1、 GPA Standard 2188-14 Method for the Determination Of Ethyl Mercaptan in LP-Gas Using Length of Stain Tubes Adopted as a Standard 1988 Revised 2014 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 DISCLAIMER GPA publications necessarily address problems of a general nature and
2、may be used by anyone desiring to do so. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. Every effort has been made by GPA to assure accuracy and reliability of the information contained in its publications. It is not the intent of GPA to
3、assume the duties of employers, manufacturers, 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 liability
4、 or responsibility for loss or damage resulting from its use or for the violation 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 Processor
5、s Association. All rights reserved. No part of this Report may be reproduced without the written consent of the Gas Processors Association. Impact Statement Standard for Determination of Ethyl-Mercaptan in LP Gas Using a Length of Stain Tubes GPA 2188 Purpose This standard covers the determination o
6、f the concentration of Ethyl-Mercaptan in LP Gas using a length of stain tube. While other mercaptans and sulfur species may be indicated in this stain tube it is important to remember that the tubes are calibrated for and intended to be used to determine the concentrations of Ethyl Mercaptan in LP
7、Gas. This standard has seen only reprinting and no revisions since 1989, when it was adopted as a tentative method. It was revised in 1989 and reprinted in 1990 and 1998. In 1989 major studies were performed by Sung I. Johnson and W.J. Hines. At that time the team evaluated four brands of tubes and
8、developed accuracy statements which would hold true today and were actually better than, in most cases, the manufacturers stated accuracies. The current revision updates some materials used in the procedures which by todays best practices would be found obsolete in terms of what we would use to coll
9、ect 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 followed implicitly. Each lot of stain tube
10、s 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, indicating colors or contaminants, etc. Further the manufacturers instructions will state the exponent t
11、o 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 required and chromatography is not indicated o
12、r feasible, then there is many portable handheld electronic devices which offer these things, as well as low power usage and intrinsic safety. So the user should consider if a length of stain tube is really the best commercially available option based on their situation and requirements. Impact It i
13、s not expected that the use of this Standard will have any new or further impact. The users of this Standard should perform their own economic evaluation to include the use of handheld portable electronic devices as applicable. Method for the Determination Of Ethyl Mercaptan in LP-Gas Using Length o
14、f Stain Tubes 1.0 Cautionary Statement The need to determine the concentration of Ethyl Mercaptan in Propane can be driven by numerous factors, including safety related incidents. The user should be aware that if definitive, highly accurate and repeatable readings are needed, length of stain tubes m
15、ay not be the first choice as a commercially viable solution. In recent years there have been advancements in the determination of components of interest in various gases and liquids by means of portable electronic devices (which are outside the scope of this document). Concentration level accuracie
16、s 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 concentration levels of components of interest. The analytical results are highly dependent on many factors. Therefore, it is imperative that the
17、user read, understand and follow the instructions in each box of tubes implicitly (since every box or lot of tubes potentially has its own individual characteristics). Interferences may include temperature and moisture content of the material under test, ambient conditions, and a number of other dil
18、uents and contaminants. Ambient or operational conditions may preclude the use of a stain tube. It may be too hot or humid outside or the conditions in the Propane may not be appropriate for stain tube use. Manufacturers instructions will direct the user. Stain tube accuracies as stated by most manu
19、facturers is 25%. While GPA testing shows 10% accuracy in controlled conditions, it is important for the user to determine if the stated accuracy by the stain tube manufacturer is acceptable, based on the operational conditions, ambient conditions and operational and safety requirements for the dete
20、rmination of the concentration level of Ethyl Mercaptan in Propane. 2.0 Scope 2.1 This method covers the determination of ethyl mercaptan in LP-gas. Limited data (Appendix A) show a precision of 10% of the measured amount. The suggested range of tubes to be used is 0.5 to 25 ppmv. A chromatographic
21、method should be used for more precise, quantitative determination of ethyl mercaptan in propane. CAUTION: This test procedure requires the release of combustible LP-Gas vapors. The user is warned to take proper safety precautions to prevent the accumulation of a combustible mixture and prevent igni
22、tion. This procedure should never be performed in an enclosed building unless a properly designed vapor recovery exhaust hood is provided. Liquid propane evaporates rapidly and can freeze flesh quickly. If any liquid propane is encountered, avoid any skin contact. 1 3.0 Summary of Method 3.1 The sam
23、ple is passed through the detector tube made specifically for the detection of ethyl mercaptan by specially prepared chemicals. 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 ethyl mercaptan pr
24、esent in the sample being tested. A bellows or piston type pump is used to draw a measured volume of sample through the tube at a controlled rate of flow. The length of stain produced is converted to volume by comparison with a calibration scale supplied by the manufacturer with each box of detector
25、 tubes. Some tubes have a pre-determined calibration constant which is multiplied by the observed length of stain to arrive at a volume percent concentration in the sample being tested. The apparatus is easily portable and is completely suitable for making spot checks for ethyl mercaptan in LP-gas u
26、nder field conditions. 4.0 Apparatus 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 break-off tips sized to fit the orific
27、e of the pump used. (Tubes and pumps from different manufacturers should not be interchanged.) The chemical sealed in the tube must be specific for ethyl mercaptan and produce a distinct color change when exposed to a sample of LP-gas containing ethyl mercaptan. Any substances known to interfere mus
