Hydrocarbon dew-point is to prevent the condensation and accumulation of hydrocarbon liquids at low points in the gas distribution grid. The hydrocarbon dew-point specification is usually achieved by removing a proportion of the higher hydrocarbons (typically butanes and heavier) from the process gas stream by using different refrigeration, absorption, adsorption or other technologies.

There are three common techniques for the measurement of hydrocarbon dew-point. The first is by far the most commonly accepted method – a fundamental chilled mirror technique which relies on the cooling of a mirrored surface in contact with the gas stream. As the dew-point temperature is reached, condensate starts to appear on the mirrored surface. The temperature of the mirror at the point where condensate is first observed is, by definition, the dew point. However, this manual, optical technique is not particularly appropriate because the observation of hydrocarbon condensate is very subjective to the human eye.

The second technique involves the analysis of gas composition using a process gas chromatograph, combined with a calculation method based upon equations of state that estimates the resultant hydrocarbon dew-point temperature. Whilst this methodology is capable of giving a reasonable degree of accuracy in terms of the hydrocarbon dew-point temperature, to do so requires analysis of a large number of hydrocarbon components which is expensive and complex.

The third technique, not widely used within industry, is the measurement of potential hydrocarbon liquid content (PHLC). This technique effectively measures the amount of hydrocarbon liquid, in milligrams per cubic meter, which will condense out of a natural gas stream at a particular operating temperature. It is therefore a parallel measurement to hydrocarbon dew-point temperature. It has the advantage that it is a fundamental measurement that can be related back to mass and flow standards, but has the considerable disadvantage that it is quite difficult to realize in practice and is therefore more suited to a periodic validation technique than to an on-line measurement process.

Although the three techniques above have their particular weaknesses, all are used to varying levels of success to determine the hydrocarbon dew-point (or PHLC) of natural gas.