The importance of moisture analysis in pipelines and gas transportation

Introduction

Water is one of the most common substances on Earth, covering roughly 75% of the planet’s surface as a liquid and being the fourth most prevalent component of air as a gas. Due to its prevalence and impact on operations, moisture is the most commonly measured impurity across various industries. Moisture can cause major operational challenges, including process blockages, infrastructure damage, environmental harm, and reduced product yield. Gas hydrates in pipelines pose significant safety risks and cause production and transmission stoppages. Moisture forms corrosive acids with substances in the gas stream, damaging internal pipe walls and seals. It also lowers the heat value of natural gas, reducing the end product's value and potentially leading to financial penalties. Advanced moisture analysis technologies are essential for accurate and reliable moisture measurement.

Gas hydrates

Gas hydrates are complex, ice-like formations found both naturally and in industrial processes. Hydrates develop under specific environmental pressure and temperature conditions. Under these conditions, water molecules form hydrogen bonds in a lattice structure that traps natural gas inside. Hydrates often form near valves and other restrictions where pressure decreases rapidly, causing gas to expand and cool, even at temperatures above 0°C.

The formation of gas hydrates in pipelines can result in significant financial losses. Once formed, hydrates create blockages in transmission lines reducing or stopping flow. This can lead to pressure build-up and even stop the flow of gas through a pipeline.

The receiver station plays a vital role in the pigging process, a method used to prevent the formation of hydrates and scale inside pipelines.

Corrosion

Corrosion occurring on the inner surface of pipeline systems can result in dangerous conditions and potential regulatory consequences. This can be mitigated by choosing appropriate materials and maintaining strict quality control over the gas composition in the pipeline. Analysing moisture levels plays a fundamental role in managing internal corrosion.

Moisture in natural gas pipelines forms corrosive acids when it interacts with other impurities such as hydrogen sulfide (H2S) and carbon dioxide (CO2). Corrosive acid formation can have significant environmental and financial consequences. These acids react with pipeline components, reducing wall integrity, weakening pressure seals, and damaging instrumentation and equipment.

Quality standards

Moisture in natural gas reduces the quality and overall value of the product. Natural gas quality is measured by its oxygen content, hydrocarbon dew point, and water dew point. Elevated moisture levels decrease the heating value and the energy content of the gas.

At custody transfer points, where natural gas is sold from one party to another, moisture analysis is essential. Standards are contractually agreed upon for the quality of natural gas, including the limit of moisture content. In the US, the agreed upon standard, or ‘tariff,’ is typically 7 lbs H2O/mmscf natural gas. Suppliers who exceed this limit may face penalties, such as ‘shut-ins’, which temporarily halt gas delivery, causing revenue loss and higher costs. This can create a cascading effect on upstream operations, forcing the closure of producing wells or the flaring of gas, potentially resulting in fines or additional regulatory requirements.

Moisture measurement

Various technologies have been developed to measure moisture in gas, including chilled mirror instruments, aluminium oxide moisture sensor, and laser-based moisture analysers. Each of these technologies offers unique advantages and has been used and developed over several decades.

Quartz crystal microbalance (QCM) technology is particularly effective for pipeline applications due to its numerous advantageous characteristics. The response speed of QCM analysers is nearly instantaneous, providing operators with real-time moisture concentration data. These analysers are also extremely precise and can detect moisture at sub-ppm levels. Lastly, QCM analysers have the ability to self-verify, eliminating the need for removal from the process for performance validation or calibration.

In QCM, a quartz crystal oscillator is coated with a hygroscopic polymer that selectively attracts water molecules. Changes in mass from the adsorption or desorption of water directly impact the crystal's frequency, proportionally altering the sensor's output.

Ultimately, a wide range of moisture measurement technologies are available to pipeline operators. The choice of technology and type of analyser will be based on application requirements like measurement range, location, and cost.

The AMETEK 3050 moisture analyser uses QCM technology to measure trace moisture in pipeline natural gas and can also perform online sensor verification.

Conclusion

Moisture analysis is vital in the natural gas industry, playing a critical role in preventing operational disruptions, infrastructure damage, and quality degradation. Effective moisture management allows operators to prevent hydrate formation, mitigate corrosion, and maintain quality standards. As energy demands grow, the role of precise and reliable moisture analysis will only become more critical, ensuring the safe and efficient transportation of natural gas.

Written by Ryan Cumpston, AMETEK Process Instruments, US.

For more information visit www.ametekpi.com/products.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/01072024/the-importance-of-moisture-analysis-in-pipelines-and-gas-transportation/