ASTM D1988-20

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of the standard as published by ASTM is to be considered the official document.
Designation: D1988 − 06 (Reapproved 2015) D1988 − 20
Standard Test Method for
Mercaptans in Natural Gas Using Length-of-Stain Detector
Tubes
This standard is issued under the fixed designation D1988; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers a rapid and simple field determination of mercaptans in natural gas pipelines. Available detector tubes
provide a total measuring range of 0.5 to 160 ppm by volume of mercaptans, although the majority of applications will be on the
lower end of this range (that is, under 20 ppm). Besides total mercaptans, detector tubes are also available for methyl mercaptan
(0.5 to 100 ppm), ethyl mercaptan (0.5 to 120 ppm), and butyl mercaptan (0.5 to 30 mg/M or 0.1 to 8 ppm).
NOTE 1—Certain detector tubes are calibrated in terms of milligrams per cubic metre (mg/M ) instead of parts per million by volume. The conversion
is as follows for 25°C (77°F)25 °C (77 °F) and 760 mm Hg.
ppm 3molecular weight
mg/M 5 (1)
24.45
1.2 Detector tubes are usually subject to interferences from gases and vapors other than the target substance. Such interferences
may vary among brands because of the use of different detection principles. Many detector tubes will have a precleanse layer
designed to remove interferences up to some maximum level. Consult manufacturer’s instructions for specific interference
information. Hydrogen sulfide and other mercaptans are usually interferences on mercaptan detector tubes. See Section 56 for
interferences of various methods of detection.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. For specific hazard statements, see 7.38.3.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D4150 Terminology Relating to Gaseous Fuels
This test method is under the jurisdiction of ASTM Committee D03 on Gaseous Fuels and is the direct responsibility of Subcommittee D03.07 on Analysis of Chemical
Composition of Gaseous Fuels.
Current edition approved Nov. 1, 2015Dec. 15, 2020. Published December 2015January 2021. Originally approved in 1991. Last previous edition approved in 20112015
as D1988D1988 – 06 –06 (2011). (2015). DOI: 10.1520/D1988-06R15.10.1520/D1988-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1988 − 20
2.2 Gas Processors Association GPA Standard:
GPA Standard 21882188 Tentative Method for the Determination of Ethyl Mercaptan in LP Gas Using Length-of-Stain Detector
Tubes, Appendix B, Test for Ethyl Mercaptan Odourant in Propane, Field Method, 1988LP-Gas Using Length of Stain Tubes
3. Terminology
3.1 Definitions—For definitions of general terms used in D03 Gaseous Fuels standards, refer to Terminology D4150.
4. Summary of Test Method
4.1 The sample is passed through a detector tube filled with a specially prepared chemical. Any mercaptan present in the sample
reacts with the chemical to produce a color change, or stain. The length of the stain produced in the detector tube, when exposed
to a measured volume of sample, is directly proportional to the amount of mercaptan present in the sample. A hand-operated piston
or bellows-type pump is used to draw a measured volume of sample through the tube at a controlled rate of flow. The length of
stain produced is converted to parts per million (ppm) by volume mercaptan by comparison to a calibration scale supplied by the
manufacturer for each box of detection tubes. The system is direct reading, easily portable, and completely suited to making rapid
spot checks for mercaptans under field conditions (see Note 1).
5. Significance and Use
5.1 The measurement of mercaptans in natural gas is important, because mercaptans are often added as odorants to natural gas
to provide a warning property. The odor provided by the mercaptan serves to warn consumers (for example, residential use) of
natural gas leaks at levels that are well below the flammable or suffocating concentration levels of natural gas in air. Field
determinations of mercaptans in natural gas are important because of the tendency of the mercaptan concentration to fadedecline
over time.
5.2 This test method provides inexpensive field screening of mercaptans. The system design is such that it may be used by
nontechnical personnel, with a minimum of proper training.
6. Interferences
6.1 Interference from hydrogen sulfide gas (H S) is a common problem with mercaptan detector tubes, and its extent potential
impact should be understood to make use of prior to making decisions based upon tube readings. There are at least three common
detection principleschemistries used in mercaptan detector tubes, and each is summarized below.
6.1.1 Palladium sulfate is used by at least one manufacturer. It has tubes based upon this substance have a positive interference
from H S, but H S may be removed in a preconditioning layer at the in front of the tube. If this is the case, the manufacturer will
2 2
state some finite level of H S at which interference initiates (for example, greater than 500-ppm500 ppm H S causes a positive
2 2
error). Consult manufacturers’ instruction sheets for this information. Propylene and hydrocarbons of five or more carbon atoms
higher hydrocarbons will cause interfering discolorations making the palladium sulfate detection principle ineffective for liquefied
petroleum gas (LPG). (Palladium chloride is used by at least one manufacturer, and it exhibits similar H S interference as with
the palladium sulfate detection principle. sulfate. Palladium chloride may also exhibit the hydrocarbon interference described for
the palladium sulfate detection principle. chemistry. Contact the manufacturer for specific interference information.)
6.1.2 Mercuric Chloride—Mercuric chloride is used by at least one manufacturer. It has Tubes based upon this substance have a
positive interference from H S but doesdo not have the hydrocarbon interference described above for palladium sulfate. This
detection principle is preferred for LPG applications. H S will produce a stain on mercuric chloride tubes even if mercaptans are
not present. The approximate H S sensitivity ratio is typically as follows: One part per million H S will produce a reading of
2 2
0.4-0.4 to 0.7-ppm0.7 ppm mercaptans. Consult manufacturers for exact information if it does not appear in tube instruction sheets.
6.1.3 A Two-stage Copper Salt/Sulfur Reaction—A two-stage copper salt/sulfur reaction is used by at least one manufacturer. This
detection principle has Tubes based upon this chemistry have a positive interference from H S, with H S being twice as sensitive
2 2
as for mercaptans (that is, 10-ppm10 ppm H S will appear as 20-ppm20 ppm mercaptan). Ammonia or amines also interfere with
this principlechemistry, producing a second color.
Available from Gas Processors Association, 6526 E. 60th St., Tulsa, OK 74145.Association (GPA), 6060 American Plaza, Suite 700, Tulsa, OK 74135,
http://www.gpaglobal.org.
D1988 − 20
7. Apparatus
7.1 Length-of-Stain Detector Tube—A sealed glass tube with breakoff tips sized to fit the tube holder of the pump. The reagent
layer inside the tube, typically a silica gel substrate coated with the active chemicals, must be specific to mercaptans and produce
a distinct color change when exposed to a sample of gas containing mercaptans. Any substances known to interfere must be listed
in the instructions accompanying the tubes. A calibration scale printed on the glass tube shall correlate mercaptan concentration
to the length of the color stain. A separate calibration scale supplied with the tubes shall be is acceptable. Shelf life of the detector
tubes must beis typically a minimum of two years from date of manufacturer,manufacture, when stored according to
manufacturer’s recommendations.
7.2 Detector Tube Pump—A hand-operated pump of a piston or bellows type. It must be capable of drawing 100 mL per stroke
of sample through the detector tube with a volume tolerance of 65 mL. It must be specifically designed for use with detector
tubes.
NOTE 2—A detector tube and pump together form a unit and must be used as such. Each manufacturer calibrates detector tubes to match the flow
characteristics of their specific pump. Crossing brands of pumps and tubes is not permitted, as considerable loss of system accuracy is likely to occur.
7.3 Gas Sampling Chamber—Any container that provides for access of the detector tu
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