1、Designation: D1988 06 (Reapproved 2011)Standard Test Method forMercaptans in Natural Gas Using Length-of-Stain DetectorTubes1This standard is issued under the fixed designation D1988; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、 the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers a rapid and simple field deter-mination of mercaptans in natural gas pipelines. A
3、vailabledetector tubes provide a total measuring range of 0.5 to 160ppm by volume of mercaptans, although the majority ofapplications will be on the lower end of this range (that is,under 20 ppm). Besides total mercaptans, detector tubes arealso available for methyl mercaptan (0.5 to 100 ppm), ethyl
4、mercaptan (0.5 to 120 ppm), and butyl mercaptan (0.5 to 30mg/M3or 0.1 to 8 ppm).NOTE 1Certain detector tubes are calibrated in terms of milligramsper cubic metre (mg/M3) instead of parts per million by volume. Theconversion is as follows for 25C (77F) and 760 mm Hg.mg/M35ppm 3 molecular weight24.45(
5、1)1.2 Detector tubes are usually subject to interferences fromgases and vapors other than the target substance. Such inter-ferences may vary among brands because of the use of differentdetection principles. Many detector tubes will have a pre-cleanse layer designed to remove interferences up to some
6、maximum level. Consult manufacturers instructions for spe-cific interference information. Hydrogen sulfide and othermercaptans are usually interferences on mercaptan detectortubes. See Section 5 for interferences of various methods ofdetection.1.3 This standard does not purport to address all of the
7、safety concerns, if any, associated with its use. It is theresponsibility 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. For specific hazardstatements, see 7.3.2. Referenced Documents2.1 Gas
8、Processors Association Standard:GPAStandard 2188 Tentative Method for the Determinationof Ethyl Mercaptan in LP Gas Using Length-of-StainDetector Tubes, Appendix B, Test for Ethyl MercaptanOdourant in Propane, Field Method, 198823. Summary of Test Method3.1 The sample is passed through a detector tu
9、be filled witha specially prepared chemical. Any mercaptan present in thesample reacts with the chemical to produce a color change, orstain. The length of the stain produced in the detector tube,when exposed to a measured volume of sample, is directlyproportional to the amount of mercaptan present i
10、n the sample.A hand-operated piston or bellows-type pump is used to drawa measured volume of sample through the tube at a controlledrate of flow. The length of stain produced is converted to partsper million (ppm) by volume mercaptan by comparison to acalibration scale supplied by the manufacturer f
11、or each box ofdetection tubes. The system is direct reading, easily portable,and completely suited to making rapid spot checks for mer-captans under field conditions (see Note 1).4. Significance and Use4.1 The measurement of mercaptans in natural gas is im-portant, because mercaptans are often added
12、 as odorants tonatural gas to provide a warning property. The odor providedby the mercaptan serves to warn consumers (for example,residential use) of natural gas leaks at levels that are well belowthe flammable or suffocating concentration levels of natural gasin air. Field determinations of mercapt
13、ans in natural gas areimportant because of the tendency of the mercaptan concen-tration to fade over time.4.2 This test method provides inexpensive field screening ofmercaptans. The system design is such that it may be used bynontechnical personnel, with a minimum of proper training.5. Interferences
14、5.1 Interference from hydrogen sulfide gas (H2S) is acommon problem with mercaptan detector tubes and its extentshould be understood to make use of tube readings. There areat least three detection principles used in mercaptan detectortubes and each is summarized below.5.1.1 Palladium sulfate is used
15、 by at least one manufacturer.It has a positive interference from H2S, but H2S may be1This test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is the direct responsibility of Subcommittee D03.07 on Analysis ofChemical Composition of Gaseous Fuels.Current edition approved N
16、ov. 1, 2011. Published March 2012. Originallyapproved in 1991. Last previous edition approved in 2006 as D1988 06. DOI:10.1520/D1988-06R11.2Available from Gas ProcessorsAssociation, 6526 E. 60th St., Tulsa, OK 74145.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken
17、, PA 19428-2959, United States.removed in a preconditioning layer at the front of the tube. Ifthis is the case, the manufacturer will state some finite level ofH2S at which interference initiates (for example, greater than500-ppm H2S causes a positive error). Consult manufacturersinstruction sheets
