Wet Gas Measurement Terms
There are a number of specific terms used to describe the characteristics of wet gas flow:
Superficial gas velocity is the gas velocity if there were no liquid present in the wet gas flow. In wet gas flows the gas velocity is higher due to a reduction in the pipe area caused by the presence of the liquid.
Superficial liquid velocity is the liquid velocity if there were no gas present in the wet gas flow.
Liquid load is the ratio of the liquid mass flow rate to the gas mass flow rate and is normally expressed as a percentage.
GVF – Gas volume fraction is the ratio of the gas volumetric flow rate to the total volumetric flow rate.
LVF – Liquid volume fraction is the ratio of the liquid volumetric flow rate to the total volumetric flow rate.
Hold up is the cross sectional area occupied by the liquid in the pipe carrying the wet gas flow.
Void fraction is the ratio of the flow area occupied by the gas to the total flow area.
Lockhart–Martinelli parameter. Gas is compressible and the density changes significantly with changes in pressure. Liquids, on the other hand, are considered to be incompressible and so their density does not tend to change with a change in pressure. If the pressure of a wet gas system increases, the density of the gas will increase but the density of the liquid will not change. The densities of the flow components are an important consideration in flow measurement as they relate to the actual mass quantities of the fluids present. To account for both the flow rates and densities of the liquid and gas phases it is common practice to define the wetness or liquid loading of the gas using the Lockhart–Martinelli parameter, referred to as χ (Greek letter chi), which is a dimensionless number. This parameter can be calculated from the mass flow rate or volumetric flow rate and the density of the fluids. It is defined as:
where
- is the liquid phase mass flow rate;
- is the gas phase mass flow rate;
- is the gas density;
- is the liquid density.
This Lockhart–Martinelli parameter χ can be used to define a completely dry gas when the value is zero. A wet gas flow has a value of χ between zero and about 0.3 and values above 0.3 are usually defined as multiphase flows.
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