Converting pounds per million standard cubic feet (lbs/MMSCF) to parts per mole (ppm) is an important mathematical calculation when determining the proper water content for a gas pipeline. If you have too much water in your pipeline, gas hydrates will start to develop, and these can turn into a clog and block the pipeline.
Measure the water presence in your gas pipeline using a single-channel dew point hygrometer. Set the units desired to lbs/MMSCF.
Multiply your lbs/MMSCF reading by 21 to get parts per million of water. This constant applies regardless of temperature.
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Compare your findings with the norms for the country or region through which your pipeline travels. In the United States, the accepted maximum water presence is 7 lbs/MMSCF, or 147 ppm. In Canada, though, it's 4 lbs/MMSCF, or 84 ppm.