1、Designation: D 1142 95 (Reapproved 2006)Standard Test Method forWater Vapor Content of Gaseous Fuels byMeasurement of Dew-Point Temperature1This standard is issued under the fixed designation D 1142; the number immediately following the designation indicates the year oforiginal adoption or, in the c
2、ase of revision, 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.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This te
3、st method covers the determination of the watervapor content of gaseous fuels by measurement of the dew-point temperature and the calculation therefrom of the watervapor content.NOTE 1Some gaseous fuels contain vapors of hydrocarbons or othercomponents that easily condense into liquid and sometimes
4、interfere withor mask the water dew point. When this occurs, it is sometimes veryhelpful to supplement the apparatus in Fig. 1 with an optical attachmentthat uniformly illuminates the dewpoint mirror and also magnifies thecondensate on the mirror. With this attachment it is possible, in somecases, t
5、o observe separate condensation points of water vapor, hydrocar-bons, and glycolamines as well as ice points. However, if the dew point ofthe condensable hydrocarbons is higher than the water vapor dew point,when such hydrocarbons are present in large amounts, they may flood themirror and obscure or
6、 wash off the water dew point. Best results indistinguishing multiple component dew points are obtained when they arenot too closely spaced.NOTE 2Condensation of water vapor on the dew-point mirror mayappear as liquid water at temperatures as low as 0 to 10F (18to 23C). At lower temperatures an ice
7、point rather than a water dewpoint likely will be observed. The minimum dew point of any vapor thatcan be observed is limited by the mechanical parts of the equipment.Mirror temperatures as low as 150F (100C) have been measured,using liquid nitrogen as the coolant with a thermocouple attached to the
8、mirror, instead of a thermometer well.1.2 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 li
9、mitations prior to use.2. Terminology2.1 Definitions of Terms Specific to This Standard:2.1.1 saturated water vapor or equilibrium watervaporcontentthe water vapor concentration in a gas mixture that isin equilibrium with a liquid phase of pure water that issaturated with the gas mixture. When a gas
10、 containing watervapor is at the water dew-point temperature, it is said to besaturated at the existing pressure.2.1.2 specific volumeof a gaseous fuel, the volume of thegas in cubic feet per pound.2.1.3 water dew-point temperatureof a gaseous fuel, thetemperature at which the gas is saturated with
11、water vapor atthe existing pressure.3. Significance and Use3.1 Generally, contracts governing the pipeline transmissionof natural gas contain specifications limiting the maximumconcentration of water vapor allowed. Excess water vapor cancause corrosive conditions, degrading pipelines and equipment.I
12、t can also condense and freeze or form methane hydratescausing blockages. Watervapor content also affects the heat-ing value of natural gas, thus influencing the quality of the gas.This test method permits the determination of water content ofnatural gas.4. Apparatus4.1 Any properly constructed dew-
13、point apparatus may beused that satisfies the basic requirements that means must beprovided:4.1.1 To permit a controlled flow of gas to enter and leavethe apparatus while the apparatus is at a temperature at least3F above the dew point of the gas.4.1.2 To cool and control the cooling rate of a porti
14、on(preferably a small portion) of the apparatus, with which theflowing gas comes in contact, to a temperature low enough tocause vapor to condense from the gas.1This test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is the direct responsibility of Subcommittee D03.05 on
15、Determination ofSpecial Constituents of Gaseous Fuels.Current edition approved June 1, 2006. Published June 2006. Originallyapproved in 1950. Last previous edition approved in 2000 as D 1142 95(2000).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,
16、 United States.4.1.3 To observe the deposition of dew on the cold portionof the apparatus.4.1.4 To measure the temperature of the cold portion on theapparatus on which the dew is deposited, and4.1.5 To measure the pressure of the gas within the appara-tus or the deviation from the known existing bar
17、ometricpressure.4.1.6 The apparatus should be constructed so that the “coldspot,” that is, the cold portion of the apparatus on which dewis deposited, is protected from all gases other than the gasunder test. The apparatus may or may not be designed for useunder pressure.4.2 The Bureau of Mines type
18、 of dew-point apparatus2shown in Fig. 1 fulfills the requirements specified in 4.1.Within the range of conditions in Section 1, this apparatus issatisfactory for determining the dew point of gaseous fuels.Briefly, this apparatus consists of a metal chamber into and outof which the test gas is permit
