Grain sampling systems
May 1, 2007
Kevin B. Mason
The following article is the second in a two-part series on grain sampling systems. This article focuses on crosscut and point type grain samplers. The first article, which appeared in the April 2007 issue of World Grain, examined probe samplers used for sampling trucks and railcars.
Quality and quantity are typically key considerations in any commercial grain commodity transaction. Quantity can be easily determined by measuring or weighing the lot of product, but gauging the quality is more difficult since it is not practical to inspect the entire lot.
It is therefore critical that a representative sample be used to allow a reasonably accurate assessment of the quality of the entire lot. When it comes to grain quality, storage and handling facilities are primarily concerned with the characteristics and consistency of grain kernels themselves, as well as the presence and proportion of any contaminants.
The most common types of sampling equipment used in the grain industry today are probe samplers, cross-cut samplers and point samplers. This article will examine the advantages and disadvantages of using cross-cut samplers and point samplers.
Cross-cut samplers are usually considered the “most accurate” type of samplers, which is why some government groups, such as the Canadian Grain Commission (CGC) and U.S. Department of Agriculture’s Federal Grain Inspection Service (FGIS), require that they be used when taking “official” grain samples.
A cross-cut sampler consists of a slotted opening that rapidly traverses the entire product stream, withdrawing a cross-section of the stream. This process is repeated periodically during the product flow to withdraw multiple cross-sections and obtain samples spaced evenly throughout the lot of the product.
Usually installed in vertical or sloped gravity spouts or at the discharges of belt conveyors, cross-cut samplers withdraw a relatively large amount of product from the stream. In fact, the amount taken is far too large to test and grade, so a sample divider is used to take the product extracted by the main cross-cut sampler and further divide it to get the resulting sample down to a workable size. The divider splits the stream from the cross-cut into a final sample stream of about 5% to 10% of the total sample flow, with the balance being excess sample for return to the main product flow.
The excess sample must be returned at a location that does not affect the weighing of the lot. The return location must not be dusty enough to cause plugging of the sample lines.
Some characteristics of cross-cut samplers are fairly standard between manufacturers, therefore the choice of the sampler usually centers on the planned installation geometry as well as manufacturer-specific features.
Samplers installed in the spout are often preferred because they are cheaper, simpler and usually easier to install. My experience has also been that belt samplers have more opportunity for distorted samples due to failure, misadjustment or idiosyncrasy in the product flow.
After you have narrowed your choice to models that will physically fit, consider the construction of the sampler itself. Diverters and seals that are fully or partially protected from grain flow when not sampling will be more durable, as will models with smooth surfaces rather than steps in the spouting. The sealing mechanism for the diverter opening should be protected and durable, as well as easy to inspect and change. Pay careful attention to the size of the sealing area and the tolerance available for the “parking” position. Travel limits on samplers tend to be somewhat finicky to set, and a larger parking tolerance makes this easier.
Inspect the switch mechanism used to set the travel and understand its adjustment and operation. Also, check the construction of the drive mechanisms and the access to them.
Samplers where the diverter remains relatively square to the product stream as it travels across are recommended over diverters that rotate in a relatively tight arc. The rotating diverters may end up with a significant angle to the product stream at the edges, which reduces the effective opening size and theoretically reduces the amount of sample taken at the edge of the spout relative to the middle.
The design of the diverter mechanism should result in all portions of the stream being sampled equally. Samplers are generally available lined or unlined. Unlined samplers are not recommended except for very light duty applications. Linings, such as ceramic or urethane, should be used to lengthen the life of samplers.
It is also important that the sampler diverter have enough capacity to hold the sample accumulated during the “sweep” through the spout, particularly with inclined spouts. Without the necessary capacity, you can end up obtaining significantly less sample (grams of sample per tonne of grain moved) when tested at a high flow rate verses a lower flow rate. In such cases, the diverter can fill up before it makes its way across the spout at the higher speeds, causing under sampling at the latter part of its travel.
In principle, cross-cut samplers should be installed where there is minimal likelihood that the quality of grain will change between the sampler and the receiving or discharge point. FGIS, for example, says this should be after final elevation and as near the end of the shipping spout as practical for shipping samplers.
