1、Designation: D 7256 / D 7256M 06aStandard Practice forMechanical Collection and Within-System Preparation of aGross Sample of Coal from Moving Streams1This standard is issued under the fixed designation D 7256 / D 7256M; the number immediately following the designation indicates theyear of original
2、adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONAnalysis data obtained from coal samples are used in establishing p
3、rice, controlling mine andcleaning plant operations, allocating production costs, and determining plant or component efficiency.The task of obtaining a sample of reasonable weight to represent an entire lot presents a number ofproblems and emphasizes the necessity for using standard sampling procedu
4、res.Coal is one of the most difficult of materials to sample, varying in composition from noncombus-tible particles to those which can be burned completely, with all gradations in between. The task isfurther complicated by the use of the analytical results, the sampling equipment available, the quan
5、tityto be represented by the sample, and the degree of precision required.This practice gives the overall requirements for the collection and within-system preparation of coalsamples through the use of mechanical sampling systems. The wide varieties of coal-handlingfacilities preclude the publicatio
6、n of detailed procedures for every sampling situation. The propercollection of the sample involves an understanding and consideration of the physical character of thecoal, the number and weight of increments, and the overall precision required.1. Scope1.1 This practice covers procedures for the mech
7、anicalcollection of a sample under Classification I-B-1 and I-B-2(Practice D 2234) and the within-system preparation (reductionand division) of gross samples utilizing various components ofthe mechanical sampling system.1.2 This practice describes mechanical sampling proceduresfor coals (1) by size
8、and condition of preparation (for example,mechanically cleaned coal or raw coal) and (2) by samplingcharacteristics.1.3 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. Within the text theinch-pound units are shown in brackets. The values stated inea
9、ch system may not be exact equivalents; therefore, use eachsystem independently of the other. Combining values from thetwo systems may result in nonconformance with the standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is therespons
10、ibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 121 Terminology of Coal and CokeD 2013 Practice for Preparing Coal Samples for AnalysisD 223
11、4/D 2234M Practice for Collection of a Gross Sampleof CoalD 4702 Practice for Quality Management of MechanicalCoal Sampling SystemsD 6518 Practice for Bias Testing a Mechanical Coal Sam-pling SystemE 177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE 456 Terminology Relating t
12、o Quality and Statistics3. Terminology3.1 Definitions of Terms Specific to This StandardManyterms used in this practice may be found in TerminologiesD 121 and E 456 and in Practice E 177.1This practice is under the jurisdiction of ASTM Committee D05 on Coal andCoke and is the direct responsibility o
13、f Subcommittee D05.23 on Sampling.Current edition approved Dec. 15, 2006. Published January 2007. Originallypublished in 2006. Last previous edition approved in 2006 as D 7256 / D 7256M-06.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at servic
14、eastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.1 activation intervalfor a falling-stream or cross-be
15、ltcutter, the time from the beginning of movement for taking anincrement, to the beginning of movement for taking of the nextincrement.3.1.2 cross-belt samplera single sampling machine orcomponent of a mechanical sampling system designed toextract an increment directly from a conveyor belt surface b
16、ysweeping a sampling device (cutter) through the material onthe conveyor.3.1.3 falling-stream samplera single sampling machine orcomponent of a mechanical sampling system designed toextract an increment from a falling stream of coal at thedischarge end of a conveyor or chute by moving a samplingdevi
17、ce (cutter) through the falling stream of material.3.1.4 mechanical sampling systema single machine orseries of interconnected machines whose purpose is to extractmechanically, or process (divide and reduce), or a combinationthereof, a sample of coal.3.1.5 within-system preparationthe process of gro
18、sssample preparation carried out mechanically by sequentialcrushing (reduction) equipment and/or division equipment. Itmay be carried out by processing increments individually or bybatching increments together and processing them together asa group. In any case, within-system preparation is conducte