18、for this information. Propylene and hydro-carbons of five or more carbon atoms will cause interferingdiscolorations making the palladium sulfate detection principleineffective for liquefied petroleum gas (LPG). (Palladiumchloride is used by at least one manufacturer, and it exhibitssimilar H2S inter
19、ference as with the palladium sulfate detectionprinciple. Palladium chloride may also exhibit the hydrocarboninterference described for the palladium sulfate detectionprinciple. Contact the manufacturer for specific interferenceinformation.)5.1.2 Mercuric chloride is used by at least one manufacture
20、r.It has a positive interference from H2S but does not have thehydrocarbon interference described above for palladium sul-fate. This detection principle is preferred for LPG applications.H2S will produce a stain on mercuric chloride tubes even ifmercaptans are not present. The approximate H2S sensit
21、ivityratio is as follows: One part per million H2S will produce areading of 0.4- to 0.7-ppm mercaptans. Consult manufacturersfor exact information if it does not appear in tube instructionsheets.5.1.3 A two-stage copper salt/sulfur reaction is used by atleast one manufacturer. This detection princip
22、le has a positiveinterference from H2S with H2S being twice as sensitive (thatis, 10-ppm H2S will appear as 20-ppm mercaptan). Ammoniaor amines also interfere with this principle producing a secondcolor.6. Apparatus6.1 Length-of-Stain Detector TubeA sealed glass tubewith breakoff tips sized to fit t
23、he tube holder of the pump. Thereagent layer inside the tube, typically a silica gel substratecoated with the active chemicals, must be specific to mercap-tans and produce a distinct color change when exposed to asample of gas containing mercaptans. Any substances knownto interfere must be listed in
24、 the instructions accompanying thetubes. A calibration scale printed on the glass tube shallcorrelate mercaptan concentration to the length of the colorstain. A separate calibration scale supplied with the tubes shallbe acceptable. Shelf life of the detector tubes must be aminimum of two years from
25、date of manufacturer, when storedaccording to manufacturers recommendations.6.2 Detector Tube PumpA hand-operated pump of apiston or bellows type. It must be capable of drawing 100 mLper stroke of sample through the detector tube with a volumetolerance of 65 mL.3It must be specifically designed for
26、usewith detector tubes.NOTE 2A detector tube and pump together form a unit and must beused as such. Each manufacturer calibrates detector tubes to match theflow characteristics of their specific pump. Crossing brands of pumps andtubes is not permitted, as considerable loss of system accuracy is like
27、ly tooccur.36.3 Gas Sampling ChamberAny container that providesfor access of the detector tube into a uniform flow of samplegas at atmospheric pressure and isolates the sample from thesurrounding atmosphere. A stainless steel needle valve (orpressure regulator) is placed between the source valve and
28、 thesampling chamber for the purpose of throttling the sampleflow. Flow rate should approximate one to two volume changesper minute or, at minimum, provide a positive exit gas flowthroughout the detector tube sampling period.NOTE 3A suitable sampling chamber may be devised from a poly-ethylene wash
29、bottle of nominal 500-mL or 1-L size. The wash bottlesinternal delivery tube provides for delivery of sample gas to the bottom ofthe bottle.A12-in. (13-mm) hole cut in the bottles cap provides access forthe detector tube and vent for the purge gas (see Fig. 1). Purge gas mustbe vented at a sufficien
30、t rate so that pressure does not build up within thesampling chamber and increase the flow rate through the detector tube.(An alternative flow-through sampler may be fashioned using a 1-galzipper-type food storage bag. The flexible line enters one corner of thebags open end and extends to the bottom
31、 of the bag. The opposite cornerof the open end is used for tube access and sample vent. The remainder ofthe bags top is sealed shut. The basic procedure for the sampler in Fig. 1applies.)NOTE 4An alternative sampling container is a collection bag made ofa material suitable for the collection of nat
32、ural gas (for example, polyesterfilm). The sampling bag should have a minimum capacity of 2 L.7. Procedure7.1 Select a sampling point that will provide access to arepresentative sample of the gas to be tested (source valve onthe main line). The sample point should be on top of thepipeline and equipp
33、ed with a stainless steel sample probeextending into the middle third of the pipeline. Open the sourcevalve momentarily to clear the valve and connecting nipple offoreign materials.3Direct Reading Colorimetric Indicator Tubes Manual , American IndustrialHygiene Association, Akron, OH, 1976. FIG. 1 A
34、pparatus SchematicD1988 06 (2011)27.2 Install needle valve (or pressure regulator) at the sourcevalve outlet. Connect sampling chamber using the shortestlength of flexible tubing possible (see Fig. 1). Avoid usingtubing that reacts with or absorbs mercaptans, such as copperor natural rubber. Use mat
