Moisture Measurement in Natural Gas

Moisture Measurement in Natural Gas

The Problem
Most of the gas companies demand a continuous control of the humidity in the natural gas on drying, transfer and storage ( feed-in and feed-out ) for the following reasons:

  • Protection of the plant
    Too much humidity in the gas leads on the one hand to high corrosion in the pipe, on the other hand to the formation of hydrates that causes pipelines and valves to freeze. The consequences are damages of the plant and very high costs because of shutdown.
  • Reduce of expense
    Because of the measurement of humidity, the dryer can be controlled and optimized. Both the costs of over-drying (energy) and the use of drying agent can be reduced to a minimum.
  • Constant Quality
    Every gas company has to grant for their natural gas a certain dew point value at all transfer Points.

The solution
In general HYGROPHIL F 5673 takes inline and in-situ measurements within a range of dew point temperatures from –80°C to +20°C. Due to a certified measurement in natural gas and under pressure ( at an accredited institute ) a very high accuracy can be achieved in that application. HYGROPHIL F 5673 shows no cross-sensitivity against any Hydrocarbons, Methanol or Glycol. The sensor can be placed directly in the pressured pipe up to 200bar (with the BARTEC Retraction Tool you are able to take the sensor out under pressure at any time) or in a BARTEC Sample System. The humidity sensor itself is not only very accurate because of the natural gas certification, but also long-term stable, reproducible and maintenance-free ( no DRIFT ) because of the physical principle. Experiential an annual technical inspection is sufficient. The length of the fiber–optic cable to connect the sensor and the evaluation unit can be up to 800m.

The measurement principle
The sensor consists of a coated, very resistant and microporous substrate of quartz glass with a pore diameter in which only water molecules can enter. This penetration happens because of the equilibrium (all gases balance automatically).
Because of the different refraction index (nair = 1,0; nwater = 1,33) the water molecules cause a shift in wavelength. With increasing moisture, the refraction spectrum is shifted towards larger light wavelength and read out by a polychromator. This shift (no measurement of intensity => independence of fibreoptic cable length) is proportional to the dewpoint temperature of the gas.

The sensor can be used in hazardous areas (Zone 0, or Class I / Div. 1).
The whole system is ATEX, CSA / UL and GOST approved.

Advantages of the retraction tool:

  • The installation and/or dismantling of the sensor can be done without interrupting the operating procedure, which guarantees a high availability of the facility.
  • Direct measurement in the pressured pipe up to 200 bar ensures precise measuring results and offers a high measurement accuracy.
  • On the upstream side the sensor is protected against impurities. Result: Long-term availability and service-life.