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Ash Hopper used for bulk storage and fly ash slurry out of wet scrubber.

Mass Flow Hopper

Core Flow Hopper

Hopper Discharge ConsiderationsThe flow of material cannot be allowed to take freely but must be “throttled” to the required rate by some type of a “feeder”The product flows from the hopper to the feeder must flow continuously at an adequate rate.Stored Volume Height of the Cylindrical Section Recommended proportions of mass-flow hoppers to store a given volume of bulk solidD < H < 4D Jenike’s ‘No Flow’ Criterion Jenike’s ‘Flow’ Criterion Effect of storage time on the minimum outlet dimensions CakingCaking is the result of the change in moisture level or chemical reaction in bulk material. Friction characteristics of brown flour against epoxy-coated mild steel and stainless steel linings Common causes of failure in SilosShock loads during arch collapseBending in cylinder walls Overpressure due to change in cross section Belt Feeders A belt feeder consists essentially of continuous rubber or polymer belt running between end pulleys and supported by a number of idler rollers. Belt feeders are typically 0.5-2m wide and 2- m in length, maximum speed of belt is 17m/min or higher. Maximum capacity of 2000 tonnes/hr. Suitable to transport fine granular materials such as small coal or ores Belt feeders Apron feeders and Rotary feeders These two devices is used to regulate the discharge from a hopper by passing a continuous series of pockets across the hopper outlet at controlled rate. Apron feeders are typically 0.6-3m wide and 3-5m long, operates at speeds 3-16 m/min, capacities about 100 – 2000 tonnes/hr. It can operate on an upward gradient, however, it requires high maintenance cost.
Apron feeders Rotary table feeder Designed for the unobstructed discharge of poorly-flowing materials . The diameter of the table is some 50% of the hopper outlet diameter. Blades rotates typically at 2-10 rpm. Screw feederMost common mechanical method of discharging/extracting and feeding products from storage containers. Discharging AidesPneumatic – relying on air (or gas)Vibrational– relying on mechanical vibration of the hopper and/or the productMechanical – physically extracting the product from the hopper Pneumatic Method Air flow rates usually as little as 0.1 m3/min/m2A pressure of 7 bars or 100 psi Vibrational MethodVibrational frequency can range from 14 Hz to 1300 Hz and amplitude from zero to more than 60 mm. Vibrational Method Mechanical Method Assignment:Discuss and enumerate the safety systems, standards and procedures implemented in storage bins, silos or hoppers.What are the causes of silos to lose slow?What are the different methods of cleaning a silo, explain each.What are the different types of coatings that can be placed in a silo. Designing silos without bulk solid properties All too often, silos are designed using minimal flow properties information like bulk density and particle size. While the design procedure for a liquid pumping application requires properties of density and viscosity, the design procedure for a silo requires key flow properties, like cohesive strength, coefficient of sliding friction, and bulk density over a range of pressures, for specification of a hopper’s outlet size/shape and its slope.
It is imperative for an engineer to consider the factors that affect bridging and rat-holing tendencies for a bulk solid including variables of moisture, particle size, and storage at rest. Knowing this information will allow proper selection of a silo discharge pattern, namely funnel or mass flow. Funnel flow occurs when the sloping hopper walls of a silo are not sufficiently steep and low enough in friction for material to flow along the walls. Under these conditions, particles slide on themselves rather than on the hopper walls, and usually a small internal flow channel develops. Funnel flow is only suitable for solids that are coarse, free flowing, and not prone to caking or segregation.
Mass flow in a silo results when all of the material moves when any material is discharged. Mass flow prevents ratholing, provides usable (live) capacity equal to the silo’s design volume, provides a first-in-first-out flow sequence, eliminates stagnant material, and reduces sifting segregation. The mass flow silo in Figure 2 was designed using limestone properties. Limestone can vary tremendously from hard, coarse consistency to soft, fine particles surrounded by clay.
Using incorrect ‘rules of thumb
Incorrect design rules, like ‘selecting a hopper angle at least as steep as the angle of repose,’ continue to be used for silo design. Fluid mechanics principles are frequently employed by engineers to estimate bulk solids flow behavior – this generally does not work because bulk solids are different than liquids, given they have internal friction and are usually compressible.
Pyramidal hoppers are actively selected because they theoretically have a greater volume than conical hoppers and are easier to build. Yet, from a flow perspective, pyramidal hoppers yield the most flow problems because the shallow valley angles discourage solids flow. Even though they may be the cheapest option to build, from an operational standpoint, they are usually the most costly.
Underestimating material-induced loads
The first step for the proper structural design of a silo is to determine the loads exerted by the stored material under initial fill and flow conditions, as well as the external loads (e.g., wind, seismic). Bulk solids do not behave like liquids since they develop frictional forces against the wall in their static and sliding conditions; this considerably affects the loads on the silo structure.
Commonly, structural engineers will incorrectly assume a hydrostatic loading condition for a mass flow silo. This mistake yields the maximum load at the hopper outlet, whereas the largest lateral load is actually at the cylinder/hopper interface. If these loads are not considered, structural failure of the silo can occur, leading to loss-of-life, costly production loss, and high costs to repair/retrofit the structure. It is also important for engineers to consider eccentric withdrawal conditions in a silo, which can result in large bending moments in the structure’s walls.
Designing rock-box chutes for cohesive solids
The design of belt-to-belt transfer chutes in cement plants is often neglected since chutes are generally considered ‘low technology’ equipment. Unfortunately, if one cannot convey raw materials into the plant via the belt and chute systems, then the plant cannot produce cement! Given that many raw materials handled are abrasive, a rock-box arrangement in the chute head box is typically used to minimise wear. However, most raw materials are also cohesive (sticky), thus using a rock-box design that encourages material build up for protective reasons can actually lead to frequent chute plugging. A chute should be designed to gently transfer solids from one belt to another, while preventing pluggages, spillage, and excessive dust generation.