ASTM D1142-95(2000)
(Test Method)Standard Test Method for Water Vapor Content of Gaseous Fuels by Measurement of Dew-Point Temperature
Standard Test Method for Water Vapor Content of Gaseous Fuels by Measurement of Dew-Point Temperature
SCOPE
1.1 This test method covers the determination of the water vapor content of gaseous fuels by measurement of the dew-point temperature and the calculation therefrom of the water vapor content.
Note 1--Some gaseous fuels contain vapors of hydrocarbons or other components that easily condense into liquid and sometimes interfere with or mask the water dew point. When this occurs, it is sometimes very helpful to supplement the apparatus in with an optical attachment that uniformly illuminates the dew-point mirror and also magnifies the condensate on the mirror. With this attachment it is possible, in some cases, to observe separate condensation points of water vapor, hydrocarbons, and glycolamines as well as ice points. However, if the dew point of the condensable hydrocarbons is higher than the water vapor dew point, when such hydrocarbons are present in large amounts, they may flood the mirror and obscure or wash off the water dew point. Best results in distinguishing multiple component dew points are obtained when they are not too closely spaced.
Note 2--Condensation of water vapor on the dew-point mirror may appear as liquid water at temperatures as low as 0 to -10oF (-18 to -23oC). At lower temperatures an ice point rather than a water dew point likely will be observed. The minimum dew point of any vapor that can be observed is limited by the mechanical parts of the equipment. Mirror temperatures as low as -150oF (-100oC) have been measured, using liquid nitrogen as the coolant with a thermocouple attached to the mirror, instead of a thermometer well.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 1142 – 95 (Reapproved 2000)
Standard Test Method for
Water Vapor Content of Gaseous Fuels by
Measurement of Dew-Point Temperature
This standard is issued under the fixed designation D1142; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript 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. Scope 2.1.1 saturated water vapor or equilibrium water–vapor
content—thewatervaporconcentrationinagasmixturethatis
1.1 This test method covers the determination of the water
in equilibrium with a liquid phase of pure water that is
vapor content of gaseous fuels by measurement of the dew-
saturated with the gas mixture. When a gas containing water
point temperature and the calculation therefrom of the water
vapor is at the water dew-point temperature, it is said to be
vapor content.
saturated at the existing pressure.
NOTE 1—Some gaseous fuels contain vapors of hydrocarbons or other
2.1.2 specific volume—of a gaseous fuel, the volume of the
components that easily condense into liquid and sometimes interfere with
gas in cubic feet per pound.
or mask the water dew point. When this occurs, it is sometimes very
2 2.1.3 water dew-point temperature—of a gaseous fuel, the
helpful to supplement the apparatus in Fig. 1 with an optical attachment
temperature at which the gas is saturated with water vapor at
that uniformly illuminates the dew–point mirror and also magnifies the
condensate on the mirror. With this attachment it is possible, in some the existing pressure.
cases, to observe separate condensation points of water vapor, hydrocar-
3. Significance and Use
bons,andglycolaminesaswellasicepoints.However,ifthedewpointof
the condensable hydrocarbons is higher than the water vapor dew point,
3.1 Generally,contractsgoverningthepipelinetransmission
when such hydrocarbons are present in large amounts, they may flood the
of natural gas contain specifications limiting the maximum
mirror and obscure or wash off the water dew point. Best results in
concentration of water vapor allowed. Excess water vapor can
distinguishing multiple component dew points are obtained when they are
causecorrosiveconditions,degradingpipelinesandequipment.
not too closely spaced.
NOTE 2—Condensation of water vapor on the dew-point mirror may It can also condense and freeze or form methane hydrates
appear as liquid water at temperatures as low as 0 to−10°F (−18
causing blockages. Water–vapor content also affects the heat-
to−23°C). At lower temperatures an ice point rather than a water dew
ingvalueofnaturalgas,thusinfluencingthequalityofthegas.
point likely will be observed. The minimum dew point of any vapor that
This test method permits the determination of water content of
can be observed is limited by the mechanical parts of the equipment.
natural gas.
Mirror temperatures as low as−150°F (−100°C) have been measured,
using liquid nitrogen as the coolant with a thermocouple attached to the
4. Apparatus
mirror, instead of a thermometer well.
4.1 Any properly constructed dew-point apparatus may be
1.2 This standard does not purport to address all of the
used that satisfies the basic requirements that means must be
safety concerns, if any, associated with its use. It is the
provided:
responsibility of the user of this standard to establish appro-
4.1.1 To permit a controlled flow of gas to enter and leave
priate safety and health practices and determine the applica-
the apparatus while the apparatus is at a temperature at least
bility of regulatory limitations prior to use.
3°F above the dew point of the gas.
4.1.2 To cool and control the cooling rate of a portion
2. Terminology
(preferably a small portion) of the apparatus, with which the
2.1 Definitions of Terms Specific to This Standard:
flowing gas comes in contact, to a temperature low enough to
cause vapor to condense from the gas.
