08.07.2009 | Author / Editor: E. McGee / Marcel Dröttboom
Achieving mass flow discharge, and preventing bridging and formation of rat holes, in large silos can be problematic when an existing silo is used to store new materials.
In this article Ajax Equipment describes the approach used to modify a large silo ahead of its new use to store a poor flowing plastic powder. It reports on how the results of flow property measurements were applied to silo design to provide a practical solution to the prevention of bridging and rat hole occurrence.
The plastics company discovered from bitter experience that the plastic powder was very poor flowing particularly under storage conditions. Large silos were already used for storing similar plastics powder which had experienced flow problems but none on the scale of the new powder they had to now handle. A silo with a fluidised bottom cone with twin outlets of modest size was used but severe difficulties were immediately encountered after the new material was put inside. Unassisted gravity flow did not occur and a lot of manual rodding was required to get any discharge. A stable rathole was formed comparable with the outlet size and near constant operator intervention with rodding sticks was needed to generate any flow at all. Due to the nature of the product it was forbidden to inject air into and use the conventional discharge aid associated with this silo. Emergency action was required and the fastest safe discharge method that could be used was to vacuum the contents out from above - an expensive process that took the company over 2 weeks to fully clear out the contents of the silo. From a nest of silos at the site an alternative silo was identified which had the most scope for modifying to this new use.
The silo was 7.6 m diameter by 14.2 m tall parallel section and a maximum holding capacity was of the order of 300 tonnes. It had a conical converging section 3.5 m deep inclined at 56 degrees to the horizontal and a 3 m diameter outlet. Installed 29 years ago, the silo was constructed from aluminium and fitted with a bin activator that focused the output to a nom. 250 mm diameter down spout. The downstream equipment comprised a diverter valve to direct product to either a positive pressure pneumatic conveying system via a rotary valve or a vacuum pipeline directly.
Initial use with the product in this silo (only partly filled, mindful of previous experience) generated some flow. However, the activator only emptied the container in the central region leaving large amounts of residue resting on the surrounding cone by forming a steep walled rat-hole to the surface of the material. Consulting with plant supervisors and operators confirmed that similar difficulties have arisen when the plastics powder had been tried in the large silo. The powder failed to respond to the vibrations of the bin activator and formed a very stable structure in the silo.
Whilst bin activators can be a reasonably effective discharge mechanism they rely on vibration disturbing the bulk to commence flow through a relatively narrow annular gap. The performance is very product dependent and the effectiveness in discharging from a silo is sensitive to the proportions of the bin, the bin activator and the final outlet diameter. Moreover, products capable of absorbing vibration can be difficult to discharge in this manner. And even if the activator successfully disturbs the bulk, the flow rate can be largely unpredictable and swamp downstream equipment.
Before consideration was given to the suitability of the large silo for storage of the powder, its past performance was analysed, material flow characteristics measured and the operating and process requirements adequately understood. Flow property tests were carried out by Ajax Equipment on the plastic powder. The tests included measuring the wall friction characteristics of the powder against various contact materials and the shear strength under compaction. The results of these tests enabled wall geometry and outlet sizes for reliable flow to be determined with confidence.
The test data identified the unsuitability of the existing silo ‘as is‘ for storage of the plastic powder. The aluminium cone of the large silo was too shallow for mass flow and the powder was so strong that it arched over significant spans especially when the consolidation pressures are high such as exist in a large silo with substantial inventory level. Furthermore the springy nature of the product indicated potential for absorbing vibration and this confirmed the bin activator would not be a suitable discharge device. The shear strength results indicate the potential for the product to form stable rat holes approaching 3m diameter so it was clear that the existing cone would have to be converted
Ajax carried out test work to examine the merits of stainless steel 2B finish as a contact surface for slip. The results identified that stainless steel 2B offers significant slip advantages over aluminium (see Fig.1). However even with stainless steel as a contact surface the existing wall angle of the silo would be insufficient for reliable mass flow. Ultra high Molecular Weight Polyethylene (UHMWPE) was found to offer even more superior slip behaviour flow however the copolymer powders had to be handled carefully due to their potential dust explosion hazards. Static is a major concern and a liner had to be sourced which not only met the slip criteria but had suitable electrical properties too.
Flow property measurements indicated that the critical rat-hole diameter was larger than the dimension cleared by the bin activator. The bin activator was replaced by a multiple screw discharge system to control the feed rate and allow a suitable mass flow hopper bottom section to be fitted to the bottom of the silo where the bin activator had been. The multi screw feeder formed part of a system to facilitate distribution to either one of two pneumatic conveying lines.
The proposed plant modifications meant that removing the bin activator, diverter mechanism and chute work would give almost 4 m of headroom into which the new hopper bottom could be fitted. As well as having the correct wall angle and shape for mass flow, a new feeder was required. It was very important that the feeder was able to fully draw from the entirety of the outlet cross section. If it did not then the actual flow path developed in the silos contents would be narrower than is needed for reliable flow. Uncertainty in the ability to discharge would be created and ultimately complete blockages could occur. Furthermore there would be dead areas in the silo where product could consolidate into a poor flowing condition making it even more reluctant to flow.
At the end of the feeder a new reversible conveyor was needed to distribute product on demand to either one of two blow lines.
The original silo cone was lined with UHMWPE sheeting, to provide a lower friction surface to the powder and inserts were fitted to encourage flow from multiple local regions adjacent to the central region. The powder flow property tests indicated the need to fit a mass flow silo bottom section which focused product towards a more suitable feeder mechanism.
Recommended design criteria were as follows:
• Use the slip benefits of stainless steel with a good quality 2B mill finish to exploit the benefits of single plane convergence to ensure reliable flow through orifice,
Make sure the feeder mechanism offers a fully live extraction pattern at the silo outlet, and
Guarantee the fullest retrieval of contents fit a liner and insert system in the silo, which would destabilise any rat hole that might form in that section.
For the transition hopper section with a stainless steel construction (2B finish), a 70 degrees wall angle to the horizontal was used, providing the transition to single plane convergence e.g. chisel or Vee shaped hopper section, to produce mass flow. From the data of the vertical shear tests, and the headroom constraints, the final outlet slot was 0.9m wide. Ideally this slot should have been 2.7m long, however by maintaining steep end converging sections this could be reduced to between 1.8m and 2.2m whilst still maintaining mass flow potential and an ample outlet size.
To get the product to flow reliably from the mass flow transition section then the outlet had to be fully live with product flowing along the full slot length. This was achieved by using a screw feeder featuring multiple screws. Uniform outlet geometry has the tendency to draw down from only one portion of the hopper and can narrow the effective flow channel. In this case the long slot length into each screw demanded both variable pitch and stepped shaft diameters to give fully progressive extraction potential.
A triple 300 mm diameter unit was installed that delivered the 6 TPH design rate, with a collecting conveyor that was reversible to direct to either of the pneumatic conveying lines. With the effectively round to rectangular transition shape of the hopper, the orientation of the screw feeder system was chosen to provide minimal re-routing of the pneumatic conveying lines.
The modifications to the large silo have been effective in ensuring mass flow of the plastics powder. By lining the original silo cone with UHMWPE sheeting, to provide a lower friction surface to the powder, and fitting inserts to encourage flow from multiple local regions adjacent to the central region, an enlarged central flow channel was achieved. This was larger than the critical rathole size and reduced the restraining friction acting on the annular contents, thereby aiding the residual material to self-clear. Finally the combination of multiple screw feeder fitted on the new, hopper bottom, mass flow section provided effective discharge from the silo. The techniques emphasised the multi stage approach to efficient silo design.
Ajax Equipment Ltd
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