ASTM D7904-21

Standard Test Method for Determination of Water Vapor (Moisture Concentration) in Natural Gas by Tunable Diode Laser Spectroscopy (TDLAS)

SIGNIFICANCE AND USE
5.1 Moisture measurement in natural gas is performed to ensure sufficiently low levels for gas purchase contracts and to prevent corrosion. Moisture may also contribute to the formation of hydrates.  
5.2 The significance of applying TDLAS for the measurement of moisture in natural gas is TDLAS analyzers may have a very high degree of selectivity and minimal interference in many natural gas streams. Additionally, the sensing components of the analyzer are not wetted by the natural gas, limiting the potential damage from corrosives such as hydrogen sulfide (H2S) and liquid contaminants such as ethylene glycol or compressor oils. As a result, the TDLAS analyzer is able to detect changes in concentration with relatively rapid response. It should be noted that the mirrors of a TDLAS analyzer may be fouled if large quantities of condensed liquids enter the sample cell. In most cases the mirror can be cleaned without the need for recalibration or realignment.  
5.3 Primary applications covered in this method are listed in 5.3.1 – 5.3.3. Each application may have differing requirements and methods for gas sampling. Additionally, different natural gas applications may have unique spectroscopic considerations.  
5.3.1 Raw natural gas is found in production, gathering sites, and inlets to gas-processing plants characterized by potentially high levels of water (H2O), carbon dioxide (CO2), hydrogen sulfide (H2S), and heavy hydrocarbons. Gas-conditioning plants and skids are normally used to remove H2O, CO2, H2S, and other contaminants. Typical moisture concentration after dehydration is roughly 20 to 200 ppmv. Protection from liquid carryover such as heavy hydrocarbons and glycols in the sample lines is necessary to prevent liquid pooling in the cell or the sample components.  
5.3.2 Underground gas storage facilities are high-pressure caverns used to store large volumes of gas for use during peak demand. Underground storage caverns can reach pressures as high as 275 bar. ...
SCOPE
1.1 This test method covers online determination of vapor phase moisture concentration in natural gas using a tunable diode laser absorption spectroscopy (TDLAS) analyzer also known as a “TDL analyzer.” The particular wavelength for moisture measurement varies by manufacturer; typically between 1000 and 10 000 nm with an individual laser having a tunable range of less than 10 nm.  
1.2 Process stream pressures can range from 700-mbar to 700-bar gage. TDLAS is performed at pressures near atmospheric (700- to 2000-mbar gage); therefore, pressure reduction is typically required. TDLAS can be performed in vacuum conditions with good results; however, the sample conditioning requirements are different because of higher complexity and a tendency for moisture ingress and are not covered by this test method. Generally speaking, the vent line of a TDL analyzer is tolerant to small pressure changes on the order of 50 to 200 mbar, but it is important to observe the manufacturer’s published inlet pressure and vent pressure constraints. Large spikes or steps in backpressure may affect the analyzer readings.  
1.3 The typical sample temperature range is -20 to 65 °C in the analyzer cell. While sample system design is not covered by this standard, it is common practice to heat the sample transport line to around 50 °C to avoid concentration changes associated with adsorption and desorption of moisture along the walls of the sample transport line.  
1.4 The moisture concentration range is 1 to 10 000 parts per million by volume (ppmv). It is unlikely that one spectrometer cell will be used to measure this entire range. For example, a TDL spectrometer may have a maximum measurement of 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv with varying degrees of accuracy and different lower detection limits.  
1.5 TDL absorption spectroscopy measures molar ratios such as ppmv or mole percentage. Volumetric ratios (ppmv and %) are not pressure d...

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 75.060 - Natural gas
• Technical Committee: D03 - Gaseous Fuels
• Drafting Committee: D03.12 - On-Line/At-Line Analysis of Gaseous Fuels

Standards Content (Sample)

