1、Designation: D 4888 06Standard Test Method forWater Vapor in Natural Gas Using Length-of-Stain DetectorTubes1This standard is issued under the fixed designation D 4888; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of la
2、st revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers a procedure for rapid andsimple field determination of water vapor in natural gaspipelines. Ava
3、ilabel detector tubes provide a total measuringrange of 0.1 to 40 mg/L, although the majority of applicationswill be on the lower end of this range (that is, under 0.5 mg/L).At least one manufacturer provides tubes that read directly inpounds of water per million cubic feet of gas. See Note 1.1.2 De
4、tector 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 methods. Consult manufacturers instructions forspecific interference information. Alcohols and glycols willcause
5、interferences on some water vapor tubes because of thepresence of the hydroxyl group on those molecules.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and
6、health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Test Method2.1 The sample is passed through a detector tube filled witha specially prepared chemical. Any water vapor present in thesample reacts with the chemical to produce a color change orstain
7、. The length of the stain produced in the detector tube,when exposed to a measured volume of sample, is directlyproportional to the amount of water vapor present in thesample. A hand-operated piston or bellows-type pump is usedto draw a measured volume of sample through the tube at acontrolled rate
8、of flow. The length of stain produced isconverted to milligrams per litre of H2O by comparison to acalibration scale supplied by the manufacturer for each box ofdetection tubes. The system is direct reading, easily portable,and completely suited to making rapid spot checks for watervapor under field
9、 conditions.NOTE 1Detector tubes are available with calibration scales printed inpounds of water per million cubic feet of gas (lb/MMCF). The conversionfactor is 1 mg/L = 62.3 lb/MMCF (7 lb/MMCF = 0.11 mg/L).3. Significance and Use3.1 The measurement of water vapor in natural gas isimportant because
10、 of the gas quality specifications, the corro-sive nature of water vapor on pipeline materials, and the effectsof water vapor on utilization equipment.3.2 This test method provides inexpensive field screening ofwater vapor. The system design is such that it may be used bynontechnical personnel with
11、a minimum of proper training.4. Apparatus4.1 Length-of-Stain Detector TubesA sealed glass tubewith the breakoff tips sized to fit the tube holder of the pump.The reagent layer inside the tube, typically a silica gel substratecoated with the active chemical, must be specific for watervapor and produc
12、e a distinct color change when exposed to asample of gas containing water vapor. Any substances knownto interfere must be listed in the instructions accompanying thetubes. A calibration scale should be marked directly on thetube; however, other markings that provide for ready interpre-tation of wate
13、r vapor content from a separate calibration scalesupplied with the tubes shall be acceptable. The calibrationscale shall correlate water vapor concentration to the length ofthe color stain. Shelf life of the detector tubes must be aminimum of two years from date of manufacture when storedaccording t
14、o manufacturers recommendations.4.2 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.2It must be specifically designed for usewith detector tubes.NOTE 2A detector tube
15、 and pump together form a unit and must beused as such. Each manufacturer calibrates detector tubes to match theflow characteristics of its specific pump. Crossing brands of pumps andtubes is not permitted, as considerable loss of system accuracy is likely tooccur.24.3 Gas Sampling ChamberAny contai
16、ner that providesfor access of the detector tube into a uniform flow of sample1This test method is under the jurisdiction of Committee D03 on Gaseous Fuelsand is the direct responsibility of Subcommittee D03.07 Analysis of ChemicalComposition of Gaseous Fuels.Current edition approved June 1, 2006. P
17、ublished June 2006. Originallyapproved in 1988. Last previous edition approved in 1999 as D 4888 88 (1999).2Direct Reading Colorimetric Indicator Tubes Manual , First ed., AmericanIndustrial Hygiene Association, Akron, OH 44311, 1976.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
18、 West Conshohocken, PA 19428-2959, United States.gas at atmospheric pressure and isolates the sample from thesurrounding atmosphere. A stainless steel needle valve (orpressure regulator) is placed between the source valve and thesampling chamber for the purpose of throttling the sampleflow. Flow rat
19、e should approximate one to two volume changesper minute or, at minimum, provide exit gas flow throughoutthe detector tube sampling period.NOTE 3Asuitable chamber may be devised from a polyethylene washbottle of nominal 500-mL (16-oz) or 1-L (32-oz) size. The wash bottlesinternal delivery tube provi
20、des for delivery of sample gas to the bottom ofthe bottle. A12-in. hole cut in the bottles cap provides access for thedetector tube and vent for the purge gas (Fig. 1).5. Procedure5.1 Select a sampling point that will provide access to arepresentative sample of the gas to be tested (source valve ont
