Sampling Standards
All sampling standards, whether ISO, ASTM International, or an individual nation’s standard, recognize that the best time to collect the a sample is when the material (in this case coal) is being moved from one location to another via conveyor belt. This provides the best chance for any and all particles in the consignment to be selected for inclusion in the sample.
The sampling standards also agree that the best way to collect sample increments is by stopping the moving conveyor under load and taking full cross sections by collecting all the material (lumps and fines) between two parallel planes — whose width is usually three times the nominal top size of the material. This is the best way to ensure that the fundamental objective of sampling is achieved — which is that the size distribution in the sample is the same as the size distribution in the consignment.
As the number of sample increments required to make up a full sample is large, the stopped belt sampling method is not practical for day to day commerce, or quality control. The frequent stopping of the conveyor belts under load is too disruptive to normal operations and causes too much stress to the conveyor drive motors. As such, the next best type of sampling is to collect the full cross section increments from the coal without stopping the conveyors. This is where mechanical sampling comes in.
Location
Selecting the location for your sampling system is one of the first steps. Keeping in mind some of the considerations below, the system should be located as far downstream as possible so that all possible blending scenarios take place prior to reaching the sampling system. Take into account potential plans for expansion or future changes in material handling characteristics and arrangements.
A second consideration for location is access to the conveyor belt. The ideal location is always a mix of the availability of space on the conveyor to locate the primary sampler and space on the ground to locate the processing equipment. Most modern sampling systems have very small footprints and are housed in standard cargo containers. Access to the area by vehicle is very important. In some locations the availability of the appropriate electrical service is an issue.
The location of the primary sampler on the main conveyor can be a cost issue and a quality issue.
If the primary sampler is located at a high elevation, then the length of the conveying system will be longer (and therefore more expensive) for both the sample material and the unused sample returned to the conveyor. In addition, it is not good sampling practice to allow sample increments to drop from extreme heights as it can cause drying of the sample as it falls and breakage of the larger particles upon impact. Since the sample will need to be fed to a crusher via gravity or a conveyor, if the primary sampler elevation is too low, excavation of the area underneath it will be necessary create enough room.
An additional consideration is whether there is a conveyor belt scale located on the main conveyor system. The vibration from the primary sampler can affect scale performance, so it is always best to locate the primary sampler on a different belt, if possible. It is not a problem to locate them on the same belt but you should space them as far apart as possible and no less than 20 feet.
Finally, keep an eye out for housekeeping issues. Locating a sampler near dust suppression systems or conveyor belt wipers will cause a cleaning nightmare and shorten the effective life of the equipment through corrosion. Under certain circumstances it could also compromise the collected sample.
Overview of Equipment Design
When selecting the design and layout of a mechanical sampling system it is important to understand that there are a variety of ways to accomplish the objective of effectively collecting a sample; and a number of issues that need to be understood prior to sending out your RFQ. While it is the goal of this article to provide a survey of the factors that should be taken into consideration, we wish to emphasize that there is no substitute for doing your own research in order to clearly spell out what you want from the manufacturers.
While price is always an important consideration, there are many others. It is therefore a mistake to assume that all manufacturers and designs are equivalent. It is critical to be specific in your design application. Once purchased and installed, it can be difficult and expensive to correct major problems. And many problems will not show until after many hours of operation.
Some of the basic information any sampling system manufacturer will need can be found in Table One. But this is just the starting point. Your RFQ should include as much detail as possible.
Below you will find some features to consider in designing yours.
- Main Conveyor Width
- Main Conveyor Travel Speed
- Drawing of Main Conveyor Idler Racks Assemblies
- Maximum Flow Rate To Be Sampled
- Top Size of Material To Be Sampled
- Consignment Sizes To Be Sampled
- Elevation of Conveyor
- Space Limitations at Sampling System Location
- Access to Electrical Power (i.e. 440/480 Three Phase, 200 amps)
- Belt Scale Located On Main Conveyor?