4.1.3 To observe the deposition of dew on the cold portion
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous
Fuels and is the direct responsibility of Subcommittee D03.05 on Determination of
of the apparatus.
Special Constituents of Gaseous Fuels.
4.1.4 To measure the temperature of the cold portion on the
Current edition approved Feb. 15, 1995. Published April 1995. Originally
apparatus on which the dew is deposited, and
published as D1142–50. Last previous edition D1142–90.
Several pieces of apparatus for this purpose are commercially available.
Information concerning this apparatus is available from ASTM Headquarters.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 1142 – 95 (2000)
FIG. 1 Bureau of Mines Dew-Point Apparatus
4.1.5 To measure the pressure of the gas within the appara- refrigerant such as liquid butane, propane, carbon dioxide, or
tus or the deviation from the known existing barometric
some other liquefied gas in the chiller, G. The refrigerant is
pressure. throttled into the chiller through valve H and passes out at J.
4.1.6 The apparatus should be constructed so that the “cold
The chiller body is made of copper and has brass headers on
spot,” that is, the cold portion of the apparatus on which dew
either end. The lower header is connected with the upper
is deposited, is protected from all gases other than the gas
header by numerous small holes drilled in the copper body
under test. The apparatus may or may not be designed for use
through which the vaporized refrigerant passes. The chiller is
under pressure.
attached to the cooling rod, F, by means of a taper joint. The
4.2 The Bureau of Mines type of dew-point apparatus
temperature of the target mirror, C, is indicated by a calibrated
shown in Fig. 1 fulfills the requirements specified in 4.1.
mercury-in-glass thermometer, K, whose bulb fits snugly into
Within the range of conditions in Section 1, this apparatus is
the thermometer well. Observation of the dew deposit is made
satisfactory for determining the dew point of gaseous fuels.
through the pressure-resisting transparent window, E.
Briefly, this apparatus consists of a metal chamber into and out
4.2.1 Note that only the central portion of the stainless steel
ofwhichthetestgasispermittedtoflowthroughcontrolvalves
target mirror, C, is thermally bonded to the fitting, I, through
A and D. Gas entering the apparatus through valve A is
which C is cooled. Since stainless steel is a relatively poor
deflectedbynozzleBtowardsthecoldportionoftheapparatus,
thermal conductor, the central portion of the mirror is thus
C. The gas flows across the face of C and out of the apparatus
maintained at a slightly lower temperature than the outer
through valve D. Part C is a highly polished stainless steel
portion, with the result that the dew first appears on the central
“target mirror,” cooled by means of a copper cooling rod, F.
portionofthemirroranditsdetectionisaidedmateriallybythe
The mirror, C, is silver-soldered to a nib on the copper
contrast afforded.The arrangement for measuring the tempera-
thermometer well fitting, I, which is soft-soldered to the
ture of the target mirror, C, also should be noted. The
cooling rod, F. The thermometer well is integral with the
temperature is read with a thermometer or RTD, K, inserted in
fitting, I. Cooling of rod F is accomplished by vaporizing a
the cooling rod, F, so that the bulb of the temperature
measuring device is entirely within the thermometer well in
fitting, I. The stud to which the stainless steel mirror is
Deaton, W. M., and Frost, E. M., Jr., “Bureau of Mines Apparatus for
silver-soldered is a part of the base of the thermometer well,
Determining the Dew Point of Gases Under Pressure,” Bureau of Mines Report of
Investigation 3399, May 1938. and as there is no metallic contact between the thermometer
D 1142 – 95 (2000)
well and the cooling tube, other than through its base, the conditions as nearly as possible. The most satisfactory method
thermometer or RTD indicates the temperature of the mirror is to cool or warm the target mirror stepwise. Steps of about
rather than some compromise temperature influenced by the 0.2°F (0.1°C) allow equilibrium conditions to be approached
temperature gradient along the cooling tube as would be the closely and favor an accurate determination. When dew has
case if this type of construction were not used. The RTD will been deposited, allow the target mirror to warm up at a rate
include suitable electronics and display. comparable to the recommended rate of cooling. The normal
4.2.2 Tests with the Bureau of Mines type of dew-point warming rate usually will be faster than desired. To reduce the
apparatus are reported to permit a determination with a rate, “crack” valve H momentarily at intervals to supply
precision (reproducibility) of 60.2°F (60.1°C) and with an coolingtothecoolingtube, F.Repeatthecoolingandwarming
accuracy of 60.2°F (60.1°C) when the dew-point tempera- cycles several times. The arithmetic average of the tempera-
tures range from room temperature to a temperature of 32°F tures at which dew is observed to appear and disappear is
(0°C). It is estimated that water dew points may be determined considered to be the observed dew point.
with an accuracy of 60.5°F (0.3°C) when they are below 32°F
NOTE 3—If the water–vapor content is to be calculated as described in
(0°C) and not lower than 0°F (−17.8°C), provided ice crystals
6.2,thegasspecimenshouldbethrottledattheinletvalve, A,toapressure
do not form during the determination.
within the apparatus approximately equal to atmospheric pressure. The
outlet valve may be left wide open or restricted, as desired. The pressure
5. Procedure
existing within the apparatus must, however, be known to the required
accuracy.
