• Dense Phase: Dense phase conveying is characterised by all the material being conveyed below the suspension velocity. The material moves through the pipeline in a series of constantly forming and re-forming packets (Figure 2). With some materials these packets look like solid plugs, and with other materials they appear more like fluidised moving beds. Conveying velocities are typically in the range of 2-10m/s. • High Power Consumption • Transport Capacities below Expectations Compared to natural mineral gypsum, the pneumatic transport of dried DSG requires more energy. Research carried out by Clyde typically shows that for a 15 tonnes per hour (tph) lean phase system conveying over 100 metres (m), the power consumption per tonne for DSG is almost 2.2 times that of ground mineral (taking the worst case DSG), or 1.6 times that of ground mineral (with a more typical grade of DSG).The downside of using dried DSG as raw material is therefore increased power consumption, detracting somewhat from the environmental benefits of its utilisation in place of natural mineral. • Frequent Pipeline Blockages Many gypsum companies today obtain their DSG from a variety of sources. As part of a flexible supply chain, it is becoming ever more critical for the process equipment, including pneumatic conveying systems, to be able to cope with different sources of DSG, as well as being able to convey mineral gypsum as well, with minimal disruption or change to the conveying system settings.Whilst most lean phase pneumatic conveying systems designed to convey DSG can also convey natural gypsum, it is almost certainly not the case when the system was originally designed for mineral. More importantly, different grades of DSG have differing minimum conveying speed requirements.Some grades of DSG require a minimum speed, whilst other grades require a higher conveying velocity. The effect of this, with fixed air supply systems, is to reduce the available conveying capacity of the systems when handling the more challenging grades. Depending on other criteria, this can, and often does, have negative effects on the production capacity of a plant, or at best restrict the potential sources of DSG available to the plant. Many plants suffer from excessive conveying pipeline blockages when pneumatically conveying DSG. Such blockages typically arise when the grade of DSG is changed. With lean phase systems these can be easily solved by reducing the conveying rate through the system, which has the effect of increasing the conveying speed, thereby avoiding the problems, but contributing to higher wear within the system. To restore the required conveying rates, it is also possible to re-gear blowers, to provide more energy to the system.However, with dense phase conveying systems, differing grades of DSG can require more complex re-setting of the machine. In frequent cases, especially with machines designed to operate with mineral, successful re-setting of the machines to maintain transfer rates is impossible, leading to serious operational difficulties with conventional dense phase systems. Clyde, however, is able to provide a self-adaptive dense phase technology capable of conveying all grades of DSG dense phase, which is discussed later in this paper.Difficulties with dense phase conveying DSG begin to arise especially when the mean particle size is greater than 35 microns.
Figure 2
DSG with Recycled Plasterboard Content
Benefits of Pneumatic Conveying
Clyde Materials Handling’s pneumatic conveying solutions have the ability to generate the following business benefits: • Sustainable Economic Advantages • Environmentally Sound by reducing spillages and noise levels
• Low Maintenance, Long Life
• Reduction in Operating Costs • Greater Process Control - Agility to react to the needs of your business
• High System Availability, Reliability and Performance
• Cost Savings through Process Efficiencies
• Flexibility to Integrate with Existing and Emerging Technologies A dense phase conveying system also brings further advantages to the user, often improving the cost effectiveness of the proposed plant, as a dense phase system compared to a lean phase system of the same duty will be operating with smaller conveying pipelines, and therefore absorbing less energy and wear. The lower the conveying velocity means that a dense phase system is much less likely to damage the material, bringing benefits in quality control. Further advantages are: • Low wear and maintenance, with associated low labour costs • Reliable conveying brings high process availability
• Low particle damage
• Low energy consumption compared to lean phase conveying • Wear of Rotary Valves
Typical Problems when Pneumatically Conveying Dried DSG
DSG is abrasive and can often cause excessive wear on the tips of
rotary valve rotors, presenting problems such as conveying air leakage through the
rotary valve, compromising the conveying efficiency rate, silo discharge rate and starting an ever increasing circle of valve wear, leading ultimately to valve repair/replacement, and sometimes unplanned lost production, and therefore, revenue. DSG is frequently conveyed either lean phase or dense phase, depending on operator experience and preferences. However, many gypsum companies do not pneumatically convey dried DSG at all because of previous poor experience. By following the guidelines outlined in this brochure, which have been implemented successfully on several gypsum plants by Clyde, the pneumatic transportation of DSG should never be problematic.The following are, in Clyde’s opinion, the main problems experienced by plant operators when pneumatically conveying DSG. • Crystalline Deposits of DSG in the pipeline With lean phase pneumatic conveying, the conveying velocity for DSG can vary between 12 and 16 m/s pick up velocity, which increases to 25 to 30 m/s terminal velocity in typical 150mm pipeline systems operating at pressures suitable for standard
rotary valves. These velocities are sufficiently high to cause the crystals within DSG to adhere to the steel conveying pipe work, which over time eventually builds up to a thick coating that must be chiselled out, leading to excessive costs of maintenance, equipment down time and lost production.
CMH Gypsum Brochure - 32078 54