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.
Designation: D7904 − 21
Standard Test Method for
Determination of Water Vapor (Moisture Concentration) in
1
Natural Gas by Tunable Diode Laser Spectroscopy (TDLAS)
This standard is issued under the fixed designation D7904; 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 1.5 TDL absorption spectroscopy measures molar ratios
such as ppmv or mole percentage.Volumetric ratios (ppmv and
1.1 This test method covers online determination of vapor
%) are not pressure dependent. Weight-per-volume units such
phase moisture concentration in natural gas using a tunable
as milligrams of water per standard cubic metre or pounds of
diode laser absorption spectroscopy (TDLAS) analyzer also
water per standard cubic foot can be derived from ppmv at a
known as a “TDL analyzer.” The particular wavelength for
specific condition such as standard temperature and pressure
moisture measurement varies by manufacturer; typically be-
(STP). Standard conditions may be defined differently for
tween 1000 and 10 000 nm with an individual laser having a
different regions and entities. The dew point can be estimated
tunable range of less than 10 nm.
from ppmv and pressure. Refer toTest Method D1142 and ISO
1.2 Process stream pressures can range from 700-mbar to
18453.
700-bar gage. TDLAS is performed at pressures near atmo-
1.6 Units—The values stated in SI units are to be regarded
spheric (700- to 2000-mbar gage); therefore, pressure reduc-
as standard. No other units of measurement are included in this
tion is typically required.TDLAS can be performed in vacuum
standard.
conditionswithgoodresults;however,thesampleconditioning
1.7 This standard does not purport to address all of the
requirements are different because of higher complexity and a
safety concerns, if any, associated with its use. It is the
tendency for moisture ingress and are not covered by this test
responsibility of the user of this standard to establish appro-
method. Generally speaking, the vent line of aTDLanalyzer is
priate safety, health, and environmental practices and deter-
tolerant to small pressure changes on the order of 50 to
mine the applicability of regulatory limitations prior to use.
200 mbar, but it is important to observe the manufacturer’s
Some specific hazards statements are given in Section 8 on
published inlet pressure and vent pressure constraints. Large
Hazards.
spikes or steps in backpressure may affect the analyzer read-
1.8 This international standard was developed in accor-
ings.
dance with internationally recognized principles on standard-
1.3 The typical sample temperature range is -20 to 65 °C in
ization established in the Decision on Principles for the
the analyzer cell. While sample system design is not covered
Development of International Standards, Guides and Recom-
by this standard, it is common practice to heat the sample
mendations issued by the World Trade Organization Technical
transport line to around 50 °C to avoid concentration changes
Barriers to Trade (TBT) Committee.
associated with adsorption and desorption of moisture along
the walls of the sample transport line.
2. Referenced Documents
2
1.4 The moisture concentration range is 1 to 10 000 parts
2.1 ASTM Standards:
per million by volume (ppmv). It is unlikely that one spec-
D1142 Test Method for Water Vapor Content of Gaseous
trometer cell will be used to measure this entire range. For
Fuels by Measurement of Dew-Point Temperature
example, a TDL spectrometer may have a maximum measure-
D4150 Terminology Relating to Gaseous Fuels
ment of 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv with
D5503 Practice for Natural Gas Sample-Handling and Con-
varying degrees of accuracy and different lower detection
ditioning Systems for Pipeline Instrumentation (With-
3
limits.
drawn 2017)
1 2
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Fuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-Line contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Analysis of Gaseous Fuels. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Nov. 1, 2021. Published November 2021. Originally the ASTM website.
3
approved in 2015. Last previous edition approved in 2015 as D7904 – 15. DOI: The last approved version of this historical standard is referenced on
10.15
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7904 − 15 D7904 − 21
Standard Test Method for
Determination of Water Vapor (Moisture Concentration) in
1
Natural Gas by Tunable Diode Laser Spectroscopy (TDLAS)
This standard is issued under the fixed designation D7904; 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 online determination of vapor phase moisture concentration in natural gas using a tunable diode laser
absorption spectroscopy (TDLAS) analyzer also known as a “TDL analyzer.” The particular wavelength for moisture measurement
varies by manufacturer; typically between 1000 and 10 000 nm 10 000 nm with an individual laser having a tunable range of less
than 10 nm. 10 nm.
1.2 Process stream pressures can range from 700-mbar to 700-bar gage. TDLAS is performed at pressures near atmospheric (700-
to 2000-mbar gage); therefore, pressure reduction is typically required. TDLAS can be performed in vacuum conditions with good
results; however, the sample conditioning requirements are different because of higher complexity and a tendency for moisture
ingress and are not covered by this test method. Generally speaking, the vent line of a TDL analyzer is tolerant to small pressure
changes on the order of 50 to 200 mbar, but it is important to observe the manufacturer’s published inlet pressure and vent pressure
constraints. Large spikes or steps in backpressure may affect the analyzer readings.
1.3 The typical sample temperature range is -20 to 65°C65 °C in the analyzer cell. While sample system design is not covered by
this standard, it is common practice to heat the sample transport line to around 50°C50 °C to avoid concentration changes
associated with adsorption and desorption of moisture along the walls of the sample transport line.
1.4 The moisture concentration range is 1 to 10 000 10 000 parts per million by volume (ppmv). It is unlikely that one
spectrometer cell will be used to measure this entire range. For example, a TDL spectrometer may have a maximum measurement
of 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv with varying degrees of
accuracy and different lower detection limits.
1.5 TDL absorption spectroscopy measures molar ratios such as ppmv or mole percentage. Volumetric ratios (ppmv and %) are
not pressure dependant.dependent. Weight-per-volume units such as milligrams of water per standard cubic metre or pounds of
water per standard cubic foot can be derived from ppmv at a specific condition such as standard temperature and pressure (STP).
Standard conditions may be defined differently for different regions and entities. The moisture dew point can be estimated from
ppmv and pressure. Refer to Test Method D1142 and ISO 18453.
1.6 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1
This test method is under the jurisdiction of ASTM Committee D03 on Gaseous Fuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-Line
Analysis of Gaseous Fuels.
Current edition approved Jan. 1, 2015Nov. 1, 2021. Published February 2015November 2021. Originally approved in 2015. Last previous edition approved in 2015 as
D7904 – 15. DOI: 10.1520/D7904/D7904–1510.1520/D7904-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1

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D7904 − 21
1.7 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. Some specific hazards statements are given in Section 8 on Hazards.Some
specific hazards statements are given in Section 8 on Hazards.
1.8 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
2.1 ASTM Standards:
D1142 Test Method for Water Va
...