19、ted to flow through control valvesA and D. Gas entering the apparatus through valve A isdeflected by nozzle B towards the cold portion of the apparatus,C. The gas flows across the face of C and out of the apparatusthrough valve D. Part C is a highly polished stainless steel“target mirror,” cooled by
20、 means of a copper cooling rod, F.The mirror, C, is silver-soldered to a nib on the copperthermometer well fitting, I, which is soft-soldered to thecooling rod, F. The thermometer well is integral with thefitting, I. Cooling of rod F is accomplished by vaporizing arefrigerant such as liquid butane,
21、propane, carbon dioxide, orsome other liquefied gas in the chiller, G. The refrigerant isthrottled into the chiller through valve H and passes out at J.The chiller body is made of copper and has brass headers oneither end. The lower header is connected with the upperheader by numerous small holes dr
22、illed in the copper bodythrough which the vaporized refrigerant passes. The chiller isattached to the cooling rod, F, by means of a taper joint. Thetemperature of the target mirror, C, is indicated by a calibratedmercury-in-glass thermometer, K, whose bulb fits snugly intothe thermometer well. Obser
23、vation of the dew deposit is madethrough the pressure-resisting transparent window, E.4.2.1 Note that only the central portion of the stainless steeltarget mirror, C, is thermally bonded to the fitting, I, throughwhich C is cooled. Since stainless steel is a relatively poorthermal conductor, the cen
24、tral portion of the mirror is thusmaintained at a slightly lower temperature than the outerportion, with the result that the dew first appears on the centralportion of the mirror and its detection is aided materially by thecontrast afforded. The arrangement for measuring the tempera-ture of the targ
25、et mirror, C, also should be noted. Thetemperature is read with a thermometer or RTD, K, inserted inthe cooling rod, F, so that the bulb of the temperature2Deaton, W. M., and Frost, E. M., Jr., “Bureau of Mines Apparatus forDetermining the Dew Point of Gases Under Pressure,” Bureau of Mines Report o
26、fInvestigation 3399, May 1938.FIG. 1 Bureau of Mines Dew-Point ApparatusD 1142 95 (2006)2measuring device is entirely within the thermometer well infitting, I. The stud to which the stainless steel mirror issilver-soldered is a part of the base of the thermometer well,and as there is no metallic con
27、tact between the thermometerwell and the cooling tube, other than through its base, thethermometer or RTD indicates the temperature of the mirrorrather than some compromise temperature influenced by thetemperature gradient along the cooling tube as would be thecase if this type of construction were
28、not used. The RTD willinclude suitable electronics and display.4.2.2 Tests with the Bureau of Mines type of dew-pointapparatus are reported2to permit a determination with aprecision (reproducibility) of 60.2F (60.1C) and with anaccuracy of 60.2F (60.1C) when the dew-point tempera-tures range from ro
29、om temperature to a temperature of 32F(0C). It is estimated that water dew points may be determinedwith an accuracy of 60.5F (0.3C) when they are below 32F(0C) and not lower than 0F (17.8C), provided ice crystalsdo not form during the determination.5. Procedure5.1 General ConsiderationsTake the spec
30、imen so as to berepresentative of the gas at the source. Do not take at a pointwhere isolation would permit condensate to collect or wouldotherwise allow a vapor content to exist that is not inequilibrium with the main stream or supply of gas, such as thesorption or desorption of vapors from the sam
31、pling line or fromdeposits therein. The temperature of the pipelines leading thespecimen directly from the gas source to the dew-pointapparatus, and also the temperature of the apparatus, shall be atleast 3F (1.7C) higher than the observed dew point. Thedetermination may be made at any pressure, but
32、 the gaspressure within the dew-point apparatus must be known withan accuracy appropriate to the accuracy requirements of thetest. The pressure may be read on a calibrated bourdon-typepressure gage; for very low pressures or more accuratemeasurements, a mercury-filled manometer or a dead-weightgage
33、should be used.5.2 Detailed Procedure for Operation of Bureau of MinesDew-Point ApparatusIntroduce the gas specimen throughvalve A (Fig. 1), opening this valve wide if the test is to bemade under full source pressure (Note 3), and controlling theflow by the small outlet valve, D. The rate of flow is
34、 not criticalbut should not be so great that there is a measurable orobjectionable drop in pressure through the connecting lines anddew-point apparatus. A flow of 0.05 to 0.5 ft3/min (1.4 to 14L/min) (measured at atmospheric pressure) usually will besatisfactory. With liquefied refrigerant gas piped