5.1 General Considerations—Takethespecimensoastobe
representative of the gas at the source. Do not take at a point
6. Calculation
where isolation would permit condensate to collect or would
6.1 If an acceptable chart showing the variation of water-
otherwise allow a vapor content to exist that is not in
vapor content with saturation or water dew-point temperatures
equilibrium with the main stream or supply of gas, such as the
over a suitable range of pressures for the gas being tested is
sorptionordesorptionofvaporsfromthesamplinglineorfrom
available, the water-vapor content may be read directly, using
deposits therein. The temperature of the pipelines leading the
the observed water dew-point temperature and the pressure at
specimen directly from the gas source to the dew-point
which the determination was made.
apparatus,andalsothetemperatureoftheapparatus,shallbeat
6.2 If such a chart is not available, the water–vapor content
least 3°F (1.7°C) higher than the observed dew point. The
of the gas may be calculated from the water dew-point
determination may be made at any pressure, but the gas
temperature and the pressure at which it was determined (see
pressure within the dew-point apparatus must be known with
Note 3), as follows:
an accuracy appropriate to the accuracy requirements of the
test. The pressure may be read on a calibrated bourdon-type
pressure gage; for very low pressures or more accurate
measurements, a mercury-filled manometer or a dead-weight
gage should be used.
5.2 Detailed Procedure for Operation of Bureau of Mines
Dew-Point Apparatus—Introduce the gas specimen through
valve A (Fig. 1), opening this valve wide if the test is to be
made under full source pressure (Note 3), and controlling the
flowbythesmalloutletvalve, D.Therateofflowisnotcritical
but should not be so great that there is a measurable or
objectionabledropinpressurethroughtheconnectinglinesand
dew-point apparatus. A flow of 0.05 to 0.5 ft /min (1.4 to 14
L/min) (measured at atmospheric pressure) usually will be
satisfactory. With liquefied refrigerant gas piped to the chiller
throttle valve, H, “crack” the valve momentarily, allowing the
refrigerant to vaporize in the chiller to produce suitable
lowering in temperature of the chiller tube, F, and target
mirror, C, as indicated by the thermometer, K. The rate of
cooling may be as rapid as desired in making a preliminary
test. After estimating the dew-point temperature, either by a
preliminary test or from other knowledge, control the cooling
orwarmingratesothatitdoesnotexceed1°F/min(0.5°C/min)
when this temperature is approached. For accurate results, the
cooling and warming rates should approximate isothermal FIG. 2 Equilibrium Water Vapor Content of Natural Gases
D 1142 – 95 (2000)
6.3 A correlation of the available data on the equilibrium
W 5 w 310 3 ~P /P 3 ~T/T !! (1)
b b
water content of natural gases has been reported by Bukacek.
where:
This correlation is believed to be accurate enough for the
W = lb of water/million ft of gaseous mixture at pressure
requirements of the gaseous fuels industry, except for unusual
P and temperature T ;
b b
situations where the dew point is measured at conditions close
w = weight of saturated water vapor, lb/ft , at the water
to the critical temperature of the gas. The correlation is a
dew-point temperature, that is, the reciprocal of the
modified form of Raoult’s law having the following form:
specific volume of saturated vapor (see Table 1);
W 5 ~A/P! 1 B (2)
P = pressure-base of gas measurement, psia;
b
P = pressure at which the water dew point of gas was
where:
determined, psia; 3
W = water–vapor content, lb/million ft ;
t = observed water dew-point temperature, °F;
P = total pressure, psia;
T = Rankine (absolute Fahrenheit scale) water dew point,
A = aconstantproportionaltothevaporpressureofwater;
t +460, at pressure P; and
and
T = base temperature of gas measurement, t +460.
b b
B = a constant depending on temperature and gas compo-
sition.
NOTE 4—Example 1:
NOTE 5—Values of B were computed from available data on methane,
Given: Water dew point=37°F at 15.0-psia pressure.
methane-ethane mixtures, and natural gases.
What is the water–vapor content million ft of gas (gas measurement
6.3.1 Table2listsvaluesoftheconstants Aand Bfornatural
base of 60°F and 14.7-psia pressure)?
From Table 1 the specific volume of saturated water at 37°F is 2731.9
gases in the temperature range from−40 to 460°F (−40 to
ft /lb, from which:
238°C).
w =(1/2731.9)=0.0003660 lb/ft
6.3.2 Tables 3-5 list values of water–vapor content
and
from−40to250°F(−40°to121°C)andfrom14.7to5000psia
W =0.0003660 310 3(14.7/15.0) 3[(460+37)/(460+60)]
(101 to 34475 kPa), covering the r
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