Most engineers on a gas plant know the cost of wet gas: hydrates, corrosion, and ice formation in the coldbox. However, the cost of over-dried gas is discussed far less often. When a dehydration unit is run harder than the specification requires, it consumes additional reboiler heat, fuel gas and molecular sieve bed life every operating day. The difference between the two outcomes comes down to measurement. An online moisture analyser for natural gas dehydration replaces an assumed margin with a measured value, so the plant can hold its specification without paying for margin it does not need.
Why we overdehydrate natural gas, and what it costs
Over-drying is rarely a deliberate design decision. It is what operators do when they cannot measure the actual moisture content of the dried gas with confidence. Without a trusted trace measurement, uncertainty is managed conservatively. For example, molecular sieve beds are regenerated on a fixed clock set for the worst case, glycol (TEG) units are run harder than the water load requires, and in most of these cases, the gap between actual moisture at the end of dehydration and the required specification is left unmeasured. Each of those decisions is rational on its own. However, they build an unaccounted energy cost into the plant that is significant cumulatively.
Regenerating a molecular sieve bed requires heat from the fuel gas. Switching beds earlier than the moisture front requires means more regeneration cycles per year, and every additional cycle ages the bed unnecessarily. Molecular sieve degradation is cumulative, so conservative switching shortens bed life and burns through more fuel gas than required. On glycol units, circulating and reboiling beyond the demand also provide a continuous cost with no matching benefit. In some cases, there may be a commercial impact associated with the moisture value as specified in the contract. The end customer requires an accurate moisture analysis and moisture balancing for optimum contract value. Gas delivered far drier than the contract requires gives that value away with every delivery.
You should be cautious about how you manage the dehydration process as well, because the downside of wet gas is equally catastrophic. Water vapour reacts with acidic impurities to form corrosive compounds that attack pipeline integrity, and in cold or high-pressure conditions moisture forms hydrates that block lines, valves and instrument connections. On LNG and FLNG plants, the moisture in the feed gas must be held below 1 ppmV, because residual moisture freezes in coldboxes, process pipelines and components at cryogenic temperatures. Nobody trims margin against consequences like these on guesswork, and nobody should.
Measurement is what lets you operate close to spec
A continuous trace moisture measurement converts the unknown margin into a measured number. For example, molecular sieve beds can be switched based on measured moisture rather than a worst-case clock, which reduces unnecessary early regenerations while keeping breakthrough visible. The same measurement also detects molecular sieve degradation and breakthrough as they develop, so you can reduce the conservatism in the regeneration schedule without reducing the protection. At start-up, fast and accurate analysis validates process conditions and shortens the time to on-spec production. At custody transfer, the measured value provides the evidence that the gas meets its contractual specification at the moisture level the contract actually requires.
Reducing the margin only works if you can trust the reading, and that sets the specification for the analyser itself. The measurement range must match the installation point: parts per million (ppmV) for pipeline gas, and below 1 ppmV for LNG feed gas. The response must also be fast enough to support bed changeover decisions in real time. Calibration should be multi-point and traceable, with ISO17025 calibration available where the measurement supports custody transfer. In most gas plants, the sample point sits in a classified hazardous area, so the analyser must carry certification for the zone, named on its certificate rather than implied. Finally, the analyser should validate itself, with self-checks that flag a fault openly rather than quietly reporting a number that operators then act on.
Moisture analysers for gas dehydration applications
Measurement Solutions supplies the MicroView range of silicon sensor hygrometers for natural gas and process gas measurement. For sample points in non-hazardous areas, the MicroView Mini Online Hygrometer provides online moisture measurement in a standard range of 0.1 to 100 ppmV, with configurations available from 0.1 ppbV to 6000 ppmV. Calibration is multi-point in ppbV or ppmV, with ISO17025 calibration as an option, and the analyser runs automatic configuration checks at start-up and periodic self-tests during normal operation. Ranges in °C dew point are also available on request.
For classified areas, the MicroView Mini Ex-d Hygrometer System carries ATEX, IECEx and UKEX certification with execution II 2 GD; Ex db IIC T4 Gb; Ex tb IIIC T135°C Db, and IP66 ingress protection. It operates in ambient temperatures from -20 to +60 °C, measures down to a 0.01 to 10 ppmV range (lower detection limit below 0.01 ppmV), and is suited to sweet natural gas and LNG applications. Where the process gas is contaminating, the MicroView Mini Ex-d A/Z upgrades the standard Ex-d with in-built desiccant dryers for 24/7 zero gas, together with automated self-validation and zero-adjust capability, so the analyser checks its own zero rather than waiting for a technician to do it.
Measurement Solutions Pty Ltd, A Bestech Company, has supported Australian process industries since 1985 and is now integrated into Bestech Australia, with nationwide sales and application engineering support. Our engineers assist with analyser selection, installation planning, commissioning and through-life support. Send us the gas composition, operating pressure and temperature, and the moisture range you need to measure, and we will recommend the right configuration.
Request a quote or call +61 2 9894 4300 to speak with an application engineer. (https://measurement-solutions.com/contact-us/)