19、d ina manner to minimize moisture changes and without removingthe gross sample or its increments from the sampling system.4. Summary of Practice4.1 The general-purpose sampling procedures are intendedto provide, in 19 of 20 cases, dry ash results that are within aninterval of 6110 of the average dry
20、 ash results that would beobtained in hypothetical repeated sampling.4.2 Special-purpose sampling procedures apply to the sam-pling of coal when other precision limits are required, or whenother constituents are used to specify precision, or for perfor-mance tests.4.3 For coals of known size and con
21、dition of preparation, atable (Table 1) is given for the determination of the number andweight of increments required for a gross sample for bothgeneral- and special-purpose sampling.4.4 The only processes of sample division and reductioncovered in this document are the use of mechanical sampledivid
22、ers for the division of the sample, and mechanicalcrushing equipment for the reduction of the sample, both ofwhich are within-system components of the mechanical sam-pling system.4.5 The procedures appear in the following order:Test Method SectionSampling of Coals Based on Size and Condition of Prep
23、aration 8.1General-Purpose Sampling Procedure 8.1.1Number and Weight of Increments 8.1.1.2Number of Gross Samples 8.1.1.4Special-Purpose Sampling 8.1.2Number and Weight of Increments 8.1.2.2Number of Gross Samples 8.1.2.3Division of the Gross Samples Before Crushing 8.2Reduction and Division 8.35. S
24、ignificance and Use5.1 It is intended that this practice be used to provide asample representative of the coal from which it is collected.Because of the variability of coal and the wide variety ofmechanical sampling equipment available, caution should beused in all stages of the sample collection pr
25、ocess, the designof sampling system specifications, the equipment procurementand the acceptance testing of installed equipment.5.2 After removal from the sampling system and furtherpreparation (Practice D 2013), the sample may be analyzed fora number of different parameters. These parameters may def
26、inethe lots value, its ability to meet specifications, its environ-mental impact, as well as other properties.6. Increment Collection Classification6.1 The type of selection, the conditions under whichindividual increments are collected, and the method of spacingof increments from the coal consignme
27、nt or lot are classifiedaccording to the following descriptions and Table 1 in PracticeD 2234.6.2 Types of Incrementsthe only type of selection ofincrements covered by this document are Type I where there isno human discretion in the selection of the pieces of coal orportions of the coal stream. Typ
28、e I selection incrementsgenerally yield more accurate results than Type II wherehuman discretion is exercised in the selection of specific piecesof coal or of specific portions of the stream, pile, or shipment.6.3 Conditions of Increment CollectionThe conditionsunder which individual increments are
29、collected are the con-ditions of the main body of coal relative to the portionwithdrawn. Only Condition B (Full-Stream Cut), in which afull cross-section cut is removed from a moving stream of coalis covered by this document.6.4 Spacing of IncrementsThe spacing of incrementspertains to the kind of i
30、ntervals between increments. TwoTABLE 1 Number and Weight of Increments for General-Purpose Sampling ProcedureATopSize 16mm58 in. 50 mm 2 in. 150 mm 6 in.BMechanically Cleaned CoalCMinimum number of increments 15 15 15Minimum weight of increments, kg lb 1 2 3 6 7 15Raw (Uncleaned Coal)CMinimum numbe
31、r of increments 35 35 35Minimum weight of increments, kg lb 1 2 3 6 7 15AConditions C and D are not addressed in this standard.BFor coals above 150-mm 6-in. top size, the sampling procedure should be mutually agreed upon in advance by all parties concerned.CIf there is any doubt as to the condition
32、of preparation of the coal (for example, mechanically cleaned coal or raw coal), the number of increments for raw coal shallapply. Similarly, although a coal has been mechanically cleaned, it may still show great variation because of being a blend of two different portions of one seam or a blendof t
33、wo different seams. In such cases, the number of increments should be as specified for raw (uncleaned) coal.D 7256 / D 7256M 06a2spacing methods are recognized: systematic and random.Systematic spacing is usually preferable.6.4.1 Systematic Spacing 1, in which the movements ofindividual increment co
34、llection are spaced evenly in time or inposition over the lot. This standard allows both time-based andmass-based distribution of increments.6.4.2 Random Spacing 2, in which the increments arespaced at random in time or in position over the lot.7. Organization and Planning of Sampling Operations7.1
