Wet gas flow measurements

Natural gas that is produced is under high pressure and contains liquid fractions in addition to the gas. Those liquid fractions consist of water and gas condensate, which is a mixture of various light hydrocarbons.

Wet gas flow measurement methods

The flow of wet gas can be measured in various ways. The most common measurement methods are based on differential pressure flow measurement. Sometimes an ultrasonic flow meter can be applied. The liquid fraction always has a negative effect on the performance of the flowmeter, regardless of which measurement principle is applied.

The dP element for wet gas measurements is often a venturi. Measuring flanges are less suitable because wet gas is erosive: the high flow rate at high pressure causes wear from liquid droplets. And the gas also contains sand particles. Venturis are more expensive but have significantly better long-term stability.

Dual stream venturi differential pressure gauge

A "dual stream venturi" has a third on the outlet side of the flowmeter in addition to the two normal pressure taking points. Here the permanent pressure loss across the venturi is determined. This gives an indication of the liquid volume, which allows for compensation for deviations in the venturi measurement technique.

Wet gas flowmeter based on V-Cone

Another popular differential pressure element is the V-Cone. This element is able to deal well with the fluid fractions present. Because the droplets easily pass through the cone because it is more aerodynamically efficient. The deviation in the dP measurement with this measuring element is comparable to that of a measuring flange (orifice) and does not conform to ISO 5167. Depending on the Lockhart-Martinelli factor a correction must be made in a flow computer.

The V-Cone can be equipped with a measuring technique to determine the fluid fraction. This is done using a high-frequency technique, allowing correction for the fluid fraction within the instrument itself.

Photo: V-Cone differential pressure element for wet gas flow measurements.

Photo: Dual stream venturi differential pressure gauge for wet gas.

Photo: Venturi differential pressure meter with RTJ flanges for wet gas flow measurements.