35、 to the chillerthrottle valve, H, “crack” the valve momentarily, allowing therefrigerant to vaporize in the chiller to produce suitablelowering in temperature of the chiller tube, F, and targetmirror, C, as indicated by the thermometer, K. The rate ofcooling may be as rapid as desired in making a pr
36、eliminarytest. After estimating the dew-point temperature, either by apreliminary test or from other knowledge, control the coolingor warming rate so that it does not exceed 1F/min (0.5C/min)when this temperature is approached. For accurate results, thecooling and warming rates should approximate is
37、othermalconditions as nearly as possible. The most satisfactory methodis to cool or warm the target mirror stepwise. Steps of about0.2F (0.1C) allow equilibrium conditions to be approachedclosely and favor an accurate determination. When dew hasbeen deposited, allow the target mirror to warm up at a
38、 ratecomparable to the recommended rate of cooling. The normalwarming rate usually will be faster than desired. To reduce therate, “crack” valve H momentarily at intervals to supplycooling to the cooling tube, F. Repeat the cooling and warmingcycles several times. The arithmetic average of the tempe
39、ra-tures at which dew is observed to appear and disappear isconsidered to be the observed dew point.NOTE 3If the watervapor content is to be calculated as described in6.2, the gas specimen should be throttled at the inlet valve, A, to a pressurewithin the apparatus approximately equal to atmospheric
40、 pressure. Theoutlet valve may be left wide open or restricted, as desired. The pressureexisting within the apparatus must, however, be known to the requiredaccuracy.6. Calculation6.1 If an acceptable chart showing the variation of water-vapor content with saturation or water dew-point temperatureso
41、ver a suitable range of pressures for the gas being tested isavailable, the water-vapor content may be read directly, usingthe observed water dew-point temperature and the pressure atwhich the determination was made.6.2 If such a chart is not available, the watervapor contentof the gas may be calcul
42、ated from the water dew-pointtemperature and the pressure at which it was determined (seeNote 3), as follows:FIG. 2 Equilibrium Water Vapor Content of Natural GasesD 1142 95 (2006)3W 5 w 3 1063 Pb/P 3 T/Tb! (1)where:W = lb of water/million ft3of gaseous mixture at pressurePband temperature Tb;w = we
43、ight of saturated water vapor, lb/ft3, at the waterdew-point temperature, that is, the reciprocal of thespecific volume of saturated vapor (see Table 1);Pb= pressure-base of gas measurement, psia;P = pressure at which the water dew point of gas wasdetermined, psia;t = observed water dew-point temper
44、ature, F;T = Rankine (absolute Fahrenheit scale) water dew point,t + 460, at pressure P; andTb= base temperature of gas measurement, tb+ 460.NOTE 4Example 1:Given: Water dew point = 37F at 15.0-psia pressure.What is the watervapor content million ft3of gas (gas measurementbase of 60F and 14.7-psia p
45、ressure)?From Table 1 the specific volume of saturated water at 37F is 2731.9ft3/lb, from which:w = (1/2731.9) = 0.000 366 0 lb/ft3andW = 0.000 366 0 3 1063 (14.7/15.0) 3 (460 + 37)/(460 + 60)= 342.8 lb/million ft3Example 2:Given: Water dew point = 5F at 14.4 psia.From Table 2, the specific volume o
46、f saturated water vapor with respectto ice at 5F is 11 550 ft3/lb from which Wice, 5F= 0.000 086 6, but theobserved water dew point was in equilibrium with subcooled liquid waterat 5F. From Table 2 (data from International Critical Tables3), the vaporpressures of subcooled liquid water and of ice at
47、 5F (15C) are 1.436mm and 1.241 mm Hg, respectively.Since the vapor pressure of subcooled liquid water is greater than ice atthe same temperature, the weight per cubic foot of water vapor inequilibrium with liquid water will be proportionately larger than the valuecalculated from the specific volume
48、 read from the table, which is forequilibrium with ice.Hence,Wliq., 5F= Wice 5F3 (1.436/1.241)= 0.000 086 6 3 1.157= 0.000 100 2 andW = 0.000 100 2 3 1063 (14.7/14.4) 3 (460 + 5)/460 + 60)= 91.5 lb/million ft36.3 A correlation of the available data on the equilibriumwater content of natural gases ha
49、s been reported by Bukacek.4This correlation is believed to be accurate enough for therequirements of the gaseous fuels industry, except for unusualsituations where the dew point is measured at conditions closeto the critical temperature of the gas. The correlation is amodified form of Raoults law having the following form:W 5 A/P! 1 B (2)where:W = watervapor content, lb/million ft3;P = total pressure, psia;A = a constant proportional to the vapor pressure of water;andB = a constant depending on temperature and gas comp
