Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D1142-95 – Standard Test Method Technical Guide
ASTM D1142-95 (Reapproved 2021) is a standard test method that establishes a procedure for determining the water vapor content of gaseous fuels by measuring the dew-point temperature. This method is heavily relied upon in the natural gas industry to ensure that pipeline gas meets contractual quality specifications regarding moisture content.
⚗️ Principle and Scope of the Test Method
This test method covers the direct measurement of the water vapor dew-point temperature in a gas stream. When a gas containing water vapor is at the water dew-point temperature, it is said to be saturated at the existing pressure. The water vapor content is subsequently calculated from this measured temperature.
✅ Key Significance: Contracts governing the pipeline transmission of natural gas contain specifications limiting the maximum concentration of water vapor. Excess water vapor can cause corrosive conditions, degrade pipelines and equipment, and condense to form blockages, ice, or methane hydrates. It also affects the heating value of the gas.
The standard also acknowledges that some gaseous fuels contain vapors of hydrocarbons or other components (e.g., glycolamines) that can easily condense into liquids and interfere with or mask the water dew point. An optical attachment is recommended to uniformly illuminate the dew-point mirror and magnify the condensate to help distinguish between the different condensation points.
🛠️ Apparatus, Coolants, and Temperature Limits
The core of the apparatus is a dew-point mirror that is cooled until condensation forms. The temperature at which the first trace of condensate appears is the dew-point temperature. The choice of coolant dictates the minimum achievable mirror temperature.
| 🟦 Condensate Phase | 📏 Temperature Range | 🎯 Observation Notes |
|---|---|---|
| Liquid Water | 32°F to -10°F (0°C to -23°C) | Standard water dew point observation range. |
| Ice Point | Below -10°F (-23°C) | Condensation of water vapor appears as ice rather than liquid water. |
| Hydrocarbons | Variable | Can flood the mirror; an optical attachment is required for differentiation. |
| Minimum Achievable | -150°F (-100°C) | Uses liquid nitrogen as coolant with a thermocouple attached directly to the mirror. |
Standard mechanical refrigeration units have practical limits, but for extremely low dew points, liquid nitrogen is utilized. The standard notes that mirror temperatures as low as -150°F (-100°C) have been measured using this method, substituting a thermometer well with a directly attached thermocouple.
⚙️ Managing Hydrocarbon Interference
Interference from other condensable components is a primary challenge in this test method. The standard provides crucial guidance on identifying and mitigating these effects.
⚠️ Flooding Risk: If the dew point of condensable hydrocarbons is higher than the water vapor dew point, and the hydrocarbons are present in large amounts, they may flood the mirror. This can obscure or entirely wash off the water dew point, rendering the standard observation method ineffective.
💡 Optimization Technique: Best results in distinguishing multiple component dew points (e.g., water, hydrocarbons, glycolamines) are obtained when their condensation temperatures are not too closely spaced. Uniform illumination of the mirror significantly improves the ability to observe separate condensation points.
An optical attachment capable of magnifying the mirror condensate allows the operator to observe separate condensation points for water vapor, hydrocarbons, and glycolamines, as well as ice points. This differentiation is critical for accurately reporting the water vapor content of the fuel.
| 🔍 Interference Type | ⚡ Operational Impact | 📐 Recommended Solution |
|---|---|---|
| Hydrocarbon Flooding | Obscures water dew point on mirror. | Optical magnification and careful cooling rate control. |
| Multiple Component Condensation | Difficult to distinguish which condensate is water. | Uniform mirror illumination and optical attachment. |
| Ice Formation Misidentification | Incorrectly reported as water dew point. | Recognizing that ice points occur below -10°F (-23°C). |
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D1142-95?
It is a standard test method for determining the water vapor content of gaseous fuels by measuring the dew-point temperature. This calculation is essential for verifying compliance with pipeline specifications regarding moisture.
💡 How does the standard handle interference from hydrocarbon condensates?
The standard recommends the use of an optical attachment that uniformly illuminates and magnifies the condensate on the dew-point mirror. This allows the operator to visually distinguish separate condensation points for water versus hydrocarbons or glycolamines.
⚡ What is the lowest temperature achievable with this method?
While mechanical parts of standard equipment limit the range, mirror temperatures as low as -150°F (-100°C) have been measured. This requires the use of liquid nitrogen as a coolant and a thermocouple attached directly to the mirror.
📌 Why is controlling water vapor in natural gas so critical?
Excess water vapor can cause corrosive conditions that degrade pipelines and equipment. It can also condense and freeze or form methane hydrates, leading to blockages, and it negatively impacts the heating value of the gas.