1、 Test for Hydrogen Sulfide and Carbon Dioxide in Natural Gas Using Length of Stain Tubes Adopted as a Standard 1984 Revised 2014 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 GPA Standard 2377-14 DISCLAIMER GPA publications necessarily address problems of a general nature an
2、d may be used by anyone desiring to do so. Every effort has been made by GPA to assure accuracy and reliability of the information con-tained in its publications. With respect to particular circumstances, local, state, and federal laws and reg-ulations should be reviewed. It is not the intent of GPA
3、 to assume the duties of employers, manufactur-ers, or suppliers to warn and properly train employees, or others exposed, concerning health and safety risks or precautions. GPA makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liab
4、ility or responsibility for loss or damage resulting from its use or for the vio-lation of any federal, state, or municipal regulation with which this publication may conflict, or for any infringement of letters of patent regarding apparatus, equipment, or method so covered. Copyright2014 by Gas Pro
5、cessors Association. All rights reserved. No part of this Report may be reproduced without the written consent of the Gas Processors Association. 2 Impact Statement GPA 2377- Test for Hydrogen Sulfide and Carbon Dioxide in Natural Gas Using a Length of Stain Tubes PURPOSE This standard covers the de
6、termination of the concentration of H2S and CO2in Natural Gas using a length of stain tube. The ranges covered is 3 ppmv to 5% by volume H2S and 0.25 to 10% by volume of CO2. The Method as written is applicable to the determination of H2S and CO2in hydrocarbons and in air. This standard has seen onl
7、y reprinting and few revisions since 1977 when it was adopted as a tentative method. It was adopted as a Standard in 1984, revised in 1986 and reprinted in 1988, 1997, 1998 and 1999. The current revision updates some materials used in the procedures which by todays best practices would be found obso
8、lete in terms of what we would use to collect or transport a sample to the stain tube. I.e. gum rubber tubing etc. The most significant addition to the standard is a “Cautionary Statement”. The intention of the statement is to inform the user that it is critical that manufacturers instructions be fo
9、llowed implicitly. Each lot of stain tubes is tested by the manufacturer for compliance to stated accuracy and each lot or box of stain tubes quite possibly is slightly different in terms of how they are to be used i.e. exposure times, indicat-ing colors or contaminants etc. Further the manufacturer
10、s instructions will state the exponent to be used for multiple or partial testing “pulls” of the pump or if these are allowed at all. This information may vary from lot to lot and from one brand of tube to another. Also of significance is the concept that if high accuracy and repeatability is requir
11、ed and chromatog-raphy is not indicated or feasible, there are many portable handheld electronic devices available which operate on low power and are intrinsically safe. Therefore, the user should consider if a length of stain tube is really the best commercially available option based on their situ
12、ation and requirements. IMPACT It is not expected that the use of this Standard will have any new or increased impact. The users of this Standard should perform their own economic evaluation to include the use of handheld portable electronic devices as applicable. 3 Test for Hydrogen Sulfide and Car
13、bon Dioxide in Natural Gas Using Length of Stain Tubes 1. CAUTIONARY STATEMENT H2S or CO2Determination in Natural Gas by Length of Stain Tubes 1.1 The need to determine the concentration of Hydrogen Sulfide or Carbon Dioxide in Natu-ral Gas can be driven by numerous factors, including safety related
14、 incidents. The user should be aware that if definitive, highly accurate and repeatable readings are needed, length of stain tubes may not be the first choice as a commercially viable solution. In recent years there have been advancements in the determination of components of in-terest in various ga
15、ses and liquids by means of portable or handheld electronic devices which are outside the scope of this document. Concentration level accuracies can be greatly enhanced by the use of these electronic devices and of course chromatographic analysis can be the most accurate means of determining the con
16、centration levels of com-ponents of interest. The analytical results from a length of stain tubes are highly dependent on many factors. Therefore, it is imperative that the user read, understand and follow the instructions in each box of tubes implicitly since every box or lot of tubes potentially h
17、as its own indi-vidual characteristics. Interferences may include temperature and moisture content of the product under test, ambient conditions, and a number of other diluents and contaminants. Ambient or operational conditions may preclude the use of a stain tube. It may be too hot or humid outsid
18、e or the conditions in the Natural Gas may not be appropriate for stain tube use. Manufacturers instructions will direct the user. Stain tube accuracies as stated by most manufacturers is +-25%. While GPA testing shows no worse than +-16% accuracy in controlled conditions, it is important for the us
19、er to determine if the stated accuracy by the stain tube manufacturer is acceptable, based on the operational conditions, ambient conditions and safety requirements for the determina-tion of the concentration level of H2S or CO2in Natural Gas. 2. SCOPE 2.1. This method covers the determination of hy
20、drogen sulfide in natural gas in the range of 3 ppmv to 5 vol %. 2.2. This method covers the determination of carbon dioxide in natural gas in the range of 0.25 to 10 vol %. 2.3. This method as written is applicable to the determination of hydrogen sulfide and carbon dioxide in hydrocarbons and in a
21、ir. 4 3. SUMMARY OF METHOD 3.1. The sample is passed through the detector tube made specifically for the detection of hy-drogen sulfide or carbon dioxide by specially prepared chemicals. The hydrogen sulfide or carbon dioxide present in the sample reacts with the chemical to produce a color change.
