Sampling Flowing Streams

Most powder systems are transported at some time during their manufacture as flowing streams: Hoppers are emptied by screw or belt conveyors, powders are transferred to bagging operations by screw or pneumatic conveyors, and many solids are transported through pipes. A general rule in all sampling is that whenever possible, the sample should be taken while the powder is in motion. This is usually easy with continuous processes; with consignment sampling it may be possible during filling and emptying of storage containers.

Sampling from a Conveyor Belt. 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. The particles at the edge of the belt may not be the same as those at the center, and particles at the top of the belt may not be the same as those at the bottom. If the belt can be stopped, the sample may be collected by inserting into the stream a frame consisting of two parallel plates shaped to fit the belt; the whole of the material between the plates is then swept out. A scoop can be used to scoop out an increment, but this operation can be hazardous if the belt is moving.

When sampling from a continuous stream, the sampling may be continuous or intermittent. In continuous sampling, a portion of the flowing stream is split off and frequently further divided subsequently. In intermittent sampling, the whole stream is taken for many short increments of time at fixed time intervals. These increments are usually compounded and samples for analysis are taken from this gross sample. Continuous sampling is deprecated because if there is segregation on the belt, the extracted sample may not be representative.

It is common practice, in sampling from a blender, to extract three samples: the first after the blender has been emptying for a few minutes, the second when the blender is half empty, and the third when the blender is almost empty. Note that blenders sometimes have a "heel" of unmixed material that is first out of the blender. Powder from the whole cross section of the blender discharge steam should be collected for each sample. This practice should only be used after the mixing efficiency of the blender has been established for each product by taking multiple samples and analyzing these separately.

Point Samplers. Samples can be extracted from the product stream by projecting a sample tube, containing a nozzle or orifice, into the flow. The particles impact the tube and fill the open cavity. The sampling head is out of the stream when not sampling Snorkel-type samplers are available for vertical or inclined applications and can be preprogrammed for sampling frequency. It is not possible to sample nonhomogeneous streams representatively with this type of device. With the auger-type sampler, a slot inside the process stream is rotated to capture a cross section of the process stream, which is then delivered into a sample container. This type of device does not collect a representative sample unless the stream is homogeneous, and it has the added disadvantage that it obstructs flow.

Sampling from Falling Streams. In collecting from a falling stream of power, care should be taken to offset the effects of segregation. Each increment should be obtained by collecting the whole of the stream for a short time. Care must be taken in putting the sampler in and out of the stream. Figure 3 shows correct and incorrect ways of doing this. Unless the time during which the receiver is stationary in its receiving position is long compared with the time taken to insert and withdraw the sampler, the method shown in Fig. 3(a) will lead to an excess of coarse particles, because the surface region of the stream, usually rich in coarse particles, is sampled for a longer time than the rest of the stream. The method shown in Fig. 3(b) is not subject to this objection. If this method is not possible due to some obstruction, the ratio of stationary to moving time for the receiver should be made as large as possible. In many cases it is not possible to collect the whole of the stream as this would give too large an amount to be handled. The best procedure in this case is to pass a sample collector of the form shown in Fig. 3(c) through the stream.


Fig. 3 Sampling from falling streams. (a) Bad sampling technique. (b) Good sampling technique. (c) Sampling procedure to be adopted for high mass flow rate

The width of the receiver, b, should be chosen to give an acceptable weight of sample but must not be made so small that the biggest particles have any difficulty in entering the receiver. Particles that strike the edges of the receiver are likely to bounce out and not be collected, so that the effective width is (b-d), where d is the particle diameter. The effective width is therefore greater for small particles than for large ones. To reduce this error to an acceptable level, the ratio of receiver width to the diameter of the largest particle should be made as large as possible with a minimum value of 3:1. The depth, a, must be great enough to ensure that the receiver is never full of powder. If the receiver fills before it finishes its traverse through the powder, a wedge-shaped heap will form that is size selective. As more powder falls on top of the heap, the fine particles will percolate through the surface and be retained, whereas the coarse particles will roll down the sloping surface and be lost. The length of the receiver, c, should be sufficient to ensure that the full depth of the stream is collected.

Stream Sampling Ladles. Powder may be manually withdrawn from a moving stream of powder using one of the several commercially available ladles. These are suitable for occasional use, but automatic on-line stream sampling samplers are preferred for frequent applications.

Traverse Cutters. With large tonnages, samples taken from conveyors can represent large quantities of material that need to be further reduced. With the action shown in Fig. 4(a) and 4(b), uniform increments are withdrawn to give a representative sample, but with the action shown in Fig. 4(c), a biased sample results if the inner and outer arcs of the container are significantly different and the powder is segregated horizontally on the belt.

Fig. 4 Traversing cutters. (a) Straight path action, in line. (b) Cross line. (c) Oscillating or swinging arc path

Often, a traversing cutter is used as a primary sampler, and the extracted sample is further cut into a convenient quantity by a secondary sampling device. The secondary sampler must also conform with the golden rules of sampling.

A traversing cutter is satisfactory for many applications, but it has limitations that restrict its use:

• Although a traversing cutter is comparatively readily designed into a new plant, it is frequently difficult and expensive to retrofit an existing plant because of the space requirements.

• The quantity of sample obtained is proportional to product flow rate, and this can be inconvenient when the plant flow rate is subject to wide variations. On the other hand, where the daily average of a plant is required, this is a necessary condition.

• It is difficult to enclose the sampler to the extent required to prevent the escape of dust and fumes when handling dusty powders.

Sampling Dusty Material. Figure 5 shows a sampler designed to sample a dusty material, sampling taking place only on the return stroke. This is suitable provided that the trough extends the whole length of the stream and does not overfill. The radial cutter or Vezin sampler shown in Fig. 6 is suitable for sample reduction. These samples vary in size from a 15 cm laboratory unit to a 152 cm commercial unit.

Fig. 5 Full-stream trough sampler

Fig. 6 Schematic of a primary and secondary system based on Denver Equipment Company's type C and Vezin samplers

Diverter Valve Sampler. The diverter valve shown in Fig. 7 is suitable for online intermittent sampling when there is limited head room. It can also be operated manually.

Fig. 7 Diverter valve sampler

0 0

Post a comment