Why moisture matters in natural gas processing

If you work anywhere in the natural gas value chain, including production, processing or pipeline transport, you will already know that water is a problem.

However, it is worth being clear about why.

The failure modes caused by moisture are among the most expensive and disruptive issues an operator can face.

Four ways moisture in natural gas causes damage

If left unchecked, water vapour can condense out of the gas stream.

That single event can trigger a chain of problems.

Corrosion

Natural gas often carries trace CO₂ and, in sour service, sometimes H₂S.

When free water condenses in their presence, corrosive compounds can form.

These attack carbon steel from the inside, and over time this slow, hidden damage can lead to unplanned pipeline inspection or repair.

Hydrate blockages

Pressure and temperature changes are typical in transmission systems.

Under these conditions, water and hydrocarbon molecules can join together to form solid, ice-like hydrates.

These can lodge in valves, orifice plates and instrumentation.

As a result, they can completely block flow at a compressor station or metering skid.

Reduced gas quality

Water vapour displaces hydrocarbon content.

This reduces the delivered heating value of the gas.

As a result, a buyer may receive less energy content than they are paying for, even when nothing downstream has visibly failed.

Custody transfer disputes

Pipeline contracts set a maximum water content limit, typically 1 to 7 lb/MMSCF, at the point where ownership changes hands.

If gas arrives wetter than the tariff allows, the receiving pipeline can reject it.

This can lead to a commercial dispute between the shipper and the pipeline.

Where water dewpoint in natural gas is checked

Operators typically measure water dewpoint at two key points.

The first is at the dehydration unit outlet. This confirms that the glycol contactor or molecular sieve bed is meeting its target.

The second is at the custody transfer point. This confirms that the gas still meets tariff requirements after transport.

Portable dewpoint meters also play an important role at remote or unmanned stations.

Technicians can use them to verify dewpoint during commissioning or troubleshooting without needing a permanent transmitter at every tap.

A measurement mistake worth avoiding

One of the quickest ways to get a false reading is to sample directly from an unconditioned high-pressure line.

When gas is stepped down to sensor pressure, it cools.

Unless the pressure reduction stage is heated, the sample can fall through its own dewpoint before it even reaches the sensor.

As a result, the reading may show condensation that was never actually present in the pipeline.

A proper sample system, with a heated regulator, filtration and controlled flow, is designed specifically to prevent this.

It is also important to keep water dewpoint and hydrocarbon dewpoint separate.

Water vapour condenses in one, while heavier hydrocarbons condense through retrograde behaviour in the other.

Because these are different phenomena, a dewpoint transmitter is not a substitute for tracking hydrocarbon liquid dropout.

Applicable standards

Accurate dewpoint measurement helps keep a natural gas system free from corrosion, hydrate blockages and tariff disputes.

The key is to take the measurement in the vapour phase with a properly conditioned sample.

When the sampling is correct, operators can trust the reading at every point it is taken.

The full Shaw Moisture Meters application note includes standards references, including ASTM D5454, ASTM D1142 and ISO 18453, as well as suitable product options.

Contact Shaw Moisture Meters

For more information on dewpoint measurement in natural gas processing and transmission applications, contact the Shaw Moisture Meters team.