ASTM D7904-15

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 pppmv. 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 dep...

General Information

• Status: Historical
• Publication Date: 31-Dec-2014
• 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)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D7904 − 15
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 %) are not pressure dependant. Weight-per-volume units such
as milligrams of water per standard cubic metre or pounds of
1.1 This test method covers online determination of vapor
water per standard cubic foot can be derived from ppmv at a
phase moisture concentration in natural gas using a tunable
specific condition such as standard temperature and pressure
diode laser absorption spectroscopy (TDLAS) analyzer also
(STP). Standard conditions may be defined differently for
known as a “TDL analyzer.” The particular wavelength for
different regions and entities. The moisture dew point can be
moisture measurement varies by manufacturer; typically be-
estimated from ppmv and pressure. Refer to Test Method
tween 1000 and 10 000 nm with an individual laser having a
D1142 and ISO 18453.
tunable range of less than 10 nm.
1.6 The values stated in SI units are to be regarded as
1.2 Process stream pressures can range from 700-mbar to
standard. No other units of measurement are included in this
700-bar gage. TDLAS is performed at pressures near atmo-
standard.
spheric (700- to 2000-mbar gage); therefore, pressure reduc-
1.7 This standard does not purport to address all of the
tion is typically required.TDLAS can be performed in vacuum
safety concerns, if any, associated with its use. It is the
conditionswithgoodresults;however,thesampleconditioning
responsibility of the user of this standard to establish appro-
requirements are different because of higher complexity and a
priate safety and health practices and determine the applica-
tendency for moisture ingress and are not covered by this test
bility of regulatory limitations prior to use. Some specific
method. Generally speaking, the vent line of aTDLanalyzer is
hazards statements are given in Section 8 on Hazards.
tolerant to small pressure changes on the order of 50 to
200 mbar, but it is important to observe the manufacturer’s
2. Referenced Documents
published inlet pressure and vent pressure constraints. Large
2
spikes or steps in backpressure may affect the analyzer read-
2.1 ASTM Standards:
ings.
D1142 Test Method for Water Vapor Content of Gaseous
Fuels by Measurement of Dew-Point Temperature
1.3 The typical sample temperature range is -20 to 65°C in
the analyzer cell. While sample system design is not covered D4150 Terminology Relating to Gaseous Fuels
D5503 Practice for Natural Gas Sample-Handling and Con-
by this standard, it is common practice to heat the sample
transport line to around 50°C to avoid concentration changes ditioning Systems for Pipeline Instrumentation
D5454 Test Method for Water Vapor Content of Gaseous
associated with adsorption and desorption of moisture along
the walls of the sample transport line. Fuels Using Electronic Moisture Analyzers
3
2.2 ISO Standards:
1.4 The moisture concentration range is 1 to 10 000 parts
ISO 10715 Natural Gas Sampling Guidelines
per million by volume (ppmv). It is unlikely that one spec-
ISO 18453 Natural Gas—Correlation Between Water Con-
trometer cell will be used to measure this entire range. For
tent and Water Dew Point
example, a TDL spectrometer may have a maximum measure-
ment of 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv with
varying degrees of accuracy and different lower detection 3. Terminology
limits.
3.1 Definitions: Also refer to Terminology D4150.
1.5 TDL absorption spectroscopy measures molar ratios
such as ppmv or mole percentage.Volumetric ratios (ppmv and
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
1
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Fuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-Line Standards volume information, refer to the standard’s Document Summary page on
Analysis of Gaseous Fuels. the ASTM website.
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Current edition approved Jan. 1, 2015. Published February 2015. DOI: 10.1520/ Available from International Organization for Standardization (ISO), 1, ch. de
D7904/D7904–15 la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D7904 − 15
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