whenever two dissimilar materials come into contact, a charge is transferred between them. The amount of charge transfer depends upon the type of contact made, as well as on the nature of the materials. Almost all bulk solids acquire an electrostatic charge in conveying and handling operations. In a large number of cases the amount of charge generated is too small to have any noticeable effect, but in many cases appreciable charge generation can occur, resulting in high electric fields. Very often these are just a nuisance, but occasionally they can attain hazardous levels. In all cases where dust clouds are present the build-up of an electrostatic charge should be prevented.
Pneumatic conveying systems are prolific generators of static electricity. Frictional charging of the particles moving along the walls of a pipeline can lead to a carry-over of net charge into the receiving hopper. In the case of non-conducting materials a build-up of charge might occur in the receiving vessel, because of the difficulties of leakage through an insulating medium. In the case of conducting solids, electrostatic problems can still arise when the particles are suspended in air. In such a case the air prevents the electric charge on each individual particle from leaking away.
It is possible, therefore, for high electric fields to exist in receiving hoppers. In many cases the charge may reach the breakdown level for air and produce a spark. Such a spark may have sufficient energy to provide the necessary source of ignition for the dust cloud in the vessel, and hence cause an explosion. A ‘rule of thumb’ value of 25 mJ is often taken, and materials with ignition energies less than this may be regarded as being particularly prone to ignition by static electricity (see Table 26.2). In these cases special precautions should be taken.
From an electrostatic point of view, pneumatic conveying lines should be constructed of metal and be securely bonded to earth. All flanged joints in the pipe-work should main- tain electrical continuity across them, to reduce the chance of arc-over within the pipe. Particular attention should be given to areas where rubber or plastic is inserted for anti- vibration purposes, and where sight glasses are positioned in pipelines. Regular routine checks of the integrity of the earthing of all metal parts of the system should be carried out. The use of well grounded facilities can help to reduce these potential hazards.
Although certainly safer than systems that have plastic sections, where charge can build up, earthed metal systems will not ensure that the system is safe. Metal pipes pro- vide a very effective source of charge for particles conveyed through them. The charge created on the pipe will flow instantly to earth, but that on the particles may remain for long periods. The storage potential is particularly important with regard to operations subsequent to conveying, for it is quite possible for such a charge on a material to be transferred to operatives.
If this occurred in the presence of an appropriate concentration of the material, the spark could provide the necessary ignition energy to cause an explosion. In this case special precautions should be taken, including the use of anti-static clothing and con- ducting footwear by all people in direct contact with a dust cloud. These, however, would be quite useless if they were to be used on a highly insulated floor, such as is often found in modern buildings. The operatives should stand on an earthed metal grid or plate at the point of operation.
Static generation on a material increases as the relative humidity of the surrounding air decreases and since it is more difficult to generate and store charges under more humid conditions, increasing the relative humidity of the conveying air to 60–75 per cent may also be used as a means of controlling the problem. The use of humidity for charge control is obviously not suitable for hygroscopic materials, and must be considered in relation to the possibility of condensation and freezing in any application.
Of those materials that are explosible, research has shown that it is only the fraction with a particle size less than about 200 11m that poses the problem. If a size analysis of a material to be conveyed shows that there is no significant amount of material below this size, the possibility of an explosion occurring during its conveying should not be dismissed. Degradation caused by pneumatic conveying can result in the generation of a considerable number of fines, particularly if the material is friable. This point was illustrated earlier in Figure 24.2 which shows the possible fractional size distribution of a material both before and after conveying.
Initially the material had a mean particle size of about 350 11m, with a typically Gaussian distribution, and contained essentially no material below 200 11m in size. After conveying the mean particle size of the material was about 280 11m. The really significant effect, however, is shown in the fractional size plot in Figure 24.2. A con- siderable number of fines can be produced and even on a percentage mass basis these can cause a significant secondary peak in the particle size distribution. This is likely to occur with a very friable material, such as granulated sugar, when conveyed at high velocity over a long distance in a pipeline with a large number of bends. In terms of explosion risks the material after conveying could be a serious contender.