35、erials such as TFE-fluorocarbon,vinyl, polyethylene, or stainless steel.7.3 Open source valve. Open needle valve enough to obtainpositive flow of gas through the chamber, in accordance with6.3. Purge the container for at least 3 min (see Fig. 1).(WarningTake precautions to vent the gas away frompers
36、ons collecting the sample such that the exposure to the gasis minimal. Escaping gases will produce flammable mixtures inair. Keep sources of heat, spark, or flame away from thesampler.)NOTE 5If a collection bag is used instead of a sampling chamber,follow 7.1 and 7.2, substituting the bag for the ch
37、amber. Follow 7.3,disconnecting the bag when filled. Deflate the bag to provide a purge andfill a second time to provide a sample. The bag must be flattenedcompletely before each filling (Note 4).7.4 Before each series of measurements, test the pump forleaks by operating it with an unbroken tube in
38、place. Consultmanufacturers instructions for leak check procedure detailsand for maintenance instruction, if leaks are detected. The leakcheck typically takes 1 min. A leaking pump used in fieldtesting will bias sample results low.7.5 Select the tube range that best encompasses mercaptanconcentratio
39、n. Reading accuracy is improved when the stainlength extends into the upper half of the calibration scale.Consult manufacturer guidelines for using multiple strokes toachieve a lower range on a given tube.7.6 Break off the tube tips and insert the tube into the pump,observing the flow direction indi
40、cation on the tube. Place thedetector tube into the sampling chamber through the accesshole, such that the tube inlet is near the chamber center (seeFig. 1).NOTE 6Detector tubes have temperature limits from 0 to 40C (32 to104F), and sample gases must remain in that range throughout the test.Cooling
41、probes are available for sample temperatures exceeding 40C.7.7 Operate the pump to draw the measured sample volumethrough the detector tube. Observe tube instructions whenapplying multiple strokes. Ensure that a positive flow ismaintained throughout the sample duration at the samplingchamber gas exi
42、t vent. Observe tube instructions for propersampling time per pump stroke. The tube inlet must remain inposition inside the sampling chamber until the sample iscompleted. Many detector tube pumps will have stroke finishindicators that eliminate the need to time the sample.(WarningIt is very importan
43、t to ensure that ambient air isnot being drawn into the sample. Intrusion of ambient air intothe sample will tend to bias the mercaptan readings low.)NOTE 7If a collection bag is used, the sample is drawn from the bagby way of a flexible tubing connection. Do not squeeze the bag duringsampling. Allo
44、w the bag to collapse under pump vacuum, so that thepumps flow characteristics are not altered.7.8 Remove the tube from the pump and immediately readthe mercaptan concentration from the tubes calibration scaleor from the charts provided in the box of tubes. Read the tubeat the maximum point of the s
45、tain. If channeling has occurred(nonuniform stain length), read the maximum and minimumstain lengths and average the two.NOTE 8If the calibration scale is not printed directly on the detectortube, be sure that any separate calibration chart is the proper match for thetube in use.7.9 If the number of
46、 strokes used differs from the number ofstrokes specified for the calibration scale, correct the reading,as follows:A 5 B 3 C/D! (2)where:A = ppm (corrected),B = ppm (reading),C = specified strokes, andD = actual strokes.7.10 Record the reading immediately, along with the gastemperature and the baro
47、metric pressure. Observe any tem-perature corrections supplied in the tube instruction. Altitudecorrections become significant at elevations above 2000 ft.Correct for barometric pressure, as follows:A 5 B 3 E/F! (3)where:E = barometric pressure, 760 mm Hg, andF = ambient barometric pressure, mm Hg.N
48、OTE 9Even though the amount of chemicals contained in detectortubes is very small, the tubes should not be disposed of carelessly. Ageneral disposal method includes soaking the opened tubes in water beforetube disposal. The water should be pH neutralized before its disposal.Observe all local, state,
49、 and federal regulations for small-scale chemicaldisposal.8. Quality Assurance8.1 Detector tubes from each batch or lot of tubes should betested to conform the published accuracy, (generally 6 25 %).8.2 The tubes should continue to meet the published accu-racy until the expiration date, if the tubes are shipped andstored per manufacturers instructions.9. Precision and Bias9.1 The accuracy of detector tube systems is generallyconsidered to be 625 % of reading. This is based mainly onprograms conducted by the National Institute of OccupationalSafety and H
