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coal sampling continued • • • • Begin control charting the sampling ratio. Identify and fix causes of out-of-control conditions. Causes include design problems, mechanical problems and operator problems. Until you have a stable, in-control system, you will not have reliable sample results. After a stable condition has been reached as indicated by 20 or more successive sampling ratios being in control, check the coefficient of variation (% CV) of the sampling ratio. If the % CV is greater than 15, identify and fix the causes of excessive variation. Continue to chart the sampling ratio and take action when indicated by the charts. After the system is stable, the % CV is less than 20 (actually, for some systems less than 10 is an achievable goal), and the average of 20 or more sampling ratios average observed value is within 10% of the design ratio, bias test the system for the purpose of documenting your successful improvement process. Table 3: Acme Plant Example Extraction Tables To properly understand the operation of a sampling system an extraction table is essential to verify sampling system operating parameters and system flow rates. By using an extraction table, the system operation can be mathematically evaluated. Also, one can exercise "what ifs" such as the impact of increased or decreased tons per hour flow rate to the primary sampler. By using the extraction table, as preparation plant personnel use the plant flow sheet to evaluate flow rates and equipment capacities, the sampling system inspector can formulate mathematical recommendations versus estimates, which can cost time and money if not correct. The extraction table also results in total sample mass and design sampling ratio values. This evaluation and planning tool is very useful, but is not discussed in ASTM D7430. Table 2 displays an as-found extraction versus a recommended extraction. Sampling Ratio Control Charts Use of the sampling ratio control chart is recommended in ASTM D7430. Currently some coal supply agreements specify their use. These charts have rules and limits and are process control charts, which provide mathematical evidence that the system is operating correctly. The example control chart in Table 3, which is very easy to use, is the result of a visual basic adding to Excel. By using the chart consistently, one can May 2013 verify the correct extraction of increments from the coal stream to be sampled. Also one can show consistent operation and accumulate an operational history to explain special cause occurrences such as crusher pluggage or other issues. In this manner the operator is provided with current information as to each sample's conformance to the ASTM limits stated in D7430 i.e., whether the system is within the 15% limit for percent coefficient of variation and 10% difference between the design (calculated) sampling ratio and the average observed sampling ratio. Coal sampling systems are mechanical devices that operate in a predictable way to extract and accumulate increments from coal streams. The risks presented and mathematical methods for evaluation of mechanical sampling systems presented above can provide the confidence of correct system operation. These methods enable evidence to be provided to inspectors, management and others who may question or just need assurance of the system's correct operation. Using these methods we raise the level of what constitutes a "good" sampling system from "doesn't plug up and puts coal in the bag." to one evaluated using mathematical methods. This prevents issues being presented by auditors or inspectors which can surprise and embarrass operation and management personnel. Renner is the manager of inspection and performance testing for SGS Minerals Services. This article was adapted from a presenation he gave at Coal Prep 2012. He can be reached at: chuck.renner@sgs.com. www.coalage.com 43