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Europe’s renewable energy targets and diminishing natural gas resources require the diversification of energy sources to include non-conventional gases, such as biogas, methane and hydrogen. These gases have different chemical and physical properties to traditional natural gas and need to be well characterised before entering the gas grid or transmission networks and during custody transfer between different commercial operators. This is crucial to ensuring safe operation and enabling fair trade and environmental decision-making.
Gas pipeline networks are highly complex transmission and distribution systems used to move gas across states, countries or even continents. It is important that gas quality is maintained throughout the network and remains within the requirements set by international natural gas quality guidelines. The presence of water in energy gases is a particular problem.
The need for quality control of natural gas
Moisture content is a key parameter of gas quality and it is assessed against industry guidelines when gas enters the network and at custody transfer. For this reason, accurate and cost-effective moisture measuring instrumentation is essential to pipeline operators.
This can be anywhere in the gas supply chain – from processing plants, to pipeline transmission entry and exit points. Water content must be within limits to avoid risk of condensation, corrosion or even hugely disruptive blockages. Water content is also of interest to large consumers of gas, such as electricity generation companies, for reasons of efficiency, emissions control, and avoidance of potentially damaging effects of condensates. The problem condensate can be water, or hydrocarbon, or it can be methane hydrate formed in the presence of methane and water, depending on temperature, pressure and gas composition. Thus, measurement of water content is an essential part of control of the process.
The choice of conditions for measurement depends on operating at the right pressure-temperature combination. This is essential to ensure all components are in gas phase and requires measurements at pressures from atmospheric to above 8 MPa (above 80 bar).
The challenge – measurement and calibration
Water vapour, or humidity, can be measured using a wide variety of principles. Perhaps the most commonly used in natural gas are electronic capacitive sensor ‘probes’, which operate either at gas line pressure, or at atmospheric pressure. Although relatively simple to operate, these devices can suffer significant drift, especially in the harsh environments concerned. For sampled gas expanded to near atmospheric pressure, a wider set of instruments can be used. These include electrolytic phosphorous pentoxide sensors and, increasingly, spectrometers based on absorption of infrared light by water vapour.
Unfortunately, the measurements don’t always directly give the information needed. Firstly, in natural gas some sensing principles are not purely selective for water vapour – they can have sensitivities to components in the background gas mixture, or to influences of pressure or temperature. This means that compensation is needed to correct for those effects. Secondly, the instruments don’t always directly measure the quantities and units of interest. Sensing principles variously measure water dew point (temperature at which liquid water or solid ice would form), or partial pressure, or water fraction by mass or volume, or mass of water per unit volume, or others. To interpret and apply measurement results, conversions are often needed, but this is not straightforward.
Problems of instrument drift, and sensitivities to gas species, pressure and temperature, can all be reduced through access to reliable calibrations and measurement checks. A calibration traceable to an authoritative reference can identify instrument errors, and associated uncertainties. Readings can be compensated, and measurement uncertainty can be taken into account in deciding whether tolerances
are met. But until now there has been a problem in obtaining calibrations that are relevant to hygrometer use in natural gas. Most humidity calibration laboratories can only perform calibrations in air at atmospheric pressure. This is not enough to test the performance of instruments for use in other gas species and at higher pressures, where these could affect the measurement results.
The solution – validity in the gas medium of use
Although industrial humidity measurements may be at high pressures, or in other gases, many laboratories can only perform humidity calibrations in air at atmospheric pressure. The UK’s National Physical Laboratory (NPL) has developed a novel facility to enable humidity calibrations at the highest levels of accuracy in a range of real-world energy gases and gas mixtures in an extended pressure range.
“the NPL Multi-Gas Multi-Pressure Humidity Generator is a primary standard based on the saturation of gas with water vapour under controlled conditions”
The NPL Multi-Gas Multi-Pressure Humidity Generator is a primary standard based on the saturation of gas with water vapour under controlled conditions. The facility covers a wide humidity and pressure range (with plans to extend these further), and results are obtained in terms of typically either dew point (frost point below 0°C) or volume fraction (ppmv).
For best validity, humidity calibrations should ideally be in the gas medium of use, because the performance of some hygrometer types can depend on gas species and pressure. Calibrations at NPL can be performed at pressures up to 3 MPa (30 bar), in air, inert gases, methane and pre-made cylinder gas blends. Where gas species are not suitable for direct saturation in the generator (due to corrosion risk or hydrate formation), humidification is possible by blending the gas in appropriate flow ratios with nitrogen or another suitable inert gas that has been passed through the saturator. Dew-point probes, condensation hygrometers, water vapour spectrometers and other hygrometers can be calibrated. Calibrations can cover other humidity quantities and units directly related to dew point, such as vapour pressure, volume fraction, amount fraction, vapour density (mass of water per unit volume), and others.
Studies and trial results
The facility has already been used for a number of studies and calibrations, for oil and gas industry companies, instrument companies,
and other organisations. Examples are:
- Tests of alternative hygrometer types used on a natural gas extraction platform – explaining discrepancy between instruments
- Tests of prototype hygrometers under development by instrument manufacturers for use in natural gas – especially water vapour spectrometers, but also various electronic probe-type sensors
- Detailed study of water vapour spectrometers for monitoring nuclear CO2 heat-exchange gas • Study of hygrometers for use in hydrogen, testing for sensitivity to background gas and pressure
- Humidity-controlled testing of free-space leak detectors for natural gas platforms
Operation is with a growing variety of gases, including methane-ethane mixtures, CO2-N2 mixtures, argon and most recently hydrogen. As part of a European-funded project, NPL has worked with humidity sensors for use in hydrogen refuelling stations. Hydrogen for vehicles is generated by electrolysis of water, emerging with high water content and needing to be dried to just 5 parts per million of water vapour. This is to prevent condensation at tank pressures of up to 70 MPa (700 bar). By providing hygrometer testing and calibration in hydrogen, NPL is contributing to the viability of hydrogen refuelling networks, and to the potential for greater uptake of hydrogen-powered transport.
What next
The NPL facility provides a unique capability to generate defined water content in a wide range of gases and mixtures used in different applications. Plans are being devised for future research that could provide new measurements of gas non-ideality for a variety of gases and gas mixtures. This will allow more accurate conversions between different humidity quantities – conversions which, for non-air gases, currently have some uncertainty. Overall, by enabling users to obtain calibrations that are appropriate to the conditions of use, this facility supports the accurate use of industrial equipment in process conditions, making sure that industry needs are met.
