ASTM D1988-06(2011)
(Test Method)Standard Test Method for Mercaptans in Natural Gas Using Length-of-Stain Detector Tubes
Standard Test Method for Mercaptans in Natural Gas Using Length-of-Stain Detector Tubes
SIGNIFICANCE AND USE
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 fade over time.
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.
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/M3 or 0.1 to 8 ppm).
Note 1—Certain detector tubes are calibrated in terms of milligrams per cubic metre (mg/M3) instead of parts per million by volume. The conversion is as follows for 25°C (77°F) and 760 mm Hg.
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 5 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 and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see 7.3.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D1988 − 06(Reapproved 2011)
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 Detector Tubes, Appendix B, Test for Ethyl Mercaptan
Odourant in Propane, Field Method, 1988
1.1 This test method covers a rapid and simple field deter-
mination of mercaptans in natural gas pipelines. Available
3. Summary of Test Method
detector tubes provide a total measuring range of 0.5 to 160
3.1 The sample is passed through a detector tube filled with
ppm by volume of mercaptans, although the majority of
a specially prepared chemical. Any mercaptan present in the
applications will be on the lower end of this range (that is,
sample reacts with the chemical to produce a color change, or
under 20 ppm). Besides total mercaptans, detector tubes are
stain. The length of the stain produced in the detector tube,
also available for methyl mercaptan (0.5 to 100 ppm), ethyl
when exposed to a measured volume of sample, is directly
mercaptan (0.5 to 120 ppm), and butyl mercaptan (0.5 to 30
proportional to the amount of mercaptan present in the sample.
mg/M or 0.1 to 8 ppm).
A hand-operated piston or bellows-type pump is used to draw
NOTE 1—Certain detector tubes are calibrated in terms of milligrams
3 a measured volume of sample through the tube at a controlled
per cubic metre (mg/M ) instead of parts per million by volume. The
rate of flow. The length of stain produced is converted to parts
conversion is as follows for 25°C (77°F) and 760 mm Hg.
per million (ppm) by volume mercaptan by comparison to a
ppm 3molecular weight
calibration scale supplied by the manufacturer for each box of
mg/M 5 (1)
24.45
detection tubes. The system is direct reading, easily portable,
1.2 Detector tubes are usually subject to interferences from
and completely suited to making rapid spot checks for mer-
gases and vapors other than the target substance. Such inter-
captans under field conditions (see Note 1).
ferencesmayvaryamongbrandsbecauseoftheuseofdifferent
detection principles. Many detector tubes will have a pre-
4. Significance and Use
cleanse layer designed to remove interferences up to some
4.1 The measurement of mercaptans in natural gas is
maximum level. Consult manufacturer’s instructions for spe-
important, because mercaptans are often added as odorants to
cific interference information. Hydrogen sulfide and other
natural gas to provide a warning property. The odor provided
mercaptans are usually interferences on mercaptan detector
by the mercaptan serves to warn consumers (for example,
tubes. See Section 5 for interferences of various methods of
residentialuse)ofnaturalgasleaksatlevelsthatarewellbelow
detection.
theflammableorsuffocatingconcentrationlevelsofnaturalgas
1.3 This standard does not purport to address all of the
in air. Field determinations of mercaptans in natural gas are
safety concerns, if any, associated with its use. It is the
important because of the tendency of the mercaptan concen-
responsibility of the user of this standard to establish appro-
tration to fade over time.
priate safety and health practices and determine the applica-
4.2 This test method provides inexpensive field screening of
bility of regulatory limitations prior to use. For specific hazard
mercaptans. The system design is such that it may be used by
statements, see 7.3.
nontechnical personnel, with a minimum of proper training.
2. Referenced Documents
5. Interferences
2.1 Gas Processors Association Standard:
GPAStandard 2188 Tentative Method for the Determination 5.1 Interference from hydrogen sulfide gas (H S) is a
of Ethyl Mercaptan in LP Gas Using Length-of-Stain common problem with mercaptan detector tubes and its extent
should be understood to make use of tube readings. There are
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous at least three detection principles used in mercaptan detector
Fuels and is the direct responsibility of Subcommittee D03.07 on Analysis of
tubes and each is summarized below.