- Inclement Weather Effects at Sampling System Location (Flooding, Freezing)
- Proximity to Dust Suppression Water Spraying
- Explosion Proof Requirements Applicable?
Do Your Research
Know the products that your system will sample. Be sure to think through the range of qualities (i.e. HGI and moisture content) and consignment sizes the system will face. This is especially important at terminals with multiple users and frequent blending. This will greatly assist the manufacturer to design a better system for you.
We recommend adopting the attitude that the system you are buying is mechanical not automatic. Buyers are frequently disappointed by the amount of human attention mechanical sampling systems require in order to function properly. Thoughtful selection of hardware and features will help to manage the amount of future effort required.
When it comes to the design of the sampling systems and its components, attention to detail always pays off. You should start with a review of the local building and operating codes at the particular location.
An example in the USA is whether the location of the sampler falls under Occupational Safety and Health Administration (OSHA) or Mine Safety and Health Administration (MSHA). Some codes require explosion proof components and some do not. Different codes will involve different design and costs.
Being specific in your RFQ is not only beneficial to you in terms of getting what you need; it will also make it easier to compare the quotations from different manufacturers. The manufacturers are trying to provide you with the lowest price. If your RFQ lacks specificity, it is more difficult for them to know what you really require and you are more likely to get the short cuts and compromises that the absolutely lowest priced system inevitably requires.
The sampling system manufacturers can be an invaluable source of information on these and other issues with sampling systems. The manufacturers should be very happy to educate you on the many sampling system issues and we find that extensive discussions with the various vendors reveal which are the better ones.
Popular Sampling Solutions: U.S. vs. International
All sampling systems consist of mechanical components to collect and composite together a series of initial, or primary, increments and then process those increments to a smaller mass, laboratory ready sample, while at the same time preserving the integrity of the sample.
There are two different designs to collect this primary increment. The first is by collecting the cross section from the falling coal stream at a transfer point between two conveyors. This is known as Cross-Stream Sampling. The second method is by collecting the primary increment by sweeping the cross section from the material on a single conveyor belt. This is known as Cross-Belt Sampling.
Both Cross-Stream and Cross-Belt designs collect a good primary increment when well maintained. However, Cross-Belt Samplers have several significant advantages. The first is that Cross-Belt Samplers collect a much smaller mass of primary increment than do Cross-Stream samplers. Even though the width of the primary sampler opening on both designs is the same, Cross-Stream samplers cannot move through the falling stream of coal very rapidly without starting to selectively reject larger particles. This slow movement translates into a longer transit time through the coal stream and large primary increments.
Cross-Belt Samplers, on the other hand, can be very rapid as they move through the coal laying on the conveyor belt — in fact, the faster the better. This means that the transit time through the coal stream is quick and less sample mass is collected. By way of example, the individual primary increments for a high flow rate sampling system can be easily over 1,000 lbs, while the commensurate Cross-Belt primary increment for the same flow rate would well under 220 lbs. This difference in primary increment mass translates into a significantly lower scale on the “downstream” material handling and processing components. Consequently, Cross-Belt systems have a significant price advantage.
Secondly, Cross-Belt Samplers are installed on the top of conveyor structures and therefore do not require extra engineering and construction required to install their primary samplers in the chute work at the transfer point between two conveyors. This is not only another price advantage but it also means that Cross-Belt Sampling Systems can be easily retrofitted to existing conveyor belts. This means that the choice between selecting a Cross-Stream or Cross-Belt mechanical sampler usually only occurs during the design phase of a new conveyor line.
Due to the above factors, the vast majority of the mechanical sampling systems installed today are of the Cross-Belt design. As a result, the remainder of this article will focus solely on Cross-Belt Sampling Systems.
How Vital Is Maintenance to Proper Sampling Operations?
Maintenance is critical to sampling system performance and the design should maximize the ease of access to components for preventive maintenance and repair. While there is a legitimate concern that easy access can cause safety risks if not well designed, components that are difficult to access will not get the attention they need. Maintaining and repairing sampling systems is unpopular enough without making it more difficult.
