Sampling from a conveyor belt

Sampling from a conveyor belt is used for sampling both individual aggregate fractions and aggregate mixture. In most production plants, conveyor belt sampling presupposes stoppage 

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For instance, the glass tube must always be full of solids with a constant flow. To ensure this, the end of each tube was squeezed and the bottom of these tubes were placed at a short distance from a conveyor belt operating at constant speed (5 m min-1), thus gently deposing the mixture as a layer onto the belt. This set up also allows a validation of the method by comparing the sample compositions in the belt to the compositions given by the closest sensor. With this special set up, all particle flow rates become quickly equal from the vibrating channel to the conveyor belt (in fact, the amplitude of the vibrations can be adjusted for this task). 

When a sample is to be collected from a conveyor belt, the best position for collecting the increments is where the material falls in a stream from the end of the belt. If access at such a point is not possible the sample must be collected from the belt. The whole of the powder on a short length of the belt must be collected. It must be borne in mind that the particles at the edge of the belt may be different to those at the center, and particles at the top of the bed may not be the same as those at the bottom. If the belt can be stopped, inserting into the stream a frame consisting of two parallel plates shaped to fit the belt may collect the sample, the whole of the material between the plates is then swept out. A scoop, such as the one shown in Figure 1.7, can be used to scoop out an increment, but this operation can be hazardous if the belt is moving. 

Figure l.l Schematic representation of a heap of granular material of different particle sizes which has dropped from a conveyor belt. (Adapted from EN 932-1, Tests for general properties of aggregates - Part I Methods for sampling, 1996.)... 

Extractions performed with the conveyor stopped allow more assured accuracy by the certainty of including fines in the sample increment. Sampler design to extract increments from a flat belt or rotaiw table sampler while the conveyor is stopped minimizes potential for residual fine particles remaining on the conveyor surface in carrying out extractions. See Fig. 19-6 for rotary table sampler extraction diagram. 

Before a size analysis can be carried out, it is necessary to collect a representative sample of the solids, and then to reduce this to the quantity which is required for the chosen method of analysis. Again, the work of Allen gives information on how this is best carried out. Samples will generally need to be taken from the bulk of the powder, whether this is in a static heap, in the form of an airborne dust, in a flowing or falling stream, or on a conveyor belt, and in each case the precautions which need to be taken to obtain a representative sample are different. 

Transportation (by belt, rail, or truck) can initiate (due to movement of the coal) processes that result in size and density segregation. Thus, variations from one side of a conveyor belt to the other, from side-to-side, end-to-end, and top-to-bottom locations in individual cars or trucks, and between one location and another in a coal pile, must be anticipated (ASTM D-346 ASTM D-2234 ASTM D-4182 ASTM D-4702 ASTM D-4915 ASTM D-4916 ASTM D-6315 ASTM D-6518 ASTM D-6543 ISO 1988). Therefore, the challenge in sampling coal from a source or shipment is to collect a relatively small portion of the coal that accurately represents the composition of the coal. This requires that sample increments be collected such that no piece, regardless of position (or size) relative to the sampling position and implement, is collected or rejected selectively. Thus, the coal sample must be representative of the composition of the whole coal (i.e., coal in a pile or coal in a railcar or truck) as represented by the properties or quality of the sample. 

Figure 3.1 Examples of grab sampling from the side of a conveyor belt or from the bottom of a pipe.

If the powder has been charged on the conveyor, fines concentrate at the center of the belt while coarse particles roll to the outer edges such as the segregation seen when sampling from a heap. 

FIGURE 5 Material taken from only one side of a stream (such as a conveyor belt or enclosed pipe) will result in a sample with bta.s, which will increase with more heterogeneity across the stream. 

Fig. 9.3.15 Analysis nf a section from a conveyor bell with steel cords by the CPMG method starting from thermodynamic equilibrium and from dynamic equilibrium corresponding to partial. saturation, (a) Sketch of the sample and positions of measurements, (b) The transverse relasaiion times for one side of the belt reveal homogeneous matcriat. (c) The transverse relaxation limes for the other side indicate considerable inhomogeneity.

The cutters in use in mechanical sampling are divided into diverter types and bucket types. Both types are used to cut a stream falling due to gravity off the end of a conveyor belt or from the discharge end of a pneumatic conveying pipe or a chute. 

Aggregate sampling for conducting the required tests may be carried out from (a) a stockpile, (b) a conveyor belt and (c) hot bins. 

After washing, the excess water is pressed from the laundry and it is transported via a conveyor belt to a tumbler (not a tumble dryer) to separate the washing items from eaeh other. The samples that were only washed were loaded direetly back into the washing-machine, while the samples that were also tumble dried were transported to the tumble dryer. 

The alternative method of sample extrac tion is termed the ctoss-sti eam samphng method, or ci o.s.s-belt when used in conjunc tiou with a belt conveyor. Sample extraction typically take place with a belt conveyor in motion. However, with a rotary table-feeder conveyor, extractions are made with the table stopped. A cutter can perform extractions by this means from a machined flat surface with neghgible... 

Conveyor-belt speeds above approximately 300 ft per minute (1.5 meters per second) impart sufficient momentum to material discharging at its head pulley to cause lifting of material streams in a trajectory from the head pulley. A trajectory is illustrated in Fig. 19-7. Blades of the sample cutter are positioned to intersect the trajectoiy. See Fig. 19-7 for an example of a linear-traversing bottom-dump cutter installation. Calculation of trajectory profiles are described in the Conveyor Equipment Manufacturers Association publications and similar references. 

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