1、Designation: D 3284 05Standard Practice forCombustible Gases in the Gas Space of ElectricalApparatus Using Portable Meters1This standard is issued under the fixed designation D 3284; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This field practice covers the detection and estimation ofcombustible gases in the gas blanket above the
3、oil or in gasdetector relays in transformers using portable instruments. It isapplicable only with transformers using mineral oil as thedielectric fluid. Gases dissolved in the oil and noncombustiblegases are not determined. A method of calibrating the instru-ments with a known gas mixture is includ
4、ed.1.2 This practice affords a semi-quantitative estimate of thetotal combustible gases present in a gas mixture. If a moreaccurate determination of the total amount of combustiblegases or a quantitative determination of the individual compo-nents is desired, use a laboratory analytical method, such
5、 asTest Method D 3612.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior
6、to use. Specific precau-tionary statements are given in Section 7.2. Referenced Documents2.1 ASTM Standards:2D 3612 Test Method for Analysis of Gases Dissolved inElectrical Insulating Oil by Gas Chromatography2.2 IEEE Standard:3C57.104 Guide for the Interpretation of Gases Generated inOil-Immersed T
7、ransformers3. Summary of Practice3.1 A sample of gas is diluted to a fixed ratio with air andintroduced into the meter at a pressure of approximately oneatmosphere. Any combustible gases present are catalyticallyoxidized on the surface of a sensor which is an element of aWheatstone bridge. When comb
8、ustible gases oxidize on thesurface, they increase the temperature of the element, whichchanges its resistance and upsets the balance of the bridge.3.2 The change in the resistance of the indicating elementsin the bridge circuit is indicated on a meter, which is usuallycalibrated to read in percent
9、total combustible gas.4. Significance and Use4.1 Arcing, partial discharge, and localized overheating inthe insulation system of transformers result in chemical de-composition of the insulating oil and other insulating materials.This may generate various gases, some of which are combus-tible. Typica
10、lly, gases are generated in the oil and thenpartitioned into the gas space according to their individualsolubilities. Gases which are highly oil-soluble, such as acety-lene, may not be in significant quantities in the gas space untilan incipient fault has progressed to a very serious condition orfai
11、lure of the transformer. Gases such as carbon monoxide andhydrogen which have low solubilities in oil can make up alarge fraction of the combustible gases in the gas space.Detection of these gases is frequently the first availableindication of a malfunction. Portable combustible gas metersare a conv
12、enient means of detecting the presence of generatedgases.4.2 Normal operation of a transformer may result in theformation of some combustible gases. The detection of anincipient fault by this method involves an evaluation of theamount of combustible gases present, the rate of generation ofthese gase
13、s, and their rate of escape from the transformer. Referto IEEE C57.104 for detailed information on interpretation ofgassing in transformers.5. Interferences5.1 In this practice it is essential that sufficient oxygen bepresent in the gas mixture to oxidize the combustible gases.Since the gas blanket
14、in a transformer is usually an inert gas, itis necessary to dilute the sample gas with a known amount ofair. This is usually accomplished by either introducing air andthe sample gas into the instrument in known ratios throughfixed orifices, or by mixing known quantities of air and testspecimen exter
15、nally by displacement over water before intro-duction into the instrument. The working range of these1This practice is under the jurisdiction of ASTM Committee D27 on ElectricalInsulating Liquids and Gases and is the direct responsibility of SubcommitteeD27.03 on Analytical Tests.Current edition app
16、roved May 1, 2005. Published June 2005. Originallyapproved in 1974. Last previous edition approved in 1999 as D 3284 99.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer
17、 to the standards Document Summary page onthe ASTM website.3Available from IEEE, 445 Hoes Lane, Piscataway, NJ 08855-1331.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.instruments is between the low limit of sensitivity and aboutth
18、e lower explosive limit. They generally read off-scale at thehigh end between the lower explosive limit and the upperexplosive limit and may indicate zero when the combustiblegas content is above the upper explosive limit.5.2 Contamination of the sensor can seriously impair thesensitivity and respon
19、se of the meter. This loss of responsedoes not affect the normal balancing of the bridge circuit, andis not detected by an electrical check of the instrument. It is,therefore, essential to check and calibrate the instrumentfrequently against a standard combustible gas mixture.5.3 Since each combusti
20、ble gas has its own individualinstrument response, the values determined by an instrumentare dependent upon the kind of combustible gas used forcalibration. In order to compare results, a mixture of methanein nitrogen has been selected as the standard calibrating gas.NOTE 1Sensor fouling generally i
21、mpairs the instruments response tohydrocarbon gases, including methane, but has little effect on response tohydrogen. A comparison of the analyses of known mixtures of methane innitrogen and hydrogen in nitrogen can therefore be used to indicate thedegree of sensor fouling.6. Apparatus and Materials
22、6.1 Portable Combustible-Gas Meter, consisting of thefollowing components:6.1.1 Sensor, incorporated in a Wheatstone bridge circuit,complete with a suitable battery and controls for balancing andvoltage adjustment.6.1.2 Meter, preferably calibrated to read directly in percenttotal combustible gases.
