Health and safety
Introduction
Pneumatic conveying systems are basically quite simple and are eminently suitable for the safe transport of powdered and granular materials in factory, site and plant situations. The system requirements are a source of compressed gas, usually air, a feed device, a conveying pipeline, and a receiver to disengage the conveyed material and carrier gas. The system is totally enclosed, and if it is required, the system can operate entirely without moving parts coming into contact with the conveyed material. High, low or negative pressure air can be used to convey materials. For explosible materials, an inert gas such as nitrogen can be employed instead of air.
System flexibility
With a suitable choice and arrangement of equipment, materials can be conveyed from a hopper or silo in one location, to another location some distance away. Considerable flexibility in both plant layout and operation are possible, such that multiple point feeding can be made into a common line, and a single line can be discharged into a number of receiving hoppers. With vacuum systems, materials can be picked up from open storage or stockpiles, and they are ideal for clearing dust accumulations and spillages.
Pneumatic conveying systems are particularly versatile. A very wide range of materials can be handled, and they are totally enclosed by the system and pipeline. This means that potentially hazardous materials can be conveyed quite safely, with the correct choice of system and components. There is minimal risk of dust generation, and so these systems generally meet the requirements of any local Health and Safety legislation with little or no difficulty.
Industries and materials
A wide variety of materials are handled in powdered and granular form, and a large number of different industries have processes that involve their transfer and storage. Some of the industries in which bulk materials are conveyed include agriculture, min- ing, chemicals, pharmaceuticals, paint, rubber 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. 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 and milk powder, are conveyed pneumatically in numerous manufacturing processes. Confectionery is an industry in which many of these materials are also handled.
Mode of conveying
Much confusion exists over how materials are conveyed through a pipeline and to the terminology given to the mode of flow. First it must be recognized that materials can either be conveyed in batches through a pipeline, or they can be conveyed on a continuous basis, 24 h a day if necessary. In batch conveying the material may be conveyed as a single plug if the batch size is relatively small. For continuous conveying, and batch conveying if the batch size is large, two modes of conveying are recognized. These are dilute and dense phase flow and are considered in detail in Section 1.2.3.
System integration
Dust, mess and spillage that are often found surrounding bulk solids handling plant are not generally caused by pneumatic conveying systems. Feeders for pneumatic conveying systems, for example, usually fit under hoppers, and these in turn are fed from above by other systems, such as belts, bucket elevators and chain and flight (en-masse) conveyors. Dust and mess in the area often comes from poor integration of the mechanical conveyor with the hopper, and not with the pneumatic conveyor. In terms of plant safety, therefore, due consideration must be given to the interfacing of different systems, particularly if they are operating in series.
Pneumatic conveying systems provide a totally enclosed environment throughout for the transport of materials, and along the conveying route there are no moving parts at all, unless diverter valves are employed for multiple point off-loading. Some feeding devices, such as blow tanks, venturis and vacuum nozzles have no moving parts, apart from valves opening and closing at the start and end of the process. Although pneumatic conveying systems are capable of releasing dust into the atmosphere, it generally occurs only as a result of a fault situation, and is not an endemic problem with the conveying system.