ASTM D1988-91(2000)
(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
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 25oC (77oF) and 760 mm Hg. Equation 1 - mg/M3 = ppm X molecular weight /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 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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Designation:D1988–91(Reapproved2000)
Standard Test Method for
Mercaptans in Natural Gas Using Length-of-Stain Detector
Tubes
This standard is issued under the fixed designation D 1988; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope Odourant in Propane, Field Method, 1988
1.1 This test method covers a rapid and simple field deter-
3. Summary of Test Method
mination of mercaptans in natural gas pipelines. Available
3.1 The sample is passed through a detector tube filled with
detector tubes provide a total measuring range of 0.5 to 160
a specially prepared chemical. Any mercaptan present in the
ppm by volume of mercaptans, although the majority of
sample reacts with the chemical to produce a color change, or
applications will be on the lower end of this range (that is,
stain. The length of the stain produced in the detector tube,
under 20 ppm). Besides total mercaptans, detector tubes are
when exposed to a measured volume of sample, is directly
also available for methyl mercaptan (0.5 to 100 ppm), ethyl
proportional to the amount of mercaptan present in the sample.
mercaptan (0.5 to 120 ppm), and butyl mercaptan (0.5 to 30
A hand-operated piston or bellows-type pump is used to draw
mg/M or 0.1 to 8 ppm).
a measured volume of sample through the tube at a controlled
NOTE 1—Certain detector tubes are calibrated in terms of milligrams
rate of flow. The length of stain produced is converted to parts
per cubic metre (mg/M ) instead of parts per million by volume. The
per million (ppm) by volume mercaptan by comparison to a
conversion is as follows for 25°C (77°F) and 760 mm Hg.
calibration scale supplied by the manufacturer for each box of
ppm 3 molecular weight
detection tubes. The system is direct reading, easily portable,
mg/M 5 (1)
24.45
and completely suited to making rapid spot checks for mer-
1.2 Detector tubes are usually subject to interferences from
captans under field conditions (see Note 1).
gases and vapors other than the target substance. Such inter-
4. Significance and Use
ferencesmayvaryamongbrandsbecauseoftheuseofdifferent
detection principles. Many detector tubes will have a pre- 4.1 The measurement of mercaptans in natural gas is im-
cleanse layer designed to remove interferences up to some
portant, because mercaptans are often added as odorants to
maximum level. Consult manufacturer’s instructions for spe- natural gas to provide a warning property. The odor provided
cific interference information. Hydrogen sulfide and other by the mercaptan serves to warn consumers (for example,
mercaptans are usually interferences on mercaptan detector residentialuse)ofnaturalgasleaksatlevelsthatarewellbelow
tubes. See Section 5 for interferences of various methods of theflammableorsuffocatingconcentrationlevelsofnaturalgas
detection. in air. Field determinations of mercaptans in natural gas are
1.3 This standard does not purport to address all of the important because of the tendency of the mercaptan concen-
safety concerns, if any, associated with its use. It is the tration to fade over time.
responsibility of the user of this standard to establish appro- 4.2 This test method provides inexpensive field screening of
priate safety and health practices and determine the applica- mercaptans. The system design is such that it may be used by
bility of regulatory limitations prior to use. For specific hazard nontechnical personnel, with a minimum of proper training.
statements, see 7.3.
5. Interferences
2. Referenced Documents
5.1 Interference from hydrogen sulfide gas (H S) is a
2.1 Gas Processors Association Standard:
common problem with mercaptan detector tubes and its extent
GPAStandard 2188 TentativeMethodfortheDetermination should be understood to make use of tube readings. There are
of Ethyl Mercaptan in LP Gas Using Length-of-Stain
at least three detection principles used in mercaptan detector
Detector Tubes, Appendix B, Test for Ethyl Mercaptan tubes and each is summarized below.
5.1.1 Palladium sulfate is used by at least one manufacturer.
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
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous
Fuels and is the direct responsibility of Subcommittee D03.05 on Determination of
Special Constituents of Gaseous Fuels.
Current edition approved March 15, 1991. Published May 1991. 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
this is the case, the manufacturer will state some finite level of 6.3 Gas Sampling Chamber—Any container that provides
H S at which interference initiates (for example, greater than for access of the detector tube into a uniform flow of sample
500-ppm H S causes a positive error). Consult manufacturers’ gas at atmospheric pressure and isolates the sample from the
instruction sheets for this information. Propylene and hydro- surrounding atmosphere. A stainless steel needle valve (or
carbons of five or more carbon atoms will cause interfering pressure regulator) is placed between the source valve and the
discolorations making the palladium sulfate detection principle sampling chamber for the purpose of throttling the sample
ineffective for liquefied petroleum gas (LPG). (Palladium flow.Flowrateshouldapproximateonetotwovolumechanges
chloride is used by at least one manufacturer, and it exhibits per minute or, at minimum, provide a positive exit gas flow
similarH Sinterferenceaswiththepalladiumsulfatedetection throughout the detector tube sampling period.
principle. Palladium chloride may also exhibit the hydrocarbon
NOTE 3—A suitable sampling chamber may be devised from a poly-
interference described for the palladium sulfate detection
ethylene wash bottle of nominal 500-mL or 1-L size. The wash bottle’s
principle. Contact the manufacturer for specific interference
internal delivery tube provides for delivery of sample gas to the bottom of
information.)
thebottle.A ⁄2-in.(13-mm)holecutinthebottle’scapprovidesaccessfor
the detector tube and vent for the purge gas (see Fig. 1). Purge gas must
5.1.2 Mercuricchlorideisusedbyatleastonemanufacturer.
be vented at a sufficient rate so that pressure does not build up within the
It has a positive interference from H S but does not have the
sampling chamber and increase the flow rate through the detector tube.
hydrocarbon interference described above for palladium sul-
(An alternative flow-through sampler may be fashioned using a 1-gal
fate. This detection principle is preferred for LPG applications.
zipper-type food storage bag. The flexible line enters one corner of the
H S will produce a stain on mercuric chloride tubes even if
2 bag’s open end and extends to the bottom of the bag. The opposite corner
mercaptans are not present. The approximate H S sensitivity
of the open end is used for tube access and sample vent. The remainder of
the bag’s top is sealed shut. The basic procedure for the sampler in Fig. 1
ratio is as follows: One part per million H S will produce a
applies.)
reading of 0.4- to 0.7-ppm mercaptans. Consult manufacturers
NOTE 4—An alternative sampling container is a collection bag made of
for exact information if it does not appear in tube instruction
a material suitable for the collection of natural gas (for example, polyester
sheets.
film). The sampling bag should have a minimum capacity of 2 L.
5.1.3 A two-stage copper salt/sulfur reaction is used by at
least one manufacturer. This detection principle has a positive
7. Procedure
interference from H S with H S being twice as sensitive (that
2 2
7.1 Select a sampling point that will provide access to a
is, 10-ppm H S will appear as 20-ppm mercaptan). Ammonia
representative sample of the gas to be tested (source valve on
or amines also interfere with this principle producing a second
the main line). The sample point should be on top of the
color.
pipeline and equipped with a stainless steel sample probe
extendingintothemiddlethirdofthepipeline.Openthesource
6. Apparatus
valve momentarily to clear the valve and connecting nipple of
6.
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