Powder Thieves and Powder Samplers Information
Powder thieves and powder samplers are used to pull a representative sample from a large quantity of powder or solids in a container, pile, or moving stream. Examples of powder or solids include granules, pellets, seeds, grains, seeds, soil, pills, flour, tablets, sand, gravel, aggregate, coal, gravel, ceramic powders, fertilizers, pesticides, soils, fruit, sugar, minerals, ores, metal powders, glass cullet, and other particulate materials. A representative sample should mirror or duplicate the particle size, shape, and composition distribution of the production stream or stored bulk material.
Improper sampling techniques such as scooping a sample from the top of a container could result in a sample with lower fines or larger average particle size compared to the actual product, because settling occurs during transport and storage. Extraction or “thieving” of a sample using a powder thief is preferable to scooping, but sampling from a moving stream is the best option when possible. If the powder product is already in storage, all of the containers should be thieved at several locations within the bins or supersacks.
Ideally, several samples should be taken from a moving stream of the powder batch across the whole area of the stream and at various times while the powder product is flowing. Assuming the whole batch is blended before use, the samples obtained should be blended and then passed through a splitter or riffler to provide a representative sample. Some powder sampling systems will have integral blenders and splitters. If the quantity of powder product is too large to blend, then the individual samples should be tested separately to determine the variation across the batch from container to container. Individual portions of the batch could then be adjusted to normalize the particle size or compositional variations.
The sampling methods and sampler applied should be based on proper “theory of sampling (TOS)” principles to provide practical and effective sampling of the powder or solids to be monitored.
Manual samplers are typically used to sample a stationary powder or solid in a bin, sack, ground (soil, sand), pile, silo, or truck bed. A powder thief, trier, or lance consists of a tube or tubes with a sharp tip, which can be plunged into a container or pile of flowable powder. When the tip has been inserted to the desired depth, a port or door is opened and the powder fills in the sample pocket. The door is closed and then the sampling thief is removed. Some powder sampling thieves will have multiple openings along their length to collect samples at multiple depths. Some common manual samplers include:
- Auger / spiral sampler
- Core sampler / borer
- Pocket sampler
- Powder trier
- Sack probe / bag trier
- Slot sampler
- Tablet / granular solids
- Thief sampler
- Probe/tube (e.g., truck probe)
- Unit dosage / pharmaceutical sleeve
Automatic/inline samplers can sample continuously or at timed intervals. Inline samplers are used in process applications where sampling needs to occur while a stream of solids or powders are flowing or moving. For example, conveyor belt samplers sample powders moving on a conveyor belt. The powder may be sample while on the belt, while dropping onto the belt or while falling off of a conveyor belt. Over the belt samplers or under belt samplers are types of conveyor belt samplers. Pipe samplers are designed for sampling powders or solids flowing through a pipe or chute such as gravity pipe sampler, pressure pipe sample, vertical pipe sampler, or horizontal pipe sampler. Common automatic sampler types include:
- Arcual / rotary sampler
- Bulk / diverter
- Conveyor belt sampler
- Cross cut / cross stream sampler
- Linear / strip sampler
- Moving inlet sampler
- Pipe / chute sampler
- Pneumatic chute/conveyor sampler
- Pressure pipe sampler
- Poppet sampler
- Suction / vacuum sampler
- Vezin sampler
Automatic samplers can be grouped by sampling rate or method:
- Continuous — The sampler continuously takes samples of the product.
- Manual — The device takes samples of the product based on operator control or on manual input, such as when an operator or monitoring device notices process parameters are not within normal operating conditions or specifications.
- Timed/interval — The inline or automatic sampler takes samples of the product at timed intervals.
Automatic samplers can also be classified by their sample extraction mechanism:
- Gravity — The sampler utilizes gravity to capture samples of the product.
- Pressure/pneumatic — The inline sampler utilizes the pressure within the pipe or chute to extract a sample.
- Vacuum/suction — The sampler uses vacuum or suction to sample the product.
- Container — Typically, manual type samplers are used to sample powders in containers such as bins, bags, sacks, drums, hopper, tanks, or silos. Inline samplers can be used during filling or discharging of the powders or solids from the containers.
- Process equipment — The sampler is designed for sampling powders or solids within a pipe, chute, blender, power coater, reactor, or other process vessel.
- Transport vehicle — The sampler operates in a truck bed, rail car, barge, or ship hold. A truck probe is an example of this type of sampler.
Sampling Method / Process
- Point/spot — The sampler collects powder or solids at a single location in the granular product.
- Line/strip — The sampler collects powder or solids at a across a length or line within the powder stream or body.
- Cross cut / areal — A cross-cut sampler pulls a sample from across the whole area of flowing powder or solids stream. The total cross sectional area of the stream is sampled. A rotary sampler may sample the whole area by sweeping a line or area of collection across the stream.
- Multi-point: Port averaged / blended — The sampler collects powder or solids at a multiple locations in the granular product and then the individual sample are combined or blended to provide a port averaged sample.
- Multi-point: Compartmentalized / partitioned ports — The sampler collects powder or solids at a multiple locations in the granular product and keep the individual samples as separate sample, which may allow identification of points in a process or stream where inconsistencies in particle size or composition occur.
- Abrasive powders / solids — Abrasive powders or solids require handling and sampling equipment with good abrasion wear resistance. Rubber, ceramics, cemented carbides, and hardfacing materials are used on internal powder-contacting surfaces to reduce wear.
- Agglomerated powder — The solids or powders requiring sampling have a degree of agglomeration. The particles may need to be de-agglomerated to before testing through a screen, sifting, or light milling operation. Particle sizing might be performed before and after.
- Dry powder / solids — Dry powder and solids are free flowing and typically less trouble to handle and sample compared to wet or sticky powders.
- Granular / coarse (grains) — Granular or coarse grains are typically free flowing and typically less trouble to handle and sample compared to wet, sticky, or very fine powders.
- Fine powders / flours — Fine powders or flours may not exhibit consistent solids flow characteristic, which can cause problems in handling and sampling.
- Sticky / cohesive powders — Sticky or cohesive powders may not exhibit consistent solids flow characteristic, which can cause problems in handling and sampling due to bridging and pluggage. Cohesive powders may stick to the walls of the sampler requiring more vigilant cleaning to avoid cross contamination between samples.
- Slurries / wet particles — The solids or powders requiring sampling are wet powders, wet solids, slurries, pastes, or contain wet particles. Specialized sampling equipment that can handle wet powders or slurries might be required. The wet powders may stick to the walls of the sampler requiring more vigilant cleaning to avoid cross contamination between samples.