One valuable feature that enhances both safety and access is the ability to disconnect electrical for each component. Unless otherwise specified, there will usually be only one place to safely lock out the equipment — at the electrical panel. By placing a lockable disconnect locally at each component, the operators of the mechanical sampling system can quickly and effectively make the component safe to clean and inspect.
Another valuable feature is an easy access crusher, which thankfully is becoming the industry norm. Crushers are high wear items that are quite prone to plugging. Selecting the crusher with the best access will pay off many times over in reducing downtime and ease of the replacement of crusher hammers and screens.
Even the small things help. We recommend ordering sampling systems with greasing lines run to a central location so simplify the process. We recommend fabricating conveyor and component covers from aluminum so that they are not only corrosion resistant but lighter to handle during maintenance activities.
One very important example of maintenance access is adequate decking and access to the primary sampler. We have seen many systems installed that did not supply any access to the primary sampler because it was not asked for in the RFQ. As a result, many primary samplers are never inspected or cleaned. On Cross-Belt Samplers there is a critical wear component on the edge of the primary sampler that wipes the conveyor belt clean of fine particle sizes. This needs regular adjustment and cannot be done without access. Due to the lack of proper access, it is the number one maintenance failure we see and it is on the most important component.
Sampling coal and coke is a rigorous application. Both products can be very abrasive and corrosive and the movement of this material through metal chutes and sampling devices inevitably takes its toll on the equipment. Cutting costs through less robust metals and materials will always reveal themselves as the equipment operating hours mount.
Thickness and grade selection of steel is important. Stainless steel is essential is high wear areas or places where coal will stick and start to corrode mild steel. Review design plans with an eye towards protecting key components such as the crusher from rainfall and dust accumulations.
Think About the Long Run
Often overlooked are such issues as primer coats and paint type — where poor materials and application will reveal themselves as the years go by.
Special attention is needed for the primary sampler which will need to collect thousands of increments from a moving coal stream on the conveyor belt. Ensure that the conveyor structure is as robust as possible, as constant imperceptible flexing can cause metal fatigue. One recent innovation that we highly recommend is the use of a hydraulic clutch and brake, combined with a constantly running motor. We believe this to be an excellent example of thinking about the long run through countless cycles of the equipment.
A good reputation for reliability and after sales service should be one of the criteria for selection of a sampling system manufacturer. An under-engineered or a poor design will be a headache with no easy cure.
Maximize Your Flexibility
The installation is not complete until the sampling system has been calibrated and certified. Calibration starts with setting the operating parameters such as cutter frequencies, cutter speeds and conveyor speeds. These parameters then need to be tested using coal under expected operating conditions. As long as the sampling standard minimums are met, there is latitude to set the operating parameters to achieve a good flow of material through the sampling system. It is important to achieve a balance between enough flow to prevent moisture loss from the sample (especially in the crusher) and enough unused capacity for the sampling system to handle the highest peak flow rates on the main conveyor.
For each set of sampling parameters (such as for different lot sizes), the sampling system is designed to produce a set mass of sample per one thousand tons sampled (known as the Sampling Ratio). The first measure of proper operation is that the actual observed Sampling Ratio mass is within 10% of the Design Ratio.
The sampling ratio should then be assessed using Statistical Process Control (SPC) to determine whether the sampling process is under control.
Once control is achieved, defined under most standards as having a Coefficient of Variation of less than 15%, the sampling system can be considered to be calibrated and is ready for the certification test — known as a bias test.
