Introduction
Discussions involving sampling system design are often focused on the correct design of a complete sampling plan for a specific application, from the overall system design and operation to the design of the individual pieces of equipment needed to complete the system design in question. While the correct design of a complete system and its associated equipment is certainly the primary goal, the inherent value of a sampling system can be greater than its ability to meet the requirements of a specific application; this value can be improved by incorporating provisions that will allow for future adjustments of sampling machines and system operation. These adjustments may become necessary over time to adjust or fine-tune a sampling protocol or to account for changes in the process from which samples are extracted.
A certain amount of flexibility should be an inherent part of the design of both sampling systems and individual pieces of sampling equipment in order to realize their full potential.
The right balance
In design, there are nearly always compromises to be made – a modification in one area may result in a more positive outcome in that particular area, but it may also result in a negative outcome in another area. The key then is to find ways to design adjustability into sampling systems and equipment with minimal impact on function, maintenance and, of course, cost.
Sampling systems are (usually) small-scale material handling plants that include several types of equipment. Aside from the samplers themselves, belt conveyors, crushers and sample collectors are also commonly used. Regardless of the machine in question, each one offers an opportunity to improve the flexibility of a sampling solution.
Examples of flexibility
An internal view of a flow gate.
While equipment design can allow for system flexibility, a great deal of flexibility can also be included in sampling system programming. Some examples are multiple sets of system settings that automatically change based on different lot sizes, different modes of operation that can adjust to process requirements (e.g. continuous flow vs batch), or programming provisions that easily allow the addition of signal inputs (e.g. flow sensors, belt scales). Sampling system programming offers what is likely the best value for added flexibility in design. Many changes can be realized using relatively basic hardware simply by applying some creativity in programming.
Many possibilities
There are many possibilities for building flexibility into a sampling solution - too many to address here. The key takeaway is that when developing a sampling system, it is important to consider the possible ways in which your process may change as well as the possible ways that your sampling equipment may be used during the service life of the equipment, and then incorporate adjustments that can account for these potential changes, if this is feasible. Working closely with your sampling system manufacturer can ensure the system and equipment selected for your application best suits your current situation while accounting for any anticipated future needs.
One of the most common changes that we see with sampling equipment involves sample cutters. The width of sample cutters is usually specific to the material size that is handled in a particular application. Therefore, changes in an application that affect the nominal top size of the material to be sampled will in turn change the minimum width that is required for that application. Small adjustments are often handled by using adjustable cutter lips on sample cutters. This can be an effective option, though it cannot be universally applied. In some applications, adjustable cutter lip designs can negatively affect the material flow into and around the cutter, which introduces error into the sampling process or material handling issues related to material buildup or excessive wear.
Adjustable, replaceable cutter lips.
In other cases, adjustable cutter lips are not suited to the operation of a machine, as is the case with cross belt samplers, where the impact is very likely to damage adjustable cutter lips. So, what can be done if adjustable cutter lips aren’t used? In these circumstances, the answer would be to take a more modular approach to the sampler design. Basically, this involves designing a sampler to accept as wide a range of cutters as possible while requiring replacement of as few parts as possible. For cross belt type samplers, this modularization results in sets of cutters and their corresponding baffle plates that can be substituted if cutter width requirements change. In a falling stream machine, this may be a removable front section of a cutter that can be replaced with a new section, allowing for a different opening width.
Another cutter-related change we see is the belt speed of a conveyor feeding a falling stream type sampler. The speed of the belt determines the trajectory through which material being discharged from the conveyor will travel. Generally, faster belt speeds will raise the trajectory and slower speed speeds will lower it. If the cutter assembly is designed with modularized sub-assemblies, the position of the cutter can be changed by substituting smaller sub-assemblies related to cutter mounting, without the need for to replace the entire cutter assembly (or a least a larger and more expensive portion of a cutter assembly).
For sample conveyors, a relatively easy way to add flexibility is to simply include a variable frequency drive (VFD) on the conveyor motor. As the requirements of a sampling protocol change, the amount of sample material that is processed within a mechanical sampling system will generally also change. The change in the amount of material handled will, in turn, typically require a change in the speed of sample conveyors. A VFD offers an easy means of changing conveyor speed. If frequent speed changes will be necessary, it is also possible to automate the functions of a VFD, where changes in user accessible settings will trigger an automatic change in VFD settings to alter conveyor speed. Conveyor speed is one way in which material feed through a sampling system can be controlled.
Variable frequency drive.
Another option is the use of a flow gate. A flow gate is a vertically adjustable plate (or similar arrangement) that is used to control the volume of material that passes a particular point on a conveyor. Since skirtboard width is fixed, the height at which the flow gate is set will determine the burden depth on the conveyor and therefore the volume of material that is flowing downstream of the gate. This is useful in fine-tuning sampling system operation because it allows control of the time that material is flowing on a sample conveyor, and this is useful in ensuring that the correct number of increments can be extracted from the flow from the conveyor.