Many materials that require conveying are abrasive. These include some of the larger bulk commodities such as cement, alumina, fly ash and silica sand. With a conveying air velocity of only 20 m/s silica sand is capable of wearing a hole in a conventional steel bend in a pipeline in less than 2 h. Erosive wear can be reduced with wear resist- ant materials and special bends, but it cannot be eliminated. Even straight pipeline is prone to wear under some circumstances.
If an abrasive material has to be conveyed, therefore, consideration must be given to the possibility of a bend or some other component in the system failing, with the consequent release of dust, particularly with a positive pressure conveying system.
Bends are available that have detectors embedded into them so that notice can be given in advance of an impending failure.
In long straight horizontal pipe runs, and large diameter pipelines, there is the possibility of material coming out of suspension in dilute phase conveying and depositing on the bottom of the pipeline. Accumulations of material such as pulverized coal in a pipeline could result in a fire, through spontaneous combustion, and possibly an explosion. An increase in conveying air velocity will generally help to reduce the problem but this is not an ideal solution. A disturbance to the flow with a turbulence generator usually cures the problem.
Food products, of course, will deteriorate if left in pipelines, and contamination of subsequent product could result. Since it is unlikely to be known whether such deposition occurs or not, it is necessary to physically clean all lines periodically. For food and pharmaceutical products, pipelines and all valves and components that could possibly come into contact with the material being conveyed are likely to be made of stainless steel. A particular problem with carbon steel is that it is liable to rust, as a result of condensation in the pipeline, and so contaminate the material.
If a pipeline is to be purged with the conveying air, in order to clear it of material, radiused bends should be used rather than blind tees. Blind tees are used in pipelines because they will trap the conveyed material and so provide protection to a bend from abrasive particles, since the particles will impact against each other rather than the bend wall. Material will require a much longer purging time to be completely cleared from blind tees. If additional air is available for purging, the process will be more effective. Air stored in a receiver will help here, particularly if it is at pressure, but care must be taken not to overload the filtration plant during this operation.
In dense phase conveying, air velocities employed are very much lower than those required for dilute phase conveying. Pipeline purging can be a major problem if add- itional air is not available. If high pressure air is used for conveying a material it is common for the pipeline to be stepped to a larger bore along its length once or twice in order to allow the air to expand and so prevent excessive velocities from occurring towards the end of the pipeline. This does, however, create problems if such a pipeline needs to be purged clear, for the purging velocity will decrease at each step to a larger bore and so considerably more air would be needed for the purpose (see Figure 9.16).
The consequences of a power failure on system operation need to be considered at the design stage so that back-up systems and preventative measures can be incorporated at the time of installation. With a pneumatic conveying system the plant will generally shut itself down safely on loss of power, but whether it can be started up again will depend upon the type of conveying system, pipeline routing, mode of conveying and material properties.
In many cases the pipeline will block and the only method of restarting the system will be to physically remove the material from where the pipeline is blocked, usually at the bottom of a vertical lift. If this is not an option then a stand-by power system must be available to take over. Alternatively an air receiver can be built into the air supply system, and this will provide air to purge the lines sufficiently clear of material so that the system can be restarted when power is reinstated (see Figure 20.9).
If the possibility of the pipeline becoming blocked from any eventuality must be avoided, consideration should be given to the use of an innovatory system. In ‘conventional systems’ the material is simply blown or sucked through the pipeline. In ‘innovatory systems’ the material is either conditioned as it is fed into the pipeline, or along the length of the pipeline. There is no difference in any of the basic system components employed.
Air pulsing or trace air lines are generally employed. Parallel lines are used either to inject air into the pipeline, to give the material artificial air retention, or to allow the air within the pipeline to by-pass short sections of material, to give the material artificial permeability. Depending upon the properties of the material to be conveyed, one or other of these innovatory systems will generally guarantee that the pipeline can be restarted on full load.