35、This practice provides definitive procedures for thecollection of a gross sample. Parties claiming to use thispractice must adhere to the procedures as set out in thisstandard. If the sampling is not done in accordance with theprocedures set out in this practice then that sample may not besuitable f
36、or comparison with a sample collected by the proce-dures described in this practice. Since it may be impracticableor impossible to take another sample of a given lot of coal it isessential that parties agree on sampling procedures prior toundertaking sampling.7.2 Selection of Appropriate Sampling Pr
37、ocedureVariations in coal-handling facilities make it impossible topublish rigid rules covering every sampling situation in com-plete and exact detail. Proper sampling involves an understand-ing and proper consideration of the minimum number andweight of increments, the size consist of the coal, the
38、 conditionof preparation of the coal, the variability of the constituentsought, and the degree of precision required.7.2.1 Number and Weight of IncrementsThe number andweight of increments required for a given degree of precisiondepends upon the variability of the coal. This variabilityincreases wit
39、h an increase in free impurity. A coal high ininherent impurity and with comparatively little free impuritymay exhibit much less variability than a coal with a lowinherent impurity and a relatively high proportion of freeimpurity. For most practical purposes, an increase in the ashcontent of a given
40、 coal usually indicates an increase invariability. It is imperative that not less than the minimumspecified number of increments of not less than the minimumspecified weight be collected from the lot.7.3 Distribution of IncrementsIt is essential that theincrements be distributed throughout the lot t
41、o be sampled.This distribution is related to the entire volume of the lot, notmerely its surface or any linear direction through it or over it.If circumstances prevent the sampler from applying thisprinciple, the lot is sampled only in part, and the gross sampleis representative only of this part. T
42、he spacing of the incre-ments shall be varied if the possibility exists that incrementcollection may get “in phase” with the sequence of coalvariability. Example: routine sampling of commercial coalfrom a continuous stream (conveyor belt) in which incrementcollection is automatic and its sequence co
43、incides with the“highs” or “lows” in the content of fines.7.4 Dimensions of Sampling DeviceThe opening of thesampling device shall be no less than 2.5 times the nominal topsize of the coal and no less than 30 mm 1.25 in. Thesampling device shall be of sufficient capacity to completelyretain or entir
44、ely pass the increment without spillage at themaximum rate of coal flow.7.5 Characteristics and Movement of Sampling DeviceInsampling from moving streams of coal, the sampling deviceshall be designed to collect each increment with no selectiverejection of material by size and with no contamination b
45、ynonsample material.7.5.1 Falling-Stream SamplerIn collecting an increment,the falling-stream cutter should move at a constant velocitythrough the entire cross section of the stream of coal. The massm, in kg lb of material collected in one pass through thestream by a falling-stream cutter, with cutt
46、ing edges and cuttervelocity perpendicular to the stream flow, is calculated fromthe following equation:m 5Cw3.6vcFm 5Cw1.8vcG(1)where:C = the stream flow rate in Mg/h ton/h,w = the tip-to-tip cutter aperture width in mm in., andvc= the average cutter speed in mm/s in./s.NOTE 1Falling stream cutter
47、speeds of 460 mm/s 18 in./s or lesshave been found to produce acceptable results.NOTE 2The constant value 3.6 1.8 in the denominator of Eq 1converts Mg/h to kg/s ton/h to lb/s.NOTE 3If the falling-stream cutter velocity is not constant as ittraverses the material stream, the mass of collected materi
48、al may not agreewith that calculated using Eq 1.7.5.2 Cross-Belt SamplerThe cross-belt cutter should bedesigned and operated at a velocity across the conveyor surfacethat is high enough to prevent selective rejection of material bysize, prevent contamination of the sample with material notcollected
49、within the cutter, and avoid mechanical problems dueto damming of conveyed material against the outside of thecutter body as the cutter travels though the stream. Further-more, the design should assure a complete increment extrac-tion, and the arc of travel of the sweep-arm cutter shouldclosely fit the configuration of the conveyor belt. The mass m,in kg lb, of material collected in one pass through the movingstream by a cutter with cutting edges and cutter velocityperpendicular to the stream flow is calculated from the f