The bias test is a controlled test whereby a series of samples (usually around 30) is compared to a similar series of reference samples to confirm that there is close statistical agreement. The method for collecting the reference samples for the bias test is none other than the stopped belt samples generally accepted as the best sampling method possible. In short, the bias test confirms that the best practical sampling method (mechanical sampling) is comparable to the best possible sampling method (stopped belt sampling)
As we pointed out in the beginning of this article, stopped belt sampling is disruptive to normal operations and, due to the large number of test sets needed to demonstrate that a sampling system is free of bias; this certification test is an arduous and expensive process. This is the reason that it is good practice to confirm the sampling process is under control before starting the test. In many cases, the final payment for the sampling system is conditional on passing a bias test — which is often performed by an independent third party. In cases where a the buyer and seller utilize a mutually agreed upon third party, the manufacturer does not control the schedule any longer and it is therefore fair practice to agree on a time frame during which the test must be completed or final payment becomes due.
Calibration and Certification
Another important area where advance planning can pay off is designing your mechanical sampling system with operational flexibility in mind. Whenever possible allow for changes in the size and make up of consignments that are sampled. Consider potential plans for expansion at the plant or terminal where sampling is taking place. A common productivity enhancing event to consider is a future increase in the speed of the conveyor belt from which the samples are collected.
A key tool in maximizing operational flexibility is to take advantage of the capabilities of the Programmable Logic Controller (PLC) with which all modern sampling systems are equipped. First of all, be sure to have the software to modify the program included in your purchase. This will allow for changes in the case of unanticipated operational requirements of the addition of new equipment in the future. Be sure to allow for the expansion of the PLC to gain more inputs and outputs which are essential to add more of the feedback capabilities mentioned below. Whenever possible, set up the sampling system to receive inputs on flow rate and tons loaded directly from a conveyor scale.
Variable speed drives on sampling system feed conveyors should be part of the design make up. Variable speed offers the flexibility to convey coal at the correct travel speed when flow rate and increment frequencies may fluctuate. These drive units can be programmed through the PLC to change the speed of the conveyor at pre-determined feed rates. Consistent flow to the crusher makes for good sampling practice in many conditions and flexibility with conveyor travel offers this option. Conveyor travel should have the flexibility to convey primary increments for large consignments that are less frequent (less volume in a small time period) while at the same time move increments for small consignments that may be more frequent with more volume in small time period.
It is critical that expansion be available and be sure your system is installed and equipped with appropriate feedback loops, detection devices and electronic information components. Vibrators and Plugged Chute Indicators should be installed at strategic locations on chutes throughout a sampling system. Vibrators will help reduce system plugging when sampling wet or fine coal. The vibrators should be electronically looped to plugged chute detection indicators. If a plugged chute indicator detects coal buildup, it can activate an electronic signal to start a vibrator in an effort to clear the buildup before an actual plug occurs. If the vibrator clears the buildup, the indicator will detect this and the system will continue to process sample without interruption.
Primary samplers must always have a relay interlock with the main conveyor from which sample is being extracted. This interlock is a mechanism to prevent damage and excessive coal spillage if the primary sampler should fail while in the middle of the coal stream. In addition, primary sampler activation should always be interconnected with the scale or a material on conveyor signal. Electronic devices can detect material flow and communicate this to the primary sampler activation timer. There is no need for the primary sampler to perform an increment cycle if material flow is not present.
Tramp metal can sometimes be picked up in sampling devices and the resultant damage involved can be severe. Electronic metal detection devices can be part of the system design to interrupt cycling of the primary sampler when metal is detected on the main conveyor. A programmed interval of delay time will allow the metal to pass beyond the sampling device before resuming the sampling cycle.
Further Considerations
There are many considerations involved in the design and procurement of mechanical sampling systems. This article outlines many of them but there will always be factors in a given situation that are unique. Purchasers of sampling systems should do their homework and not only understand the variety of options available but be able to clearly communicate their needs in the RFQ. Advance planning and oversight throughout the process are important to a successful installation.
In many cases the sampling system will be used to produce samples used to make critical operational decisions and/or commercial payment. We find it useful to think of procuring a mechanical sampling system as purchasing a cash register to track your money. Be sure to get a good one from a manufacturer with a proven track record and a reputation for standing by their product.