22、The length of stain (or color change) produced in the detector tube when exposed to a measured volume of sample is directly proportional to the amount of hydrogen sul-fide or carbon dioxide present in the sample being tested. A bellows or piston type pump is used to draw a measured volume of sample
23、through the tube at a controlled rate of flow. The length of stain produced is converted to volume by comparison with a calibra-tion scale supplied by the manufacturer with each box of detector tubes. Some tubes have a predetermined calibration constant which is multiplied by the observed length of
24、stain to arrive at a volume percent concentration in the sample being tested. The apparatus is easily portable and is suitable for making spot checks for hydrogen sulfide or carbon di-oxide under the field conditions when applied within the manufacturers published limits and instructions. 4. APPARAT
25、US 4.1. Piston or Bellows Pump The pump is hand-operated and must be capable of drawing a minimum of 100 ml per stroke of sample through the detector tube with an accuracy of 2.0 ml. 4.2. Detector Tube Tubes must be made of glass with break-off tips sized to fit the orifice of the pump. The chemical
26、 sealed in the tube must be specific for hydrogen sulfide or carbon dioxide and produce a distinct color change when exposed to a sample of gas con-taining hydrogen sulfide or carbon dioxide. Any substances known to interfere must be listed in instructions accompanying the tubes. A calibration scale
27、 or other markings refer-enced to a scale should be etched or printed directly on the tube to provide for easy inter-pretation of hydrogen sulfide or carbon dioxide content. Shelf life and storage conditions of the detector tubes must be clearly stated by the manufacturer and each tube or unit of sa
28、le package should be clearly marked with an expiration date. Note 1 The various brands of bellows pumps and detector tubes should not be commingled. The same brand of detector tubes and bellows pumps should be used together. 4.3. Gas Sampling Container Any non-reactive container (polyethylene; stain
29、less steel; mylar; nylon; sulfinert; silco steel; tedlar) which provides for access of the detector tube into a uniform flow of sample gas at atmospheric pressure and isolated from the sur-rounding atmosphere. Sampling containers must be maintained in clean and dry condi-tion, to avoid retention or
30、absorption of contaminants and moisture. 4.3.1. A suitable container may be devised from a one pint polyethylene bottle. A 1/4 inch OD polyethylene tubing sealed into the bottle and discharging near the bot-tom provides for flow of sample gas into the bottle. A 1/4 inch hole cut into the 5 cap of th
31、e bottle provides both access for the detector tube and a vent for gas flow. (Figure 1.) Note 2 A one pint polyethylene wash bottle is easily adapted to a suitable sample container. Vented gas sampling containers are also commercially available in stainless steel and are non-reactive. 4.3.2. Mylar g
32、as collection bags are useful as gas sample containers when the supply of sample gas is limited. Mylar bags with a minimum capacity of two liters are an acceptable substitute for the bottle described in 4.3.1 4.4. Barometer Any barometer equipped with a scale graduated in 1 mm of mercury sub-divisio
33、ns and a range including the expected atmospheric pressure condition at the sam-pling site. 4.5. Thermometer Standard laboratory thermometer graduated in 1C subdivisions and in-cluding the range of sample temperatures expected during the test. 4.6. Needle Value and Tubing Any stainless steel needle
34、valve which can be adjusted to control the flow of gas from source pressure into the gas sampling container. Polyeth-ylene or similar non-reactive tubing may be used to connect the gas sampling container to the needle valve outlet. Note 3 A non-reactive pressure regulator may be used to control flow
35、 of the sample gas, in lieu of a nee-dle valve. 5. SAMPLING 5.1. Select a sampling point which affords access to a representative sample of the gas to be tested (i.e., a point on the main flow line). Flow line connections should have a centerline tap. 5.1.1. Open source valve (Valve A), Figure 1, an