The sampler location must also be in a position where unsampled product cannot be introduced into the grain stream after the sampler. It also should be located at a point where flow is relatively smooth with little turbulence. Practically, this means not locating the sampler immediately after a significant bend in the spouting and not locating it after a steep vertical drop where the product speed would be very high.
It has been my experience that the installation and operation of the sample divider and sample transfer system have the largest influence on sample quality due to the relatively small volumes of product involved.
The system used to transport the final sample to the sample receiver location must be installed with great care. Small amounts of hung-up product can carry over between samples and skew results if this is not done carefully.
All joints must be square and flush. Any transitions in size must have the top edges tapered so that product cannot sit on them. Steel or aluminum piping is preferred so that product does not stick to the piping due to static. Where flexible plastic tubing is desired, it must have an embedded ground wire, grounded to limit static buildup.
When handling fragile products, self-cleaning flow retarders or extra bends in the pipe should be used to limit speed and damage in gravity lines. Lastly, a steel sample pail should be used, especially during testing, since light dockage tends to cling to plastic sample pails.
If pneumatic systems are used to transport product, make sure all joints are flush and smooth. Compression type couplings are recommended. The ends of all pipes must be square and deburred and smoothed before connection. Bends should be minimized and the conveying speed should be only fast enough to convey the product without plugging. Also, the feed of the product into the pneumatic line must be metered to avoid overfeeding.
POINT TYPE SAMPLERS
Point samplers generally come in two fundamental styles. One is installed in a spout and takes samples from flowing product. The other is installed in a hopper and takes samples from a relatively stationary grain mass. The latter style is common in the lower hopper of bulkweighers.
Point samplers are theoretically less accurate than a cross-cut sampler, because smaller samples are obtained and they are taken from an area that may or may not be representative of the entire product stream or mass.
The advantage of using point samplers are that they are relatively small, inexpensive and easy to install compared to cross-cuts. Also, the sample size is relatively small, which means a sample divider usually is not needed. On the other hand, some point type spout samplers remain in the spout at all times, which means they tend to wear out relatively quickly.
So why use a point sampler if there are questions about its accuracy? If the characteristics of the product being sampled are believed to be fairly uniform, with little or no blending occurring, point samplers may be suitable. They also may be suitable if the application’s tolerance for inaccuracy is higher than commercial sales, and they are useful for confirming grade if dockage is unimportant.
Beyond price, several factors can influence your choice of point samplers, such as:
• For cold outdoor applications, electrically or hydraulically actuated models are recommended over pneumatically operated styles;
• Some samplers extend into the product flow at all times, making them wear out faster than those that retract out of the flow when not in use;
• Some styles take a continuous sample stream as long as they are active;
• Some styles take a fixed volume of sample each time they are activated;
• Some styles work only in vertical spouts, while others work in inclined spouts or on the bottom of conveyors;
• Some styles may have more “pinch points” than others and may contribute to breaking the sample.
With this many choices, it is important to review your application with the equipment manufacturer and take time to understand the operating principle and limitations of each sampler model.
After deciding on the appropriate point sampler, it may be wise to see how much additional cost would be necessary for a cross-cut, particularly if the point sampler is a sophisticated electric auger or retracting tube sampler. After factoring in electrical and sample delivery installation costs, you may find the additional cost is not a big percentage, and the crosscut premium may be warranted because of its durability and better sampling accuracy.
In the case of spout samplers, careful consideration must be given regarding the location for installation. It should be in a location where the flow is smooth and turbulence is minimal. You must also ensure that the product is flowing where the sampler is installed rather than flowing against one side of the spout where it will miss the sampler.
Ideally, the location should be in a section where the grain is moving relatively slowly to minimize wear. As well, it needs to be in a location that is accessible for service, as occasional adjustment and replacement of wearing parts will be necessary.
It is important to understand how the sampler operates and make sure it is properly installed and adjusted. Auger equipped rotating tube samplers are frequently misunderstood and installed or adjusted incorrectly. These are fairly complex devices with clutches, switches and solenoids, in addition to the basic motor drive. It all must function properly for the sampler to work.
Kevin B. Mason is senior project manager and division engineer for Agricore United, Winnipeg, Manitoba, Canada. He may be contacted at: firstname.lastname@example.org. This article is modified from his presentation at the 2007 GEAPS Exchange. For the full presentation, visit www.geaps.com.