Natural Gas Purging Calculations

Purging is a process of removing the gas (methane) from the pipeline. Controlled purging of air from pipelines by direct displacement with natural gas has been safely practiced for many years with the recognition that some flammable mixture is present. Purging of natural gas from pipelines by direct displacement with air or another gas such as nitrogen also has been similarly practiced. It works both ways; however, there will always be an atmosphere of type of a mixture. This is due to the densities of the gases.

The replacement of one gas by another in an enclosed space or chamber takes place by means of two distinct actions: (I) displacement and (2) dilution or mixing. In a purge that is effected entirely by displacement, the gas or the air that is originally present is pushed out of the escape vents by the entering purge gas with little or no mixing of the purge gas and the original contents. Thus, the quantity of purge gas required for purging by displacement approximates the quantity of gas or air being replaced.

In addition, the contraction in the volume of gases caused by decrease in temperature is another temperature effect that should be considered. A positive, though slight, pressure must be maintained within a pipeline being purged.

However, experience has shown that the quantity will be approximately 1.5 or 2.5 times greater compared with complete displacement.