Moisture measurement in natural gas is very much essential. There are various moisture measurement instruments for natural gas applications such as transmitters, portables and online analyzers with multi-point NIST traceable factory calibration. In transmission pipeline, measurements are generally done in terms of lbs/MMSCF and in cryogenic gas plants measurements will be in terms of dew point. If there is methanol in the sample, it should be ensured that methanol-ageing factor is added to the sensor to have an accurate measurement. The main challenge during the moisture measurement is the contaminants like H2S, Mercury, glycol carry over etc. In most cases, a properly designed sample system is a requirement. Sample conditioning is the most important factor in accurate moisture analysis, and usually the most neglected.

Natural gas poses a unique situation since it can have very high levels of solid and liquid contaminants as well as corrosives in varying concentrations. Moisture Measurements & Analysis in natural gas are typically performed with one of the following techniques:

  • Moisture Color Indicator Tubes – The color indicator tube (also referred to as the Drager Tube or Stain Tube) is a device many natural gas pipelines use for a quick and rough measurement of moisture. Each tube contains chemicals that react to a specific compound to form a stain or color when passed through the gas. The tubes are used once and discarded.
  • Chilled Mirrors – When gas flows over a chilled surface, or chilled mirror, the moisture will condense on it. The exact temperature at which this condensation begins is known as the dew point. The temperature of this mirror is reduced from high to low, and the temperature is read exactly when the dew is observed. By obtaining the dew point temperature, one can calculate moisture content in the gas.
  • Electrolytic Moisture Sensor – The Electrolytic sensor uses two closely spaced, parallel windings coated with a thin film of phosphorus pentoxide. As this coating absorbs incoming water vapor, an electrical potential is applied to the windings that electrolyzes the water to hydrogen and oxygen. The current consumed by the electrolysis determines the mass of water vapor entering the sensor.
  • Piezoelectric Sorption (also known as Quartz Crystal Microbalance) Moisture Analyzer – The piezoelectric sorption instrument compares the changes in frequency of hydroscopically coated quartz oscillators. As the mass of the crystal changes due to adsorption of water vapor, the frequency of the oscillator changes. The sensor is a relative measurement, so an integrated calibration system with desiccant dryers, permeations tubes and sample line switching is used to correlate the system on a frequent basis.
  • Aluminum Oxide and Silicon Oxide Moisture Sensors – The oxide sensor is made up of an inert substrate material and two dielectric layers, one of which is sensitive to humidity. The moisture molecules pass through the pores on the surface and cause a change to a physical property of the layer beneath it.
  • Moisture Absorption Spectroscopy – Absorption spectroscopy is a relatively simple method of passing light through a gas sample and measuring the amount of light absorbed at the specific wavelength. Traditional spectroscopic techniques have not been successful at doing this in natural gas because methane absorbs light in the same wavelength regions as water. But if one uses a very high resolution spectrometer, it is possible to find some water peaks that are not overlapped by other gas peaks.