PRODUCTION • PROCESSING • HANDLING
GAS NETWORKS Future-proofing our Dr Stephanie Bell tackles the problem of humidity G
as 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. Te presence of water in energy gases is a particular problem. 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. Tis 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. Te 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. Tus measurement of water content is an essential part of control of the process. Te choice of conditions for
measurement depends on operating at the right pressure-temperature combination. Tis is essential to ensure all components are in gas phase and requires measurements at pressures from atmospheric to above 8MPa (above 80 bar).
MEASUREMENT AND CALIBRATION Water vapour, or humidity, can be measured using a wide variety of principles.
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www.engineerlive.com Inside the multi-gas multi-pressure humidity generator
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. Tese 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. Tis 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 or solid 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. Tis 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.
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