How to Design and Implement Chutes in Bulk Solids Handling Systems
Chute Design Essentials
How to Design and Implement Chutes in Bulk Solids Handling Systems
Chutes are in use in almost every bulk solids handling plant. Although everybody knows them, they are mostly overlooked, except for those cases where they cause extra-attention and -work due to malfunctioning. This article attempts to give the reader some simple rules to apply to chute design.
Adi Frittella, Abri Smit
(ed. WoMaMarcel - 20/4/2016)
4.4. Chute Liners
Chute Hood: Under normal conditions, the chute hood side plates are not lined. If the material trajectory is such that the hood front plate or side plates will experience impact, then only the areas subject to impact should be lined, in order to minimise the mass (and cost) of the liners.
Chute Body: Only the areas where the material impacts or slides should be lined.
Chute Exit and Skirts: The exit of the chute is normally lined wherever the material impacts or slides. Skirts should be lined full length. The depth of liners should be at least equal to the depth of material contacting the skirts. In the impact and acceleration zones, the skirts should be lined full depth.
Types of Liners: The following types of liners may be considered for selection:
Mild Steel (Grade 300W). This may be used on material that is sized and washed. The liners may be used in areas of sliding contact or light impact only.
VRN-500. This is preferred at locations where impact is high, or where the material lump size is greater than 100 mm – 150 mm.
Ti-Hard, Rio-Carb or other harder grades of liner steel have high wearing properties and should be considered, based on the specific application.
Solidur or equivalent UHMWPE. This may be specified in locations where the material is sliding. These liners should not be specified in areas of high impact, or material with large lumps and sharp edges. This material is especially useful for lining the back plates of chutes and for lining dribble chutes, in order to improve material flow.
Ceramics. This is useful where the action of the material is largely sliding and there is a significant moisture content in the material. These liners should not be specified in areas of high impact, or where the material lump size is greater than 100 mm to 150 mm. Ceramics are best utilised where the body of the chute has a long sliding portion and where water is introduced to wash down fines collection areas.
Rubber. These liners are best utilised in primary crushed material bins and hoppers, or where impact is likely to be high. The location of the liner with respect to the trajectory of the material must be carefully considered, in order to present the material flow normal to the surface of the liners as far as possible. Other locations where rubber may be used are in the body of the chute which may be subject to material splash, in order to reduce noise levels. Rubber liners should not be specified in areas where sliding takes place without the introduction of wash water.
For other methods of attachment, such as adhesives or riveting, the surface of the chute to be lined should be prepared in accordance with the requirements of the liner supplier.
All liner plates must be sized for ease of handling, with an average mass of 30 kg and a maximum mass not exceeding 35 kg. Keep in mind that liner plates are often difficult to manipulate within the confines of the chute body. A recommendation is that liner plate could be provided with removable ‘handles’ to facilitate handling.
Metal liners should be secured with countersunk bolts. The maximum bolt diameter is usually determined by the thickness of the liner. The countersunk holes should allow a base thickness of about 3 mm between the back of the liner and the underside of the countersink. The maximum securing bolt diameter may then be determined as
where t is liner plate thickness.
Liner plates of thickness 12 mm and above should be secured with M16 countersunk bolts.
The minimum number of securing bolts per liner plate should be as follows:
• For triangular sections: 3 bolts
• For any other shape: 4 bolts
For other lining materials, the securing bolts or rivets should be in accordance with the requirements of the liner supplier.
Nib head bolts may be used in areas that are not subject to flexing or heavy impact. Note that cracks in the harder steel liners originate at the notch for the bolt head nib. For this reason, liners in hardened steels that are subject to flexing ought to be secured with conventional countersunk bolts, with slot heads or hexagonal socket heads. The securing bolts may be grade 4.4.
In areas where wash down water is used, the bolt joints should be made water tight.
The liners ought to be so patterned that the gaps between the liners are staggered in the direction of flow, in order to prevent the material fines ‘channelling’, and creating ‘pagging’ areas (the rapid build-up of very fine material). In corners, the liners must be so arranged that the edges overlap and the corners of the bare chute are protected.
The welding of liners is unacceptable.
The recommended thickness of steel liners shall be as follows, subject to input from the liner supplier:
• 20 mm: on high wear, heavy impact areas and chutes handling material of average lump size greater than 100 mm to 150 mm
• 12 mm: on surfaces subject to light impact and material sliding only, and on skirts
• 10 mm: on fines chutes that are not subject to impact.
The thickness of other lining materials, such as ceramics and Solidur should be as determined by the liner supplier.
4.5. The ‘Between Skirts’ Dimension
A very commonly applied standard for the dimension between conveyor skirts is that the dimension between skirts should be 2/3 of the belt width.
This dimension was developed for flat feeder belts and remains applicable in this case. However with ever increasing trough angles applying this simple rule often results in a very small clearance between the belt edge and the skirt rubber. A small lateral movement of the belt causes the belt to push the skirt rubber out with resultant spillage and constant belt tracking problems.
Shortt has proposed a modified rule which is based on retaining the free-board (dimension between belt edge and skirt in this case) distance instead of the ‘between skirts’ dimension .
In this case the free-board dimension is premised on the reliable rule for the flat belt condition of one sixth (W/6) of belt width (Table 2).
Table 2: Skirt width for constant W/6 free-board.
5. Material Properties
Knowing the inherent properties of the material being conveyed is critical to the successful design of transfer chutes.
The obvious properties which would probably have been used in the selection of the required belt parameters to suit the duty are:
The type of material (e.g. coal) and whether it is abrasive or corrosive
The particle size and particle size distribution [mm] – highly dependent on process
The bulk density ρ [kg/m3]
The belt surcharge angle λ [°]
The angle of repose θr [°].
Whilst many of the above material characteristics are published in manuals and catalogues it is always best to run typical bulk flow tests on the specific material to be conveyed.
An important property is ascertaining the point at which the material begins to slide down the chute face for different types of liner material. This is typically established by testing, utilising a system similar to the Jenike Johannsen Shear testing system – however in this case, a force is applied to a block of the material and then the pressure is released and the block tilted until it begins to slide. The test is performed at different loads and with different wall materials, effectively simulating the impact of material on the chute face and the angle at which the impacted material will slide (Fig. 8).
Fig. 8: Test for wall friction angle.
6. Material Trajectory – The Starting Point
The starting point is the point at which the material leaves the discharge pulley. It is important here to identify this position as at this point mechanical interaction between material and belt is lost, and the material acts like a projectile with initial velocity subject only to the action of gravity (excluding the effects of air resistance).