TDLAS analysers: Revolutionising moisture measurement in natural gas

Water is an inevitable component of natural gas, and its content can vary widely depending on the source and the production method. In addition to its corrosive effect, water can cause operational problems for pipeline operators and petrochemical plants. For example, it can freeze and block pipelines or valves, leading to operational shutdowns and costly repairs. It can also react with some contaminants to form solid deposits, further exacerbating corrosion problems.

The composition of natural gas is regulated to some extent, and there are usually limits on its chemical content and attributes such as calorific value. Sulfur and its many compounds are the most widely encountered contaminant in all fossil fuels, including natural gas, and are known for their toxicity and the pollutants produced during combustion.

Oxygen can degrade amine and some mercaptans used in natural gas treatment, and carbon dioxide can dilute the overall heat value. When combined with water, these contaminants can produce acids that attack carbon steel piping, valves and other equipment, causing internal corrosion and metal loss over time. Internal metal loss is more difficult to recognise and measure, making it a greater problem than external corrosion.

The presence of even small amounts of water in natural gas can still pose a risk for corrosion and blockages. The variations in temperature and pressure that the gas experiences during transport can cause water to change phase multiple times, which can lead to the accumulation of liquid water in low spots of the pipeline. This can cause blockages and, if not addressed, can lead to corrosion and ultimately pipeline failure.

Analysers based on tunable diode laser absorption spectroscopy (TDLAS) possess inherent stability, requiring infrequent calibration due to resistance to drift and the absence of issues stemming from chemical contamination affecting the sensor itself. Nonetheless, ensuring effective operation encounters difficulties with the auxiliary components. As an optical device, a TDLAS analyser employs mirrors and lenses to direct and focus the laser beam emitted from the source to the detector. The sample gas flows through the measurement cell, which acts as the pathway for the beam. Instances of liquid presence in the sample create the potential for cell flooding. Moreover, if the mirrors become coated the beam can be attenuated or completely obstructed. The occurrence of such problems necessitates servicing of the measurement cell, but operators might take some time to recognise these issues.

A TDLAS instrument.

Compared to other techniques such as quartz crystal microbalance (QCM), infrared absorption, capacitance, and dew point measurement, TDLAS has several advantages, which include high sensitivity and selectivity; non-intrusive and real-time measurements; wide dynamic range; and low maintenance.

High sensitivity and selectivity

The TDLAS technique relies on the absorption of infrared light by water molecules, allowing for precise quantification of moisture content. TDLAS is capable of detecting moisture levels in the parts-per-million (ppm) range, making it suitable for applications requiring stringent moisture control. Furthermore, TDLAS can differentiate between water vapour and other gases, ensuring accurate measurements even in the presence of interfering substances. This high sensitivity and selectivity make TDLAS an ideal choice for moisture analysis in complex samples and environments.

Non-intrusive and real-time measurements compared. For a larger image click here.

One significant advantage of TDLAS is its non-intrusive measurement capability. The technique allows moisture analysis without physical contact, minimising sample contamination and disturbance. TDLAS also enables real-time measurements, providing instantaneous feedback for process control and optimisation. This real-time capability is particularly advantageous in applications where moisture levels need to be continuously monitored and adjusted. TDLAS can facilitate rapid decision-making, allowing operators to take immediate corrective actions when moisture deviations occur, thus preventing product quality issues and minimising downtime.

Wide dynamic range

Whether the moisture content is low or high, TDLAS can accurately quantify moisture levels across a broad range of concentrations. This versatility is especially beneficial in applications where moisture content can vary significantly, such as drying processes or environmental monitoring. By covering a wide dynamic range, TDLAS eliminates the need for multiple instruments or calibration adjustments, streamlining the measurement process and reducing overall costs.

Robustness and low maintenance

The absence of moving parts in TDLAS analysers minimises the risk of mechanical failures, ensuring long-term reliability. The non-contact measurement principle also eliminates the need for sample conditioning, reducing maintenance tasks and associated costs. Additionally, TDLAS analysers are often designed with self-diagnostic capabilities, enabling proactive maintenance and troubleshooting. These features make TDLAS a cost-effective solution for moisture measurement, with reduced downtime and lower overall ownership costs compared to alternative technologies.

Tunable diode laser absorption spectroscopy (TDLAS) offers numerous advantages for moisture measurement over other technologies. Its high sensitivity, selectivity, non-intrusive nature, wide dynamic range, and robustness make it a preferred choice for precise and reliable moisture analysis in various industries compared to other techniques.

To find out more or get in touch, visit the Endress+Hauser website.