Designing a solids handling system requires many factors to be taken into consideration and balanced against each other; by using our ‘Top Tips’ you can avoid many of the common mistakes that lead to the production of poor performing equipment.
1.Make yourself familiar with the full range of conditions of the material to be stored. Secure a 'fully representative' sample to check that the material is of consistent quality, from homogeneous stock, stable over time and under the full range of relevant operating conditions. If the material varies, identify how and design around the 'worst' condition appropriate to each part of the equipment. See the 'Guide to the Specification of Bulk Materials for Storage and Handling Applications' published by the Process Industries Division of the IMechE for further guidance.
2.Measure wall friction every time. For Mass Flow, wall friction mainly determines the wall inclination required for a given geometric construction. For non-Mass Flow, the value indicates the lowest slope that may be used for the container to self-clear. Remember there is no ubiquitous 'Low Friction' material for a hopper wall; it depends upon the product and the contact surface. As wall slip depends almost entirely on wall friction, it is much easier to do something about promoting flow than it is to generate wall slip if the slope of the hopper wall is inadequate. Note that the both the surface material and the sample of bulk material must reflect those prevailing in all conceivable operating conditions.
(double click on picture to open in new window)
Ratholing
3.Check if the bulk material has cohesive properties and is likely to 'arch' or 'bridge' over an outlet. This depends upon many factors, such as the size and shape of the outlet, the time and degree of compaction and whether the storage system is designed for Mass Flow or not. Even if the material does not form a stable arch, a 'rathole' may still form in a conical hopper, leaving a large proportion of the volume in dead regions. The opening size that will generate reliable flow is determined by finding the shear strength of the material and applying a design formula. If the outlet size allows unrestricted flow, a feeder may be fitted to control the feed rate.
(double click on picture to open in new window)
Arching
4.Review the range of operating conditions, including unusual and rare circumstances the plant may have to work under. Check whether the ambient conditions alter according to production conditions, weather, season or events such as site wash-downs, and allow for these effects on the material to be stored. Circumstances such as restarting operation after shut-downs or holidays for example, may call for a larger orifice size than is needed for use under normal conditions. Ambient temperature variations can initiate deposition of condensate on the inside walls of containers, or cause progressive 'ratchet compaction' of the static contents by alternate expansion and contraction of the container boundaries.
It is also important to select the right form and type of feeder to avoid jeopardising flow from an otherwise well designed hopper.
If you need help or advice in achieving reliable flow, contact Ajax.
Achieving Reliable Flow 2 – Designing for Flow
AJAX Top Tips:
Achieving Reliable Flow 2 – Designing for Flow
Designing a solids handling system requires many factors to be taken into consideration and balanced against each other; by using our ‘Top Tips’ you can avoid many of the common mistakes that lead to the production of poor performing equipment.
1.Make yourself familiar with the full range of conditions of the material to be stored. Secure a 'fully representative' sample to check that the material is of consistent quality, from homogeneous stock, stable over time and under the full range of relevant operating conditions. If the material varies, identify how and design around the 'worst' condition appropriate to each part of the equipment. See the 'Guide to the Specification of Bulk Materials for Storage and Handling Applications' published by the Process Industries Division of the IMechE for further guidance.
2.Measure wall friction every time. For Mass Flow, wall friction mainly determines the wall inclination required for a given geometric construction. For non-Mass Flow, the value indicates the lowest slope that may be used for the container to self-clear. Remember there is no ubiquitous 'Low Friction' material for a hopper wall; it depends upon the product and the contact surface. As wall slip depends almost entirely on wall friction, it is much easier to do something about promoting flow than it is to generate wall slip if the slope of the hopper wall is inadequate. Note that the both the surface material and the sample of bulk material must reflect those prevailing in all conceivable operating conditions.
(double click on picture to open in new window)
Ratholing
3.Check if the bulk material has cohesive properties and is likely to 'arch' or 'bridge' over an outlet. This depends upon many factors, such as the size and shape of the outlet, the time and degree of compaction and whether the storage system is designed for Mass Flow or not. Even if the material does not form a stable arch, a 'rathole' may still form in a conical hopper, leaving a large proportion of the volume in dead regions. The opening size that will generate reliable flow is determined by finding the shear strength of the material and applying a design formula. If the outlet size allows unrestricted flow, a feeder may be fitted to control the feed rate.
(double click on picture to open in new window)
Arching
4.Review the range of operating conditions, including unusual and rare circumstances the plant may have to work under. Check whether the ambient conditions alter according to production conditions, weather, season or events such as site wash-downs, and allow for these effects on the material to be stored. Circumstances such as restarting operation after shut-downs or holidays for example, may call for a larger orifice size than is needed for use under normal conditions. Ambient temperature variations can initiate deposition of condensate on the inside walls of containers, or cause progressive 'ratchet compaction' of the static contents by alternate expansion and contraction of the container boundaries.
It is also important to select the right form and type of feeder to avoid jeopardising flow from an otherwise well designed hopper.
If you need help or advice in achieving reliable flow, contact Ajax.
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