21、he main line). The sample point should be on top of thepipeline and equipped with a stainless steel sample probeextending to the middle third of the pipeline. Open the sourcevalve momentarily to clear the valve and connecting nipple offoreign materials.5.2 Install needle valve (or pressure regulator
22、) at the sourcevalve outlet. Connect sampling chamber using the shortestlength of tubing possible (Fig. 1). Many flexible tubingmaterials will be suitable for water vapor sampling; however, ifthe sampler is also used for other constituents such as hydrogensulfide, then tubing materials should be cho
23、sen carefully.Avoidusing tubing that reacts with or absorbs hydrogen sulfide, suchas copper or natural rubber. Use materials such as TFE-fluorocarbon, polyethylene, or stainless steel. Stainless steeltubing is preferred. (Warning Exiting gases are highlyflammable and may contain toxic levels of hydr
24、ogen sulfide.Consider venting the exit gases away from the user whenpossible.)5.3 Open source valve. Open needle valve enough to obtainpositive flow of gas chamber, in accordance with 4.3. Purge thecontainer for at least 3 min (Fig. 1). Purge for at least 10 minif a polyethylene bottle is used.5.4 B
25、efore each series of measurements, test the pump forleaks by operating it with an unbroken tube in place. Consultmanufacturers instructions for leak check procedure detailsand for maintenance instruction if leaks are detected. The leakcheck typically takes 1 min.5.5 Select the tube range that best e
26、ncompasses water vaporconcentration. 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.5.6 Break off the tube tips and insert the tube into the pump
27、,observing the flow direction indication on the tube. Place thedetector tube into the sampling chamber through the accesshole, such that the tube inlet is near the chamber center (Fig. 1).NOTE 4Detector tubes have temperature limits of 0 to 40C (32 to104F), and sample gases must remain in that range
28、 throughout the test.Cooling probes are available for sample tempertures exceeding 40C.5.7 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
29、 the samplingchamber gas exit 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
30、.NOTE 5It is very important to ensure that ambient air is not beingdrawn into the sample. Ambient humidity is often much higher than thewater vapor level in the gas sample, and intrusion could bias the readingshigh (for example, at 60F and 10 % relative humidity air contains about83-lb H2O/MMCF or a
31、bout 1.33 mg/L).5.8 Remove the tube from the pump and immediately readthe water vapor concentration from the tubes calibration scaleor from the charts provided in the box of tubes. Read the tubeat the maximum point of the stain. If “channeling” hasoccurred (nonuniform stain length), read the maximum
32、 andminimum stain lengths and average the two. Consult tubeinstructions for any special information in the event of multi-colored stains.NOTE 6If 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.5.9
33、If the number of strokes used differs from the number ofstrokes specified for the calibration scale, correct the readingfor water vapor concentration (WVC) as shown below (see also5.5):WVC corrected!5WVC reading!3specified strokesactual strokes(1)FIG. 1 Pump and Tube ApparatusD48880625.10 Record the
34、 reading immediately, along with the gastemperature and the barometric pressure. Observe any tem-perature corrections supplied in the tube instructions. Altitudecorrections become significant at elevations above 2000 ft.Correct for barometric pressure, as shown below:WVC corrected!5WVC reading!3750
35、mm Hgbarometric pressure, mm Hg(2)NOTE 7Even 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 treated to a neutral pH b
36、efore itsdisposal.6. Quality Assurance6.1 Detector tubes from each batch or lot of tubes should betested to conform the published accuracy, (generally 6 25 %).6.2 The tubes should continue to meet the published accu-racy until the expiration date, if the tubes are shipped andstored per manufacturer
37、instructions.7. Precision and Bias7.1 The accuracy of detector tube systems is generallyconsidered to be 625 %. This is based mainly on programsconducted by the National Institute of Occupational Safety andHealth (NIOSH) in certifying detector tubes for low-levelcontaminants in air adapted to worker
38、 exposure monitoring.2NIOSH tested tubes at12 , 1, 2, and 5 times the threshold limitvalue (TLV), requiring 625 % accuracy at the three higherlevels and 635 % at the12 TLV level (for example, H2S witha TLV level of 10 ppm was tested at levels of 5, 10, 20, and 50ppm). The higher tolerance allowed at
39、 the low level was due tothe loss accuracy for shorter stain lengths.3NIOSH discontin-ued this program in 1983, and it was picked up by the SafetyEquipment Institute (SEI) in 1986.8. Keywords8.1 gaseous fuels; natural gasASTM International takes no position respecting the validity of any patent righ
40、ts asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any
41、time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments wil
42、l receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by AST
43、M International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).3“NIOSH Certification Requirements for Gas Detector Tube Units,” NIOSH/TC/A-012, July 1978.D4888063
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