With around four trillion cubic metres of natural gas in demand for 2024, the natural gas supply chain is vast and must adhere to strict supply legislation and guidance. One key parameter for the processing, storage and transportation of natural gas is the measurement of moisture within the gas itself.
Before transportation, natural gas is processed and purified through a range of different methods designed to remove contaminants such as solids, water, carbon dioxide (CO2), hydrogen sulphides, mercury and other hydrocarbons to produce pipeline quality dry natural gas.
However, to minimise dehydration in natural gas there arises a critical challenge: finding the balance between reducing dehydration and maintaining gas quality, while also keeping maintenance costs, transportation expenses and safety concerns in check. This requires precise and reliable measurement of the water content in natural gas.
During the transfer of natural gas ownership, whether between current and prospective owners, there are established limits on water content. These limits, usually expressed as absolute humidity (in mg/m3 or lbs/mmscfh) or dew point temperature, are regulated by tariffs.
Various technologies are available for both online measurement and spot sampling of moisture content, catering the needs of the industry. In a gasworld exclusive, we preview the release of a new white paper from Panametrics, a Baker Hughes business.
Created by Gerard McKeogh, Global Product & Sales Manager at Panametrics, the white paper reviews the most commonly used moisture measuring instruments and provides a comparison of those technologies.
Technologies for measuring water vapour in natural gas
Most technologies for measuring the amount of water vapour in natural gas rely on sample conditioning systems, where a gas sample is extracted, filtered, the pressures regulated and flow controlled.
Installing a sensor directly in a natural gas pipeline is ill-advised due to the presence of physical contaminants, additives and liquid hydrocarbons. A sampling system offers the advantage of isolation from the main pipeline. However, it’s crucial to ensure that the sample system does not alter the moisture concentration through leaks or desorption/adsorption from wetted components.
The most commonly used measurement technologies include chilled mirror, impedance sensors, quartz microbalance, Fabry-Perot interferometer and tunable diode lasers, each with its own set of pros and cons.
Chilled mirror
Chilled mirror hygrometers are divided into two main types: manual and automated. Automated systems include cycling and equilibrium chilled mirrors, measuring dew/frost point temperatures directly by cooling a surface until condensation forms. They can also determine hydrocarbon dew points in gas mixtures.
Manual systems, like the ASTM-1142 apparatus, use high-pressure gas for cooling and require careful observation to identify condensation onset. Automatic systems utilise thermoelectric cooling with feedback control loops, offering precise measurements within a wide temperature range.
While traditionally used in laboratories, some models like Panametrics Optica and OptiSonde can now be used in non-hazardous areas, expanding their practical applications beyond the lab. These instruments provide high accuracy and repeatability, suitable for on-site calibration of other sensors.
Impedance sensors
The most common impedance-based moisture sensor technology in natural gas applications is the aluminium oxide sensor. These sensors feature an aluminium base with a thin layer of aluminium oxide, allowing water molecules to permeate and cause micro-condensation in its porous structure.
By measuring impedance, these sensors act as water molecule counters and are calibrated across multiple dew/frost points for accurate temperature readings. However, they exhibit slower response times in the wet-to-dry direction and are prone to drift, typically around 2°C per year, necessitating periodic recalibration.
Despite their compact size and capability to withstand high pressure, they are typically not directly installed in pipelines but integrated into extraction type sampling systems for gas filtration. Users often rotate sensors to maintain accuracy within recommended recalibration intervals.
Quartz microbalance hygrometers
Quartz microbalance hygrometers utilise a quartz substrate coated with a hygroscopic polymer film. When exposed to water vapour, the film adsorbs water, altering the sensor’s resonant frequency proportionally to the vapour’s partial pressure.
Periodic re-zeroing is necessary due to hysteresis, requiring a ‘zero gas’ closer to zero water content. Accuracy typically ranges within ±10% from 1-2,500 ppmv. Some methods alternate between zero gas and process gas to minimise errors and contamination of the sensor surface necessitates a clean sampling system. Despite these challenges, quartz microbalance analysers offer relatively fast response times.
Fabry-Perot hygrometers
Fabry-Perot hygrometers feature a sensor head with high and low refractive index materials like SiO2 and ZrO2, coated with a glass substrate tailored for water molecules. A light beam, typically from an LED, passes through fiber optics, with water molecules altering its refractive index and causing a wavelength shift.
This shift, detected by a Polychromator, correlates with water concentration and is calibrated to dew point temperature. The sensor, mounted on a stainless steel probe, connects to a control unit via fiber optics, needing temperature and pressure compensation for ppmv readings.
TDLAS
Tunable Diode Laser Absorption Spectrometers (TDLAS) provide non-contact continuous moisture measurement in natural gas based on the Beer-Lambert Law. A diode laser emits light at a specific wavelength, modulated for precision, passing through the gas sample.
Water absorption alters the light, measured to determine water content. TDLAS sweeps laser wavelength while holding temperature constant, measuring absorption peaks to determine water concentration. By measuring total pressure and temperature, TDLAS calculates absolute humidity, dew point and pressure dew point with high accuracy.
Typical accuracy is ±2% of reading for mole fraction or ppmv. Response time for optical measurement is <2 seconds, though system response time is typically under 5 minutes for a 90% step change.
White paper: In-depth exploration of technologies
With so many technologies available for measuring moisture content in natural gas, each with their respective pros and cons, navigating the world of hygrometers can be made easier with Panametrics new white paper.
For the full conclusions of the white paper, in addition to an in-depth examination of each technology complete with a comprehensive scorecard of moisture measurement technologies for natural gas, keep a lookout for the new white paper coming soon from Panametrics.