23、6.1.3 Gas-Mixing Device, for mixing air and sample gas atfixed ratios.6.2 Standard Reference GasThe standard reference gasshall be a mixture of methane and nitrogen, with the methanecontent usually between 1 and 3 %. The methane contentshould be known to the nearest 0.1 %.7. Hazards7.1 Safety Hazard
24、s:7.1.1 Gases generated in transformers can be explosive.Therefore, the combustible gas meter should be equipped witha flame arrester.7.1.2 Dilute the gas sample with air to avoid the possibilityof explosion. Never use pure oxygen for this purpose.7.1.3 Observe normal safety precautions when attachi
25、ngportable meters to transformers.7.1.4 Verify that the gas space being sampled is at a positivepressure before attempting to draw a sample.8. Calibration8.1 Prepare the instrument for operation and make zerobalance and voltage adjustments in accordance with the in-structions of the instrument manuf
26、acturer and Section 9.8.2 Mix the standard reference gas with air, and introduce itinto the meter in exactly the same manner as used for thesample gas from the transformer.8.3 Turn the calibration adjustment so the meter indicatesthe known methane content of the reference standard when theair and re
27、ference gas mixture is being tested. Purge the meterwith air to remove traces of reference gas and proceed inaccordance with Section 9.9. Procedure9.1 Prepare the instrument for operation in accordance withthe instrument manufacturers instructions. This may include awarm-up period to allow the batte
28、ry voltage to stabilize, avoltage adjustment, and an instrument balance or zero adjust-ment.9.2 Calibrate the instrument against the standard methanemixture in accordance with Section 8. The frequency ofcalibration required for individual instruments can be deter-mined by experience. Low readings, d
29、ue to sensor contamina-tion, can only be detected by a check with a gas of knowncomposition. In any case of doubt, the instrument should berecalibrated.9.3 If the instrument has a range switch of more than oneair-dilution orifice, set it for maximum range or dilution toavoid damage to the instrument
30、. Purge the instrument so it isfree of gas from previous tests.9.4 Introduce the mixture of air and sample in accordancewith the instructions of the instrument manufacturer. The flowrate should be moderate and reproducible. The pressure of gasin the oxidation chamber should be atmospheric, and thete
31、mperature should be above the dew point of the gas beingtested to prevent condensation of moisture on the oxidizingelements.9.5 Read the percent combustible gas indicated on themeter. If the reading is low, change to a lower range or changethe orifice to one admitting less dilution air. If the instr
32、umentgoes momentarily off-scale on the high side and then returns tozero, the combustible gas content is probably above the upperexplosive limit. This can be verified by additional dilution ofthe sample gas with air to bring the combustible-gas contentwithin the range of the instrument.9.6 Recheck t
33、he zero balance and voltage adjustments of theinstrument.9.7 Record the reading as percent total combustible gases(% TCG). Report digital meter readings to two decimal places,and analog meter readings to the nearest small scale division.9.8 Purge the instrument to remove sample gas, in order tominim
34、ize corrosion and element contamination.10. Precision and Bias10.1 PrecisionIt is not possible to specify the precision ofthe procedure for measuring combustible gases using portablemeters because this is a field method in which the electricalapparatus is the sample container, making it impractical
35、toperform interlaboratory testing.10.2 BiasNo information can be presented on the bias ofthis procedure because this is a field method in which theelectrical apparatus is the sample container, making the use ofa reference material impractical.11. Keywords11.1 combustible gases; gas space; gases; por
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