36、d blow down vigorously to clear for-eign materials from source valve and connecting nipple. Close source valve. 5.1.2. Install control valve (Valve B) or pressure regulator on outlet of source valve. Connect outlet of control valve (Valve B) to gas sampling container using shortest length practicabl
37、e of polyethylene or other suitable tubing. 5.1.3. Open source valve (Valve A) and crack control valve (Valve B) to obtain positive flow of gas through gas sample container venting to atmosphere through tube ac-cess and vent (Vent C). 5.1.4. Purge gas sample container until all air is displaced. A m
38、inimum purge time of three minutes is recommended. Note 4 When using collection bags the same procedure is followed except that the deflated bag 6 is attached directly to control value (Valve B). The bag is filled once, disconnected and deflated. The bag is filled a second time and is then ready for
39、 the analysis. 6. PROCEDURE Note 5 There is tremendous variation in recommended usage, instructions, time intervals, interferences, cross sensitivities, and temperature corrections between different model tubes, including different models from a particu-lar manufacturer. It is the users responsibili
40、ty to assure they are using the proper tube and strictly conforming to the manufacturers instructions. Failure to do so can dramatically impact accuracy of results. 6.1. Immediately, before each series of measurements, test the pump (following manufactur-ers instructions) for tightness by inserting
41、an unopened tube and operating the pump. A loss in vacuum on the pump after 30 seconds indicates a leak. 6.1.1. Select the tube range that includes the expected amount of hydrogen sulfide or carbon dioxide present in the sample. Reading accuracy is improved when the stain extends at least 50% of the
42、 tube length. Consider multiple strokes and/or a lower range tube to achieve this length of stain. Note 6 Follow manufacturers instructions regarding stroke counts for a particular stain tube. Multiple or half strokes may not be allowed or may skew results. 6.1.2. Break off tips and immediately inse
43、rt outlet end of tube snugly into the pump head. Temperature of tube must remain in the stated manufacturers temperature range throughout the test period. 6.1.3. Place detector tube well into gas sampling container through the tube access and vent (Vent C). Note 7 Gas sample container must be comple
44、tely purged of air and with control valve (Valve B) adjusted to maintain a positive flow of gas leaving the tube access and vent (Vent C) for the dura-tion of the test. 6.1.4. Operate the pump to draw a measured volume of gas through the detector tube. Within limits set by manufacturers instructions
45、, use multiple strokes to maximize length of stain. 6.1.5. Remove the tube from the pump and immediately read the concentration of hy-drogen sulfide or carbon dioxide from graduations on the tube or charts supplied with the tubes. The scale reading even with the end of the stain is the approximate h
46、ydrogen sulfide or carbon dioxide concentration. Interpolation can be made be-tween scale readings. If the number of strokes used is different from the number specified by the manufacturer for a particular concentration, a correction must be made as following manufacturers instructions. The correcti
47、on is generally; 7 Corr. H2S/CO2Conc. = Scale reading X Specified Strokes Actual Strokes X = the linearity correction for No. of pump strokes. Figure 1. Sampling manifold to be used with H2S and CO2detector tubes. 6.1.6 Record temperature of gas flowing through gas sample container and barometric pr
48、essure to provide data for gas volume corrections if required. 7. CALCULATIONS 7.1. Gas volume corrections may be desirable to improve precision of results. The effect of temperature is usually negligible; however, the barometric pressure becomes significant at altitudes above 2000. Correction for b
49、arometric pressure is done as follows: 8 Corr. Volume % = Vol % (read from tube) x 760 mm Hg Baro. Press. mm Hg 7.2. The manufacturers instructions should be consulted for correction calculations for tem-perature and interferences. These corrections may require interpolation of graphs or ta-bles provided by the manufacturer. 7.3. Check with manufacturer if it becomes necessary to test at gas temperatures outside the range specified in the manufacturers instructions. 8. PRECISION 8.1. The following criteria should be used for judging the acceptabilit
