Natural gas is a vital energy source used globally for various applications, including power generation, heating, and industrial processes. Its abundance and relatively low carbon emissions have made it a popular choice for meeting energy needs. However, to ensure the safe and efficient use of natural gas, it is essential to measure and control any impurities present in the gas that can affect the intended end use. Moisture, or water vapor, is one of the most common impurities in natural gas. In this blog, we will delve into the significance of measuring moisture in natural gas, the impact of water vapor on the properties of natural gas, the technologies used for moisture measurement, and best practices for installation and maintenance to ensure accurate readings and overall system efficiency.
The Impact of Water in Natural Gas
Water is an unavoidable impurity in natural gas as it is naturally occurring and will be present in the gas no matter where or how the gas is extracted from the Earth. Water vapor in natural gas reduces the heating value of the gas and decreases the energy content of the gas. Depending on the end use of the gas, this can have adverse impacts for the applications. Moreover, water can also act as a solvent for other impurities present in natural gas, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). When combined, these impurities can form corrosive acids that can damage gas pipelines, the mechanical infrastructure and equipment.
One of the most concerning consequences of water in natural gas is the formation of natural gas hydrates. These hydrates are solid or semi-solid masses that occur when methane molecules, the primary component of natural gas, combine with the hydrogen-bonded water molecules under specific temperature and pressure conditions. These hydrates can clog gas pipelines, slowing or completely blocking gas flow, leading to costly shutdowns and disruptions in energy supply.
To transport natural gas safely and efficiently, it must be effectively processed and maintained to achieve pipeline quality. This process involves the removal of water and other impurities, ensuring that the gas meets specific standards and regulations.
Technologies for Moisture Measurement
Accurate moisture measurement is vital for ensuring the quality and safety of natural gas. Various technologies are available for this purpose, each with its advantages and limitations. Some of the most used technologies include:
- Quartz Crystal Microbalance (QCM): QCM sensors utilize a thin film of hygroscopic polymer on a quartz crystal. Water vapor adsorption by the polymer causes a change in the crystal's vibration frequency, enabling precise moisture concentration measurement. The QCM system offers an accuracy of up to 10% reading and is commonly used in applications with moisture levels ranging from tens to hundreds of parts per million (ppm).
- Tunable Diode Laser (TDL): TDL technology utilizes a laser diode emitting light tuned over an extremely narrow wavelength where moisture molecules can absorb the light energy. The energy absorbed is proportional to the water concentration, providing fast and accurate moisture measurements. TDL analyzers are versatile and can measure multiple gases, including moisture, CO2, and H2S.
- Aluminum Oxide: This technology uses an impedance or capacitance sensor to measure moisture. It offers a lower cost per measurement compared to QCM and TDL but comes with inherent limitations in accuracy and reliability. As a result, QCM and TDL are preferred for most applications requiring precise moisture measurements.
Best Practices for Installation and Maintenance
Proper installation and maintenance are crucial to obtaining accurate moisture measurements and ensuring the long-term reliability of the analyzer. Implementing best practices can help optimize the moisture measurement system and minimize potential issues. Some key best practices include:
- Short Sample Lines: Keeping the sample line as short as possible reduces surface area, minimizing the potential for moisture molecules to “stick” or adsorb on the surface, which slows response time of the measurement.
- Smaller Diameter Sample Lines: Using smaller diameter sample lines reduces the internal volume, leading to faster response times and reduced moisture accumulation.
- Heated Sample Lines: Heated sample lines prevent moisture sticking to the internal surfaces and minimize the impact of ambient temperature variations, ensuring more consistent readings.
- Eliminate Dead Legs: Dead legs, which are sections of the sample system where moisture can accumulate and be released during changes in pressure or flow, should be avoided to maintain accurate measurements.
- Use Bypass Flow: Installing a bypass flow at the analyzer’s inlet can increase the speed of response by increasing the overall sample gas consumption.
- Plug Analyzer Ports During Maintenance: When performing maintenance that involves disconnecting the sample system, it is essential to plug all wetted surface parts, including vents, to prevent atmospheric exposure and moisture intrusion.
- Purge Sample System Before Reconnecting Analyzer: Thoroughly purging the sample system before reconnecting the analyzer reduces the risk of exposing the dry analyzer to atmospheric moisture and other contaminants that may be present in the sampling system.
Conclusion
Measurement of moisture in natural gas is critical for ensuring safety, efficiency, and compliance with quality standards. Various technologies, such as QCM and TDL, offer reliable and accurate measurements. By following best practices for installation and maintenance, gas operators can optimize their moisture measurement systems and ensure the smooth operation of natural gas processes. Proper moisture management not only enhances the quality of natural gas but also helps prevent costly disruptions and equipment damage caused by water impurities. As the demand for natural gas continues to grow, accurate moisture measurement will remain a crucial aspect of the energy industry's safety and sustainability efforts.