This explains why misaligned flanges and poorly welded joints can cause such problems. Any situation in which turbulence or deflecting flows can occur could cause low angle impact and, hence, rapid failure of a ductile material. It also points up why brittle materials should not be used for short radius bends. The impact angle of particles against a short radius bend will be very high, generally resulting in rapid failure of a brittle material.
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 many chemicals, plastics, foodstuffs, metal powders and fuels such as coal and wood. Research has shown that the particle size must be below about 200 for a hazard to exist.
It is virtually impossible to avoid dust cloud formations in pneumatic conveying. Even when the product being conveyed consists of particles larger than that threshold, you must consider the possibility of the production of fines during conveying. Such fines may result in an explosion hazard being created in the receiving vessel.
Two sources of ignition frequently encountered in industrial plant are a hot surface and a spark. For example, rotary valve bearings, if not properly protected and maintained, could overheat and thus provide the necessary source of ignition. There is always the possibility of spark generation by metal-to-metal contact; therefore, all valves and feeding devices with moving parts should be checked. Sparks are often associated with electrostatic generation, as discussed above.
In truly dense phase systems, the concentration of the product in the air is well above the upper explosive limit and so explosions are unlikely. Solids concentrations in cyclones, filters or receiving hoppers, however, could be in the explosion risk range. Also during startup and shutdown, dilute phase conditions are likely to exist in the conveying line.
Explosions can either be prevented by reducing the percentage of oxygen in the conveying air to an acceptable level, or they can be contained. Detection and suppression equipment can be employed or relief venting can be used with appropriate safety measures.
In the previous section, we discussed some problems that directly result from the product being conveyed but that have an impact on the system. This section covers problems the system can cause in the product being conveyed. These include:
Angel hairs. The formation of angel hairs is a problem that can occur with plastic pellets such as nylon, polyethylene and polyesters. The presence of angel hairs is undesirable because they can cause blockages at line diverters and in filters. The problem can be overcome to a large extent by pipeline treatment. Conveying air velocity is a major variable; decreasing the velocity at which the product is conveyed will help to reduce the problem.
Cohesive products. Such materials may experience problems in hopper discharge. If difficulties are encountered in achieving flow rates with a system and the conveying line pressure drop is below the expected value, the problem could well relate to the discharge of the product from the hopper rather than the capability of the feeding device. In this case, the use of a suitable bin-discharge aid should be considered. In the case of rotary valves, a blow-through type should be used if there is any difficulty in discharging a cohesive product.
Granular products. If a granular product has to be conveyed, difficulties may arise in discharging the product into the conveying line. Rotary valves and blow tanks may cause problems here, as discussed previously .
Hygroscopic products. A hygroscopic product may absorb moisture from the air used to convey it. Although the specific humidity of air will decrease if it is compressed isothermally beyond the saturation point, its relative humidity will increase and is likely to be 100 percent after compression. The added moisture will not only affect product quality but could cause subsequent handling problems. The problem can be overcome by drying the air used for conveying the product.
Large particles. Such particles can be conveyed quite successfully in pneumatic conveying systems. It is generally recommended that the diameter of the pipeline should be about three times greater than that of the particles. This is simply an expedient measure to ensure that the pipeline will not block by the wedging action of two rigid particles. There are exceptions to this rule, of course. For instance, with very pliable products such as fish, it is possible to convey "particles" that actually are larger than the pipeline bore. With rigid particles, a problem may arise if a mean particle value is used in sizing and particles have an irregular shape. (Care must be exercised in feeding in all cases).
Particle degradation. Pneumatic conveying can cause the fracture and breakage of friable materials. Even if the presence of fines in a product is not a problem with respect to product quality, the fines produced will add unnecessarily to the duty of the filtration unit. The problem is influenced to a large extent by conveying air velocity. Any possible reduction in the velocity at which the product is conveyed will help to decrease the problem.
Product quality. If a conveying system is dedicated to a single product and has been optimized to the lowest specific energy, a change in product quality can cause operating difficulties. Handling a product of a slightly different shape or size could be sufficient to cause the pipeline to be blocked.