An Approximate Practical Correlation of the Effect of Pressure on the Dew Point Temperature of Pipeline Natural Gases

The dew point temperatures of normal pipeline gases are influenced not onlyby the hydrocarbon composition of the gas but also by the presence of traces ofcontaminants such as hydrate depressants and compressor oil. For this reasondew point temperatures calculated from gas analyses are frequently lower thanobserved dew points and the effect of pressure on dew point temperature is notthe same as for a pure hydrocarbon gas. Actual dew point measurements on 16 gases were compared with calculated dewpoints and differences up to 50°F were observed. Notwithstanding these seriousdifferences the shape of the actual and the calculated dew pointtemperature-pressure curves were similar. This similarity has permitted thedevelopment of a practical correlation of the effect of pressure on the dewpoint temperature of the gases. The correlation permits the estimation of the dew point temperature at anypressure within the range of 100 to 800 psi from its knowledge at anyonepressure within this range. The accuracy is about ±5°F for pressures in therange of 400 to 800 psi. The correlation, along with calculations of thetheoretical dew point, may also be used to estimate the degree of contaminationof pipeline gases. Introduction The contracts for the pipeline transportation of natural gas usually limitthe higher hydrocarbon content of the gas by specifying a maximum hydrocarbondew point. In Alberta contracts specify that the hydrocarbon dew point belimited to 15°F. up to the maximum operating pressure of the pipeline. In orderstrictly to comply with this specification the dew point temperature must bemeasured directly at the maximum operating pressure, Usually, however, thegases are available only at pressures below the maximum pipeline operatingpressure and a direct measurement is not practical. Under these conditions theengineer must either (a) calculate the dew point at the desired higher pressure directly from theavailable gas analysis, or (b) estimate the dew point at the higher pressure from dew pointmeasurements at some lower pressure. A companion paper by Aziz and Govier has presented a semi-theoreticalcorrelation which facilitates the calculation. from gas analysis, ofhydrocarbon dew points of clean pipeline gases. The measured dew pointtemperatures of normal pipeline gases arc influenced not only by thehydrocarbon composition of the gas but also by the presence of traces ofcontaminants such as hydrate depressants and compressor oil. For this reason.dew point temperatures calculated from gas analyses are frequently lower thanthe observed dew points and the effect of pressure on the dew point temperatureis not the same as for a natural gas composed entirely of hydrocarbons.