Bulk Materials Flow Discover the Most Important Design Considerations for Mass Flow in Hoppers

Author / Editor: Atanu Banerjee; L.P. Biswas / Dominik Stephan

A steel plant faces various difficulties while handling raw material. This article gives an idea for designing the geometry of bins/ bunkers suitable for the mass flow. Read on to know more.

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Hoppers/bins used in a steel plant.
Hoppers/bins used in a steel plant.
(Picture: M N Dastur & Company (P))

In raw material handling systems for the steel industry, flow of the material in bins, bunkers, chute, etc., is a nightmare for material handling engineers. Specially maintaining flow of materials like iron ore, fines, flux and coal fines during the monsoon in India is an extremely difficult task. Recently, various flow aids have come into the market and these have been used at various steel plants like Tata Steel, Vizag Steel Plant (VSP) and RSP; however, the result is not up to the mark.

Various type of flow pattern Depending on the shape of bins, the roughness of its interior surfaces, properties of the stored material, several patterns of the flow are possible during the discharge of materials. The air blaster / air cannon, bin vibrator and manual hammer are examples of common types of flow aid devices.

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Mass flow is defined as the flow that occurs when the entire volume of stored material is emptied from the bin. In flat-bottom bins and bins with hoppers that are not steep enough for the entire material to be emptied in one go, funnel flow occurs.

Properties of Bulk Solids

In order to find the solution for this problem, it is important to define the properties of bulk solids. The solution mentioned in this article is applicable only to bulk solids.

  • a) Bulk density: It is the weight of bulk material per unit volume. There are two types—loose bulk density and compacted bulk density. Compacted bulk density is determined by weighing a compacted sample, i.e., where compaction has been done through vibrating the container that has the sample.
  • b) Angle of repose: When an unconsolidated bulk solid falls freely to a horizontal surface from a height, the particle rolls down from a pile. The angle made with horizontal surface is called angle of repose.
  • c) Internal angle of friction: Unconsolidated material has no strength, but it gains strength when it is stored in bins or is compacted. Cohesive materials gain strength with consolidation and hence, develop the pressure, as a result one faces flow problem.
  • d) External angle of friction: Flow in a bin also depends on the co-efficient of friction between the materials and side wall of the bin.
  • e) Flow factor: Force acting on the stored material in a hopper/bin tends to compact the material and shear stress in the material tends to make it flow. Jenike & Johanson showed that for an element at any position inside a mass flow hopper/bin, the ratio of compacting stress to the shear stress is constant and he called it as the flow factor (ff).
  • f) Valley angle of hopper/bin: Valley angle in a hopper/bin has great influence in the flow of material. It is expressed as:

(Picture: Dastur)

Common Problems in Bin Flow

In the steel industry, a common problem in bin flow is arching. It takes place about 2–2.5 m from the bin outlet. Tata’s Iron & Steel plant at Jamshedpur faced a problem in the flux bin (-3 mm mixture of lime stone and dolomite). To solve the problem, the company used an air blaster along with a local hammer at a 2.5 m level. This solution offered a good result.

Conditions of Mass Flow

For mass flow to occur, the following basic conditions have to be satisfied:

  • a) The hopper wall must be sufficiently steep/inclined. This has got a significant influence on the flow pattern. Although a standard practice is to make slope of the hopper at least as steep as or little more the angle of repose of the material being stored, the same shall only guarantee emptying of the hopper and may or may not develop a condition suitable for mass flow. It has been established that the profile of the hopper is a function of (a) effective kinematic angle of internal friction ‘δ’ of the bulk material being stored and (b) angle of friction ‘ø’ between hopper wall surface (liner) and the bulk material. The critical hopper angle cr measured from the vertical, can be expressed mathematically as

(Picture: M N Dastur & Company (P))

  • Extensive researches carried out by Jenike & Johanson have established that the hopper angle ‘ ’ should be less than or equal to 60° – 1.33 ø ..............(II)
  • b) The hopper wall must be smooth enough so that the mechanical interlocking force between the particles of material is more than the sliding friction between the material and the hopper wall. In other words, the frictional force between the material and hopper wall should be less than the frictional force developed between the material particles during motion. Mathematically, this can be expressed as

(Picture: M N Dastur & Company (P))

  • c) Hopper outlet dimension is also an important factor for mass flow to occur. As per Jenike & Johanson, the minimum outlet dimension, i.e., width of hopper ‘B’ shall be

(Picture: M N Dastur & Company (P))

  • Where, fc = Unconfined yield strength in kg/m2 ; = Bulk density in kg/m3. To ensure mass flow in hoppers, all the
  • above conditions have to be fulfilled.

Case Study for Hopper of a Raw Material Unloading System

The profile of a hopper designed for a raw material unloading system has been checked with different materials handled and the conditions are stipulated in table 1.

(Picture: M N Dastur & Company (P))

The critical hopper angles calculated with above values of & for the materials are given as

(Picture: M N Dastur & Company (P))

The hopper angle is critical for iron ore fines. Hence, the design of the hopper has to be based on the material. The hopper angle provided is 20° with the vertical.

Check for condition in (II)

(Picture: M N Dastur & Company (P))

Check for condition in (III) above

(Picture: M N Dastur & Company (P))

To calculate the hopper opening dimension, the following values are considered. The opening provided is 650 mm Hence, the hopper dimensions and profile are suitable for mass flow of materials.

This article is a general guide line for designing the mass flow bins/bunkers. Apart from this, various flow aids have been developed along with suitable liners, which can be used to combat flow problems. Extensive experiment work is still going on in India as well as other countries to overcome these difficulties.

(Picture: M N Dastur & Company (P))

* Both authors are with M N Dastur & Company (P)