Chemical Composition of Gaseous Fuels.
Current edition approved Nov. 1, 2011. Published March 2012. Originally
approved in 1991. Last previous edition approved in 2006 as D1988 –06. DOI:
10.1520/D1988-06R11. Available from Gas ProcessorsAssociation, 6526 E. 60th St.,Tulsa, OK 74145.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1988 − 06 (2011)
tubes is not permitted, as considerable loss of system accuracy is likely to
5.1.1 Palladium sulfate is used by at least one manufacturer.
occur.
It has a positive interference from H S, but H S may be
2 2
removed in a preconditioning layer at the front of the tube. If 6.3 Gas Sampling Chamber—Any container that provides
this is the case, the manufacturer will state some finite level of for access of the detector tube into a uniform flow of sample
H S at which interference initiates (for example, greater than gas at atmospheric pressure and isolates the sample from the
500-ppm H S causes a positive error). Consult manufacturers’ surrounding atmosphere. A stainless steel needle valve (or
instruction sheets for this information. Propylene and hydro- pressure regulator) is placed between the source valve and the
carbons of five or more carbon atoms will cause interfering sampling chamber for the purpose of throttling the sample
discolorations making the palladium sulfate detection principle flow.Flowrateshouldapproximateonetotwovolumechanges
ineffective for liquefied petroleum gas (LPG). (Palladium per minute or, at minimum, provide a positive exit gas flow
chloride is used by at least one manufacturer, and it exhibits throughout the detector tube sampling period.
similarH Sinterferenceaswiththepalladiumsulfatedetection
NOTE 3—A suitable sampling chamber may be devised from a poly-
principle. Palladium chloride may also exhibit the hydrocarbon
ethylene wash bottle of nominal 500-mL or 1-L size. The wash bottle’s
interference described for the palladium sulfate detection
internal delivery tube provides for delivery of sample gas to the bottom of
thebottle.A ⁄2-in.(13-mm)holecutinthebottle’scapprovidesaccessfor
principle. Contact the manufacturer for specific interference
the detector tube and vent for the purge gas (see Fig. 1). Purge gas must
information.)
be vented at a sufficient rate so that pressure does not build up within the
5.1.2 Mercuricchlorideisusedbyatleastonemanufacturer.
sampling chamber and increase the flow rate through the detector tube.
It has a positive interference from H S but does not have the
2 (An alternative flow-through sampler may be fashioned using a 1-gal
hydrocarbon interference described above for palladium sul- zipper-type food storage bag. The flexible line enters one corner of the
bag’s open end and extends to the bottom of the bag. The opposite corner
fate. This detection principle is preferred for LPG applications.
of the open end is used for tube access and sample vent. The remainder of
H S will produce a stain on mercuric chloride tubes even if
the bag’s top is sealed shut. The basic procedure for the sampler in Fig. 1
mercaptans are not present. The approximate H S sensitivity
applies.)
ratio is as follows: One part per million H S will produce a
2 NOTE 4—An alternative sampling container is a collection bag made of
reading of 0.4- to 0.7-ppm mercaptans. Consult manufacturers a material suitable for the collection of natural gas (for example, polyester
film). The sampling bag should have a minimum capacity of 2 L.
for exact information if it does not appear in tube instruction
sheets.
7. Procedure
5.1.3 A two-stage copper salt/sulfur reaction is used by at
least one manufacturer. This detection principle has a positive
7.1 Select a sampling point that will provide access to a
interference from H S with H S being twice as sensitive (that representative sample of the gas to be tested (source valve on
2 2
is, 10-ppm H S will appear as 20-ppm mercaptan). Ammonia
the main line). The sample point should be on top of the
or amines also interfere with this principle producing a second pipeline and equipped with a stainless steel sample probe
color.
6. Apparatus
6.1 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 mercap-
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