Hopper Flow Tests

Testing standards involving timed discharges have little relevance to extruder hoppers. Under normal extrusion conditions, the slowly moving pellets lack the momentum and entrained air assistance associated with the standard tests. 

Butters, Plastics Pneumatic Conveying and Bulk Storage, Applied Science Publishers, Barking, UK, 1981. 

ASTM D1895-96, Standard Test Methods for Apparent Density, Bulk Factor, and pour ability of Plastics Materials, 2003. 

For many years, plastic extruder feed zones had smooth barrels, but feed zone inserts with grooves to restrain pellet rotation were introduced in Europe in the 1960s and have grown in application, particularly in combination with barrier screws, but less so in North America. 

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Figure 5.10 Hopper flow test using controlled discharge rate.

The use of irradiated particles as tracers provides an accurate way to measure particle velocity it allows one to calibrate low-cost flowmeters such as the capacitive flowmeter. Unfortunately, in most industrial environments, it is not possible to produce short-lived radioactive tracers. At ANL, a nuclear research reactor was available for the production of radioactive particles. Ideally, the density and size of the tracer particles and the solids in the flow should be the same so the tracers can be uniformly distributed in the flow and represent the solids velocity distribution. For that purpose, the sample particles to be activated were fabricated from resin, hardener, and indium oxide powder to closely duplicate the size and density of the glass beads used in the flow tests. The particles, after irradiated in the reactor, had a 54-min half-life of y activity. Twenty particles were injected into the flow stream during the reloading of the feed hopper, becoming randomly mixed with the glass beads. 

A particular problem is that a material that has excellent hopper flow may well exhibit poor machine screw conveying. For hopper flow, it might be expected that measurement of flow using a standard hopper would be a simple test that avoided the need for friction measurements, but unfortunately this is not the case. 

Due to the desired thin layer of the mortar (about 10 mm), a very fluid but still sedimentation free mortar is needed. It was planned to fill the mortar into a textile formwork which is wrapped around the steel tower. Whether a certain mortar mixture is suitable to fill such a narrow space can be tested with a specifically developed testing formwork [8]. The initial mortar composition was successfully filled into the testing formwork, confirming the fact that the fresh mortar properties should not show dramatic changes by the addition of polymers. An important factor to determine a successful filling process is the hopper flow time of the mortar, from which conclusions concerning the viscosity and the workability of the mortar can be drawn. 

Several bolted silos storing lubricated plastic pellets split apart along a radial seam near the top of the hopper section. Although the silos were designed structurally for funnel flow, no flow tests were performed to see if this flow pattern would occur. Lab tests performed after the failure showed that mass flow developed along the 45 cone walls. See Figure 3. 

Tables 1 and 2 below show results for vertical and inclined flow (test section at 15° fix)m vertical). In the first case, both ERT and the Miniature Conductivity Probe (MCP) show a relatively uniform distribution of the void flaction and velocity. The results for both parameters are compared with the reference values obtained fl om the weighing hoppers and gradiomanometer (see Section 3.1). Both sets of measmements compare well with the reference measurements, although the void flaction reading for the MCP is rather low.

In order to characterize this bonding tendency, the flow function of a material must be deterrnined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk soHd as a function of consoHdation pressure appHed to it. Such strength is directly related to the abihty of the material to form arches and ratholes in bins and hoppers. 

A useful approximation of B for a conical hopper is B = 22f/a, where a is the bulk density of the stored product. The apparatus for determining the properties of solids has been developed and is offered for sale by the consulting firm of Jenike and Johansen, Winchester, Massachusetts, which also performs these tests on a contract basis. The flow-factor FF tester, a constant-rate-of-strain, direct-shear-type machine, gives the locus of points for the FF cui ve as well as ( ), the... 

Conclusions concerning the causes of the fugitive emissions were developed from extensive model testing. The emissions escaped from the enclosure by direct plume trajectory and wind flow patterns. Lime dropped into the back of the grizzly creates a plume towards the front of the enclosure, whereas a drop near the front produces a plume to the rear. The plume is caused by the rapid displacement of air and dust from the hopper. 

The mechanical and electrostatic ash samples were obtained by emptying the specified vacuum-operated hoppers just prior to the start of the run. When the test was completed, a cap plug was unscrewed from the bottom of the hopper permitting the hot ash to flow out. A 10-gal sample of each of these ashes was collected and split, maintaining a 16-oz sample of each ash. With the amount of ash involved, it was impossible to obtain a representative sample of the total amount of ash collected in the hopper during the test period. 

Two experimental rigs were used for this study. The first rig comprised a double-feed hopper arrangement to ensure a virtually constant mass flow rate of powder during each test. The hopper arrangement was mounted on a frame that could be raised or lowered to change drop height and was suspended on cables connected to three load cells to enable the mass flow rate to be recorded. The bulk material dropped through a hole in a shelf above the stockpile and air was extracted from underneath the shelf at a rate such that the air pressure under the shelf was kept exactly equal... 

It is required to design a mass flow conical hopper with the volume capacity of 100 m3 to store a cohesionless material of bulk density 1,700 kg/m3 and an angle of internal friction 40°. Four sets of shear tests have been conducted on the material and results for the unconfined yield strength and the corresponding consolidating stress are as follows ... 

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