28、t be listed in instructions accompanying the tubes. A calibration scale or other markings referenced to a scale should be etched directly on the tube to provide for easy interpretation of ethyl mercaptan content. Shelf life of the detector tubes must be a minimum of two years when stored according t
29、o the manufacturers recommendations. 4.3 Gas Sampling Container Any non-reactive container which provides for access of the detector tube into a uniform flow of sample gas at atmospheric pressure and isolated from the surrounding atmosphere. 4.3.1 A suitable container may be devised from a one-pint
30、polyethylene bottle. A 1/2 in. hole cut into the cap of the bottle provides both access for the detector tube and a vent for gas flow. (Figure 1.) Note 1 Suitable containers for this application are readily commercially available. Note 2 Mylar gas collection bags with a minimum capacity of two liter
31、s are an acceptable substitute for the bottle described in 4.3.1. 4.4 Needle Valve and Tubing Any stainless steel needle valve, which can be adjusted to control the flow of gas from source pressure into the gas sampling container, can be used. Polyethylene or non-reactive tubing may be used to conne
32、ct the gas sampling container to the needle valve outlet. 2 Note 3 A non-reactive pressure regulator may be used to control flow of the sample gas, in lieu of a needle valve. Note 4 Equipment used to develop this test method are listed in Appendix B. Note 5 Avoid carbon steel anywhere in the samplin
33、g process including fittings, cylinders and collection devices. Note 6 Avoid any moisture in the sampling process as this will ultimately affect the reading. 5.0 Sampling 5.1 Before sampling, all sampling equipment should be thoroughly cleaned and dried. Note 7 Sampling containers must be maintained
34、 in clean and dry condition, to avoid retention or absorption of contaminants and moisture. 5.1.1 Select a sampling point which affords access to a representative sample of LP-gas vapors from the tank to be tested. 5.1.2 Open source valve (Valve A, Figure 1) and blow down vigorously to clear foreign
35、 materials from source valve and connecting nipple. Close source valve. 5.1.3 Install control valve (Valve B, Figure 1) or pressure regulator, on outlet of source valve. Connect outlet of control valve (Valve B) to the gas sampling container using shortest length practicable of polyethylene or other
36、 suitable tubing. 5.1.4 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 access and vent (Vent C, Figure 1). 5.1.5 Purge gas sample container until all air is displaced. A minimum purge time o
37、f three minutes is recommended. Note 8 When using gas collection bags the same procedure is followed. The bag is filled once, disconnected and deflated. The bag is filled a second time and is then ready for the analysis. 3 Figure 1 6.0 Procedure Note 9 There is tremendous variation in recommended us
38、age, instructions, time intervals, interferences, cross sensitivities, and temperature corrections between different model tubes, including different models from a particular manufacture. It is the users responsibility to assure they are using the proper tube and strictly conforming to the manufactu
39、rers instructions. Failure to do so can dramatically impact accuracy of the results. 6.1 Immediately before each series of measurements, test the pump for tightness by inserting an unopened tube and operating the pump. A loss in vacuum on the pump within 30 seconds indicates a leak. 6.1.1 Select the
40、 tube range that includes the expected amount of ethyl mercaptan present in the sample. Reading accuracy is improved when the stain extends at least 50% of the tube length. Consider multiple strokes and/or a lower range tube to achieve this length of stain. NOTE 10 Follow manufacturers instructions
41、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 insert outlet end of tube (end to which the arrow points) snugly into the pump head. Temperature of tube must remain in the manufacturers specified ranges th
42、roughout the test period. 4 6.1.3 Place detector tube well into gas sampling container through the tube access and vent (Vent C). Note 11 Gas sample container must be completely purged of air and with control valve (Valve B) adjusted to maintain a positive flow of gas leaving the tube access and ven
43、t (Vent C) for the duration 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, use multiple strokes to maximize length of stain. 7.0 Calculations 7.1 Remove the tube from the pump and follow the manufacture
44、rs instructions if further handling of the tube is necessary. 7.1.1 Immediately read the concentration of ethyl mercaptan from graduations on the tube or charts supplied with the tubes. The scale reading even with the end of the stain is the approximate ethyl mercaptan concentration. If the number o
45、f pump strokes used is different from the number specified by the manufacturer for a particular concentration, a correction must be made as follows: Corr. Ethyl Mercaptan Concentration = Scale reading X Specified Strokes Actual Strokes 8.0 Interpretation 8.1 This test is a direct measure of the conc
46、entration of ethyl mercaptan in propane vapors. It is also an indirect measure of the concentration of ethyl mercaptan in propane liquid. Ethyl mercaptan is very soluble in propane, but has a lower vapor pressure. Therefore, the equilibrium concentration of ethyl mercaptan in the vapor phase is much
47、 lower than its concentration in the liquid phase. The exact ratio of concentrations is dependent on the temperature of the system and the liquid level of the tank. 9.0 Intereferences 9.1 The manufacturers literature should be consulted for possible interfering substances. Most detector tubes will s
48、how all mercaptans present in LP-gas. Hydrogen Sulfide, ammonia and propylene will interfere with the test results. 9.2 The Propane Gas Association of Canada test method for determination of ethyl Mercaptan in propane liquid is Canadian General Standards Board (CGSB) 18.5-2006 “Test for Ethyl Mercap
49、tan Odorant in Propane, Field Method” April 2006. 5 APPENDIX A EVALUATION OF LENGTH OF STAIN DETECTOR TUBES FOR MEASURING ETHYL MERCAPTAN IN PROPANE Sung I. Johnson* and W. J. Hines* SUMMARY Four commercial detector tubes were evaluated for measuring ethyl mercaptan in the liquid and vapor phases of odorized propane. The investigation used a prepared blend of EthSH in propane at a calculated concentration level of 1.32 lbs/ 10,000 gal. The test results showed the liquid phase to contain 1.09 lbs/ 10,000 gas with a precision of about 7% of th
